Nonmetabolic Reduction of Cr(VI) by Bacterial Surfaces Under Nutrient-Absent Conditions

We have measured the ability of nonmetabolizing cells of the bacterial species Bacillus subtilis, Sporosarcina ureae , and Shewanella putrefaciens to reduce aqueous Cr(VI) to Cr(III) in the absence of externally supplied electron donors. Each species can remove significant amounts of Cr(VI) from solution, and the Cr(VI) reduction rate is strongly dependent on solution pH. The fastest reduction rates occur under acidic conditions, with decreasing rates with increasing pH. XANES data demonstrate that Cr(VI) reduction to Cr(III) occurs within the experimental systems. Control experiments indicate that the Cr removal is not a purely adsorptive process. Reduction appears to occur at the cell wall, and is not coupled to the oxidation of bacterial organic exudates. Detailed kinetic data suggest that the reduction involves at least a two-stage process, involving an initial rapid removal mechanism followed by a slower process that follows first-order reaction kinetics. Due to the prevalence of nonmetabolizing cells and cell wall fragments in soils and deeper geologic environments, our results suggest that the observed nonmetabolic reduction of Cr(VI) to Cr(III) may significantly affect the environmental distribution of Cr in bacteria-bearing systems.