Aquatic Redox Chemistry

Oxidation-reduction (redox) reactions are among the most important and interesting chemical reactions that occur in aquatic environmental systems, including soils, sediments, aquifers, rivers, lakes, and water treatment systems. Redox reactions are central to major element cycling, to many sorption processes, to trace element mobility and toxicity, to most remediation schemes, and to life itself. Over the past 20 years, a great deal of research has been done in pursuit of process-level understanding aquatic redox chemistry, but the field is only beginning to converge around a unified body of knowledge. This chapter provides a very broad overview of the state of this convergence, including clarification of key terminology, some relatively novel examples of core thermodynamic concepts (involving redox ladders and Eh-pH diagrams), and some historical perspective on the persistent challenges of how to characterize redox intensity and capacity of real, complex, environmental materials. Finally, the chapter attempts to encourage further convergence among the many facets of aquatic redox chemistry by briefly reviewing major themes in this volume and several past volumes that overlap partially with this scope.

In oxygenated waters, chloride and bromide are the thermodynamically stable halogen species that exist whereas iodate, the thermodynamically stable form of iodine, and iodide can co-exist. The stability and oxidation of halides in the environment is related to the unfavorable thermodynamics for the first electron transfer with oxygen to form X· atoms. However, reactive oxygen species (ROS) such as ¹O₂, H₂O₂ and O₃ can oxidize the halides to X₂ and perhaps HOX in two electron transfer processes; these reactions become less favorable with increasing pH. Fe(III) and Mn(III,IV) solid phases can oxidize halides with similar patterns as ROS. The ease of oxidation increases from Cl^- < Br^- < I^-. X₂ can also form HOX in water, and both halogen species can react with natural organic matter with formation of organo-halogen (R-X) compounds. During the treatment of drinking water, unwanted R-X disinfectant byproducts can form when the oxidant is not capable of quantitatively converting iodide to iodate. Natural and anthropogenic volatile R-X compounds are photochemically active and lead to X· atoms in the atmosphere which undergo reaction with O₃ via an O atom transfer step (two electron transfer step) resulting in O₃ destruction. In the case of iodine, iodine oxide species form aerosol nanoparticles leading to cloud condensation nuclei.

Many redox reactions of importance in aquatic chemistry involve elementary steps that occur by single-electron transfer (SET). This step is often the first and rate limiting step in redox reactions of environmental contaminants, so there has been a great deal of interest in the corresponding one-electron reduction potentials (E ¹). Although E ¹ can be obtained by experimental methods, calculation from first-principles chemical structure theory is becoming an increasingly attractive alternative. Sufficient data are now available to perform a critical assessment of these methods—and their results—for two types of contaminant degradation reactions: dehalogenation of chlorinated aliphatic compounds (CACs) and reduction of nitro aromatic compounds (NACs). Early datasets containing E ¹’s for dehalogenation of CACs by dissociative SET contained a variety of errors and inconsistencies, but the preferred datasets show good agreement between values calculated from thermodynamic data and quantum mechanical models. All of the datasets with E ¹’s for reduction of NACs by SET are relatively new, were calculated with similar methods, and yet yield a variety of systematic differences. Further analysis of these differences is likely to yield computational methods for E ¹’s of NAC nitro reduction that are similar in reliability to those for CAC dechlorination. However, comparison of the E ¹ data compiled here with those calculated with a more universal predictive model (like SPARC) highlight a number of challenges with implementation of models for predicting properties over a wide range of chemical structures.

Terminal electron accepting processes (TEAPs) control the fate of elements in anoxic environments. This study focuses on thermodynamic regulation of H₂-dependent TEAPs. H₂-dependent methanogenesis and sulfate reduction operate near free energy thresholds (ΔG_c) and can be inhibited by changes in thermodynamic conditions, whereas more ‘potent’ TEAPs occur far from their energy thresholds and lower H₂ concentrations to levels that exclude other TEAPs. Metabolic free energy thresholds depend on microbial physiology and occur when the energy conserved by ATP generation approaches the thermodynamic driving force. A model analysis for peat-sand mixtures suggests that acetoclastic methanogenesis can be inhibited by CH₄ and dissolved inorganic carbon (DIC) accumulation, lowering the free energy (ΔG_r) toward an energy threshold (ΔG_c), which was identified by inverse modeling near - 25 kJ mol^-1. Inhibition was sensitive to ΔG_c and acetate concentrations, so that ΔG_c ± 5 kJ mol^-1 and a range of 1 to 100 µmol L^-1 acetate lead to strongly differing steady state CH₄ concentrations in the model results.

Natural organic matter (NOM) is an inherently complex mixture of polyfunctional organic molecules. Because of their universality and chemical reversibility, oxidation/reductions (redox) reactions of NOM have an especially interesting and important role in geochemistry. Variabilities in NOM composition and chemistry make studies of its redox chemistry particularly challenging, and details of NOM-mediated redox reactions are only partially understood. This is in large part due to the analytical difficulties associated with NOM characterization and the wide range of reagents and experimental systems used to study NOM redox reactions. This chapter provides a summary of the ongoing efforts to provide a coherent comprehension of aqueous redox chemistry involving NOM and of techniques for chemical characterization of NOM. It also describes some attempts to confirm the roles of different structural moieties in redox reactions. In addition, we discuss some of the operational parameters used to describe NOM redox capacities and redox states, and describe nomenclature of NOM redox chemistry. Several relatively facile experimental methods applicable to predictions of the NOM redox activity and redox states of NOM samples are discussed, with special attention to the proposed use of fluorescence spectroscopy to predict relevant redox characteristics of NOM samples.

The progress of science shows completeness and a logical development only in the settings provided by textbooks and review articles. Living science, like living human beings, invariably exhibits partial truths, tentativeness, trial-and-error, and subjectivity. The development of the concept of electron shuttling by natural organic matter is not an exception to this paradigm. In the following essay, the story of this development will be retold heuristically to endow it with a more logical structure, providing both a template for future research and a perspective on the contributions made by Donald Macalady.

Natural organic matter (NOM) plays an important role in a variety of environmental redox processes, ranging from fueling the global carbon cycle to mediating microbial interactions with minerals. However, the complex and indeterminant composition of NOM makes characterization of its redox activity challenging. Approaches that have been taken to address these challenges include chemical probe reactions, potentiometric titrations, chronocoulometry, and voltammetry. Advantages of the latter include that it can be diagnostic and quantitative, but applying voltammetry to the characterization of NOM has been challenging. Improved results have been obtained recently by using aprotic solvents, microelectrodes, and various applied potential waveforms. Results obtained with several voltammetric methods and DMSO as the solvent strongly suggest that quinone-like moieties are the dominant redox active groups. Correcting the associated peak potentials for comparison with estimates of NOM redox potentials in aqueous solutions shows that the range of peak potentials resolved by voltammetry spans most of the redox potentials obtained by other means that have been reported for various types of NOM or NOM model compounds. The multiplicity of electron-transfer steps that are distinguishable by voltammetry, and the likelihood that there is a degree of redox-coupling among these moieties, suggests that the redox potential of NOM might best be modeled as a continuum of redox potentials. The kinetics of electron exchange along this continuum will vary with factors such as the complex tertiary structure of NOM. The kinetic limitations created by this tertiary structure may be overcome with organic solvents (which allow the structure to unravel) or electron shuttles (which can pass into the structure), which accounts for the improved resolution of methods that use these strategies in electrochemical characterization of NOM.

Pathways contributing to the formation and decay of Fe(II) in sunlit natural waters are investigated in this chapter with insights drawn from both laboratory experiments and kinetic modelling. Our results support previous findings that superoxide-mediated iron reduction (SMIR) is the main pathway for photochemical reduction of Fe(III) at pH 8 while light-induced ligand to metal charge transfer (LMCT) is important in low pH environments. Furthermore, our work shows that triplet oxygen and photo-produced species are the main oxidants of Fe(II) while hydrogen peroxide, a relatively stable end product of SRFA photolysis, is not involved in Fe(II) oxidation. A kinetic model based on these observations is presented which provides an excellent description of the experimental results and is consistent with observations from a wide range of studies investigating the redox cycling of iron.

A new picture of the Fenton reaction has emerged over the last two decades that extends our understanding beyond the acidic conditions studied previously. In the absence of ligands, the reaction produces hydroxyl radical under acidic conditions and a less reactive oxidant, presumed to be the ferryl ion (Fe[IV]), at circumneutral pH values. Formation of complexes between Fe(II) and organic ligands alters the reaction mechanism, resulting in production of hydroxyl radical over a wide pH range. As a result, iron coordination and pH determine the oxidants produced by the Fenton reaction. Consideration of the reactive oxidant produced by the Fenton reaction under environmentally- and biologically-relevant conditions is necessary to develop more effective treatment systems, to predict the fate of iron and carbon in natural waters, and to assess iron-mediated oxidative damage.

Photo-induced redox chemical reactions occurring on irradiated semiconductor surfaces have been utilized for the purification of water contaminated with various inorganic and organic chemicals. Here, we focus on TiO₂ as the most popular photocatalyst and briefly describe its characteristics and applications mainly in relation with the photochemical redox conversion of aquatic pollutants. The photoexcitation of TiO₂ induces electron-hole pair formation and subsequent charge separation/migration/transfer leads to the production of highly reactive oxygen species (ROS) such as OH radical and superoxide on the surface of TiO₂. Aquatic organic pollutants subsequently react with ROS, holes, or electrons, and they undergo a series of redox chemical reactions, eventually leading to mineralization. The photo-induced ROS generation on TiO₂ is exploitable for bacterial/viral inactivation as well, while TiO₂ particles at the nano- and microscale possibly induce adverse biological effects in the absence of light. Photo-induced redox reactions on TiO₂ can also transform a variety of inorganic pollutants such as oxyanions (arsenite, chromate, bromate, etc.), ammonia, and metal ions. On the other hand, the photocatalytic degradation mechanism can be actively controlled by modifying the surface of TiO₂ to change the products. For example, the photocatalytic degradation of phenolic compounds can be accompanied by the simultaneous production of hydrogen when the surface of TiO₂ is modified with both platinum and fluoride. Finally, the photocatalytic activity of TiO₂ is highly dependent on the kind of substrates and the activity assessed with a specific test substrate is difficult to generalize. Therefore, the photocatalytic activities of TiO₂ should be assessed using multiple substrates to obtain balanced information.

This chapter discusses the main aspects of chlorine and bromine speciation in systems with varying pHs, concentrations of bromide, chloride and total active chlorine. In the absence of ammonia, formal consideration of equilibria in solutions containing hypohalogenous acids, Cl₂, Br₂, BrCl and trihalogenide ions Br_iCl_3-i ^- can be carried out based on two reference species (OCl^-, Br^-). Formal constants necessary for implementing such an approach are presented in the paper. While haloamine formation constants can be determined based on the consideration of OCl^-, Br^- and NH₄ ⁺ as reference species, this approach is deemed to be applicable only when monochloramine is prevalent. Properties of systems with halogens can be examined based on the electrochemical potential of the HOCl/Cl^- couple.

The quantitative removal of chemical contaminants in water is one of the most pressing problems facing water utilities today. To augment traditional water treatments that are usually based on adsorptive and chemical-physical processes, radical-based advanced oxidation and reduction processes (AO/RPs) are now being considered. While most AO/RPs utilize the hydroxyl radical in treatment the use of oxidizing sulfate radicals is also gaining interest. To help assess the applicability of sulfate radical based AO/RPs in remediating contaminated waters, here we have determined absolute rate constants and reaction mechanisms for SO₄ ^-• reaction with four β-lactam antibiotics (amoxicillin, penicillin-G, piperacillin, tircarcillin), three estrogenic steroids (ethynylestradiol, estradiol, and progesterone) and one personal care product (isoborneol). For the four antibiotics of this study the relatively fast rate constant values suggests that the majority of the SO₄ ^-• oxidation occurs at the sulfur atom in the ring adjacent to the β-lactam moiety, as opposed to the hydroxyl radical reaction which occurs at peripheral aromatic rings. The measured sulfate radical rate constants for estradiol and progesterone are identical, with the slightly faster value for ethynylestradiol suggesting significant oxidation occurring at its ethynyl moiety. For isoborneol, the sulfate radical reactivity was slightly lower, but still fast enough that AO/RP treatment utilizing this radical might be feasible at large-scale. Piperacillin was also chosen for a detailed investigation of its degradation by both SO₄ ^-• and ^•OH in a laboratory scale homogeneous UV photochemical system. It was found that although the absolute reaction rate constant for piperacillin reaction with SO₄ ^-• was lower than for ^•OH, the overall removal of this antibiotic was more effective when using UV/S₂O₈ ^2- than UV/H₂O₂. For an initial oxidant dose of 1 mM and an antibiotic concentration of 50 µM, percentage removals of 65.2% and 33.0%, respectively, at a UV fluence of 320 mJ/cm² were obtained. This difference was attributed to the higher quantum yield of sulfate radical production from persulfate under UV 254 nm irradiation.

Voltammetry at Hg drop electrodes is a promising method for detecting sulfide nanoparticles in natural waters. Recent research suggests that such nanoparticles might affect organisms in unforeseen ways. Sulfide nanoparticles diffusing to Hg surfaces are captured selectively even from unfiltered waters that contain larger amounts of other nanoscale materials, such as organic macromolecules or clay minerals. Optimum size sensitivity for capture is roughly 5-100 nm at Hg drop electrodes. Sulfide nanoparticles are stabilized at Hg surfaces by transformation to adlayers, whose accumulation can be quantified electrochemically. Study of FeS adlayers has led to new insights regarding the puzzling -1.1 V vs. Ag/AgCl signal observed in sulfidic natural waters. This signal has been attributed previously to Fe sulfide clusters or complexes. New evidence shows that it arises from reduction of Fe²⁺ at FeS adlayers formed by sorption of FeS nanoparticles on Hg electrodes. Partial coverage of Hg with FeS creates in essence two electrodes. These reduce Fe²⁺ at separate potentials.

Extracellular organic ligands and ligand functional groups within macromolecular natural organic matter can significantly influence the speciation and kinetic redox reactivity of Fe(II) with aquatic contaminants. Fe(II) complexation by Fe(III)-stabilizing ligands (e.g., carboxylate, catecholate, thiol) leads to formation of Fe(II) species with low standard reduction potentials (E_H ⁰) and enhanced reactivity with reducible contaminants (e.g., nitroaromatics and halogenated alkanes). Rates of contaminant reduction by Fe(II) are highly variable and dependent upon the identity and concentration of specific organic ligands as well as environmental conditions that affect the extent of complex formation. Linear free energy relationships have been developed to predict the aqueous reactivity of individual Fe(II) species with contaminants. Studies on the reactivity of Fe(II) complexes with model ligands also provide mechanistic insights into the potential mechanisms responsible for contaminant transformations observed in more complex aquatic systems where Fe(II) co-accumulates with more poorly defined natural organic matter.

Sorption of aqueous Fe²⁺ at the Fe oxide-water interface has traditionally been viewed in the classic framework of sorption at static oxide surface sites as formulated in surface complexation models (SCMs). Significant experimental and theoretical evidence has accumulated, however, to indicate that the reaction of aqueous Fe²⁺ with Fe³⁺ oxides is much more complex and is comprised of sorption, electron transfer, conduction, dissolution, and, in some cases, atom exchange and/or transformation to secondary minerals. Here, we provide a brief historical review of Fe²⁺ sorption on Fe oxides and present a revised conceptual model based on the semiconducting properties of Fe oxides that incorporates recent experimental evidence for Fe²⁺ - Fe³⁺ _oxide electron transfer, bulk electron conduction, and Fe atom exchange. We also discuss the implications of this revised conceptual model for important environmental processes, such as trace metal cycling and contaminant fate.

Electrochemical reduction/oxidation (redox) reactions have been observed to drive stable isotope fractionation in many metal systems, where the lighter isotopes of a metal are typically partitioned into the reduced chemical species. While physical processes such as diffusive mass transport lead to small isotope fractionations, charge transfer processes can lead to isotope fractionations up to ten times larger and twice that predicted by equilibrium stable isotope theory. Control over physical and chemical variables during electrochemical experiments, such as overpotential and temperature, allow for the isotopic composition of deposited metals to be fine tuned to a specific value.

Redox reactions of structural Fe in clay minerals play important roles in biogeochemical processes and for the fate of contaminants in the environment. Many of the redox properties of Fe in clay minerals are, however, poorly understood, thus limiting the knowledge of the factors that make structural Fe participate in electron transfer reactions. This chapter summarizes the current state of knowledge on the redox properties of structural Fe in clay minerals. In the first part, we review the various spectroscopic observations associated with structural Fe reduction and oxidation and how changes in Fe oxidation state affect the clay mineral structure and the binding environment of Fe in the octahedral sheet of planar 2:1 clay minerals. In the second part, we show how information on the structural alterations and arrangement of Fe can be interpreted to assess the apparent reactivity and the thermodynamic redox properties of structural Fe in clay minerals.

Granular, reactive zerovalent metals (ZVMs)—especially iron (ZVI)—form the basis for model systems that have been used in fundamental and applied studies of a wide variety of environmental processes. This has resulted in notable advances in many areas, including the kinetics and mechanisms of contaminant reduction reactions, theory of filtration and transport of colloids in porous media, and modeling of complex reactive-transport scenarios. Recent emphasis on nano-sized ZVI has created a new opportunity: to advance the understanding of how coatings of organic polyelectrolytes—like natural organic matter (NOM)—influence the reactivity of environmental surfaces. Depending on many factors, organic coatings can be activating or passivating with respect to redox reactions at particle-solution interfaces. In this study, we show the effects of organic coatings on nZVI vary with a number of factors including: (i) time (i.e., “aging” is evident not only in the structure and composition of the nZVI but also in the interactions between nZVI and NOM) and (ii) the type of organic matter (i.e., suspensions of nZVI are stabilized by NOM and the model polyelectrolyte carboxymethylcellulose (CMC), but NOM stimulates redox reactions involving nZVI while CMC inhibits them).

Degradation of chlorinated organic contaminants by natural and engineered reductive dechlorination reactions can occur via numerous biotic and abiotic transformation pathways giving rise to either benign or more toxic products. To assess whether dechlorination processes may lead to significant detoxification (a) the thermodynamic feasibility of a reaction, (b) rates of transformation, and (c) product formation routes need to be understood. To this end, fundamental knowledge of chlorohydrocarbon (CHC) reaction mechanisms is essential. We review insight from reaction thermodynamics, structure-reactivity relationships, and applications of radical and carbene traps, as well as of synthetic probe molecules. We summarize the state-of-knowledge about intermediates and reductive dechlorination pathways of vicinal and geminal haloalkanes, as well as of chlorinated ethenes. Transformation conditions are identified under which problematic products may be avoided. In an outlook, we discuss the potential of stable carbon and chlorine isotope fractionation to identify initial transformation mechanisms, competing transformation pathways, and common branching points.

RDX, hexahydro-1,3,5-trinitro-1,3,5-triazine, is one of the most widely used nitro-organic explosives that presently contaminate various terrestrial and aquatic systems. It is a highly oxidized molecule whose degradation is governed by the electronic structure and redox chemistry of its –CH₂–N(NO₂)– multifunctional group. In the present chapter, we discuss how the chemical reacts in various redox systems with particular emphasis on the initial steps involved in RDX decomposition. Three important redox reactions are analyzed and commented: 1) 1e–transfer to –N–NO₂ leading to denitration, 2) 2e–transfer to –NO₂ leading to the formation of the corresponding nitroso derivatives (MNX, DNX and TNX), and 3) H^●-abstraction from one of the –CH₂– groups by OH^● and O₂ ^●ˉ. We will analyze and identify knowledge gaps in the transformation pathways of RDX to highlight future research needs.

Arsenic occurrence in groundwater, particularly in South and Southeast Asia, has had profoundly deleterious impacts on human health. To address this tragedy, extensive research has been conducted on the biogeochemical cycling of arsenic and its consequences for arsenic mobilization and sequestration. This research has elucidated a key role of microorganisms in redox transformations and the importance of iron and sulfur minerals as carrier phases for arsenic. Research gaps remain, particularly with regard to determining in situ rates of redox transformations and the coupled influence of hydrologic and biogeochemical processes on arsenic occurrence and mobility. Despite these gaps, the insights of this research can be applied to mitigate human exposure through improved water resources management as well as through treatment and remediation.

Understanding the processes controlling the chemical speciation of radionuclide contaminants is key for predicting their fate and transport in aquatic and terrestrial environments, and is a critical consideration in the design of nuclear waste storage facilities and the development of remediation strategies for management of nuclear legacy sites. The redox processes that influence the chemical speciation, and thus mobility, of Tc, U, Np, and Pu in surface and near‑subsurface environments are reviewed, with a focus on coupled biotic-abiotic reactions driven by microbial activity. A case study of U^VI reduction under Fe^III- and sulfate-reducing conditions is presented, using a laboratory-based experimental system to simulate potential electron transfer pathways in natural systems. The results suggest that U^VI was reduced to nanoparticulate uraninite (UO₂) and complexed mononuclear U^IV via multiple pathways including direct microbial reduction and coupled biotic-abiotic processes. These results highlight the potential importance of coupled biotic-abiotic processes in determining the speciation and mobility of Tc, U, Np, and Pu in natural and engineered environments.

While in situ dechlorination of chlorinated aliphatic contaminants such as tetrachloroethylene (PCE) and carbon tetrachloride (CT) has been studied extensively, rate controlling processes in the transformation of these compounds remain uncertain. The objectives of this work were (1) to compare the relative rates of abiotic and microbial transformation of PCE and CT in microcosms designed to simulate natural conditions, and (2) for CT, to measure the relative rates of reactive mineral formation and CT transformation by these reactive minerals. While the rates of microbial dechlorination exceeded the rates of abiotic dechlorination of PCE in the microcosms, the opposite trend was observed for CT. The times required for microbial sulfate reduction, the first step in formation of many reactive minerals, were significantly longer than those required for transformation of CT by these reactive minerals, indicating possible rate control of abiotic CT transformation by microbial respiration under natural conditions.

Identifying the predominant chemical reductants and pathways for electron transfer in anaerobic systems is paramount to the development of environmental fate models that incorporate pathways for abiotic reductive transformations. Currently, such models do not exist. In this chapter we address the approaches based on the use of probe chemicals that have been successfully implemented for this purpose. The general approach has been to identify viable pathways for electron transfer based on the study of probe chemicals in well-defined model systems. The subsequent translation of these findings to natural systems has been based primarily on laboratory studies of probe chemicals in anaerobic sediments and aquifers. In summary, the results of these studies support a scenario in which pathways for reductive transformations in these systems are dominated by surface-mediated processes (i.e., reaction with Fe(II) associated with Fe(III) mineral oxides and clay minerals), and through the aqueous phase by reduced dissolved organic matter (DOM) (i.e., reduced quinone moieties) and Fe(II)/DOM complexes.

Heterogeneous redox processes with mobile and immobile reactants can be significantly affected by transport processes in porous media. In this study, we have performed a sensitivity analysis on the effect of transport on the turnover of the reaction between H₂S and FeOOH that is based on a surface complexation model implemented into a reactive transport code. The analysis considered two reactive surface species (≡FeS^- and ≡FeSH), the amount of surface sites and the buffering capacity. Turnover depends on the ratio between residence time and characteristic reaction time, expressed as Da (Damköhler) numbers. Two completely different relationships between turnover and Da numbers were observed for the two reactive surface species which is due to the pH dependence of their speciation. Calibration of the kinetic model in a column experiment suggests the surface species ≡FeSH to be responsible for the turnover. The characteristic reaction time depends on the concentration of reactive surface sites and the amount of ferric(hydr)oxides. In natural systems, spatial distributions of these parameters exist along with that of residence times. We, therefore, postulate that Da numbers are also spatially distributed reflecting zones of high and low turnover with implications for product accumulation and competition with other reactions involving iron oxides.

Reduction/oxidation (redox) processes affect the chemical quality of groundwater in all aquifer systems. The evolution of redox processes in groundwater is dependent on many factors such as the source and distribution of electron donors and acceptors in the aquifer, relative rates of redox reaction and groundwater flow, aquifer confinement, position in the flow system, and groundwater mixing. Redox gradients are largely vertical in the recharge areas of unconfined aquifers dominated by natural sources of electron donors, whereas substantial longitudinal redox gradients can develop in unconfined aquifers when anthropogenic sources of electron donors are dominant. Longitudinal redox gradients predominate in confined aquifers. Electron-donor limitations can result in the preservation of oxic groundwater over flow distances of many kilometers and groundwater residence times of several thousand years in some aquifers. Where electron donors are abundant, redox conditions can evolve from oxygen reducing to methanogenic over substantially shorter flow distances and residence times.