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A Critical Review on the Multiple Roles of Manganese in Stabilizing and Destabilizing Soil Organic Matter

  • Hui Li
    Hui Li
    Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
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  • , 
  • Fernanda Santos
    Fernanda Santos
    Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
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  • , 
  • Kristen Butler
    Kristen Butler
    Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
    Department of Earth and Planetary Sciences, College of Arts & Sciences, University of Tennessee, Knoxville, Tennessee 37996, United States
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  • , and 
  • Elizabeth Herndon*
    Elizabeth Herndon
    Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
    Department of Earth and Planetary Sciences, College of Arts & Sciences, University of Tennessee, Knoxville, Tennessee 37996, United States
    *Phone: 865-341-0330; email: [email protected]
    More by Elizabeth Herndon
    Orcidhttps://orcid.org/0000-0002-9194-5493
Cite this: Environ. Sci. Technol. 2021, 55, 18, 12136–12152
Publication Date (Web):September 1, 2021
https://doi.org/10.1021/acs.est.1c00299

Copyright © 2021 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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Manganese (Mn) is a biologically important and redox-active metal that may exert a poorly recognized control on carbon (C) cycling in terrestrial ecosystems. Manganese influences ecosystem C dynamics by mediating biochemical pathways that include photosynthesis, serving as a reactive intermediate in the breakdown of organic molecules, and binding and/or oxidizing organic molecules through organo-mineral associations. However, the potential for Mn to influence ecosystem C storage remains unresolved. Although substantial research has demonstrated the ability of Fe- and Al-oxides to stabilize organic matter, there is a scarcity of similar information regarding Mn-oxides. Furthermore, Mn-mediated reactions regulate important litter decomposition pathways, but these processes are poorly constrained across diverse ecosystems. Here, we discuss the ecological roles of Mn in terrestrial environments and synthesize existing knowledge on the multiple pathways by which biogeochemical Mn and C cycling intersect. We demonstrate that Mn has a high potential to degrade organic molecules through abiotic and microbially mediated oxidation and to stabilize organic molecules, at least temporarily, through organo-mineral associations. We outline research priorities needed to advance understanding of Mn–C interactions, highlighting knowledge gaps that may address key uncertainties in soil C predictions.

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1. Introduction

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Soils are a dynamic global C reservoir that contain at least three times more C than the atmosphere and four times more C than terrestrial plants. (1,2) Even small changes in the stability and bioavailability of soil C can alter atmospheric CO2 concentrations and further influence global climate. Indeed, approximately ∼14% of anthropogenic CO2 released to the atmosphere over the past decade is attributed to land use changes that destabilized soil carbon. (3) Organic matter (OM) that is intimately associated with minerals constitutes an important soil C stock. (4,5) While an extensive literature exists on biotic and abiotic factors controlling inputs and outputs of soil C in terrestrial ecosystems, (6−8) there have been indications that C storage depends on interactions between climatic and geochemical factors. (9) Predicting the magnitude and direction of changes in global C cycling with climate change requires a fundamental understanding of how intersecting climatic, biological, and geochemical factors influence soil C storage.
Manganese is a highly reactive soil constituent that is known to interact with organic compounds but remains one of the least understood factors influencing soil C storage. This critical review provides an ecosystem-level examination of interactions between Mn and OM that regulate soil C storage. This topic, despite growing interest and increasingly recognized relevance to C cycling, has never been reviewed. Remucal and Ginder-Vogel (10) provide a comprehensive assessment of how a broad suite of organic contaminants react with Mn-oxides but focus on contaminant transformation rather than Mn–C interactions in an ecosystem context. Other recent reviews have focused on properties of Mn-oxides, (11) mechanisms of Mn-oxide reduction by inorganic species, (12) microbial transformation of Mn in marine systems, (13) and mineral–organic associations that mostly exclude Mn-oxides. (4) Additionally, Berg et al. (2015) (14) discuss the role of enzymatically produced Mn(III) in driving broad trends in litter decomposability. Our aim is to synthesize existing knowledge and identify critical gaps in understanding of coupled Mn–C cycling. Our ultimate objective is to evaluate in what circumstances Mn serves to increase C storage in soils through mineral protection or decrease C storage through oxidative degradation of organic compounds. To address this unknown, we review mechanisms underlying Mn–C interactions that stabilize or destabilize organic compounds and place these mechanisms within the broader context of terrestrial environments.

2. Manganese in Terrestrial Ecosystems: Occurrence and Characterization

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2.1. Manganese Geochemistry in Soils

Manganese is a redox-sensitive element that comprises about 0.1% of weight of crustal rocks (15) and is commonly found in air, soils and sediments, and surface and groundwater. (16−18) Mn(II) that is released during weathering of bedrock minerals is readily oxidized to Mn(III/IV) to form more than 30 known Mn (hydr)oxide (i.e., Mn-oxides) minerals with more than 15 phases known in soils (e.g., birnessite, manganite, and hausmannite; Supporting Information (SI) Table S1). (19,20) Manganese oxides are ubiquitous in soils and sediments as fine-grained aggregates, veins, nodules and concretions, crusts, dendrites, and coatings on rocks or other mineral particles. (19) Due to their high oxidation potentials and relatively large surface areas, Mn-oxides exert strong controls over the distribution, transport, and transformation of other elements, far out of proportion to their natural abundance. Here, we briefly introduce Mn redox cycling and the formation, properties, and transformation of Mn-oxide minerals and defer to existing reviews (10,13,19,21−23) for more detailed processes.
The majority of naturally present Mn-oxides are formed during biotic Mn(II) oxidation to a poorly crystalline phyllomanganate phase that subsequently transforms to more ordered phases (SI Figure S1). (21,24−26) Mn(II) is oxidized by a wide range of phylogenetically diverse microorganisms (bacteria, fungi, algae, and diatoms) using multiple enzymatic and nonenzymatic pathways. (11,13,21,27−29) Various microorganisms use multicopper oxidases and/or animal haem peroxidases (AHPs) to directly mediate a one-electron transfer from Mn(II) to generate Mn(III), which may be followed by disproportionation to Mn(II) and Mn(IV) (13,21) or a second electron transfer reaction to form Mn(IV). (30) Biotic Mn(II) oxidation also occurs through interaction with reactive oxygen species (ROS) produced by bacteria (e.g., Roseobacter (27,31)), Ascomycete fungi, (32) and phototrophic algae and diatoms. (28) The physiological function of Mn(II) oxidation remains unclear since most known organisms do not obtain energy from the reaction, and Mn(II) oxidation has only been demonstrated as a viable energy-yielding metabolism in select bacteria. (33,34)
Abiotic Mn(II) oxidation is another important and well-studied pathway for Mn-oxide formation. Homogeneous oxidation of aqueous Mn(II) by dissolved O2 is thermodynamically unfavorable (at pH < 8) and kinetically hindered due to high activation energy of the reaction, (12,18,35) yet there are many physical and chemical factors that drive abiotic formation and transformation of Mn-oxides. Mn(II) can be oxidized to Mn(III) by ROS (e.g., O2•–, 1O2, OH•) produced through abiotic pathways including Fe(II) oxidation, (36) nitrate photolysis, (37,38) illumination of humic substances, (39) and illumination of metal oxides in desert varnish. (40) Further Mn(III) oxidation by ROS and/or disproportionation results in rapid Mn(IV)-oxide formation. (37,38) Mineral surfaces, such as Fe-oxides and Mn-oxides, also catalyze Mn(II) oxidation via interfacial catalysis and/or electrochemical reactions (41−43) at rates equivalent to or faster than biological oxidation. (44,45) Products of Mn(II) oxidation and Mn-oxide phase transformations are highly dependent on pH conditions (e.g., SI Figure S1) and mineral surface properties (e.g., semiconductivity, particle size, ions). (35,42,46−48) For instance, the presence of transition metals (e.g., Co and Ni), (49) high concentrations of uranyl, (50) and thallium (51) have all been shown to mediate changes in Mn-oxide crystallinity or phase transformation.
Microbial Mn(IV) reduction, well-understood as a primary driver of Mn-oxide dissolution under reducing conditions, is attributed primarily to bacteria with lesser contributions from fungi and archaea. (13) Mn(IV)-reducing bacteria respire Mn(IV) using H2 and various organic compounds as electron donors, but this process may be hindered in the presence of other electron acceptors such as oxygen, nitrate, and sulfur compounds. (34) Microbial Mn(IV) reduction initiates via a one-electron transfer step to form soluble Mn(III) as intermediate product. (12,52) Mn(III,IV)-oxides are also abiotically reduced by a wide range of organic (53,54) and inorganic compounds, (12) for example, Fe(II) and sulfide. Mn reduction rates vary with environmental factors including soil temperature, water potential, pH, and redox conditions. (55)

2.2. Ecological Function of Mn as a Micronutrient and in Relation to Carbon Storage

Manganese plays essential yet complex roles in influencing soil C storage. Specific mechanisms influencing C dynamics include Mn’s key function as a biotic and abiotic oxidizer of soil organic matter (SOM), as discussed in more detail in following sections. In this subsection, we introduce its broader roles as a micronutrient and in microbial decomposition.
Manganese is a critical micronutrient for plants and can impair growth when present in either limiting or excess quantities. In acidic soils where Mn is readily soluble, cycling through vegetation (i.e., uptake and litterfall) results in Mn accumulation in surface soils after a few decades. (56,57) Dissolved Mn(II) in soil solution is moved through roots and transported primarily to leaves where it is utilized in the water-splitting complex of photosystem II and as a cofactor for superoxide dismutase, an enzyme that detoxifies byproducts of photosynthesis. (58) Manganese accumulates in leaves as aqueous or organic-bound Mn(II) (59,60) and is not translocated to other plant tissues or reabsorbed during senescence; consequently, Mn is largely returned to soil each year in throughfall or litterfall. (61) Mn(II) species in litter are oxidized to insoluble Mn(III/IV)-oxides during decomposition, facilitating their accumulation in surface soils. (59,62,63) Many plants accumulate Mn in linear proportion to the quantity of soluble and exchangeable Mn(II) in soil. (64) As such, foliar Mn concentrations often exceed nutritional requirements, resulting in oxidative stress and impaired photosynthesis in sensitive plant species (e.g., sugar maples). (58,65−67) At the ecosystem-scale, Mn toxicity can lead to declines in certain plant types where soil Mn is highly phytoavailable. For example, increases in Mn(II) solubility driven by chronic nitrogen deposition led to the decline of forbs in a temperate steppe ecosystem. (68) The resulting shift to an exclusively grassland system was attributed to the greater sensitivity of forbs to reduced photosynthetic rates and growth driven by high Mn(II) concentrations. Decreases in plant biomass and shifts in plant communities driven by Mn bioavailability may in turn affect the quantity and composition of C inputs to soil.
During SOM decomposition, facultative anaerobic microorganisms can utilize Mn(IV) as a terminal electron acceptor via dissimilatory Mn(IV) reduction, particularly when more thermodynamically favorable dissolved O2 and nitrate are absent or limited. (22,69) Microorganisms couple Mn(IV) reduction to their metabolic oxidation of organic compounds (e.g., acetate, sugars, amino acids, long chain fatty acids, and aromatics) to release CO2. (70,71) Dissimilatory Mn(IV) reduction depends on pH and redox potential and predominates where anoxic or suboxic conditions develop, such as pores within soil microaggregates and soil horizons that experience large fluctuations in moisture and/or are poorly drained. Specifically, Mn(IV)-oxides are reduced by microorganisms during periods of anoxia when O2 is depleted but regenerated during subsequent oxic periods. High concentrations of OM also promote reducing conditions that favor Mn reduction. As one example, applications of biodegradable OM to crop soils over 19 years coupled to high soil moisture and temperature promoted anaerobic respiration that mobilized high concentrations of Mn in subsoils. (72) Hence, Mn is expected to serve as an important terminal electron acceptor in soils, especially in organic-rich soils with absent or low levels of O2 and nitrate that are strongly affected by fluctuating water tables. Reducing conditions that drive Mn-reduction can also increase dissolved organic C (DOC) mobilization in soils, (73) potentially increasing OM bioavailability to microorganisms.
Manganese may serve other ecological functions that remain poorly recognized. For example, Macrotermitinae are a subfamily of termites that can depolymerize lignin in their guts (74) and cultivate specialized Basidiomycete fungi in their colonies to aid in digestion of lignocellulosic material. (75) Certain Basidiomycetes are highly effective wood degraders due in part to their production of Mn peroxidase enzymes that act as biocatalysts to degrade lignin. (76) Cultivated fungal combs can concentrate Mn to levels nearly 103× the surrounding soil, and termites that ingest these combs contain 100× more Mn than other insects and store unique Mn nodules in their abdomens. (77) Given that fungal-farming termites are important ecosystem engineers in many arid environments, Mn may be an unrecognized supporter of these ecosystems through its ability to enhance lignin degradation.

2.3. Mn Characterization Techniques and Limitations

Studies on Mn in soils are limited by its low abundance, monoisotopic nature, and the poor crystallinity of a diverse array of Mn-oxides. Poorly crystalline minerals are difficult to identify and characterize due to limitations of conventional techniques. Techniques used to study Fe-oxides and Fe–C interactions, for example, Fe stable isotope labeling and fractionation and Mössbauer spectroscopy, cannot be applied to Mn-oxides. Furthermore, commonly used chemical extractions (e.g., dithionite) are nonspecific and coextract Mn-oxides with Fe-oxides. Even chemicals that selectively dissolve Mn-oxides (hydroxylamine hydrochloride) may extract small quantities of Fe-oxide (<5%) that are equal in concentration to extracted Mn. (78)
Despite these challenges, a suite of techniques applied to natural and synthetic Mn-oxides have provided insight into their formation and properties. Synchrotron-source X-ray absorption spectroscopy (XAS) remains one of the key techniques for characterizing Mn within complex environmental samples, (21) including both minerals (25,79,80) and organic materials (59,62) in bulk or at the micrometer-scale, by probing its oxidation state and local bonding environment. (81) X-ray diffraction (XRD) has also been widely used to characterize synthetic, biogenic, and natural Mn-oxide structures, (11,19) but application of even synchrotron-source XRD is challenging for natural samples that are low in Mn-oxide abundance or lack long-range crystalline order. (25,80) Where XRD is limiting, Fourier transform infrared (FTIR) spectroscopy can probe short-range order by measuring Mn–O and O–H bond vibrations, yielding information on vacancies and proportions of distinct phases within mixtures (e.g., triclinic and hexagonal birnessite). (80,82) Additional phase identification has been performed using nondestructive Raman spectroscopy, although application of this technique has been limited by low Raman activity and experimental artifacts. (83,84) X-ray photon spectroscopy (XPS) provides key information on Mn oxidation states at mineral surfaces that mediate geochemical reactions but may differ from bulk composition. (85) Finally, new insights into fractionation of stable oxygen isotopes (δ18O) during biogenic Mn(II) oxidation indicate the potential for δ18OMnOx to record oxygen sources and bio-oxidation pathways. (86,87) These techniques, particularly in combination, can provide new insights into coupled Mn–C interactions in natural systems.

3. Mn-Promoted Protection of Organic Compounds

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Organic compounds are known to directly interact with mineral surfaces through various sorption reactions (88) or can be stabilized through coprecipitation and aggregate formation. The mineral–OM association is one essential mechanism in stabilizing and protecting SOM against microbial metabolism. (4,89) Iron oxides are common soil minerals that have been extensively studied for their abilities to bind OM through mechanisms such as anion exchange, ligand exchange/surface complexation, hydrophobic interaction, entropic effects, hydrogen bonding, and cation bridging. (90) Mn-oxides possess many similar properties to Fe-oxides but are less well studied for their interactions with OM. However, Mn-oxides, particularly poorly crystalline phases with relatively large specific surface area (SSA), have unusually high adsorption capacities and scavenging capabilities for organic (91,92) and inorganic compounds (e.g., heavy metals), (20) thereby potentially protecting OM against decay. This section explores mechanisms for OM stabilization by minerals including Mn-oxides and discusses current evidence for these associations in terrestrial (91,93) and engineered systems. (94) A summary of Mn–C interactions, both stabilizing reactions discussed here and destabilizing reactions reviewed in Section 4, is presented in Figure 1. Our aim is to present an overview of these processes, while more detailed reaction mechanisms are reviewed elsewhere. (10)

Figure 1

Figure 1. Conceptual diagram summarizing stabilizing and destabilizing interactions between Mn(III,IV)-oxides and organic compounds. Organic compounds are represented as unspecified organic matter (OM) or by specific functional groups and/or compounds. The processes shown here serve as examples for each category and are streamlined representations of detailed mechanisms. Stabilization reactions include outersphere interactions such as cation bridging (103) and electrostatic interactions, inner-sphere ligand exchange reactions that release hydroxyl ions, (94,106) physical trapping between Mn-oxide layers, (94) coprecipitation within the oxide structure, and polymerization of degraded phenolic structures. (116) Destabilization reactions include reductive dissolution of Mn-oxides through abiotic interactions with reducing compounds (e.g., catechol degradation (116)) or microbially mediated dissimilatory Mn reduction (22), fragmentation of phenolic radicals, (116) ligand-promoted extraction of Mn(III) (e.g., phosphonoformic acid), (54) and Mn(III)-promoted degradation of organic chelators through internal electron transfer (e.g., NTMP degradation (146)). Degradation reactions often release byproducts such as carbon dioxide or phosphate through cleavage of functional groups.

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3.1. Mechanisms for Organic Matter Protection

Soil OM is protected from microbial degradation in part through mineral associations. (4) Adsorption of organic compounds to mineral surfaces involves physical forces such as hydrophobic partitioning, electrostatic outer-sphere complexation, and cation bridging, and chemical interactions including inner-sphere complexation (ligand exchange). (95) Organic matter can also be protected when it is incorporated into (coprecipitation) or encapsulated by (physical trapping) precipitating minerals. Polymerization of small organic compounds into more complex structures may increase chemical recalcitrance.

Hydrophobic Partitioning

Hydrophobic compounds partition from aqueous solution onto Mn-oxides due to incompatibility of nonpolar compounds with water. (96) Organic compounds with hydrophobic moieties (e.g., −OCH3, −CH3, −CF3, −CN) have shown a higher tendency to precipitate on Mn-oxides surface than compounds with hydrophilic moieties. (96) As one example, during adsorption of several phenylarsonic acid analogues on birnessite, molecules with higher hydrophobicity showed higher adsorption rates. (97)

Electrostatic Interaction

pH-dependent electrostatic interaction is one of the most important mechanisms driving OM adsorption to soil minerals. (98,99) Minerals and OM are net positively charged at lower pH than their isoelectric points (point of zero charge = PZC) due to surface protonation but net negatively charged at higher pH due to deprotonation. (100) Most Mn-oxides are net negatively charged on the surface at neutral pH (20) (SI Table S1) and have high potential to sorb positively charged ions and functional groups. For example, protein adsorption on soil minerals surface normally initiates through electrostatic interaction, during which positively charged functional groups of proteins complex with negatively charged mineral surfaces. (100,101) However, this weak interaction is generally reversible. (101) Similar to Mn-oxides, natural organic matter (NOM) is generally net negatively charged at circumneutral pH, (95) limiting electrostatic interactions between OM and Mn-oxides under many common soil conditions. However, electrostatic interactions between Mn-oxides and organic compounds increase with decreasing pH as organic compounds protonate and associate with Mn-oxide surfaces. (97,99)

Cation Bridging

Polyvalent cations can balance negative charges on mineral surfaces and OM functional groups, thus acting as a bridge to connect them. (102) For example, adsorption of Mn2+ ions to Mn-oxide surfaces can partially neutralize negative surface charges and facilitate adsorption of complex humic-like compounds. (103) Dissolved Ca2+ has similarly been shown to promote adsorption of river-isolated NOM to poorly crystalline and crystalline Mn-oxides (lithiophorite, birnessite, and cryptomelane). (96) This mechanism has been frequently observed for clay minerals, which remain negatively charged over a wide pH range. For example, metal cation-saturated montmorillonites can adsorb up to 5× more DNA than pure montmorillonites, (102) and divalent cations (e.g., Ca2+ and Mg2+) are generally more efficient in promoting adsorption than monovalent cations (e.g., Na+). (104)

Ligand Exchange

Ligand exchange is a strong adsorption mechanism that occurs between hydroxyl (−OH) functional groups on mineral surfaces and carboxyl or phenolic −OH groups of OM. (90,105) Significant −OH release during NOM adsorption to birnessite serves as an indicator of this ligand exchange reaction. (106) It is likely that carboxylate groups form inner-sphere complexes with birnessite via ligand exchange with −OH on the Mn-oxide surface. (94,106) Ligand exchange is particularly favored for NOM with high molecular weight, acidity, and aromaticity. (90,106) Unlike weaker electrostatic and cation bridging reactions, specific adsorption through ligand exchange is not readily reversible.

Coprecipitation

Under aeration, Mn(II) can be rapidly oxidized to Mn(III/IV)-oxides that coprecipitate with dissolved OM. (107) Coprecipitation with OM, primarily studied in Fe-oxides and Al-oxides, leads to quantities and compositions of stabilized OM that differ from adsorption. (108−110) Coprecipitation of SOM with Fe-oxides has resulted in higher C stabilization (108) and higher enrichment of aromatic components (110) than surface complexation (adsorption). Manganese widely coexists with Fe in soils, though at lower abundance, and precipitates under similar conditions. It is likely that SOM coprecipitates with Mn-oxides to generate C stabilization similar to Fe-oxides.

Physical Trapping

Mn-oxides can physically protect OM by coating organic structures or serving as cementing agents that stabilize OM in microaggregates. (111) Mn(II)-oxidizing bacteria (e.g., Leptothrix discophora and Pseudomonas putida), fungi (e.g., Acremonium and Ascomycete) and phototrophic organisms (e.g., algae and diatoms) are often encrusted in the Mn(III,IV)-oxides generated during microbial Mn(II) oxidation, (21,28,32,112,113) which can trap cellular components or whole cells. Dissolved organic compounds that bind on Mn-oxide surfaces can also be trapped when a new layer of Mn-oxide precipitates. (94)

Polymerization

Polymerization reactions stabilize soil organic carbon either physically, ascribed to increased size and complexity of micro/macro-aggregates, or chemically, via increasing the energy threshold to be overcome in microbial respiration. (114) Mn(IV)-oxides are Lewis acids that serve as efficient oxidants that can degrade organic compounds into smaller molecules, as discussed in the next section, or accelerate oxidative polymerization of polyphenols under common soil pH conditions (e.g., 4–8). The ability of Mn-oxides to drive polyphenol polymerization, sometimes referred to as “browning”, far exceeds similar effects of Fe-, Al-, or Si-oxides. (115) This accelerating effect is more efficient at near neutral pH than acidic conditions. Birnessite rapidly oxidizes hydroquinone to 1,4-benzoquinone at pH 7, which then undergoes dimerization and/or ring cleavage reactions that ultimately polymerize degradation products. (116) Associated decarboxylation reactions produce CO2, which can be released as a gas or react with aqueous Mn(II) to form rhodochrosite (MnCO3) at neutral pH. (116) In one case, NOM adsorbed to Mn-oxide largely via Mn-carboxylate bonds; subsequently, phenol groups within the bound OM were oxidatively transformed to phenoxy radicals that further polymerized (e.g., converted to quinones or produced dimers or polymeric products) in the interlayers of Mn-oxides. (94)

3.2. Evidence for Organic Matter Protection in Natural Systems

While many studies have investigated how individual organic compounds adsorb to and react with Mn-oxides, few have examined these interactions in natural or engineered systems with environmentally formed minerals and/or complex mixtures of organic compounds. In two studies, DOM leached from forest organic soils was reacted with synthetic goethite and Mn-oxides (birnessite and hydrous Mn-oxide). (92,106) Birnessite adsorbed low amounts of C relative to goethite but more effectively oxidized and transformed OM. (106) In contrast, hydrous Mn-oxide had a greater capacity for OM sorption than goethite, most likely due to its high surface area. However, OM bound to hydrous Mn-oxide was also more readily desorbed, possibly because goethite formed carboxylic-metal bonds with OM while bond with hydrous Mn-oxide were weaker. (92)
Interactions between Mn-oxides and OM have been reported for wastewater treatment systems where Mn-oxides are used as sorbents in the treatment process. Substantial OM was associated with birnessite-coated sand obtained from a water treatment filter bed. (94) Organic compounds that adsorbed to birnessite surfaces were physically trapped under progressive layers of freshly precipitated birnessite. Trapped OM was more aromatic than the initial OM, indicating the potential for Mn-mediated polymerization of OM within the oxide layers. In another study, (96) NOM was reacted with both synthetic birnessite and natural Mn-oxides (mixture of lithiophorite, birnessite, and cryptomelane) used for drinking water treatment. Organic matter adsorption increased as pH decreased and was facilitated by hydrophobic partitioning. High molecular weight molecules with high aromaticity preferentially adsorbed to the minerals and were partially oxidized to low molecular weight compounds.
There is also evidence that Mn-oxides serve as meaningful reservoirs for OM in natural environments that include brackish pond water, ferromanganese cave deposits, and soils. (91,93) Estes et al. (2017) (91) examined how Mn-associated OM in cave deposits and pond water compared to Mn–C associations formed in cultures of Mn-oxidizing terrestrial fungi and marine bacteria. In pond water and cave deposits, Mn-oxides trapped heterogeneous mixtures of OM that reflected the natural complexity of OM in those systems. In microbial cultures, Mn-oxides hosted relatively homogeneous OM enriched in proteinaceous and/or lipopolysaccharide compounds (amide, carboxylic, and o-alkyl functional groups), likely derived from the microbial components used in Mn-oxide formation.
In soils, the contribution of Mn-oxides to OM preservation may be historically underestimated by bulk techniques that fail to differentiate between Mn- and Fe-oxides; however, spatial associations between Fe, Mn, and OM are important to consider. (93) Within a soil ferromanganese nodule, Fe- and Mn-oxides were shown to form distinct accumulation zones following cycles of mobilization and reprecipitation during alternating wetting and drying conditions. (93) These oxide accumulations preserve OM by acting as cements that limit microbial access to small pores. Organic C was highly enriched where Mn-oxides accumulated but relatively depleted in Fe-rich zones, indicating an important role for Mn-oxides in preserving OM in these nodules.

4. Mn-Promoted Degradation of Organic Compounds

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4.1. Oxidation of Organic Compounds by Mn-Oxides

Mn-oxides are among the strongest natural oxidants in soils (95) and are capable of oxidizing OM (53,106,117) and inorganic compounds. (20,118) Mn-oxides can decompose OM via oxidative reactions into smaller compounds that can ultimately be transformed to CO2. (119) Mn-promoted OM oxidation also directly produces CO2 through decarboxylation reactions. (116) Although extensive research has addressed oxidation of organic contaminants by Mn-oxides, (10) particularly as a remediation strategy, the role of Mn-oxides in degrading soil OM remains unresolved. In this section, we discuss Mn-oxide dissolution by organic compounds via proton-promoted (organic acids) and ligand-assisted (e.g., oxalate, citrate) mechanisms as well as the oxidation and degradation of organic compounds (53) by reductive dissolution (e.g., catechol, phosphonoformic acid, oxalic acid) (Figure 1). (54,120)

Proton-Promoted Reaction

Decreases in pH accelerate mineral dissolution and transformation. (46) Organic acids are a potential source of protons that bind to oxygen atoms on the mineral surface, weaken their bonds with metal cations, (95) and facilitate Mn-oxide dissolution. For example, manganite (MnIIIOOH) dissolves via proton-promoted disproportionation at pH < 6 to produce MnIVO2 solids and dissolved Mn(II). (121) Similarly, disproportion of hausmannite (Mn3O4) is initiated by proton-promoted dissolution at pH < 7 and accelerates with decreasing pH. (46)

Ligand-Assisted Reaction

Ligands that complex onto structural Mn centers via exchange for hydroxyl functional groups can change mineral surface coordination, polarize and weaken the Mn–OH bonds, and cause detachment of the Mn cation into solution. (95,105) MnOOH dissolution by citrate, pyrophosphate (for β-MnOOH), (105) and phosphonoformic acid (for γ-MnOOH) (54) initiates via ligand-assisted dissolution reactions that form aqueous Mn(III)-complexes.

Redox Reaction

Mn(IV) and Mn(III) in Mn-oxides are both capable of accepting electrons from organic molecules and being reduced to lower valence states, for example, to Mn(III) and Mn(II). The reducing organic compound is concurrently oxidized to produce various degradation products. For example, in contrast to manganite, dissolution of birnessite by phosphonoformic acid (H2PO3COOH) occurs via reductive dissolution to generate aqueous Mn(II), orthophosphate, and CO2. (54) Birnessite efficiently oxidizes catechol into benzoquinone and eventually to CO2, forming three moles of Mn(II) for each mole of CO2 released. (122) Oxidative transformation of organic compounds by Mn-oxides is also evident for complex organic mixtures. Dissolved OM from forest floor leachate reacted with Mn-oxide to release aqueous Mn(II), acetic acid, and formic acid as major products. (106)
Reductive dissolution is the predominant mineral dissolution mechanism induced by most organic reactants; (54) however, many SOM compounds interact with Mn-oxides via a combination of the reaction mechanisms discussed above. For example, oxalic acid, phosphonoformic acid, and citric acid strongly adsorb to several Mn-oxides. (54,120) These organic compounds complex with Mn-oxides through ligand exchange, initiate ligand-assisted dissolution, and then undergo intramolecular electron transfer in the solubilized Mn(III)–organic complex to generate aqueous Mn(II) and oxidized organic compounds.
Plant and microbial siderophores, organic chelating compounds produced by soil organisms to acquire Fe, also effectively dissolve and are degraded by Mn-oxides through a combination of dissolution mechanisms. Siderophores bind structural Mn(III) atoms to functional groups such as hydroxyamate, catecholamide, hydroxycarboxylate, and/or catecholate. (123,124) At circumneutral to high pH, siderophores extract Mn(III) and drive ligand-promoted dissolution. (125−127) At circumneutral to low pH, siderophores and their subsequent oxidation products reduce Mn(III) to Mn(II), either at the mineral surface or in solution following Mn(III) extraction. (124−127) In Mn(II)-bearing oxides such as hausmannite, depletion of Mn(III) ultimately destabilizes the mineral structure and induces additional release of Mn(II) ions into solution. (127) These compounds may act in concert with low molecular weight organic compounds to dissolve Mn-oxides in soils, (128) with the added significance that Mn-oxides may compete with Fe-oxides and act as an important soil sink for siderophores.

4.2. Oxidation of Organic Compounds by Mn(II)/Mn(III) Cycling

4.2.1. Fungal-Mediated Mn Cycling and Litter Decomposition

Manganese is a critical component of the microbial decomposition of lignin compounds and has been identified as a key factor influencing litter decomposition rates. Much of what is known about the mechanisms for Mn-promoted lignin degradation is based on pure culture studies of lignin-degrading fungi and their enzymes. (76) However, correlational relationships at the ecosystem-scale suggest positive relationships between foliar Mn and both litter mass loss (14,129) and the activities of lignin-degrading enzymes. (130) Similarly, exchangeable soil Mn(II) is negatively correlated with soil C storage across multiple biomes. (131−133)
During litter decomposition, a select group of saprotrophic fungi (Agaricomycetes within the phylum Basidiomycota) release the extracellular enzyme Mn peroxidase (MnP) which degrades lignin by oxidizing its phenolic bonds. (134) Other ligninolytic enzymes (e.g., lignin peroxidase, laccase) are less efficient lignin degraders because they only attack nonphenolic bonds. Specifically, MnP oxidizes aqueous Mn(II) to chelated-Mn(III), which then serves as a diffusible reactant that oxidizes lignin. (135) As the microbial oxidation of lignin proceeds, (hemi)cellulose bound within the lignocellulose matrix becomes readily accessible to soil microbes. Given that lignin is the next most abundant natural polymer on Earth after (hemi)cellulose, understanding how Mn(II) and Mn(III) cycling in soils contributes to litter degradation (130,136) is essential to determine its role in the global C cycle. It is important to note that MnP can oxidize more than lignin. For example, certain basidiomycetes can oxidize a suite of polycyclic aromatic hydrocarbons. (137−139) Chelated-Mn(III) can also oxidize unsaturated fatty acids to lipid radicals (140,141) which further oxidize and cleave nonphenolic lignin structures. (142)
These observations suggest that Mn availability is a key regulator of fungal-mediated litter decomposition and plays a relevant role in stimulating soil CO2 fluxes and influencing soil C storage. To assess net impacts of Mn enrichment on ecosystem C storage, we compiled available field and laboratory data reporting ecosystem response to simulated Mn addition (Figure 2; SI Table S2). Despite experimental differences in substrate composition, Mn addition, and incubation time, these studies demonstrate that Mn addition stimulates production of ligninolytic enzymes including MnP (+20 ± 6% change relative to control), depletes lignin in decomposing litter (−14 ± 5%), and increases C loss as both DOC (+17 ± 12%) and CO2 (+14 ± 3%). As one example, litter and organic horizons that received Mn additions were reported to have 10–35% higher rates of soil-respired CO2 effluxes (143,144) and produce higher DOC concentrations (145) than control treatments. The mass of leaf litter that remained following decomposition also generally decreased in response to Mn addition over the incubation time (−5 ± 6%), but this effect was less clear across treatments.

Figure 2

Figure 2. (top) Mn-mediated enzymatic oxidation of lignin by Agaricomycete fungi (phylum Basidiomycota). Fungi produce manganese peroxidase (MnP) that reacts with hydrogen peroxide (H2O2) to convert Mn2+ into reactive Mn3+, which in turn is stabilized by small organic molecules (i.e., organic acids). The attack of phenolic lignin structures by chelated Mn3+ leads to the production of CO2 and DOC. (bottom) Responses (in % difference relative to control) of ligninolytic enzyme activity, leaf litter mass, lignin content, CO2 fluxes (or respiration), and dissolved organic carbon to Mn additions in studies that simulated Mn addition to litter and/or soils (see data in SI Table S2). Data in the violin plots include the median (white dot), interquartile range (horizontal black bar), lower and upper adjacent values (black whiskers), outliers (black crosses), and probability density (colored area).

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4.2.2. Organic–Mn(III) Complexes in Soil Solution

Organic–Mn(III) complexes formed through abiotic processes have also been implicated as a driver of OM degradation. (43,146) Chelated Mn(III) can form through either oxidation of aqueous Mn(II), extraction of solid-phase Mn(III) by ligands, or surface-promoted oxidation of sorbed Mn(II). For the latter, minerals such as goethite facilitate the oxidation of aqueous Mn(II) to reactive Mn(III) species that subsequently oxidize organic molecules to regenerate Mn(II). (43)
Aqueous Mn(II) ions form complexes with a variety of organic molecules that accelerate their oxidation to Mn(III). For example, phosphonic acids such as nitrilostris(methylene-phosphonic acid) (NTMP) form Mn(II)–NTMP complexes that oxidize to Mn(III)–NTMP in air. (146) Mn(III) subsequently extracts an electron from the compound and oxidizes NTMP to form degradation products. A diverse suite of siderophores, including desferrioxamine B (DFOB), rhizoferrin, and protochelin, similarly bind aqueous Mn(II) and facilitate air oxidation to Mn(III)–siderophore complexes. (123,147,148) These Mn(III) complexes are stable at high pH (∼7–11) but undergo internal electron transfer to generate aqueous Mn(II) and oxidized degradation products at pH < 7. (147) Aqueous organic-Mn(III) complexes can also form when ligands (e.g., phosphonates, siderophores) extract Mn(III) from Mn-oxides. (149) Siderophores drive ligand-promoted dissolution of hausmannite, (127) manganite, (125) Mn(III/IV)-oxides, (126) and δ-MnO2 (128) under circumneutral to alkaline pH by either directly extracting Mn(III) from Mn(III)-bearing minerals or reducing Mn(IV) atoms to Mn(III) prior to extraction.
Although the occurrence of Mn(III) species at redox interfaces is increasingly recognized in aqueous and terrestrial systems, (150,151) the environmental interactions of these Mn(III) species are relatively unexplored. Mn(III) can form complexes with organic ligands under oxic and anoxic conditions, (123,148,152) and evidence regarding their subsequent reactivity is newly emerging. For example, Mn(III) catalyzes N-dealkylation of atrazine and can also oxidatively degrade 4-chlorophenol through one electron transfer. (153) Mn(III) species formed by Mn(II) oxidation at redox interfaces also oxidize and depolymerize OM in soils. (154) Indeed, oxidative capability was shown to be strongly correlated with Mn(III) abundance in the litter layer of a temperate deciduous forest, from which Jones et al. (2020) (155) contend that Mn(III)-mediated reactions may dominant oxidative processes in litter and organic soils. Mn(III)–complexes can also oxidize nitrite to nitrate under anoxic conditions, (156) indicating an important role for Mn in driving oxidation of both organic and inorganic species.

5. Intrinsic and Environmental Factors Influencing Mn-Oxide–C Interactions

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This section discusses the roles of intrinsic characteristics of Mn-oxides and SOM and external environmental conditions on regulating Mn-oxide–C interactions.

5.1. Characteristics of Mn-Oxides That Influence Organo–Mineral Interactions

Biogenic and abiotic Mn-oxides that form in soils (157) transform to more stable and oxidized crystalline phases over time (SI Figure S1). (157,158) The properties of these Mn-oxide minerals are important for regulating reactivity with organic compounds. For example, the oxidation state, SSA, redox potential, and structure of Mn-oxides determine OM adsorption capacity and oxidation efficiency. (12,54) Here, we discuss how characteristics of Mn-oxides control their stability and reactivity. (157)
Specific surface area varies from <10 to >200 m2 g–1 among Mn-oxides that differ in structure, particle sizes, and crystallinity (SI Table S1). (20,159,160) In general, Mn-oxides with smaller particle sizes and poor crystallinity exhibit higher SSA and more exposed reactive surface sites. Biogenic Mn-oxides that consist of phyllomanganates with dominant structural Mn(IV) generally have higher SSA than more crystalline synthetic Mn-oxides. (20,160,161) High SSA and disordered crystalline structure generally confer higher reactivity, as has been demonstrated with poorly crystalline Fe-minerals that are more susceptive to reductive dissolution and subsequent mineralization of associated OM than crystalline Fe-oxide phases. (162,163)
Different Mn-oxides also possess different types and abundances of reactive surface sites, for example, planar octahedral vacancy sites and edge sites, (19,26,164) which provide different levels of binding energies. Edge sites contain unsaturated singly and doubly coordinated oxygen groups that confer a large fraction of the total surface charge and have high ion adsorption capacity. (26) In low-vacancy (<10%) δ-MnO2, edge sites were shown to contribute ≥50% of the total reactive sites for adsorbing Pb(II). (165) Edge sites also dominate electron transfer reactions for Mn-oxides such as birnessite. (166) In birnessite, edges serve as main adsorption sites for ligands such as fulvic acid (164) and for oxyanions such as As(III) and As(V) while vacancies have higher affinity for metal ions (e.g., Mn(II), Mn(III), Zn2+). (166)
Oxidation state largely determines the forms of Mn present and their environmental interactions. Mn(III) and Mn(IV) precipitate in Mn(III,IV)-oxides, which have among the highest standard reduction potentials of natural minerals (e.g., E° = 1.29 V for the synthetic vernadite-analog MnO2) and a higher capacity to oxidize organic compounds than MnIIIOOH. (54,95) Birnessite, the most dominant class of MnO2 minerals in soils, oxidizes a wide range of organic compounds. (54) Within a single MnO2 phase (e.g., birnessite or vernadite), higher Mn(IV) content generally results in stronger capacity to oxidize reduced species (e.g., As(III) and Cr(III)) (167,168) and organic compounds (e.g., hydroquinone). (169) The relative oxidizing activity between Mn(III) and Mn(IV) within Mn-oxides may vary due to multiple mechanisms. For example, Mn(III) sites are less effective at oxidizing As(III) due to their lower affinity for As(III) adsorption onto δ-MnO2 (167) and Cr(III) due to surface site occupation by Mn(III) on vernadite. (168) Although Mn(IV) sites confer higher oxidation capacity, Mn(III) undergoes more rapid ligand exchange. Surface functional groups OH– and H2O easily dissociate from relatively labile Mn(III) due to Jahn–Teller distortion but barely from Mn(IV). (12) Mn(III) in Mn-oxides can accept one electron from OM or inorganic reactants and be reduced to Mn(II). Mn(II) is present in certain Mn-oxides (e.g., hausmannite) but is highly soluble and often found as an aqueous ion. (120) The presence of Mn(II) in solution improves OM adsorption to mineral surfaces via cation bridging. Mn(II) also complexes with humic acid to promote OM aggregation and precipitation, although this effect is more strongly induced by Mn(III). (43,152)
The structure, phase transformation, and morphology of Mn-oxides also regulate their oxidizing reactivity. Biogenic Mn-oxides typically undergo transformation from hexagonal to triclinic birnessite which decreases their oxidizing capability, as observed for Cr(III) oxidation in the presence of OM and light that drove photoinduced regeneration of hexagonal birnessite. (170) Rates that various MnO2 structures were able to oxidize bisphenol A decreased in the order δ-MnO2 > α-MnO2≈ γ-MnO2 > λ-MnO2 > β-MnO2. (171) Similarly, the rate that α-MnO2 oxidized the anti-inflammatory compound naproxen was shown to increase from nanorod to flower-like nanostructure to nanoparticle morphologies. (172) Reactions between Mn-oxides and organic compounds can also induce structural changes, for example, a buildup of interlayer Mn(II/III), that reduce reactivity over time. (173)

5.2. Characteristics of Organic Compounds That Influence Organo–Mineral Interactions

Molecular properties of OM, including molecular size, polarity, composition and position of various functional groups, also control OM affinity to Mn-oxides and their reactivity. (88,97) In this section, we discuss how intrinsic OM characteristics and various complexation mechanisms (e.g., inner-sphere and outer-sphere) determine reactivity with Mn-oxides. (53)
Soil OM is comprised of various functional groups, including acidic (e.g., carboxyl), neutral (e.g., alcohol), and basic groups (e.g., amine), and is, in general, negatively charged at near neutral pH. (95) Functional groups exert a strong control over how organic molecules adsorb to and react with mineral surfaces. The affinities of functional groups for metal ions vary, (95,97) generally decreasing in the order –O– > −NH2 > −N = N >COO– > −C=O. (95) Most DOC binds on Fe-oxide and clay minerals (e.g., goethite and montmorillonite) preferentially via carboxylate moieties, but N-rich organic compounds may have higher adsorption affinity to Mn-oxides. (94) Phenolic groups are particularly susceptible to oxidation by Mn-oxides. (43)
The effects of functional groups on organo-mineral interactions have been assessed by comparing adsorption and oxidation of several phenylarsonic acid analogues on birnessite. It has been observed that −OH and −NH2 groups promote adsorption via deprotonation and H-bonding and also donate electrons to birnessite to generate radicals, with −NH2 demonstrating a higher electron-donating ability than −OH. (97,99) In contrast, the electron-withdrawing −NO2 group decreases compound adsorption due to steric hindrance and also inhibits electron transfer and radical formation. (97) The position of these functional moieties also influences the reaction. For instance, −NH2 and −OH groups at ortho and para positions of the aromatic ring, which are the dominant positions of functional moieties, increase electron density at C1 and accelerate electron transfer from OM to birnessite. (97)
Differences in preferential adsorption between various organic compounds and minerals can lead to certain types of organic molecules being selectively removed from complex mixtures. For example, catechol forms predominately outer-sphere complexes on MnO2 but inner-sphere complexes on Fe2O3, TiO2, and Cr2O3 minerals, which may be ascribed to their different surface charges (negative vs positive, respectively). (174) Goethite preferentially adsorbs high molecular weight (MW) aromatic components from natural OM, while montmorillonite adsorbs low MW constituents without a preference toward aromatic or aliphatic OM. (106) In contrast, birnessite preferentially adsorbs aromatic moieties with no significant fractionation effects on the MW. (98,106)

5.3. Environmental Factors That Influence Organo–Mineral Interactions

Environmental factors influence the properties of both Mn-oxides and organic compounds, and consequently their interaction mechanisms and reaction rates. Here, we briefly elaborate on several environmental factors that influence organo-mineral interactions, including soil and soil solution pH (which influences surface charges, adsorption mechanisms, and redox potential), (95) redox conditions, (11) C:Mn ratios, (92) and pre-existing sorbed OM (175) and cations. (43)
Environmental pH alters surface charges of minerals and OM, consequently influencing adsorption mechanisms, controlling redox potential of Mn-oxides, and regulating interaction rates. (95) Oxygen atoms on Mn-oxide surfaces present as > Mn–O– under neutral or alkaline conditions but > Mn–OH or > Mn–OH2+ under acidic conditions. (12) In general, Mn-oxides present net negative surface charges that electrostatically repel negatively charged moieties of OM at circumneutral to high pH, resulting in decreased OM adsorption capacity as pH increases. (96,97,99) In addition to charge effects, the redox potential of birnessite decreases as pH increases following the Nernst equation, leading to decreases in reaction rates with increasing pH. (176)
Redox conditions control the phases as well as reactivity of Mn-oxides. (11) Under oxic conditions, Mn(II) produced through reductive dissolution can sorb to Fe- or Mn-oxide surfaces and be reoxidized to Mn(IV), (43,106,177) accelerating degradation of organic compounds. (176) Mn-oxides can also degrade organic compounds under anoxic conditions, although possibly through different mechanisms than observed under oxic conditions. For example, in the presence of O2, NTMP degrades through intramolecular electron transfer following O2-promoted oxidation of NTMP–Mn(II) to NTMP–Mn(III). (146,178) Under anoxic conditions, NTMP directly transfers an electron to Mn(III) centers in MnOOH. (178)
C:Mn molar ratios determine the complexation capacity of Mn-oxides for OM. (92) For instance, sorption of DOC, leached from the O horizon of an Ultisol forest soil, to hydrous Mn-oxides was shown to increase with increasing C:Mn molar ratios (up to 10), ascribed to higher affinity of the oxide for organic C at higher C:Mn molar ratios. (92) Increasing C:Mn ratios were observed to slow adsorption kinetics but increase total adsorption of fulvic acid on birnessite, consequently increasing reductive dissolution. (98) For NOM–Mn(III) complexes, lower C:Mn ratios promote aggregation and removal from solution whereas higher C:Mn ratios (10–50) enhance the stability of these complexes in aqueous systems. (152)
Mineral coatings and sorbed compounds can also influence OM complexation by altering the surface properties of Mn-oxides. (106,175) Surface complexation of humic acid was shown to strongly influence products of Mn(II) oxidation on goethite surfaces, inhibiting β-MnOOH generation but not affecting γ-MnOOH and Mn3O4 formation. (43) Metal cation (e.g., Mn2+, Ca2+) sorption can balance negative surface charges and facilitate OM complexation. (96,103) For example, Zn2+, Mg2+, and Ca2+ sorption on birnessite surfaces enhances fulvic acid adsorption to birnessite and increases reductive dissolution. (164) Metal ion adsorption to mineral surfaces also affects Mn-oxide transformation, thus influencing the rates and mechanisms of interactions between SOM and Mn-oxides. Mn(II) adsorption to octahedral vacancies in phyllomanganates induces disproportionation with structural Mn(IV) that drives mineral transformation under acidic conditions, with more subtle effects observed at neutral pH. (179) Moreover, the transformation of Mn(II)-bearing birnessite to diverse sheet and tunneled Mn-oxide structures (γ-MnO2, δ-MnO2, triclinic birnessite, 4 × 4 tunneled Mn-oxides) is regulated by the presence of specific ions (e.g., Cu2+, Mg2+, Ca2+, Li+, Na+, or K+). (180)
Although many studies investigate rates and mechanisms of reactions between single phase Mn-oxides and individual organic compounds, (10,53,105,116,120) few studies examine interactions with NOM or organic mixtures, for example, preferential adsorption of high versus low molecular weight compounds or aliphatic versus aromatic structures. (96,106) Given the complexity of organic molecules in soil solution, more efforts are needed to determine how Mn-oxides interact with specific organic compounds in complex OM mixtures under varied environmental conditions.

6. Organic Matter Stabilization and Destabilization Potential in Soils

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Major soil minerals (clays, Fe-oxides, Al-oxides) are well-studied for their interactions with OM (4,181−184) relative to Mn-oxides. Here, we synthesize data quantifying the ability of Mn(III/IV)-oxides and Fe(III)-oxides to stabilize (immobilize) or destabilize (degrade) organic molecules. We compile existing data from laboratory studies that span a range of mineralogies, reactions conditions, and organic constituents (SI Table S3). We also evaluate quantities of organic carbon associated with metal oxides in soils and sediments.
Given the nonspecificity and heterogeneity in data availability (see SI for further discussion), our strategy is to present information on “stabilization” and “destabilization” of organic compounds (Figure 3; SI Table S3). Stabilization (S, μmol C g-oxide–1) refers to removal of organic C from solution during reaction with the oxide but does not consider underlying mechanisms (e.g., sorption, polymerization into insoluble phases) and does not reflect any potential transformation that occurs during stabilization. These values represent interactions between either individual compounds or NOM with either synthetic or natural oxides. Destabilization (D, μmol C g-oxide–1 h–1) refers to the transformation of organic molecules during reaction with the oxide, that is, OM oxidation coupled to reductive dissolution of the oxide. These values primarily reflect interactions between individual compounds and synthetic minerals but largely exclude reactions with NOM due to difficulties quantifying transformation in these complex systems. A broad suite of compounds that largely consists of small organic molecules is considered.

Figure 3

Figure 3. Compilation of (de)stabilization potentials of a broad suite of organic compounds by Mn-oxides (blue boxes) and Fe-oxides (orange boxes) (SI Table S3). (a) Destabilization rates (log-scale) are presented per mass oxide (μg C g-oxide–1 h–1) and per mass soil (μg C g-soil–1 h–1). (b) Comparison of destabilization rates (μg C g-oxide–1 h–1) of individual organic compounds by Mn- and Fe-oxides. Compound structures are shown below each name. (c) Stabilization (log-scale) for Mn-oxides and Fe-oxides are presented per mass oxide (μg C g-oxide–1) and per mass soil (μg C g-soil–1) and are compared with C stabilization by reactive minerals (dithionite-soluble) in soils and sediments. Differences between Mn-oxides and Fe-oxides within each group are shown as significant (* = p < 0.05) or highly significant (*** = p < 0.001) based on oneway ANOVA.

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To encompass the wide variety of experimental conditions, we report the highest rates of stabilization or destabilization from each study and compile these across commonly used Mn(III/IV)-oxides (e.g., birnessite, Manganite) and Fe(III)-oxides (e.g., ferrihydrite, goethite, hematite). For all minerals, adsorption and reaction rates tend to be higher at low pH (<4) where increasingly positive surface charge facilitates interactions with negatively charged organic compounds. (99,185) The focus on low pH systems also reduces complexities associated with the sorption or reoxidation of reduced metals given that Mn2+ and Fe2+ ions released through reductive dissolution remain in solution at low pH. (186) However, many studies conducted only at circumneutral pH (∼7) are still included with careful consideration for comparability. For both stabilization and destabilization, we excluded compounds that did not show measurable effects. For example, compounds that only adsorbed to mineral surfaces were excluded from compiled destabilization values unless directly comparing interactions of specific compounds. To scale these observations to soil, we also multiplied oxide stabilization (μg C g-oxide–1) and destabilization (μg C g-oxide–1 h–1) by approximate oxide concentrations in surface soils (Fe-oxide ∼20 000 μg g-soil–1 and Mn-oxide ∼2000 μg g-soil–1) to obtain soil stabilization (μg C g-soil–1) and soil destabilization rates (μg C g-soil–1 h–1), with the caveat that reaction rates reported for synthetic minerals are typically higher than those reported for environmental samples. (10)

Destabilization

Mn-oxides demonstrated a higher average oxide destabilization potential (log D = 4.08 ± 0.14 μg C g-oxide–1 h–1) and broader reactivity than Fe-oxides (log D = 3.52 ± 0.19 μg C g-oxide–1 h–1) (p < 0.001) (Figure 3a). In contrast, Fe-oxides showed higher average soil destabilization potential (log D = 1.82 ± 0.19 μg C g-soil–1 h–1) than Mn-oxides (log D = 1.38 ± 0.14 μg C g-soil–1 h–1) due to the higher abundance of Fe-oxides in soils, although this difference was not considered significant (p = 0.06). Importantly, destabilization rates reported for Mn-oxides largely represent direct oxidation of diverse organic compounds on the mineral surface. Mn(III/IV)-oxides are known to oxidize a large suite of organic molecules over a wide range of environmentally relevant pH conditions, whereas Fe(III)-oxides are thermodynamically constrained to interactions with strong reductants at low pH (<4). (187) As such, transformation rates reported here for Fe(III)-oxides are limited to select molecules (e.g., ascorbate, quinones, flavins) that serve as electron shuttles and are not degraded.
To more directly compare destabilization potentials between these minerals, we also examined a subset of organic molecules (catechol, glyphosate, hydroquinone, oxalic acid, pyrogallol, and resorcinol) for which reactions with both Fe(III)- and Mn(III/IV)-oxides are reported (Figure 3b; SI Table S3). All examined organic molecules reacted with Mn(III/IV)-oxides at rates ≥103 μg C g-oxide–1 h–1 (pH 4–7), whereas Fe(III)-oxides were only reactive with phenolic compounds at pH ≤ 4 with rates ≤103 μg C g-oxide–1 h–1.

Stabilization

Mn-oxides exhibited higher oxide stabilization potential (log S = 4.33 ± 0.21 μg C g-oxide–1) than Fe-oxides (log S = 3.69 ± 0.17 μg C g-oxide–1) (p = 0.02) but with a broader spread (Figure 3c). The potential for soil stabilization was similar for Mn-oxides (log S = 1.63 ± 0.21 μg C g-soil–1) and Fe-oxides (log S = 1.99 ± 0.17 μg C g-soil–1) (p = 0.20), indicating a role for C stabilization by Mn-oxides that is comparable to Fe-oxides. However, stabilization by Fe-oxides may persist for longer than Mn-oxides given that OM can readily desorb from Mn-oxides, (92) whereas release from Fe-oxide may depend on mineral dissolution.
We also examined concentrations of organic C associated with reactive minerals as reported for soils and sediments. (188−190) Reactive minerals represent those extracted from soils with either dithionite and/or hydroxylamine hydrochloride and are broadly interpreted to represent Fe-oxides; as such, we present oxide and soil concentrations for comparison with mineral studies. Soil concentrations (μg C g-soil–1) were reported in the literature while oxide concentrations (μg C g-oxide–1) were calculated based on concentrations of extractable Fe and the assumption that Fe-oxides are dominantly in the FeOOH stoichiometry.
Organic C stabilization by reactive minerals, calculated as log S = 5.5 ± 0.1 μg C g-oxide–1 and log S = 3.4 ± 0.1 μg C g-soil–1, was 10–100× higher than stabilization reported for individual minerals, representing an apparent ability for Fe-oxides to stabilize substantially more organic C in soil than in lab studies. However, it is likely that “Fe-bound” OM is also derived from Mn-oxides and potentially Al-oxides, which are also at least partially extracted by dithionite. (191) Thus, this stabilization may represent organic C bound to multiple minerals. Furthermore, lab studies are typically designed to probe sorption processes, whereas soil minerals are associated with organic C through multiple mechanisms. Sorption processes may represent only a small portion of the total C stabilization in soils, (192) and thus certain lab studies may underestimate stabilization potential.
Our analyses demonstrate that Mn- and Fe-oxides may serve as reservoirs for organic C in soils, whereas Mn-oxides also possess a high potential to oxidize and degrade organic molecules in a manner not yet understood in natural systems. Further characterization of oxide–OM interactions in soils is needed to validate the environmental relevance of these trends. Laboratory experiments summarized here indicate that Fe- and Mn-oxides can bind organic compounds and theoretically protect them from microbial degradation; however, the persistence of these associations over time are unknown.

7. Future Framework for Investigating Mn–C Interactions in Terrestrial Ecosystems

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Mn and C biogeochemistry intersect through diverse processes that degrade or protect organic molecules. Manganese demonstrates a potential to stabilize organic molecules via multiple organo-mineral associations (Figure 1) at scales on par with Fe (Figure 3c) but also acts as a transformer that processes organic molecules through biotic (Figure 2) and abiotic (Figure 3a,b) pathways. From these observations, we hypothesize that increasing Mn availability may decrease ecosystem C storage. However, the spatiotemporal variability of these interactions in soils and the magnitude of their effects on soil C storage remain unknown. Similar to Fe, (163) the ability of Mn to protect or degrade OM depends on the stability and reactivity of Mn(III) and Mn(IV) phases under particular environmental conditions at a given time. Although SOM is often considered to be stabilized by Fe-minerals, Fe cycling can also stimulate soil C loss, specifically in anoxic or redox-dynamic soils. Even under oxic conditions, Fe-driven Fenton reactions promote SOM mineralization. (163) Mn has similar interactions with OM: it can physically bind OM to form aggregates, adsorb or coprecipitate with SOM, and transfer electrons that destabilize and/or degrade SOM. The contribution of each role is highly dependent on environmental conditions (soil moisture, redox potential, oxygen availability) and the crystallinity and stability of Mn phases.
Here, we present open questions in Mn–C interactions and highlight opportunities to further explore how Mn regulates C cycling in terrestrial ecosystems. In particular, we focus on Mn–C interactions in soils, while also noting that plant–soil interactions are essential for placing soil processes within a broader ecosystem context. For example, to constrain ecosystem C dynamics, it is necessary to parameterize species-dependent abilities of plants to recycle and retain Mn within surface soils, the influence of varied Mn uptake on biochemical processes such as C fixation, and combined effects of lignin and Mn content on litter decomposition.

Over What Time Scales do Mn-Oxides Stabilize Organic Matter?

It is necessary to establish how long and under what environmental conditions Mn-oxide minerals serve as carbon reservoirs. Although other minerals (e.g., clays, Al-oxides, Fe-oxides) may store organic C for decades to millennia, (6) the magnitude and persistence of OM stabilization by Mn-oxides remains unknown. It is possible that these associations are relatively fleeting. Organic compounds that sorb onto a Mn-oxide surface may undergo redox transformation or be desorbed back into solution. Reductive dissolution of Mn-oxides may release associated organic molecules, as has been widely observed for Fe-oxides. (163) Isotopic techniques have been used to constrain ages (14C) and sources (13C) of organic molecules associated with other minerals (6,188) but have not been applied to Mn-oxides. Indeed, isolating Mn-oxides from soil and characterizing associated OM is technically challenging given its similarities with Fe-oxides. In order to assess the contribution of Mn-oxides to carbon storage in soils, it is necessary to quantify the amount, composition, and age of organic molecules associated with Mn-oxides across diverse environments.

How Do Mn–C Interactions Vary in Space and Time?

Many studies have focused on abiotic transformation of synthetic Mn-oxide phases under specified pH, temperature, and/or solution conditions. (35,46,48,121,193) However, the stability and phase transformation of various Mn-oxides in soils remain poorly studied. Long-term weathering of Mn-oxides in complex soil environments likely results in Mn translocation and mineral phase changes that modify mineral surface properties and OM complexation capacity. Manganese is subject to redistribution at pore-scales (93) and across soil profiles as Mn-oxides dissolve and reprecipitate under fluctuating redox. Different Mn-oxides exhibit diverse properties (e.g., SSA, PZC, Mn oxidation states) (20,21) that strongly regulate their interactions with OM. Thus, it is necessary to evaluate mineral transformation in soil environments to determine the ability of Mn-oxides to stabilize or destabilize OM across wide-ranging ecosystems. Additionally, Mn-oxides strongly adsorb a large variety of metal cations (e.g., Ca2+, Pb2+, Zn2+, Co2+) (49,118,161) and oxyanions (e.g., AsO33–, PO43–) (161,168,194) that compete with organic compounds for reactive surface sites. Further research will be necessary to determine practical reactivity of Mn-oxides in complex soil environments when interacting with SOM.

What Role Does Mn(III) Play in Oxidizing Organic Compounds in Soils?

Although dissolved Mn(III) has long been considered a negligible species in the environment, several recent studies have identified abundant dissolved Mn(III) in marine, estuarine, sediment, and sediment porewater systems. (150,151,195) Diverse compounds including small organic acids, siderophores, and humic acids can form Mn(III)-complexes that are stable relative to the aqueous ion. These Mn(III)-complexes can in turn oxidize organic and inorganic species. (76,123,148,155,195) However, despite their known occurrence and reactivity, the formation, persistence, and reactivity of Mn(III)-complexes in soil environments are not established.

How Do Microbial Mn–C Interactions Influence Soil C Processing?

Increased Mn(II) availability reportedly increases MnP activity and decreases lignin content in litter and organic soils, (130,144,196) demonstrating the importance of Mn(II) in stimulating MnP-mediated lignin oxidation (Figure 2, SI Table S2). However, it remains unclear how relationships among Mn(II) supply, MnP expression and activity, and microbial community composition vary across biomes. Accounting for MnP activity and microbial communities capable of using MnP to degrade complex OM is an important step toward elucidating mechanisms driving the negative relationship between foliar Mn and litter mass loss reported by multiple studies. Furthermore, various microorganisms use extracellular multicopper oxidases and reactive oxygen species to oxidize Mn(II) to reactive Mn(III) species that may be important SOM oxidizers independent of MnP activity.

How Do Mn–C Interactions Influence Soil C Turnover and Storage?

Current knowledge of the influence of Mn on C turnover is limited to Mn manipulation studies conducted in limited laboratory and field studies. These studies show increased C losses (as CO2 and DOC) in response to Mn addition to soils during litter decomposition (Figure 2, SI Table S2), (143−145) suggesting that increased Mn supply to soil would lead to decreased soil C storage. It is possible that these effects are restricted to surface soil where biological activity and the supply of OM and Mn(II) from litterfall are high compared with subsoils. (155) However, the net effect of Mn on C storage has not been validated experimentally. Progress toward a better understanding of Mn effects on C persistence in soils would be enhanced with studies designed to capture changes in C stocks over time and to account for other environmental variables that are likely to influence Mn and C cycling, such as temperature, precipitation, and the plant community.
These individual questions provide context for a larger objective: quantifying the influence of Mn-mediated processes on C storage across diverse ecosystems. Addressing this objective will require detailed investigation of complex soil processes as well as coupled modeling studies that can extend observations through space and time. These efforts potentially fill a knowledge gap in our understanding of processes that regulate ecosystem C dynamics. Earth system models assessed by the Coupled Model Intercomparison Project Phase 5 (CMIP5) present large uncertainty with respect to current soil C storage and turnover rates, limiting our ability to predict how soil C will respond to changing climate. (197) Improved understanding and quantification of key geochemical processes, such as the coupled Mn–C interactions presented here, may better inform and constrain predictions into the future.

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  • Corresponding Author
    • Elizabeth Herndon - Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Department of Earth and Planetary Sciences, College of Arts & Sciences, University of Tennessee, Knoxville, Tennessee 37996, United States;  Orcidhttps://orcid.org/0000-0002-9194-5493;  Email: [email protected]
  • Authors
    • Hui Li - Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
    • Fernanda Santos - Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States
    • Kristen Butler - Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States;  Department of Earth and Planetary Sciences, College of Arts & Sciences, University of Tennessee, Knoxville, Tennessee 37996, United States
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LCC for the U.S. Department of Energy under contract DE-AC05-00OR22725. We gratefully acknowledge Nathan Armistead (ORNL) for graphics development and five anonymous reviewers for their constructive comments.

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    Ferrihydrite and a vernadite-like mineral, in samples collected from the riverbeds and floodplains of the river draining the largest mining-contaminated site in the United States (the Clark Fork River Superfund Complex), have been studied with transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) anal. These poorly cryst. minerals are environmentally important in this system because contaminant heavy metals (As, Cu, Pb, and/or Zn) are always assocd. with them. Both two- and six-line ferrihydrite have been identified with selected-area electron diffraction. For the vernadite-like mineral, the two d values obsd. are approx. between 0.1 and 0.2 Å larger than those reported for vernadite, the Mn hydrous oxide that is thought to have a birnessite-like structure, but which is disordered in the layer stacking direction. In several field specimens, the ferrihydrite and vernadite-like minerals are intimately mixed on the nanoscale, but they also occur sep. It is suggested that the vernadite-like mineral, found sep., is produced biogenically by Mn-oxidizing bacteria, whereas the same mineral assocd. with ferrihydrite is produced abiotically via the heterogeneous oxidn. of Mnaq2+ initially on ferrihydrite surfaces. Evidence from this study demonstrates that the vernadite-like mineral sorbs considerably more toxic metals than does ferrihydrite, demonstrating that it may be a good candidate for application to heavy-metal sorption in permeable reactive barriers.
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  18. 18
    Morgan, J. J. Kinetics of reaction between O2 and Mn(II) species in aqueous solutions. Geochim. Cosmochim. Acta 2005, 69, 35– 48,  DOI: 10.1016/j.gca.2004.06.013
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    18
    Kinetics of reaction between O2 and Mn(II) species in aqueous solutions
    Morgan, James J.
    Geochimica et Cosmochimica Acta (2004), 69 (1), 35-48CODEN: GCACAK; ISSN:0016-7037. (Elsevier Inc.)
    The objective of this research is to assess critically the exptl. rate data for O2 oxidn. of dissolved Mn(II) species at 25° and to interpret the rates in terms of the soln. species of Mn(II) in natural waters. A species kinetic rate expression for parallel paths expresses the total rate of Mn(II) oxidn. as Σki aij, where ki is the rate const. of species i and aij is the species concn. fraction in soln. j. Among the species considered in the rate expression are Mn(II) hydrolysis products, carbonate complexes, ammonia complexes, and halide and sulfate complexes, in addn. to the free aq. ion. Expts. in three different lab. buffers and in seawater yield an apparent rate const. for Mn(II) disappearance, kapp,j ranging from 8.6 × 10-5 to 2.5 × 10-2 (M-1s-1), between pH 8.03 and 9.30, resp. Obsd. values of kapp exceed predictions based on Marcus outer-sphere electron transfer theory by more than four orders of magnitude, lending strong support to the proposal that Mn(II) + O2 electron transfer follows an inner-sphere path. A multiple linear regression anal. fit of the obsd. rates to the species kinetic rate expression yields the following oxidn. rate consts. (M-1/s-1) for the most reactive species: MnOH+, 1.66 × 10-2; Mn(OH)2, 2.09 × 101; and Mn(CO3)22-, 8.13 × 10-2. The species kinetic rate expression accounts for the influence of pH and carbonate on oxidn. rates of Mn(II), through complex formation and acid-base equil. of both reactive and unreactive species. At pH ∼8, the greater fraction of the total rate is carried by MnOH+. At pH greater than ∼8.4, the species Mn(OH)2 and Mn(CO3)22- make the greater contributions to the total rate.
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    Post, J. E. Manganese oxide minerals: Crystal structures and economic and environmental significance. Proc. Natl. Acad. Sci. U. S. A. 1999, 96, 3447– 3454,  DOI: 10.1073/pnas.96.7.3447
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    Manganese oxide minerals: crystal structures and economic and environmental significance
    Post, Jeffrey E.
    Proceedings of the National Academy of Sciences of the United States of America (1999), 96 (7), 3447-3454CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    A review with 105 refs. More than 30 Mn oxide minerals occur in a wide variety of geol. settings. They are major components of Mn nodules that pave huge areas of the ocean floor and bottoms of many fresh-water lakes. Mn oxide minerals are ubiquitous in soils and sediments and participate in a variety of chem. reactions that affect groundwater and bulk soil compn. Their typical occurrence as fine-grained mixts. makes it difficult to study their at. structures and crystal chemistries. In recent years, however, investigations using TEM and powder x-ray and neutron diffraction methods have provided important new insights into the structures and properties of these materials. The crystal structures for todorokite and birnessite, two of the more common Mn oxide minerals in terrestrial deposits and ocean nodules, were detd. by using powder x-ray diffraction data and the Rietveld refinement method. Because of the large tunnels in todorokite and related structures there is considerable interest in the use of these materials and synthetic analogs as catalysts and cation exchange agents. Birnessite-group minerals have layer structures and readily undergo redox and cation-exchange reactions and play a major role in controlling groundwater chem.
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    Feng, X. H.; Zhai, L. M.; Tan, W. F.; Liu, F.; He, J. Z. Adsorption and redox reactions of heavy metals on synthesized Mn oxide minerals. Environ. Pollut. 2007, 147, 366– 373,  DOI: 10.1016/j.envpol.2006.05.028
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    20
    Adsorption and redox reactions of heavy metals on synthesized Mn oxide minerals
    Feng, Xiong Han; Zhai, Li Mei; Tan, Wen Feng; Liu, Fan; He, Ji Zheng
    Environmental Pollution (Amsterdam, Netherlands) (2007), 147 (2), 366-373CODEN: ENPOEK; ISSN:0269-7491. (Elsevier B.V.)
    Several Mn oxide minerals commonly occurring in soils were synthesized by modified or optimized methods. The morphologies, structures, compns., and surface properties of the synthetic Mn oxide minerals were characterized. Adsorption and redox reactions of heavy metals on these minerals in relation to the mineral structures and surface properties were also investigated. The synthesized birnessite, todorokite, cryptomelane, and hausmannite were single-phased minerals and had the typical morphologies from analyses of XRD and TEM/ED. The PZCs of the synthesized birnessite, todorokite and cryptomelane were 1.75, 3.50 and 2.10, resp. The magnitude order of their surface variable neg. charge was: birnessite ≥ cryptomelane > todorokite. The hausmannite had a much higher PZC than others with the least surface variable neg. charge. Birnessite exhibited the largest adsorption capacity on heavy metals Pb2+, Cu2+, Co2+, Cd2+ and Zn2+, while hausmannite the smallest one. Birnessite, cryptomelane and todorokite showed the greatest adsorption capacity on Pb2+ among the tested heavy metals. Hydration tendency (pK1) of the heavy metals and the surface variable charge of the Mn minerals had significant impacts on the adsorption. The ability in Cr(III) oxidn. and concomitant release of Mn2+ varied greatly depending on the structure, compn., surface properties and crystallinity of the minerals. The max. amts. of Cr(III) oxidized by the Mn oxide minerals in order were (mmol/kg): birnessite (1330.0) > cryptomelane (422.6) > todorokite (59.7) > hausmannite (36.6). The characteristics of heavy metal adsorption and Cr(III) oxidn. on Mn oxide minerals are detd. by their structure, compn., surface property and crystallinity.
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  21. 21
    Tebo, B. M.; Bargar, J. R.; Clement, B. G.; Dick, G. J.; Murray, K. J.; Parker, D.; Verity, R.; Webb, S. M. Biogenic mManganese Oxides: Properties and Mechanisms of Formation. Annu. Rev. Earth Planet. Sci. 2004, 32 (1), 287– 328,  DOI: 10.1146/annurev.earth.32.101802.120213
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    21
    Biogenic manganese oxides: Properties and mechanisms of formation
    Tebo, Bradley M.; Bargar, John R.; Clement, Brian G.; Dick, Gregory J.; Murray, Karen J.; Parker, Dorothy; Verity, Rebecca; Webb, Samuel M.
    Annual Review of Earth and Planetary Sciences (2004), 32 (), 287-328, 2 platesCODEN: AREPCI; ISSN:0084-6597. (Annual Reviews Inc.)
    A review. Manganese(IV) oxides produced through microbial activity, i.e., biogenic Mn oxides or Mn biooxides, are believed to be the most abundant and highly reactive Mn oxide phases in the environment. They mediate redox reactions with org. and inorg. compds. and sequester a variety of metals. The major pathway for bacterial Mn(II) oxidn. is enzymic, and although bacteria that oxidize Mn(II) are phylogenetically diverse, they require a multicopper oxidase-like enzyme to oxidize Mn(II). The oxidn. of Mn(II) to Mn(IV) occurs via a sol. or enzyme-complexed Mn(III) intermediate. The primary Mn(IV) biooxide formed is a phyllomanganate most similar to δ-MnO2 or acid birnessite. Metal sequestration by the Mn biooxides occurs predominantly at vacant layer octahedral sites.
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    Lovley, D. R.; Holmes, D. E.; Nevin, K. P. Dissimilatory Fe (III) and Mn (IV) reduction. Adv. Microb. Physiol. 2004, 49 (2), 219– 286,  DOI: 10.1016/S0065-2911(04)49005-5
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    22
    Dissimilatory Fe(III) and Mn(IV) reduction
    Lovley, Derek R.; Holmes, Dawn E.; Nevin, Kelly P.
    Advances in Microbial Physiology (2004), 49 (), 219-286CODEN: AMIPB2; ISSN:0065-2911. (Elsevier)
    A review. Dissimilatory Fe(III) and Mn(IV) redn. has an important influence on the geochem. of modern environments, and Fe(III)-reducing microorganisms, most notably those in the Geobacteraceae family, can play an important role in the bioremediation of subsurface environments contaminated with org. or metal contaminants. Microorganisms with the capacity to conserve energy from Fe(III) and Mn(IV) redn. are phylogenetically dispersed throughout the Bacteria and Archaea. The ability to oxidize hydrogen with the redn. of Fe(III) is a highly conserved characteristic of hyperthermophilic microorganisms, and one Fe(III) reducing Archaea grows at the highest temp. yet recorded for any organism. Fe(III)- and Mn(IV)-reducing microorganisms have the ability to oxidize a wide variety of org. compds., often completely to carbon dioxide. Typical alternative electron acceptors for Fe(III) reducers include oxygen, nitrate, U(VI) and electrodes. Unlike other commonly considered electron acceptors, Fe(III) and Mn(IV) oxides, the most prevalent form of Fe(III) and Mn(IV) in most environments, are insol. Thus, Fe(III)- and Mn(IV)-reducing microorganisms face the dilemma of how to transfer electrons derived from central metab. onto an insol., extracellular electron acceptor. Although microbiol. and geochem. evidence suggests that Fe(III) redn. may have been the first form of microbial respiration, the capacity for Fe(III) redn. appears to have evolved several times as phylogenetically distinct Fe(III) reducers have different mechanisms for Fe(III) redn. Geobacter species, which are representative of the family of Fe(III) reducers that predominate in a wide diversity of sedimentary environments, require direct contact with Fe(III) oxides in order to reduce them. In contrast, Shewanella and Geothrix species produce chelators that solubilize Fe(III) and release electron-shuttling compds. that transfer electrons from the cell surface to the surface of Fe(III) oxides not in direct contact with the cells. Electron transfer from the inner membrane to the outer membrane in Geobacter and Shewanella species appears to involve an electron transport chain of inner membrane, periplasmic, and outer membrane c-type cytochromes, but the cytochromes involved in these processes in the two organisms are different. In addn., Geobacter species specifically express flagella and pili during growth on Fe(III) and Mn(IV) oxides and are chemotactic to Fe(II) and Mn(II), which may lead Geobacter species to the oxides under anoxic conditions. The physiol. characteristics of Geobacter species appear to explain why they have consistently been found to be the predominant Fe(III)- and Mn(IV)-reducing microorganisms in a variety of sedimentary environments. In comparison with other respiratory processes, the study of Fe(III) and Mn(IV) redn. is in its infancy, but genome-enabled approaches are rapidly advancing our understanding of this environmentally significant physiol.
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    Brock, S. L. D. N.; Tian, Z. R.; Giraldo, O.; Zhou, H.; Suib, S. L. A review of porous manganese oxide materials. Chem. Mater. 1998, 10, 2619– 2628,  DOI: 10.1021/cm980227h
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    23
    A Review of Porous Manganese Oxide Materials
    Brock, Stephanie L.; Duan, Niangao; Tian, Zheng Rong; Giraldo, Oscar; Zhou, Hua; Suib, Steven L.
    Chemistry of Materials (1998), 10 (10), 2619-2628CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
    A review with 62 refs. on the synthesis, characterization, and applications of porous Mn oxides during the last two years. The synthesis of porous tunnel structures, layered structures, and related materials is discussed. Both microporous and mesoporous systems materials are covered here. Characterization discussed here focuses around structural studies. The focus of the application sections include electrochem. and catalytic studies.
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  24. 24
    Learman, D. R.; Wankel, S. D.; Webb, S. M.; Martinez, N.; Madden, A. S.; Hansel, C. M. Coupled biotic-abiotic Mn(II) oxidation pathway mediates the formation and structural evolution of biogenic Mn oxides. Geochim. Cosmochim. Acta 2011, 75, 6048– 6063,  DOI: 10.1016/j.gca.2011.07.026
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    24
    Coupled biotic-abiotic Mn(II) oxidation pathway mediates the formation and structural evolution of biogenic Mn oxides
    Learman, D. R.; Wankel, S. D.; Webb, S. M.; Martinez, N.; Madden, A. S.; Hansel, C. M.
    Geochimica et Cosmochimica Acta (2011), 75 (20), 6048-6063CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Manganese oxides are among the strongest oxidants and sorbents in the environment, impacting the transport and speciation of metals, cycling of carbon, and flow of electrons within soils and sediments. The oxidn. of Mn(II) to Mn(III/IV) oxides has been primarily attributed to biol. processes, due in part to the faster rates of bacterial Mn(II) oxidn. compared to obsd. mineral-induced and other abiotic rates. The authors explore the reactivity of biogenic Mn oxides formed by a common marine bacterium (Roseobacter sp. AzwK-3b), which has been previously shown to oxidize Mn(II) via the prodn. of extracellular superoxide. The oxidn. of Mn(II) by superoxide results in the formation of highly reactive colloidal birnessite with hexagonal symmetry. The colloidal oxides induce the rapid oxidn. of Mn(II), with dramatically accelerated rates in the presence of orgs., presumably due to mineral surface-catalyzed org. radical generation. Mn(II) oxidn. by the colloids is further accelerated in presence of both orgs. and light, implicating reactive oxygen species in aiding abiotic oxidn. Indeed, the enhancement of Mn(II) oxidn. is negated when the colloids are reacted with Mn(II) in the presence of superoxide dismutase, an enzyme that scavenges the reactive oxygen species (ROS) superoxide. The reactivity of the colloidal phase is short-lived due to the rapid evolution of the birnessite from hexagonal to pseudo-orthogonal symmetry. The secondary particulate triclinic birnessite phase exhibits a distinct lack of Mn(II) oxidn. and subsequent Mn oxide formation. Thus, the evolution of initial reactive hexagonal birnessite to non-reactive triclinic birnessite imposes the need for continuous prodn. of new colloidal hexagonal particles for Mn(II) oxidn. to be sustained, illustrating an intimate dependency of enzymic and mineral-based reactions in Mn(II) oxidn. Further, the coupled enzymic and mineral-induced pathways are linked such that enzymic formation of Mn oxide is requisite for the mineral-induced pathway to occur. Mn(II) oxidn. is shown to involve a complex network of abiotic and biotic processes, including enzymically produced superoxide, mineral catalysis, org. reactions with mineral surfaces, and likely photo-prodn. of ROS. The complexity of coupled reactions involved in Mn(II) oxidn. highlights the need for further investigations of microbially-mediated Mn oxide formation, including identifying the role of Mn oxide surfaces, orgs., reactive oxygen species, and light in Mn(II) oxidn. and Mn oxide phase evolution.
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  25. 25
    Santelli, C. M.; Webb, S. M.; Dohnalkova, A. C.; Hansel, C. M. Diversity of Mn oxides produced by Mn(II)-oxidizing fungi. Geochim. Cosmochim. Acta 2011, 75 (10), 2762– 2776,  DOI: 10.1016/j.gca.2011.02.022
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    25
    Diversity of Mn oxides produced by Mn(II)-oxidizing fungi
    Santelli, Cara M.; Webb, Samuel M.; Dohnalkova, Alice C.; Hansel, Colleen M.
    Geochimica et Cosmochimica Acta (2011), 75 (10), 2762-2776CODEN: GCACAK; ISSN:0016-7037. (Elsevier B.V.)
    Microscopic and spectroscopic techniques are coupled to characterize the Mn oxides produced by 4 different species of Mn(II)-oxidizing Ascomycete fungi (Plectosphaerella cucumerina strain DS2psM2a2, Pyrenochaeta sp. DS3sAY3a, Stagonospora sp SRC1lsM3a, and Acremonium strictum strain DS1bioAY4a) isolated from acid mine drainage treatment systems in central Pennsylvania. The site of Mn oxide formation varies greatly among the fungi, including deposition on hyphal surfaces, at the base of reproductive structures (e.g., fruiting bodies), and on envisaged extracellular polymers adjacent to the cell. The primary product of Mn(II) oxidn. for all species growing under the same chem. and phys. conditions is a nanoparticulate, poorly-cryst. hexagonal birnessite-like phase resembling synthetic δ-MnO2. The phylogeny and growth conditions (planktonic vs. surface-attached) of the fungi, however, impact the conversion of the initial phyllomanganate to more ordered phases, such as todorokite (A. strictum strain DS1bioAY4a) and triclinic birnessite (Stagonospora sp. SRC1lsM3a). The authors' findings reveal that the species of Mn(II)-oxidizing fungi impacts the size, morphol., and structure of Mn bio-oxides, which will likely translate to large differences in the reactivity of the Mn oxide phases.
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    Villalobos, M.; Lanson, B.; Manceau, A.; Toner, B.; Sposito, G. Structural model for the biogenic Mn oxide produced by pseudomonas putida. Am. Mineral. 2006, 91, 489– 502,  DOI: 10.2138/am.2006.1925
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    26
    Structural model for the biogenic Mn oxide produced by Pseudomonas putida
    Villalobos, Mario; Lanson, Bruno; Manceau, Alain; Toner, Brandy; Sposito, Garrison
    American Mineralogist (2006), 91 (4), 489-502CODEN: AMMIAY; ISSN:0003-004X. (Mineralogical Society of America)
    X-ray diffraction (XRD) and Mn K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy were combined to elaborate a structural model for phyllomanganates (layer-type Mn oxides) lacking 3D ordering (turbostratic stacking). These techniques were applied to a sample produced by a common soil and freshwater bacterium (Pseudomonas putida), and to two synthetic analogs, δ-MnO2 and acid birnessite, obtained by the redn. of potassium permanganate with MnCl2 and HCl, resp. To interpret the diffraction and spectroscopic data, we applied an XRD simulation technique utilized previously for well-crystd. birnessite varieties, complementing this approach with single-scattering-path simulations of the Mn K-edge EXAFS spectra. Our structural analyses revealed that all three Mn oxides have an hexagonal layer symmetry with layers comprising edge-sharing Mn4+O6 octahedra and cation vacancies, but no layer Mn3+O6 octahedra. The proportion of cation vacancies in the layers ranged from 6 to 17%, these vacancies being charge-compensated in the interlayer by protons, alkali metals, and Mn atoms, in amts. that vary with the phyllomanganate species and synthesis medium. Both vacancies and interlayer Mn were most abundant in the biogenic oxide. The diffracting crystallites contained three to six randomly stacked layers and have coherent scattering domains of 19-42 Å in the c* direction, and of 60-85 Å in the a-b plane. Thus, the Mn oxides investigated here are nanoparticles that bear significant permanent structural charge resulting from cation layer vacancies and variable surface charge from unsatd. O atoms at layer edges.
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  27. 27
    Learman, D. R.; Voelker, B. M.; Vazquez-Rodriguez, A. I.; Hansel, C. M. Formation of manganese oxides by bacterially generated superoxide. Nat. Geosci. 2011, 4 (2), 95– 98,  DOI: 10.1038/ngeo1055
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    27
    Formation of manganese oxides by bacterially generated superoxide
    Learman, D. R.; Voelker, B. M.; Vazquez-Rodriguez, A. I.; Hansel, C. M.
    Nature Geoscience (2011), 4 (2), 95-98, S95/1-S95/6CODEN: NGAEBU; ISSN:1752-0894. (Nature Publishing Group)
    Manganese oxide minerals are among the strongest sorbents and oxidants in the environment. The formation of these minerals controls the fate of contaminants, the degrdn. of recalcitrant C, the cycling of nutrients, and the activity of anaerobic-based metabs. Oxidn. of sol. Mn(II) ions to Mn(III/IV) oxides has been primarily attributed to direct enzymic oxidn. by microorganisms. However, the physiol. reason for this process remains unknown. Here we assess the ability of a common species of marine bacteria, Roseobacter sp. AzwK-3b, to oxidize Mn(II) in the presence of chem. and biol. inhibitors. We show that Roseobacter AzwK-3b oxidizes Mn(II) by producing the strong and versatile redox reactant superoxide. The oxidn. of Mn(II), and concomitant prodn. of manganese oxides, was inhibited in both the light and dark in the presence of enzymes and metals that scavenge superoxide. Oxidn. was also inhibited by various proteases, enzymes that break down bacterial proteins, confirming that the superoxide was bacterially generated. We conclude that bacteria can oxidize Mn(II) indirectly, through the enzymic generation of extracellular superoxide radicals. We suggest that dark bacterial prodn. of superoxide may be a driving force in metal cycling and mineralization in the environment.
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  28. 28
    Chaput, D. L.; Fowler, A. J.; Seo, O.; Duhn, K.; Hansel, C. M.; Santelli, C. M. Mn oxide formation by phototrophs: Spatial and temporal patterns, with evidence of an enzymatic superoxide-mediated pathway. Sci. Rep. 2019, 9 (1), 18244,  DOI: 10.1038/s41598-019-54403-8
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    28
    Mn oxide formation by phototrophs: Spatial and temporal patterns, with evidence of an enzymatic superoxide-mediated pathway
    Chaput, Dominique L.; Fowler, Alexandre J.; Seo, Onyou; Duhn, Kelly; Hansel, Colleen M.; Santelli, Cara M.
    Scientific Reports (2019), 9 (1), 18244CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
    Manganese (Mn) oxide minerals influence the availability of org. carbon, nutrients and metals in the environment. Oxidn. of Mn(II) to Mn(III/IV) oxides is largely promoted by the direct and indirect activity of microorganisms. Studies of biogenic Mn(II) oxidn. have focused on bacteria and fungi, with phototrophic organisms (phototrophs) being generally overlooked. Here, we isolated phototrophs from Mn removal beds in Pennsylvania, USA, including fourteen Chlorophyta (green algae), three Bacillariophyta (diatoms) and one cyanobacterium, all of which consistently formed Mn(III/IV) oxides. Isolates produced cell-specific oxides (coating some cells but not others), diffuse biofilm oxides, and internal diatom-specific Mn-rich nodules. Phototrophic Mn(II) oxidn. had been previously attributed to abiotic oxidn. mediated by photosynthesis-driven pH increases, but we found a decoupling of Mn oxide formation and pH alteration in several cases. Furthermore, cell-free filtrates of some isolates produced Mn oxides at specific time points, but this activity was not induced by Mn(II). Manganese oxide formation in cell-free filtrates occurred via reaction with the oxygen radical superoxide produced by sol. extracellular proteins. Given the known widespread ability of phototrophs to produce superoxide, the contribution of phototrophs to Mn(II) oxidn. in the environment may be greater and more nuanced than previously thought.
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  29. 29
    Bohu, T.; Santelli, C. M.; Akob, D. M.; Neu, T. R.; Ciobota, V.; Rosch, P.; Popp, J.; Nietzsche, S.; Kusel, K. Characterization of pH dependent Mn(II) oxidation strategies and formation of a bixbyite-like phase by Mesorhizobium australicum T-G1. Front. Microbiol. 2015, 6, 734,  DOI: 10.3389/fmicb.2015.00734
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    29
    Characterization of pH dependent Mn(II) oxidation strategies and formation of a bixbyite-like phase by Mesorhizobium australicum T-G1
    Bohu Tsing; Santelli Cara M; Akob Denise M; Neu Thomas R; Ciobota Valerian; Rosch Petra; Popp Jurgen; Nietzsche Sandor; Kusel Kirsten
    Frontiers in microbiology (2015), 6 (), 734 ISSN:1664-302X.
    Despite the ubiquity of Mn oxides in natural environments, there are only a few observations of biological Mn(II) oxidation at pH < 6. The lack of low pH Mn-oxidizing bacteria (MOB) isolates limits our understanding of how pH influences biological Mn(II) oxidation in extreme environments. Here, we report that a novel MOB isolate, Mesorhizobium australicum strain T-G1, isolated from an acidic and metalliferous uranium mining area, can oxidize Mn(II) at both acidic and neutral pH using different enzymatic pathways. X-ray diffraction, Raman spectroscopy, and scanning electron microscopy with energy dispersive X-ray spectroscopy revealed that T-G1 initiated bixbyite-like Mn oxide formation at pH 5.5 which coincided with multi-copper oxidase expression from early exponential phase to late stationary phase. In contrast, reactive oxygen species (ROS), particularly superoxide, appeared to be more important for T-G1 mediated Mn(II) oxidation at neutral pH. ROS was produced in parallel with the occurrence of Mn(II) oxidation at pH 7.2 from early stationary phase. Solid phase Mn oxides did not precipitate, which is consistent with the presence of a high amount of H2O2 and lower activity of catalase in the liquid culture at pH 7.2. Our results show that M. australicum T-G1, an acid tolerant MOB, can initiate Mn(II) oxidation by varying its oxidation mechanisms depending on the pH and may play an important role in low pH manganese biogeochemical cycling.
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  30. 30
    Butterfield, C. N.; Soldatova, A. V.; Lee, S.-W.; Spiro, T. G.; Tebo, B. M. Mn(II,III) oxidation and MnO2 mineralization by an expressed bacterial multicopper oxidase. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 11731– 11735,  DOI: 10.1073/pnas.1303677110
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    30
    Mn(II,III) oxidation and MnO2 mineralization by an expressed bacterial multicopper oxidase
    Butterfield, Cristina N.; Soldatova, Alexandra V.; Lee, Sung-Woo; Spiro, Thomas G.; Tebo, Bradley M.
    Proceedings of the National Academy of Sciences of the United States of America (2013), 110 (29), 11731-11735,S11731/1-S11731/5CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Reactive Mn(IV) oxide minerals are ubiquitous in the environment and control the bioavailability and distribution of many toxic and essential elements and org. compds. Their formation is thought to be dependent on microbial enzymes, because spontaneous Mn(II) to Mn(IV) oxidn. is slow. Several species of marine Bacillus spores oxidize Mn(II) on their exosporium, the outermost layer of the spore, encrusting them with Mn(IV) oxides. Mol. studies have identified the mnx (Mn oxidn.) genes, including mnxG, encoding a putative multicopper oxidase (MCO), as responsible for this two-electron oxidn., a surprising finding because MCOs only catalyze single-electron transfer reactions. Characterization of the enzymic mechanism has been hindered by the lack of purified protein. By purifying active protein from the mnxDEFG expression construct, we found that the resulting enzyme is a blue (absorption max. 590 nm) complex contg. MnxE, MnxF, and MnxG proteins. Further, by analyzing the Mn(II)- and (III)-oxidizing activity in the presence of a Mn(III) chelator, pyrophosphate, we found that the complex facilitates both electron transfers from Mn(II) to Mn(III) and from Mn(III) to Mn(IV). X-ray absorption spectroscopy of the Mn mineral product confirmed its similarity to Mn(IV) oxides generated by whole spores. Our results demonstrate that Mn oxidn. from sol. Mn(II) to Mn(IV) oxides is a two-step reaction catalyzed by an MCO-contg. complex. With the purifn. of active Mn oxidase, we will be able to uncover its mechanism, broadening our understanding of Mn mineral formation and the bioinorg. capabilities of MCOs.
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  31. 31
    Andeer, P. F.; Leadbetter, J. R.; Mcllvin, M.; Dunn, J. A.; Hansel, C. M. Extracellular haem peroxidases mediate Mn(II) oxidation in a marine Roseobacter bacterium via superoxide production. Environ. Microbiol. 2015, 17, 3925– 3936,  DOI: 10.1111/1462-2920.12893
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    31
    Extracellular heme peroxidases mediate Mn(II) oxidation in a marine Roseobacter bacterium via superoxide production
    Andeer, Peter F.; Learman, Deric R.; McIlvin, Matt; Dunn, James A.; Hansel, Colleen M.
    Environmental Microbiology (2015), 17 (10), 3925-3936CODEN: ENMIFM; ISSN:1462-2912. (Wiley-Blackwell)
    Summary : Manganese (Mn) oxides are among the strongest sorbents and oxidants in environmental systems. A no. of biotic and abiotic pathways induce the oxidn. of Mn(II) to Mn oxides. Here, we use a combination of proteomic analyses and activity assays, to identify the enzyme(s) responsible for extracellular superoxide-mediated Mn oxide formation by a bacterium within the ubiquitous Roseobacter clade. We show that animal hem peroxidases (AHPs) located on the outer membrane and within the secretome are responsible for Mn(II) oxidn. These novel peroxidases have previously been implicated in direct Mn(II) oxidn. by phylogenetically diverse bacteria. Yet, we show that in this Roseobacter species, AHPs mediate Mn(II) oxidn. not through a direct reaction but by producing superoxide and likely also by degrading hydrogen peroxide. These findings point to a eukaryotic-like oscillatory oxidative-peroxidative enzymic cycle by these AHPs that leads to Mn oxide formation by this organism. AHP expression appears unaffected by Mn(II), yet the large energetic investment required to produce and secrete these enzymes points to an as yet unknown physiol. function. These findings are further evidence that bacterial peroxidases and secreted enzymes, in general, are unappreciated controls on the cycling of metals and reactive oxygen species (ROS), and by extension carbon, in natural systems.
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  32. 32
    Tang, Y. Z. C. A.; Santelli, C. M.; Hansel, C. M. Fungal oxidative dissolution of the Mn(II)-bearing mineral rhodochrosite and the role of metabolites in manganese oxide formation. Environ. Microbiol. 2013, 15, 1063– 1077,  DOI: 10.1111/1462-2920.12029
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    32
    Fungal oxidative dissolution of the Mn(II)-bearing mineral rhodochrosite and the role of metabolites in manganese oxide formation
    Tang, Yuanzhi; Zeiner, Carolyn A.; Santelli, Cara M.; Hansel, Colleen M.
    Environmental Microbiology (2013), 15 (4), 1063-1077CODEN: ENMIFM; ISSN:1462-2912. (Wiley-Blackwell)
    Summary : Microbially mediated oxidn. of Mn(II) to Mn(III/IV) oxides influences the cycling of metals and remineralization of carbon. Despite the prevalence of Mn(II)-bearing minerals in nature, little is known regarding the ability of microbes to oxidize mineral-hosted Mn(II). Here, we explored oxidn. of the Mn(II)-bearing mineral rhodochrosite (MnCO3) and characteristics of ensuing Mn oxides by six Mn(II)-oxidizing Ascomycete fungi. All fungal species substantially enhanced rhodochrosite dissoln. and surface modification. Mineral-hosted Mn(II) was oxidized resulting in formation of Mn(III/IV) oxides that were all similar to δ-MnO2 but varied in morphol. and distribution in relation to cellular structures and the MnCO3 surface. For four fungi, Mn(II) oxidn. occurred along hyphae, likely mediated by cell wall-assocd. proteins. For two species, Mn(II) oxidn. occurred via reaction with fungal-derived superoxide produced at hyphal tips. This pathway ultimately resulted in structurally unique Mn oxide clusters formed at substantial distances from any cellular structure. Taken together, findings for these two fungi strongly point to a role for fungal-derived org. mols. in Mn(III) complexation and Mn oxide templation. Overall, this study illustrates the importance of fungi in rhodochrosite dissoln., extends the relevance of biogenic superoxide-based Mn(II) oxidn. and highlights the potential role of mycogenic exudates in directing mineral pptn.
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  33. 33
    Yu, H.; Leadbetter, J. R. Bacterial chemolithoautotrophy via manganese oxidation. Nature 2020, 583, 453– 458,  DOI: 10.1038/s41586-020-2468-5
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    33
    Bacterial chemolithoautotrophy via manganese oxidation
    Yu, Hang; Leadbetter, Jared R.
    Nature (London, United Kingdom) (2020), 583 (7816), 453-458CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Abstr.: Manganese is one of the most abundant elements on Earth. The oxidn. of manganese has long been theorized1-yet has not been demonstrated2-4-to fuel the growth of chemolithoautotrophic microorganisms. Here we refine an enrichment culture that exhibits exponential growth dependent on Mn(II) oxidn. to a co-culture of two microbial species. Oxidn. required viable bacteria at permissive temps., which resulted in the generation of small nodules of manganese oxide with which the cells assocd. The majority member of the culture-which we designate 'Candidatus Manganitrophus noduliformans'-is affiliated to the phylum Nitrospirae (also known as Nitrospirota), but is distantly related to known species of Nitrospira and Leptospirillum. We isolated the minority member, a betaproteobacterium that does not oxidize Mn(II) alone, and designate it Ramlibacter lithotrophicus. Stable-isotope probing revealed 13CO2 fixation into cellular biomass that was dependent upon Mn(II) oxidn. Transcriptomic anal. revealed candidate pathways for coupling extracellular manganese oxidn. to aerobic energy conservation and autotrophic CO2 fixation. These findings expand the known diversity of inorg. metabs. that support life, and complete a biogeochem. energy cycle for manganese5,6 that may interface with other major global elemental cycles.
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    Hansel, C. M.; Learman, D. R. Chapter eighteen: Geomicrobiology of manganese. In Ehrlich’s Geomicrobiology; CRC Press: Boca Raton, FL, 2015 401 452
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    Elzinga, E. J. Reductive transformation of birnessite by aqueous Mn(II). Environ. Sci. Technol. 2011, 45, 6366– 6372,  DOI: 10.1021/es2013038
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    Reductive Transformation of Birnessite by Aqueous Mn(II)
    Elzinga, Evert J.
    Environmental Science & Technology (2011), 45 (15), 6366-6372CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Reaction of aq. Mn(II) with hexagonal birnessite at pH 7.5 causes reductive transformation of birnessite into feitknechtite (β-MnIIIOOH) and manganite (γ-MnIIIOOH) through interfacial electron transfer from adsorbed Mn(II) to structural Mn(IV) atoms and arrangement of product Mn(III) into MnOOH, summarized by Mn(II) + Mn(IV)O2 + 2 H2O → 2 Mn(III)OOH + 2 H+. Feitknechtite is the initial transformation product, and subsequently converted into the more stable manganite polymorph during ongoing reaction with Mn(II). Feitknechtite prodn. is obsd. at Mn(II) concns. 2 orders of magnitude below thermodn. thresholds, reflecting uncertainty in thermodn. data of Mn-oxide minerals and/or specific interactions between Mn(II) and birnessite surface sites facilitating electron exchange. Under oxic conditions, feitknechtite formation through surface-catalyzed oxidn. of Mn(II) by O leads to addnl. Mn(II) removal from soln. relative to anoxic systems. These results indicate that Mn(II) may be an important moderator of the reductive arm of Mn-oxide redox cycling, and suggest a controlling role of Mn(II) in regulating the soly. and speciation of phyllomanganate-reactive metal pollutants including Co, Ni, As, and Cr in geochem. environments.
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    van Genuchten, C. M.; Pena, J. Mn(II) Oxidation in Fenton and Fenton Type Systems: Identification of Reaction Efficiency and Reaction Products. Environ. Sci. Technol. 2017, 51 (5), 2982– 2991,  DOI: 10.1021/acs.est.6b05584
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    36
    Mn(II) Oxidation in Fenton and Fenton Type Systems: Identification of Reaction Efficiency and Reaction Products
    van Genuchten, Case M.; Pena, Jasquelin
    Environmental Science & Technology (2017), 51 (5), 2982-2991CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Efficient and low-cost methods of removing aq. Mn(II) are required to improve the quality of impacted groundwater supplies. In this work, we show that Fe(0) electrocoagulation (EC) permits the oxidative removal of Mn(II) from soln. by reaction with the reactive oxidant species produced through Fe(II) oxidn. Manganese(II) removal was enhanced when the accumulation of aq. Fe(II) was minimized, which was achieved at low Fe(II) prodn. rates, high pH, the presence of H2O2 instead of O2 as the initial Fe(II) oxidant, or a combination of all three. In addn., in the EC-H2O2 system, Mn(II) removal efficiency increased as pH decreased from 6.5 to 4.5 and as pH increased from 6.5 to 8.5, which implicates different reactive oxidants in acidic and alk. solns. Chem. analyses and x-ray absorption spectroscopy revealed that Mn(II) removal during Fe(0) EC leads to the formation of Mn(III) (0.02 to >0.26 Mn·Fe-1 molar ratios) and its incorporation into the resulting Fe(III) coppts. (lepidocrocite and hydrous ferric oxide for EC-O2 and EC-H2O2, resp.), regardless of pH and Fe(II) prodn. rate. The Mn(II) oxidn. pathways elucidated in this study set the framework to develop kinetic models on the impact of Mn(II) during EC treatment and in other Fenton type systems.
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  37. 37
    Jung, H.; Chadha, T. S.; Kim, D.; Biswas, P.; Jun, Y. S. Photochemically assisted fast abiotic oxidation of manganese and formation of delta-MnO2 nanosheets in nitrate solution. Chem. Commun. (Cambridge, U. K.) 2017, 53 (32), 4445– 4448,  DOI: 10.1039/C7CC00754J
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    37
    Photochemically assisted fast abiotic oxidation of manganese and formation of δ-MnO2 nanosheets in nitrate solution
    Jung, Haesung; Chadha, Tandeep S.; Kim, Doyoon; Biswas, Pratim; Jun, Young-Shin
    Chemical Communications (Cambridge, United Kingdom) (2017), 53 (32), 4445-4448CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    This study introduces a new and previously unconsidered fast abiotic formation of Mn(IV) oxides. We report photochem. assisted fast abiotic oxidn. of Mn2+ (aq) to Mn(IV) (s) by superoxide radicals generated from nitrate photolysis. This photochem. pathway generates randomly stacked layered birnessite (δ-MnO2) nanosheets.
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  38. 38
    Zhang, T.; Liu, L.; Tan, W.; Suib, S. L.; Qiu, G. Formation and transformation of manganese(III) intermediates in the photochemical generation of manganese(IV) oxide minerals. Chemosphere 2021, 262, 128082,  DOI: 10.1016/j.chemosphere.2020.128082
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    38
    Formation and transformation of manganese(III) intermediates in the photochemical generation of manganese(IV) oxide minerals
    Zhang, Tengfei; Liu, Lihu; Tan, Wenfeng; Suib, Steven L.; Qiu, Guohong
    Chemosphere (2021), 262 (), 128082CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
    As important natural oxidants and adsorbents, manganese (Mn) oxide minerals affect the speciation, bioavailability and fate of pollutants and nutrient elements. It was found that birnessite-type Mn(IV) oxide minerals can be formed in the presence of NO-3 and solar irradn. However, the photochem. formation and transformation processes from Mn2+ to Mn(IV) oxide minerals remain unclear. In this work, the Mn(IV) oxide minerals were confirmed to be photochem. formed mainly due to the disproportionation of Mn(III) intermediates generated from the oxidn. of Mn2+ in the presence of NO-3 under UV light irradn. The oxidn. rate of Mn2+ to Mn(IV) oxide minerals decreased with increasing initial Mn2+ concn. due to the lower disproportionation rate. The increase in NO-3 concn., pH and temp. promoted Mn2+ photochem. oxidn. The photochem. formation rate of Mn(IV) oxide minerals increased with increasing ligand concns. at low ligand concns. Ligands affected the formation of Mn(IV) oxide minerals by promoting the formation and reducing the reactivity of Mn(III) intermediates. Overall, this work reveals the important role of Mn(III) intermediates in the formation of natural Mn oxide minerals.
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  39. 39
    Nico, P.; Anastasio, C.; Zasoski, R. Rapid photo-oxidation of Mn(II) mediated by humic substances. Geochim. Cosmochim. Acta 2002, 66 (23), 4047– 4056,  DOI: 10.1016/S0016-7037(02)01001-3
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    39
    Rapid photo-oxidation of Mn(II) mediated by humic substances
    Nico, Peter S.; Anastasio, Cort; Zasoski, Robert J.
    Geochimica et Cosmochimica Acta (2002), 66 (23), 4047-4056CODEN: GCACAK; ISSN:0016-7037. (Elsevier Science Inc.)
    The oxidn. of Mn(II) by O2 to Mn(III) or Mn(IV) is thermodynamically favored under the pH and pO2 conditions present in most near surface waters, but the kinetics of this reaction are extremely slow. This work investigated whether reactive oxygen species, produced through illumination of humic substances, could oxidize Mn at an environmentally relevant rate. The simulated sunlight illumination of a soln. contg. 200 μM Mn(II) and 5 mg/L Aldrich humic acid buffered at pH 8.1 produced ∼19 μM of oxidized Mn (MnOx where x is between one and two) after 45 min. The major oxidants responsible for this reaction appear to be photoproduced superoxide radical anion, O2-, and singlet mol. oxygen, 1O2. The dependencies of MnOx formation on Mn(II), humic acid, and H+ concn. were characterized. A kinetic model based largely on published rate consts. was established and fit to the exptl. data. As expected, anal. of the model indicates that the key reaction rate controlling MnOx prodn. is the rate of decompn. of a MnO2+ complex formed from the reaction of Mn(II) with O2-. This rate is strongly dependent on the Mn(II) complexing ligands in soln. The MnOx prodn. in the seawater sample taken from Bodega Bay, USA and spiked with 200 μM Mn(II) was well reproduced by the model. Extrapolations from the model imply that Mn photo-oxidn. should be a significant reaction in typical surface seawaters. Calcd. rates, 5.8 to 55 pM h-1, are comparable to reported rates of biol. Mn oxidn., 0.07 to 89 pM h-1. Four fresh water samples that were spiked with 200 μM Mn(II) also showed significant MnOx prodn. Based on these results, it appears that Mn photo-oxidn. could constitute a significant, and apparently unrecognized geochem. pathway in natural waters.
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  40. 40
    Xu, X.; Li, Y.; Li, Y.; Lu, A.; Qiao, R.; Liu, K.; Ding, H.; Wang, C. Characteristics of desert varnish from nanometer to micrometer scale: A photo-oxidation model on its formation. Chem. Geol. 2019, 522, 55– 70,  DOI: 10.1016/j.chemgeo.2019.05.016
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    40
    Characteristics of desert varnish from nanometer to micrometer scale: A photo-oxidation model on its formation
    Xu, Xiaoming; Li, Yan; Li, Yanzhang; Lu, Anhuai; Qiao, Ruixi; Liu, Kaihui; Ding, Hongrui; Wang, Changqiu
    Chemical Geology (2019), 522 (), 55-70CODEN: CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
    Rock varnish is a widespread Mn-rich rock coating commonly developed in arid environments, but the mechanism for its formation is still under debate. In this study, rock varnish and adjacent soil dust collected from Gobi Desert were analyzed for exploring the possible abiotic oxidn. mechanism of Mn oxides. The occurrence of rock varnish shows a direct and close relationship with strong irradn. of sunlight, giving the first evidence of photochem. genesis. Abundant caves and tunnels with av. diam. of ∼1-5 μm are obsd. on varnish surface regions, which facilitate the penetration of sunlight and water. Metal (oxyhydr)oxides, including birnessite, hematite, goethite, rutile and anatase, account for the major components of rock varnish, which are all solar light-responsive semiconducting minerals. The trace elements enrichment patterns revealed by LA-ICP-MS provide the indication of an aq. origin. The pos. Ce anomalies in varnish in contrast to the rock substrate, as well as the pos. correlation between Ce and Mn suggest a strong oxidizing environment in the genesis of rock varnish. Therefore, the photo-generated holes and reactive oxygen species (ROSs) on metal oxides in varnish can promote Mn(II) oxidn., which are further demonstrated from thermodn. and kinetic considerations. ROSs including 1O2 and OH· with strong oxidizing capability are detected by EPR in varnish suspension, providing the direct evidence for Mn(II) oxidn. Lab. exptl. simulations show the photocatalysis of metal oxides in rock varnish greatly promote Mn(II) oxidn. by 2.10-7.97 times in comparison with homogenous soln. oxidn. All these lines of evidence suggest that light-induced abiotic process may play important roles in the formation of rock varnish together with other abiotic and biotic pathways, which can even provide implications for the evolution of Mn oxides on surface of terrestrial planets.
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    Lan, S.; Wang, X.; Xiang, Q.; Yin, H.; Tan, W.; Qiu, G.; Liu, F.; Zhang, J.; Feng, X. Mechanisms of Mn(II) catalytic oxidation on ferrihydrite surfaces and the formation of manganese (oxyhydr)oxides. Geochim. Cosmochim. Acta 2017, 211, 79– 96,  DOI: 10.1016/j.gca.2017.04.044
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    41
    Mechanisms of Mn(II) catalytic oxidation on ferrihydrite surfaces and the formation of manganese (oxyhydr)oxides
    Lan, Shuai; Wang, Xiaoming; Xiang, Quanjun; Yin, Hui; Tan, Wenfeng; Qiu, Guohong; Liu, Fan; Zhang, Jing; Feng, Xionghan
    Geochimica et Cosmochimica Acta (2017), 211 (), 79-96CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Oxidn. of Mn(II) is an important process that controls the mobility and bioavailability of Mn, as well as the formation of Mn (oxyhydr)oxides in natural systems. It was found that the surfaces of minerals, such as iron (oxyhydr)oxides, can accelerate Mn(II) oxidn. to a certain degree, but the underlying mechanism has not been clearly understood. This study explores the reaction pathways and mechanisms of Mn(II) oxidn. on ferrihydrite surfaces at neutral pH, commonly found in natural environments, by comparisons with montmorillonite, amorphous Al(OH)3, goethite, and magnetite using macroscopic expts. and spectroscopic analyses. Results show that when Mn(II) concns. are below 4 mM, macroscopic Mn(II) adsorption on the three iron (oxyhydr)oxide surfaces conforms well to the Langmuir equation, with ferrihydrite showing the highest adsorption capacity. With Mn(II) concns. ranging within 6-24 mM, the adsorbed Mn(II) is mainly oxidized into manganite (γ-MnOOH) and/or feitknechtite (β-MnOOH) by dissolved O2, and Mn(II) removal on a unit mass basis in the presence of magnetite is the highest compared with ferrihydrite and goethite. Ferrihydrite, a semiconductor material, shows stronger catalytic ability for Mn(II) oxidn. on the same surface area than insulator minerals (i.e., montmorillonite and amorphous Al(OH)3). Addnl., the products of Mn(II) oxidn. in the presence of semiconductor iron (oxyhydr)oxides (i.e., ferrihydrite, goethite, or magnetite) at the same Fe/Mn molar ratio include both manganite and a small amt. of Mn(IV) minerals, and the Mn av. oxidn. states (Mn AOSs) of these products follow the order: magnetite > goethite > ferrihydrite. Magnetite and goethite, with relatively smaller SSAs and lower band gap energies, exhibit greater catalysis for Mn(II) oxidn. than ferrihydrite at the same Fe/Mn ratio, which goes against the conventional interfacial effect and is related to the electrochem. properties. Thus, the Mn(II) catalytic oxidn. by O2 on ferrihydrite surfaces should include an electrochem. pathway, i.e., electron transfer (ET) in the Mn(II)-Conduction Band (CB)Ferrihydrite-O2 complexes, in addn. to the conventional two interfacial catalytic pathways, i.e., ET in the Mn(II)-Fe(II, III)-O2 complexes and direct ET in the Mn(II)-O2 complexes. These results reveal new implications for understanding the processes and mechanisms of Mn(II) oxidn. on iron (oxyhydr)oxide surfaces and the abiotic formation of Mn (oxyhydr)oxides in surface environments.
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    Wang, X.; Lan, S.; Zhu, M.; Ginder-Vogel, M.; Yin, H.; Liu, F.; Tan, W.; Feng, X. The presence of ferrihydrite promotes abiotic formation of manganese (oxyhydr)oxides. Soil Sci. Soc. Am. J. 2015, 79, 1297– 1305,  DOI: 10.2136/sssaj2014.12.0502
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    The presence of ferrihydrite promotes abiotic formation of manganese (oxyhydr)oxides
    Wang, Xiaoming; Lan, Shuai; Zhu, Mengqiang; Ginder-Vogel, Matthew; Yin, Hui; Liu, Fan; Tan, Wenfeng; Feng, Xionghan
    Soil Science Society of America Journal (2015), 79 (5), 1297-1305CODEN: SSSJD4; ISSN:1435-0661. (Soil Science Society of America)
    Iron (Fe) and manganese (Mn) occur in various geol. settings and are often assocd. with each other. In the present study, we examd. the impacts of ferrihydrite surfaces on the oxidn. of dissolved Mn(II) and concurrent formation of Mn(III/IV) (oxyhydr)oxides under various conditions. In the absence of ferrihydrite, the oxidn. products of 24 mM Mn(II) by atm. O2 are manganite (γ-MnOOH) at pH 7.5 and 8.5, feitknechtite (β-MnOOH), groutite (α-MnOOH) and manganite at pH 8, and hausmannite (Mn3O4) only at pH 9. In contrast, in the presence of ferrihydrite, manganite is formed at pH 6.5 to 8, manganite and hausmannite at pH 8.5, hausmannite and birnessite (δ-MnO2) at pH 9. When 24 mM Mn(II) is oxidized by pure O2 (i.e., at higher dissolved O2 level) at pH 9 in the absence of ferrihydrite, the products are mainly hausmannite and birnessite with a small amt. of feitknechtite, while only birnessite is obtained in the presence of ferrihydrite. These results suggest that the presence of ferrihydrite promotes the formation of manganite, birnessite, and hausmannite, disfavoring the formation of feitknechtite and groutite. The three favored phases in the presence of ferrihydrite are the commonly obsd. Mn (oxyhydr)oxides in the environment, suggesting the important role of mineral surfaces in their formation. The resulting solids are intimate mixts. of Fe and Mn oxides, which could contribute to the formation of Fe/Mn complex mineral assemblage in soils and sediments.
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  43. 43
    Ma, D.; Wu, J.; Yang, P.; Zhu, M. Coupled Manganese Redox Cycling and Organic Carbon Degradation on Mineral Surfaces. Environ. Sci. Technol. 2020, 54 (14), 8801– 8810,  DOI: 10.1021/acs.est.0c02065
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    Coupled manganese redox cycling and organic carbon degradation on mineral surfaces
    Ma, Dong; Wu, Juan; Yang, Peng; Zhu, Mengqiang
    Environmental Science & Technology (2020), 54 (14), 8801-8810CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Minerals, natural org. matter (NOM), and divalent manganese (Mn(II)) often coexist in suboxic/oxic environment. Multiple adsorption and oxidn. processes occur in this ternary system, which are coupled to affect the fate of both OM and Mn therein and alter their chem. reactivity toward metals and other pollutants. However, the details about the coupling are poorly known although much has been gained for the binary systems. We detd. the mutual influence of surface-catalyzed Mn(II) oxidn. and humic acid (HA) adsorption and oxidn. in a Fe(III) oxide (goethite)-HA-Mn(II) system at pH 5-8. The presence of Mn(II) substantially increased HA adsorption whereas HA greatly impaired the extent and rate of Mn(II) oxidn. by O2 on goethite surfaces. The impacts were more pronounced at higher pH. Mn(II) oxidn. produced β-MnOOH, γ-MnOOH, and Mn3O4 which in turn oxidized HA, producing small org. acids. The presence of HA markedly altered the compn. of Mn(II) oxidn. products by inhibiting the formation of β-MnOOH while favoring the prodn. of γ-MnOOH and Mn(II) adsorbed on the HA-mineral assemblage. Nonconducting γ-Al2O3 exhibited similar but weaker effects than semiconducting goethite in the above processes. Our results suggest that similar to Mn-oxidizing microorganisms, mineral surfaces can drive the coupling of the Mn redox cycle with NOM oxidative degrdn. under suboxic/oxic and circumneutral/alk. conditions.
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  44. 44
    Learman, D. R.; Wankel, S. D.; Webb, S. M.; Martinez, N.; Madden, A. S.; Hansel, C. M. Coupled biotic-abiotic Mn(II) oxidation pathway mediates the formation and structural evolution of biogenic Mn oxides. Geochim. Cosmochim. Acta 2011, 75 (20), 6048– 6063,  DOI: 10.1016/j.gca.2011.07.026
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    44
    Coupled biotic-abiotic Mn(II) oxidation pathway mediates the formation and structural evolution of biogenic Mn oxides
    Learman, D. R.; Wankel, S. D.; Webb, S. M.; Martinez, N.; Madden, A. S.; Hansel, C. M.
    Geochimica et Cosmochimica Acta (2011), 75 (20), 6048-6063CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Manganese oxides are among the strongest oxidants and sorbents in the environment, impacting the transport and speciation of metals, cycling of carbon, and flow of electrons within soils and sediments. The oxidn. of Mn(II) to Mn(III/IV) oxides has been primarily attributed to biol. processes, due in part to the faster rates of bacterial Mn(II) oxidn. compared to obsd. mineral-induced and other abiotic rates. The authors explore the reactivity of biogenic Mn oxides formed by a common marine bacterium (Roseobacter sp. AzwK-3b), which has been previously shown to oxidize Mn(II) via the prodn. of extracellular superoxide. The oxidn. of Mn(II) by superoxide results in the formation of highly reactive colloidal birnessite with hexagonal symmetry. The colloidal oxides induce the rapid oxidn. of Mn(II), with dramatically accelerated rates in the presence of orgs., presumably due to mineral surface-catalyzed org. radical generation. Mn(II) oxidn. by the colloids is further accelerated in presence of both orgs. and light, implicating reactive oxygen species in aiding abiotic oxidn. Indeed, the enhancement of Mn(II) oxidn. is negated when the colloids are reacted with Mn(II) in the presence of superoxide dismutase, an enzyme that scavenges the reactive oxygen species (ROS) superoxide. The reactivity of the colloidal phase is short-lived due to the rapid evolution of the birnessite from hexagonal to pseudo-orthogonal symmetry. The secondary particulate triclinic birnessite phase exhibits a distinct lack of Mn(II) oxidn. and subsequent Mn oxide formation. Thus, the evolution of initial reactive hexagonal birnessite to non-reactive triclinic birnessite imposes the need for continuous prodn. of new colloidal hexagonal particles for Mn(II) oxidn. to be sustained, illustrating an intimate dependency of enzymic and mineral-based reactions in Mn(II) oxidn. Further, the coupled enzymic and mineral-induced pathways are linked such that enzymic formation of Mn oxide is requisite for the mineral-induced pathway to occur. Mn(II) oxidn. is shown to involve a complex network of abiotic and biotic processes, including enzymically produced superoxide, mineral catalysis, org. reactions with mineral surfaces, and likely photo-prodn. of ROS. The complexity of coupled reactions involved in Mn(II) oxidn. highlights the need for further investigations of microbially-mediated Mn oxide formation, including identifying the role of Mn oxide surfaces, orgs., reactive oxygen species, and light in Mn(II) oxidn. and Mn oxide phase evolution.
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  45. 45
    Madden, A. S.; Hochella, M. F. A test of geochemical reactivity as a function of mineral size: Manganese oxidation promoted by hematite nanoparticles. Geochim. Cosmochim. Acta 2005, 69 (2), 389– 398,  DOI: 10.1016/j.gca.2004.06.035
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    45
    A test of geochemical reactivity as a function of mineral size: manganese oxidation promoted by hematite nanoparticles
    Madden, Andrew S.; Hochella, Michael F., Jr.
    Geochimica et Cosmochimica Acta (2005), 69 (2), 389-398CODEN: GCACAK; ISSN:0016-7037. (Elsevier Inc.)
    Mn2+ (aq) oxidn. as promoted by hematite in the presence of mol. oxygen has been studied as a function of hematite particle size. This system is a good candidate to serve as a test of the change of particle reactivity as a function of size due not only to its importance in Earth/environmental processes, but also because it involves electronic coupling between the hematite and adsorbed manganese. The properties of nanoscale hematite, including size quantization of the electronic structure and the relative proportions of terrace vs. edge/kink sites, are expected to change significantly with the particle size in this size range. Exptl. results from this study suggest that the heterogeneous manganese oxidn. rate is approx. one to one and a half orders of magnitude greater on hematite particles with an av. diam. of 7.3 nm than with those having an av. diam. of 37 nm, even when normalized to the surface areas of the particles. The acceleration of electron transfer rate for the reactions promoted by the smallest particles is rationalized in the framework of electron transfer theory. According to this theory, for a reaction such as heterogeneous Mn oxidn., the rate depends on three factors: the electronic coupling between initial and final electronic states, the substantial reorganization energy for solvent and coordinated ligands between initial and final states, and the free energy of reaction (cor. for work required to bring reactants together). The adsorbed Mn is electronically coupled with the solid during the electron transfer, and changes in the electronic structure of the solid would be expected to influence the rate. The Lewis base character of surface oxygen atoms increases as the electronic structure becomes quantized, which should allow increased coupling with adsorbed Mn. Finally, as demonstrated previously by in situ AFM observations, the reaction proceeds most readily at topog. features that distort the octahedral Mn2+ coordination environment. This has the effect of lowering the reorganization energy, which effectively controls the magnitude of the transition state barrier. Previous studies of < 10 nm diam. hematite nanoparticles have demonstrated a decrease of symmetry in the av. coordination environment of surface atoms, supporting the idea that smaller sizes should correspond to a decrease in reorganization energy.
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  46. 46
    Luo, Y. T. W.; Suib, S. L.; Qiu, G.; Liu, F. Dissolution and phase transformation processes of hausmannite in acidic aqueous systems under anoxic conditions. Chem. Geol. 2018, 487, 54– 62,  DOI: 10.1016/j.chemgeo.2018.04.016
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    Dissolution and phase transformation processes of hausmannite in acidic aqueous systems under anoxic conditions
    Luo, Yao; Tan, Wenfeng; Suib, Steven L.; Qiu, Guohong; Liu, Fan
    Chemical Geology (2018), 487 (), 54-62CODEN: CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
    Hausmannite is the most widely distributed spinel-structured manganese oxide in soils and sediments. The transformation of this metastable manganese oxide to Mn(IV) oxides with higher adsorption capacity has attracted much research interest, while the transformation mechanisms and influencing factors still remain largely unknown, esp. under acidic condition. In this work, the transformation processes of hausmannite at different pH values and the influence of cations were studied. Results indicated that hausmannite was transformed into manganite at pH 5.0-9.0. The dissoln. of hausmannite was initiated and promoted by protons (≤ 7.0), and the decrease of pH accelerated its conversion to Mn(IV) oxides. The tunnel-structured Mn(IV) oxide was generated via two steps during the dissoln. process of hausmannite at pH ≤ 3.0. Hausmannite was disproportionated to δ-MnO2 at first, which was then transformed to nsutite in the presence of Na+ and H+ through the transfer of electrons from adsorbed Mn(II) to structural Mn(IV). The disproportionation of hausmannite to δ-MnO2 was not affected by other cations, while the presence of K+ promoted the further transformation of δ-MnO2 to cryptomelane. The structural rearrangement process of δ-MnO2 was the rate-detg. step for the formation of final products. This work expands the understanding of the formation, transformation and geochem. processes of manganese oxides in supergene environments.
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  47. 47
    Tu, S. R. G. J.; Goh, T. B. Transformation of synthetic birnessite as affected by pH and manganese concentration. Clays Clay Miner. 1994, 42, 321– 330,  DOI: 10.1346/CCMN.1994.0420310
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    Transformations of synthetic birnessite as affected by pH and manganese concentration
    Tu, Shihua; Racz, Geza J.; Goh, Tee Boon
    Clays and Clay Minerals (1994), 42 (3), 321-30CODEN: CLCMAB; ISSN:0009-8604.
    The amt. of Mn2+ adsorbed or removed from sol. by birnessite is several times greater than its reported cation exchange capacity. Extractability of the sorbed Mn2+ decreases with aging. It is uncertain whether the sorbed Mn2+ is oxidized on the surface or incorporated into the structure of birnessite. Using x-ray powder diffractometry and transmission electron microscopy, a study was conducted to examine the mineralogical alteration of birnessite after treatment with various concns. of MnSO4 and sol. pH. The sorbed Mn2+ was not directly oxidized and remained on the birnessite surface. The sorption of Mn2+ was followed by alteration of birnessite with the formation of new Mn minerals. The specific Mn minerals formed were governed by the pH of the reaction, and the rate of the transformation was detd. by Mn2+ concn. and pH. Nsutite and ramsdellite were identified at pH 2.4, cryptomelane at pH 4, groutite at pH 6, and manganite at pH 8. Other Mn minerals formed at these and other pH levels could not be identified. As the concn. of Mn in the soln. decreased, the time required to form new minerals from the birnessite increased. The newly formed phases were the result of structural conversion since dissoln. of birnessite and repptn. of new phases were not obsd.
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    Lefkowitz, J. P.; Rouff, A. A.; Elzinga, E. J. Influence of pH on the reductive transformation of birnessite by aqueous Mn(II). Environ. Sci. Technol. 2013, 47, 10364– 10371,  DOI: 10.1021/es402108d
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    Influence of pH on the Reductive Transformation of Birnessite by Aqueous Mn(II)
    Lefkowitz, Joshua P.; Rouff, Ashaki A.; Elzinga, Evert J.
    Environmental Science & Technology (2013), 47 (18), 10364-10371CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    We investigated the effect of pH (5.5-8.5) on the mineralogical transformation of hexagonal birnessite induced by reaction with aq. Mn-(II) (50-2200 μM), using batch sorption expts., X-ray diffraction analyses, X-ray absorption and IR spectroscopic measurements. Samples reacted at pH < 7.0 exhibited disrupted stacking of birnessite sheets, but no mineralogical transformation products were obsd. At pH 7.0 and 7.5, reaction with Mn-(II) under anoxic conditions caused reductive transformation of birnessite into manganite (γ-MnOOH), whereas at pH 8.0 and 8.5, conversion into hausmannite (Mn3O4) occurred. Feitknechtite (β-MnOOH) is a major transformation product at low Mn-(II) inputs at pH 7.0-8.5, and represents a metastable reaction intermediate that is converted into manganite and possibly hausmannite during further reaction with Mn-(II). Thermodn. calcns. suggest that conversion into hausmannite at alk. pH reflects a kinetic effect where rapid hausmannite pptn. prevents formation of thermodynamically more favorable manganite. In oxic systems, feitknechtite formation due to surface catalyzed oxidn. of Mn-(II) by O2 increases Mn-(II) removal relative to anoxic systems at pH ≥ 7. The results of this study suggest that aq. Mn-(II) is an important control on the mineralogy and reactivity of natural Mn-oxides, particularly in aq. geochem. environments with neutral to alk. pH values.
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  49. 49
    Yin, H.; Li, H.; Wang, Y.; Ginder-Vogel, M.; Qiu, G.; Feng, X.; Zheng, L.; Liu, F. Effects of Co and Ni co-doping on the structure and reactivity of hexagonal birnessite. Chem. Geol. 2014, 381, 10– 20,  DOI: 10.1016/j.chemgeo.2014.05.017
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    Effects of Co and Ni co-doping on the structure and reactivity of hexagonal birnessite
    Yin, Hui; Li, Hui; Wang, Yan; Ginder-Vogel, Matthew; Qiu, Guohong; Feng, Xionghan; Zheng, Lirong; Liu, Fan
    Chemical Geology (2014), 381 (), 10-20CODEN: CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
    Natural hexagonal birnessites are enriched in various transition metals (TMs). Many studies have examd. the effects of single metal doping on the structures and properties of birnessites, but none focused on the simultaneous interaction mechanism of copptn. of two different TMs with birnessite. In this work Co and Ni co-doped hexagonal birnessites were synthesized and characterized by powder X-ray diffraction (XRD), elemental anal., field emission SEM (FE-SEM), XPS and X-ray absorption fine structure (XAFS) spectroscopy to investigate the effects of co-doping on the structure and reactivity of birnessite and the crystal chem. of Co and Ni. These co-doped birnessites have lower crystallinity, i.e., fewer manganese layers stacking in the c* direction, larger sp. surface areas (SSAs) and increased Mn av. oxidn. states (AOSs) than the undoped birnessite, and Co exists in a valence of + 3. Co, Ni and Mn K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) spectra demonstrate an increase in edge-sharing Ni-Me (Me = Ni, Co and Mn) distances in birnessite layers with the increase of the contents of dopants while Mn-Me distances first decrease and then increase while those of Co-Me pairs are nearly const., coupled with first a decrease and then increase of the in-plane unit-cell parameter b. The effect of co-doping on the amts. of structural Mn and K+, nos. of [MnO6] layers stacked in c* axis, and SSAs, is larger than the effects of doping with Co alone, but less than singly Ni doping. In birnessites doped with both Co and Ni, ∼ 74-79% of the total Co and ∼ 23-39% of the total Ni are present within the manganese layers. Compared with the spatial distribution of TM in singly doped birnessites, the coexistence of Ni hinders the incorporation of Co into the layers during birnessite crystn.; however, copptn. with Co has little effects, neither hindrance nor promotion, on the insertion of Ni into the layers. These results provide insight into the interaction mechanism between coexisting Co, Ni within layered Mn oxides. It further helps us to interpret the geochem. characteristics of multi-metal incorporation into natural Mn oxides and their effects on the structures and physicochem. properties of these minerals.
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    Webb, S. M.; Fuller, C. C.; Tebo, B. M.; Bargar, J. R. Determination of uranyl incorporation into biogenic manganese oxides using X-ray absorption spectroscopy and scattering. Environ. Sci. Technol. 2006, 40, 771– 777,  DOI: 10.1021/es051679f
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    Determination of uranyl incorporation into biogenic manganese oxides using X-ray absorption spectroscopy and scattering
    Webb, S. M.; Fuller, C. C.; Tebo, B. M.; Bargar, J. R.
    Environmental Science and Technology (2006), 40 (3), 771-777CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Biogenic manganese oxides are common and an important source of reactive mineral surfaces in the environment that may be potentially enhanced in bioremediation cases to improve natural attenuation. Expts. were performed in which the uranyl ion, UO22+ (U(VI)), at various concns. was present during manganese oxide biogenesis. At all concns., there was strong uptake of U onto the oxides. Synchrotron-based extended X-ray absorption fine structure (EXAFS) spectroscopy and X-ray diffraction (XRD) studies were carried out to det. the mol.-scale mechanism by which uranyl is incorporated into the oxide and how this incorporation affects the resulting manganese oxide structure and mineralogy. The EXAFS expts. show that at low concns. (<0.3 mol % U, <1 μM U(VI) in soln.), U(VI) is present as a strong bidentate surface complex. At high concns. (>2 mol % U, >4 μM U(VI) in soln.), the presence of U(VI) affects the stability and structure of the Mn oxide to form poorly ordered Mn oxide tunnel structures, similar to todorokite. EXAFS modeling shows that uranyl is present in these oxides predominantly in the tunnels of the Mn oxide structure in a tridentate complex. Observations by XRD corroborate these results. Structural incorporation may lead to more stable U(VI) sequestration that may be suitable for remediation uses. These observations, combined with the very high uptake capacity of the Mn oxides, imply that Mn-oxidizing bacteria may significantly influence dissolved U(VI) concns. in impacted waters via sorption and incorporation into Mn oxide biominerals.
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    Ruiz-Garcia, M. V. M.; Voegelin, A.; Pi-Puig, T.; Martínez-Villegas, N.; Göttlicher, J. Transformation of hexagonal birnessite upon reaction with thallium(I): Effects of birnessite crystallinity, pH, and thallium concentration. Environ. Sci. Technol. 2021, 55, 4862– 4870,  DOI: 10.1021/acs.est.0c07886
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    51
    Transformation of Hexagonal Birnessite upon Reaction with Thallium(I): Effects of Birnessite Crystallinity, pH, and Thallium Concentration
    Ruiz-Garcia, Mismel; Villalobos, Mario; Voegelin, Andreas; Pi-Puig, Teresa; Martinez-Villegas, Nadia; Gottlicher, Jorg
    Environmental Science & Technology (2021), 55 (8), 4862-4870CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    We examd. the uptake of Tl(I) by two hexagonal birnessites and related phase transformations in lab. expts. over 12 sequential addns. of 0.01 M Tl(I)/Mn at pH 4.0, 6.0, and 8.0. The Tl-reacted Mn oxides were characterized for their structure, Tl binding, and morphol. using X-ray diffraction, X-ray photoelectron and X-ray absorption spectroscopies, and transmission electron microscopy. Very limited Tl oxidn. was obsd. in contrast to previous works, where equal Tl(I)/Mn was added in a single step. Instead, both birnessites transformed into a 2 x 2 tunneled phase with dehydrated Tl(I) in its tunnels at pH 4, but only partially at pH 6, and at pH 8.0 they remained layered. The first four to nine sequential Tl(I)/Mn addns. resulted in lower residual dissolved Tl+ concns. than when the same amts. of Tl(I)/Mn were added in single steps. This study thus shows that the repeated reaction of hexagonal birnessites with smaller Tl(I)/Mn at ambient temp. triggers a complete phase conversion with Tl(I) as the sole reacting cation. The novel pathway found may be more relevant for contaminated environments and may help explain the formation of minerals like thalliomelane [Tl+(Mn7.54+Cu0.52+)O16]; it also points to the possibility that other reducing species trigger similar Mn oxide transformation reactions.
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    Lin, H.; Szeinbaum, N. H.; DiChristina, T. J.; Taillefert, M. Geochim. Cosmochim. Acta 2012, 99, 179– 192,  DOI: 10.1016/j.gca.2012.09.020
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    Microbial Mn(IV) reduction requires an initial one-electron reductive solubilization step
    Lin, Hui; Szeinbaum, Nadia H.; DiChristina, Thomas J.; Taillefert, Martial
    Geochimica et Cosmochimica Acta (2012), 99 (), 179-192CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Mn(IV) and Mn(II) are the most stable and prevalent forms of manganese in natural environments. The occurrence of Mn(III) in minerals and the detection of sol. Mn(III) in natural waters, however, suggest that Mn(III) is an intermediate in both the oxidn. of Mn(II) and the redn. of Mn(IV). Mn(III) has recently been proposed as an intermediate during the oxidn. of Mn(II) by Mn-oxidizing bacteria but has never been considered as an intermediate during the bio-redn. of Mn(IV). Here we show for the first time that microbial Mn(IV) redn. proceeds step-wise via two successive one-electron transfer reactions with prodn. of sol. Mn(III) as transient intermediate. Incubations with mutant strains demonstrate that the redn. of both solid Mn(IV) and sol. Mn(III) occurs at the outer membrane of the cell. In addn., pseudo-first order rate consts. obtained from these incubations indicate that Mn(IV) respiration involves only one of the two potential terminal reductases (c-type cytochrome MtrC and OmcA) involved in Fe(III) respiration. More importantly, only the second electron transfer step is coupled to prodn. of dissolved inorg. carbon, suggesting that the first electron transfer reaction is a reductive solubilization step that increases Mn bioavailability. These findings oppose the long-standing paradigm that microbial Mn(IV) redn. proceeds via a single two-electron transfer reaction coupled to org. carbon oxidn., and suggest that diagenetic models should be revised to correctly account for the impact of manganese redn. in the global carbon cycle.
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    Lin, H.; Szeinbaum, N. H.; DiChristina, T. J.; Taillefert, M. Microbial Mn(IV) reduction requires an initial one-electron reductive solubilization step. Geochim. Cosmochim. Acta 2012, 99, 179– 192,  DOI: 10.1016/j.gca.2012.09.020
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    Microbial Mn(IV) reduction requires an initial one-electron reductive solubilization step
    Lin, Hui; Szeinbaum, Nadia H.; DiChristina, Thomas J.; Taillefert, Martial
    Geochimica et Cosmochimica Acta (2012), 99 (), 179-192CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Mn(IV) and Mn(II) are the most stable and prevalent forms of manganese in natural environments. The occurrence of Mn(III) in minerals and the detection of sol. Mn(III) in natural waters, however, suggest that Mn(III) is an intermediate in both the oxidn. of Mn(II) and the redn. of Mn(IV). Mn(III) has recently been proposed as an intermediate during the oxidn. of Mn(II) by Mn-oxidizing bacteria but has never been considered as an intermediate during the bio-redn. of Mn(IV). Here we show for the first time that microbial Mn(IV) redn. proceeds step-wise via two successive one-electron transfer reactions with prodn. of sol. Mn(III) as transient intermediate. Incubations with mutant strains demonstrate that the redn. of both solid Mn(IV) and sol. Mn(III) occurs at the outer membrane of the cell. In addn., pseudo-first order rate consts. obtained from these incubations indicate that Mn(IV) respiration involves only one of the two potential terminal reductases (c-type cytochrome MtrC and OmcA) involved in Fe(III) respiration. More importantly, only the second electron transfer step is coupled to prodn. of dissolved inorg. carbon, suggesting that the first electron transfer reaction is a reductive solubilization step that increases Mn bioavailability. These findings oppose the long-standing paradigm that microbial Mn(IV) redn. proceeds via a single two-electron transfer reaction coupled to org. carbon oxidn., and suggest that diagenetic models should be revised to correctly account for the impact of manganese redn. in the global carbon cycle.
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    Stone, A. T.; Morgan, J. J. Reduction and Dissolution of Manganese(III) and Manganese(IV) Oxides by Organics: 2. Survey of the Reactivity of Organics. Environ. Sci. Technol. 1984, 18, 617– 624,  DOI: 10.1021/es00126a010
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    Reduction and dissolution of manganese(III) and manganese(IV) oxides by organics: 2. Survey of the reactivity of organics
    Stone, Alan T.; Morgan, James J.
    Environmental Science and Technology (1984), 18 (8), 617-24CODEN: ESTHAG; ISSN:0013-936X.
    Redn. and dissoln. of Mn(III,IV) oxide suspensions by 27 arom. and nonarom. compds. resembling natural orgs. were examd. in order to detn the solubilization reaction in nature. At pH 7.2, 10-3M formate, fumarate, glycerol, lactate, malonate, phthalate, PrOH, EtCHO, propionate, and sorbitol did not dissolve appreciable amts. of oxide after 3 h of reaction. The following orgs. did dissolve Mn oxides under these conditions and are listed in order of decreasing reactivity: 3-methoxycatechol ∼ catechol ∼ 3,4-dihydroxybenzoic acid ∼ ascorbate > 4-nitrocatechol > thiosalicylate > hydroquinone > 2,5-dihydroxybenzoic acid > syringic acid > o-methoxyphenol > vanillic acid > orcinol ∼ 3,5-dihydroxybenzoic acid > resorcinol > oxalate ∼ pyruvate ∼ salicylate. Relative reactivities of org. substrates are discussed in terms of surface complex formation prior to electron transfer. Dissoln. of Mn oxides by marine fulvic acid was enhanced by illumination, verifying that the reaction is photocatalyzed.
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  54. 54
    Wang, Y.; Stone, A. T. Reaction of MnIII,IV (hydr)oxides with oxalic acid, glyoxylic acid, phosphonoformic acid, and structurally-related organic compounds. Geochim. Cosmochim. Acta 2006, 70 (17), 4477– 4490,  DOI: 10.1016/j.gca.2006.06.1548
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    54
    Reaction of MnIII,IV (hydr)oxides with oxalic acid, glyoxylic acid, phosphonoformic acid, and structurally-related organic compounds
    Wang, Yun; Stone, Alan T.
    Geochimica et Cosmochimica Acta (2006), 70 (17), 4477-4490CODEN: GCACAK; ISSN:0016-7037. (Elsevier)
    Phosphonoformic acid, oxalic acid, glyoxylic acid, and 10 addnl. org. compds. that are structurally related to them have been reacted with synthetic MnO2 (birnessite), consisting of 22% MnIII and 78% MnIV, and synthetic MnOOH (manganite), consisting solely of MnIII. Significant concns. of dissolved MnIII were detected in reactions of phosphonoformic acid with MnOOH, indicating that ligand-assisted dissoln. took place. Reaction of phosphonoformic acid with MnO2, and reaction of all other org. reactants with either MnOOH or MnO2, yielded only MnII, indicating that reductive dissoln. was predominant. As far as reductive dissoln. reactions are concerned, MnO2 yields a range of reactivity that is nearly 20-times greater than that of MnOOH. Oxidn. converts phosphonoformic acid into orthophosphate ion, glyoxylic acid into formic acid, pyruvic acid into acetic acid, and 2,3-butanedione into acetic acid. When differences in surface area loading are accounted for, oxalic acid, pyruvic acid, and 2,3-butanedione yield virtually the same dissoln. rates for the two (hydr)oxides. At pH 5.0, glyoxylic acid reacts 14-times faster with MnO2 than with MnOOH. MnO2 reacts more slowly than MnOOH by a factor of 1/16th with oxamic acid, 1/20th with lactic acid, and 1/33rd with di-Me oxalate. Adsorptive, complexant, and reductant properties of the 13 org. reactants are believed responsible for the obsd. reactivity differences.
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  55. 55
    Sparrow, L. A.; Uren, N. C. Manganese oxidation and reduction in soils: Effects of temperature, water potential, pH and their interactions. Soil Res. 2014, 52, 483– 494,  DOI: 10.1071/SR13159
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    Manganese oxidation and reduction in soils: effects of temperature, water potential, pH and their interactions
    Sparrow, L. A.; Uren, N. C.
    Soil Research (Collingwood, Australia) (2014), 52 (5), 483-494CODEN: SRCACP; ISSN:1838-6768. (CSIRO Publishing)
    Manganese (Mn) toxicity is a potential limitation to plant growth on acidic and poorly drained soils. Five lab. expts. using such soils were conducted to examine the influence of soil temp., pH and water potential on the redox reactions of Mn and the potential for Mn toxicity. The microbial inhibitor sodium azide was used in some expts. to assess the role of microorganisms in these reactions. The redn. of Mn oxides (MnOx) during waterlogging was faster at 20°C and 30°C than at 10°C or 4°C. Sodium azide slowed the redn. of Mn oxides at 20°C and 30°C during waterlogging but had little effect at 4°C and 10°C, suggesting that microbial MnOx redn. during waterlogging was minimal at the lower temps. Re-oxidn. of Mn2+ in soil drained after severe waterlogging was only obsd. in soil not treated with sodium azide, indicating that even when very high concns. of Mn2+ were present, Mn2+ oxidn. was still microbial. Prior liming of aerobic soil established lower starting concns. of water-sol. plus exchangeable (WS+E) Mn2+ and slowed the redn. of Mn oxides during subsequent waterlogging. After drainage, rapid re-oxidn. of Mn2+ was obsd. in all lime treatments but was fastest at the two highest lime rates. In the fourth and fifth expts., interactions between temp. and water potential were obsd. When waterlogged soils were drained to -5 and -10kPa, re-oxidn. of Mn2+ occurred at both 10°C and 20°C. At -1kPa, there was no net change in WS+E Mn2+ at 10°C, whereas at 20°C, the concn. of WS+E Mn2+ increased, possibly due to the lower concn. of O2 in the soil water at the higher temp. In the fifth expt., at 4°C and 10°C there was little or no effect on Mn reactions of varying water potential from -1 to -1500kPa, but at 20°C and esp. at 30°C, both Mn2+ oxidn. and Mn oxide redn. were slowed at -1500kPa compared with the higher water potentials. Overall, the expts. show that a delicate balance between the microbial oxidn. of Mn2+ and the redn. of Mn oxides can exist, and that it can be shifted by small changes in soil water potential along with changes in temp. and pH.
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  56. 56
    Jobbágy, E. J.; Jackson, R. B. The uplift of soil nutrients by plants: biogeochemical consequences across scales. Ecology 2004, 85 (9), 2380– 2389,  DOI: 10.1890/03-0245
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    Oh, N.-H.; Richter, D. D. Elemental translocation and loss from three highly weathered soil-bedrock profiles in the southeastern United States. Geoderma 2005, 126 (1–2), 5– 25,  DOI: 10.1016/j.geoderma.2004.11.005
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    Elemental translocation and loss from three highly weathered soil-bedrock profiles in the southeastern United States
    Oh, Neung-Hwan; Richter, Daniel D.
    Geoderma (2005), 126 (1-2), 5-25CODEN: GEDMAB; ISSN:0016-7061. (Elsevier B.V.)
    The authors used geochem. mass-balance equations to quantify the net result of pedogenic weathering, i.e. elemental loss and gain, in three residual soil-bedrock profiles on the Piedmont of North Carolina. Soils are located on interfluves and derived directly from the bedrock below: a Kanhapludult (Tarrus series) from phyllite, a Kanhapludult (Cecil series) from granitic gneiss, and a Hapludalf (Enon series) from diabase. Bulk d. ratios of soils and bedrock as well as elemental concns. referenced to Zr, Ti, Y, and V were used to est. strain and open-system mass-transport functions through the soil profiles. Estd. strains of the three soils indicated substantial volumetric changes during C horizon and saprolite formation. Overall, desilication was the most predominant pedogenic process removing chem. elements from the three soils. Losses of Si were about 50% of total elemental molar losses in the 8.5-m deep Tarrus profile, 75% of total losses in the 3.8-m deep Cecil profile, and 39% of total losses in the 4-m deep Enon profile. Base cations were also lost in great amts. following desilication. Losses of base cations accounted for about 50% of the total elemental losses in the Tarrus, 20% of the total losses in the Cecil, and 37% of the total losses in the Enon profiles. The specific base cations lost in greatest amts. differed among the three soils and depended on bedrock mineralogy. Sodium and Mg accounted for 24% and 16% of total elemental loss from Tarrus profiles, Na and K accounted for 14% and 4% of total elemental loss from Cecil profiles, and Ca and Mg accounted for 19% and 12% of total elemental loss from Enon profiles. The vertical pattern of loss of base cations was not always gradual from surface soil horizons to saprolite to bedrock. For example, almost 100% of Ca in the bedrock had been lost from throughout the upper 4.5-m deep Cecil and 8-m deep Tarrus profiles. Aluminum and iron were lost from A and E horizons but were accumulated in B and C horizons due to translocation as well as secondary clay and sesquioxide formation at depth. Phys. and chem. data from all three soils and geol. substrata indicate that the entire regolith profile (A through C horizons or solum plus saprolite) is formed by pedogenic processes of elemental inputs, transformations, translocations, and removals.
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  58. 58
    Broadley, M.; Brown, P.; Cakmak, I.; Rengel, Z.; Zhao, F., Function of nutrients: micronutrients. In Marschner’S Mineral Nutrition of Higher Plants; Elsevier, 2012; pp 191– 248.
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    Herndon, E. M.; Martínez, C. E.; Brantley, S. L. Spectroscopic (XANES/XRF) characterization of contaminant manganese cycling in a temperate watershed. Biogeochemistry 2014, 121 (3), 505– 517,  DOI: 10.1007/s10533-014-0018-7
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    59
    Spectroscopic (XANES/XRF) characterization of contaminant manganese cycling in a temperate watershed
    Herndon, Elizabeth M.; Martinez, Carmen E.; Brantley, Susan L.
    Biogeochemistry (2014), 121 (3), 505-517CODEN: BIOGEP; ISSN:0168-2563. (Springer)
    Many soils around the globe are contaminated with metals due to inputs from anthropogenic activities; however, the long-term processes that retain these metals in soils or flush them into river systems remain unclear. Soils at the Susquehanna/Shale Hills Crit. Zone Observatory, a headwater catchment in central Pennsylvania, USA, are enriched in manganese due to past atm. deposition from industrial sources. To investigate how Mn is retained in the catchment, we evaluated the spatial distribution and speciation of Mn in the soil-plant system using X-ray fluorescence and X-ray Absorption Near Edge Structure spectroscopies. Weathered soils near the land surface were enriched in both amorphous and cryst. Mn(III/IV)-oxides, presumably derived from biogenic pptn. and atm. deposition, resp. In contrast, mineral soils near the soil-bedrock interface contained Mn(II) in clays and cryst. Mn(III/IV)-oxides that formed as Mn(II) was leached from the parent shale and oxidized. Roots, stems, and foliar tissue were dominated by org.-bound and aq. Mn(II); however, a small portion of foliar Mn was concd. as org.-bound Mn(III) in dark spots that denote Mn toxicity. During decompn. of leaves and roots, sol. Mn(II) stored in vegetation was rapidly oxidized and immobilized as mixed-valence Mn-oxides. We propose that considerable uptake of Mn by certain plant species combined with rapid oxidn. of Mn during org. matter decompn. contributes to long-term retention in soils and may slow removal of Mn contamination from watersheds.
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  60. 60
    Fernando, D. R.; Mizuno, T.; Woodrow, I. E.; Baker, A. J.; Collins, R. N. Characterization of foliar manganese (Mn) in Mn (hyper)accumulators using X-ray absorption spectroscopy. New Phytol. 2010, 188 (4), 1014– 27,  DOI: 10.1111/j.1469-8137.2010.03431.x
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    60
    Characterization of foliar manganese (Mn) in Mn (hyper)accumulators using X-ray absorption spectroscopy
    Fernando, D. R.; Mizuno, T.; Woodrow, I. E.; Baker, A. J. M.; Collins, R. N.
    New Phytologist (2010), 188 (4), 1014-1027CODEN: NEPHAV; ISSN:0028-646X. (Wiley-Blackwell)
    Plant hyperaccumulation of the essential nutrient manganese (Mn) is a rare phenomenon most evident in the Western Pacific region, and differs from hyperaccumulation of other elements. My hyperaccumulators employ a variety of species-dependent spatial distribution patterns in sequestering excess foliar Mn, including primary sequestration in both nonphotosynthetic and photosynthetic tissues. This investigation employed synchrotron X-ray absorption spectroscopy (XAS) in a comparative study of Mn (hyper)accumulators, to elucidate in situ the chem. form(s) of foliar Mn in seven woody species from Australia, New Caledonia and Japan. Foliar Mn was found to predominate as Mn(II) in all samples, with strong evidence of the role of carboxylic acids, such as malate or citrate, as complexing ligands. Overall, the X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine-structure spectroscopy (EXAFS) data appeared weighted against previous observations that oxalate binds excess Mn in Mn-(hyper)accumulating species.
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  61. 61
    McCain, D. C.; Markley, J. L. More manganese accumulates in maple sun leaves than in shade leaves. Plant Physiol. 1989, 90 (4), 1417– 1421,  DOI: 10.1104/pp.90.4.1417
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    More manganese accumulates in maple sun leaves than in shade leaves
    McCain, Douglas C.; Markley, John L.
    Plant Physiology (1989), 90 (4), 1417-21CODEN: PLPHAY; ISSN:0032-0889.
    NMR and neutron activation anal. were used to measure Mn concns. in leaves of Acer platanoides. Mn accumulated in both the vacuoles and the chloroplasts, with more Mn (per unit area) in sun leaves than in shade leaves. No Mn was lost at senescence. Different seasonal patterns of Mn accumulation were found in sun and shade leaves. The quantity of chloroplast reserve Mn (bound to the outer surface of thylakoid membranes) increased rapidly in sun leaves from bud-break through midsummer, and then remained approx. const. through senescence. In shade leaves, however, the quantity of reserve Mn increased slowly, and at approx. a const. rate throughout the growing season.
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    Keiluweit, M.; Nico, P.; Harmon, M. E.; Mao, J.; Pett-Ridge, J.; Kleber, M. Long-term litter decomposition controlled by manganese redox cycling. Proc. Natl. Acad. Sci. U. S. A. 2015, 112 (38), E5253– 60,  DOI: 10.1073/pnas.1508945112
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    Long-term litter decomposition controlled by manganese redox cycling
    Keiluweit, Marco; Nico, Peter; Harmon, Mark E.; Mao, Jingdong; Pett-Ridge, Jennifer; Kleber, Markus
    Proceedings of the National Academy of Sciences of the United States of America (2015), 112 (38), E5253-E5260CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Litter decompn. is a keystone ecosystem process impacting nutrient cycling and productivity, soil properties, and the terrestrial carbon (C) balance, but the factors regulating decompn. rate are still poorly understood. Traditional models assume that the rate is controlled by litter quality, relying on parameters such as lignin content as predictors. However, a strong correlation has been obsd. between the manganese (Mn) content of litter and decompn. rates across a variety of forest ecosystems. Here, we show that long-term litter decompn. in forest ecosystems is tightly coupled to Mn redox cycling. Over 7 years of litter decompn., microbial transformation of litter was paralleled by variations in Mn oxidn. state and concn. A detailed chem. imaging anal. of the litter revealed that fungi recruit and redistribute unreactive Mn2+ provided by fresh plant litter to produce oxidative Mn3+ species at sites of active decay, with Mn eventually accumulating as insol. Mn3+/Mn4+ oxides. Formation of reactive Mn3+ species coincided with the generation of arom. oxidn. products, providing direct proof of the previously posited role of Mn3+-based oxidizers in the breakdown of litter. Our results suggest that the litter-decompg. machinery at our coniferous forest site depends on the ability of plants and microbes to supply, accumulate, and regenerate short-lived Mn3+ species in the litter layer. This observation indicates that biogeochem. constraints on bioavailability, mobility, and reactivity of Mn in the plant-soil system may have a profound impact on litter decompn. rates.
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    Herndon, E. M.; Jin, L.; Andrews, D. M.; Eissenstat, D. M.; Brantley, S. L. Importance of vegetation for manganese cycling in temperate forested watersheds. Global Biogeochem. Cy. 2015, 29 (2), 160– 174,  DOI: 10.1002/2014GB004858
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    Marschner, H., Mechanisms of manganese acquisition by roots from soils. In Manganese in Soils and Plants; Springer, 1988; pp 191– 204.
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    Horsley, S. B.; Long, R. P.; Bailey, S. W.; Hallett, R. A.; Hall, T. J. Factors associated with the decline disease of sugar maple on the Allegheny Plateau. Can. J. For. Res. 2000, 30 (9), 1365– 1378,  DOI: 10.1139/x00-057
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    Factors associated with the decline disease of sugar maple on the Allegheny Plateau
    Horsley, Stephen B.; Long, Robert P.; Bailey, Scott W.; Hallett, Richard A.; Hall, Thomas J.
    Canadian Journal of Forest Research (2000), 30 (9), 1365-1378CODEN: CJFRAR; ISSN:0045-5067. (National Research Council of Canada)
    Mortality of sugar maple (Acer saccharum Marsh.) has reached unusually high levels across northern Pennsylvania since the early to mid-1980s. The authors evaluated the influence of glaciation, topog. position, foliage chem., defoliation history, and stand characteristics (species compn., structure, d.) on the health of sugar maple in 43 stands at 19 sites on the northern Allegheny Plateau. Using percent dead sugar maple basal area as the measure of health, it was found that all moderately to severely declining stands were on unglaciated summits, shoulders, or upper backslopes. Stands on glaciated sites and unglaciated lower topog. positions were not declining. The most important factors assocd. with sugar maple health were foliar levels of Mg and Mn and defoliation history. The lowest foliar Mg, highest foliar Mn, and highest no. and severity of insect defoliations were assocd. with unglaciated summits, shoulders, and upper backslopes. Declining stands had less than ∼700 mg·kg-1 Mg and two or more moderate to severe defoliations in the past 10 yr; both conditions were assocd. with moderately to severely declining stands. The decline disease of sugar maple seems to result from an interaction between Mg (and perhaps Mn) nutrition and stress caused by defoliation.
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    Gonzalez, A.; Lynch, J. P. Effects of manganese toxicity on leaf CO2 assimilation of contrasting common bean genotypes. Physiol. Plant. 1997, 101 (4), 872– 880,  DOI: 10.1034/j.1399-3054.1997.1010427.x
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    Effects of manganese toxicity on leaf CO2 assimilation of contrasting common bean genotypes
    Gonzalez, Alonso; Lynch, Jonathan P.
    Physiologia Plantarum (1997), 101 (4), 872-880CODEN: PHPLAI; ISSN:0031-9317. (Munksgaard International Publishers Ltd.)
    Parameters related to leaf photosynthesis were evaluated in three genotypes of common bean (Phaseolus vulgaris L.) with contrasting tolerance to Mn toxicity. Two short-term studies in soln. culture were used to assess the effect of excess Mn on CO2 assimilation in mature and immature leaves. Mn toxicity decreased total chlorophyll content only in immature leaves, with a consequent redn. of leaf CO2 assimilation. Mature leaves that showed brown speckles characteristic of Mn toxicity, did not suffer any detriment in their capacity to assimilate CO2, at least in a 4-day expt. Stomatal conductance and transpiration were not affected by the presence of high levels of Mn in leaf tissue. Lower stomatal conductance and transpiration rates were obsd. only in leaves with advanced chlorosis. Differences among genotypes were detected as increased chlorosis in the more sensitive genotype ZPV-292, followed by A-283 and less chlorosis in the tolerant genotype CALIMA. Since CO2 assimilation expressed per unit of chlorophyll was not different between high-Mn plants and control plants, we conclude that the neg. effect of Mn toxicity on CO2 assimilation can be explained by a redn. in leaf chlorophyll content.
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    Fernando, D. R.; Lynch, J. P. Manganese phytotoxicity: new light on an old problem. Ann. Bot. 2015, 116 (3), 313– 319,  DOI: 10.1093/aob/mcv111
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    Manganese phytotoxicity: new light on an old problem
    Fernando, Denise R.; Lynch, Jonathan P.
    Annals of Botany (Oxford, United Kingdom) (2015), 116 (3), 313-319CODEN: ANBOA4; ISSN:0305-7364. (Oxford University Press)
    Background: Manganese (Mn) is an essential micronutrient that is phytotoxic under certain edaphic and climatic conditions. Multiple edaphic factors regulate Mn redox status and therefore its phytoavailability, and multiple environmental factors including light intensity and temp. interact with Mn phytotoxicity. The complexity of these interactions coupled with substantial genetic variation in Mn tolerance have hampered the recognition of Mn toxcity as an important stress in many natural and agricultural systems. Scope: Conflicting theories have been advanced regarding the mechanism of Mn phytotoxicity and tolerance. One line of evidence suggests that Mn toxicity occurs in the leaf apoplast, while another suggests that toxicity occurs by disruption of photosynthetic electron flow in chloroplasts. These conflicting results may at least in part be attributed to the light regimes employed, with studies conducted under light intensities approximating natural sunlight showing evidence of photo-oxidative stress as a mechanism of toxicity. Excessive Mn competes with the transport and metab. of other cationic metals, causing a range of induced nutrient deficiencies. Compartmentation, exclusion and detoxification mechanisms may all be involved in tolerance to excess Mn. The strong effects of light, temp., pptn. and other climate variables on Mn phytoavailability and phytotoxicity suggest that global climate change is likely to exacerbate Mn toxicity in the future, which has largely escaped scientific attention. Conclusions: Given that Mn is terrestrially ubiquitous, it is imperative that the heightened risk of Mn toxicity to both managed and natural plant ecosystems be factored into evaluation of the potential impacts of global climate change on vegetation. Large inter- and intraspecific genetic variation in tolerance to Mn toxicity suggests that increased Mn toxicity in natural ecosystems may drive changes in community compn., but that in agroecosystems crops may be developed with greater Mn tolerance. These topics deserve greater research attention.
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  68. 68
    Tian, Q.; Liu, N.; Bai, W.; Li, L.; Chen, J.; Reich, P. B.; Yu, Q.; Guo, D.; Smith, M. D.; Knapp, A. K.; Cheng, W.; Lu, P.; Gao, Y.; Yang, A.; Wang, T.; Li, X.; Wang, Z.; Ma, Y.; Han, X.; Zhang, W.-H. A novel soil manganese mechanism drives plant species loss with increased nitrogen deposition in a temperate steppe. Ecology 2016, 97 (1), 65– 74,  DOI: 10.1890/15-0917.1
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    A novel soil manganese mechanism drives plant species loss with increased nitrogen deposition in a temperate steppe
    Tian Qiuying; Liu Nana; Bai Wenming; Li Linghao; Chen Jiquan; Reich Peter B; Yu Qiang; Guo Dali; Smith Melinda D; Knapp Alan K; Cheng Weixin; Lu Peng; Gao Yan; Yang An; Wang Tianzuo; Li Xin; Wang Zhengwen; Ma Yibing; Han Xingguo; Zhang Wen-Hao
    Ecology (2016), 97 (1), 65-74 ISSN:0012-9658.
    Loss of plant diversity with increased anthropogenic nitrogen (N) deposition in grasslands has occurred globally. In most cases, competitive exclusion driven by preemption of light or space is invoked as a key mechanism. Here, we provide evidence from a 9-yr N-addition experiment for an alternative mechanism: differential sensitivity of forbs and grasses to increased soil manganese (Mn) levels. In Inner Mongolia steppes, increasing the N supply shifted plant community composition from grass-forb codominance (primarily Stipa krylovii and Artemisia frigida, respectively) to exclusive dominance by grass, with associated declines in overall species richness. Reduced abundance of forbs was linked to soil acidification that increased mobilization of soil Mn, with a 10-fold greater accumulation of Mn in forbs than in grasses. The enhanced accumulation of Mn in forbs was correlated with reduced photosynthetic rates and growth, and is consistent with the loss of forb species. Differential accumulation of Mn between forbs and grasses can be linked to fundamental differences between dicots and monocots in the biochemical pathways regulating metal transport. These findings provide a mechanistic explanation for N-induced species loss in temperate grasslands by linking metal mobilization in soil to differential metal acquisition and impacts on key functional groups in these ecosystems.
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    Lovley, D. R. Dissimilatory Fe (III) and Mn (IV) reduction. Microbiol. Rev. 1991, 55 (2), 259– 287,  DOI: 10.1128/mr.55.2.259-287.1991
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    Dissimilatory iron(III) and manganese(IV) reduction
    Lovley, Derek R.
    Microbiological Reviews (1991), 55 (2), 259-87CODEN: MBRED3; ISSN:0146-0749.
    A review with 342 refs concerning types of dissimilatory Fe(III) and Mn(IV) redn., pathway for microbial oxidn. of sedimentary org. matter coupled to Fe(III) and Mn(IV) redn., relative potential for enzymic and nonenzymic redn. of Fe(III) and Mn(IV), environmental significance of microbial Fe(III) and Mu(IV) redn., misc. applied aspects of the metab. of Fe(III)- and Mn(IV)-reducing microorganisms, enumeration and culturing of Fe(III) and Mn(IV) reducers, and electron transport to Fe(III) and Mn(IV).
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    Lovley, D. R.; Phillips, E. J. Novel mode of microbial energy metabolism: organic carbon oxidation coupled to dissimilatory reduction of iron or manganese. Appl. Environ. Microbiol. 1988, 54 (6), 1472– 1480,  DOI: 10.1128/aem.54.6.1472-1480.1988
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    Novel mode of microbial energy metabolism: organic carbon oxidation coupled to dissimilatory reduction of iron or manganese
    Lovley, Derek R.; Phillips, Elizabeth J. P.
    Applied and Environmental Microbiology (1988), 54 (6), 1472-80CODEN: AEMIDF; ISSN:0099-2240.
    A dissimilatory Fe(III)- and Mn(IV)-reducing microorganism was isolated from freshwater sediments of the Potomac River, Maryland. The isolate, designated GS-15, grew in defined anaerobic medium with acetate as the sole electron donor and Fe(III), Mn(IV), or nitrate as the sole electron acceptor. GS-15 oxidized acetate to carbon dioxide with the concomitant redn. of amorphic Fe(III) oxide to magnetite (Fe3O4). When Fe(III) citrate replaced amorphic Fe(III) oxide as the electron acceptor, GS-15 grew faster and reduced all of the added Fe(III) to Fe(II). GS-15 reduced a natural amorphic Fe(III) oxide but did not significantly reduce highly cryst. Fe(III) forms. Fe(III) was reduced optimally at pH 6.7 to 7 and at 30 to 35°. Ethanol, butyrate, and propionate could also serve as electron donors for Fe(III) redn. A variety of other org. compds. and hydrogen could not. MnO2 was completely reduced to Mn(II), which pptd. as rhodochrosite (MnCO3). Nitrate was reduced to ammonia. Oxygen could not serve as an electron acceptor, and it inhibited growth with the other electron acceptors. This is the first demonstration that microorganisms can completely oxidize org. compds. with Fe(III) or Mn(IV) as the sole electron acceptor and that oxidn. of org. matter coupled to dissimilatory Fe(III) or Mn(IV) redn. can yield energy for microbial growth. GS-15 provides a model for how enzymically catalyzed reactions can be quant. significant mechanisms for the redn. of iron and manganese in anaerobic environments.
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    Lovley, D. R.; Chapelle, F. H. Deep subsurface microbial processes. Rev. Geophys. 1995, 33 (3), 365– 381,  DOI: 10.1029/95RG01305
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    Warrinnier, R.; Bossuyt, S.; Resseguier, C.; Cambier, P.; Houot, S.; Gustafsson, J. P.; Diels, J.; Smolders, E. Anaerobic Respiration in the Unsaturated Zone of Agricultural Soil Mobilizes Phosphorus and Manganese. Environ. Sci. Technol. 2020, 54 (8), 4922– 4931,  DOI: 10.1021/acs.est.9b06978
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    Anaerobic Respiration in the Unsaturated Zone of Agricultural Soil Mobilizes Phosphorus and Manganese
    Warrinnier, Ruben; Bossuyt, Sara; Resseguier, Camille; Cambier, Philippe; Houot, Sabine; Gustafsson, Jon Petter; Diels, Jan; Smolders, Erik
    Environmental Science & Technology (2020), 54 (8), 4922-4931CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Anaerobic conditions mobilize phosphorus (P) in soils and sediments. The role of anaerobic microsites in well-drained soil on P migration is unknown. This study aimed to identify mechanisms that control field-scale vertical P mobility as affected by org. fertilizers that may trigger variable redox conditions. Soils were sampled at different depths in a well-drained Luvisol after 19 years of application of org. fertilizers. The concns. of P and manganese (Mn) in 0.45-μm-filtered exts. (10-3 M CaCl2) of field-moist soil samples were strongly correlated (r = + 0.95), and both peaked in and below the compacted plough pan, suggesting that reductive processes mobilize P. Waterlogged soil incubations confirmed that anaerobic respiration comobilizes Mn and P and that this leads to the release of colloidal P and iron (Fe). The long-term applications of farmyard manure and immature compost enhanced the concns. of Mn, Fe, and aluminum (Al) in the soil soln. of subsurface samples, whereas less such effect was found under the application of more stable org. fertilizers. Farmyard manure application significantly enhanced soil P stocks below the plough layer despite a small P input. Overall, multiple lines of evidence confirm that anaerobic respiration, sparked by labile org. matter, mobilizes P in this seemingly well-drained soil.
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    Grybos, M.; Davranche, M.; Gruau, G.; Petitjean, P.; Pédrot, M. Increasing pH drives organic matter solubilization from wetland soils under reducing conditions. Geoderma 2009, 154 (1–2), 13– 19,  DOI: 10.1016/j.geoderma.2009.09.001
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    Increasing pH drives organic matter solubilization from wetland soils under reducing conditions
    Grybos, Malgorzata; Davranche, Melanie; Gruau, Gerard; Petitjean, Patrice; Pedrot, Mathieu
    Geoderma (2009), 154 (1-2), 13-19CODEN: GEDMAB; ISSN:0016-7061. (Elsevier B.V.)
    In wetlands, large quantities of dissolved org. matter (DOM) are solubilized under reducing conditions. Controlled incubations of a wetland soil were performed under oxic and anoxic conditions to investigate the extent to which the following processes account for this phenomenon: (i) prodn. of org. metabolites by microbes during soil redn.; (ii) release of org. matter (OM) from Mn- and Fe-oxyhydroxides that undergo reductive dissoln.; and (iii) desorption of OM from soil minerals due to pH changes. Anaerobic incubation releases 2.5% of the total soil org. carbon (OC) as dissolved org. carbon (DOC), and is accompanied by a pH rise from 5.5 to 7.4 and by the soil Mn- and Fe-redn. The three processes above all take place. However, anaerobic incubation at a const. pH of 5.5 (preventing OM desorption) releases only 0.5% of the total soil OC, while aerobic incubation at pH 7.4 (preventing Mn- and Fe-redn.) releases 1.7% of the total soil OC. By contrast, aerobic incubation at pH 5.5 (preventing both Mn- and Fe-redn. and pH rise) does not solubilize any DOC. The DOC released is markedly arom., indicating little contribution from microbial metabolites, but, rather, the presence of microbes leading to OM mineralization. The pH rise is the key factor controlling OM solubilization under reducing conditions. This rise of pH accounts for > 60% of the total released DOC, which is not due to reductive dissoln. as such.
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    Li, H.; Yelle, D. J.; Li, C.; Yang, M.; Ke, J.; Zhang, R.; Liu, Y.; Zhu, N.; Liang, S.; Mo, X.; Ralph, J.; Currie, C. R.; Mo, J. Lignocellulose pretreatment in a fungus-cultivating termite. Proc. Natl. Acad. Sci. U. S. A. 2017, 114 (18), 4709– 4714,  DOI: 10.1073/pnas.1618360114
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    Lignocellulose pretreatment in a fungus-cultivating termite
    Li, Hongjie; Yelle, Daniel J.; Li, Chang; Yang, Mengyi; Ke, Jing; Zhang, Ruijuan; Liu, Yu; Zhu, Na; Liang, Shiyou; Mo, Xiaochang; Ralph, John; Currie, Cameron R.; Mo, Jianchu
    Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (18), 4709-4714CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Depolymg. lignin, the complex phenolic polymer fortifying plant cell walls, is an essential but challenging starting point for the lignocellulosics industries. The variety of ether- and carbon-carbon interunit linkages produced via radical coupling during lignification limit chem. and biol. depolymn. efficiency. In an ancient fungus-cultivating termite system, we reveal unprecedentedly rapid lignin depolymn. and degrdn. by combining lab. feeding expts., lignocellulosic compositional measurements, electron microscopy, 2D-NMR, and thermochemolysis. In a gut transit time of under 3.5 h, in young worker termites, poplar lignin sidechains are extensively cleaved and the polymer is significantly depleted, leaving a residue almost completely devoid of various condensed units that are traditionally recognized to be the most recalcitrant. Subsequently, the fungus-comb microbiome preferentially uses xylose and cleaves polysaccharides, thus facilitating final utilization of easily digestible oligosaccharides by old worker termites. This complementary symbiotic pretreatment process in the fungus-growing termite symbiosis reveals a previously unappreciated natural system for efficient lignocellulose degrdn.
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  75. 75
    Poulsen, M.; Hu, H.; Li, C.; Chen, Z.; Xu, L.; Otani, S.; Nygaard, S.; Nobre, T.; Klaubauf, S.; Schindler, P. M.; Hauser, F.; Pan, H.; Yang, Z.; Sonnenberg, A. S.; de Beer, Z. W.; Zhang, Y.; Wingfield, M. J.; Grimmelikhuijzen, C. J.; de Vries, R. P.; Korb, J.; Aanen, D. K.; Wang, J.; Boomsma, J. J.; Zhang, G. Complementary symbiont contributions to plant decomposition in a fungus-farming termite. Proc. Natl. Acad. Sci. U. S. A. 2014, 111 (40), 14500– 5,  DOI: 10.1073/pnas.1319718111
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    Complementary symbiont contributions to plant decomposition in a fungus-farming termite
    Poulsen, Michael; Hu, Haofu; Li, Cai; Chen, Zhensheng; Xu, Luohao; Otani, Saria; Nygaard, Sanne; Nobre, Tania; Klaubauf, Sylvia; Schindler, Philipp M.; Hauser, Frank; Pan, Hailin; Yang, Zhikai; Sonnenberg, Anton S. M.; Wilhelm de Beer, Z.; Zhang, Yong; Wingfield, Michael J.; Grimmelikhuijzen, Cornelis J. P.; de Vries, Ronald P.; Korb, Judith; Aanen, Duur K.; Wang, Jun; Boomsma, Jacobus J.; Zhang, Guojie
    Proceedings of the National Academy of Sciences of the United States of America (2014), 111 (40), 14500-14505CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Termites normally rely on gut symbionts to decomp. org. matter but the Macrotermitinae domesticated Termitomyces fungi to produce their own food. This transition was accompanied by a shift in the compn. of the gut microbiota, but the complementary roles of these bacteria in the symbiosis have remained enigmatic. We obtained high-quality annotated draft genomes of the termite Macrotermes natalensis, its Termitomyces symbiont, and gut metagenomes from workers, soldiers, and a queen. We show that members from 111 of the 128 known glycoside hydrolase families are represented in the symbiosis, that Termitomyces has the genomic capacity to handle complex carbohydrates, and that worker gut microbes primarily contribute enzymes for final digestion of oligosaccharides. This apparent division of labor is consistent with the Macrotermes gut microbes being most important during the second passage of comb material through the termite gut, after a first gut passage where the crude plant substrate is inoculated with Termitomyces asexual spores so that initial fungal growth and polysaccharide decompn. can proceed with high efficiency. Complex conversion of biomass in termite mounds thus appears to be mainly accomplished by complementary cooperation between a domesticated fungal monoculture and a specialized bacterial community. In sharp contrast, the gut microbiota of the queen had highly reduced plant decompn. potential, suggesting that mature reproductives digest fungal material provided by workers rather than plant substrate.
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    Hofrichter, M. Review: lignin conversion by manganese peroxidase (MnP). Enzyme Microb. Technol. 2002, 30 (4), 454– 466,  DOI: 10.1016/S0141-0229(01)00528-2
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    Review: lignin conversion by manganese peroxidase (MnP)
    Hofrichter, Martin
    Enzyme and Microbial Technology (2002), 30 (4), 454-466CODEN: EMTED2; ISSN:0141-0229. (Elsevier Science Ireland Ltd.)
    A review. Manganese peroxidase (MnP) is the most common lignin-modifying peroxidase produced by almost all wood-colonizing basidiomycetes causing white-rot and various soil-colonizing litter-decompg. fungi. Multiple forms of this glycosylated heme protein with mol. wts. normally at 40 to 50 kDa are secreted by ligninolytic fungi into their microenvironment. There, MnP preferentially oxidizes manganese(II) ions (Mn2+), always present in wood and soils, into highly reactive Mn3+, which is stabilized by fungal chelators such as oxalic acid. Chelated Mn3+ in turn acts as low-mol. wt., diffusible redox-mediator that attacks phenolic lignin structures resulting in the formation of instable free radicals that tend to disintegrate spontaneously. MnP is capable of oxidizing and depolymg. natural and synthetic lignins as well as entire lignocelluloses (milled straw or wood, pulp) in cell-free systems (in vitro). In vitro depolymn. is enhanced in the presence of co-oxidants such as thiols (e.g. glutathione) or unsatd. fatty acids and their derivs. (e.g. Tween 80). The review summarizes and discusses different approaches to prove lignin decompn. in vitro and lists, in addn., other recalcitrant substances oxidizable by MnP.
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  77. 77
    Verspoor, R. L.; Soglo, M.; Adeoti, R.; Djouaka, R.; Edwards, S.; Fristedt, R.; Langton, M.; Moriana, R.; Osborne, M.; Parr, C. L.; Powell, K.; Hurst, G. D. D.; Landberg, R. Mineral analysis reveals extreme manganese concentrations in wild harvested and commercially available edible termites. Sci. Rep. 2020, 10 (1), 6146,  DOI: 10.1038/s41598-020-63157-7
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    Mineral analysis reveals extreme manganese concentrations in wild harvested and commercially available edible termites
    Verspoor, Rudi L.; Soglo, Murielle; Adeoti, Razack; Djouaka, Rousseau; Edwards, Sam; Fristedt, Rikard; Langton, Maud; Moriana, Rosana; Osborne, Matthew; Parr, Catherine L.; Powell, Kathryn; Hurst, Gregory D. D.; Landberg, Rikard
    Scientific Reports (2020), 10 (1), 6146CODEN: SRCEC3; ISSN:2045-2322. (Nature Research)
    Abstr.: Termites are widely used as a food resource, particularly in Africa and Asia. Markets for insects as food are also expanding worldwide. To inform the development of insect-based foods, we analyzed selected minerals (Fe-Mn-Zn-Cu-Mg) in wild-harvested and com. available termites. Mineral values were compared to selected com. available insects. Alate termites, of the genera Macrotermes and Odontotermes, showed remarkably high manganese (Mn) content (292-515 mg/100 gdw), roughly 50-100 times the concns. detected in other insects. Other mineral elements occur at moderate concns. in all insects examd. On further examn., the Mn is located primarily in the abdomens of the Macrotermes subhyalinus; with SEM revealing small spherical structures highly enriched for Mn. We identify the fungus comb, of Macrotermes subhyanus, as a potential biol. source of the high Mn concns. Consuming even small quantities of termite alates could exceed current upper recommended intakes for Mn in both adults and children. Given the widespread use of termites as food, a better understanding the sources, distribution and bio-availability of these high Mn concns. in termite alates is needed.
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    Chao, T. T. Selective Dissolution of Manganese Oxides from Soils and Sediments with Acidified Hydroxylamine Hydrochloride. Soil Sci. Soc. Am. J. 1972, 36, 764– 768,  DOI: 10.2136/sssaj1972.03615995003600050024x
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    Herndon, E.; Havig, J. R.; Singer, D.; McCormick, M.; Kump, L. Manganese and iron geochemistry in sediments underlying the redox-stratified Fayetteville Green Lake. Geochim. Cosmochim. Acta 2018, 231, 50– 63,  DOI: 10.1016/j.gca.2018.04.013
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    Manganese and iron geochemistry in sediments underlying the redox-stratified Fayetteville Green Lake
    Herndon, Elizabeth M.; Havig, Jeff R.; Singer, David M.; McCormick, Michael L.; Kump, Lee R.
    Geochimica et Cosmochimica Acta (2018), 231 (), 50-63CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Manganese and iron are redox-sensitive elements that yield clues about biogeochem. and redox conditions both in modern environments and in the geol. past. Here, we investigated Mn and Fe-bearing minerals preserved in basin sediments underlying Fayetteville Green Lake, a redox-stratified lake that serves as a geochem. analog for Paleoproterozoic oceans. Synchrotron-source microprobe techniques (μXRF, μXANES, and μXRD) and bulk geochem. analyses were used to examine the microscale distribution and speciation of Mn, Fe, and S as a function of depth in the top 48 cm of anoxic lake sediments. Manganese was primarily assocd. with calcite grains as a manganese-rich carbonate that pptd. in the chemocline of the water column and settled through the euxinic basin to collect in lake sediments. Iron was preserved in framboidal iron sulfides that pptd. in euxinic bottom waters and underwent transformation to pyrite and marcasite in the sediments. Previous studies attribute the formation of manganese-rich carbonates to the diagenetic alteration of manganese oxides deposited in basins underlying oxygenated water. Our study challenges this paradigm by providing evidence that Mn-bearing carbonates form in the water column and accumulate in sediments below anoxic waters. Consequently, manganoan carbonates preserved in the rock record do not necessarily denote the presence of oxygenated bottom waters in ocean basins.
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    Ling, F. T.; Post, J. E.; Heaney, P. J.; Santelli, C. M.; Ilton, E. S.; Burgos, W.; Rose, A. W. A multi-method characterization of natural terrestrial birnessites. Am. Mineral. 2020, 105, 833– 847,  DOI: 10.2138/am-2020-7303
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    Newville, M. Fundamentals of XAFS. Rev. Mineral. Geochem. 2004, 78 (1), 33– 74,  DOI: 10.2138/rmg.2014.78.2
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    Ling, F. T.; Post, J. E.; Heaney, P. J.; Kubicki, J. D.; Santelli, C. M. Fourier-transform infrared spectroscopy (FTIR) analysis of triclinic and hexagonal birnessites. Spectrochim. Acta, Part A 2017, 178, 32– 46,  DOI: 10.1016/j.saa.2017.01.032
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    Fourier-transform infrared spectroscopy (FTIR) analysis of triclinic and hexagonal birnessites
    Ling, Florence T.; Post, Jeffrey E.; Heaney, Peter J.; Kubicki, James D.; Santelli, Cara M.
    Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (2017), 178 (), 32-46CODEN: SAMCAS; ISSN:1386-1425. (Elsevier B.V.)
    The characterization of birnessite structures is particularly challenging for poorly cryst. materials of biogenic origin, and a detn. of the relative concns. of triclinic and hexagonal birnessite in a mixed assemblage has typically required synchrotron-based spectroscopy and diffraction approaches. In this study, Fourier-transform IR spectroscopy (FTIR) is demonstrated to be capable of differentiating synthetic triclinic Na-birnessite and synthetic hexagonal H-birnessite. Furthermore, IR spectral deconvolution of peaks resulting from Mn-O lattice vibrations between 400 and 750 cm- 1 yield results comparable to those obtained by linear combination fitting of synchrotron X-ray absorption fine structure (EXAFS) data when applied to known mixts. of triclinic and hexagonal birnessites. D. functional theory (DFT) calcns. suggest that an IR absorbance peak at ∼ 1628 cm- 1 may be related to OH vibrations near vacancy sites. The integrated intensity of this peak may show sensitivity to vacancy concns. in the Mn octahedral sheet for different birnessites.
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    Post, J. E.; McKeown, D. A.; Heaney, P. J. Raman spectroscopy study of manganese oxides: Tunnel structures. Am. Mineral. 2020, 105 (8), 1175– 1190,  DOI: 10.2138/am-2020-7390
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    Bernardini, S.; Bellatreccia, F.; Casanova Municchia, A.; Della Ventura, G.; Sodo, A. Raman spectra of natural manganese oxides. J. Raman Spectrosc. 2019, 50 (6), 873– 888,  DOI: 10.1002/jrs.5583
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    Raman spectra of natural manganese oxides
    Bernardini, Simone; Bellatreccia, Fabio; Casanova Municchia, Annalaura; Della Ventura, Giancarlo; Sodo, Armida
    Journal of Raman Spectroscopy (2019), 50 (6), 873-888CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
    Manganese oxides occur typically as cryptocryst. and fine grained mixts. of different Mn-phases, carbonates, silicates, and Fe oxides/hydroxides; thus their characterization by std. methods, such as X ray diffraction, is extremely challenging. These materials have been widely used in various applications over the millennia, for example, in art works as pigments for pottery, mural paintings, stained glass, and recently, as nanostructured materials with very attractive physicochem. properties. Furthermore, they are important geomaterials that could play a key role in environmental applications, by controlling the partitioning of arsenic and heavy metals between rocks, soils, and aq. systems. Raman spectroscopy, which is a punctual and nondestructive technique, has been widely used to characterize these materials. However, literature data are often conflicting and contradictory, usually not allowing a proper identification of the Mn species. In this work, we characterize the most common natural manganese oxides by combining X-ray powder diffraction, Fourier-transform IR spectroscopy, and Raman spectroscopy. Our data show that some manganese oxides have characteristic Raman spectra and can be easily recognized by using Raman spectroscopy alone, whereas integration of Raman data with other techniques is mandatory to characterize minerals that have almost identical Raman spectra. With this respect, Raman spectroscopy is the only technique allowing an easy discrimination between hollandite [Ba(Mn4+6,Mn3+2)O16] and cryptomelane [K(Mn4+7,Mn3+)O16]. The final goal of this work is to provide ref. Raman spectra, acquired on previously well-characterized Mn samples to facilitate the application of Raman spectroscopy in the study of these geomaterials.
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    Ilton, E. S.; Post, J. E.; Heaney, P. J.; Ling, F. T.; Kerisit, S. N. XPS determination of Mn oxidation states in Mn (hydr)oxides. Appl. Surf. Sci. 2016, 366, 475– 485,  DOI: 10.1016/j.apsusc.2015.12.159
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    XPS determination of Mn oxidation states in Mn (hydr)oxides
    Ilton, Eugene S.; Post, Jeffrey E.; Heaney, Peter J.; Ling, Florence T.; Kerisit, Sebastien N.
    Applied Surface Science (2016), 366 (), 475-485CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Hydrous manganese oxides are an important class of minerals that help regulate the geochem. redox cycle in near-surface environments and are also considered to be promising catalysts for energy applications such as the oxidn. of water. A complete characterization of these minerals is required to better understand their catalytic and redox activity. In this contribution an empirical methodol. using XPS is developed to quantify the oxidn. state of hydrous multivalent manganese oxides with an emphasis on birnessite, a layered structure that occurs commonly in soils but is also the oxidized endmember in biomimetic water-oxidn. catalysts. The Mn2p3/2, Mn3p, and Mn3s lines of near monovalent Mn(II), Mn(III), and Mn(IV) oxides were fit with component peaks; after the best fit was obtained the relative widths, heights and binding energies of the components were fixed. Unknown multivalent samples were fit such that binding energies, intensities, and peak-widths of each oxidn. state, composed of a packet of correlated component peaks, were allowed to vary. Peak-widths were constrained to maintain the difference between the stds. Both av. and individual mole fraction oxidn. states for all three energy levels were strongly correlated, with close agreement between Mn3s and Mn3p analyses, whereas calcns. based on the Mn2p3/2 spectra gave systematically more reduced results. Limited stoichiometric analyses were consistent with Mn3p and Mn3s. Further, evidence indicates the shape of the Mn3p line was less sensitive to the bonding environment than that for Mn2p. Consequently, fitting the Mn3p and Mn3s lines yielded robust quantification of oxidn. states over a range of Mn (hydr)oxide phases. In contrast, a common method for detg. oxidn. states that utilizes the multiplet splitting of the Mn3s line was found to be not appropriate for birnessites.
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    Sutherland, K. M.; Wankel, S. D.; Hansel, C. M. Oxygen isotope analysis of bacterial and fungal manganese oxidation. Geobiology 2018, 16 (4), 399– 411,  DOI: 10.1111/gbi.12288
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    86
    Oxygen isotope analysis of bacterial and fungal manganese oxidation
    Sutherland, K. M.; Wankel, S. D.; Hansel, C. M.
    Geobiology (2018), 16 (4), 399-411CODEN: GEOBCZ; ISSN:1472-4669. (Wiley-Blackwell)
    The ability of micro-organisms to oxidize manganese (Mn) from Mn(II) to Mn(III/IV) oxides transcends boundaries of biol. clade or domain. Many bacteria and fungi oxidize Mn(II) to Mn(III/IV) oxides directly through enzymic activity or indirectly through the prodn. of reactive oxygen species. Here, we det. the oxygen isotope fractionation factors assocd. with Mn(II) oxidn. via various biotic (bacteria and fungi) and abiotic Mn(II) reaction pathways. As oxygen in Mn(III/IV) oxides may be derived from precursor water and mol. oxygen, we use a twofold approach to det. the isotope fractionation with respect to each oxygen source. Using both 18O-labeled water and closed-system Rayleigh distn. approaches, we constrain the kinetic isotope fractionation factors assocd. with O atom incorporation during Mn(II) oxidn. to -17.3% to -25.9± for O2 and -1.9% to +1.8% for water. Results demonstrate that stable oxygen isotopes of Mn(III/IV) oxides have potential to distinguish between two main classes of biotic Mn(II) oxidn.: direct enzymic oxidn. in which O2 is the oxidant and indirect enzymic oxidn. in which superoxide is the oxidant. The fraction of Mn(III/IV) oxide-assocd. oxygen derived from water varies significantly (38%-62%) among these bio-oxides with only weak relationship to Mn oxidn. state, suggesting Mn(III) disproportionation may account for differences in the fraction of mineral-bound oxygen from water and O2. Addnl., direct incorporation of mol. O2 suggests that Mn(III/IV) oxides contain a yet untapped proxy of δ18OO2 of environmental O2, a parameter reflecting the integrated influence of global respiration, photorespiration, and several other biogeochem. reactions of global significance.
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    Sutherland, K. M.; Wostbrock, J. A. G.; Hansel, C. M.; Sharp, Z. D.; Hein, J. R.; Wankel, S. D., Ferromanganese crusts as recorders of marine dissolved oxygen. Earth Planet. Sci. Lett. 2020, 533. 116057 DOI: 10.1016/j.epsl.2019.116057
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    87
    Ferromanganese crusts as recorders of marine dissolved oxygen
    Sutherland, Kevin M.; Wostbrock, Jordan A. G.; Hansel, Colleen M.; Sharp, Zachary D.; Hein, James R.; Wankel, Scott D.
    Earth and Planetary Science Letters (2020), 533 (), 116057CODEN: EPSLA2; ISSN:0012-821X. (Elsevier B.V.)
    The distinct triple oxygen isotope compn. of tropospheric O2 relative to seawater is the result of biogeochem. reactions (e.g. primary productivity, respiration), exchange with the stratosphere, and the relative size of different oxygen-contg. reservoirs, namely O2, O3, and CO2. This difference in isotopic compn. gives tropospheric O2 utility as a record of biogeochem. and atm. processes and may also be used for detg. where in the rock record isotopic fingerprints of tropospheric oxygen may be preserved. The isotopic record of tropospheric oxygen in previous studies is largely limited to analyses of gas trapped in continental glaciers and a patchwork of other proxies, most notably the triple oxygen signature of sulfate. Here we show the uppermost layers of hydrogenetic, deep-ocean ferromanganese crusts from each of the major ocean basins have a triple oxygen isotope compn. consistent with the direct incorporation of dissolved oxygen. The range of δ18O and Δ'17O in ferromanganese crusts suggests the Mn oxide endmember contains a near 50:50 mixt. of oxygen from water and dissolved O2. Our data indicate this signal also persists into older layers of the crusts, potentially preserving near 75 million years of the oxygen isotopic compn. of the lower troposphere and subsequent deep-ocean respiration. Our anal. of oxygen isotope values, bulk chem., and estd. local dissolved oxygen for crust top samples reveals that variations in bulk chem. ultimately exhibit more influence on the oxygen mass balance than changes in dissolved oxygen, presenting a challenge for unambiguous detn. of local dissolved oxygen. Although anal. challenges remain, these widespread, layered deposits of ferromanganese crust may offer a viable path for future interrogation of the history or relative history of the oxygen cycle of the troposphere and deep ocean millions of years into the past.
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  88. 88
    Lützow, M. V.; Kögel-Knabner, I.; Ekschmitt, K.; Matzner, E.; Guggenberger, G.; Marschner, B.; Flessa, H. Stabilization of organic matter in temperate soils: Mechanisms and their relevance under different soil conditions—a review. Eur. J. Soil Sci. 2006, 57, 426– 445,  DOI: 10.1111/j.1365-2389.2006.00809.x
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    88
    Stabilization of organic matter in temperate soils: mechanisms and their relevance under different soil conditions - a review
    Luetzow, M. V.; Koegel-Knabner, I.; Ekschmitt, K.; Matzner, E.; Guggenberger, G.; Marschner, B.; Flessa, H.
    European Journal of Soil Science (2006), 57 (4), 426-445CODEN: ESOSES; ISSN:1351-0754. (Blackwell Publishing Ltd.)
    A review. Mechanisms for C stabilization in soils have received much interest recently due to their relevance in the global C cycle. The mechanisms that are currently, but often contradictorily or inconsistently, considered to contribute to org. matter (OM) protection against decompn. in temperate soils were: (i) selective preservation due to recalcitrance of OM, including plant litter, rhizodeposits, microbial products, humic polymers, and charred OM; (ii) spatial inaccessibility of OM against decomposer organisms due to occlusion, intercalation, hydrophobicity and encapsulation; and (iii) stabilization by interaction with mineral surfaces (Fe-, Al-, Mn-oxides, phyllosilicates) and metal ions. The goal is to assess the relevance of these mechanisms to the formation of soil OM during different stages of decompn. and under different soil conditions. The view that OM stabilization is dominated by the selective preservation of recalcitrant org. components that accumulate in proportion to their chem. properties can no longer be accepted. In contrast, anal. of mechanisms shows that: (i) the soil biotic community is able to disintegrate any OM of natural origin; (ii) mol. recalcitrance of OM is relative, rather than abs.; (iii) recalcitrance is only important during early decompn. and in active surface soils; while (iv) during late decompn. and in the subsoil, the relevance of spatial inaccessibility and organo-mineral interactions for SOM stabilization increases. Thus, the major difficulties in the understanding and prediction of SOM dynamics originate from the simultaneous operation of several mechanisms. Knowledge gaps and promising directions of future research are discussed.
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  89. 89
    Porras, R. C.; Hicks Pries, C. E.; Tom, M. S.; Nico, P. S. Synthetic iron (hydr)oxide-glucose associations in subsurface soil: Effects on decomposability of mineral associated carbon. Sci. Total Environ. 2018, 613–614, 342– 351,  DOI: 10.1016/j.scitotenv.2017.08.290
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    89
    Synthetic iron (hydr)oxide-glucose associations in subsurface soil: Effects on decomposability of mineral associated carbon
    Porras, R. C.; Hicks Pries, C. E.; Torn, M. S.; Nico, P. S.
    Science of the Total Environment (2018), 613-614 (), 342-351CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
    Soils are a globally important reservoir of org. carbon. There is a growing understanding that interactions with soil mineral phases contribute to the accumulation and retention of otherwise degradable org. matter (OM) in soils and sediments. However, the bioavailability of org. compds. in mineral-org.-assocns. (MOAs), esp. under varying environmental conditions is not well known. To assess the impact of mineral assocn. and warming on the decompn. of an easily respirable org. substrate (glucose), we conducted a series of lab. incubations at different temps. with field-collected soils from 10 to 20 cm, 50-60 cm, and 80-90 cm depth. We added 13C-labeled glucose either directly to native soil or sorbed to one of two synthetic iron (hydr)oxide phases (goethite and ferrihydrite) that differ in crystallinity and affinity for sorbing glucose. We found that: (1) assocn. with the Fe (hydr)oxide minerals reduced the decompn. rate of glucose by > 99.5% relative to rate of decompn. for free glucose in soil; (2) the respiration rate per g carbon did not differ appreciably with depth, suggesting a similar degree of decomposability for native C across depths and that under the incubation conditions total carbon availability represents the principal limitation on respiration under these conditions as opposed to reduced abundance of decomposers or moisture and oxygen limitations; (3) addn. of free glucose enhanced native carbon respiration at all soil depths with the largest effect at 50-60 cm; (4) in general respiration of the organo-mineral complex (glucose and iron-(hydr)oxide) was less temp. sensitive than was respiration of native carbon; (5) the addn. of org. free mineral decreased the rate of soil respiration in the intermediate 50-60 cm depth soil. The results emphasize the key role of MOAs in regulating the fluxes of carbon from soils to the atm. and in turn the stocks of soil carbon.
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  90. 90
    Gu, B.; Schmitt, J.; Chen, Z.; Liang, L.; McCarthy, J. F. Adsorption and desorption of natural organic matter on iron oxide: Mechanisms and models. Environ. Sci. Technol. 1994, 28, 38– 46,  DOI: 10.1021/es00050a007
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    90
    Adsorption and desorption of natural organic matter on iron oxide: mechanisms and models
    Gu, Baohua; Schmitt, Juergen; Chen, Zhihong; Liang, Liyuan; McCarthy, John F.
    Environmental Science and Technology (1994), 28 (1), 38-46CODEN: ESTHAG; ISSN:0013-936X.
    This study was undertaken to elucidate the interaction mechanisms between natural org. matter(NOM) and iron oxide surfaces and to develop a predictive model for NOM adsorption and desorption. Results indicated that ligand exchange between carboxyl/hydroxyl functional groups of NOM and iron oxide surfaces was the dominant interaction mechanism, esp. under acidic or slightly acidic pH conditions. This conclusion was supported by the measurements of heat of adsorption (microcalorimetry), FTIR and 13C NMR anal., and competitive adsorption between NOM and some specifically adsorbed anions. A modified Langmuir model was proposed in which a surface excess-dependent affinity parameter was defined to account for a decreasing adsorption affinity with surface coverage due to the heterogeneity of NOM and adsorbent surfaces. With three adjustable parameters, the model is capable of describing a variety of adsorption isotherms. A hysteresis coeff., h, was used to describe the hysteretic effect of adsorption reactions that, at h = 0, the reaction is completely reversible, whereas at h = 1, the reaction is completely irreversible. Fitted values of h for NOM desorption on iron oxide surfaces ranged from 0.72 to 0.93, suggesting that the adsorbed NOM was very difficult to be desorbed at a given pH and ionic compn. The results imply that a better mechanistic understanding of the interaction between NOM and oxide surfaces is needed to improve the predictive capabilities in NOM transport and cotransport of contaminants assocd. with NOM or iron oxides.
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    Estes, E. R.; Andeer, P. F.; Nordlund, D.; Wankel, S. D.; Hansel, C. M. Biogenic manganese oxides as reservoirs of organic carbon and proteins in terrestrial and marine environments. Geobiology 2017, 15 (1), 158– 172,  DOI: 10.1111/gbi.12195
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    91
    Biogenic manganese oxides as reservoirs of organic carbon and proteins in terrestrial and marine environments
    Estes, E. R.; Andeer, P. F.; Nordlund, D.; Wankel, S. D.; Hansel, C. M.
    Geobiology (2017), 15 (1), 158-172CODEN: GEOBCZ; ISSN:1472-4669. (Wiley-Blackwell)
    Manganese (Mn) oxides participate in a range of interactions with org. carbon (OC) that can lead to either carbon degrdn. or preservation. Here, we examine the abundance and compn. of OC assocd. with biogenic and environmental Mn oxides to elucidate the role of Mn oxides as a reservoir for carbon and their potential for selective partitioning of particular carbon species. Mn oxides pptd. in natural brackish waters and by Mn(II)-oxidizing marine bacteria and terrestrial fungi harbor considerable levels of org. carbon (4.1-17.0 mol OC per kg mineral) compared to ferromanganese cave deposits which contain 1-2 orders of magnitude lower OC. Spectroscopic analyses indicate that the chem. compn. of Mn oxide-assocd. OC from microbial cultures is homogeneous with bacterial Mn oxides hosting primarily proteinaceous carbon and fungal Mn oxides contg. both protein- and lipopolysaccharide-like carbon. The bacterial Mn oxide-hosted proteins are involved in both Mn(II) oxidn. and metal binding by these bacterial species and could be involved in the mineral nucleation process as well. By comparison, the compn. of OC assocd. with Mn oxides formed in natural settings (brackish waters and particularly in cave ferromanganese rock coatings) is more spatially and chem. heterogeneous. Cave Mn oxide-assocd. org. material is enriched in aliph. C, which together with the lower carbon concns., points to more extensive microbial or mineral processing of carbon in this system relative to the other systems examd. in this study, and as would be expected in oligotrophic cave environments. This study highlights Mn oxides as a reservoir for carbon in varied environments. The presence and in some cases dominance of proteinaceous carbon within the biogenic and natural Mn oxides may contribute to preferential preservation of proteins in sediments and dominance of protein-dependent metabs. in the subsurface biosphere.
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  92. 92
    Stuckey, J. W.; Goodwin, C.; Wang, J.; Kaplan, L. A.; Vidal-Esquivel, P.; Beebe, T. P., Jr.; Sparks, D. L. Impacts of hydrous manganese oxide on the retention and lability of dissolved organic matter. Geochem. Trans. 2018, 19, 6,  DOI: 10.1186/s12932-018-0051-x
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    92
    Impacts of hydrous manganese oxide on the retention and lability of dissolved organic matter
    Stuckey, Jason W.; Goodwin, Christopher; Wang, Jian; Kaplan, Louis A.; Vidal-Esquivel, Prian; Beebe, Thomas P., Jr.; Sparks, Donald L.
    Geochemical Transactions (2018), 19 (), 6/1-6/19CODEN: GETRF9; ISSN:1467-4866. (Springer International Publishing AG)
    Minerals constitute a primary ecosystem control on org. C decompn. in soils, and therefore on greenhouse gas fluxes to the atm. Secondary minerals, in particular, Fe and Al (oxyhydr)oxides-collectively referred to as "oxides" hereafter-are prominent protectors of org. C against microbial decompn. through sorption and complexation reactions. However, the impacts of Mn oxides on org. C retention and lability in soils are poorly understood. Here we show that hydrous Mn oxide (HMO), a poorly cryst. d-MnO2, has a greater max. sorption capacity for dissolved org. matter (DOM) derived from a deciduous forest composite Oi, Oe, and Oa horizon leachate ("O horizon leachate" hereafter) than does goethite under acidic (pH 5) conditions. Nonetheless, goethite has a stronger sorption capacity for DOM at low initial C:(Mn or Fe) molar ratios compared to HMO, probably due to ligand exchange with carboxylate groups as revealed by attenuated total reflectance-Fourier transform IR spectroscopy. XPS and scanning transmission X-ray microscopy-near-edge X-ray absorption fine structure spectroscopy coupled with Mn mass balance calcns. reveal that DOM sorption onto HMO induces partial Mn reductive dissoln. and Mn redn. of the residual HMO. XPS further shows increasing Mn(II) concns. are correlated with increasing oxidized C (C= O) content (r = 0.78, P < 0.0006) on the DOM-HMO complexes. We posit that DOM is the more probable reductant of HMO, as Mn(II)-induced HMO dissoln. does not alter the Mn speciation of the residual HMO at pH 5. At a lower C loading (2 × 102 μg C m-2), DOM desorption-assessed by 0.1 M NaH2PO4 extn.-is lower for HMO than for goethite, whereas the extent of desorption is the same at a higher C loading (4 × 102 μg C m-2). No significant differences are obsd. in the impacts of HMO and goethite on the biodegradability of the DOM remaining in soln. after DOM sorption reaches steady state. Overall, HMO shows a relatively strong capacity to sorb DOM and resist phosphate-induced desorption, but DOM-HMO complexes may be more vulnerable to reductive dissoln. than DOM-goethite complexes.
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  93. 93
    Rennert, T.; Handel, M.; Hoschen, C.; Lugmeier, J.; Steffens, M.; Totsche, K. U. A NanoSIMS study on the distribution of soil organic matter, iron and manganese in a nodule from a Stagnosol. Eur. J. Soil Sci. 2014, 65, 684– 692,  DOI: 10.1111/ejss.12157
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    93
    A NanoSIMS study on the distribution of soil organic matter, iron and manganese in a nodule from a Stagnosol
    Rennert, T.; Haendel, M.; Hoeschen, C.; Lugmeier, J.; Steffens, M.; Totsche, K. U.
    European Journal of Soil Science (2014), 65 (5), 684-692CODEN: ESOSES; ISSN:1351-0754. (Wiley-Blackwell)
    The development of Stagnosols is the consequence of perched water tables, which induce periodic oxidizing and reducing conditions. These cause the spatial distribution of iron (Fe) and manganese (Mn) between the soil matrix and ferromanganese concretions or nodules. Since oxides of these metals may interact with org. matter, we studied their spatial distribution in bulk material from the Bg horizon of a Stagnosol and in a nodule sepd. from the horizon. We used wet-chem. analyses and X-ray diffractometry together with microscopic techniques and nano-scale secondary ion mass spectrometry (NanoSIMS), the latter allowing for a submicrometre-scale spatial resoln. X-ray diffractometry revealed the presence of quartz, clay minerals, micas and feldspars as the dominant minerals and indicated the presence of lepidocrocite. Relative to the bulk horizon material, the nodule was strongly enriched in org. C (by a factor of 31) and pedogenic (dithionite-extractable) Fe and Mn (by factors of 2.2 and 62). We selected two regions on a thin section of the nodule for NanoSIMS investigations after studying the element distribution by scanning-electron microscopy (SEM): one was located in an almost closed pore, the other one along an elongated pore. The NanoSIMS measurements allowed a clearer distinction of Fe- and Mn-accumulation zones than SEM-EDS. The evaluation of the NanoSIMS measurements by unsupervised classification revealed that zones contg. silicates and Mn oxides and the transitional zones between Fe and Mn oxides were particularly enriched in soil org. matter, while, with one exception, the pure Fe-accumulation zones did not indicate the presence of soil org. matter.
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  94. 94
    Johnson, K.; Purvis, G.; Lopez-Capel, E.; Peacock, C.; Gray, N.; Wagner, T.; Marz, C.; Bowen, L.; Ojeda, J.; Finlay, N.; Robertson, S.; Worrall, F.; Greenwell, C. Towards a mechanistic understanding of carbon stabilization in manganese oxides. Nat. Commun. 2015, 6, 7628,  DOI: 10.1038/ncomms8628
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    Towards a mechanistic understanding of carbon stabilization in manganese oxides
    Johnson, Karen; Purvis, Graham; Lopez-Capel, Elisa; Peacock, Caroline; Gray, Neil; Wagner, Thomas; Marz, Christian; Bowen, Leon; Ojeda, Jesus; Finlay, Nina; Robertson, Steve; Worrall, Fred; Greenwell, Chris
    Nature Communications (2015), 6 (), 7628CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Minerals stabilize org. carbon (OC) in sediments, thereby directly affecting global climate at multiple scales, but how they do it is far from understood. Here, we show that manganese oxide (Mn oxide) in a water treatment works filter bed traps dissolved OC as coatings build up in layers around clean sand grains at 3%w/wC. Using spectroscopic and thermogravimetric methods, we identify two main OC fractions. One is thermally refractory (>550 °C) and the other is thermally more labile (<550 °C). We postulate that the thermal stability of the trapped OC is due to carboxylate groups within it bonding to Mn oxide surfaces coupled with phys. entrapment within the layers. We identify a significant difference in the nature of the surface-bound OC and bulk OC. We speculate that polymn. reactions may be occurring at depth within the layers. We also propose that these processes must be considered in future studies of OC in natural systems.
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    Sparks, D. L. Environmental Soil Chemistry, 2nd ed.; Academic Press: San Diego, CA, 2002.
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    Allard, S.; Gutierrez, L.; Fontaine, C.; Croué, J.; Gallard, H. Organic matter interactions with natural manganese oxide and synthetic birnessite. Sci. Total Environ. 2017, 583, 487– 495,  DOI: 10.1016/j.scitotenv.2017.01.120
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    96
    Organic matter interactions with natural manganese oxide and synthetic birnessite
    Allard, Sebastien; Gutierrez, Leonardo; Fontaine, Claude; Croue, Jean-Philippe; Gallard, Herve
    Science of the Total Environment (2017), 583 (), 487-495CODEN: STENDL; ISSN:0048-9697. (Elsevier B.V.)
    Redox reactions of inorg. and org. contaminants on manganese oxides have been widely studied. However, these reactions are strongly affected by the presence of natural org. matter (NOM) at the surface of the manganese oxide. Interestingly, the mechanism behind NOM adsorption onto manganese oxides remains unclear. Therefore, in this study, the adsorption kinetics and equil. of different NOM isolates to synthetic manganese oxide (birnessite) and natural manganese oxide (Mn sand) were investigated. Natural manganese oxide is composed of both amorphous and well-crystd. Mn phases (i.e., lithiophorite, birnessite, and cryptomelane). NOM adsorption on both manganese oxides increased with decreasing pH (from pH 7 to 5), in agreement with surface complexation and ligand exchange mechanisms. The presence of calcium enhanced the rate of NOM adsorption by decreasing the electrostatic repulsion between NOM and Mn sand. Also, the adsorption was limited by the diffusion of NOM macromols. through the Mn sand pores. At equil., a preferential adsorption of high mol. wt. mols. enriched in arom. moieties was obsd. for both the synthetic and natural manganese oxide. Hydrophobic interactions may explain the adsorption of org. matter on manganese oxides. The formation of low mol. wt. UV absorbing mols. was detected with the synthetic birnessite, suggesting oxidn. and redn. processes occurring during NOM adsorption. This study provides a deep insight for both environmental and engineered systems to better understand the impact of NOM adsorption on the biogeochem. cycle of manganese.
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  97. 97
    Zhao, W.; Cheng, H.; Tao, S. Structure-reactivity relationships in the adsorption and degradation of substituted phenylarsonic acids on birnessite. Environ. Sci. Technol. 2020, 54, 1475– 1483,  DOI: 10.1021/acs.est.9b04203
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    Structure-Reactivity Relationships in the Adsorption and Degradation of Substituted Phenylarsonic Acids on Birnessite (δ-MnO2)
    Zhao, Wei; Cheng, Hefa; Tao, Shu
    Environmental Science & Technology (2020), 54 (3), 1475-1483CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Phenylarsonic acid compds. could be oxidized by manganese oxides in surface soils, resulting in quick release of inorg. arsenic. This study investigated the structure-reactivity relationships in the adsorption and oxidative degrdn. of six substituted phenylarsonic acids on the surface of a major type of manganese oxides, birnessite (δ-MnO2), using batch expts. conducted under acidic to neutral conditions. The initial adsorption rates of the substituted phenylarsonic acids on δ-MnO2 decreased in the order of phenylarsonic acid (PAA) > 4-aminophenylarsonic acid (p-ASA) ≈ 2-aminophenylarsonic acid (2-APAA) > 4-hydroxyphenylarsonic acid (4-HPAA) > 2-nitrophenylarsonic acid (2-NPAA) > 4-hydroxy-3-nitrophenylarsonic acid (ROX), which could be attributed to steric hindrance of the substituents and the hydrophobicity of these compds. The oxidn. rates of these structural analogs by δ-MnO2 decreased in the order of p-ASA ≈ 2-APAA > 4-HPAA > ROX, while 2-NPAA and PAA were non-reactive due to the lack of electron-donating substituents on their arom. rings. The redox reactivity of these compds. agrees well with the electron d. at C1, which is detd. by the types and position of the substituents on the arom. ring. Although cleavage of arsonic acid group from the arom. ring was the predominant transformation pathway, a range of adduct products also formed through cross-coupling of the radicals and radical substitution. The contribution of radical coupling and substitution in overall degrdn. decreased in the order of p-ASA > 2-APAA > 4-HPAA > ROX, which resulted from the varying reactivity and steric hindrance of the substituents. These insights could help better understand and predict the fate of substituted phenylarsonic acids in manganese oxide-rich surface soils, and the assocd. environmental risk of arsenic pollution.
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  98. 98
    Wang, Q.; Yang, P.; Zhu, M. Structural transformation of birnessite by fulvic acid under anoxic conditions. Environ. Sci. Technol. 2018, 52, 1844– 1853,  DOI: 10.1021/acs.est.7b04379
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    98
    Structural Transformation of Birnessite by Fulvic Acid under Anoxic Conditions
    Wang, Qian; Yang, Peng; Zhu, Mengqiang
    Environmental Science & Technology (2018), 52 (4), 1844-1853CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The structure and Mn(III) concn. of birnessite dictate its reactivity and can be changed by birnessite partial redn., but effects of pH and reductant/birnessite ratios on the changes by redn. remain unclear. We found that the two factors strongly affect the structure of birnessite (δ-MnO2) and its Mn(III) content during its redn. by fulvic acid (FA) at pH 4-8 and FA/solid mass ratios of 0.01-10 under anoxic conditions over 600 h. During the redn., the structure of δ-MnO2 is increasingly accumulated with both Mn(III) and Mn(II) but much more with Mn(III) at pH 8, whereas the accumulated Mn is mainly Mn(II) with little Mn(III) at pH 4 and 6. Mn(III) accumulation, either in layers or over vacancies, is stronger at higher FA/solid ratios. At FA/solid ratios ≥1 and pH 6 and 8, addnl. hausmannite and MnOOH phases form. The altered birnessite favorably adsorbs FA because of the structural accumulation of Mn(II, III). Like during microbially mediated oxidative pptn. of birnessite, the dynamic changes during its redn. are ascribed to the birnessite-Mn(II) redox reactions. Our work suggests low reactivity of birnessite coexisting with org. matter and severe decline of its reactivity by partial redn. in alk. environment.
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  99. 99
    Joshi, T. P.; Zhang, G.; H, C.; Liu, R.; Liu, H.; Qu, J. Transformation of para arsanilic acid by manganese oxide: Adsorption, oxidation, and influencing factors. Water Res. 2017, 116, 126– 134,  DOI: 10.1016/j.watres.2017.03.028
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    99
    Transformation of para arsanilic acid by manganese oxide: Adsorption, oxidation, and influencing factors
    Joshi, Tista Prasai; Zhang, Gong; Cheng, Hanyang; Liu, Ruiping; Liu, Huijuan; Qu, Jiuhui
    Water Research (2017), 116 (), 126-134CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Arom. organoarsenic compds. tend to transform into more mobile toxic inorg. arsenic via several processes, and can inadvertently spread toxic inorg. arsenic through the environment to water sources. To gain insight into the transformation mechanisms, we herein investigated how the process of para arsanilic acid (p-ASA) transformation works in detail on the surface of adsorbents by comparing it with phenylarsonic acid (PA) and aniline, which have similar chem. structures. In contrast to the values of 0.23 mmol g-1 and 0.68 mmol g-1 for PA and aniline, the max. adsorption capacity was detd. to be 0.40 mmol g-1 for p-ASA at pH 4.0. The results of FTIR and XPS spectra supported the presence of a protonated amine, resulting in a suitable condition for the oxidn. of p-ASA. Based on the combined results of UV-spectra and UPLC-Q-TOF-MS, we confirmed that the adsorbed p-ASA was first oxidized through the transfer of one electron from p-ASA on MnO2 surface to form a radical intermediate, which through further hydrolysis and coupling led to formation of benzoquinone and azophenylarsonic acid, which was identified as a major intermediate. After that, p-ASA radical intermediate was cleaved to form arsenite (III), and then further oxidized into arsenate (V) with the release of manganese (Mn) into soln., indicating a heterogeneous oxidn. process.
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  100. 100
    Rabe, M.; Verdes, D.; Seeger, S. Understanding protein adsorption phenomena at solid surfaces. Adv. Colloid Interface Sci. 2011, 162, 87– 106,  DOI: 10.1016/j.cis.2010.12.007
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    100
    Understanding protein adsorption phenomena at solid surfaces
    Rabe, Michael; Verdes, Dorinel; Seeger, Stefan
    Advances in Colloid and Interface Science (2011), 162 (1-2), 87-106CODEN: ACISB9; ISSN:0001-8686. (Elsevier B.V.)
    A review. Protein adsorption at solid surfaces plays a key role in many natural processes and has therefore promoted a widespread interest in many research areas. Despite considerable progress in this field there are still widely differing and even contradictive opinions on how to explain the frequently obsd. phenomena such as structural rearrangements, cooperative adsorption, overshooting adsorption kinetics, or protein aggregation. In this review recent achievements and new perspectives on protein adsorption processes are comprehensively discussed. The main focus is put on commonly postulated mechanistic aspects and their translation into math. concepts and model descriptions. Relevant exptl. and computational strategies to practically approach the field of protein adsorption mechanisms and their impact on current successes are outlined.
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    Keil, R. G.; Mayer, L. M. Mineral matrices and organic matter. Treatise Geochem. 2014, 12, 337– 359,  DOI: 10.1016/B978-0-08-095975-7.01024-X
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    Sheng, X.; Qin, C.; Yang, B.; Hu, X.; Liu, C.; Waigi, M. G.; Li, X.; Ling, W. Metal cation saturation on montmorillonites facilitates the adsorption of DNA via cation bridging. Chemosphere 2019, 235, 670– 678,  DOI: 10.1016/j.chemosphere.2019.06.159
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    Metal cation saturation on montmorillonites facilitates the adsorption of DNA via cation bridging
    Sheng, Xue; Qin, Chao; Yang, Bing; Hu, Xiaojie; Liu, Cun; Waigi, Michael Gatheru; Li, Xuelin; Ling, Wanting
    Chemosphere (2019), 235 (), 670-678CODEN: CMSHAF; ISSN:0045-6535. (Elsevier Ltd.)
    Extracellular DNA (eDNA) is widely present in soil, with potential ecol. impacts. Metal cations are naturally present on the surface of soil clay minerals, although the adsorption mechanism of eDNA on clay minerals satd. with metal cations is still not fully understood. The research investigated the adsorption of eDNA, using salmon sperm DNA as a representative, on metal cation (Na+, Ca2+, and Fe3+)-satd. montmorillonites (Mt). Metal cation-satd. Mt have higher adsorption capacities for DNA. Compared with Mt (3500 mg.kg-1), the amts. of DNA adsorption on metal cation-satd. Mt (pH = 7.0) were increased by 0.74-5.38 times, and followed the descending order of Fe-Mt > Na-Mt > Ca-Mt > Mt. A temp. of 25°C was found to be more suitable than 15 and 35°C for DNA adsorption, while an increasing pH value (3.0-9.0) reduced DNA adsorption on Mt and metal cation-satd. Mt. Microscopic and spectroscopic analyses, together with a model computation technique, confirmed that metal cations satd. on the surface of Mt work like a 'cation bridge' linking oxygen atoms in the phosphate groups of DNA and the neg. charged moieties of Mt, which were predominantly bound through electrostatic forces, thus, facilitating DNA adsorption at pH > 5. The results of this investigation provide valuable insight into eDNA adsorption on soil clay minerals and the transport and fate of eDNA in the natural soil environment.
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  103. 103
    Cheng, H.; Yang, T.; Jiang, J.; Li, X.; Wang, P.; Ma, J. Mn2+ effect on manganese oxides (MnOx) nanoparticles aggregation in solution: Chemical adsorption and cation bridging. Environ. Pollut. 2020, 267, 115561,  DOI: 10.1016/j.envpol.2020.115561
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    Mn2+ effect on manganese oxides (MnOx) nanoparticles aggregation in solution: Chemical adsorption and cation bridging
    Cheng, Haijun; Yang, Tao; Jiang, Jin; Lu, Xiaohui; Wang, Panxin; Ma, Jun
    Environmental Pollution (Oxford, United Kingdom) (2020), 267 (), 115561CODEN: ENPOEK; ISSN:0269-7491. (Elsevier Ltd.)
    Manganese oxides (MnOx) and Mn2+ usually co-exist in the natural environment, as well as in water treatments for Mn2+ removal. Therefore, it is necessary to investigate the influence of Mn2+ on the stability of MnOx nanoparticles, as it is vital to their fate and reactivity. In this study, we used the time-resolved dynamic light scattering technique to study the influence of Mn2+ on the initial aggregation kinetics of MnOx nanoparticles. The results show that Mn2+ was highly efficient in destabilizing MnOx nanoparticles. The crit. coagulation concn. ratio of Mn2+ (0.3 mM) to Na+ (30 mM) was 2-6.64, which is beyond the ratio range indicated by the Schulze-Hardy rule. This is due to the coordination bond formed between Mn2+ and the surface O of MnOx, which could efficiently decrease the neg. surface charge of MnOx. As a result, in the co-presence of Mn2+ and Na+, a small amt. of Mn2+ (5μM) could efficiently neutralize the neg. charge of MnOx, thereby decreasing the amt. of Na+, which mainly destabilized nanoparticles through elec. double-layer compression, required to initiate aggregation. Further, Mn2+ behaved as a cation bridge linking both the neg. charged MnOx and humic acid, thereby increasing the stability of the MnOx nanoparticles as a result of the steric repulsion of the adsorbed humic acid. The results of this study enhance the understanding of the stability of the MnOx nanoparticles in the natural environment, as well as in water treatments.
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    Franchi, M.; Ferris, J. P.; Callori, E. Cations as mediators of the adsorption of nucleic acids on clay surfaces in prebiotic environments. Origins Life Evol. Biospheres 2003, 33, 1– 16,  DOI: 10.1023/A:1023982008714
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    Cations as mediators of the adsorption of nucleic acids on clay surfaces in prebiotic environments
    Franchi, Marco; Ferris, James P.; Gallori, Enzo
    Origins of Life and Evolution of the Biosphere (2003), 33 (1), 1-16CODEN: OLEBEM; ISSN:0169-6149. (Kluwer Academic Publishers)
    Monovalent ([Na+] > 10 mM) and divalent ([Ca2+], [Mg2+] > 1.0 mM) cations induced the pptn. of nucleic acid mols. In the presence of clay minerals (montmorillonite and kaolinite), there was adsorption instead of pptn. The cation concn. needed for adsorption depended on both the valence of the cations and the chem. nature of the nucleic acid mols. Double-stranded nucleic acids needed higher cation concns. than single-stranded ones to be adsorbed to the same extent on clay. Divalent cations were more efficient than monovalent ones in mediating adsorption. Adsorption to the clay occurred only when both nucleic acids and cations were present. However, once the complexes were formed, the cations could not be removed from the system by washing, indicating that they are directly involved in the assocn. between nucleic acids and mineral surfaces. These observations indicate that cations take part directly in the formation of nucleic acid-clay complexes, acting as a bridge' between the neg. charges on the mineral surface and those of the phosphate groups of the genetic polymer. The relatively low cation concns. needed for adsorption and the ubiquitous presence of clay minerals in the environment suggest that the adsorption of nucleic acids on mineral surfaces could have taken place in prebiotic habitats. This may have played an important role in the formation and preservation of nucleic acids and/or their precursor polymers.
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    Klewicki, J. K.; Morgan, J. J. Dissolution of β-MnOOH particles by ligands: Pyrophosphate, ethylenediaminetetraacetate, and citrate. Geochim. Cosmochim. Acta 1999, 63, 3017– 3024,  DOI: 10.1016/S0016-7037(99)00229-X
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    Dissolution of β-MnOOH particles by ligands: pyrophosphate, ethylenediaminetetraacetate, and citrate
    Klewicki, J. K.; Morgan, J. J.
    Geochimica et Cosmochimica Acta (1999), 63 (19/20), 3017-3024CODEN: GCACAK; ISSN:0016-7037. (Elsevier Science Inc.)
    Expts. describing the extent of dissoln. of β-MnOOH(s) suspensions vs. time using measurements of both total dissolved Mn and Mn(III) complex species show that the ligands pyrophosphate (P2O74-), ethylenediaminetetraacetate (EDTA4-), and citrate (CIT3-), each in large excess with respect to solids concn. and estd. surface sites, promote dissoln. by different mechanisms and at different rates. Pyrophosphate dissoln. of β-MnOOH yields predominately the Mn(III)HP2O7(aq) complex. EDTA4- causes reductive dissoln., with no Mn(III) complexes of the ligand obsd. in soln. Citrate causes ligand-promoted dissoln. in the initial stages of reaction, after which soln. phase reactions bring about redn. of Mn(III)CIT(aq) to Mn(II) soln. species. Rates of dissoln. at circum-neutral pH show somewhat higher rates at lower pH. Initial rates of ligand-promoted dissoln., R0, by pyrophosphate were in the range ∼ 8 × 10-8 to 6 × 10-7 mol g-1 s-1 between pH 8 and 6.5. For reductive dissoln. by EDTA, initial rates, R0, ranged from ∼ 8 × 10-7 to 2 × 10-6 mol g-1 s-1 between pH 8 and 7. At pH 6.3, initial dissoln. rate with citrate was ∼ 6 × 10-6 mol g-1 s-1. Results of this exploratory study of MnOOH dissoln. by ligands at circum-neutral pH suggest that manganese cycling in natural waters may involve loss of MnOOH particles promoted by naturally-occurring or pollutant ligands with formation of Mn(III) aq. complexes.
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  106. 106
    Chorover, J.; Amistadi, M. K. Reaction of forest floor organic matter at goethite, birnessite and smectite surfaces. Geochim. Cosmochim. Acta 2001, 65 (1), 95– 109,  DOI: 10.1016/S0016-7037(00)00511-1
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    Reaction of forest floor organic matter at goethite, birnessite and smectite surfaces
    Chorover, J.; Amistadi, M. K.
    Geochimica et Cosmochimica Acta (2001), 65 (1), 95-109CODEN: GCACAK; ISSN:0016-7037. (Elsevier Science Inc.)
    Expts. were conducted to compare the affinity and reactivity of three different minerals for natural org. matter (NOM) in forest floor leachate (FFL) from hardwood and pine forests. The FFLs were acidic (pH 4) with ionic strengths of 1.4 mM (hardwood) and 1.1 mM (pine), and they contained larger org. mols. (wt. av. mol. wts. [Mw] = 5-6 kDa) than has been reported recently for surface waters using similar methods. A synthetic diluent soln. was prepd. to match the inorg. chem. of the FFL and to provide a range of initial dissolved org. carbon (DOC) concns. (0-140 g C m-3) for reaction with goethite (α-FeOOH), birnessite (δ-MnO2) and smectite (montmorillonite, SWy-2) in suspension, and in corresponding blanks. A variety of macroscopic and spectroscopic methods were employed to show that reaction with the three minerals resulted in distinctly different NOM adsorption, fractionation and transformation patterns. Goethite exhibited a steep initial slope in the adsorption isotherm and a max. retention of 10.5 g C kg-1. The isotherm for montmorillonite was more linear, but equal amts. of C were adsorbed to goethite and montmorillonite (per unit sorbent mass) at max. DOC. Whereas preferential uptake of high Mw, arom. constituents via ligand exchange was obsd. for goethite, compds. of lower than av. Mw were retained on montmorillonite and no preference for arom. moieties was obsd. Birnessite, which has an isoelec. point of pH < 2, retained low amts. of org. C (<2 g C kg-1) but exhibited the highest propensity for oxidative transformation of the NOM. The data indicate that fractionation behavior of NOM is dependent on mineral surface chem. in addn. to sorbent affinity for org. C. This work also emphasizes the fact that abiotic transformation reactions must be considered in studies of NOM interaction with Fe(III) and Mn(IV) contg. solid phases.
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    Tamrat, W. Z.; Rose, J.; Grauby, O.; Doelsch, E.; Levard, C.; Chaurand, P.; Basile-Doelsch, I. Soil organo-mineral associations formed by co-precipitation of Fe, Si and Al in presence of organic ligands. Geochim. Cosmochim. Acta 2019, 260, 15– 28,  DOI: 10.1016/j.gca.2019.05.043
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    Soil organo-mineral associations formed by co-precipitation of Fe, Si and Al in presence of organic ligands
    Tamrat, Wuhib Zewde; Rose, Jerome; Grauby, Olivier; Doelsch, Emmanuel; Levard, Clement; Chaurand, Perrine; Basile-Doelsch, Isabelle
    Geochimica et Cosmochimica Acta (2019), 260 (), 15-28CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Weathering of silicates supplies a range of cations (mainly Si, Al, Fe, but also Ca, Mg, Na, K, Mn) to the soil soln. There, cations can interact with charged functional groups of dissolved soil org. matter (OM). Unlike Al and Fe, Si does not directly bind to natural OM. However, the role of Si in the mechanisms of OM stabilization by copptn. with short range order mineral phases (SRO) may have been underestimated. The formation of coppts. was tested by titrating a biotite-weathering soln. up to pH 5 in presence of 3,4-Dihydroxy-L-phenylalanine (DOPA) with initial (Fe + Al):C ratio ranging from 3 to 0.003. Size, crystallinity, chem. compn. and the local structure of the coppts. were analyzed by TEM-EDX and Fe K-edge EXAFS. Coppts. are amorphous particles whatever the (Fe + Al):C ratio, but their size, compn. and local structure were nevertheless seen to progressively vary with increasing C content. In low C samples (high (Fe + Al):C), coppts. were 2-40 nm in size and were dominated by Si (30-70%). Fe represented only 20-50% of the mineral phase and was structured in small oligomers of Fe octahedra. Around 20% of the Fe of the coppts. were bound to C. Conversely, in high C samples (low (Fe + Al):C), coppts. were 10-90 nm in size and Fe was the main component (45-70%). Fe was almost exclusively linked to OM by monomeric Fe-O-C bonds. Si (5-40%) and Al (15-35%) were able to form oligomers occluded in the Fe-OM network. In samples with intermediate C content ((Fe + Al):C = 0.3), the coppts. had 5-200 nm size particles. We suggest these coppts. are structured in a loose irregular 3D network of amorphous small oligmers of Fe (25-75%), Si (15-50%), and Al (10-35%), forming an amorphous and open-structured mineral skeleton. Within this mineral network, we suggest the org. compds. are linked either by bonds with Fe and Al to the skeleton, by monomeric Fe-O-C in the porosity of the network, or by weak bonds with other OM. This conceptual model provides an alternative to the std. view that SRO-OM is formed by ferrihydrite and amorphous Al(OH)3. We suggest naming the structure "Nanosized Coppts. of inorg. oLIgomers with orgs." with "nanoCLICs" as acronym. The presence of Si in the inorg. structures may have an impact not only on the amt. of OM stabilized by the nanoCLICs, but in the longer term, on the persistence of the OM stabilization potential by metallic oligomers.
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    Chen, C.; Dynes, J. J.; Wang, J.; Sparks, D. L. Properties of Fe-organic matter associations via coprecipitation versus adsorption. Environ. Sci. Technol. 2014, 48, 13751– 13759,  DOI: 10.1021/es503669u
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    Properties of Fe-Organic Matter Associations via Coprecipitation versus Adsorption
    Chen, Chunmei; Dynes, James J.; Wang, Jian; Sparks, Donald L.
    Environmental Science & Technology (2014), 48 (23), 13751-13759CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The assocn. of org. matter (OM) with minerals is recognized as the most important stabilization mechanism for soil org. matter. This study compared the properties of Fe-OM complexes formed from adsorption (reaction of OM to postsynthesis ferrihydrite) vs. copptn. (formation of Fe solids in the presence of OM). Coppts. and adsorption complexes were synthesized using dissolved org. matter (DOM) exts. from a forest little layer at varying molar C/Fe ratios of 0.3-25.0. Sample properties were studied by N2 gas adsorption, XRD, FTIR, Fe EXAFS, and STXM-NEXAFS techniques. Copptn. resulted in much higher max. C contents (∼130 mg/g C difference) in the solid products than adsorption, which may be related to the formation of pptd. insol. Fe(III)-org. complexes at high C/Fe ratios in the coppts. as revealed by Fe EXAFS anal. Copptn. led to a complete blockage of mineral surface sites and pores with ≥177 mg/g C and molar C/Fe ratios ≥2.8 in the solid products. FTIR and STXM-NEXAFS showed that the copptd. OM was similar in compn. to the adsorbed OM. An enrichment of arom. C was obsd. at low C/Fe ratios. Assocn. of carboxyl functional groups with Fe was shown with FTIR and STXM-NEXAFS anal. STXM-NEXAFS anal. showed a continuous C distribution on minerals. Desorption of the copptd. OM was less than that of the adsorbed OM at comparable C/Fe ratios. These results are helpful to understand C and Fe cycling in the natural environments with periodically fluctuating redox conditions, where copptn. can occur.
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  109. 109
    Mikutta, R.; Zang, U.; Chorover, J.; Haumaier, L.; Kalbitz, K. Stabilization of extracellular polymeric substances (bacillus subtilis) by adsorption to and coprecipitation with Al forms. Geochim. Cosmochim. Acta 2011, 75, 3135– 3154,  DOI: 10.1016/j.gca.2011.03.006
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    Stabilization of extracellular polymeric substances (Bacillus subtilis) by adsorption to and coprecipitation with Al forms
    Mikutta, Robert; Zang, Ulrich; Chorover, Jon; Haumaier, Ludwig; Kalbitz, Karsten
    Geochimica et Cosmochimica Acta (2011), 75 (11), 3135-3154CODEN: GCACAK; ISSN:0016-7037. (Elsevier B.V.)
    Extracellular polymeric substances (EPS) are continuously produced by bacteria during their growth and metab. In soils, EPS are bound to cell surfaces, assocd. with biofilms, or released into soln. where they can react with other solutes and soil particle surfaces. If such reaction results in a decrease in EPS bioaccessibility, it may contribute to stabilization of microbial-derived org. carbon (OC) in soil. The authors examd.: (i) the chem. fractionation of EPS produced by a common Gram pos. soil bacterial strain (Bacillus subtilis) during reaction with dissolved and colloidal Al species and (ii) the resulting stabilization against desorption and microbial decay by the resp. copptn. (with dissolved Al) and adsorption (with Al(OH)3(am)) processes. Coppts. and adsorption complexes obtained following EPS-Al reaction as a function of pH and ionic strength were characterized by FTIR spectroscopy and XPS. The stability of adsorbed and copptd. EPS against biodegrdn. was assessed by mineralization expts. for 1100 h. Up to 60% of the initial 100 mg/L EPS-C was adsorbed at the highest initial molar Al:C ratio (1.86), but this still resulted only in a moderate OC mass fraction in the solid phase (17 mg/g Al(OH)3(am)). In contrast, while copptn. by Al was less efficient in removing EPS from soln. (max. values of 33% at molar Al:C ratios of 0.1-0.2), the OC mass fraction in the solid product was substantially larger than that in adsorption complexes. Org. P compds. were preferentially bound during both adsorption and copptn. Data are consistent with strong ligand exchange of EPS phosphoryl groups during adsorption to Al(OH)3( am ), whereas for copptn. weaker sorption mechanisms are also involved. X-ray photoelectron analyses indicate an intimate mixing of EPS with Al in the coppts., which is not obsd. in the case of EPS adsorption complexes. The incubation expts. showed that both processes result in overall stabilization of EPS against microbial decay. Stabilization of adsorbed or copptd. EPS increased with increasing molar Al:C ratio and biodegrdn. was correlated with EPS desorption, implying that detachment of EPS from surface sites is a prerequisite for microbial utilization. Results indicate that the mechanisms transferring EPS into Al-org. assocns. may significantly affect the compn. and stability of biomol. C, N and P in soils. The obsd. efficient stabilization of EPS might explain the strong microbial character of org. matter in subsoils.
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    Eusterhues, K.; Rennert, T.; Knicker, H.; Kogel-Knabner, I.; Totsche, K. U.; Schwertmann, U. Fractionation of organic matter due to reaction with ferrihydrite: Coprecipitation versus adsorption. Environ. Sci. Technol. 2011, 45, 527– 533,  DOI: 10.1021/es1023898
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    Fractionation of Organic Matter Due to Reaction with Ferrihydrite: Coprecipitation versus Adsorption
    Eusterhues, Karin; Rennert, Thilo; Knicker, Heike; Kogel-Knabner, Ingrid; Totsche, Kai U.; Schwertmann, Udo
    Environmental Science & Technology (2011), 45 (2), 527-533CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    In soil and water, ferrihydrite frequently forms in the presence of dissolved org. matter. This disturbs crystal growth and gives rise to copptn. of ferrihydrite and org. matter. To compare the chem. fractionation of org. matter during copptn. with the fractionation involved in adsorption onto pristine ferrihydrite surfaces we prepd. ferrihydrite-org. matter assocns. by adsorption and copptn. using (1) a forest-floor ext. or (2) a sulfonated lignin. The reaction products were studied by 13C CPMAS NMR, FTIR, and anal. of hydrolyzable neutral polysaccharides. Relative to the original forest-floor ext., the ferrihydrite-assocd. org. matter was enriched in polysaccharides, esp. when adsorption took place. Mannose and glucose were bound preferentially to ferrihydrite, while fucose, arabinose, xylose, and galactose accumulated in the supernatant. This fractionation of sugar monomers was more pronounced during copptn. and led to an enhanced ratio of (galactose + mannose)/(arabinose + xylose). Expts. with lignin revealed that the ferrihydrite-assocd. material was enriched in its arom. components but had a lower ratio of phenolic C to arom. C than the original lignin. A compositional difference between the adsorbed and copptd. lignin is obvious from a higher contribution of methoxy C in the copptd. material. Copptd. org. matter may thus differ in amt. and compn. from adsorbed org. matter.
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  111. 111
    Totsche, K. U.; Amelung, W.; Gerzabek, M. H.; Guggenberger, G.; Klumpp, E.; Knief, C.; lehndorff, E.; Mikutta, R.; Peth, S.; Prechtel, A.; Ray, N.; Kögel-Knabner, I. Microaggregates in soils. J. Plant Nutr. Soil Sci. 2018, 181, 104– 136,  DOI: 10.1002/jpln.201600451
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    Microaggregates in soils
    Totsche, Kai Uwe; Amelung, Wulf; Gerzabek, Martin H.; Guggenberger, Georg; Klumpp, Erwin; Knief, Claudia; Lehndorff, Eva; Mikutta, Robert; Peth, Stephan; Prechtel, Alexander; Ray, Nadja; Koegel-Knabner, Ingrid
    Journal of Plant Nutrition and Soil Science (2018), 181 (1), 104-136CODEN: JNSSFZ; ISSN:1436-8730. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. All soils harbor microaggregates, i.e., compd. soil structures smaller than 250 μm. These microaggregates are composed of diverse mineral, org. and biotic materials that are bound together during pedogenesis by various phys., chem. and biol. processes. Consequently, microaggregates can withstand strong mech. and physicochem. stresses and survive slaking in water, allowing them to persist in soils for several decades. Together with the physiochem. heterogeneity of their surfaces, the three-dimensional structure of microaggregates provides a large variety of ecol. niches that contribute to the vast biol. diversity found in soils. As reported for larger aggregate units, microaggregates are composed of smaller building units that become more complex with increasing size. In this context, organo-mineral assocns. can be considered structural units of soil aggregates and as nanoparticulate fractions of the microaggregates themselves. The mineral phases considered to be the most important as microaggregate forming materials are the clay minerals and Fe- and Al-(hydr)oxides. Within microaggregates, minerals are bound together primarily by physicochem. and chem. interactions involving cementing and gluing agents. The former comprise, among others, carbonates and the short-range ordered phases of Fe, Mn, and Al. The latter comprise org. materials of diverse origin and probably involve macromols. and macromol. mixts. Work on microaggregate structure and development has largely focused on org. matter stability and turnover. However, little is known concerning the role microaggregates play in the fate of elements like Si, Fe, Al, P, and S. More recently, the role of microaggregates in the formation of microhabitats and the biogeog. and diversity of microbial communities has been investigated. Little is known regarding how microaggregates and their properties change in time, which strongly limits our understanding of micro-scale soil structure dynamics. Similarly, only limited information is available on the mech. stability of microaggregates, while essentially nothing is known about the flow and transport of fluids and solutes within the micro- and nanoporous microaggregate systems. Any quant. approaches being developed for the modeling of formation, structure and properties of microaggregates are, therefore, in their infancy. We respond to the growing awareness of the importance of microaggregates for the structure, properties and functions of soils by reviewing what is currently known about the formation, compn. and turnover of microaggregates. We aim to provide a better understanding of their role in soil function, and to present the major unknowns in current microaggregate research. We propose a harmonized concept for aggregates in soils that explicitly considers the structure and build-up of microaggregates and the role of organo-mineral assocns. We call for expts., studies and modeling endeavors that will link information on aggregate forming materials with their functional properties across a range of scales in order to better understand microaggregate formation and turnover. Finally, we hope to inspire a novel cohort of soil scientists that they might focus their research on improving our understanding of the role of microaggregates within the system of aggregates and so help to develop a unified and quant. concept of aggregation processes in soils.
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    Toner, B.; Fakra, S.; Villalobos, M.; Warwick, T.; Sposito, G. Spatially resolved characterization of biogenic manganese oxide production within a bacterial biofilm. Appl. Environ. Microbiol. 2005, 71 (3), 1300– 1310,  DOI: 10.1128/AEM.71.3.1300-1310.2005
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    Spatially resolved characterization of biogenic manganese oxide production within a bacterial biofilm
    Toner, Brandy; Fakra, Sirine; Villalobos, Mario; Warwick, Tony; Sposito, Garrison
    Applied and Environmental Microbiology (2005), 71 (3), 1300-1310CODEN: AEMIDF; ISSN:0099-2240. (American Society for Microbiology)
    Pseudomonas putida strain MnB1, a biofilm-forming bacterial culture, was used as a model for the study of bacterial Mn oxidn. in freshwater and soil environments. The oxidn. of aq. Mn+2 [Mn+2(aq)] by P. putida was characterized by spatially and temporally resolving the oxidn. state of Mn in the presence of a bacterial biofilm, using scanning transmission X-ray microscopy (STXM) combined with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy at the Mn L2,3 absorption edges. Subsamples were collected from growth flasks contg. 0.1 and 1 mM total Mn at 16, 24, 36, and 48 h after inoculation. Immediately after collection, the unprocessed hydrated subsamples were imaged at a 40-nm resoln. Manganese NEXAFS spectra were extd. from X-ray energy sequences of STXM images (stacks) and fit with linear combinations of well-characterized ref. spectra to obtain quant. relative abundances of Mn(II), Mn(III), and Mn(IV). Careful consideration was given to uncertainty in the normalization of the ref. spectra, choice of ref. compds., and chem. changes due to radiation damage. The STXM results confirm that Mn+2(aq) was removed from soln. by P. putida and was concd. as Mn(III) and Mn(IV) immediately adjacent to the bacterial cells. The Mn ppts. were completely enveloped by bacterial biofilm material. The distribution of Mn oxidn. states was spatially heterogeneous within and between the clusters of bacterial cells. Scanning transmission X-ray microscopy is a promising tool for advancing the study of hydrated interfaces between minerals and bacteria, particularly in cases where the structure of bacterial biofilms needs to be maintained.
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  113. 113
    Yang, W.; Zhang, Z.; Zhang, Z.; Chen, H.; Liu, J.; Ali, M.; Liu, F.; Li, L. Population structure of manganese-oxidizing macteria in stratified soils and properties of manganese oxide aggregates under manganese-complex medium enrichment. PLoS One 2013, 8, e73778  DOI: 10.1371/journal.pone.0073778
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    Population structure of manganese-oxidizing bacteria in stratified soils and properties of manganese oxide aggregates under manganese-complex medium enrichment
    Yang, Weihong; Zhang, Zhen; Zhang, Zhongming; Chen, Hong; Liu, Jin; Ali, Muhammad; Liu, Fan; Li, Lin
    PLoS One (2013), 8 (9), e73778CODEN: POLNCL; ISSN:1932-6203. (Public Library of Science)
    Manganese-oxidizing bacteria in the aquatic environment have been comprehensively studied. However, little information is available about the distribution and biogeochem. significance of these bacteria in terrestrial soil environments. Stratified soils were initially examd. to study the community structure and diversity of manganese-oxidizing bacteria. Total 344 culturable bacterial isolates from all substrata exhibited Mn(II)-oxidizing activities at the range of 1 μM to 240 μM of the equiv. MnO2. The high Mn(II)-oxidizing isolates (>50 mM MnO2) were identified as the species of phyla Actinobacteria, Firmicutes and Proteobacteria. Seven novel Mn(II)-oxidizing bacterial genera (species), namely, Escherichia, Agromyces, Cellulomonas, Cupriavidus, Microbacterium, Ralstonia, and Variovorax, were revealed via comparative phylogenetic anal. Moreover, an increase in the diversity of soil bacterial community was obsd. after the combined enrichment of Mn(II) and carbon-rich complex. The phylogenetic classification of the enriched bacteria represented by predominant denaturing gradient gel electrophoresis bands, was apparently similar to culturable Mn(II)-oxidizing bacteria. The expts. were further undertaken to study the properties of the Mn oxide aggregates formed by the bacterial isolates with high Mn(II)-oxidizing activity. These bacteria were closely encrusted with their Mn oxides and formed regular microspherical aggregates under prolonged Mn(II) and carbon-rich medium enrichment for three weeks. The biotic oxidn. of Mn(II) to Mn(III/IV) by these isolates was confirmed by kinetic examns. X-ray diffraction assays showed the characteristic peaks of several Mn oxides and rhodochrosite from these aggregates. Leucoberbelin blue tests also verified the Mn(II)-oxidizing activity of these aggregates. Mn oxides were formed at certain amts. under the enrichment conditions, along with the formation of rhodochrosite in such aggregates. Therefore, this study provides insights into the structure and diversity of soil-borne bacterial communities in Mn(II)-oxidizing habitats and supports the contribution of soil-borne Mn(II)-oxidizing bacteria to Mn oxide mineralization in soils.
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    Piccolo, A.; Spaccini, R.; Nebbioso, A.; Mazzei, P. Carbon sequestration in soil by in situ catalyzed photo-oxidative polymerization of soil organic matter. Environ. Sci. Technol. 2011, 45, 6697– 6702,  DOI: 10.1021/es201572f
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    Carbon Sequestration in Soil by in Situ Catalyzed Photo-Oxidative Polymerization of Soil Organic Matter
    Piccolo, Alessandro; Spaccini, Riccardo; Nebbioso, Antonio; Mazzei, Pierluigi
    Environmental Science & Technology (2011), 45 (15), 6697-6702CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    An innovative mechanism for C sequestration in soil by in-situ photo-polymn. of soil org. matter via biomimetic catalysis is described. A synthetic water-sol. iron-porphyrin was added to 3 Mediterranean soils, irradiated by solar light, and subjected first to 5 days incubation, then 15 and 30 wetting/drying (w/d) cycles. In-situ catalyst-assisted photo-polymn. of soil org. C (SOC) increased water stability of soil aggregates after 5 days incubation and 15 w/d cycles, but not after 30 w/d cycles. Particle size distribution of all treated soil confirmed the induced soil phys. improvement, by displaying a concomitant lower yield of the clay-sized fraction and larger yields of coarse sand- or fine sand-size fractions, depending on soil texture, though only after 5 days incubation. The gain in soil phys. quality was reflected by the shift of OC content from small to large soil aggregates, suggesting that photo-polymn. stabilized OC by chem. and phys. processes. Further evidence of C sequestration capacity of the photocatalytic treatment was provided by a significant redn. of CO2 respired by all soils after both incubation and w/d cycles. Results suggested green catalytic technologies may potentially be the bases for future practices to increase soil C stabilization and mitigate CO2 emissions from arable soils.
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    Shindo, H.; Huang, P. M. Role of Mn(IV) oxide in abiotic formation of humic substances in the environment. Nature 1982, 298, 363– 365,  DOI: 10.1038/298363a0
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    Role of manganese(IV) oxide in abiotic formation of humic substances in the environment
    Shindo, H.; Huang, P. M.
    Nature (London, United Kingdom) (1982), 298 (5872), 363-5CODEN: NATUAS; ISSN:0028-0836.
    The effect of inorg. oxides on the formation of humic substances in soils by oxidative polymn. of phenols was studied. Mn(IV) oxides (birnessite and δ-MnO2) are very effective oxidants with respect to the abiotic browning of hydroquinone soln. at pH 4-8 (common in soil). However, the effect of Fe oxide is very small. The degree of browning in the Mn oxide system was not affected by the presence of antiseptic PhMe and inoculum. Agar plates innoculated with the browning soln. did not show any growth of microorganisms.
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    Liu, M. M.; Cao, X. H.; Tan, W. F.; Feng, X. H.; Qiu, G. H.; Chen, X. H.; Liu, F. Structural controls on the catalytic polymerization of hydroquinone by birnessite. Clays Clay Miner. 2011, 59, 525– 537,  DOI: 10.1346/CCMN.2011.0590510
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    Structural controls on the catalytic polymerization of hydroquinone by birnessites
    Liu, Ming-Ming; Cao, Xing-Hui; Tan, Wen-Feng; Feng, Xiong-Han; Qiu, Guo-Hong; Chen, Xiu-Hua; Liu, Fan
    Clays and Clay Minerals (2011), 59 (5), 525-537CODEN: CLCMAB; ISSN:0009-8604. (Clay Minerals Society)
    The role of Mn oxide in the abiotic formation of humic substances has been well demonstrated. However, information on the effect of crystal structure and surface-chem. characteristics of Mn oxide on this process is limited. In the present study, hexagonal and triclinic birnessites, synthesized in acidic and alkali media, were used to study the influence of the crystal-structure properties of birnessites on the oxidative polymn. of hydroquinone and to elucidate the catalytic mechanism of birnessites in the abiotic formation of humic-like polymers in hydroquinone-birnessite systems. The intermediate and final products formed in soln. and solid-residue phases were identified by UV/Visible spectroscopy, at. absorption spectrometry, Fourier-transform IR spectroscopy, X-ray diffraction, solid-phase microextn.-gas chromatog.-mass spectrometry, ion chromatog., and ultrafiltration. The degree of oxidative polymn. of hydroquinone was enhanced with increase in the interlayer hydrated H+, the av. oxidn. state (AOS), and the sp. surface area of birnessites. The nature of the functional groups of the humic-like polymers formed was, however, almost identical when hydroquinone was catalyzed by hexagonal and triclinic birnessites with similar AOS of Mn. The results indicated that crystal structure and surface-chem. characteristics have significant influence on the oxidative activity of birnessites and the d.p. of hydroquinone, but have little effect on the abiotic formation mechanism of humic-like polymers. The proposed oxidative polymn. pathway for hydroquinone is that, as it approaches the birnessite, it forms precursor surface complexes. As a strong oxidant, birnessite accepts an electron from hydroquinone, which is oxidized to 1,4-benzoquinone. The coupling, cleavage, polymn., and decarboxylation reactions accompany the generation of 1,4-benzoquinone, lead to the release of CO2 and carboxylic acid fragments, the generation of rhodochrosite, and the ultimate formation of humic-like polymers. These findings are of fundamental significance in understanding the catalytic role of birnessite and the mechanism for the abiotic formation of humic substances in nature.
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    Xia, X.; Stone, A. T. Mandelic acid and phenyllactic acid “Reaction Sets” for exploring the kinetics and mechanism of oxidations by hydrous manganese oxide (HMO). Environ. Sci. Process. Impacts 2019, 21, 1038– 1051,  DOI: 10.1039/C9EM00128J
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    117
    Mandelic acid and phenyllactic acid "Reaction Sets" for exploring the kinetics and mechanism of oxidations by hydrous manganese oxide (HMO)
    Xia, Xiaomeng; Stone, Alan T.
    Environmental Science: Processes & Impacts (2019), 21 (6), 1038-1051CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
    At pH 4.0, hydrous manganese oxide oxidizes mandelic acid by two mole-equiv. of electrons, yielding phenylglyoxylic acid and benzaldehyde. These intermediates, in turn, are oxidized by two mole-equiv. of electrons to the same ultimate oxidn. product, benzoic acid. The four compds. of the "reaction set" just described are conveniently monitored using capillary electrophoresis and HPLC. Extents of adsorption are negligible and their sum exhibits mass balance. Concns. of phenylglyoxylic acid, benzaldehyde, and benzoic acid can therefore be used to calc. mole-equiv. delivered to HMO for comparison with exptl.-detd. dissolved MnII concns. Semi-log plots and numerical anal. can also be used to explore rates of oxidn. of the functional groups represented, i.e. an α-hydroxycarboxylic acid, an α-ketocarboxylic acid, and an aldehyde. Inserting a -CH2- group between the benzene ring and the functional groups just described yields a new reaction set comprised of phenyllactic acid, phenylpyruvic acid, and phenylacetaldehyde, plus the C-1 ultimate oxidn. product, phenylacetic acid. Phenyllactic acid was oxidized 2.7-times more slowly than mandelic acid, while phenylpyruvic acid was oxidized 12.7-times faster than phenylglyoxylic acid. Under pH 4.0 conditions, this reaction set approach was used to explore the acceleratory effects of increases in HMO loading and inhibitory effects of 500μM phosphate and pyrophosphate addns.
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    Li, H.; Liu, F.; Zhu, M.; Feng, X.; Zhang, J.; Yin, H. Structure and properties of Co-doped cryptomelane and its enhanced removal of Pb2+ and Cr3+ from wastewater. J. Environ. Sci. 2015, 34, 77– 85,  DOI: 10.1016/j.jes.2015.02.006
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    Structure and properties of Co-doped cryptomelane and its enhanced removal of Pb2+ and Cr3+ from wastewater
    Li, Hui; Liu, Fan; Zhu, Mengqiang; Feng, Xionghan; Zhang, Jing; Yin, Hui
    Journal of Environmental Sciences (Beijing, China) (2015), 34 (), 77-85CODEN: JENSEE; ISSN:1001-0742. (Science Press)
    Cryptomelane is a reactive Mn oxide and has been used in removal of heavy metal from wastewaters. Co-doped cryptomelane was synthesized by refluxing at ambient pressure and characterized by powder X-ray diffraction, SEM, XPS and extended X-ray absorption fine structure spectroscopy, and its performances for removal of Pb2+ and Cr3+ from aq. solns. were investigated. Co doping has a negligible effect on the structure and morphol. of cryptomelane but increases the sp. surface area and Mn av. oxidn. state. Mn and Co K-edge extended X-ray absorption fine structure spectroscopy (EXAFS) anal. shows that Co barely affects the at. coordination environments of Mn, and distances of edge- and comer-sharing Co-Me (Me=Co, Mn) pairs are shorter than those of the corresponding Mn-Me pairs, implying the replacement of framework Mn(III) by Co(III). These Co-doped cryptomelanes can quickly oxidize Cr3+ to be HCrO4- and remove 45% - 66% of the total Cr in the reaction systems by adsorption and fixation, and they have enhanced Pb2+ adsorption capacities. Thus these materials are promising adsorbents for heavy metal remediation. The results demonstrate the design and modification of environmental friendly Mn oxide materials and can help us understand the interaction mechanisms of transition metals with Mn oxides.
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    Sunda, W. G.; Kieber, D. J. Oxidation of humic substances by manganese oxides yields low-molecular-weight organic substrates. Nature 1994, 367, 62– 64,  DOI: 10.1038/367062a0
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    Oxidation of humic substances by manganese oxides yields low-molecular-weight organic substrates
    Sunda, William G.; Kieber, David J.
    Nature (London, United Kingdom) (1994), 367 (6458), 62-4CODEN: NATUAS; ISSN:0028-0836.
    Mn oxides lyse complex humic substances, which in general cannot be used by microorganisms directly, to form low-mol.-wt. org. compds. that can be used as substrates for microbial growth. Mn-oxidizing bacteria may thus be able to use the carbon pool in humic substances, which represent one of the largest org. reservoirs in natural waters, sediments and soils.
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    Wang, Y.; Stone, A. T. The citric acid-MnIII,IVO2 (birnessite) reaction. Electron transfer, complex formation, and autocatalytic feedback. Geochim. Cosmochim. Acta 2006, 70 (17), 4463– 4476,  DOI: 10.1016/j.gca.2006.06.1551
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    The citric acid-MnIII,IVO2(birnessite) reaction. Electron transfer, complex formation, and autocatalytic feedback
    Wang, Yun; Stone, Alan T.
    Geochimica et Cosmochimica Acta (2006), 70 (17), 4463-4476CODEN: GCACAK; ISSN:0016-7037. (Elsevier)
    Citrate released by plants, bacteria, and fungi into soils is subject to abiotic oxidn. by MnO2(birnessite), yielding 3-ketoglutarate, acetoacetate, and MnII. Citrate loss and generation of products as a function of time all yield S-shaped curves, indicating autocatalysis. Increasing the citrate concn. decreases the induction period. The max. rate (rmax) along the reaction coordinate follows a Langmuir-Hinshelwood dependence on citrate concn. Increases in pH decrease rmax and increase the induction time. Adding MnII, ZnII, orthophosphate, or pyrophosphate at the onset of reaction decreases rmax. MnII addn. eliminates the induction period, while orthophosphate and pyrophosphate addn. increase the induction period. These findings indicate that two parallel processes are responsible. The first, relatively slow process involves the oxidn. of free citrate by surface-bound MnIII,IV, yielding MnII and citrate oxidn. products. The second process, which is subject to strong pos. feedback, involves electron transfer from MnII-citrate complexes to surface-bound MnIII,IV, generating MnIII-citrate and MnII. Subsequent intramol. electron transfer converts MnIII-citrate into MnII and citrate oxidn. products.
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    Ramstedt, M.; Sjöberg, S. Phase transformation and proton promoted dissolution of hydrous Manganite (γ-MnOOH). Aquat. Geochem. 2005, 11, 413– 431,  DOI: 10.1007/s10498-005-7441-2
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    Phase transformations and proton-promoted dissolution of hydrous manganite (γ-MnOOH)
    Ramstedt, Madeleine; Sjoeberg, Staffan
    Aquatic Geochemistry (2005), 11 (4), 413-431CODEN: AQGEFP; ISSN:1380-6165. (Springer)
    The objective of this study was to describe the proton-promoted dissoln. of synthetic manganite (γ-MnOOH) and to characterize the resulting phase transformations. The soln. and remaining solid phase after dissoln. was analyzed by techniques including at. absorption spectroscopy, x-ray diffraction (XRD), at. force microscopy (AFM), and SEM. In suspensions with a pH 5-7, -log[H+] was monitored for 17 mo and equil. consts. were detd. at 9, 12, and 17 mo of reaction time for the following reaction (25°, 0.1 M (Na)NO3):2 γ-MnOOH + 2H+ .dblharw. MnO2 + Mn2+ + 2H2O. The formed MnO2 ages with time and the equil. const. for a metastable phase (ramsdellite or nsutite) as well as the most stable phase, pyrolusite (β-MnO2), was detd. Furthermore, combined pH and pe (Eh) measurements were performed to study the equil.; γ-MnOOH(s) .dblharw. β-MnO2(s) + H+ + e-. Real-time AFM measurements of the dissoln. showed shrinkage of the length of the manganite needles with time (2 h). After 1 wk, SEM images showed that this decreased length also was followed by a reduced thickness of the manganite needles. From the SEM images the morphol. of the formed Mn(IV) oxides was studied. At pH 2.6, pyrolusite (β-MnO2) and MnCl2 were found in the XRD patterns. Throughout the pH range there were indications of ramsdellite (MnO1.97) in the XRD patterns, which coincided with the existence of a fraction of needle shaped crystals with smaller dimensions (compared to manganite) in the SEM images. These observations together with the long term dissoln. expts. suggest that the dissoln. of manganite initially forms a ramsdellite or nsutite phase that over time rearranges to form pyrolusite.
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    Majcher, E. H.; Chorover, J.; Bollag, J.; Huang, P. M. Evolution of CO2 during birnessite-induced oxidation of 14C-labeled catechol. Soil Sci. Soc. Am. J. 2000, 64, 157– 163,  DOI: 10.2136/sssaj2000.641157x
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    Evolution of CO2 during birnessite-induced oxidation of 14C-labeled catechol
    Majcher, Emily H.; Chorover, Jon; Bollag, Jean-Marc; Huang, P. M.
    Soil Science Society of America Journal (2000), 64 (1), 157-163CODEN: SSSJD4; ISSN:0361-5995. (Soil Science Society of America, Inc.)
    14C-labeled catechol was reacted with birnessite (manganese oxide) in aq. suspension at pH 4. The mass of catechol-derived C in solid, soln., and gas phases was quantified as a function of time. Between 5 and 16% of the total catechol C was liberated as CO2 from oxidn. and abiotic ring cleavage under various conditions. Most of the 14C (55-83%) was incorporated into the solid phase in the form of stable org. reaction products whereas soln. phase 14C concns. increased from 16 to 39% with a doubling of total catechol added. Polymn. and CO2 evolution appear to be competitive pathways in the transformation of catechol since their relative importance was strongly dependent on initial birnessite-catechol reaction conditions. Solid phase Fourier transform IR (FTIR) spectra are consistent with the presence of phenolic, quinone, and arom. ring cleavage products. Carbon dioxide release appears to be limited by availability of reactive birnessite surface sites and it is diminished in the presence of polymd. reaction products.
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    Harrington, J. M.; Parker, D. L.; Bargar, J. R.; Jarzecki, A. A.; Tebo, B. M.; Sposito, G.; Duckworth, O. W. Structural dependence of Mn complexation by siderophores: Donor group dependence on complex stability and reactivity. Geochim. Cosmochim. Acta 2012, 88, 106– 119,  DOI: 10.1016/j.gca.2012.04.006
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    Structural dependence of Mn complexation by siderophores: Donor group dependence on complex stability and reactivity
    Harrington, James M.; Parker, Dorothy L.; Bargar, John R.; Jarzecki, Andrzej A.; Tebo, Bradley M.; Sposito, Garrison; Duckworth, Owen W.
    Geochimica et Cosmochimica Acta (2012), 88 (), 106-119CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Siderophores traditionally have been viewed as solely being involved in the biogeochem. cycling of Fe(III). This paradigm, however, ignores the diverse roles siderophores may play in the cycling of other trace metals, such as Mn, Co, Mo, and V. Recent work has shown that siderophores form complexes with high stability consts. with Mn(III), which are in some cases higher than that of the corresponding Fe(III) complex. The authors report on a structural anal. of the dissolved Fe(III)- and Mn(III)-siderophore complexes of rhizoferrin and two pyoverdin-type siderophores using X-ray spectroscopic techniques. Addnl., the stability consts. of the Mn(III)-pyoverdinPaA and Mn(III)-rhizoferrin complexes have been quantified as log β111 = 47.5 ± 0.3 and log β110 = 29.8 ± 0.3, resp. Comparisons of thermodn. stability and soln. structures of Fe(III)- and Mn(III)-complexes with a variety of siderophores demonstrate the relationship between donor group identity, siderophore structure, and strength of complex formation. Rhizoferrin and two mixed-moiety pyoverdins bind with a higher affinity for Mn(III) than Fe(III), possibly because of binding moiety compn. which makes them better able to accommodate Jahn-Teller distortion. In contrast, Fe(III) forms complexes of higher relative stability with siderophores that contain hydroxamate and catecholate moieties, more rigid donor groups that form five-membered chelate rings.
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    Akafia, M. M.; Harrington, J. M.; Bargar, J. R.; Duckworth, O. W. Metal oxyhydroxide dissolution as promoted by structurally diverse siderophores and oxalate. Geochim. Cosmochim. Acta 2014, 141, 258– 269,  DOI: 10.1016/j.gca.2014.06.024
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    Metal oxyhydroxide dissolution as promoted by structurally diverse siderophores and oxalate
    Akafia, Martin M.; Harrington, James M.; Bargar, John R.; Duckworth, Owen W.
    Geochimica et Cosmochimica Acta (2014), 141 (), 258-269CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Siderophores, a class of biogenic ligands with high affinities for Fe(III), promote the dissoln. of metal ions from sparingly sol. mineral phases. However, most geochem. studies have focused on quantifying the reactivity of DFOB, a model trishydroxamate siderophore. This study utilized three different siderophores, desferrioxamine B, rhizoferrin, and protochelin, with structures that contain the most commonly obsd. binding moieties of microbial siderophores to examine the siderophore-promoted dissoln. rates of FeOOH, CoOOH, and MnOOH in the absence and presence of the ubiquitous low mol. mass org. acid oxalate by utilizing batch dissoln. expts. at pH = 5-9. Metal-siderophore complex and total dissolved metal concns. were monitored for durations of one hour to fourteen days, depending on the metal oxyhydroxide identity and soln. pH. The results demonstrate that MnOOH and CoOOH generally dissolve more quickly in the presence of siderophores than FeOOH. Whereas FeOOH dissolved exclusively by a ligand-promoted dissoln. mechanism, MnOOH and CoOOH dissolved predominantly by a reductive dissoln. mechanism under most exptl. conditions. For FeOOH, siderophore-promoted dissoln. rates trended with the stability const. of the corresponding aq. Fe(III) complex. In the presence of oxalate, measured siderophore-promoted dissoln. rates were found to increase, decrease, or remain unchanged as compared to the obsd. rates in single-ligand systems, depending on the pH of the system, the siderophore present, and the identity of the metal oxyhydroxide. Increases in obsd. dissoln. rates in the presence of oxalate were generally greater for FeOOH than for MnOOH or CoOOH. These results elucidate potential dissoln. mechanisms of both ferric and non-ferric oxyhydroxide minerals by siderophores in the environment, and may provide further insights into the biol. strategies of metal acquisition facilitated by coordinated exudation of low mol. wt. org. acids and siderophores.
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    Duckworth, O. W.; Sposito, G. Siderophore-Manganese(III) Interactions II. Manganite Dissolution Promoted by Desferrioxamine B. Environ. Sci. Technol. 2005, 39, 6045– 6051,  DOI: 10.1021/es050276c
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    Siderophore-Manganese(III) Interactions II. Manganite Dissolution Promoted by Desferrioxamine B
    Duckworth, Owen W.; Sposito, Garrison
    Environmental Science and Technology (2005), 39 (16), 6045-6051CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Recent lab. and field studies suggest that Mn(III) forms persistent aq. complexes with high-affinity ligands. Aq. Mn(III) species thus may play a significant but largely unexplored role in biogeochem. processes. One formation mechanism for these species is the dissoln. of Mn(III)-bearing minerals. To study this mechanism, we measured the steady-state dissoln. rates of manganite (γ-MnOOH) in the presence of desferrioxamine B (DFOB), a common trihydroxamate siderophore. We find that DFOB dissolves manganite by both reductive and nonreductive reaction pathways. For pH >6.5, a nonreductive ligand-promoted reaction is the dominant dissoln. pathway, with a steady-state dissoln. rate proportional to the surface concn. of DFOB. In the absence of reductants, the aq. Mn(III)HDFOB+ complex resulting from dissoln. is stable for at least several weeks at circumneutral to alk. pH and at 25°. For pH <6.5, Mn2+ is the dominant aq. species resulting from manganite dissoln., implicating a reductive dissoln. pathway. These results have important implications for the biogeochem. cycling of both Mn and siderophores as well as Fe(III) in natural waters and soils.
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  126. 126
    Duckworth, O. W.; Sposito, G. Siderophore-promoted dissolution of synthetic and biogenic layer-type Mn oxides. Chem. Geol. 2007, 242 (3–4), 497– 508,  DOI: 10.1016/j.chemgeo.2007.05.007
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    126
    Siderophore-promoted dissolution of synthetic and biogenic layer-type Mn oxides
    Duckworth, Owen W.; Sposito, Garrison
    Chemical Geology (2007), 242 (3-4), 497-508CODEN: CHGEAD; ISSN:0009-2541. (Elsevier B.V.)
    Siderophores are biogenic chelating agents exuded in terrestrial and marine environments to increase the bioavailablity of ferric iron. Recent work suggests that both solid and aq. manganese may affect the aq. speciation of siderophores and thus siderophore-mediated iron transport. Although the interaction of the trihydroxamate siderophore desferrioxamine B (DFOB) with several lower-valence manganese oxides has been studied, the effects of siderophores on Mn(III,IV) oxide dissoln. are unknown. To remedy this situation, we measured the dissoln. rates of two synthetic layer-type Mn(IV) oxides and a biogenic oxide produced by a model organism, Pseudomonas putida GB-1. For pH 5-7, we find that all minerals studied dissolve by traditional reductive (R1) dissoln., yielding Mn(II); for pH 7-9, dissoln. yields Mn(III)-siderophore complexes, either by selective ligand-promoted dissoln. of structural Mn(III) or by redn. of >Mn(IV) to >Mn(III) followed by complexation and solubilization of Mn(III) by DFOB. Because reductive dissoln. results in siderophore oxidn., manganese oxide dissoln. at acidic pH may provide a significant abiotic sink for siderophores in natural waters. At alk. pH, Mn(III)-siderophore complexes produced may profoundly affect the aq. speciation of siderophores as well as provide a source of reactive Mn(III) complexes.
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  127. 127
    Peña, J.; Duckworth, O. W.; Bargar, J. R.; Sposito, G. Dissolution of hausmannite (Mn3O4) in the presence of the trihydroxamate siderophore desferrioxamine B. Geochim. Cosmochim. Acta 2007, 71 (23), 5661– 5671,  DOI: 10.1016/j.gca.2007.03.043
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    127
    Dissolution of hausmannite (Mn3O4) in the presence of the trihydroxamate siderophore desferrioxamine B
    Pena, Jasquelin; Duckworth, Owen W.; Bargar, John R.; Sposito, Garrison
    Geochimica et Cosmochimica Acta (2007), 71 (23), 5661-5671CODEN: GCACAK; ISSN:0016-7037. (Elsevier)
    The influence is examd. of desferrioxamine B (DFOB) on the dissoln. of hausmannite, a mixed valence Mn(II, III) oxide found in soils and freshwater sediments. Batch dissoln. expts. were conducted both in the absence (pH 4-9) and in the presence of 100 μM DFOB (pH 5-9). In the absence of the ligand, there is a sharp decrease in the extent of proton-promoted dissoln. above pH 5 and no appreciable dissoln. above pH 8. The resulting aq. Mn2+ activities were in good agreement with previous studies, indirectly supporting the accepted two-step mechanism involving the formation of manganite and repptn. of hausmannite. Desferrioxamine B enhanced hausmannite dissoln. over the entire pH range investigated, both via the formation of a Mn(III) complex and through surface-catalyzed reductive dissoln. Above pH 8, non-reductive ligand-promoted dissoln. dominated, whereas below pH 8, dissoln. was non-stoichiometric with respect to DFOB. Concurrent proton-promoted, ligand-promoted, reductive, and induced dissoln. was obsd., with Mn release by either reductive or induced dissoln. increasing linearly with decreasing pH. The fast kinetics of the DFOB-promoted dissoln. of hausmannite, as compared to iron oxides, suggest that the siderophore-promoted dissoln. of Mn(III)-bearing minerals may compete with the siderophore-promoted dissoln. of Fe(III)-bearing minerals.
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  128. 128
    Saal, L. B.; Duckworth, O. W. Synergistic Dissolution of Manganese Oxides as Promoted by Siderophores and Small Organic Acids. Soil Sci. Soc. Am. J. 2010, 74 (6), 2021– 2031,  DOI: 10.2136/sssaj2009.0465
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    128
    Synergistic dissolution of manganese oxides as promoted by siderophores and small organic acids
    Saal, Lauren B.; Duckworth, Owen W.
    Soil Science Society of America Journal (2010), 74 (6), 2021-2031CODEN: SSSJD4; ISSN:0361-5995. (Soil Science Society of America)
    Recent studies have revealed that siderophores, biogenic chelating agents that strongly complex Fe(III) and other hard metals, may function in concert with low-mol.-mass org. acids (LMMOAs) to facilitate Fe (hydr) oxide dissoln. via synergistic reactions. The siderophore desferrioxamine B (DFOB) may also participate in a no. of chem. reactions at Mn (hydr)oxide surfaces. The goal of this study was to det. the rates and products of δ-MnO2, a layer type Mn(IV) oxide, dissoln. as promoted by LMMOA-DFOB mixts. As with Fe (hydr)oxides, the rate of DFOB-promoted dissoln. of δ-MnO2 is strongly influenced by the presence of the LMMOAs citrate and oxalate. In the presence of DFOB, citrate increases the dissoln. rate relative to the sum of dissoln. rates from corresponding single-ligand systems by promoting both Mn(III)HDFOB+ complex formation and Mn(II) prodn. in a pH-dependent mechanism. In contrast, oxalate-DFOB mixts. produce predominantly Mn(II), with rates enhanced up to threefold from the sum of dissoln. rates in single-ligand systems at acidic pH values. We investigated possible mechanisms to describe this synergistic dissoln. processes by building on observations of single-ligand systems. These results suggest that biol. exudation of LMMOAs in conjunction with siderophores may allow the selection of dissoln. products, including regulation of the formation of potentially reactive aq. Mn(III) complexes.
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  129. 129
    Berg, B.; Steffen, K. T.; McClaugherty, C. Litter decomposition rate is dependent on litter Mn concentrations. Biogeochemistry 2007, 82 (1), 29– 39,  DOI: 10.1007/s10533-006-9050-6
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    129
    Litter decomposition rate is dependent on litter Mn concentrations
    Berg, B.; Steffen, K. T.; McClaugherty, C.
    Biogeochemistry (2007), 82 (1), 29-39CODEN: BIOGEP; ISSN:0168-2563. (Springer)
    A statistically significant linear relationship was found between annual mass loss of foliar litter in the late stages of decompn. and Mn concn. in the litter. We used existing decompn. data on needle and leaf decompn. of Scots pine (Pinus sylvestris L.), lodgepole pine (Pinus contorta var. contorta), Norway spruce (Picea abies (L.) Kars.), silver birch (Betula pendula L.), and gray alder (Alnus incana L.) from Sweden and Aleppo pine (Pinus halepensis Mill.) from Libya, to represent boreal, temperate, and Mediterranean climates. The later the decompn. stage as indicated by higher sulfuric-acid lignin concns., the better were the linear relationships between litter mass loss and Mn concns. We conclude that Mn concns. in litter have an influence on litter mass-loss rates in very late decompn. stages (up to 5 years), provided that the litter has high enough Mn concn. The relationship may be dependent on species as the relationship is stronger with species that take up high enough amts. of Mn.
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  130. 130
    Whalen, E. D.; Smith, R. G.; Grandy, A. S.; Frey, S. D. Manganese limitation as a mechanism for reduced decomposition in soils under atmospheric nitrogen deposition. Soil Biol. Biochem. 2018, 127, 252– 263,  DOI: 10.1016/j.soilbio.2018.09.025
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    130
    Manganese limitation as a mechanism for reduced decomposition in soils under atmospheric nitrogen deposition
    Whalen, Emily D.; Smith, Richard G.; Grandy, A. Stuart; Frey, Serita D.
    Soil Biology & Biochemistry (2018), 127 (), 252-263CODEN: SBIOAH; ISSN:0038-0717. (Elsevier B.V.)
    Long-term atm. nitrogen (N) deposition has been shown to reduce leaf litter and lignin decompn. in temperate forest soils, leading to an accumulation of soil carbon (C). Reduced decompn. has been accompanied by altered structure and function of fungal communities, the primary decomposers in forest ecosystems; however, a mechanistic understanding of fungal responses to chronic N enrichment is lacking. A redn. in soil and litter manganese (Mn) concns. under N enrichment (i.e., Mn limitation) may help explain these observations, because Mn is a cofactor and regulator of lignin-decay enzymes produced by fungi. We conducted a lab. study to evaluate the effect of Mn availability on decompn. dynamics in chronically N-enriched soils. We measured litter mass loss, lignin relative abundance, and lignin-decay enzyme activities, and characterized the litter fungal community by ITS2 metabarcoding. We obsd. a significant pos. correlation between Mn availability and lignin-decay enzyme activities. In addn., long-term (28 years) N enrichment increased the relative abundance of 'weak' decomposers (e.g., yeasts), but this response was reversed with Mn amendment, suggesting that higher Mn availability may promote fungal communities better adapted to decomp. lignin. We conclude that Mn limitation may represent a mechanism to explain shifts in fungal communities, reduced litter decompn., and increased soil C accumulation under long-term atm. N deposition.
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    Stendahl, J.; Berg, B.; Lindahl, B. D. Manganese availability is negatively associated with carbon storage in northern coniferous forest humus layers. Sci. Rep. 2017, 7 (1), 15487,  DOI: 10.1038/s41598-017-15801-y
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    131
    Manganese availability is negatively associated with carbon storage in northern coniferous forest humus layers
    Stendahl Johan; Lindahl Bjorn D; Berg Bjorn
    Scientific reports (2017), 7 (1), 15487 ISSN:.
    Carbon sequestration below ground depends on organic matter input and decomposition, but regulatory bottlenecks remain unclear. The relative importance of plant production, climate and edaphic factors has to be elucidated to better predict carbon storage in forests. In Swedish forest soil inventory data from across the entire boreal latitudinal range (n = 2378), the concentration of exchangeable manganese was singled out as the strongest predictor (R(2) = 0.26) of carbon storage in the extensive organic horizon (mor layer), which accounts for one third of the total below ground carbon. In comparison, established ecosystem models applied on the same data have failed to predict carbon stocks (R(2) < 0.05), and in our study manganese availability overshadowed both litter production and climatic factors. We also identified exchangeable potassium as an additional strong predictor, however strongly correlated with manganese. The negative correlation between manganese and carbon highlights the importance of Mn-peroxidases in oxidative decomposition of recalcitrant organic matter. The results support the idea that the fungus-driven decomposition could be a critical factor regulating humus carbon accumulation in boreal forests, as Mn-peroxidases are specifically produced by basidiomycetes.
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  132. 132
    Hou, S. L.; Hattenschwiler, S.; Yang, J. J.; Sistla, S.; Wei, H. W.; Zhang, Z. W.; Hu, Y. Y.; Wang, R. Z.; Cui, S. Y.; Lu, X. T.; Han, X. G. Increasing rates of long-term nitrogen deposition consistently increased litter decomposition in a semi-arid grassland. New Phytol. 2021, 229 (1), 296– 307,  DOI: 10.1111/nph.16854
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    132
    Increasing rates of long-term nitrogen deposition consistently increased litter decomposition in a semi-arid grassland
    Hou, Shuang-Li; Haettenschwiler, Stephan; Yang, Jun-Jie; Sistla, Seeta; Wei, Hai-Wei; Zhang, Zhi-Wei; Hu, Yan-Yu; Wang, Ru-Zhen; Cui, Shu-Yan; Lue, Xiao-Tao; Han, Xing-Guo
    New Phytologist (2021), 229 (1), 296-307CODEN: NEPHAV; ISSN:0028-646X. (Wiley-Blackwell)
    Summary : The continuing nitrogen (N) deposition obsd. worldwide alters ecosystem nutrient cycling and ecosystem functioning. Litter decompn. is a key process contributing to these changes, but the numerous mechanisms for altered decompn. remain poorly identified. We assessed these different mechanisms with a decompn. expt. using litter from four abundant species (Achnatherum sibiricum, Agropyron cristatum, Leymus chinensis and Stipa grandis) and litter mixts. representing treatment-specific community compn. in a semi-arid grassland under long-term simulation of six different rates of N deposition. Decompn. increased consistently with increasing rates of N addn. in all litter types. Higher soil manganese (Mn) availability, which apparently was a consequence of N addn.-induced lower soil pH, was the most important factor for faster decompn. Soil C : N ratios were lower with N addn. that subsequently led to markedly higher bacterial to fungal ratios, which also stimulated litter decompn. Several factors contributed jointly to higher rates of litter decompn. in response to N deposition. Shifts in plant species compn. and litter quality played a minor role compared to N-driven redns. in soil pH and C : N, which increased soil Mn availability and altered microbial community structure. The soil-driven effect on decompn. reported here may have long-lasting impacts on nutrient cycling, soil org. matter dynamics and ecosystem functioning.
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  133. 133
    Kranabetter, J. M. Increasing soil carbon content with declining soil manganese in temperate rainforests: is there a link to fungal Mn?. Soil Biol. Biochem. 2019, 128, 179– 181,  DOI: 10.1016/j.soilbio.2018.11.001
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    133
    Increasing soil carbon content with declining soil manganese in temperate rainforests: is there a link to fungal Mn?
    Kranabetter, J. M.
    Soil Biology & Biochemistry (2019), 128 (), 179-181CODEN: SBIOAH; ISSN:0038-0717. (Elsevier B.V.)
    Forest floor carbon (C) sequestration has been neg. correlated with manganese (Mn) availability, possibly due to reduced efficacy of Mn-peridoxase enzymes produced by Agaricomycete fungi. I examd. a soil C and Mn dataset from a podzolization gradient, along with fungal sporocarp Mn concns., to potentially corroborate this finding. An inverse power relationship between soil C and soil Mn content across temperate rainforests was confirmed, which provides further evidence of a Mn bottleneck in C turnover. Av. Mn concns. of saprotrophic sporocarps were greater than those of ectomycorrhizal fungi, and displayed a similar inverse correlation with increasing soil C. The absence or limited effectiveness of select saprotrophic fungi across Mn-depleted forest soils may be one mechanism behind impeded turnover of recalcitrant org. matter.
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    Kellner, H.; Luis, P.; Pecyna, M. J.; Barbi, F.; Kapturska, D.; Krüger, D.; Zak, D. R.; Marmeisse, R.; Vandenbol, M.; Hofrichter, M. Widespread occurrence of expressed fungal secretory peroxidases in forest soils. PLoS One 2014, 9, e95557  DOI: 10.1371/journal.pone.0095557
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    Hatakka, A.; Lundell, T.; Hofrichter, M.; Maijala, P. Manganese peroxidase and its role in the degradation of wood lignin. ACS Symp. Ser. 2003, 855, 230– 243,  DOI: 10.1021/bk-2003-0855.ch014
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    Manganese peroxidase and its role in the degradation of wood lignin
    Hatakka, Annele; Lundell, Taina; Hofrichter, Martin; Maijala, Pekka
    ACS Symposium Series (2003), 855 (Applications of Enzymes to Lignocellulosics), 230-243CODEN: ACSMC8; ISSN:0097-6156. (American Chemical Society)
    A review, with refs. White-rot wood-rotting and litter-decompg. fungi produce lignin-degrading enzymes the most common of which are lignin peroxidase (LiP), manganese peroxidase (MnP) and laccase. According to literature and our own results, MnP and laccase are common ligninolytic enzymes whereas LiP is less common. Mol. properties of MnPs produced by several fungi are well known. MnP oxidizes Mn(II) to Mn(III), which in chelated form is a powerful oxidizing system, and when amended with unsatd. lipids is also able to mineralize up to 16 % of 14C-labeled synthetic lignin to 14CO2. The results obtained until now indicate that the mineralization of lignin may occur outside the fungal cell wall. This points to the key role of MnP in lignin degrdn. MnP in a mixt. of Tween 80, Mn2+, Mn-chelating org. acid and hydrogen peroxide generating system resulted both in depolymn. of milled pine wood, and polymn. of the insol. part of pine wood, but depolymn. was the most prominent reaction. Many suitable fungi for biopulping produce MnP whereas the prodn. of LiP does not seem to be necessary in this application. The role of laccase is unclear. However, selective lignin degrdn. and efficiency in biopulping require a proper balance between lignin and cellulose degrdn., and therefore also thorough studies to clarify the significance of cellulose and hemicellulose degrdn. in lignin-selective fungi are needed.
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    Sun, T.; Cui, Y.; Berg, B.; Zhang, Q.; Dong, L.; Wu, Z.; Zhang, L. A test of manganese effects on decomposition in forest and cropland sites. Soil Biol. Biochem. 2019, 129, 178– 183,  DOI: 10.1016/j.soilbio.2018.11.018
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    A test of manganese effects on decomposition in forest and cropland sites
    Sun, Tao; Cui, Yalan; Berg, Bjorn; Zhang, Quanquan; Dong, Lili; Wu, Zhijie; Zhang, Lili
    Soil Biology & Biochemistry (2019), 129 (), 178-183CODEN: SBIOAH; ISSN:0038-0717. (Elsevier B.V.)
    Litter of plant origin is the main source of soil org. matter, and its phys. and chem. quality and decompn. rates are key variables in the prediction and modeling of how litter-derived carbon (C) is cycling through the ecosystem. However, the biol. control factors for decompn. are not well understood and often poorly represented in global C models. These are typically run using simple parameters, such as nitrogen (N) and lignin concns., characterizing the quality of the org. matter input to soils and its accessibility to decomposer organisms. Manganese (Mn) is a key component for the formation of manganese peroxidase (MnP), an important enzyme for lignin degrdn. However, the functional role of Mn on plant litter decompn. has been rarely exptl. examd. Here, using a forest and a cropland site we studied, over 41 mo, the effects of Mn fertilization on MnP activity and decompn. of eight substrates ranging in initial lignin concns. from 9.8 to 44.6%. Asymptotic decompn. models fitted the mass loss data best and allowed us to sep. compare the influence of Mn fertilization on different litter stages and pools. Across substrates, Mn fertilization stimulated decompn. rates of the late stage where lignin dominates decompn., resulting in smaller fraction of slowly decompg. litter. The increased MnP activity caused by Mn fertilization provided the mechanism explaining the stimulated decompn. in the Mn-addn. treatments.
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    Collins, P. J.; Dobson, A. D. Oxidation of fluorene and phenanthrene by Mn (II) dependent peroxidase activity in whole cultures of Trametes (Coriolus) versicolor. Biotechnol. Lett. 1996, 18 (7), 801– 804,  DOI: 10.1007/BF00127892
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    Oxidation of fluorene and phenanthrene by Mn(II)-dependent peroxidase activity in whole cultures of Trametes (Coriolus) versicolor
    Collins, Patrick J.; Dobson, Alan D. W.
    Biotechnology Letters (1996), 18 (7), 801-804CODEN: BILED3; ISSN:0141-5492. (Chapman and Hall)
    Manganese peroxidase (MnP) activity in 10-day-old cultures of Trametes versicolor 290 increased from 13 to 104 nmol/mL-min when the nitrogen concn. in the growth medium was increased from 0.1 to 0.4 g L-1 and supplemented with 600 μM Mn(II). When cultured under these conditions the fungus oxidized the polycyclic arom. hydrocarbons fluorene and phenanthrene, with approx. 75% oxidn. of phenanthrene after 11 days incubation and complete oxidn. of fluorene after 7 days.
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    Steffen, K. T.; Hatakka, A.; Hofrichter, M. Degradation of humic acids by the litter-decomposing basidiomycete Collybia dryophila. Appl. Environ. Microbiol. 2002, 68 (7), 3442– 3448,  DOI: 10.1128/AEM.68.7.3442-3448.2002
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    Degradation of humic acids by the litter-decomposing basidiomycete Collybia dryophila
    Steffen, Kari Timo; Hatakka, Annele; Hofrichter, Martin
    Applied and Environmental Microbiology (2002), 68 (7), 3442-3448CODEN: AEMIDF; ISSN:0099-2240. (American Society for Microbiology)
    Collybia dryophila K209, which colonizes forest soil, was found to decomp. a natural humic acid isolated from pine-forest litter (LHA) and a synthetic 14C-labeled humic acid (14C-HA) prepd. from [U-14C]catechol in liq. culture. Degrdn. resulted in the formation of polar, lower-mol.-mass fulvic acid (FA) and carbon dioxide. HA decompn. was considerably enhanced in the presence of Mn2+ (200 μM), leading to 75% conversion of LHA and 50% mineralization of 14C-HA (compared to 60% and 20%, resp., in the absence of Mn2+). There was a strong indication that manganese peroxidase (MnP), the prodn. of which was noticeably increased in Mn2+-supplemented cultures, was responsible for this effect. The enzyme was produced as a single protein with a pI of 4.7 and a mol. mass of 44 kDa. During solid-state cultivation, C. dryophila released substantial amts. of water-sol. FA (predominantly of 0.9 kDa mol. mass) from insol. litter material. The results indicate that basidiomycetes, such as C. dryophila, which colonize forest litter and soil are involved in humus turnover by their recycling of high-mol.-mass humic substances. Extracellular MnP seems to be a key enzyme in the conversion process.
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    Kadri, T.; Rouissi, T.; Brar, S. K.; Cledon, M.; Sarma, S.; Verma, M. Biodegradation of polycyclic aromatic hydrocarbons (PAHs) by fungal enzymes: A review. J. Environ. Sci. 2017, 51, 52– 74,  DOI: 10.1016/j.jes.2016.08.023
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    Biodegradation of polycyclic aromatic hydrocarbons (PAHs) by fungal enzymes: a review
    Kadri, Tayssir; Rouissi, Tarek; Brar, Satinder Kaur; Cledon, Maximiliano; Sarma, Saurabhjyoti; Verma, Mausam
    Journal of Environmental Sciences (Beijing, China) (2017), 51 (), 52-74CODEN: JENSEE; ISSN:1001-0742. (Science Press)
    A review. Polycyclic arom. hydrocarbons (PAHs) are a large group of chems. They represent an important concern due to their widespread distribution in the environment, their resistance to biodegrdn., their potential to bioaccumulate and their harmful effects. Several pilot treatments have been implemented to prevent economic consequences and deterioration of soil and water quality. As a promising option, fungal enzymes are regarded as a powerful choice for degrdn. of PAHs. Phanerochaete chrysosporium, Pleurotus ostreatus and Bjerkandera adusta are most commonly used for the degrdn. of such compds. due to their prodn. of ligninolytic enzymes such as lignin peroxidase, manganese peroxidase and laccase. The rate of biodegrdn. depends on many culture conditions, such as temp., oxygen, accessibility of nutrients and agitated or shallow culture. Moreover, the addn. of biosurfactants can strongly modify the enzyme activity. The removal of PAHs is dependent on the ionization potential. The study of the kinetics is not completely comprehended, and it becomes morem hallenging when fungi are applied for bioremediation. Degrdn. studies in soil are much more complicated than liq. cultures because of the heterogeneity of soil, thus, many factors should be considered when studying soil bioremediation, such as desorption and bioavailability of PAHs. Different degrdn. pathways can be suggested. The peroxidases are heme-contg. enzymes having common catalytic cycles. One mol. of hydrogen peroxide oxidizes the resting enzyme withdrawing two electrons. Subsequently, the peroxidase is reduced back in two steps of one electron oxidn. Laccases are copper-contg. oxidases. They reduce mol. oxygen to water and oxidize phenolic compds.
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    Bao, W.; Fukushima, Y.; Jensen Jr, K. A.; Moen, M. A.; Hammel, K. E. Oxidative degradation of non-phenolic lignin during lipid peroxidation by fungal manganese peroxidase. FEBS Lett. 1994, 354 (3), 297– 300,  DOI: 10.1016/0014-5793(94)01146-X
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    140
    Oxidative degradation of non-phenolic lignin during lipid peroxidation by fungal manganese peroxidase
    Bao, Wuli; Fukushima, Yaichi; Jensen, Kenneth A. Jr.; Moen, Mark A.; Hammel, Kenneth E.
    FEBS Letters (1994), 354 (3), 297-300CODEN: FEBLAL; ISSN:0014-5793. (Elsevier)
    A nonphenolic lignin model dimer, 1-(4-ethoxy-3-methoxyphenyl)-2-phenoxypropane-1,3-diol, was oxidized by a lipid peroxidn. system that consisted of a fungal manganese peroxidase, Mn(II), and unsatd. fatty acid esters. The reaction products included 1-(4-ethoxy-3- methoxyphenyl)-1-oxo-2-phenoxy-3-hydroxypropane and 1-(4-ethoxy-3-methoxyphenyl)-1-oxo-3-hydroxypropane, indicating that substrate oxidn. occurred via benzylic H abstraction. The peroxidn. system depolymd. both exhaustively methylated (nonphenolic) and unmethylated (phenolic) synthetic lignins efficiently. It is proposed that white-rot fungi use manganese peroxidase-mediated lipid peroxidn. to accomplish the initial delignification of wood.
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  141. 141
    Kapich, A. N.; Korneichik, T. V.; Hatakka, A.; Hammel, K. E. Oxidizability of unsaturated fatty acids and of a non-phenolic lignin structure in the manganese peroxidase-dependent lipid peroxidation system. Enzyme Microb. Technol. 2010, 46 (2), 136– 140,  DOI: 10.1016/j.enzmictec.2009.09.014
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    141
    Oxidizability of unsaturated fatty acids and of a non-phenolic lignin structure in the manganese peroxidase-dependent lipid peroxidation system
    Kapich, Alexander N.; Korneichik, Tatyana V.; Hatakka, Annele; Hammel, Kenneth E.
    Enzyme and Microbial Technology (2010), 46 (2), 136-140CODEN: EMTED2; ISSN:0141-0229. (Elsevier B.V.)
    Unsatd. fatty acids have been proposed to mediate the oxidn. of recalcitrant, non-phenolic lignin structures by fungal manganese peroxidases (MnP), but their precise role remains unknown. The oxidizability of three fatty acids with varying degrees of polyunsatn. (linoleic, linolenic, and arachidonic acids) was investigated by measuring conjugated dienes formation when lipid peroxidn. was initiated either by MnP in the presence of Mn(II) or by chelated Mn(III). An inverse relationship between the degree of fatty acid unsatn. and the rate of peroxidn. was found in both cases, but some differences were also noted between the two types of reaction. With MnP/Mn(II), the reaction developed slowly and resulted in sustained lipid peroxidn. as detd. by the formation of late-stage fatty acid degrdn. products. By contrast, the reaction with chelated Mn(III) was very rapid and did not result in the formation of these late-stage products, which suggests that this system failed to propagate the sustained radical chain reaction that is characteristic of complete lipid peroxidn. All three polyunsatd. fatty acids supported the co-oxidn. of a non-phenolic lignin model compd. by MnP, again showing an inverse relationship between the degree of unsatn. and reactivity, but chelated Mn(III) by itself supported only very low levels of fatty acid-mediated lignin model oxidn. These parallels in fatty acid reactivity are consistent with a reaction scheme in which Mn(III) acts as the proximal oxidant that initiates lipid peroxidn. by MnP, thus generating fatty acid-derived radicals which in turn oxidize lignin structures. However, the results also suggest that the initial peroxyl radicals formed may not be the ligninolytic oxidants in this system. Instead, other radical oxidants produced during late-stage reactions of lipid peroxidn. may be required.
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  142. 142
    Kapich, A.; Hofrichter, M.; Vares, T.; Hatakka, A. Coupling of manganese peroxidase-mediated lipid peroxidation with destruction of nonphenolic lignin model compounds and 14C-labeled lignins. Biochem. Biophys. Res. Commun. 1999, 259 (1), 212– 219,  DOI: 10.1006/bbrc.1999.0742
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    142
    Coupling of Manganese Peroxidase-Mediated Lipid Peroxidation with Destruction of Nonphenolic Lignin Model Compounds and 14C-Labeled Lignins
    Kapich, Alexander; Hofrichter, Martin; Vares, Tamara; Hatakka, Annele
    Biochemical and Biophysical Research Communications (1999), 259 (1), 212-219CODEN: BBRCA9; ISSN:0006-291X. (Academic Press)
    Linoleic acid, the predominant unsatd. fatty acid (UFA) in the lipids of wood-rotting fungi, was oxidized by manganese peroxidase (MnP) from the white-rot fungus Phlebia radiata through a peroxidn. mechanism. The peroxidn. was markedly stimulated by hydrogen peroxide. UFAs that are substrates for lipid peroxidn. and surfactants that emulsify water-insol. components were essential for the MnP-catalyzed destruction of a nonphenolic β-O-4-linked lignin model compd. (LMC). Moreover, both components stimulated the MnP-catalyzed mineralization of 14C-labeled synthetic lignin and 14C-labeled wheat straw. A high level of destruction was obtained in reaction systems with Tween 80 acting both as surfactant and source of UFAs. The presence of the linoleic acid in reaction systems with MnP and Tween 80 addnl. enhanced rate and level of LMC destruction and lignin mineralization. The results indicate that lipid peroxidn. may play an important role in lignin biodegrdn. by wood-rotting basidiomycetes and support the hypothesis of coupling between the processes. (c) 1999 Academic Press.
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  143. 143
    Trum, F.; Titeux, H.; Cornelis, J.-T.; Delvaux, B. Effects of manganese addition on carbon release from forest floor horizons. Can. J. For. Res. 2011, 41 (3), 643– 648,  DOI: 10.1139/X10-224
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    143
    Effects of manganese addition on carbon release from forest floor horizons
    Trum, Florence; Titeux, Hugues; Cornelis, Jean-Thomas; Delvaux, Bruno
    Canadian Journal of Forest Research (2011), 41 (3), 643-648CODEN: CJFRAR; ISSN:0045-5067. (Canadian Science Publishing)
    Lignin concn. in org. residues largely controls their decompn. Mn2+ may well play a key role in ligninolysis because it is a cofactor of manganese peroxidase, an enzyme of the lignin-degrading system. This study aims to investigate the effects of Mn2+ addn. on forest floor horizon decompn. during lab. incubation. Therefore, we sampled two distinct forest floors from European beech (Fagus sylvatica L.) stands: a mor and a moder. Lignin and Mn concns. in forest floor upper layer were significantly larger in moder than in mor. Three horizons from each forest floor were sep. incubated with or without Mn2+ addn. (250 mg Mn·kg dry matter-1) and the release of both CO2 and dissolved org. C was measured. The dissolved org. C release was not impacted by the Mn2+ addn., while a clear increase in CO2 release from specific horizons was obsd. Thus, the impact of the Mn2+ addn. depends on (i) the forest floor type and on (ii) the org. matter decompn. stage.
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  144. 144
    Subedi, P.; Jokela, E. J.; Vogel, J. G.; Bracho, R.; Inglett, K. S., The effects of nutrient limitations on microbial respiration and organic matter decomposition in a Florida Spodosol as influenced by historical forest management practices. For. Ecol. Manage. , 2021, 479, 118592. DOI: 10.1016/j.foreco.2020.118592
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    Trum, F.; Titeux, H.; Ponette, Q.; Berg, B. Influence of manganese on decomposition of common beech (Fagus sylvatica L.) leaf litter during field incubation. Biogeochemistry 2015, 125 (3), 349– 358,  DOI: 10.1007/s10533-015-0129-9
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    145
    Influence of manganese on decomposition of common beech (Fagus sylvatica L.) leaf litter during field incubation
    Trum, Florence; Titeux, Hugues; Ponette, Quentin; Berg, Bjorn
    Biogeochemistry (2015), 125 (3), 349-358CODEN: BIOGEP; ISSN:0168-2563. (Springer)
    Litter decompn. is of crucial importance for sustainable prodn. in forest ecosystems with pedoclimatic conditions and nutrients being the main factors controlling litter decompn. In particular, manganese (Mn) could accelerate decompn. of litter in the lignin-dominated (late) stage. Correlations between Mn concn. and litter decay rate were previously reported and explained by the role of Mn2+ in lignin degrdn. as a cofactor of the enzyme manganese peroxidase. However, the role of Mn in litter decay has been little exptl. tested yet. This study aims to assess the increased decompn. rate of common beech (Fagus sylvatica L.) leaf litter exptl. enriched in Mn (0.9-17.0 mg g-1) using a litterbag expt. (500 days of in situ incubation). Mass loss and acid unhydrolyzable residue, Mn, carbon (C), and dissolved org. carbon were detd. in remaining materials and litter leachates. Our results showed a pos. influence of Mn on litter and C decay and on the release of hydrophobic dissolved org. C. We explained these results by an enhanced ligninolysis leading to an increase in dissolved polyphenolic compds.
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  146. 146
    Nowack, B.; Stone, A. T. Manganese-catalyzed degradation of phosphonic acids. Environ. Chem. Lett. 2003, 1 (1), 24– 31,  DOI: 10.1007/s10311-002-0014-3
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    146
    Manganese-catalyzed degradation of phosphonic acids
    Nowack, Bernd; Stone, Alan T.
    Environmental Chemistry Letters (2003), 1 (1), 24-31CODEN: ECLNBJ; ISSN:1610-3653. (Springer)
    The non-biodegradable and chem. very stable phosphonates are used in a variety of industrial applications including cooling waters, oil prodn. and textile industry. We show here that they are degraded in the presence of Mn(II) and oxygen. The half-life for the reaction is 9 min near neutral pH. The presence of other cations such as Ca(II) and Zn(II) considerably slows down the reaction by competition with Mn(II) for the phosphonate. The reaction involves the oxidn. of complexed Mn(II) by oxygen to Mn(III) and the subsequent oxidn. of phosphonate by Mn(III) thus yielding two stable phosphonic acid breakdown products. The oxidn. also proceeds in the presence of the mineral manganite (Mn(III)OOH), and yields the same breakdown products. The use of a newly developed chromatog. method revealed the presence of the breakdown products in wastewater. The results show that manganese-catalyzed oxidn. might be an important pathway for phosphonate degrdn. in natural waters.
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  147. 147
    Duckworth, O. W.; Sposito, G. Siderophore-Manganese(III) Interactions. I. Air-Oxidation of Manganese(II) Promoted by Desferrioxamine B. Environ. Sci. Technol. 2005, 39, 6037– 6044,  DOI: 10.1021/es050275k
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    147
    Siderophore-Manganese(III) Interactions. I. Air-Oxidation of Manganese(II) Promoted by Desferrioxamine B
    Duckworth, Owen W.; Sposito, Garrison
    Environmental Science and Technology (2005), 39 (16), 6037-6044CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Recent studies suggest that aq. Mn(III) complexes, particularly those with non-carboxylated ligands such as microbial siderophores, may be stable in soil and aquatic environments. We det. the stability consts. for Mn(II) and Mn(III) complexes with the common trihydroxamate siderophore, desferrioxamine B (DFOB). Base and redox titrns. were conducted to det. DFOB conditional protonation consts. and conditional stability consts. for 1:1 DFOB complexes with Mn(II) and Mn(III). The conditional protonation consts. agree well with literature values. We detd. stability consts. for three Mn(II)-DFOB species and one Mn(III)-DFOB species at 25° in 0.1M NaCl. The Mn(III) HDFOB+ complex can be formed readily by air-oxidn. of Mn(II)-DFOB. This reaction exhibits pseudo 1st-order kinetics with a rate coeff. that can be characterized as the product of O concn. with a linear combination of the concns. of the 3 Mn(II)-DFOB complexes. The 2nd-order rate coeffs. appearing in this linear combination are 1-2 orders of magnitude smaller than that assocd. with oxidn. of the hydrolytic species Mn(OH)20. The Mn(III)HDFOB+ complex is stable for pH 7.0-11.3; but, at pH <7.0 it decomps. by internal electron transfer, yielding oxidized DFOB products and Mn(II). For p[H+] >11.3, the complex degrades by disproportionation, yielding Mn(II) and solid MnO2. This range of pH stability supports the hypothesis that aq. Mn(III) may play a vital role in the biogeochem. cycling of not only manganese, but also other elements, such as C, S, N, O, and redox-active metals.
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  148. 148
    Harrington, J. M.; Bargar, J. R.; Jarzecki, A. A.; Roberts, J. G.; Sombers, L. A.; Duckworth, O. W. Trace metal complexation by the triscatecholate siderophore protochelin: structure and stability. BioMetals 2012, 25 (2), 393– 412,  DOI: 10.1007/s10534-011-9513-7
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    148
    Trace metal complexation by the triscatecholate siderophore protochelin: structure and stability
    Harrington, James M.; Bargar, John R.; Jarzecki, Andrzej A.; Roberts, James G.; Sombers, Leslie A.; Duckworth, Owen W.
    BioMetals (2012), 25 (2), 393-412CODEN: BOMEEH; ISSN:0966-0844. (Springer)
    Although siderophores are generally viewed as biol. iron uptake agents, recent evidence has shown that they may play significant roles in the biogeochem. cycling and biol. uptake of other metals. One such siderophore that is produced by A. vinelandii is the triscatecholate protochelin. In this study, we probe the soln. chem. of protochelin and its complexes with environmentally relevant trace metals to better understand its effect on metal uptake and cycling. Protochelin exhibits low soly. below pH 7.5 and degrades gradually in soln. Electrochem. measurements of protochelin and metal-protochelin complexes reveal a ligand half-wave potential of 200 mV. The Fe(III)Proto3- complex exhibits a salicylate shift in coordination mode at circumneutral to acidic pH. Coordination of Mn(II) by protochelin above pH 8.0 promotes gradual air oxidn. of the metal center to Mn(III), which accelerates at higher pH values. The Mn(III)Proto3- complex was found to have a stability const. of log β110 = 41.6. Structural parameters derived from spectroscopic measurements and quantum mech. calcns. provide insights into the stability of the Fe(III)Proto3-, Fe(III)H3Proto, and Mn(III)Proto3- complexes. Complexation of Co(II) by protochelin results in redox cycling of Co, accompanied by accelerated degrdn. of the ligand at all soln. pH values. These results are discussed in terms of the role of catecholate siderophores in environmental trace metal cycling and intracellular metal release.
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  149. 149
    Wang, Y.; Stone, A. T. Phosphonate- and Carboxylate-Based Chelating Agents that Solubilize (Hydr)oxide-Bound MnIII. Environ. Sci. Technol. 2008, 42, 4397– 4403,  DOI: 10.1021/es7032668
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    149
    Phosphonate- and Carboxylate-Based Chelating Agents that Solubilize (Hydr)oxide-Bound MnIII
    Wang, Yun; Stone, Alan T.
    Environmental Science & Technology (2008), 42 (12), 4397-4403CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Recent field studies suggest that dissolved MnIII should be ubiquitous at oxic/anoxic interfaces in all natural waters and may play important roles in biogeochem. redox processes. We uncovered environmentally relevant synthetic phosphonate-based chelators that solubilize (hydr)oxide-bound MnIII via ligand-promoted dissoln. at circumneutral pHs and that their ability to release aq. MnIII can be predicted based on the chem. structure. For 2 (hydr)oxides (manganite and birnessite) reacting with excess concns. of pyrophosphoric acid (PP), methylenediphosphonic acid (MDP), and phosphonoacetic acid (PAA), ligand-promoted dissoln. is predominant: from pH 6 to 8, initial dissoln. rates and plateau concns. for dissolved MnIII decrease in the order PP > MDP > PAA, and at pH 5, MDP reacts equally well (with birnessite) or more efficiently (with manganite) than PP, and PAA remains the least reactive chelator. For manganite reacting with an excess concn. of aminophosphonate/carboxylate-based chelators, the aminophosphonate-contg. iminodimethylenephosphonic acid and glyphosate yield appreciable amts. of dissolved MnIII, but the aminocarboxylate-based methyliminodiacetic acid yields solely dissolved MnII via MnIII redn.
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  150. 150
    Madison, A. S.; Tebo, B. M.; Luther, G. W., 3rd Simultaneous determination of soluble manganese(III), manganese(II) and total manganese in natural (pore)waters. Talanta 2011, 84 (2), 374– 81,  DOI: 10.1016/j.talanta.2011.01.025
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    150
    Simultaneous determination of soluble manganese(III), manganese(II) and total manganese in natural (pore)waters
    Madison, Andrew S.; Tebo, Bradley M.; Luther, George W., III
    Talanta (2011), 84 (2), 374-381CODEN: TLNTA2; ISSN:0039-9140. (Elsevier B.V.)
    A new spectrophotometric protocol was developed for the simultaneous detn. of sol. Mn(III), Mn(II) and total Mn [sum of sol. Mn(III) and Mn(II)] in sediment porewaters using a water sol. meso-substituted porphyrin [α,β,γ,δ-tetrakis(4-carboxyphenyl)porphine (T(4-CP)P)]. A simple kinetic rate model is used to quantify sol. Mn(II), Mn(III) and total Mn concns. during a metal substitution reaction. Under optimized conditions, the method accurately dets. sol. Mn(II) and Mn(III) within a concn. range of 100 nM-10 μM. The detection limit of total sol. Mn is 50 nM. Using this method, sol. Mn(II) and Mn(III) concns. were detd. in std. solns. within 0.4-2% of the known values and agreed closely with results of inductively coupled plasma mass spectrometric and voltammetric analyses. The procedure was successfully applied to det. sol. Mn(II), Mn(III) and total Mn in sediment porewaters of the Lower St. Lawrence Estuary. Mn(III) represented up to 85% of the total sol. Mn pool in surface sediments.
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  151. 151
    Madison, A. S.; Tebo, B. M.; Mucci, A.; Sundby, B.; Luther, G. W. Abundant porewater Mn (III) is a major component of the sedimentary redox system. Science 2013, 341 (6148), 875– 878,  DOI: 10.1126/science.1241396
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    151
    Abundant Porewater Mn(III) Is a Major Component of the Sedimentary Redox System
    Madison, Andrew S.; Tebo, Bradley M.; Mucci, Alfonso; Sundby, Bjorn; Luther, George W., III
    Science (Washington, DC, United States) (2013), 341 (6148), 875-878CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Sol. Mn(III) [Mn(III)] can potentially serve as both oxidant and reductant in 1-electron-transfer reactions with other redox species. In near-surface sediment porewater, it is often overlooked as a major component of Mn cycling. Applying a spectrophotometric kinetic method to hemipelagic sediments from the Laurentian Trough (Quebec, Canada), sol. Mn(III), likely stabilized by org. or inorg. ligands, accounts for up to 90% of the total dissolved Mn pool. Vertical profiles of dissolved O and dissolved and solid Mn suggest that sol. Mn(III) is primarily produced via oxidn. of Mn(II) diffusing upwards from anoxic sediments with lesser contributions from biotic and abiotic reductive dissoln. of MnO2. The conceptual model of the sedimentary redox cycle should therefore explicitly include dissolved Mn(III).
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  152. 152
    Li, Q.; Xie, L.; Jiang, Y.; Fortner, J. D.; Yu, K.; Liao, P.; Liu, C. Formation and stability of NOM-Mn(III) colloids in aquatic environments. Water Res. 2019, 149, 190– 201,  DOI: 10.1016/j.watres.2018.10.094
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    152
    Formation and stability of NOM-Mn(III) colloids in aquatic environments
    Li, Qianqian; Xie, Lin; Jiang, Yi; Fortner, John D.; Yu, Kai; Liao, Peng; Liu, Chongxuan
    Water Research (2019), 149 (), 190-201CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Sol. Mn(III) species stabilized by natural org. matter (NOM) plays a crucial role in a no. of biogeochem. processes. To date, current understanding of these phenomena has been primarily concerned on the occurrence and chem. of sol. NOM-Mn(III) complexes; much less is known regarding the formation and stability of NOM-Mn(III) colloids in the environment. This presents a crit. knowledge gap with regard to biogeochem. cycling of manganese and assocd. carbon, and for predicting the fate and transport of colloid-assocd. contaminants, nutrients, and trace metals. In this work, we have characterized the chem. and phys. properties of humic acid based (HA)-Mn(III) colloids formed over a range of environmentally relevant conditions and quantified their subsequent aggregation and stability behaviors. Results show that molar C/Mn ratios and HA types (Aldrich HA (AHA) and Pahokee peat soil HA (PPSHA)) are crit. factors influencing HA-Mn(III) colloidal properties. Both the amt. and the stability of HA-Mn(III) colloids increased with increasing initial molar C/Mn ratios, regardless of HA type. The correlation between the crit. coagulation concn. (CCC) and zeta potential (R2 > 0.97) suggests that both Derjaguin-Landau-Verwey-Overbeek (DLVO) type and non-DLVO interactions are responsible for enhanced stability of HA-Mn(III) colloids. For a given C/Mn ratio, PPSHA-Mn(III) colloids are significantly more stable against aggregation than AHA-Mn(III) colloids, which is likely due to stronger electrostatic interactions, hydration interactions, and steric hindrance. Further examn. in real-world waters indicates that the HA-Mn(III) colloids are highly stable in surface river water, but become unstable (i.e. extensive aggregation) in solns. representing a groundwater-seawater interaction zone. Overall, this study provides new insights into the formation and stability of NOM-Mn(III) colloids which are crit. for understanding Mn-based colloidal behavior(s), and thus Mn cycling processes, in aquatic systems.
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  153. 153
    Hu, E.; Zhang, Y.; Wu, S.; Wu, J.; Liang, L.; He, F. Role of dissolved Mn(III) in transformation of organic contaminants: Non-oxidative versus oxidative mechanisms. Water Res. 2017, 111, 234– 243,  DOI: 10.1016/j.watres.2017.01.013
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    153
    Role of dissolved Mn(III) in transformation of organic contaminants: Non-oxidative versus oxidative mechanisms
    Hu, Erdan; Zhang, Ya; Wu, Shuyan; Wu, Jun; Liang, Liyuan; He, Feng
    Water Research (2017), 111 (), 234-243CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Mn(III) is a strong oxidant for one electron transfer, which may be important in the transformation of org. contaminants during water/wastewater treatment and biogeochem. redox processes. This study explored the reaction mechanisms of dissolved Mn(III) with orgs. The role of dissolved Mn(III) either as a catalyst or an oxidant in reactions with orgs. was recognized. Aquo and/or hydroxo (or free) Mn(III), generated from the bisulfite activated permanganate process, facilitated efficient N-dealkylation of atrazine via a β-elimination mechanism, resulting no net redox reaction. In contrast, free Mn(III) degraded 4-chlorophenol via intramol. redox processes, the same as hydroxyl radical (·OH), resulting in dechlorination,·OH substitution, ring-opening and mineralization. Mn(III)-pyrophosphate compds. did not react with atrazine because complexation by pyrophosphate rendered Mn(III) unable to bond with atrazine, thus the electron and proton transfers between the reactants couldn't occur. However, it degraded 4-chlorophenol at a slower rate compared to free Mn(III), due to its reduced oxidn. potential. These results showed two distinct mechanisms on the degrdn. of org. contaminants and the insights may be applied in natural manganese-rich environments and water treatment processes with manganese compds.
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  154. 154
    Jones, M. E.; Nico, P. S.; Ying, S.; Regier, T.; Thieme, J.; Keiluweit, M. Manganese-Driven Carbon Oxidation at Oxic-Anoxic Interfaces. Environ. Sci. Technol. 2018, 52 (21), 12349– 12357,  DOI: 10.1021/acs.est.8b03791
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    154
    Manganese-Driven Carbon Oxidation at Oxic-Anoxic Interfaces
    Jones, Morris E.; Nico, Peter S.; Ying, Samantha; Regier, Tom; Thieme, Jurgen; Keiluweit, Marco
    Environmental Science & Technology (2018), 52 (21), 12349-12357CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The formation of reactive manganese (Mn) species is emerging as a key regulator of carbon oxidn. rates, and thus CO2 emissions, in soils and sediments. Many subsurface environments are characterized by steep oxygen gradients, forming oxic-anoxic interfaces that enable rapid redox cycling of Mn. Here, we examd. the impact of Mn(II)aq oxidn. along oxic-anoxic interfaces on carbon oxidn. in soils using lab.-based diffusion reactors. A combination of cyclic voltammetry, X-ray absorption spectroscopy, and X-ray microprobe imaging revealed a tight coupling between Mn(II)aq oxidn. and carbon oxidn. at the oxic-anoxic interface. Specifically, zones of Mn(II)aq oxidn. across the oxic-anoxic transition also exhibited the greatest lignin oxidn. potential, carbon solubilization, and oxidn. Microprobe imaging further revealed that the generation of Mn(III)-dominated ppts. coincided with carbon oxidn. Combined, our findings demonstrate that biotic Mn(II)aq oxidn., specifically the formation of Mn(III) species, contributes to carbon oxidn. along oxic-anoxic interfaces in soils and sediments. Our results suggest that we should regard carbon oxidn. not merely as a function of mol. compn., which insufficiently predicts rates, but in relation to microenvironments favoring the formation of critically important oxidants such as Mn(III).
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  155. 155
    Jones, M. E.; LaCroix, R. E.; Zeigler, J.; Ying, S. C.; Nico, P. S.; Keiluweit, M. Enzymes, Manganese, or Iron? Drivers of Oxidative Organic Matter Decomposition in Soils. Environ. Sci. Technol. 2020, 54 (21), 14114– 14123,  DOI: 10.1021/acs.est.0c04212
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    155
    Enzymes, Manganese, or Iron? Drivers of Oxidative Organic Matter Decomposition in Soils
    Jones, Morris E.; LaCroix, Rachelle E.; Zeigler, Jacob; Ying, Samantha C.; Nico, Peter S.; Keiluweit, Marco
    Environmental Science & Technology (2020), 54 (21), 14114-14123CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Oxidative decompn. of soil org. matter dets. the proportion of carbon that is either stored or emitted to the atm. as CO2. Full conversion of org. matter to CO2 requires oxidative mechanisms that depolymerize complex mols. into smaller, sol. monomers that can be respired by microbes. Current models attribute oxidative depolymn. largely to the activity of extracellular enzymes. Here we show that reactive manganese (Mn) and iron (Fe) intermediates, rather than other measured soil characteristics, best predict oxidative activity in temperate forest soils. Combining bioassays, spectroscopy, and wet-chem. anal., we found that oxidative activity in surface litters was most significantly correlated to the abundance of reactive Mn(III) species. In contrast, oxidative activity in underlying mineral soils was most significantly correlated to the abundance of reactive Fe(II/III) species. Pos. controls showed that both Mn(III) and Fe(II/III) species are equally potent in generating oxidative activity, but imply conventional bioassays have a systematic bias toward Fe. Combined, our results highlight the coupled biotic-abiotic nature of oxidative mechanisms, with Mn-mediated oxidn. dominating within Mn-rich org. soils and Fe-mediated oxidn. dominating Fe-rich mineral soils. These findings suggest microbes rely on different oxidative strategies depending on the relative availability of Fe and Mn in a given soil environment.
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  156. 156
    Karolewski, J. S.; Sutherland, K. M.; Hansel, C. M.; Wankel, S. D. An isotopic study of abiotic nitrite oxidation by ligand-bound manganese (III). Geochim. Cosmochim. Acta 2021, 293, 365– 378,  DOI: 10.1016/j.gca.2020.11.004
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    156
    An isotopic study of abiotic nitrite oxidation by ligand-bound manganese (III)
    Karolewski, Jennifer S.; Sutherland, Kevin M.; Hansel, Colleen M.; Wankel, Scott D.
    Geochimica et Cosmochimica Acta (2021), 293 (), 365-378CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Redox transformations of nitrogen (N) play a crit. role in detg. its speciation and biol. availability, thus defining the magnitude and extent of productivity in many ecosystems. A range of important nitrogen transformations often co-occur in regions hosting other redox-active elements, including sulfur, iron, and manganese (Mn), esp. along sharp redox gradients within aquatic sediments. This proximity in "redox real estate" produces conditions under which multi-element interactions and coupled cycling are thermodynamically favored. While previous work has reported anoxic nitrification linked to the presence of manganese (Mn) oxides in sediments, a clear connection between the cycling of Mn and N has remained elusive. Sol. Mn(III), which is stabilized via ligand-complexation, has recently been shown to represent the dominant dissolved Mn species in many environments. Here, we examd. the reactivity of ligand-stabilized Mn(III) with nitrite, using natural abundance stable nitrogen and oxygen isotopes to explore reaction dynamics under a range of conditions. Oxidn. of nitrite to nitrate by Mn(III)-pyrophosphate proceeded abiotically under both oxygen replete and nitrogen-purged conditions. Kinetics and isotope systematics of this reaction were measured over a range of pH (5-8), with reaction rates decreasing with increasing pH. Under all treatments, an inverse kinetic isotope effect of -19.9 ± 0.7‰ was obsd. for N, remarkably similar to previously documented fractionation by nitrite-oxidizing organisms. Expts. using 18O-labeled water confirmed that the source of the addnl. oxygen atom was from water. These findings suggest that nitrite oxidn. in environments hosting abundant ligand-bound Mn(III), including porewaters, estuaries, and coastal waters, may be facilitated in part by abiotic reactions with Mn, even under functionally anoxic conditions.
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  157. 157
    Birkner, N.; Navrotsky, A. Thermodynamics of manganese oxides: Sodium, potassium, and calcium birnessite and cryptomelane. Proc. Natl. Acad. Sci. U. S. A. 2017, 114, E1046– E1053,  DOI: 10.1073/pnas.1620427114
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    157
    Thermodynamics of manganese oxides: Sodium, potassium, and calcium birnessite and cryptomelane
    Birkner, Nancy; Navrotsky, Alexandra
    Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (7), E1046-E1053CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Manganese oxides with layer and tunnel structures occur widely in nature and inspire technol. applications. Having variable compns., these structures often are found as small particles (nanophases). This study explores, using exptl. thermochem., the role of compn., oxidn. state, structure, and surface energy in the their thermodn. stability. The measured surface energies of cryptomelane, sodium birnessite, potassium birnessite and calcium birnessite are all significantly lower than those of binary manganese oxides (Mn3O4, Mn2O3, and MnO2), consistent with added stabilization of the layer and tunnel structures at the nanoscale. Surface energies generally decrease with decreasing av. manganese oxidn. state. A stabilizing enthalpy contribution arises from increasing counter-cation content. The formation of cryptomelane from birnessite in contact with aq. soln. is favored by the removal of ions from the layered phase. At large surface area, surface-energy differences make cryptomelane formation thermodynamically less favorable than birnessite formation. In contrast, at small to moderate surface areas, bulk thermodn. and the energetics of the aq. phase drive cryptomelane formation from birnessite, perhaps aided by oxidn.-state differences. Transformation among birnessite phases of increasing surface area favors compns. with lower surface energy. These quant. thermodn. findings explain and support qual. observations of phase-transformation patterns gathered from natural and synthetic manganese oxides.
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  158. 158
    Hem, J. D.; Lind, C. J. Nonequilibrium models for predicting forms of precipitated manganese oxides. Geochim. Cosmochim. Acta 1983, 47, 2037– 2046,  DOI: 10.1016/0016-7037(83)90219-3
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    158
    Nonequilibrium models for predicting forms of precipitated manganese oxides
    Hem, John D.; Lind, Carol J.
    Geochimica et Cosmochimica Acta (1983), 47 (11), 2037-46CODEN: GCACAK; ISSN:0016-7037.
    Mn oxides pptd. by bubbling air through 0.01 M solns. of MnCl2, Mn(NO3)2, MnSO4, or Mn(ClO4)2 at a constantly maintained pH of 8.5-9.5 at >25° consisted mainly of hausmannite [1309-55-3], Mn3O4. At ∼0°, but with other conditions the same, the product is feitknechtite [12195-46-9], β-MnOOH, except that if the initial soln. is MnSO4 and the temp. is ∼0° the product is a mixt. of manganite [1310-98-1] (λ-MnOOH) and groutite [12025-98-8] (α-MnOOH). All these oxides are metastable in aerated soln. and alter by irreversible processes to more highly oxidized species during aging. A 2-step non-equil. thermodn. model predicts that the least stable species, β-MnOOH, should be most readily converted to MnO2. Some prepns. of β-MnOOH aged in their native soln. at 5° attained a Mn oxidn. state of ≥3.3 after 7 mo. Hausmannite aged at 25° altered to λ-MnOOH. The latter is more stable than α- or β-MnOOH, and Mn oxidn. states >3.0 were not reached in hausmannite ppts. during 4 mo of aging. Initial pptn. of MnCO3 rather than a form of oxide is likely only where O availability is very low. Compn. of solns. and oxidn. state and morphol. of solids were detd. during the aging process by chem. analyses, x-ray and electron diffraction, and transmission electron micrographs.
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  159. 159
    Shaughnessy, D. A.; Nitsche, H.; Booth, C. H.; Shuh, D. K.; Waychunas, G. A.; Wilson, R. E.; Gill, H.; Cantrell, K. J.; Serne, R. J. Molecular interfacial reactions between Pu(VI) and manganese oxide minerals Manganite and hausmannite. Environ. Sci. Technol. 2003, 37, 3367– 3374,  DOI: 10.1021/es025989z
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    159
    Molecular Interfacial Reactions between Pu(VI) and Manganese Oxide Minerals Manganite and Hausmannite
    Shaughnessy, D. A.; Nitsche, H.; Booth, C. H.; Shuh, D. K.; Waychunas, G. A.; Wilson, R. E.; Gill, H.; Cantrell, K. J.; Serne, R. J.
    Environmental Science and Technology (2003), 37 (15), 3367-3374CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The sorption of Pu(VI) onto manganite (MnOOH) and hausmannite (Mn3O4) was studied as a function of time, soln. pH, and initial Pu concn. Kinetic expts. indicate that the surface complexation of Pu occurs over the 1st 24 h of contact with the mineral surface. The sorption increases with pH beginning at pH 3 until it reaches a max. value of 100% at pH 8 (0.0011-0.84 μmol of Pu/m2 of manganite and 0.98-1.2 μmol of Pu/m2 of hausmannite) and then decreases over the pH range from 8 to 10. The ratio of solid to soln. was 10 mg/mL for manganite expts. and 4 mg/mL for hausmannite samples. Carbonate was not excluded from the expts. The amt. of Pu removed from the soln. by the minerals is detd. by a combination of factors including the Pu soln. species, the surface charge of the mineral, and the mineral surface area. x-ray absorption fine structure taken at the Pu LIII edge were compared to Pu std. spectra and showed that Pu(VI) was reduced to Pu(IV) after contact with the minerals. Pu sorption to the mineral surface is consistent with an inner-sphere configuration, and no evidence of PuO2 pptn. is obsd. The redn. and complexation of Pu(VI) by Mn minerals has direct implications on possible migration of Pu(VI) species in the environment.
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  160. 160
    Tebo, B. M.; Bargar, J. R.; Clement, B. G.; Dick, G. J.; Murray, K. J.; Parker, D.; Verity, R.; Webb, S. M. Biogenic manganese oxides: Properties and mechanisms of formation. Annu. Rev. Earth Planet. Sci. 2004, 32, 287– 328,  DOI: 10.1146/annurev.earth.32.101802.120213
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    Biogenic manganese oxides: Properties and mechanisms of formation
    Tebo, Bradley M.; Bargar, John R.; Clement, Brian G.; Dick, Gregory J.; Murray, Karen J.; Parker, Dorothy; Verity, Rebecca; Webb, Samuel M.
    Annual Review of Earth and Planetary Sciences (2004), 32 (), 287-328, 2 platesCODEN: AREPCI; ISSN:0084-6597. (Annual Reviews Inc.)
    A review. Manganese(IV) oxides produced through microbial activity, i.e., biogenic Mn oxides or Mn biooxides, are believed to be the most abundant and highly reactive Mn oxide phases in the environment. They mediate redox reactions with org. and inorg. compds. and sequester a variety of metals. The major pathway for bacterial Mn(II) oxidn. is enzymic, and although bacteria that oxidize Mn(II) are phylogenetically diverse, they require a multicopper oxidase-like enzyme to oxidize Mn(II). The oxidn. of Mn(II) to Mn(IV) occurs via a sol. or enzyme-complexed Mn(III) intermediate. The primary Mn(IV) biooxide formed is a phyllomanganate most similar to δ-MnO2 or acid birnessite. Metal sequestration by the Mn biooxides occurs predominantly at vacant layer octahedral sites.
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  161. 161
    Yin, H.; Tan, W.; Zhang, L.; Cui, H.; Qiu, G.; Liu, F.; Feng, X. Characterization of Ni-rich hexagonal birnessite and its geochemical effects on aqueous Pb2+/Zn2+ and As(III). Geochim. Cosmochim. Acta 2012, 93, 47– 62,  DOI: 10.1016/j.gca.2012.05.039
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    161
    Characterization of Ni-rich hexagonal birnessite and its geochemical effects on aqueous Pb2+/Zn2+ and As(III)
    Yin, Hui; Tan, Wenfeng; Zheng, Lirong; Cui, Haojie; Qiu, Guohong; Liu, Fan; Feng, Xionghan
    Geochimica et Cosmochimica Acta (2012), 93 (), 47-62CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Hexagonal birnessite is the most ubiquitous manganese oxide in geol. environments. It is often highly enriched in trace metal ions such as Ni and plays an important role in metal(loids) geochem. Nanostructured birnessites contg. different amts. of Ni were synthesized by addn. of Ni2+ to initial reactants. Powder X-ray diffraction (XRD), element anal., field emission SEM (FE-SEM), XPS, thermogravimetric anal. (TGA), X-ray absorption spectroscopy (XAS) and isothermal adsorption and oxidn. of metal(loids) were carried out to investigate the effects of Ni doping on the substructure and physicochem. properties of birnessite, and Ni crystal chem. in birnessite. These Ni-rich birnessites have Ni contents as high as 2.99% (Ni5) and 6.08% (Ni10) in wt. EXAFS results show that Ni5 has 23.7% of the total Ni (0.71 wt.%) and Ni10 has 34.5% of the total Ni (2.10 wt.%) in Mn octahedral layer with the remaining Ni located at vacancies and edge sites. The Ni-rich birnessites have weaker crystallinity and thermal stability, fewer layers stacked along the c axis, ∼1.5-2.7 times larger surfaces areas, and a higher Mn av. oxidn. nos. (AONs) compared to the birnessite without Ni. Addnl., the doping of Ni during birnessite crystn. enhances the formation of vacancies in the layer; however, adsorption capacities for Pb2+ and Zn2+ by these Ni-rich birnessites are reduced, mainly because of vacancies and edge sites occupation by a large amt. of Ni. The Ni-rich birnessites exhibit much higher oxidn. capability and can completely oxidize As(III) in soln. at rapid initial reaction rates under the exptl. condition. The results indicate that incorporation of Ni into the natural birnessite in ferromanganese nodules may be achieved both by direct copptn. with Mn to build the layers and migration over time from adsorbed Ni on the surface into the layer structure. It is also implied that Ni doping in birnessite has great impact on the geochem. behaviors of heavy metals, either in adsorption or oxidn. reactions.
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  162. 162
    Li, H.; Bolscher, T.; Winnick, M.; Tfaily, M. M.; Cardon, Z. G.; Keiluweit, M. Simple Plant and Microbial Exudates Destabilize Mineral-Associated Organic Matter via Multiple Pathways. Environ. Sci. Technol. 2021, 55 (5), 3389– 3398,  DOI: 10.1021/acs.est.0c04592
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    162
    Simple Plant and Microbial Exudates Destabilize Mineral-Associated Organic Matter via Multiple Pathways
    Li, Hui; Bolscher, Tobias; Winnick, Matthew; Tfaily, Malak M.; Cardon, Zoe G.; Keiluweit, Marco
    Environmental Science & Technology (2021), 55 (5), 3389-3398CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Most mineral-assocd. org. matter (MAOM) is protected against microbial attack, thereby contributing to long-term carbon storage in soils. However, the extent to which reactive compds. released by plants and microbes may destabilize MAOM and so enhance microbial access, as well as the underlying mechanisms, remain unclear. Here, we tested the ability of functionally distinct model exudates-ligands, reductants, and simple sugars-to promote microbial utilization of monomeric MAOM, bound via outer-sphere complexes to common iron and aluminum (hydr)oxide minerals. The strong ligand oxalic acid induced rapid MAOM mineralization, coinciding with greater sorption to and dissoln. of minerals, suggestive of direct MAOM mobilization mechanisms. In contrast, the simple sugar glucose caused slower MAOM mineralization, but stimulated microbial activity and metabolite prodn., indicating an indirect microbially-mediated mechanism. Catechol, acting as reductant, promoted both mechanisms. While MAOM on ferrihydrite showed the greatest vulnerability to both direct and indirect mechanisms, MAOM on other (hydr)oxides was more susceptible to direct mechanisms. These findings suggest that MAOM persistence, and thus long-term carbon storage within a given soil, is not just a function of mineral reactivity but also depends on the capacity of plant roots and assocd. microbes to produce reactive compds. capable of triggering specific destabilization mechanisms.
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  163. 163
    Chen, C.; Hall, S. J.; Coward, E.; Thompson, A. Iron-mediated organic matter decomposition in humid soils can counteract protection. Nat. Commun. 2020, 11 (1), 2255,  DOI: 10.1038/s41467-020-16071-5
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    Iron-mediated organic matter decomposition in humid soils can counteract protection
    Chen, Chunmei; Hall, Steven J.; Coward, Elizabeth; Thompson, Aaron
    Nature Communications (2020), 11 (1), 2255CODEN: NCAOBW; ISSN:2041-1723. (Nature Research)
    Abstr.: Soil org. matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe also promotes SOM decompn. when oxygen (O2) becomes limited. Here we quantify Fe-mediated OM protection vs. decompn. by adding 13C dissolved org. matter (DOM) and 57FeII to soil slurries incubated under static or fluctuating O2. We find Fe uniformly protects OM only under static oxic conditions, and only when Fe and DOM are added together: de novo reactive FeIII phases suppress DOM and SOM mineralization by 35 and 47%, resp. Conversely, adding 57FeII alone increases SOM mineralization by 8% following oxidn. to 57FeIII. Under O2 limitation, de novo reactive 57FeIII phases are preferentially reduced, increasing anaerobic mineralization of DOM and SOM by 74% and 32-41%, resp. Periodic O2 limitation is common in humid soils, so Fe does not intrinsically protect OM; rather reactive Fe phases require their own physiochem. protection to contribute to OM persistence.
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    Wang, Q.; Yang, P.; Zhu, M. Effects of metal cations on coupled birnessite structural transformation and natural organic matter adsorption and oxidation. Geochim. Cosmochim. Acta 2019, 250, 292– 310,  DOI: 10.1016/j.gca.2019.01.035
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    164
    Effects of metal cations on coupled birnessite structural transformation and natural organic matter adsorption and oxidation
    Wang, Qian; Yang, Peng; Zhu, Mengqiang
    Geochimica et Cosmochimica Acta (2019), 250 (), 292-310CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Birnessite, a layered manganese (Mn) oxide, possesses extraordinary metal adsorption and oxidn. activity, and thus imposes impacts on many biogeochem. processes. The reactivity of birnessite strongly relies on its Mn oxidn. state compn. (the proportions of Mn II, III and IV), particularly by the Mn(III) proportion. Partial redn. of birnessite transforms birnessite to be Mn(II, III)-rich or to MnOOH and Mn3O4, and thus strongly affects birnessite reactivity. As a metal scavenger, naturally occurring birnessite contains abundant transition and alkali and alk. earth metal cations in its structure; however, the effects of these metal cations on the partial redn.-induced transformation of birnessite remain unknown. We examd. the effects of Zn2+, Mg2+, Ca2+ and ionic strength (controlled by NaCl) on transformation of birnessite (δ-MnO2) and adsorption and oxidn. of natural org. matter during partial redn. by fulvic acid (FA) at pH 8 and FA/MnO2 mass ratios (R) of 0.1 or 1 over 600 h under anoxic conditions. Results showed that low ionic strength (0 vs. 50 mM NaCl) disfavored FA adsorption, fractionation and oxidn., and thus disfavored formation of Mn(III) in the reacted birnessite. Compared to the 50 mM NaCl system, all divalent cations (Mg2+, Ca2+ and Zn2+) favored FA adsorption and fractionation. Both Mg2+ and Ca2+ significantly enhanced FA oxidn. at the early stage but barely at the late stage, whereas Zn2+ strongly suppressed FA oxidn. during the entire exptl. period. Due to adsorption competition, the presence of the divalent cations resulted in low concn. of Mn(II) adsorbed on vacancies of birnessite. Both Ca2+ and Mg2+ favored Mn(III) prodn. in MnO6 layers, while Zn2+ inhibited it. A small portion of birnessite also transformed to feitknechtite and hausmannite, and the transformation seemed faster in the presence of Ca2+ or Mg2+ than in NaCl soln. In the presence of Zn2+ at the high FA/MnO2 ratio (R = 1), Zn-substituted hansmannite formed extenstively. The formation of Mn(III) in the reacted birnessite can be ascribed to comproportionation between Mn(IV) and Mn(II) adsorbed on either vacancies or edge sites of birnessite. The low-valence Mn oxide phases likely formed via the comproportionation on the edges. The divalent cations affected Mn(III) concns. of birnessite and formation of the low-valence Mn oxides by competing with Mn(II) for adsorption on edge/vacancy sites or stabilizing Mn(III) in the layers. This work indicates that divalent metal cations strongly influence reactivity and transformation of birnessite in the coupled Mn and carbon redox cycles, and that birnessite contg. divalent cations can be an important adsorbent for natural org. carbon in Mn-rich environments. Overall, this study provides insights into the coupled cycles of Mn, trace metals and org. carbon in alk. and saline environments.
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    Villalobos, M.; Bargar, J.; Sposito, G. Mechanisms of Pb(II) sorption on a biogenic manganese oxide. Environ. Sci. Technol. 2005, 39, 569– 576,  DOI: 10.1021/es049434a
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    Mechanisms of Pb(II) Sorption on a Biogenic Manganese Oxide
    Villalobos, Mario; Bargar, John; Sposito, Garrison
    Environmental Science and Technology (2005), 39 (2), 569-576CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Macroscopic Pb2+ uptake expts. and Pb L3-edge extended X-ray absorption fine structure (EXAFS) spectroscopy were combined to examine mechanisms of Pb2+ sequestration by a biogenic manganese oxide and its synthetic analogs, all of which are layer-type manganese oxides (phyllomanganates). Relatively fast Pb2+ sorption was obsd. as was extremely high sorption capacity, suggesting Pb incorporation into the oxide structure. EXAFS anal. showed similar uptake mechanisms regardless of the specific nature of the phyllomanganate, electrolyte background, total Pb2+ load, or equilibration time. One Pb-O and two Pb-Mn shells at distances of 2.30, 3.53, and 3.74 Å, resp., were obsd. as was a linear relationship between Brunauer-Emmett-Teller (BET; i.e., external) sp. surface area and max. Pb2+ sorption which also encompassed data from previous work. Both observations supported the existence of 2 bonding mechanisms in Pb2+ sorption: a triple-corner sharing complex in interlayers above/below cationic sheet vacancies (N theor. = 6), and a double-corner sharing complex on particle edges at exposed, singly-coordinated -O(H) bonds (N theor. = 2). General prevalence of external over internal sorption was predicted, but the 2 simultaneous sorption mechanisms can account for the widely noted high affinity of manganese oxides for Pb2+ in natural environments.
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    Villalobos, M.; Escobar-Quiroz, I. N.; Salazar-Camacho, C. The influence of particle size and structure on the sorption and oxidation behavior of birnessite: I. Adsorption of As(V) and oxidation of As(III). Geochim. Cosmochim. Acta 2014, 125, 564– 581,  DOI: 10.1016/j.gca.2013.10.029
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    166
    The influence of particle size and structure on the sorption and oxidation behavior of birnessite: I. Adsorption of As(V) and oxidation of As(III)
    Villalobos, Mario; Escobar-Quiroz, Ingrid N.; Salazar-Camacho, Carlos
    Geochimica et Cosmochimica Acta (2014), 125 (), 564-581CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Sorption and oxidn. reactions in the environment may affect substantially the mobility of redox-sensitive toxic trace elements and compds. Investigating the environmental factors that influence these reactions is crucial in understanding and predicting the geochem. fate of these environmental species, as well as to design appropriate engineered remediation schemes. Arsenic is a widespread contaminant of concern, esp. in its oxidized forms, and Mn oxide minerals are some of the major contributors to its oxidn. The goal of this work was to investigate the influence of particle size and structural differences of environmentally-relevant Mn(IV) birnessites on the adsorption of As(V) and on the oxidn. of As(III). An acid birnessite of 39 m2/g and a δ-MnO2 of 114 m2/g were used. Both birnessites sorbed a max. Pb(II) of 0.3 Pb/Mn, indicating a significantly larger layer cationic vacancy content for acid birnessite, and a d. of reactive edge sites for both of 12 sites/nm2. As(V) forms a bidentate bridging complex on singly-coordinated surface sites at the birnessite particle edges regardless of loading, pH, birnessite type, and presence of pre-sorbed metals(II). Maximum As(V) adsorption, under repulsive electrostatic pH conditions did not yield adsorption congruency behavior between both birnessites at const. pH, presumably because the increase in internal vacancy content causes neg. electrostatic repulsion towards external As(V) oxyanion binding. At pH 4.5 As(III) oxidn. on birnessites was fast and quant. at As/Mn ratios of 0.3-0.33, the reaction being largely driven by the proton concn. At pH 6 δ-MnO2 oxidized As(III) faster and to a higher extent than acid birnessite, at equal masses; but the reverse at equal total surface areas. The oxidn. driving force (independently from protons) was higher at pH 6 than at pH 4.5 because of Mn(II) product removal by sorption to interlayer vacancies, which overcomes reactive surface site blockage by this species, provided sufficient vacancies are present. Metals(II) pre-sorbed on birnessites always decreased the initial stages of As(III) oxidn. rates as compared to the metal(II)-free systems presumably through site blockage. But after 24 h the Pb(II)-equilibrated birnessites at pH 6 reached equal and sometimes higher oxidn. extents through removal of As(V) from soln., by stabilizing its (binary) adsorption to edge sites through a decrease in electrostatic repulsion by the sorbed Pb(II). This work provides useful insights on the influence of particle size and structure (vacancy content) of birnessite minerals analogous to biogenic Mn oxides relevant to the environment, esp. as it pertains to reactivity towards sorption of Pb(II), Zn(II), and As(V), and to oxidn. of As(III), all of which are significant processes that dictate their transport and fate in aq. geochem. environments.
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  167. 167
    Zhu, M.; Paul, K. W.; Kubicki, J. D.; Sparks, D. L. Quantum chemical study of arsenic(III, V) adsorption on Mn-oxides: Implications for arsenic(III) oxidation. Environ. Sci. Technol. 2009, 43, 6655– 6661,  DOI: 10.1021/es900537e
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    167
    Quantum Chemical Study of Arsenic(III, V) Adsorption on Mn-Oxides: Implications for Arsenic(III) Oxidation
    Zhu, Mengqiang; Paul, Kristian W.; Kubicki, James D.; Sparks, Donald L.
    Environmental Science & Technology (2009), 43 (17), 6655-6661CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    D. functional theory (DFT) calcns. were used to study As(V) and As(III) surface complex structures and reaction energies on both Mn(III) and Mn(IV) sites in an attempt to better understand As(III) oxidn. by birnessite, a layered Mn-dioxide mineral. Edge-sharing dioctahedral Mn(III) and Mn(IV) clusters with different combinations of surface functional groups (>MnOH and >MnOH2) were used to mimic pH variability. Results show that As(V) adsorption was more thermodynamically favorable than As(III) adsorption on both Mn(III) and Mn(IV) surface sites under simulated acidic pH conditions. We propose that As(V) adsorption inhibits As(III) oxidn. by blocking adsorption sites. Under simulated acidic pH conditions, Mn(IV) sites exhibited stronger adsorption affinity than Mn(III) sites for both As(III) and As(V). Overall, we hypothesize that Mn(III) sites are less reactive in terms of As(III) oxidn. due to their lower affinity for As(III) adsorption, higher potential to be blocked by As(V) complexes, and slower electron transfer rates with adsorbed As(III). Results offer an explanation regarding the exptl. observations of Mn(III) accumulation on birnessite and the long residence time of As(III) adsorption complexes on manganite (r-MnOOH) during As(III) oxidn.
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  168. 168
    Zhang, S.; Chen, S.; Liu, F.; Li, J.; Liang, X.; Chu, S.; Xiang, Q.; Huang, C.; Yin, H. Effects of Mn average oxidation state on the oxidation behaviors of As(III) and Cr(III) by vernadite. Appl. Geochem. 2018, 94, 35– 45,  DOI: 10.1016/j.apgeochem.2018.05.002
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    168
    Effects of Mn average oxidation state on the oxidation behaviors of As(III) and Cr(III) by vernadite
    Zhang, Shuang; Chen, Suifeng; Liu, Fan; Li, Jiangshan; Liang, Xiaoliang; Chu, Shengqi; Xiang, Quanjun; Huang, Chuanqing; Yin, Hui
    Applied Geochemistry (2018), 94 (), 35-45CODEN: APPGEY; ISSN:0883-2927. (Elsevier Ltd.)
    Vernadite is a poorly cryst. phyllomanganate that widely distributed in natural environments, and plays a pivotal role in the geochem. transformations of heavy metal and other pollutants. Though many works have done about the reaction mechanisms between vernadite-like minerals and As(III)/Cr(III), the effects of some basic structure characteristics of the mineral, such as Mn av. oxidn. state (AOS), on the oxidn. of As(III)/Cr(III) are not fully understood. In this study, vernadite samples with different Mn AOSs but almost the same particle sizes (Ver and Ver20) were synthesized, characterized, and their reactivities towards As(III)/Cr(III) oxidn. and As(V)/Cr(VI) adsorption were compared. It is found that Ver and Ver20 have almost the same point of zero charge (PZC), but ver20 has increased contents of layer Mn(III) at particle edges and interlayer Mn(III) and Mn(II), thus greatly reduced Mn AOS. Decrease in Mn AOS greatly reduces the oxidn. capacity and initial reaction rate (Kobs) of this mineral towards As(III) on mineral-water interfaces. The apparent As(III) oxidn. amt. and Kobs by Ver are 202 ± 8 mmol kg-1 and 0.3515 min-1 while that by Ver20 are 162 ± 7 mmol kg-1 and 0.0139 min-1. The oxidn. capacity and Kobs for Cr(III) by these vernadie samples are also decreased but in a less extent compared to that in As(III) oxidn.
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  169. 169
    Liu, J.; Zhang, Y.; Gu, Q.; Sheng, A.; Zhang, B., Tunable Mn Oxidation State and Redox Potential of Birnessite Coexisting with Aqueous Mn(II) in Mildly Acidic Environments. Minerals 2020, 10, 690 DOI: 10.3390/min10080690
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    169
    Tunable Mn oxidation state and redox potential of birnessite coexisting with aqueous Mn(II) in mildly acidic environments
    Liu, Juan; Zhang, Yixiao; Gu, Qian; Sheng, Anxu; Zhang, Baogang
    Minerals (Basel, Switzerland) (2020), 10 (8), 690CODEN: MBSIBI; ISSN:2075-163X. (MDPI AG)
    As the dominant manganese oxide mineral phase in terrestrial and aquatic environments, birnessite plays an important role in many biogeochem. processes. The coexistence of birnessite with aq. Mn2+ is commonly found in the subsurface environments undergoing Mn redox cycling. This study investigates the change in Mn av. oxidn. state (AOS) of birnessite after reaction with 0.1-0.4 mM Mn2+ at pH 4.5-6.5, under conditions in which phase transformation of birnessite by Mn2+ was not detectable. The amt. of Mn2+ uptake by birnessite and the equil. concn. of Mn(III) proportionally increased with the initial concn. of Mn2+. The Mn AOS of birnessite particles became 3.87, 3.75, 3.64, and 3.53, resp., after reaction with 0.1, 0.2, 0.3, and 0.4 mM Mn2+ at pH 5.5. Oxidn. potentials (Eh) of birnessite with different AOS values were estd. using the equil. concns. of hydroquinone oxidized by the birnessite samples, indicating that Eh was linearly proportional to AOS. The oxidn. kinetics of bisphenol A (BPA), a model org. pollutant, by birnessite suggest that the logarithms of surface area-normalized pseudo-first-order initial rate consts. (log kSA) for BPA degrdn. by birnessite were linearly correlated with the Eh or AOS values of birnessite with AOS greater than 3.64.
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  170. 170
    Tang, Y.; Webb, S. M.; Estes, E. R.; Hansel, C. M. Chromium(III) oxidation by biogenic manganese oxides with varying structural ripening. Environ. Sci. Process Impacts 2014, 16, 2127– 2136,  DOI: 10.1039/C4EM00077C
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    170
    Chromium(III) oxidation by biogenic manganese oxides with varying structural ripening
    Tang, Yuanzhi; Webb, Samuel M.; Estes, Emily R.; Hansel, Colleen M.
    Environmental Science: Processes & Impacts (2014), 16 (9), 2127-2136CODEN: ESPICZ; ISSN:2050-7895. (Royal Society of Chemistry)
    Manganese (Mn) oxides, which are generally considered biogenic in origin within natural systems, are the only oxidants of Cr(III) under typical environmental conditions. Yet the influence of Mn biooxide mineral structural evolution on Cr(III) oxidn. under varying geochem. conditions is unknown. In this study we examd. the role of light, org. carbon, pH, and the structure of biogenic Mn oxides on Cr(III) oxidn. Aging of Mn oxides produced by a marine bacterium within the widespread Roseobacter clade resulted in structural ripening from a colloidal hexagonal to a particulate triclinic birnessite phase. The structurally diverse Mn oxides were then reacted with aq. Cr(III) within artificial seawater in the presence or absence of carbon and light. Here we found that Cr(III) oxidn. capacity was highest at near neutral pH and in the combined presence of carbon and light. Mn oxide ripening from a hexagonal to a triclinic birnessite phase led to decreased Cr(III) oxidn. in the presence of carbon and light, whereas no change in reactivity was obsd. in the absence of carbon and/or in the dark. As only minimal Cr(III) oxidn. was obsd. in the absence of Mn oxides, these results strongly point to coupled Mn oxide- and photo-induced generation of org. and/or oxygen radicals involved in Cr(III) oxidn. Based on Mn oxide concn. and structural trends, we postulate that Mn(II) produced from the oxidn. of Cr(III) by the primary Mn oxide is recycled in the presence of orgs. and light conditions, (re)generating secondary hexagonal birnessite and thereby allowing for continuous oxidn. of Cr(III). In the absence of this Mn oxide regeneration, Cr(III) induced structural ripening of the hexagonal birnessite precludes further Cr(III) oxidn. These results highlight the complexity of reactions involved in Mn oxide mediated Cr(III) oxidn. and suggest that photochem. carbon reactions are requisite for sustained Cr(III) oxidn. and persistence of reactive Mn oxides.
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  171. 171
    Huang, J. Z. S.; Dai, Y.; Liu, C.-C.; Zhang, H. Effect of MnO2 phase structure on the oxidative reactivity toward bisphenol A degradation. Environ. Sci. Technol. 2018, 52, 11309– 11318,  DOI: 10.1021/acs.est.8b03383
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    171
    Effect of MnO2 Phase Structure on the Oxidative Reactivity toward Bisphenol A Degradation
    Huang, Jianzhi; Zhong, Shifa; Dai, Yifan; Liu, Chung-Chiun; Zhang, Huichun
    Environmental Science & Technology (2018), 52 (19), 11309-11318CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Manganese dioxides (MnO2) are among important environmental oxidants in contaminant removal; however, most existing work has only focused on naturally abundant MnO2. We herein report the effects of different phase structures of synthetic MnO2 on their oxidative activity with regard to contaminant degrdn. Bisphenol A (BPA), a frequently detected contaminant in the environment, was used as a probe compd. A total of eight MnO2 with five different phase structures (α-, β-, γ-, δ-, and λ-MnO2) were successfully synthesized with different methods. The oxidative reactivity of MnO2, as quantified by pseudo-first-order rate consts. of BPA oxidn., followed the order of δ-MnO2-1 > δ-MnO2-2 > α-MnO2-1 > α-MnO2-2 ≈ γ-MnO2 > λ-MnO2 > β-MnO2-2 > β-MnO2-1. Extensive characterization was then conducted for MnO2 crystal structure, morphol., surface area, redn. potential, cond., and surface Mn oxidn. states and oxygen species. The results showed that the MnO2 oxidative reactivity correlated highly pos. with surface Mn(III) content and neg. with surface Mn av. oxidn. state but correlated poorly with all other properties. This indicates that surface Mn(III) played an important role in MnO2 oxidative reactivity. For the same MnO2 phase structure synthesized by different methods, higher surface area, redn. potential, cond., or surface adsorbed oxygen led to higher reactivity, suggesting that these properties play a secondary role in the reactivity. These findings provide general guidance for designing active MnO2 for cost-effective water and wastewater treatment.
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  172. 172
    Zhang, Y. Y. Y.; Zhang, Y.; Zhang, T.; Ye, M. Heterogeneous oxidation of naproxen in the presence of α-MnO2 nanostructures with different morphologies. Appl. Catal., B 2012, 127, 182– 189,  DOI: 10.1016/j.apcatb.2012.08.014
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    172
    Heterogeneous oxidation of naproxen in the presence of α-MnO2 nanostructures with different morphologies
    Zhang, Yiping; Yang, Yulong; Zhang, Yan; Zhang, Tuqiao; Ye, Miaomiao
    Applied Catalysis, B: Environmental (2012), 127 (), 182-189CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    α-MnO2 nanostructures with different morphologies including nanoparticles, flower-like nanostructures and nanorods have been successfully prepd. and used in the heterogeneous oxidn. of naproxen in water. It has been found out that the heterogeneous oxidn. process is highly pH dependent, with higher efficiency at lower pH values. The oxidn. kinetics of naproxen were modeled by Langmuir-Hinshelwood equations. Based on the kinetic consts. (k), the oxidn. efficiency follows the order of com. particles < nanorods < flower-like nanostructures < nanoparticles. The mechanism for the oxidn. of naproxen has been studied in depth, which shows that the high efficiency can be ascribed to the specific adsorption, electron transfer, and byproducts release. Dissolved anions (Cl-, CO32-, SO42-, PO43-) and cations (Mn2+) could remarkably decrease the removal rate by competitively adsorbing and reacting with MnO2, resp. In addn., a total of 7 byproducts were identified by LC-MS from which a tentative pathway was proposed.
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  173. 173
    Balgooyen, S.; Alaimo, P. J.; Remucal, C. K.; Ginder-Vogel, M. Structural Transformation of MnO2 during the Oxidation of Bisphenol A. Environ. Sci. Technol. 2017, 51 (11), 6053– 6062,  DOI: 10.1021/acs.est.6b05904
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    173
    Structural Transformation of MnO2 during the Oxidation of Bisphenol A
    Balgooyen, Sarah; Alaimo, Peter J.; Remucal, Christina K.; Ginder-Vogel, Matthew
    Environmental Science & Technology (2017), 51 (11), 6053-6062CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Bisphenol A (BPA) is an endocrine-disrupting compd. widely used in the plastic industry and found in natural waters at concns. considered harmful for aquatic life. BPA is susceptible to oxidn. by Mn(III/IV) oxides, which are commonly found in near-surface environments. Here, we quantify BPA oxidn. rates and the formation of its predominant product, 4-hydroxycumyl alc. (HCA), in tandem with transformation of a synthetic, Mn(III)-rich δ-MnO2. To investigate the effect of Mn oxide structural changes on BPA oxidn. rate, 12 sequential addns. of 80 μM BPA are performed at pH 7. During the addns., BPA oxidn. rate decreases by 3 orders of magnitude, and HCA yield decreases from 40% to 3%. This is attributed to the accumulation of interlayer Mn(II/III) produced during the reaction, as obsd. using X-ray absorption spectroscopy, as well as addnl. spectroscopic and wet chem. techniques. HCA is oxidized at a rate that is 12.6 times slower than BPA and accumulates in soln. These results demonstrate that BPA degrdn. by environmentally relevant Mn(III/IV) oxides is inhibited by the buildup of solid-phase Mn(II/III), specifically in interlayer sites. Nevertheless, Mn oxides may limit BPA migration in near-surface environments and have potential for use in drinking and wastewater treatment.
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  174. 174
    Gulley-Stahl, H.; Hogan II, P. A.; Schmidt, W. L.; Wall, S. J.; Buhrlage, A.; Bullen, H. A. Surface complexation of catechol to metal oxides: An ATR-FTIR, adsorption. and dissolution study. Environ. Sci. Technol. 2010, 44, 4116– 4121,  DOI: 10.1021/es902040u
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    174
    Surface Complexation of Catechol to Metal Oxides: An ATR-FTIR, Adsorption, and Dissolution Study
    Gulley-Stahl, Heather; Hogan, Patrick A., II; Schmidt, Whitney L.; Wall, Stephen J.; Buhrlage, Andrew; Bullen, Heather A.
    Environmental Science & Technology (2010), 44 (11), 4116-4121CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The interaction of catechol with chromium(III) oxide (Cr2O3), manganese dioxide (MnO2), iron(III) oxide (Fe2O3), and titanium dioxide (TiO2) was evaluated as a function of pH conditions (pH 3-10) and ionic strength using a combined approach of bulk adsorption, attenuated total reflectance FTIR spectroscopy (ATR-FTIR), and dissoln. anal. Adsorption of catechol showed a strong pH-dependent behavior with the metal oxides, remaining const. under acidic-neutral pH (3-7) and increasing under more basic conditions. In situ ATR-FTIR measurements indicate that catechol binds predominately as an outer-sphere complex on MnO2 and as an inner-sphere complex on Fe2O3, TiO2, and Cr2O3 substrates. Catechol complexation on Fe2O3, TiO2, and Cr2O3 promotes dissoln. at pH >5, whereas MnO2 dissoln. occurs under acidic and basic conditions (pH 3-10).
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    Johnson, S. B.; Yoon, T. H.; Kocar, B. D.; Brown, G. E., Jr. Adsorption of organic matter at mineral/water interfaces. 2. Outersphere adsorption of maleate and implications for dissolution processes. Langmuir 2004, 20, 4996– 5006,  DOI: 10.1021/la036288y
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    Adsorption of Organic Matter at Mineral/Water Interfaces. 2. Outer-Sphere Adsorption of Maleate and Implications for Dissolution Processes
    Johnson, Stephen B.; Yoon, Tae Hyun; Kocar, Benjamin D.; Brown, Gordon E., Jr.
    Langmuir (2004), 20 (12), 4996-5006CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
    The effects of the adsorption of a simple dicarboxylate low mol. wt. org. anion, maleate, on the dissoln. of a model aluminum oxide, corundum (α-Al2O3), have been examd. over a range of different maleate concns. (0.125-5.0 mM) and pH conditions (2-10). In situ attenuated total reflectance Fourier transform IR (ATR-FTIR) spectroscopic measurements indicate that maleate binds predominantly as an outer-sphere, fully deprotonated complex (≡AlOH2+---Mal2-) at the corundum surface over the entire range of maleate concns. and pH conditions investigated. In accordance with the ATR-FTIR findings, macroscopic adsorption data can be modeled as a function of maleate concn. and pH using an extended const. capacitance approach and a single ≡AlOH2+---Mal2- species. Outer-sphere adsorption of maleate is found to significantly reduce the protolytic dissoln. rate of corundum under acidic conditions (pH < 5). A likely mechanism involves steric protection of dissoln.-active surface sites, whereby strong outer-sphere interactions with maleate hinder attack on those surface sites by dissoln.-promoting species.
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    Gao, J.; Hedman, C.; Liu, C.; Guo, T.; Pedersen, J. A. Transformation of sulfamethazine by manganese oxide in aqueous solution. Environ. Sci. Technol. 2012, 46, 2642– 2651,  DOI: 10.1021/es202492h
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    Transformation of Sulfamethazine by Manganese Oxide in Aqueous Solution
    Gao, Juan; Hedman, Curtis; Liu, Cun; Guo, Tan; Pedersen, Joel A.
    Environmental Science & Technology (2012), 46 (5), 2642-2651CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    The transformation of the sulfonamide antimicrobial sulfamethazine (SMZ) by a synthetic analog of vernadite (δ-MnO2) was studied. The obsd. pseudo-first-order reaction consts. (kobs) decreased as the pH increased from 4.0 to 5.6, consistent with the decline in δ-MnO2 redn. potential with increasing pH. Mol. oxygen accelerated SMZ transformation by δ-MnO2 and influenced the transformation product distribution. Increases in the Na+ concn. produced declines in kobs. Transformation products identified by tandem mass spectrometry and the use of 13C-labeled SMZ included an azo dimer self-coupling product and SO2 extrusion products. Product anal. and d. functional theory calcns. are consistent with surface precursor complex formation followed by single-electron transfer from SMZ to δ-MnO2 to produce SMZ radical species. Sulfamethazine radicals undergo further transformation by at least two pathways: radical-radical self-coupling or a Smiles-type rearrangement with O addn. and then extrusion of SO3. Expts. conducted in H218O or in the presence of 18O2(aq) demonstrated that oxygen both from the lattice of as-synthesized δ-MnO2 and initially present as dissolved oxygen reacted with SMZ. The study results suggest that the oxic state and pH of soil and sediment environments can be expected to influence manganese oxide-mediated transformation of sulfonamide antimicrobials.
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  177. 177
    McBride, M. B. Adsorption and Oxidation of Phenolic Compounds by Iron and Manganese Oxides. Soil Sci. Soc. Am. J. 1987, 51 (6), 1466– 1472,  DOI: 10.2136/sssaj1987.03615995005100060012x
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    177
    Adsorption and oxidation of phenolic compounds by iron and manganese oxides
    McBride, M. B.
    Soil Science Society of America Journal (1987), 51 (6), 1466-72CODEN: SSSJD4; ISSN:0361-5995.
    The adsorption and oxidn. of catechol and hydroquinone by Fe and Mn oxides has been investigated by Fourier transform IR spectroscopic (FTIR) anal. of the adsorbed mols. and by the measurement of O2 consumption by aq. suspensions of these oxides. Evidence for direct coordination of catechol and salicylate to surface Fe3+ on iron oxides was obtained by FTIR. The promotion of catechol and hydroquinone oxidn. by Fe and Mn oxides was confirmed by measured rates of O2 consumption and by the appearance of Fe2+ and Mn2+ in the solns. However, only trace levels of sol. Fe2+ were detected, suggesting that oxidn. by Fe(III) oxides was catalytic in that electron transfer between the phenols and Fe3+ generated Fe2+, which was rapidly reoxidized by O2. Other adsorbates introduced into these oxide/phenol systems, such as acetate, phosphate, and Cu2+, diminished O2 consumption rates, but the effect was generally attributable to a lowered pH that inhibited oxidn. A model of surface oxidn. by Mn and Fe is presented in which coordination of the org. at the surface is a prerequisite to electron transfer. Oxidn. of orgs. can proceed with or without the uptake of O2, depending largely on pH, which dets. the rate of reoxidn. of the reduced metal ions by O2. The results emphasize the difficulty in interpreting the effects that chem. buffers have on oxidn. reactions at oxide surfaces.
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    Nowack, B.; Stone, A. T. Homogeneous and heterogeneous oxidation of nitrilotrismethylenephosphonic acid (NTMP) in the presence of manganese(II, III) and molecular oxygen. J. Phys. Chem. B 2002, 106, 6227– 6233,  DOI: 10.1021/jp014293+
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    178
    Homogeneous and Heterogeneous Oxidation of Nitrilotrismethylenephosphonic Acid (NTMP) in the Presence of Manganese(II, III) and Molecular Oxygen
    Nowack, Bernd; Stone, Alan T.
    Journal of Physical Chemistry B (2002), 106 (24), 6227-6233CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    In the absence of oxygen, NTMP (nitrilotrismethylenephosphonic acid) is oxidized by aq. suspensions of the MnIII-contg. mineral manganite (MnOOH), yielding iminodimethylenephosphonic acid (IDMP) as the sole phosphonate-contg. product, along with orthophosphate and two equiv. of MnII. Hence, both C-P and C-N bond cleavage take place. The reaction proceeds via formation of a MnIII-NTMP surface complex and oxidn. of NTMP by MnIII via a carbon-centered methylene radical. MnII strongly inhibits the reaction, presumably by competition for available surface sites. Homogeneous NTMP oxidn. in MnII- and O2-contg. solns. yields IDMP, formate, phosphate, and hydrogen peroxide. Interception of the methylene radical by O2 leads to N-formyl-IDMP and a second not-identified product. The dual presence of MnOOH and O2 yields autocatalytic NTMP oxidn., owing to the generation of dissolved MnII. Although the ultimate oxidants in the MnII-NTMP-O2 and MnOOH-NTMP systems are different (O2 vs. MnIII), both oxidn. reactions begin with C-N bond cleavage. This finding supports the hypothesis that MnIII is generated in the MnII-NTMP-O2 system.
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  179. 179
    Hinkle, M. A. G.; Flynn, E. D.; Catalano, J. G. Structural response of phyllomanganates to wet aging and aqueous Mn(II). Geochim. Cosmochim. Acta 2016, 192, 220– 234,  DOI: 10.1016/j.gca.2016.07.035
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    179
    Structural response of phyllomanganates to wet aging and aqueous Mn(II)
    Hinkle, Margaret A. G.; Flynn, Elaine D.; Catalano, Jeffrey G.
    Geochimica et Cosmochimica Acta (2016), 192 (), 220-234CODEN: GCACAK; ISSN:0016-7037. (Elsevier Ltd.)
    Naturally occurring Mn(IV/III) oxides are often formed through microbial Mn(II) oxidn., resulting in reactive phyllomanganates with varying Mn(IV), Mn(III), and vacancy contents. Residual aq. Mn(II) may adsorb in the interlayer of phyllomanganates above vacancies in their octahedral sheets. The potential for interlayer Mn(II)-layer Mn(IV) comproportionation reactions and subsequent formation of structural Mn(III) suggests that aq. Mn(II) may cause phyllomanganate structural changes that alters mineral reactivity or trace metal scavenging. Here we examine the effects of aging phyllomanganates with varying initial vacancy and Mn(III) content in the presence and absence of dissolved Mn(II) at pH 4 and 7. Three phyllomanganates were studied: two exhibiting turbostratic layer stacking (δ-MnO2 with high vacancy content and hexagonal birnessite with both vacancies and Mn(III) substitutions) and one with rotationally ordered layer stacking (triclinic birnessite contg. predominantly Mn(III) substitutions). Structural analyses suggest that during aging at pH 4, Mn(II) adsorbs above vacancies and promotes the formation of phyllomanganates with rotationally ordered sheets and mixed symmetries arranged into supercells, while structural Mn(III) undergoes disproportionation. These structural changes at pH 4 correlate with reduced Mn(II) uptake onto triclinic and hexagonal birnessite after 25 days relative to 48 h of reaction, indicating that phyllomanganate reactivity decreases upon aging with Mn(II), or that recrystn. processes involving Mn(II) uptake occur over 25 days. At pH 7, Mn(II) adsorbs and causes limited structural effects, primarily increasing sheet stacking in δ-MnO2. These results show that aging-induced structural changes in phyllomanganates are affected by aq. Mn(II), pH, and initial solid-phase Mn(III) content. Such restructuring likely alters manganese oxide reactions with other constituents in environmental and geol. systems, particularly trace metals and redox-active compds.
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  180. 180
    Yang, P.; Post, J. E.; Wang, Q.; Xu, W.; Geiss, R.; McCurdy, P. R.; Zhu, M. Metal adsorption controls stability of layered manganese oxides. Environ. Sci. Technol. 2019, 53, 7453– 7462,  DOI: 10.1021/acs.est.9b01242
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    180
    Metal Adsorption Controls Stability of Layered Manganese Oxides
    Yang, Peng; Post, Jeffrey E.; Wang, Qian; Xu, Wenqian; Geiss, Roy; McCurdy, Patrick R.; Zhu, Mengqiang
    Environmental Science & Technology (2019), 53 (13), 7453-7462CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Hexagonal birnessite, a typical layered Mn oxide (LMO), can adsorb and oxidize Mn(II) and thereby transform to Mn(III)-rich hexagonal birnessite, triclinic birnessite, or tunneled Mn oxides (TMOs), remarkably changing the environmental behavior of Mn oxides. We have detd. the effects of coexisting cations on the transformation by incubating Mn(II)-bearing δ-MnO2 at pH 8 under anoxic conditions for 25 d (dissolved Mn < 11 μM). In the Li+, Na+, and K+ chloride solns., the Mn(II)-bearing δ-MnO2 first transforms to Mn(III)-rich δ-MnO2 or triclinic birnessite (T-bir) due to the Mn(II)-Mn(IV) comproportionation, most of which eventually transform to a 4 × 4 TMO. In contrast, Mn(III)-rich δ-MnO2 and T-bir form and persist in the Mg2+ and Ca2+ chloride solns. However, in the presence of surface adsorbed Cu(II), Mn(II)-bearing δ-MnO2 turns into Mn(III)-rich δ-MnO2 without forming T-bir or TMOs. The stabilizing power of the cations on the δ-MnO2 structure pos. correlates with their binding strength to δ-MnO2 (Li+, Na+, and K+ < Mg2+ and Ca2+ < Cu(II)). Since metal adsorption decreases the surface energy of minerals, our finding suggests that the surface energy largely controls the thermodn. stability of LMOs. Our study indicates that the adsorption of divalent metal cations, particularly transition metals, can be an important cause of the high abundance of LMOs, rather than the more stable TMO phases, in the environment.
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  181. 181
    Eusterhues, K.; Rumpel, C.; Kleber, M.; Kögel-Knabner, I. Stabilisation of soil organic matter by interactions with minerals as revealed by mineral dissolution and oxidative degradation. Org. Geochem. 2003, 34 (12), 1591– 1600,  DOI: 10.1016/j.orggeochem.2003.08.007
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    181
    Stabilisation of soil organic matter by interactions with minerals as revealed by mineral dissolution and oxidative degradation
    Eusterhues, Karin; Rumpel, Cornelia; Kleber, Markus; Kogel-Knabner, Ingrid
    Organic Geochemistry (2003), 34 (12), 1591-1600CODEN: ORGEDE; ISSN:0146-6380. (Elsevier Ltd.)
    Soil org. matter is known to contain a stable fraction with an old radiocarbon age. Size and stabilization processes leading to the formation of this old soil carbon pool are still unclear. Study was done to differentiate old org. matter from young and labile carbon compds. in two acid forest soils (dystric cambisol, haplic podzol) of Bavaria, Germany. To identify such fractions soil samples were exposed to oxidn. with Na2S2O8 and to dissoln. by HF. A neg. correlation between 14C activity and carbon release after dissoln. of the mineral matrix by HF indicates a strong assocn. of stabilized carbon compds. with the mineral phase. A neg. correlation between the 14C activity and the relative proportion of carbon resistant to oxidn. by Na2S2O8 shows that young carbon is removed preferentially by this treatment. The fraction remaining after oxidn. represents a certain stabilized, long residence time carbon pool. This old fraction comprises between 1 and 30% of the total soil org. carbon in the surface horizons, but reaches up to 80% in the sub-surface horizons. Old OC is mainly stabilized by organo-mineral assocns. with clay minerals and/or iron oxides, whereas intercalation in clay minerals was not found to be important.
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  182. 182
    Mikutta, R.; Kleber, M.; Torn, M. S.; Jahn, R. Stabilization of Soil Organic Matter: Association with Minerals or Chemical Recalcitrance?. Biogeochemistry 2006, 77 (1), 25– 56,  DOI: 10.1007/s10533-005-0712-6
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    182
    Stabilization of Soil Organic Matter: Association with Minerals or Chemical Recalcitrance?
    Mikutta, Robert; Kleber, Markus; Torn, Margaret S.; Jahn, Reinhold
    Biogeochemistry (2006), 77 (1), 25-56CODEN: BIOGEP; ISSN:0168-2563. (Springer)
    Soil org. matter (OM) can be stabilized against decompn. by assocn. with minerals, by its inherent recalcitrance and by occlusion in aggregates. However, the relative contribution of these factors to OM stabilization is yet unknown. We analyzed pool size and isotopic compn. (14C, 13C) of mineral-protected and recalcitrant OM in 12 subsurface horizons from 10 acidic forest soils. The results were related to properties of the mineral phase and to OM compn. as revealed by CPMAS 13C-NMR and CuO oxidn. Stable OM was defined as that material which survived treatment of soils with 6% sodium hypochlorite (NaOCl). Mineral-protected OM was extd. by subsequent dissoln. of minerals by 10% hydrofluoric acid (HF). Org. matter resistant against NaOCl and insol. in HF was considered as recalcitrant OM. Hypochlorite removed primarily 14C-modern OM. Of the stable org. carbon (OC), amounting to 2.4-20.6 g kg-1 soil, mineral dissoln. released on av. 73%. Poorly cryst. Fe and Al phases (Feo, Alo) and cryst. Fe oxides (Fed-o) explained 86% of the variability of mineral-protected OC. At. Cp/(Fe+Al)p ratios of 1.3-6.5 suggest that a portion of stable OM was assocd. with polymeric Fe and Al species. Recalcitrant OC (0.4-6.5 g kg-1 soil) contributed on av. 27% to stable OC and the amt. was not correlated with any mineralogical property. Recalcitrant OC had lower Δ14C and δ13C values than mineral-protected OC and was mainly composed of aliph. (56%) and O-alkyl (13%) C moieties. Lignin phenols were only present in small amts. in either mineral-protected or recalcitrant OM (mean 4.3 and 0.2 g kg-1 OC). The results confirm that stabilization of OM by interaction with poorly cryst. minerals and polymeric metal species is the most important mechanism for preservation of OM in these acid subsoil horizons.
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  183. 183
    Kögel-Knabner, I.; Guggenberger, G.; Kleber, M.; Kandeler, E.; Kalbitz, K.; Scheu, S.; Eusterhues, K.; Leinweber, P. Organo-mineral associations in temperate soils: Integrating biology, mineralogy, and organic matter chemistry. J. Plant Nutr. Soil Sci. 2008, 171 (1), 61– 82,  DOI: 10.1002/jpln.200700048
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    Rasmussen, C.; Heckman, K.; Wieder, W. R.; Keiluweit, M.; Lawrence, C. R.; Berhe, A. A.; Blankinship, J. C.; Crow, S. E.; Druhan, J. L.; Hicks Pries, C. E.; Marin-Spiotta, E.; Plante, A. F.; Schädel, C.; Schimel, J. P.; Sierra, C. A.; Thompson, A.; Wagai, R. Beyond clay: towards an improved set of variables for predicting soil organic matter content. Biogeochemistry 2018, 137 (3), 297– 306,  DOI: 10.1007/s10533-018-0424-3
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    184
    Beyond clay: towards an improved set of variables for predicting soil organic matter content
    Rasmussen, Craig; Heckman, Katherine; Wieder, William R.; Keiluweit, Marco; Lawrence, Corey R.; Berhe, Asmeret Asefaw; Blankinship, Joseph C.; Crow, Susan E.; Druhan, Jennifer L.; Hicks Pries, Caitlin E.; Marin-Spiotta, Erika; Plante, Alain F.; Schadel, Christina; Schimel, Joshua P.; Sierra, Carlos A.; Thompson, Aaron; Wagai, Rota
    Biogeochemistry (2018), 137 (3), 297-306CODEN: BIOGEP; ISSN:0168-2563. (Springer)
    Improved quantification of the factors controlling soil org. matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochem. models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO2 to the atm. Emerging conceptual understanding, however, suggests other soil physicochem. properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochem. parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alk. soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochem. models to better reflect the role of soil physicochem. properties in SOM cycling.
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  185. 185
    Stone, A. T. Microbial metabolites and the reductive dissolution of manganese oxides: Oxalate and pyruvate. Geochim. Cosmochim. Acta 1987, 51, 919– 925,  DOI: 10.1016/0016-7037(87)90105-0
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    Microbial metabolites and the reductive dissolution of manganese oxides: Oxalate and pyruvate
    Stone, Alan T.
    Geochimica et Cosmochimica Acta (1987), 51 (4), 919-25CODEN: GCACAK; ISSN:0016-7037.
    Microorganisms may participate in the reductive dissoln. of Mn oxides in sediments either directly, by using Mn oxides as a terminal electron acceptor, or indirectly, through the prodn. and excretion of reductants and other mols. that perturb the chem. conditions in sediments. If the latter process is predominant, it should be possible to calc. rates of Mn mobilization from sediments from a knowledge of the kinetics of the reductive dissoln. reaction, once the structures and concns. of reductants found in sediments are known. Lab. expts. show that synthetic Mn(III, IV) oxides are reduced and dissolved by the microbial metabolites oxalate and pyruvate at appreciable rates, providing addnl. evidence that indirect involvement of microbes in the mobilization of Mn in sediments can occur. Rates of reductive dissoln. increase as the pH is decreased; the effect of pH on dissoln. rate is more pronounced for oxalate than for pyruvate. Ways in which chem. conditions in freshwater and marine sediments affect rates of Mn mobilization are discussed.
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    Stone, A. T.; Ulrich, H. Kinetics and Reaction Stoichiometry in the Reductive Dissolution of Manganese(IV) Dioxide and Co(Ill) Oxide by Hydroquinone. J. Colloid Interface Sci. 1989, 132 (2), 509– 522,  DOI: 10.1016/0021-9797(89)90265-8
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    186
    Kinetics and reaction stoichiometry in the reductive dissolution of manganese(IV) dioxide and cobalt(III) oxide by hydroquinone
    Stone, Alan T.; Ulrich, Hans Jakob
    Journal of Colloid and Interface Science (1989), 132 (2), 509-22CODEN: JCISA5; ISSN:0021-9797.
    Oxidn. of hydroquinone (QH2) to p-benzoquinone (Q) in aq. suspensions by MnO1.97 and CoOOH and subsequent reduced metal ion release were examd. in order to explore the kinetics and mechanisms of surface chem. redox reactions. When the ratio [Q(aq)]/[M2+(aq)] exceeds the value predicted from reaction stoichiometry, reduced metal ions are accumulating on oxide surfaces. This behavior was obsd. for both oxides, and is esp. pronounced early in the course of reaction and in suspension of high pH. Sorption of added Mn2+ onto MnO2 and Co2+ onto CoOOH in reductant-free suspensions resembles the retention of reduced metal ions obsd. during the reductive dissoln. reactions. Malonate facilitates Co2+(aq) release during reaction of CoOOH with hydroquinone. Rates of reduced metal ion adsorption/desorption relative to rates of prior surface chem. reactions are discussed using a numerical model. Comparisons are made with the reductive dissoln. of Fe(III) oxides by hydroquinone.
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    Stone, A. T. Oxidation and hydrolysis of ionizable organic pollutants at hydrous metal oxide surfaces. Rates of Soil Chemical Processes 2015, 27, 231– 254,  DOI: 10.2136/sssaspecpub27.c9
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    Heckman, K.; Lawrence, C. R.; Harden, J. W. A sequential selective dissolution method to quantify storage and stability of organic carbon associated with Al and Fe hydroxide phases. Geoderma 2018, 312, 24– 35,  DOI: 10.1016/j.geoderma.2017.09.043
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    A sequential selective dissolution method to quantify storage and stability of organic carbon associated with Al and Fe hydroxide phases
    Heckman, Katherine; Lawrence, Corey R.; Harden, Jennifer W.
    Geoderma (2018), 312 (), 24-35CODEN: GEDMAB; ISSN:0016-7061. (Elsevier B.V.)
    Stabilization of SOM (soil org. matter) is regulated in part by sorption and desorption reactions happening at mineral surfaces, as well as pptn. and dissoln. of organo-metal complexes. Fe and Al hydroxides play a particularly significant role in SOM stabilization in soils due to their ubiquitous distribution and their highly reactive surface properties. Iron and Al hydroxides exist in soils across a wide spectrum of crystallinity, ranging from dissolved Fe and Al cations which combine with orgs. to form organo-metal ppts. to the more cryst. end members, goethite and gibbsite, which sorb SOM through a variety of mol. interactions. Though the importance of these sorption and pptn. reactions has long been recognized, the distribution of SOM among Fe and Al hydroxides of differing crystallinity has not been well quantified, nor has the timescale over which these stabilization mechanisms operate. In an attempt to measure the distribution of org. C among (i) Al- and Fe-humus complexes (ii) short-range-order (SRO) Al and Fe hydroxide surfaces and (iii) cryst. Fe oxyhydroxide surfaces, a single method combining several selective mineral dissolns. was applied to soils of four different geneses (a tropical forest Andisol, a temperate forest basaltic Mollisol, a Mediterranean coastal prairie Mollisol, and a northern mixed hardwood forest Spodosol). The traditional reactants used in selective dissolns. were replaced with carbon-free analogs so that the carbon released along with the Fe and Al at each stage of the selective dissoln. process could be measured. Selective dissolns. were performed sequentially: Na-pyrophosphate (organo-Al and Fe complexes) followed by hydroxylamine (SRO Al and Fe hydroxides) followed by dithionite-HCl (cryst. Fe hydroxides). Carbon, Al, and Fe concns., as well as radiocarbon abundance were measured in the solns. yielded by each stage of the selective dissoln. process. Results suggest that pptn. of organo-metal complexes (Na-pyrophosphate extractable C) often accounts for the largest pool of stabilized C among the three selectively dissolved pools, but these complexes were 14C enriched in comparison to C from the other selectively dissolved pools and the residual C left on cryst. mineral surfaces after all three stages of selective dissoln. Hydroxylamine and dithionite-HCl extractable C pools were, on av., small and often below detection level in temperate soils. However, radiocarbon values for these C pools were generally depleted in comparison to other pools. These results suggest variation in organo-mineral complex stability is assocd. with degree of crystallinity of the mineral phase. Overall, this work suggests that sequential selective dissoln. methods are a promising tool for characterizing the content and isotopic compn. of soil C assocd. with distinct organo-mineral and organo-metal assocns.
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  189. 189
    Zhao, Q.; Poulson, S. R.; Obrist, D.; Sumaila, S.; Dynes, J. J.; McBeth, J. M.; Yang, Y. Iron-bound organic carbon in forest soils: quantification and characterization. Biogeosciences 2016, 13 (16), 4777– 4788,  DOI: 10.5194/bg-13-4777-2016
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    189
    Iron-bound organic carbon in forest soils: quantification and characterization
    Zhao, Qian; Poulson, Simon R.; Obrist, Daniel; Sumaila, Samira; Dynes, James J.; McBeth, Joyce M.; Yang, Yu
    Biogeosciences (2016), 13 (16), 4777-4788CODEN: BIOGGR; ISSN:1726-4189. (Copernicus Publications)
    Iron oxide minerals play an important role in stabilizing org. carbon (OC) and regulating the biogeochem. cycles of OC on the earth surface. To predict the fate of OC, it is essential to understand the amt., spatial variability, and characteristics of Fe-bound OC in natural soils. In this study, we investigated the concns. and characteristics of Fe-bound OC in soils collected from 14 forests in the United States and detd. the impact of ecogeog. variables and soil physicochem. properties on the assocn. of OC and Fe minerals. On av., Fe-bound OC contributed 37.8 % of total OC (TOC) in forest soils. Atomic ratios of OC : Fe ranged from 0.56 to 17.7, with values of 1-10 for most samples, and the ratios indicate the importance of both sorptive and incorporative interactions. The fraction of Fe-bound OC in TOC (fFe-OC) was not related to the concn. of reactive Fe, which suggests that the importance of assocn. with Fe in OC accumulation was not governed by the concn. of reactive Fe. Concns. of Fe-bound OC and fFe-OC increased with latitude and reached peak values at a site with a mean annual temp. of 6.6 °C. Attenuated total reflectance-Fourier transform IR spectroscopy (ATR-FTIR) and near-edge X-ray absorption fine structure (NEXAFS) analyses revealed that Fe-bound OC was less aliph. than non-Fe-bound OC. Fe-bound OC also was more enriched in 13C compared to the non-Fe-bound OC, but C / N ratios did not differ substantially. In summary, 13C-enriched OC with less aliph. carbon and more carboxylic carbon was assocd. with Fe minerals in the soils, with values of fFe-OC being controlled by both