Chromates, reduction

The body of research on isotopic fractionation induced by sulfate and nitrate reduction provides insight into selenate, selenite and chromate reduction. For sulfate and nitrate oxyanions, reduction is generally microbially mediated, is irreversible, and involves a fairly large but variable isotopic fractionation. As described below, Se and Cr oxyanion reduction follows suit, though abiotic reactions may have a greater role in some transformations. 

Powell, R.M., Puls, R.W., Hightower, S.K., and Sabatini, D.A., Coupled iron corrosion and chromate reduction mechanisms for subsurface remediation, Environ. Sci. Technol, 29, 1913-1922, 1995. 

Burt, T. A., Jones, H. K., Li, Z., Bowman, R. S., and Helferich, R. Perchloroethylene and chromate reduction using a surfactant-modified zeolite/zero-valent iron pellet. 1999 WERC Conf. on the Environment, Albuquerque, NM, (in press). 

Pratt AR, Blowes DW, Ptacek CJ. Products of chromate reduction on proposed subsurface remediation material. Environ Sci Technol 1997 31 2492. 

Williams AGB, Scherer MM. Kinetics of chromate reduction by carbonate green rust. 220th National Meeting, Washington, DC, American Chemical Society 2000 42(2) 666-668. 

Komori K, Toda K, Ohtake H. 1990a. Effects of oxygen stress on chromate reduction in Enterobacter cloacae strain HOI. J Ferment Bioeng 69(l) 67-69. 

The Fe3+/2+ couple has an E° = +0.77 V, which makes Fe2+ a moderate reducing agent. Fe(II) can either be added as a salt, or produced electrochem-ically using an Fe anode. A case in point is its widespread use for chromate reduction ... 

McLean, J., and Beveridge, T. J. (2001). Chromate reduction by a pseudomonad isolated from a site contaminated with chromated copper arsenate. Appl. Environ. Microbiol. 67. 1076-1084. 

As judged from our ecological model compound sea water, the bioavailability of molybdenum (104 nM) is higher than that of chromium (962pM) (see Table 1.1). While chromium is insoluble as Cr(III) in the Earth s crust, the reduction of molybdate is not as easy as chromate reduction, which leads to a factor of 10 000 when the release of chromium and molybdenum from the Earth s crust into sea water is compared. Together with its low toxicity (Nies 1999), this makes molybdate the prime choice for biochemical reactions requiring oxyanion catalysis (Williams and da Silva 2002). 

Figure 4. Rate comparison for chromate reduction by chemical and biological constituents, demonstrating the dominance (bold line) of chemical pathways at all typical pH values of natural systems. For calculated initial rates, initial [Cr(VI)] = 100 pM, [Fe(II)] = 30 pM, [S(—II)] = 10 pM. (from Fendorf et al. 2000, with permission)...

Chromate Reduction Process Andco Environ mental Processes Inc Vertical plate electrodes. Anodically generated Fe " " May be continuous via sedimentation of precipitates No ... 

FIGURE 12.15 Critical redox level for chromate reduction. 

We have also looked at the reaction of chromate with a series of other carboxylates under these same conditions, including formate, oxalate, succinate, /5-hydroxybutyrate, glycolate, malate, pyruvate and isocitrate. None of these candidate reductants reduced chromate at pH 7.4. These results emphasize the fact that chromate reduction under neutral or slightly alkaline conditions is under kinetic control, since all of the above carboxylates have favorable reduction potentials (see Table 1) and are thermodynamically capable of reducing chromate. Thus none of the carboxylic acids that have been examined, appears to be able to reduce chromate at pH 7.4 (1 M Tris HCl) and 25 °C. These results again point to the significance of the SH group in chromate metabolism. 

In comparative kinetic studies we have investigated the reaction between chromate and cysteamine, mercaptoethanol and thiolactic acid, at pH 7.4 (1 M Tris HCl) and 25 °C. All the reactions appear to be first order with respect to both chromate and thiol. The second order rate constants are listed in Table 2. The rates of chromate reduction for the various monothiols follows the order cysteine > cysteamine > glutathione (fast phase) > penicillamine > mercaptoethanol > glutathione (slow phase) > thiolactic acid. 

Thus the rate of chromate reduction for the various dithiols follows the order unithiol > DTT > lipoic acid > 2,3-dimercaptosuccinic acid. If we compare the rates of the dithiols with the monothiols (see Table 2) the addition of a second thiol group does not confer great advantage with respect to the reduction of chromate under our standard conditions. Thus the transition state proposed by McAuley and Olatunji (see Fig. 5 and Ref. 38) for chromate-thiol reactions under acidic conditions, would not appear to be relevant under physiological condition. ... 

The results of chromate reduction by the model compounds listed in Table 2, allow us to predict the environment that the SH groups in a protein will require for them to react readily with chromate. The fastest reductants (Table 2) are either neutral (e.g., cysteine, penicillamine) or have a positive charge (cysteamine). However, charge is not the only factor affecting the kinetics, since the cysteine-chromate reaction is faster than the cys-teamine-chromate reaction and the unithiol-chromate reaction is faster than the DTT-chromate reaction. The obvious advantage that cysteine has over cysteamine is its ability to chelate intermediate Gr species after the disulfide bond has been formed, as illustrated in Fig. 3. Such chelation would facilitate the change in coordination number required in going from Cr(VI) to Cr(IV) or Cr(III) (see Sect. 2). 

Chromate was reduced to chromium(III) by rat liver microsomes and NADPH in vitro ). Reduction of hexavalent chromium to trivalent chromium required the presence of both microsomal protein and NADPH cofactor. Heat denaturation of microsomes resulted in the loss of their ability to reduce chromate. Essentially no chromate reduction was observed by the cofactor in the absence of microsomal proteins. We have found that NADH also served as a cofactor for the microsomal reduction of chromate. However, at equivalent concentrations of NADPH and NADH, the rate of reduction of limiting amounts of the substrate chromate was slower with NADH than with NADPH cofactor. 

The rate of chromate reduction using NADH as a cofactor did not vary with the source of microsomes. The same kinetic parameters were found with control, phenobar-bital-induced and 3-methylcholanthrene-induced microsomes. Therefore there was no correlation of rate of chromate reduction with the activity of NADH-Cytochrome P-450 reductase in the microsomes which was 1.25 times greater in the control set than in either the phenobarbital- or 3-methylcholanthrene-induced microsomes. The cytochrome bs content of the microsomes, however, did not vary ( 5%) with the source of microsomes. 

In contrast, using NADPH as cofactor phenobarbital-induced microsomes metabolized chromate with an apparent second order rate constant 1.25 times greater than that found with control or 3-methylchoIanthrene-induced microsomes. The increased rate of chromate reduction seen with phenobarbital-induced microsomes correlated with the greater NADPH-Cytochrome P-450 reductase activity of these microsomes... 

NADPH-Cytochrome P-450 reductase has been shown to be specifically inhibited by 2 -AMP Studies with varying concentrations of the 2 -AMP inhibitor and varying concentrations of the substrate chromate showed that 2 -AMP appeared to be a competitive inhibitor of chromate reduction by microsomes and NADPH or NADH. The involvement of NADPH- and NADH-Cytochrome P-450 reductase in the microsomal metabolism of chromate was, therefore, confirmed. 

Metyrapone has been shown to inhibit Cytochrome P-450 activity and bind to the reduced and oxidized form of the enzyme Increasing concentrations of metyrapone progressively decreased the rate of chromate reduction by microsomes and NADPH. Metabolism by both control and phenobarbital-induced microsomes was affected similarly by metyrapone which appeared to be a competitive inhibitor of chromate reduction. 

Chromate reduction within cells is not expected to be random and nonselective, rather only certain small molecules and enzymes appear capable of reducing chromate at a significant rate under physiological conditions. The selective metabolism of the carcinogen chromate by cellular constituents may lead to selective damage of cell components by chromium metabolites and alteration of their normal functions. 

Eary LE, Rai D. (1991). Chromate reduction by surface soils under acidic conditions. Soil Science Society of America Journal 55 676-683. 

Fendorf SE, Li G. (1996). Kinetics of chromate reduction by ferrous iron. Environmental Science and Technology 30 1614-1617. 

Powell RM, Puls RW, Hightower SK, Sabatini DA. (1995). Coupled iron corrosion and chromate reduction Mechanisms for subsurface remediation. Environmental Science and Technology 29 1913-1922. 

Hence, since the thickness of the barrier (24 cm) is greater than the minimum length of reaction zone (0.6cm), the thickness of barrier would provide a sufficient time for Cr(VI) reduction. The above estimation does not account for solution ligands such as chloride, carbonate, sulphate, citrate, oxalate, nitrate, and phosphate that can complex with Fe° to decrease chromate reduction rate. In addition, the overall design of the barrier should consider the possible existence of dissolved oxygen in the EO flow or the O2 gas bubble produced from the electrolysis reaction at the anode that can oxidize the Fe and decrease the efficiency of the ZVI reduction performance. 

Yellow chromating The bath contains 2 to 20 g 1 chromium as chromic acid or dichromate, 1 to 5 g 1 sulfuric acid, and 0.1 to 1 g 1 sulfate, chloride or nitrate as a catalyst for the chromate reduction. The film has a yellowish color and a thickness of up to 1 (xm. This film is very efficient in corrosion protection, and a time of500 h in a salt spray testing without any corrosion spots is achieved. Moreover, the film has the property of self-healing of mechanical defects, which makes this procedure so superior to alternatives. But the film contains up to 200 mg m chromate and this will probably soon lead to an end of this very cheap and effective protection process. 

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