Microbiological reduction of chromium(VI) in presence of pyrolusite-coated sand by Shewanella alga Simidu ATCC 55627 in laboratory column experiments

Hexavalent chromium (Cr(VI)) was reduced to non-toxic trivalent chromium (Cr(III)) by a dissimilatory metal reducing bacteria, Shewanella alga Simidu (BrY-MT) ATCC 55627. A series of dynamic column experiments were conducted to provide an understanding of Cr(VI) reduction by the facultative anaerobe BrY-MT in the presence of pyrolusite (beta-MnO(2)) coated sand and uncoated-quartz sand. All dynamic column experiments were conducted under growth conditions using Cr(VI) as the terminal electron acceptor and lactate as the electron donor and energy source. Reduction of Cr(VI) was rapid (within 8 h) in columns packed with uncoated quartz sand and BrY-MT, whereas Cr(VI) reduction by BrY-MT was delayed (57 h) in the presence of beta-MnO(2)-coated sand. The role of beta-MnO(2) in this study was to provide oxidation of trivalent chromium (Cr(III)). BrY-MT attachment was higher on beta-MnO(2)-coated sand than on uncoated quartz sand at 10, 60, and 85.5 h. Results have shown that this particular strain of Shewanella did not appreciably reduce Mn(IV) to Mn(II) species nor biosorbed Cr and Mn during its metabolic activities.     

Reduction of chromate (CrO4(2-)) by an enrichment consortium and an isolate of marine sulfate-reducing bacteria

An enrichment consortium and an isolate (isolate TKW) of sulfate-reducing bacteria (SRB) have been obtained from metal-contaminated marine sediments of Tokwawan, Hong Kong SAR. These bacteria are capable of reducing highly toxic and soluble hexavalent chromium (Cr6+) enzymatically into less toxic and insoluble trivalent chromium (Cr3+) under anaerobic conditions. The enrichment consortium almost completely (98.5%) reduced 0.6 mM Cr6+ in 168 h and the rate of reduction was 0.5 g (Cr6+) g(protein)(-1)h(-1). In comparison, with Cr6+ as the sole electron acceptor (as a surrogate for SO4(2-)), isolate TKW reduced 94.5% of the initially added Cr6+ (0.36 mM) in 288 h, with the rate of 0.26 g (Cr6+) g(protein)(-1)h(-1). Adsorption by these bacteria was not the major mechanism contributing to the transformation or removal of Cr6+. The biomass and Cr3+ in the cultures increased simultaneously with the reduction of Cr6+. These indigenous SRB might have potential application in bioremediation of metal contaminated sediments.     

Characterization of a simple bacterial consortium for effective treatment of wastewaters with reactive dyes and Cr(VI)

A microbial consortia consisting of three bacteria isolated from tanning and textile wastewaters revealed high capacity to simultaneously bioaccumulate dye and Cr(VI). The identity of the bacteria were determined by 16S rRNA gene analysis to be closely related to Ochrobactrium sp., Salmonella enterica and Pseudomonas aeruginosa. Dependence of initial pH values and range of concentrations of the dye Reactive Black B (33.2-103.1 mg l(-1)) and Cr(VI) (19.9-127.6 mg l(-1)) were examined to find the effect of pH on the dye and Cr(VI) bioaccumulation. Optimal pH for growth of the consortia in media containing 35 mg l(-1) dye and 50 mg l(-1) Cr(VI) was determined to be around 8. The Cr(VI) bioaccumulation by the consortia was rapid in media containing molasses with or without reactive dye with a maximum Cr(VI) bioaccumulation yield ranging from 90% to 99% within a 2-4d period. A slightly lower yield for the dye bioaccumulation was measured with a maximum dye bioaccumulation of 80% at 59.3 mg l(-1) dye and 69.8 mg l(-1) Cr(VI). The highest specific Cr uptake value was obtained as 76.7 mg g(-1) at 117.1 mg l(-1) Cr(VI) and 50.8 mg l(-1) dye concentration. This ability to bioaccumulate dye and Cr(VI) was more efficient than the enriched sludge from which they were isolated.     

Removal of Cr(VI) and humic acid by using TiO2 photocatalysis

The removal efficiencies of Cr(VI) and HA, using a TiO(2)-mediated photocatalytic process, were investigated with variations in the pH, TiO(2) dosage and Cr(VI)/HA ratio. During the photocatalytic reaction, the total removal of Cr(VI) occurred through adsorption onto TiO(2), as well as its reduction to Cr(III). However, oxidation and adsorption were identified as important removal processes for the treatment of HA. Due to the anionic type adsorption onto TiO(2) and its acid-catalyzed photocatalytic reduction, the removal of Cr(VI) decreased with increasing pH, while that of HA increased with increasing pH. The TiO(2) dosage was also an important parameter for the removal of Cr(VI). As the TiO(2) dosage was increased to 2.5 g l(-1), the removal of Cr(VI) was continuously enhanced, but decreased at dosages above 3 g l(-1) due to the increased blockage of the incident UV light used for the photocatalytic reaction. The removal of Cr(VI) was greatly enhanced when the system contained both HA and Cr(VI) compared to Cr(VI) alone. Also, the removal of HA was greatly enhanced when the system contained both HA and Cr(VI) compared to HA alone. The removal of Cr(VI) was continuously enhanced as the HA concentration gradually increased; however, no further increase was observed above 20 mg l(-1) HA due to the increased absorption of the UV light. This result supports that the photocatalytic reaction, with illuminated TiO(2), could be applied to more effectively treat wastewater containing both Cr(VI) and HA than that containing a single species only.     

Chromium species behaviour in the activated sludge process

The purpose of this research was to compare trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) removal by activated sludge and to investigate whether Cr(VI) reduction and/or Cr(III) oxidation occurs in a wastewater treatment system. Chromium removal by sludge harvested from sequencing batch reactors, determined by a series of batch experiments, generally followed a Freundlich isotherm model. Almost 90% of Cr(III) was adsorbed on the suspended solids while the rest was precipitated at pH 7.0. On the contrary, removal of Cr(VI) was minor and did not exceed 15% in all experiments under the same conditions. Increase of sludge age reduces Cr(III) removal, possibly because of Cr(III) sorption on slime polymers. Moreover, the decrease of suspended solids concentration and the acclimatization of biomass to Cr(VI) reduced the removal efficiency of Cr(III). Batch experiments showed that Cr(III) cannot be oxidized to Cr(VI) by activated sludge. On the contrary, Cr(VI) reduction is possible and is affected mainly by the initial concentration of organic substrate, which acts as electron donor for Cr(VI) reduction. Initial organic substrate concentration equal to or higher than 1000 mgl(-1) chemical oxygen demand permitted the nearly complete reduction of 5 mgl(-1) Cr(VI) in a 24-h batch experiment. Moreover, higher Cr(VI) reduction rates were obtained with higher Cr(VI) initial concentrations, expressed in mg Cr(VI) g(-1) VSS, while decrease of suspended solids concentration enhanced the specific Cr(VI) reduction rate.     

Investigation of Cr(VI) reduction in continuous-flow activated sludge systems

The aim of this research was to investigate hexavalent chromium, Cr(VI), reduction by activated sludge and to evaluate the use of continuous-flow activated sludge systems for the treatment of Cr(VI)-containing wastewater. Three series of experiments were conducted using two parallel lab-scale activated sludge systems. During the first experiment, one system was used as a control, while the other received Cr(VI) concentrations equal to 0.5, 1, 3 and 5mg l(-1). For all concentrations added, approximately 40% of the added Cr(VI) was removed during the activated sludge process. Determination of chromium species in the dissolved and particulate phase revealed that the removed Cr(VI) was sorbed by the activated sludge flocs mainly as trivalent chromium, Cr(III), while the residual chromium in the dissolved phase was mainly detected as Cr(VI). Activated sludge ability to reduce Cr(VI) was independent of the acclimatization of biomass to Cr(VI) and it was not affected by the toxic effect of Cr(VI) on autotrophic and heterotrophic microorganisms. During the second experiment, both systems were operated under two different hydraulic residence time (theta equal to 20 and 28h) and three different initial organic substrate concentration (COD equal to 300, 150 and 0mg l(-1)). Cr(VI) reduction was favored by an increase of theta, while it was limited by influent COD concentration. Finally, at the last experiment the effect of anoxic and anaerobic reactors on Cr(VI) reduction was investigated. It was observed that the use of an anoxic zone or an anaerobic-anoxic zone ahead of the aerobic reactor favored Cr(VI) reduction, increasing mean percentage Cr(VI) reduction to almost 80%.     

Acute and chronic activity of perchlorate and hexavalent chromium contamination on the survival and development of Culex quinquefasciatus Say (Diptera: Culicidae)

Effects of water contamination with perchlorate and hexavalent chromium [Cr (VI)] on the mosquito Culex quinquefasciatus were assessed. The chronic (10-day) LC50s values for perchlorate and chromium were 74+/-8.0 mg/L and 0.41+/-0.15 mg/L, respectively. Relative Growth Index, a measure of growth and mortality rates in a population, was significantly reduced within 5 days for levels of perchlorate as low as 25 mg/L and for levels of chromium as low as 0.16 mg/L. Neither compound altered wing length of surviving adults. In combination, contaminants were synergistic, causing 14% more mortality than predicted. Acute (24-h) LC50 values for perchlorate and Cr (VI) were 17,000+/-3200 and 38+/-1.3 mg/L, respectively. Effects on mosquito larvae in contaminated environments are likely to be observed for Cr (VI) but not for perchlorate, which generally does not occur at levels as high as those shown here to affect larval mosquitoes.     

Spectroscopic investigation of magnetite surface for the reduction of hexavalent chromium

The reduction of Cr(VI) to Cr(III) by magnetite in the presence of added Fe(II) was characterized through batch kinetic experiments and the effect of Fe(II) addition and pH were investigated in this study. The addition of Fe(II) into magnetite suspension improved the reductive capacity of magnetite. Eighty percent of Cr(VI) was reduced by magnetite (6.5 g l(-1)) with Fe(II) (80 mg l(-1)) within 1 h, while 60% of Cr(VI) was removed by magnetite only. However, the extent of improved reductive capacity of magnetite with Fe(II) was less than that predicted by the summation of each reduction capacity of magnetite and Fe(II). The reduction of Cr(VI) in the magnetite suspension with Fe(II) increased with the increase of molar ratio of Fe(II) to Cr(VI) (0.6, 1, 1.5, 2.3) in the range of 0-2.3 and with the decrease of pH in the range of pH 8.0-5.5. The speciation of chromium, iron, and oxygen on the surface of magnetite was investigated by X-ray photoelectron spectroscopy. Cr 2p3/2, Fe 2p3/2, and O 1s peaks were mainly observed at 576.7 and 577.8 eV, at 711.2 eV, and at 530.2 and 531.4 eV, respectively. The results indicates that Cr(III) and Fe(III) were the dominant species on the surface of magnetite after reaction and that the dominant species covered the magnetite surface and formed metal (oxy)hydroxide.     

Accumulation of chromium (VI) from aqueous solutions using water lilies (Nymphaea spontanea)

This study describes an investigation using tropical water lilies (Nymphaea spontanea) to remove hexavalent chromium from aqueous solutions and electroplating waste. The results show that water lilies are capable of accumulating substantial amount of Cr(VI), up to 2.119 mg g(-1) from a 10 mg l(-1) solution. The roots of the plant accumulated the highest amount of Cr(VI) followed by leaves and petioles, indicating that roots play an important role in the bioremediation process. The maturity of the plant exerts a great effect on the removal and accumulation of Cr(VI). Plants of 9 weeks old accumulated the most Cr(VI) followed by those of 6 and 3 weeks old. The results also show that removal of Cr(VI) by water lilies is more efficient when the metal is present singly than in the presence of Cu(II) or in waste solution. This may be largely associated with more pronounced phytotoxicity effect on the biochemical changes in the plants and saturation of binding sites. Significant toxicity effect on the plant was evident as shown in the reduction of chlorophyll, protein and sugar contents in plants exposed to Cr(VI) in this investigation.     

Biogeochemical influence on transport of chromium in manganese sediments: experimental and modeling approaches

Hexavalent chromium (Cr(VI)) was reduced to immobile and nontoxic Cr(III) by a dissimilatory metal reducing bacteria, Shewanella alga Simidu (BrY-MT) ATCC 55627. A series of kinetic batch and dynamic column experiments were conducted to provide an understanding of Cr(VI) reduction by the facultative anaerobe BrY-MT. Reduction of Cr(VI) was rapid (within 1 h) in columns packed with quartz sand and bacteria, whereas Cr(VI) reduction by BrY-MT was delayed (57 h) in the presence of beta-MnO2-coated sand. A mathematical model was developed and evaluated against data obtained from column experiments. The model takes into account (1) advective-dispersive transport of Cr(III), Cr(VI), lactate, and protein (mobile and immobile bacteria); (2) first-order kinetic adsorption of Cr(III) and lactate; (3) conversion of solid phase beta-MnO2 to solid phase MnOOH due to oxidation of Cr(III); (4) dual-Monod kinetics, where Cr(VI) is the electron acceptor and lactate is the electron donor. The breakthrough data for Cr(III), Cr(VI), lactate, and protein (mobile and immobile bacteria) were fitted simultaneously. The breakthrough data are well described by the mathematical model that considers the above processes. This result demonstrates the ability of the coupled hydrobiogeochemical model to simulate chromium transport in complex reactive systems.     

Chromate reduction on humic acid derived from a peat soil--exploration of the activated sites on HAs for chromate removal

Humic substances are a major component of soil organic matter that influence the behavior and fate of heavy metals such as Cr(VI), a toxic and carcinogenic element. In the study, a repetitive extraction technique was used to fractionate humic acids (HAs) from a peat soil into three fractions (denoted as F1, F2, and F3), and the relative importance of O-containing aromatic and aliphatic domains in humic substances for scavenging Cr(VI) was addressed at pH 1. Spectroscopic analyses indicated that the concentrations of aromatic C and O-containing functional groups decreased with a progressive extraction as follows: F1>F2>F3. Cr(VI) removal by HA proceeded slowly, but it was enhanced when light was applied due to the production of efficient reductants, such as superoxide radical and H(2)O(2), for Cr(VI). Higher aromatic- and O-containing F1 fraction exhibited a greater efficiency for Cr(VI) reduction (with a removal rate of ca. 2.89 mmol g(-1) HA under illumination for 3 h). (13)C NMR and FTIR spectra further demonstrated that the carboxyl groups were primarily responsible for Cr(VI) reduction. This study implied the mobility and fate of Cr(VI) would be greatly inhibited in the environments containing such organic groups.     

Immobilised microbial reactor for the biotransformation of hexavalent chromium

Out of an array of bacterial strains isolated from soil contaminated with effluents from electroplating wastewater, Bacillus coagulans exhibited the maximum Cr(VI) reduction potential. The feasibility of an immobilized B. coagulans bioreactor for hexavalent chromium reduction was investigated. Experimental results demonstrated that near complete removal of Cr(VI) was achieved in the reactor with an initial Cr(VI) concentration of 26 mg/l and reactor time of 24 h. The removal efficiency in the bioreactor was significantly affected by the influent Cr(VI) concentration, the Cr(VI) loading rate, the reaction time and the amount of Cr(VI) reduced by the biomass.     

Use of waste iron metal for removal of Cr(VI) from water

Cr(VI) removal from water was evaluated using waste iron particles in batch experimental mode. The reaction rates were inversely proportional to the initial Cr(VI) concentrations, and the reaction rates of Cr(VI) removal with the waste iron metal were faster than those with Peerless iron, a commercial zero-valent iron. The loss in iron reactivity due to the oxidation, from Fe(0) to Fe(II), ultimately to Fe(III), could be recovered by adding iron-reducing consortium (IRC) to the oxidized iron. Bacterial reduction of Cr(VI) also helped to decrease the aqueous concentration of Cr(VI), but the reduction of oxidized iron by IRC and the consequent reduction of Cr(VI) to Cr(III) by the reduced iron was more significant. Thus, reusing waste iron metal for Cr(VI) removal can reduce the cost of reactive media. Furthermore, the addition of IRC to the waste iron metal can accelerate the removal rate of Cr(VI), and can recover the reactivity of irons which were oxidized by Cr(VI).     

Kinetics of the reduction of hexavalent chromium with the brown seaweed Ecklonia biomass

The dead biomass of the brown seaweed, Ecklonia sp., is capable of reducing toxic Cr(VI) into less toxic or nontoxic Cr(III). However, little is known about the mechanism of Cr(VI) reduction by the biomass. The objective of this work was to develop a kinetic model for Cr(VI) biosorption, for supporting our mechanism. The reduction rate of Cr(VI) increased with increasing total chromate concentration, [Cr(VI)], and equivalent concentration of organic compounds, [OCs], and decreasing solution pH. It was found that the reduction rate of Cr(VI) was proportional to [Cr(VI)] and [OCs], suggesting the simple kinetic equation -d[Cr(VI)]/dt=k[Cr(VI)][OCs]. When considering the consumption of organic compounds due to the oxidation by Cr(VI), an average rate coefficient of 9.33 (+/-0.65)microM(-1)h(-1) was determined, at pH 2. Although the function of the pH could not be expressed in a mechanistic manner, an empirical model able to describe the pH dependence was obtained. It is expected that the developed rate equation could likely be used for design and performance predictions of biosorption processes for treating chromate wastewaters.     

In vitro Cr(VI) reduction by cell-free extracts of chromate-reducing bacteria isolated from tannery effluent irrigated soil

Four efficient Cr(VI)-reducing bacterial strains were isolated from rhizospheric soil of plants irrigated with tannery effluent and investigated for in vitro Cr(VI) reduction. Based on 16S rRNA gene sequencing, the isolated strains SUCR44, SUCR140, SUCR186, and SUCR188 were identified as Bacillus sp. (JN674188), Microbacterium sp. (JN674183), Bacillus thuringiensis (JN674184), and Bacillus subtilis (JN674195), respectively. All four isolates could completely reduce Cr(VI) in culture media at 0.2 mM concentration within a period of 24–120 h; SUCR140 completely reduced Cr(VI) within 24 h. Assay with the permeabilized cells (treated with Triton X-100 and Tween 80) and cell-free assay demonstrated that the Cr(VI) reduction activity was mainly associated with the soluble fraction of cells. Considering the major amount of chromium being reduced within 24–48 h, these fractions could have been released extracellularly also during their growth. At the temperature optima of 28 °C and pH?7.0, the specific activity of Cr(VI) reduction was determined to be 0.32, 0.42, 0.34, and 0.28 μmol Cr(VI)?min^?1?mg^?1 protein for isolates SUCR44, SUCR140, SUCR186, and SUCR188, respectively. Addition of 0.1 mM NADH enhanced the Cr(VI) reduction in the cell-free extracts of all four strains. The Cr(VI) reduction activity in cell-free extracts of all the isolates was stable in presence of different metal ions tested except Hg²⁺. Beside this, urea and thiourea also reduced the activity of chromate reduction to significant levels.     

Chromium (VI) biosorption and bioaccumulation by chromate resistant bacteria

In this study, strains that are capable of bioaccumulating Cr(VI) were isolated from treated tannery effluent of a common effluent treatment plant. The Cr(VI) concentration in this treated effluent was 0.96 mg/l, much above the statutory limit of 0.1 mg/l for discharge of industrial effluents into inland surface waters in India. In addition to the bioaccumulation, biosorption capabilities of living and dead cells were analysed. Two strains, identified as Bacillus circulans and Bacillus megaterium were able to bioaccumulate 34.5 and 32.0 mg Cr/g dry weight, respectively and brought the residual concentration of Cr(VI) to the permissible limit in 24 h when the initial concentration was 50 mg Cr(VI)/l. Our experimental design accounts for initial as well as final residual concentration of heavy metal while selecting heavy metal accumulating strains during batch studies. Biosorption of Cr(VI) was shown by B. megaterium and an another strain, B. coagulans. Living and dead cells of B. coagulans biosorbed 23.8 and 39.9 mg Cr/g dry weight, respectively, whereas, 15.7 and 30.7 mg Cr/g dry weight was biosorbed by living and dead cells of B. megaterium, respectively. Biosorption by the dead cells was higher than the living cells. This was due to prior pH conditioning (pH 2.5 with deionized water acidified with H2SO4) of the dead cells.     

A long-term performance test on an autotrophic denitrification column for application as a permeable reactive barrier

The long-term performance of a sulfur-based reactive barrier system was evaluated using autotrophic denitrification in a large-scale column. A bacterial consortium, containing autotrophic denitrifiers attached on sulfur particles, serving as an electron donor, was able to transform 60mgNL(-1) of nitrate into dinitrogen. In the absence of phosphate, the consortium was unable to remove nitrate, but after the addition of phosphate, nitrate removal was readily evident. Once the column operation had stabilized, seepage velocities of 1.0x10(-3) and 0.5x10(-3)cms(-1), corresponding to hydraulic residence times of 24 and 48h, respectively, did not affect the nitrate removal efficiency, as determined by the nitrate concentration in the effluent. However, data on the nitrate, nitrite and sulfate distribution along the column indicated differential transformation patterns with column depths. Based on the dinitrogen concentration in the total gas collected, the denitrification efficiency of the tested column was estimated to be more than 95%. After 500d operation, the hydrodynamic characteristics of the column slightly changed, but these changes did not inhibit the nitrate removal efficiency. Data from a bacterial community analysis obtained from four parts of the column demonstrated the selective a spatial distribution of predominant species depending on available electron acceptors or donors.     

Chromium accumulation by the hyperaccumulator plant Leersia hexandra Swartz

Leersia hexandra Swartz (Gramineae), which occurs in Southern China, has been found to be a new chromium hyperaccumulator by means of field survey and pot-culture experiment. The field survey showed that this species had an extraordinary accumulation capacity for chromium. The maximum Cr concentration in the dry leaf matter was 2978 mg kg(-1) on the side of a pond near an electroplating factory. The average concentration of chromium in the leaves was 18.86 times as that in the pond sediment, and 297.41 times as that in the pond water. Under conditions of the nutrient solution culture, it was found that L. hexandra had a high tolerance and accumulation capacity to Cr(III) and Cr(VI). Under 60 mg l(-1) Cr(III) and 10 mg l(-1) Cr(VI) treatment, there was no significant decrease of biomass in the leaves of L. hexandra (p>0.05). The highest bioaccumulation coefficients of the leaves for Cr(III) and Cr(VI) were 486.8 and 72.1, respectively. However, L. hexandra had a higher accumulation capacity for Cr(III) than for Cr(VI). At the Cr(III) concentration of 10 mg l(-1) in the culture solution, the concentration of chromium in leaves was 4868 mg kg(-1), while at the same Cr(VI) concentration, the concentration of chromium in leaves was only 597 mg kg(-1). These results confirmed that L. hexandra is a chromium hyperaccumulator which grows rapidly with a great tolerance to Cr and broad ecological amplitude. This species could provide a new plant resource that explores the mechanism of Cr hyperaccumulation, and has potential for usage in the phytoremediation of Cr-contaminated soil and water.     

Chromium adsorption by aligned carbon nanotubes supported ceria nanoparticles

Ceria nanoparticles supported on aligned carbon nanotubes (CeO(2)/ACNTs), a novel adsorbent for Cr(VI) from drinking water, were prepared by chemical reaction of CeCl(3) with NaOH in aligned carbon nanotube solution and subsequent heat treatment. The best Cr(VI) adsorption effect of CeO(2)/ACNTs occurs at a pH range of 3.0-7.4. The largest adsorption capacity of CeO(2)/ACNTs reaches 30.2 mg g(-1) at an equilibrium Cr(VI) concentration of 35.3 mg l(-1) at pH 7.0. The experiment results suggest that CeO(2)/ACNTs have great potential applications in environmental protection.     

Hexavalent chromium reduction kinetics in rodent stomach contents

Reduction of hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)) in the stomach prior to absorption is a well-recognized detoxification process thought to limit the toxicity of ingested Cr(VI). However, administration of high concentrations of Cr(VI) in drinking water cause mouse small intestinal tumors, and quantitative measures of Cr(VI) reduction rate and capacity for rodent stomach contents are needed for interspecies extrapolation using physiologically-based toxicokinetic (PBTK) models. Ex vivo studies using stomach contents of rats and mice were conducted to quantify Cr(VI) reduction rate and capacity for loading rates (1-400 mg Cr(VI) L⁻¹ stomach contents) in the range of recent bioassays. Cr(VI) reduction was measured with speciated isotope dilution mass spectrometry to quantify dynamic Cr(VI) and Cr(III) concentrations in stomach contents at select time points over 1 h. Cr(VI) reduction followed mixed second-order kinetics, dependent upon concentrations of both Cr(VI) and the native reducing agents. Approximately 16 mg Cr(VI)-equivalents of reducing capacity per L of fed stomach contents (containing gastric secretions, saliva, water and food) was found for both species. The second-order rate constants were 0.2 and 0.3 L mg⁻¹ h⁻¹ for mice and rats, respectively. These findings support that, at the doses that caused cancer in the mouse small intestine (?20 mg Cr(VI) L⁻¹ in drinking water), the reducing capacity of stomach contents was likely exceeded. Thus, for extrapolation of target tissue dose in risk assessment, PBTK models are necessary to account for competing kinetic rates including second order capacity-limited reduction of Cr(VI) to Cr(III).