Send forth your light and your truth

Scientist and educator, through his research, Morgan has established key links between aquatic chemistry and large-scale water quality engineering problems.

Current methods for life-cycle assessment are only partially in line with the precautionary principle.

A review of The Skeptical Environmentalist: Measuring the Real State of the World

The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe⁰) were studied in batch reactors for a range of Fe⁰ surface area concentrations and solution pH values (5.5−9.0). At pH 7.0, there was only a modest difference (2−4×) in first-order rate coefficients (k_obs) for each contaminant among the three Fe⁰ types investigated (Fisher, Peerless, and Connelly). The k_obs values at pH 7.0 for both nitrite and Cr(VI) reduction were first-order with respect to Fe⁰ surface area concentration, and average surface area normalized rate coefficients (k_SA) of 9.0 × 10^-3 and 2.2 × 10^-1 L m^-2 h^-1 were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe⁰ surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe⁰, which reduced nitrate at slower rates at higher Fe⁰ surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe⁰ decreased with increasing pH with apparent reaction orders of 0.49 ± 0.04 for nitrate, 0.61 ± 0.02 for nitrite, and 0.72 ± 0.07 for Cr(VI). Buffer type had minimal effects on reduction rates, indicating that pH was primarily responsible for the differences in rate. At high pH values, Cr(VI) reduction ceased after a short time period, and negligible nitrite reduction was observed over 48 h.

The number of bidentate binding sites on a pristine surface (i.e., sites comprising two adjacent monodentate sites plus the space between them) can be substantially larger than the maximum number of bidentate molecules that can adsorb to the surface. When bidentate sorption occurs, the number of available bidentate sites decreases in response to direct occupation of some sites, but an even more significant loss results from the fact that several unoccupied sites immediately surrounding each adsorbed molecule can also become unavailable. Recognition of this phenomenon allows development of a model for the adsorption process that matches simulated data from Monte Carlo (MC) modeling extremely well. The model also explains the observation that, on a given surface with a given fractional occupation, the number of available bidentate sites depends on whether the occupied sites are populated by monodentate or bidentate adsorbed species. A model developed more than 60 years ago but not widely recognized by modern adsorption modelers also fits the MC simulations very well. The simulated data are also reasonably approximated by assuming that the number of available bidentate sites on a surface is proportional to the square of the number of available monodentate sites, although the fit is not as good as with the models mentioned above. By contrast, approximating the number of available sites as proportional to the number of monodentate sites to the first power yields predictions that do not match the simulations. The results have implications for modeling of both multidentate adsorption reactions and monovalent−divalent ion exchange.

Field and laboratory column experiments were performed to assess the effect of elevated pH and reduced ionic strength on the mobilization of natural colloids in a ferric oxyhydroxide-coated aquifer sediment. The field experiments were conducted as natural gradient injections of groundwater amended by sodium hydroxide additions. The laboratory experiments were conducted in columns of undisturbed, oriented sediments and disturbed, disoriented sediments. In the field, the breakthrough of released colloids coincided with the pH pulse breakthrough and lagged the bromide tracer breakthrough. The breakthrough behavior suggested that the progress of the elevated pH front controlled the transport of the mobilized colloids. In the laboratory, about twice as much colloid release occurred in the disturbed sediments as in the undisturbed sediments. The field and laboratory experiments both showed that the total mass of colloid release increased with increasing pH until the concurrent increase in ionic strength limited release. A decrease in ionic strength did not mobilize significant amounts of colloids in the field. The amount of colloids released normalized to the mass of the sediments was similar for the field and the undisturbed laboratory experiments.

Fractal dimensions of marine aggregates are often estimated from the measured slopes of particle size spectra. One technique uses dimensional analysis to determine the dependence of the spectrum's slope with fractal dimension. In this paper, we use numerical simulations to examine the assumptions underlying the dimensional analysis approach to particle size spectra. We find that the slopes of numerically computed steady-state particle size spectra disagree with those predicted by dimensional analysis. The assumptions underlying the dimensional analysis approach that are responsible for the disagreement are as follows:  only one coagulation mechanism operates in each particular particle size range, particle loss through sedimentation occurs at particle sizes larger than those for which differential sedimentation dominates, particles only interact with like-sized particles. Including disaggregation steepens the slope of the particle size spectrum for both large and small particles and changes the shape of the spectrum. These results indicate that one should exercise caution when using the measured slopes of particle size spectra to estimate aggregate fractal dimension.

The adsorption of copper, cadmium, and nickel on goethite was examined in natural groundwater samples from an infiltration site of the river Glatt at Glattfelden (Switzerland). Unfractionated dissolved organic matter was used at its natural concentrations. Metal concentrations were close to environmental conditions. Cu, Cd, and Ni presented the typical pH adsorption edge of cations. The major influence on metal adsorption was due to a strong organic ligand L_I, which inhibited adsorption of Cu, Cd, and Ni in the alkaline pH region. Complexation of Cu, Cd, and Ni by the natural organic ligands was described with a model defining a minimum number of discrete ligands:  a strong ligand L_I at low concentration and additional weaker ligands with higher concentrations. The adsorption of Cu, Cd, and Ni on the goethite surface in the presence of the natural organic ligands was adequately described by considering only surface complexation and complexation in solution by organic ligands. No ternary complexes had to be invoked in the model. The major effect was complexation by the strongest ligand, whereas interactions with other cations and anions had only a minor influence. Competition reactions between Cu and Ni for complexation with the same strong ligand L_I were observed.

This paper reports steady-state dissolution rates of synthetic low-substitution Al-goethites (mol % Al < 10) at pH 5 in the presence of the trihydroxamate siderophore, desferrioxamine B (DFO-B), and the common biological ligand, oxalate. The siderophore-promoted Fe release rate increased both with the level of Al substitution and with DFO-B concentration up to about 100 μM, after which a plateau occurred, suggesting a saturation effect from DFO-B adsorption as a precursor to dissolution. At concentrations above 200 μM, oxalate also enhanced the Fe release rate, which however was not influenced by Al substitution. For Al-goethites with mol % Al < 4, the Fe release rate in the presence of 40 μM DFO-B together with varying concentrations of oxalate was typically greater than the corresponding sum of dissolution rates in the presence of the two ligands alone. This synergism may be the combined result of the ability of oxalate to adsorb strongly at the goethite surface, thus promoting Fe release, and of the high selectivity of DFO for Fe(III). Ferric oxalate complexes formed during dissolution will likely lose Fe³⁺ by ligand substitution with DFO-B, leading to the production of Fe(HDFO-B)⁺ and uncomplexed oxalate, the latter of which, in turn, could adsorb to the goethite surface again. For Al-goethites with mol % Al > 4, synergism was not apparent, which may signal the effect of a decreased surface density of Fe−OH sites associated with Al for Fe substitution. The oxalate-promoted release rates of Al did not scale with those of Fe, indicating incongruent dissolution. However, Al release rates in the presence of DFO-B did scale approximately with those of Fe but were not affected by the concentration of siderophore. These results are consistent with the presence of Al(OH)₃ inclusions in Al-goethite.

Atomic force microscopy (AFM) is employed to directly measure colloidal surface forces between a silica particle and a smooth glass plate in an aqueous solution with or without the presence of copper ions. Without the presence of copper ions, results show that the force between these two surfaces is repulsive and that its magnitude decreases with increasing ionic strength and decreasing pH. The surface forces are calculated based on the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory for constant surface charge and are then compared with AFM force measurements. A good agreement between theory and experimental data is reported except at very small separation distances (<3 nm) between the silica particle and the glass plate. This behavior may be attributed to non-DLVO forces, such as the hydration effect that results from the bounded water molecules on the surface of the silica particle, or to surface roughness. When copper ions are present in acidic aqueous solutions, the magnitude of the force is found to be the same as that without the presence of copper ions, which indicates that no sorption of copper ions by the silica particle occurs under these conditions. Near neutral pH, sorption of copper ions causes charge reversal for the silica particle from negative to positive. Therefore, the force between the silica particle and the glass plate changes from repulsive to attractive. The transient ζ-potential of the silica particle during sorption of copper ions is determined by representing the experimental data with the DLVO theory. In alkaline solutions, where removal of copper ions is known to occur mainly by bulk precipitation, the measured force is similar to that without the presence of copper ions, which suggests that sorption does not occur under such conditions.

The technique of diffusive gradients in thin-films (DGT) accumulates metals on a Chelex resin after their diffusive transport through a hydrogel. It lowers metal concentrations in soil solution adjacent to the device and induces resupply of metal associated with the solid phase. DGT devices were deployed in an alluvial gley soil for 21 different time periods between 4 h and 19.5 d. The accumulated masses of Cu, Cd, Ni, and Zn were used to calculate the distribution coefficient for labile metal, K_dl, and adsorption and desorption rate constants. Calculations were performed using a dynamic numerical model of DGT-induced fluxes in soils (DIFS). It assumes first-order exchange between solid phase and solution and diffusional transport in both the soil solution and the hydrogel. The DIFS model fitted changes in accumulated mass with time very well. Values of K_dl calculated from DIFS of 100 (Cd), 250 (Cu), 150 (Ni), and 150 (Zn) were larger than values of distribution coefficients estimated by exchange with Ca(NO₃)₂ but similar to those estimated by isotopic exchange (Cd and Zn only). These results suggest that the solid-phase pool of metal affected by the removal of labile metal by DGT, which operates on a time scale of minutes, is similar to the solid-phase pool of metal that can isotopically exchange with solution on a time scale of 2 d. Response times of minutes were consistent with interaction rates with surfaces, and desorption rate constants agreed with other reported values. An appraisal of the DIFS model demonstrated the importance of the labile pool size in the solid phase for controlling supply to a sink, such as DGT or a plant. As values of K_dl and kinetic parameters are obtained using DGT with minimal soil disturbance and by a similar mechanism to that involved in plant uptake, they may be pertinent to bioavailability studies.

Reactive transport simulations were conducted to model chemical reactions between metal−EDTA (ethylenediaminetetraacetic acid) complexes during transport in a mildly acidic quartz−sand aquifer. Simulations were compared with the results of small-scale tracer tests wherein nickel−, zinc−, and calcium−EDTA complexes and free EDTA were injected into three distinct chemical zones of a plume of sewage-contaminated groundwater. One zone had a large mass of adsorbed, sewage-derived zinc; one zone had a large mass of adsorbed manganese resulting from mildly reducing conditions created by the sewage plume; and one zone had significantly less adsorbed manganese and negligible zinc background. The chemical model assumed that the dissolution of iron(III) from metal−hydroxypolymer coatings on the aquifer sediments by the metal−EDTA complexes was kinetically restricted. All other reactions, including metal−EDTA complexation, zinc and manganese adsorption, and aluminum hydroxide dissolution were assumed to reach equilibrium on the time scale of transport; equilibrium constants were either taken from the literature or determined independently in the laboratory. A single iron(III) dissolution rate constant was used to fit the breakthrough curves observed in the zone with negligible zinc background. Simulation results agreed well with the experimental data in all three zones, which included temporal moments derived from breakthrough curves at different distances downgradient from the injections and spatial moments calculated from synoptic samplings conducted at different times. Results show that the tracer cloud was near equilibrium with respect to Fe in the sediment after 11 m of transport in the Zn-contaminated region but remained far from equilibrium in the other two zones. Sensitivity studies showed that the relative rate of iron(III) dissolution by the different metal−EDTA complexes was less important than the fact that these reactions are rate controlled. Results suggest that the published solubility for ferrihydrite reasonably approximates the Fe solubility of the hydroxypolymer coatings on the sediments. Aluminum may be somewhat more soluble than represented by the equilibrium constant for gibbsite, and its dissolution may be rate controlled when reacting with Ca−EDTA complexes.

In this study, we examine the role of the hydroxyl (OH^•) radical as a mechanism for the photodecomposition of chromophoric dissolved organic matter (CDOM) in sunlit surface waters. Using γ-radiolysis of water, OH^• was generated in solutions of standard humic substances in quantities comparable to those produced on time scales of days in sunlit surface waters. The second-order rate coefficients of OH^• reaction with Suwannee River fulvic (SRFA; 2.7 × 10⁴ s^-1 (mg of C/L)^-1) and humic acids (SRHA; 1.9 × 10⁴ s^-1 (mg of C/L)^-1) are comparable to those observed for DOM in natural water samples and DOM isolates from other sources but decrease slightly with increasing OH^• doses. OH^• reactions with humic substances produced dissolved inorganic carbon (DIC) with a high efficiency of ∼0.3 mol of CO₂/mol of OH^•. This efficiency stayed approximately constant from early phases of oxidation until complete mineralization of the DOM. Production rates of low molecular weight (LMW) acids including acetic, formic, malonic, and oxalic acids by reaction of SRFA and SRHA with OH^• were measured using HPLC. Ratios of production rates of these acids to rates of DIC production for SRHA and for SRFA were similar to those observed upon photolysis of natural water samples. Bioassays indicated that OH^• reactions with humic substances do not result in measurable formation of bioavailable carbon substrates other than the LMW acids. Bleaching of humic chromophores by OH^• was relatively slow. Our results indicate that OH^• reactions with humic substances are not likely to contribute significantly to observed rates of DOM photomineralization and LMW acid production in sunlit waters. They are also not likely to be a significant mechanism of photobleaching except in waters with very high OH^• photoformation rates.

Shewanella putrefaciens, a heterotrophic member of the γ-proteobacteria is capable of respiring anaerobically on Fe(III) as the sole terminal electron acceptor (TEA). Recent genetic and biochemical studies have indicated that anaerobic Fe(III) respiration by S. putrefaciens requires outer-membrane targeted secretion of respiration-linked Fe(III) reductases. Thus, the availability of Fe(III) to S. putrefaciens may be governed by equilibrium chemical speciation both in the solution phase and at the bacterial cell−aqueous or cell−mineral interface. In the present study, effects of Fe(III) speciation on rates of bacterial Fe(III) reduction have been systematically examined by cultivating S. putrefaciens anaerobically on a suite of Fe(III)−organic complexes as the sole TEA. The suite of Fe(III)−organic complexes spans the range of stability constants normally encountered in natural water systems and includes Fe(III) complexed to citrate, 5-sulfosalicylate, NTA, salicylate, tiron, and EDTA. Rates of bacterial Fe(III) reduction in the presence of dissolved chelating agents correlate with the thermodynamic stability constants of the Fe(III)−organic complexes, implying that chemical speciation governs Fe(III) bioavailability. Equilibrium Fe(III) sorption experiments measured the reversible coordination of Fe(III) with S. putrefaciens as a function of cell/Fe(III) concentration, time, and activity of competing chelating agents. Results show that S. putrefaciens readily sorbs dissolved Fe(III) but that adsorption is restricted by the presence of strong Fe(III)-chelating agents. Our results indicate that dissimilatory Fe(III) reduction by S. putrefaciens is controlled by equilibrium competition for Fe(III) between dissolved organic ligands and strongly sorbing functional groups on the cell surface.

Deposition of arsenic to the sediments of Haiwee Reservoir (Olancha, CA) has dramatically increased since March 1996 as a result of an interim strategy for arsenic management in the Los Angeles Aqueduct (LAA) water supply. Ferric chloride and cationic polymer are introduced into the Aqueduct at the Cottonwood treatment plant, 27 km north of the Haiwee Reservoir. This treatment decreases the average arsenic concentration from 25 μg/L above Cottonwood to 8.3 μg/L below Haiwee. Iron- and arsenic-rich flocculated solids are removed by deposition to the reservoir sediments. Analysis of sediments shows a pronounced signature of this deposition with elevated sediment concentrations of iron, arsenic, and manganese relative to a control site. Sediment concentrations of these elements remain elevated throughout the core length sampled (ca. 4% iron and 600 and 200 μg/g of manganese and arsenic, respectively, on a dry weight basis). A pore water profile revealed a strong redox gradient in the sediment. Manganese in the pore waters increased below 5 cm; iron and arsenic increased below 10 cm and were strongly correlated, consistent with reductive dissolution of iron oxyhydroxides and concurrent release of associated arsenic to solution. X-ray absorption near-edge spectroscopy revealed inorganic As(V) present only in the uppermost sediment (0−2.5 cm) in addition to inorganic As(III). In the deeper sediments (to 44 cm), only oxygen-coordinated As(III) was detected. Analysis of the extended X-ray absorption fine structure spectrum indicates that the As(III) at depth remains associated with iron oxyhydroxide. We hypothesize that this phase persists in the recently deposited sediment despite reducing conditions due to slow dissolution kinetics.

Collision and coagulation rates between microbial aggregates and small particles were measured for individual aggregates (1.0−2.5 mm) that settled through a suspension of fluorescent yellow-green (YG) particles (2.83 μm) placed in a settling column. The microbial aggregates, with an average fractal dimension of 2.26, were generated in a lab-scale sequencing batch reactor (SBR) and also collected from a full-scale activated sludge (AS) treatment system. As calculated from comparisons between the settling velocities observed and those predicted by Stokes' law for impermeable particles, the average fluid collection efficiencies were 0.08 for the SBR aggregates and 0.14 for the AS flocs, which were much lower than those previously reported for nonbiological aggregates of latex microspheres. The collision frequency functions between microbial aggregates and small YG particles were 2 orders of magnitude lower than predicted by the rectilinear model but 1 order of magnitude greater than predicted by a curvilinear model. The overall scavenging efficiencies of suspended particles by the falling microbial aggregates compared well with those observed for the nonbiological aggregates, while the particle removal efficiencies from the flow internal to the microbial aggregates were 1 order of magnitude higher than those of the nonbiological aggregates. It is argued that the permeability of microbial aggregates could be reduced by exopolymeric material clogging the pores within the aggregates. The internal permeation through a bio-aggregate thus may not be significant enough to be included in the calculation of its settling velocity; however, the intra-aggregate flow cannot be simply neglected where coagulation is concerned. Streamlines still can penetrate the interior of microbial aggregates, allowing greater collision frequencies with other particles than predicted by the curvilinear model. The narrow and convoluted internal flow passages resulting from the collection of extracellular polymeric substances may also contribute to the higher interior particle removal efficiencies of microbial aggregates than those of more permeable, nonbiological aggregates.

Using a combination of experimental techniques, we show that Cu(II) reduction by sulfide to Cu(I) occurs in solution prior to precipitation. EPR and ⁶³Cu NMR data show that reduction to Cu(I) occurs during the reaction of equimolar amounts of Cu(II) with sulfide. ⁶³Cu solution NMR data show that Cu(I) is soluble when bound to sulfide and is in a site of high symmetry. EPR data confirm that Cu(I) forms in solution and that the mineral covellite, CuS, contains only Cu(I). Mass spectrometry data from covellite as well as laboratory prepared solid and solution CuS materials indicate that Cu₃S₃ six-membered rings form in solution. These trinuclear Cu rings are the basic building blocks for aqueous CuS molecular clusters, which lead to CuS precipitation. In controlled titration experiments where sulfide is slowly added to Cu(II), Cu₃S₃ rings and tetranuclear Cu molecular clusters (Cu₄S₅, and Cu₄S₆) form; the rings are composed primarily of Cu(II). During cluster formation from Cu₃S₃ condensation, some Cu(II) is released back into solution, indicating that Cu(II) reduction does not occur until after Cu−S bond and higher order cluster formation. Analysis of the frontier molecular orbitals for Cu(II) and sulfide indicate that an outer-sphere electron transfer is symmetry forbidden. These results are consistent with the formation of CuS bonds prior to electron transfer, which occurs via an inner-sphere process.

Contributions of biotic (cell-mediated) and abiotic (mineral-mediated) reactions to carbon tetrachloride (CT) transformation were studied in a model iron-reducing system that used hydrous ferric oxide (HFO) as the electron acceptor, acetate as the substrate, and Geobacter metallireducens as a representative dissimilative iron-reducing bacteria (DIRB). Over a period of 2−3 weeks, nanoscale magnetite particles, Fe₃O₄, were consistently formed as a product of iron respiration in this system. CT transformation rates were measured independently in resting cell suspensions of G. metallireducens or in suspensions of washed magnetite particles recovered from spent cultures. Protein and surface area-normalized expressions were derived for the biotic and abiotic reaction rates, respectively. Using the yield of total protein and magnetite surface area formed during growth in the model system as a basis for comparison, the mineral-mediated (abiotic) reaction was estimated to be 60−260-fold faster than the biotic reaction throughout the incubation period. We conclude that G. metallireducens induces CT transformation in this system primarily through the formation of reactive mineral surfaces rather than via co-metabolic mechanisms. The findings indicate that reactive biogenic minerals could play a significant role in the natural attenuation of chlorinated solvents in iron-reducing environments. A novel approach for stimulating reductive transformation of contaminants may be to enhance the formation of reactive biogenic minerals in situ.

Distinct layers with accumulated iron and manganese oxyhydroxides are found in the recent sediments of Lake Baikal (Siberia). In the South and Central Basins, these concretions accumulate close to the sediment−water interface. In northern Lake Baikal and the area of Academician Ridge, however, massive Fe/Mn crusts are formed within several thousand years at redox fronts 10 to 15 cm below the sediment surface. In some places, precipitated iron and manganese oxyhydroxides are spatially separated. The patterns are a result of secondary iron and manganese oxide precipitation. This natural long-term experiment allows the analysis of competitive adsorption and coprecipitation of trace elements with iron and manganese oxides in sediments. Background concentrations in the sediment of oxoanions (P, As, Sb, Mo); of trace metals (Cr, V, Cu, Zn, Cd, Pb); and of Mg, Ca, Sr, La, Ce, Pr, Nd, and Sm were analyzed by inductively coupled plasma mass spectrometry. Despite the differences in catchment geology of the many tributaries, they are remarkably uniform in sediment cores from different basins of Lake Baikal. Enrichment factors of P and As within Fe crusts revealed concentrations up to 14 and 58 times higher than the background, respectively. No enrichment of P and As was found in the Mn layers. By contrast, Mo accumulated exclusively in the Mn layer with up to 35-fold enrichment. Sb was only slightly enriched in both the Fe and the Mn layers. Among the trace metals studied, only Cd was found at elevated concentrations with a preference for the Mn layer. Ca and Sr were correlated with both Fe and Mn accumulations. The study quantifies the well-known specific adsorption and coprecipitation of P and As at authigenic iron oxides and of Mo on manganese oxides. In addition, the enrichment of Cd at manganese oxides in contrast to the conservative behavior of Zn and Pb reveals highly selective accumulation processes.

The effects of iron and manganese (hydr)oxide formation processes on the trace metal adsorption properties of these metal (hydr)oxides and their mixtures was investigated by measuring lead adsorption by iron and manganese (hydr)oxides prepared by a variety of methods. Amorphous iron (hydr)oxide formed by fast precipitation at pH 7.5 exhibited greater Pb adsorption (Γ_max = 50 mmol of Pb/mol of Fe at pH 6.0) than iron (hydr)oxide formed by slow, diffusion-controlled oxidation of Fe(II) at pH 4.5−7.0 or goethite. Biogenic manganese(III/IV) (hydr)oxide prepared by enzymatic oxidation of Mn(II) by the bacterium Leptothrix discophora SS-1 adsorbed five times more Pb (per mole of Mn) than an abiotic manganese (hydr)oxide prepared by oxidation of Mn(II) with permanganate, and 500−5000 times more Pb than pyrolusite oxides (βMnO₂). X-ray crystallography indicated that biogenic manganese (hydr)oxide and iron (hydr)oxide were predominantly amorphous or poorly crystalline and their X-ray diffraction patterns were not significantly affected by the presence of the other (hydr)oxide during formation. When iron and manganese (hydr)oxides were mixed after formation, or for Mn biologically oxidized with iron(III) (hydr)oxide present, observed Pb adsorption was similar to that expected for the mixture based on Langmuir parameters for the individual (hydr)oxides. These results indicate that interactions in iron/manganese (hydr)oxide mixtures related to the formation process and sequence of formation such as site masking, alterations in specific surface area, or changes in crystalline structure either did not occur or had a negligible effect on Pb adsorption by the mixtures.

A surface complexation model (SCM) for divalent metal carbonates (Ca, Mg, Sr, Ba, Mn, Fe, Co, Ni, Zn, Cd, and Pb) is developed based on new electrophoretic measurements and correlation between aqueous and surface reactions stability constants. This SCM postulates the formation of the following surface species:  >CO₃H⁰, >CO₃^-, >CO₃Me⁺, >MeOH⁰, >MeO^-, >MeOH₂⁺, >MeHCO₃⁰, and MeCO₃^- within the framework of a constant capacitance of the electric double layer. It can be used to describe the surface-controlled dissolution kinetics of divalent metal carbonates and allows determination of the order of dissolution reactions with respect to rate-controlling protonated carbonate surface groups in acid solutions (>CO₃H⁰) and hydrated metal groups (>MeOH₂⁺) in neutral to alkaline solutions. The reaction order with respect to protonated carbonate groups increases from 2 for MnCO₃ and ZnCO₃ to 4 for NiCO₃, whereas for hydrated surface metals, it augments from 2 for ZnCO₃ to ∼4 for MnCO₃ and NiCO₃. The dissolution rates at 5 ≤ pH ≤ 8 increase in the order Ni < Mg < Co < Fe < Mn < Zn < Cd < Sr ≤ Ca ≈ Ba ≈ Pb and correlate nicely with water exchange rates from the aqueous solution into the hydration sphere of the corresponding dissolved cations. Such a correlation allows the generation for all carbonates of a model describing their dissolution/precipitation kinetics, including the effect of various ligands, provided that rate constants and their activation volumes for water exchange around Me(II)−ligand dissolved complexes are available.

A detailed kinetic model has been developed to describe the oxidation of Fe(II) in seawater in both the absence and the presence of natural organic material. Experimental data were collected using a luminol chemiluminescence-based method to measure Fe(II), assuming that both the inorganic and the organically complexed species were detected. In the absence of organic matter, the data were modeled based on the Haber−Weiss mechanism with the inclusion of a back-reaction of Fe(III) with superoxide and precipitation of Fe(OH)₃. Both reactions were found to be significant using sensitivity analysis. When organic matter is present, the model was extended by organic complexation of Fe(II) and Fe(III) with the creation of a parallel oxidation pathway for Fe(II). Fe(II) oxidation at natural (nanomolar) concentrations was accurately predicted for a range of organic concentrations. The model also accounted for scavenging of superoxide by sub-nanomolar levels of dissolved copper and by organic matter when present. The presence of a relatively strong Fe(III) binding ligand was observed to significantly increase the rate of Fe(II) oxidation, while ultimately retaining most of the iron in the system in dissolved (organically complexed) form. The complexation reactions and reaction of inorganic and organically bound Fe(II) with oxygen were found to be critical reactions in the system, while Fe(III) hydrolysis became unimportant even at low organic concentrations. The superoxide radical was also observed to have a major role in the cycling of iron due to its ability to act as both an oxidant and a reductant. The model indicates that the rate constant for the reaction of Fe(II) with O₂ has generally been underestimated in previous work and that the secondary oxidation of Fe(II) by H₂O₂ and subsequently OH^• plays a relatively minor role in these systems.

The single-site Solvation, Bond Strength, and Electrostatic (SBE) model accounts for the anomalous position of silica on the surface acidity versus aqueous acidity correlation developed for metal oxides, by considering the solvation energy change in the protonation reaction implemented through the dielectric constant (1/ε_k) and the electrostatic energy change through the Pauling bond strength to bond length ratio (s/r) of the oxide. I address here why inclusion of the solid's dielectric constant brings silica into the same correlation as other oxides like TiO₂, Al₂O₃, and Fe₂O₃. The solvation and electrostatic contributions are interpreted in terms of classical concepts such as chemical hardness, polarizability, ionicity, electronegativity, and local charge densities. Silica is acidic (PZC < 7), not because of its small dielectric constant, its tetrahedral coordination, or its high bond strength alone. Surface acidity depends largely on high values of the s/r ratio. The dielectric constant of the solid affects acidity mainly by reflecting the nature of water−surface interactions. Solids with large values of ε_k are interpreted as being less polarizable and more ionic so that water, a hard polar solvent, interacts favorably with such surfaces and scales similar to water−water interactions regardless of whether the metal−oxide bond is in the solid or in the aqueous state. For these oxides, pK_a^s = pK_a^aq ± 1. Silica, with a small dielectric constant, is interpreted as being more polarizable and more covalent so that water−SiO₂ interactions scale differently than for the more ionic oxides. Such an interpretation when combined with the Partial Charge Model for metal hydrolysis suggests that the surfaces of RuO₂, WO₃, Sb₂O₅, and Ta₂O₅ should be acidic similar to silica. But, unlike silica, they would lie on the pK_a correlation defined by the other oxides because of their larger dielectric constants. The mixed oxide, AlPO₄, is predicted to behave like silica.

The biogeochemical processes controlling the speciation and transport of manganese in a Colorado mountain stream were studied using a conservative tracer approach combined with laboratory experiments. The study stream, Lake Fork Creek, receives manganese-rich inflows from a wetland contaminated by acid mine drainage. A conservative tracer experiment was conducted on a 1300-m reach of the stream. Results indicate that manganese was progressively removed from the stream, resulting in a loss of 64 ± 17 μmol day^-1 m^-1. Laboratory experiments using streamwater, mine effluent, and rocks from the stream showed the importance of surface-catalyzed oxidation and photoreduction on the speciation of manganese. The field and modeling results indicate that light generally promotes oxidation and removal of manganese from the stream, presumably through a photosynthetically enhanced oxidation process. Differences in Mn speciation within the stream suggest that reductive processes affect Mn speciation within the water column. These results identify the rapid oxidation and precipitation of MnO_x as a dominant process within this freshwater stream.

The biological-mediated redox cycle of Fe was studied in the epilimnion of Lake Kinneret (Sea of Galilee), a mesotrophic lake in Israel. Multi-annual lake water sampling and incubation experiments were carried out to study Fe(III) reduction by natural phytoplankton populations and their possible role in inhibiting Fe(II) oxidation. The reduction characteristics of the dinoflagellate Peridinium gatunense, the dominant lake alga, were further examined in the laboratory. The steady-state concentration of Fe(II) calculated from the assessed reduction and oxidation rates was compared with Fe(II) measured in the lake in order to evaluate the significance of these processes to the lake Fe redox cycle. Nanomolar concentrations of Fe(II) were measured in the oxygenated, high pH, upper water layer of the lake throughout the year. Reduction rates of Fe by natural phytoplankton assemblages ranged between 0.1 and 10 nM/h. The highest reduction rates, determined in dinoflagellate-dominated lake waters, coincided with the highest concentrations of Fe(II) measured simultaneously in the lake. Iron(II) oxidation rates calculated from the measured lake Fe(II) and the obtained reduction rates were significantly slower than published abiotic Fe(II) oxidation rates. Indeed, Fe(II) oxidation rates measured in algal-enriched lake water were 30-fold slower than Fe(II) oxidation rates in natural water, demonstrating the potential for Fe(II) stabilization by the lake phytoplankton.

The geochemical behavior of Co in aquatic systems has often been related to the presence of Fe and Mn particles. A few studies have shown that Co is exclusively associated with particulate Mn, but the dynamics of Co and Mn cycling have never been determined in real time under natural conditions. In this study, we used a combination of analytical techniques to study the temporal and spatial evolution of Mn microparticles (MnO_x) over 2 weeks in the water column of a shallow stratified lake (Paul Lake, MI). We report a temporal accumulation of dissolved Mn at the oxic−anoxic transition, and we show that this accumulation is due to the reductive dissolution of Mn particles. The reductant has not been identified, but abiotic reduction by ∑H₂S and ferrous iron is excluded because they are produced below the zone of MnO_x reduction. Hybridization of RNA isolated from Paul Lake with oligonucleotide probes targeting the δ proteobacteria, which include metal-reducing species, suggests that their activity is greatest at and just below the oxic−anoxic transition, so that Mn reduction may be influenced by bacterial activity. Mn-oxidizing bacteria were isolated from this zone as well. We also demonstrate that the dynamic evolution of MnO_x has a direct influence on the distribution of Co in the water column of this lake:  dissolved Co is released during the reductive dissolution of MnO_x and accumulates at the redox interface.

This study reports results of sediment bioslurry treatment and earthworm bioaccumulation for polycyclic aromatic hydrocarbon (PAH) contaminants found in sediment dredged from Milwaukee Harbor. A significant finding was that bioslurry treatment reduced PAHs on the sediment clay/silt fraction but not on the sediment coal-derived fraction and that PAH reduction in the clay/silt fraction correlated with substantial reduction in earthworm PAH bioaccumulation. These findings are used to infer PAH bioavailability from characterization of particle-scale PAH distribution, association, and binding among the principal particle fractions in the sediment. The results are consistent with work showing that the sediment comprised two principal particle classes for PAHs, coal-derived and clay/silt, each having much different PAH levels, release rates, and desorption activation energies. PAH sorption on coal-derived particles is associated with minimal biodegradation, slow release rates, and high desorption activation energies, while PAH sorption on clay/silt particles is associated with significant potential biodegradability, relatively fast release rates, and lower desorption activation energies. These characteristics are attributed to fundamental differences in the organic matter to which the PAHs are sorbed. Although the majority of the PAHs are found preferentially on coal-derived particles, the PAHs on the clay/silt sediment fraction are more mobile and available, and thus potentially of greater concern. This study demonstrates that a suite of tests comprising both bioassays and particle-scale investigations provide a basis to assess larger-scale phenomena of biotreatment of PAH-impacted sediments and bioavailability and potential toxicity of PAH contaminants in sediments. Improved understanding of contaminant bioavailability aids decision-making on the effectiveness of biotreatment of PAH-impacted sediments and the likelihood for possible reuse of dredged sediments as reclaimed soil or fill.

Studies have examined partitioning of trace metals onto natural particles to better understand the fate and transport of trace metals in the environment, but few studies have compared model predictions with field results. We evaluate the application of an empirical modeling approach, using surface complexation parameters available in the literature, to complex natural systems. In this work, the equilibrium speciation computer program PHREEQC was used along with the diffuse double-layer surface complexation model to simulate metal removal onto natural oxide particles formed during the mixing of acid rock drainage with ambient surface water. End-member solutions sampled in the Coeur d'Alene (CdA) Mining District in September 1999 from the Bunker Hill Mine and the South Fork Coeur d'Alene (SFCdA) River were filtered and mixed in several ratios. Solution chemistry was determined for end-members and mixed solutions, and X-ray diffraction (XRD) was used to determine the mineralogy of precipitate phases. Predicted amounts of Fe precipitates were in good agreement with measured values for particulate Fe. Surface area and reactive site characteristics were used along with surface complexation constants for ferrihydrite (Dzombak, D. A.; Morel, F. M. M. Surface Complexation Modeling:  Hydrous Ferric Oxide; John Wiley & Sons:  New York, 1990) to predict ion sorption as a function of mixing fraction. Comparisons of model predictions with field results indicate that Pb and Cu sorption are predicted well by the model, while As, Mo, and Sb sorption are less well-predicted. Additional comparisons with particulate metal and Fe data collected from the CdA Mining District in 1996 and 1997 suggest that sorption on particulate Fe, including amorphous iron oxides and schwertmannite, may be described using universal model parameters.

Solution chemical techniques were used to investigate the oxidation of As(III) to As(V) in 0.011 M arsenite suspension of well-crystallized hexagonal birnessite (H-birnessite, 2.7 g L^-1) at pH 5. Products of the reaction were studied by scanning electron microscopy coupled with energy dispersive spectroscopy (SEM-EDS), atomic force microscopy (AFM), and X-ray absorption near-edge structure spectroscopy (XANES). In the initial stage (first 74 h), chemical results have been interpreted quantitatively, and the reaction is shown to proceed in two steps as suggested by previous authors:  2>Mn^IVO₂ + H₃AsO₃ + H₂O → 2>Mn^IIIOOH + H₂AsO₄^- + H⁺ and 2>Mn^IIIOOH + H₃AsO₃ + 3H⁺ → 2Mn²⁺ + H₂AsO₄^- + 2H₂O. The As(III) depletion rate was lower (0.02 h^-1) than measured in previous studies because of the high crystallinity of the H-birnessite sample used in this study. The surface reaction sites are likely located on the edges of H-birnessite layers rather than on the basal planes. The ion activity product of Mn(II) and As(V) reached after 74 h reaction time was the solubility product of a protonated manganese arsenate, having a chemical composition close to that of krautite as identified by XANES and EDS. Krautite precipitation reaction can be written as follows:  Mn²⁺ + H₂AsO₄^- + H₂O = MnHAsO₄·H₂O + H⁺ log K_s ≈ −0.2. Equilibrium was reached after 400 h. The manganese arsenate precipitate formed long fibers that aggregated at the surface of H-birnessite. The oxidation reaction transforms a toxic species, As(III), to a less toxic aqueous species, which further precipitates with Mn²⁺ as a mixed As−Mn solid characterized by a low solubility product.

Sorption of the ionic compounds 2,4-D and quinmerac onto iron oxide-rich, variable charged soils was strongly influenced by mineralogy, particularly soil iron and aluminum oxides, whereas sorption of the neutral norflurazon was only related to total soil C. An appreciable fraction of the mass sorbed in stirred-flow studies was easily desorbed by deionized water, and desorption of ionic compounds was initially more rapid than sorption. This sorption−desorption behavior, although contrary to desorption hysteresis commonly observed in batch studies, suggests that the reversibly sorbed fraction is weakly bound to the soil surface. 2,4-D sorption to iron oxide-rich soils and pure-phase metal oxides appears to be driven by nonspecific electrostatic attraction, with specific electrostatic attraction and van der Waals interactions being secondary. Both the carboxylate and the heterocyclic N groups may participate in sorption of quinmerac, facilitated by specific and nonspecific electrostatic attraction and surface complexation. The heterocyclic N, amine, and carbonyl groups of norflurazon do not appear to interact with soil minerals.

The maintenance of monochloramine residuals in drinking water distribution systems is one technique often used to minimize microbial outbreaks and thereby maintain the safety of the water. Reactions between oxidizable species and monochloramine can however lead to undesirable losses in the disinfectant residual. Previous work has illustrated that the Fe(II) present within distribution systems is one type of oxidizable species that can exert a monochloramine demand. This paper extends this prior work by examining the kinetics of the reactions between Fe(II) and monochloramine in the presence of a variety of iron oxide surfaces. The identity of the iron oxide plays a significant role in the rate of these reactions. Surface area-normalized initial rate coefficients (k_init) obtained in the presence of each oxide at pH ≈6.9 exhibit the following trend in catalytic activity:  magnetite > goethite > hematite ≈ lepidocrocite > ferrihydrite. The differences in the activity of these oxides are hypothesized to result from variations in the amount of Fe(II) sorbed to each of the oxides and to dissimilarities in the surface site densities of the oxides. The implications of carbonate on Fe(II) sorption to iron oxides are also examined. Comparing Fe(II) sorption isotherms for goethite obtained under differential carbonate concentrations, it is apparent that as the carbonate concentration (C_T,CO3) increased from 0 to 11.7 mM that the Fe(II) sorption edge (50% sorption) shifts from a pH of approximately 5.8 to a pH of 7.8. This shift is hypothesized to be the result of the formation of aqueous and surface carbonate−Fe(II) complexes and to competition between carbonate and Fe(II) for surface sites. The implications of these changes are then discussed in light of the variable oxide studies.