Photochemical coupling reactions between Fe(III)/Fe(II), Cr(VI)/Cr(III), and polycarboxylates: inhibitory effect of Cr species

The roles of chromium species on photochemical cycling of iron and mineralization of polycarboxylates are examined in the presence of Cr(VI) or Cr(III) at pH 2.2-4.0. Under UV irradiation, Cr(III) altered the redox equilibrium of iron species, leading to the shift of the photosteady state toward Fe(II). After a longer time of illumination, total organic carbon (TOC) approached a steady state in the presence of Cr(III) or Cr(VI), whereas oxalate was thoroughly mineralized in the absence of Cr species. The TOC of steady state was closely related to the kind of polycarboxylates, Cr species dosages, pH and O2 atmosphere, but hardly affected by more addition of Fe(III). ESI-MS data indicates that several Cr-oxalate complexes formed in the photochemical reactions, which are responsible for protecting oxalate against further oxidation. A mechanism is proposed for the inhibitory effect of Cr species on oxidation of oxalate and Fe(II). The present study may provide a new insight into the dual environmental effects induced by Cr contaminants especially at heavily chromium-contaminated and dissolved organic matter (DOM)-rich sites.     

Inhibited Cr(VI) reduction by aqueous Fe(II) under hyperalkaline conditions

This study investigated Cr(VI) reduction by dissolved Fe(II) in hyperalkaline pH conditions as found in fluid wastes associated with the U.S. nuclear weapons program. The results show that Cr(VI) reduction by Fe(II) at alkaline pH solutions proceeds very quickly. The amount of Cr(VI) removed from solution and the amount reduced increases with Fe(II):Cr(VI) ratio. However, the Cr(VI) reduction under alkaline pH condition is nonstoichiometric, probably due to Fe(II) precipitation and mixed iron(III)-chromium-(III) (oxy)hydroxides blocking Fe(II) surface sites, as well as removing Fe(II) from solution through O2 oxidation. After Cr(VI) was reduced to Cr(III), it precipitated out as mixed Fe(x)Cr1-xO3(solids) and various Fe(III) precipitates with an overall Cr:Fe ratio of 1:3; all Cr remaining in the solution phase was unreduced Cr(VI). EXAFS data showed that Cr-O and Cr-Cr distances in the precipitates equal to 1.98 and 3.01 A, respectively, consistent with the spinel-type structure as chromite.     

Photocatalytic reduction of Cr(VI) in the presence of NO3- and Cl- electrolytes as influenced by Fe(III)

Photoreduction of Cr(VI) involving Fe is strongly affected by the presence of organic or inorganic compounds in an acidic environment. In this study, we have found a new pathway of Cr(VI) photoreduction in the presence of Fe-(III) that is influenced by two inorganic electrolytes (i.e., NO3- and Cl-) and the pH. In NO3- and Cl- systems without Fe(III), Cr(VI) photoreduction could occur and was independent of the Cr(VI) concentration. The zero-order rate constant of the photoreduction reaction increased when the solution pH was decreased from 2 to 1; the reaction rate was higher in the NO3- system than in the Cl- system. The higher reaction rate in the NO3- system was attributed to the photolysis of NO3-, which resulted in the formation of NO2- for reduction of Cr(VI). Conversely, the effect of Fe-(III) addition on the increase in Cr(VI) photoreduction rate in the Cl- system was more significant than that in the NO3- system. The addition of Fe(III) to the Cl- system caused the formation of [Fe(OH2)5Cl]2+, the photolysis of which subsequently resulted in the formation of Fe(II) for reduction of the Cr(VI). This study suggests that the photolysis of NO3- and Fe-Cl complex may contribute significantly to Cr(VI) reduction in surface water that receives electroplating wastewater containing high levels of NO3-, Cl-, and Fe-(III). Therefore, under the acidic conditions that are favorable for Fe-Cl complex formation or in the presence of NO3-, the effects of inorganic components on Cr(VI) photoreduction cannot be ignored for the precise evaluation of the transformation of Cr in the environment.     

Photochemical oscillation of Fe(II)/Fe(III) ratio induced by periodic flux of dissolved organic matter

Variation of iron species in the UV-irradiated aqueous solution was examined in the presence of various dissolved organic matter (DOM). Under the irradiation at constant light intensity, a regular oscillation in the ratio of Fe(II) to total iron, Fe(II)/Fe(t), was observed when DOM was periodically added into the solution. In each cycle, the Fe(II)/Fe(t) ratio increased initially and then decreased with concomitant degradation of DOM. The Fe(II)/Fe(t) ratio approached a constant value after the DOM was completely mineralized. The period and amplitude of the oscillation were dependent on DOM structure and its initial concentration, but the ultimate photosteady state was not affected by DOM. It was revealed that both DOM and photoreactive Fe(III) species were indispensable for the fluctuation in Fe(II)/Fe(t) ratio. The ultimate photosteady state originated from the equilibrium between Fe(III) photoreduction and aerobic Fe(II) photooxidation induced simultaneously by UV irradiation. It was the DOM that disturbed these two opposite processes, leading to the oscillation in Fe(II)/ Fe(t) ratio under UV irradiation.     

Kinetics of Cr(VI) reduction by carbonate green rust

The kinetics of Cr(VI) reduction to Cr(III) by carbonate green rust were studied for a range of reactant concentrations and pH values. Carbonate green rust, [FeII4FeIII2(OH)12][4H2O x CO3], was synthesized by induced hydrolysis (i.e., coprecipitation) of an Fe(ll)/Fe(III) solution held at a constant pH of 8. An average specific surface area of 47 +/- 7 m2 g(-1) was measured for five separate batches of freeze-dried green rust precipitate. Heterogeneous reduction by Fe(II) associated with the carbonate green rust appears to be the dominant pathway controlling Cr(VI) loss from solution. The apparent stoichiometry of the reaction between ferrous iron associated with green rust ([Fe(II)GR]) and Cr(VI) was slightly higherthan the expected 3:1 ratio, possibly due to the presence of other oxidants, such as oxygen, protons, or interlayer carbonate ions. The rate of Cr(VI) reduction was proportional to the green rust surface area concentration, and psuedo-first-order rate coefficients (kobs) ranging from 1.2 x 10(-3) to 11.2 x 10(-3) s(-1) were determined. The effect of pH was small with a 5-fold decrease in rate with increasing pH (from 5.0 to 9.0). At low Cr(VI) concentrations (<200 microM), the rate of reaction was first order with respect to Cr(VI) concentration, whereas, at high Cr(VI) concentrations, rates appearto deviate from first-order kinetics and approach a constant value. Estimated amounts of surface Fe(II) and total Fe(II) suggest that the deviation from first-order kinetics observed at higher Cr(VI) concentrations and the 50-fold decrease in rate observed upon three sequential exposures to Cr(VI) is due to exhaustion of available Fe(II).     

Influence of dissolved organic matter and Fe(II) on the abiotic reduction of pentachloronitrobenzene

Nitroaromatic pesticides (NAPs) are hydrophobic contaminants that can accumulate in sediments by the deposition of suspended solids from surface waters. Fe(II) and dissolved organic matter (DOM), present in suboxic and anoxic zones of freshwater sediments, can transform NAPs in natural systems. We studied the reduction of pentachloronitrobenzene (PCNB) to pentachloroaniline (PCA) in controlled studies using Fe(II) and surface water DOM isolates from Pony Lake, Antarctica, and Suwannee River, GA, in unfiltered and 0.45 microm filtered solutions. We observed rapid reduction of PCNB to PCA in the presence of Fe(II) and DOM (t(1/2) approximately = 30 min to 4 h) and very limited reduction in DOM-only systems. DOM in unfiltered systems inhibited iron colloid formation and potentially limited the formation of reactive Fe(ll)-iron colloid surface complexes, causing reductive transformation in Fe(II)-DOM media to be slower in some cases relative to Fe(ll)-only controls. Conversely, in 0.45 microm filtered solutions, PCNB reduction in Fe(III)-DOM media was faster than the Fe(II)-only controls, suggesting that DOM enhances the reductive capacity of Fe(ll) in the absence of iron colloids. This work shows that DOM may significantly affect the reactivity of Fe(ll) toward NAPs under suboxic and anoxic conditions in natural wetland sediments.     

Kinetics of nitrate, nitrite, and Cr(VI) reduction by iron metal

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 (2-4x) 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 (kSA) of 9.0 x 10(-3) and 2.2 x 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.     

Spectroscopic investigation of Cr(III)- and Cr(VI)-treated nanoscale zerovalent iron

The reaction of hexavalent chromium (Cr(VI)) with zerovalent iron (Fe0) during soil and groundwater remediation is an important environmental process. This study used several techniques including X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy to investigate nanometer scale Fe0 particles (nano Fe0) treated with Cr(III) and Cr(VI). X-ray diffraction and XPS analyses of oxidized nano Fe0 showed the crystalline Fe(III) phase is composed of lepidocrocite (gamma-FeOOH). Results of XPS Cr 2p data and Cr K-edge X-ray absorption near edge spectroscopy (XANES) provided evidence that Cr(VI) was entirely reduced to Cr(III) by nano Fe0 with no residual Cr(VI) after reaction. In addition, XPS and XANES results of Cr(III) precipitated as Cr(OH)3 in the presence of corroding nano Fe0 were nearly identical to the Cr(VI)-nano Fe0 reaction product. Detailed analysis of XPS O 1s line spectra revealed that both Cr(III)- and Cr(VI)-treated nano Fe0 yielded a predominantly hydroxylated Cr(OH)3 and/ or a mixed phase CrxFe(1 - x)(OH)3 product. The structure of the Cr(III)- and Cr(VI)-treated nano Fe0 determined using extended X-ray absorption fine structure spectroscopy (EXAFS) revealed octahedral Cr(III) with Cr-O interatomic distances between 1.97 and 1.98 A for both Cr(III) and Cr(VI) treatments and a pronounced Cr-Cr second interatomic shell at 3.01 A. Our results suggest that the reaction product of Cr(VI)-treated nano Fe0 is either a poorly ordered Cr(OH)3 precipitate or possibly a mixed phase CrxFe(1 - x)(OH)3 product, both of which are highly insoluble under environmental conditions.     

Cr stable isotopes in Snake River Plain aquifer groundwater: evidence for natural reduction of dissolved Cr(VI)

At Idaho National Laboratory, Cr(VI) concentrations in a groundwater plume once exceeded regulatory limits in some monitoring wells but have generally decreased over time. This study used Cr stable isotope measurements to determine if part of this decrease resulted from removal of Cr(VI) via reduction to insoluble Cr(III). Although waters in the study area contain dissolved oxygen, the basalt host rock contains abundant Fe(II) and may contain reducing microenvironments or aerobic microbes that reduce Cr(VI). In some contaminated locations, (53)Cr/(52)Cr ratios are close to that of the contaminant source, indicating a lack of Cr(VI) reduction. In other locations, ratios are elevated. Part of this shift may be caused by mixing with natural background Cr(VI), which is present at low concentrations but in some locations has elevated (53)Cr/(52)Cr. Some contaminated wells have (53)Cr/(52)Cr ratios greater than the maximum attainable by mixing between the inferred contaminant and the range of natural background observed in several uncontaminated wells, suggesting that Cr(VI) reduction has occurred. Definitive proof of reduction would require additional evidence. Depth profiles of (53)Cr/(52)Cr suggest that reduction occurs immediately below the water table, where basalts are likely least weathered and most reactive, and is weak or nonexistent at greater depth.     

In situ chemical reduction of Cr(VI) in groundwater using a combination of ferrous sulfate and sodium dithionite: a field investigation

A field study was conducted to evaluate the performance of a ferrous iron based in situ redox zone for the treatment of a dissolved phase Cr(VI) plume at a former industrial site. The ferrous iron based in situ redox zone was created by injecting a blend of 0.2 M ferrous sulfate and 0.2 M sodium dithionite into the path of a dissolved Cr(VI) plume within a shallow medium to fine sand unconfined aquifer formation. Monitoring data collected over a period of 1020 days after more than 100 m of linear groundwater flow through the treatment zone indicated sustained treatment of dissolved phase Cr(VI) from initial concentrations between 4 and 8 mg/L to less than 0.015 mg/L. Sustained treatment is assumed to be primarily due to the reduction of Cr(VI) to Cr(III) by ferrous iron adsorbed to, precipitated on, and/or incorporated into aquifer iron (hydr)oxide solid surfaces within the treatment zone. Precipitated phases likely include FeCO3 and FeS based on saturation index considerations and SEM/EDS analysis. The detection of solid phase sulfites and thiosulfates in aquifer sediments collected from the treatment zone more than 2 years following injection suggests dithionite decomposition products may also play a significant role in the long-term treatment of the dissolved phase Cr(VI).     

Reduction of Cr(VI) under acidic conditions by the facultative Fe(lll)-reducing bacterium Acidiphilium cryptum

The potential for biological reduction of Cr(VI) under acidic conditions was evaluated with the acidophilic, facultatively metal-reducing bacterium Acidiphilium cryptum strain JF-5 to explore the role of acidophilic microorganisms in the Cr cycle in low-pH environments. An anaerobic suspension of washed A. cryptum cells rapidly reduced 50 microM Cr(VI) at pH 3.2; biological reduction was detected from pH 1.7-4.7. The reduction product, confirmed by XANES analysis, was entirely Cr(III) that was associated predominantly with the cell biomass (70-80%) with the residual residing in the aqueous phase. Reduction of Cr(VI) showed a pH optimum similar to that for growth and was inhibited by 5 mM HgCl2, suggesting that the reaction was enzyme-mediated. Introduction of O2 into the reaction medium slowed the reduction rate only slightly, whereas soluble Fe(III) (as ferric sulfate) increased the rate dramatically, presumably by the shuttling of electrons from bioreduced Fe(II) to Cr(VI) in a coupled biotic-abiotic cycle. Starved cells could not reduce Cr(VI) when provided as sole electron acceptor, indicating that Cr(VI) reduction is not an energy-conserving process in A. cryptum. We speculate, rather, that Cr(VI) reduction is used here as a detoxification mechanism.     

Reduction of Cr(VI) at a polyaniline film: influence of film thickness and oxidation state

Hexavalent chromium [Cr(VI)] is highly toxic, carcinogenic, and mutagenic to living organisms. In this paper, the reduction of Cr(VI) to the much less toxic trivalent state [Cr(III)] was studied at polyaniline films grown to different thickness. Much higher rates of Cr(VI) reduction were observed for the "thick" polyaniline films. This was explained in terms of the morphology of the polymer and the higher surface area of polymer in contact with the Cr(VI) solution. For "thin" polyaniline films, the Cr(VI) reduction reaction was found to obey pseudo-first-order kinetics for the duration of exposure. However, in the case of thick polyaniline layers, the Cr(VI) reduction reaction followed a two-stage process, with each stage obeying pseudo-first-order kinetics. This was explained in terms of oxidation of the polymer from the leucoemeraldine to the emeraldine state and then further oxidation of the polymer from the emeraldine to the pernigraniline state. Much higher rates of Cr(VI) reduction were observed on oxidation of the polymer from the leucoemeraldine to the emeraldine state.     

Chromium isotope fractionation during reduction of Cr(VI) under saturated flow conditions

Chromium isotopes are potentially useful indicators of Cr(VI) reduction reactions in groundwater flow systems; however, the influence of transport on Cr isotope fractionation has not been fully examined. Laboratory batch and column experiments were conducted to evaluate isotopic fractionation of Cr during Cr(VI) reduction under both static and controlled flow conditions. Organic carbon was used to reduce Cr(VI) in simulated groundwater containing 20 mg L(-1) Cr(VI) in both batch and column experiments. Isotope measurements were performed on dissolved Cr on samples from the batch experiments, and on effluent and profile samples from the column experiment. Analysis of the residual solid-phase materials by scanning electron microscopy (SEM) and by X-ray absorption near edge structure (XANES) spectroscopy confirmed association of Cr(III) with organic carbon in the column solids. Decreases in dissolved Cr(VI) concentrations were coupled with increases in δ(53)Cr, indicating that Cr isotope enrichment occurred during reduction of Cr(VI). The δ(53)Cr data from the column experiment was fit by linear regression yielding a fractionation factor (α) of 0.9979, whereas the batch experiments exhibited Rayleigh-type isotope fractionation (α = 0.9965). The linear characteristic of the column δ(53)Cr data may reflect the contribution of transport on Cr isotope fractionation.     

Influence of Anionic Cosolutes and pH on Nanoscale Zerovalent Iron Longevity: Time Scales and Mechanisms of Reactivity Loss toward 1,1,1,2-Tetrachloroethane and Cr(VI)

Nanoscale zerovalent iron (NZVI) was aged over 30 days in suspension (2 g/L) with different anions (chloride, perchlorate, sulfate, carbonate, nitrate), anion concentrations (5, 25, 100 mN), and pH (7, 8). During aging, suspension samples were reacted periodically with 1,1,1,2-tetrachloroethane (1,1,1,2-TeCA) and Cr(VI) to determine the time scales and primary mode of NZVI reactivity loss. Rate constants for 1,1,1,2-TeCA reduction in Cl(-), SO(4)(2-), and ClO(4)(-) suspensions decreased by 95% over 1 month but were generally equivalent to one another, invariant of concentration and independent of pH. In contrast, longevity toward 1,1,1,2-TeCA depended upon NO(3)(-) and HCO(3)(-) concentration, with complete reactivity loss over 1 and 14 days, respectively, in 25 mN suspensions. X-ray diffraction suggests that reactivity loss toward 1,1,1,2-TeCA in most systems results from Fe(0) conversion into magnetite, whereas iron carbonate hydroxide formation limits reactivity in HCO(3)(-) suspensions. Markedly different trends in Cr(VI) removal capacity (mg Cr/g NZVI) were observed during aging, typically exhibiting greater longevity and a pronounced pH-dependence. Notably, a strong linear correlation exists between Cr(VI) removal capacities and rates of Fe(II) production measured in the absence of Cr(VI). While Fe(0) availability dictates longevity toward 1,1,1,2-TeCA, this correlation suggests surface-associated Fe(II) species are primarily responsible for Cr(VI) reduction.     

Removal of chromium (VI) by acid-washed zero-valent iron under various groundwater geochemistry conditions

The hexavalent chromium (Cr(VI)) removal capacity of acid-washed zerovalent iron (AW-Fe0) was evaluated under different groundwater geochemistry conditions through column experiments. It was found that each gram of the AW-Fe0 could remove 0.65-1.76 mg of Cr(VI) from synthetic groundwater in the absence of bicarbonate (HCO3-), magnesium and/or calcium ions. Groundwater geochemistry was found to exert various degrees of impact on Cr(VI) removal by the AW-Fe0, in which HCO3- alone gave the mildest impact whereas the copresence of calcium and HCO3- exerted the greatest impact In comparison with the unwashed Fe0, the AW-Fe0 showed a poorer Cr(VI) removal capacity and was also more susceptible to the influence of the dissolved groundwater constituents on Cr(VI) removal,thereby indicating the unsuitability of using AW-Fe0 in permeable reactive barriers for remediation of Cr(VI)-contaminated groundwater. On the AW-Fe0 surface, where the indigenous iron precipitates were almost erased, trivalent chromium including chromium (III) oxides, hydroxides, and oxyhydroxides in irregular strip, chick footmark-liked or boulder-liked forms as well as Cr(III)-Cr(VI) mixed oxides were detected.     

Using chromium stable isotope ratios to quantify Cr(VI) reduction: lack of sorption effects

Chromium stable isotope values can be effectively used to monitor reduction of Cr(VI) in natural waters. We investigate effects of sorption during transport of Cr(VI) which may also shift Cr isotopes values, complicating efforts to quantify reduction. This study shows that Cr stable isotope fractionation caused by sorption is negligible. Equilibrium fractionation of Cr stable isotopes between dissolved Cr(VI) and Cr(VI) adsorbed onto gamma-Al2O3 and goethite is less than 0.04 per thousand (53Cr/52Cr) under environmentally relevant pH conditions. Batch experiments at pH 4.0 and pH 6.0 were conducted in series to sequentially magnify small isotope fractionations. A simple transport model suggests that adsorption may cause amplification of a small isotope fractionation along extreme fringes of a plume, leading to shifts in 53Cr/52Cr values. We therefore suggest that isotope values at extreme fringes of Cr plumes be critically evaluated for sorption effects. A kinetic effect was observed in experiments with goethite at pH 4 where apparently lighter isotopes diffuse into goethite clumps at a faster rate before eventually reaching equilibrium. This observed kinetic effect may be important in a natural system that has not attained equilibrium and is in need of further study. Cr isotope fractionation caused by speciation of Cr(VI) between HCrO4- and CrO4(2-) was also examined, and we conclude that it is not measurable. In the absence of isotope fractionation caused by equilibrium speciation and sorption, most of the variation in delta53Cr values may be attributed to reduction, and reliable estimates of Cr reduction can be made.     

XANES spectroscopy studies of Cr(VI) reduction by thiols in organosulfur compounds and humic substances

The reduction of Cr(VI) by the thiol-containing compounds cysteine and glutathione and by reduced sulfur in humic substances was monitored with sulfur and chromium X-ray absorption near-edge structure (XANES) spectroscopy in chromium-contaminated soils. Reaction of humic acids with Cr(VI) resulted in a reduction of the peak area of thiols and an increase in the peak area of disulfides in the sulfur XANES spectra. Analysis of the sulfur XANES spectra in various systems indicates that the reduction of Cr(VI) by humic substances involves a thiol/disulfide redox couple analogous to that of the Cr(VI) reduction by the simple thiol-containing compounds cysteine and glutathione. A fraction of the hexavalent chromium present in industrially-contaminated soils was not reducible by thiols. Reduction of Cr(VI) to Cr(III) in soils by thiols has little effect on the pH of the system in contrast to the pH decrease resulting from reduction by Fe(II).     

Photoirradiation of dissolved humic acid induces arsenic(III) oxidation

The fate of arsenic in aquatic systems is influenced by dissolved natural organic matter (DOM). Using UV-A and visible light from a medium-pressure mercury lamp, the photosensitized oxidation of As(III) to As(V) in the presence of Suwannee River humic acid was investigated. Pseudo-first-order kinetics was observed. For 5 mg L(-1) of dissolved organic carbon (DOC) and 1.85 mEinstein m(-2) s(-1) UV-A fluence rate, the rate coefficient k degrees exp was 21.2 +/- 3.2 10(-5) s(-1), corresponding to a half-life <1 h. Rates increased linearly with DOC and they increased by a factor of 10 from pH 4 to 8. Based on experiments with radical scavengers, heavy water, and surrogates for DOM, excited triplet states and/or phenoxyl radicals seem to be important photooxidants in this system (rather than singlet oxygen, hydrogen peroxide, hydroxyl radicals, and superoxide). Photoirradiation of natural samples from freshwater lakes, rivers, and rice field water (Bangladesh) showed similar photoinduced oxidation rates based on DOC. Fe(III) (as polynuclear Fe(III)-(hydr)oxo complexes or Fe(III)-DOC complexes) accelerates the rate of photoinduced As(III) oxidation in the presence of DOC by a factor of 1.5-2.     

Chromium(VI) bioremoval by Pseudomonas bacteria: role of microbial exudates for natural attenuation and biotreatment of Cr(VI) contamination

Laboratory batch and column experiments were conducted to investigate the role of microbial exudates, e.g., exopolymeric substance (EPS) and alginic acid, on microbial Cr(VI) reduction by two different Pseudomonas strains (P. putida P18 and P. aeuroginosa P16) as a method for treating subsurface environment contaminated with Cr(VI). Our results indicate that microbial exudates significantly enhanced microbial Cr(VI) reduction rates by forming less toxic and highly soluble organo-Cr(III) complexes despite the fact Cr(III) has a very low solubility under the experimental conditions studied (e.g., pH 7). The formation of soluble organo-Cr(III) complexes led to the protection of the cells and chromate reductases from inactivation. In systems with no organic ligands, soluble organo-Cr(III) end products were formed between Cr(III) and the EPS directly released by bacteria due to cell lysis. Our results also provide evidence that cell lysis played an important role in microbial Cr(VI) reduction by Pseudomonas bacteria due to the release of constitutive reductases that intracellularly and/or extracellularly catalyzed the reduction of Cr(VI) to Cr(III). The overall results highlight the need for incorporation of the release and formation of organo-Cr(III) complexes into reactive transport models to more accurately design and monitor in situ microbial remediation techniques for the treatment of subsurface systems contaminated with Cr(VI).