Microbial Reduction of Cr(VI) during Anaerobic Degradation of Benzoate

Abstract

A series of microcosms and enrichments were conducted to evaluate the potential for microbially mediated Cr(VI) reduction linked to benzoate catabolism. Bacterial degradation of benzoate with the transport of electrons to Cr(VI) was achieved using nitrate or molecular oxygen as an initial stimulator. After depletion of nitrate or oxygen, microcosms and enrichments still retained the capacity for benzoate degradation linked to Cr(VI) reduction. Experiments demonstrated that benzoate degradation occurred concurrently with the reduction of Cr(VI) and was strongly dependent on the presence of Cr(VI). Benzoate degradation proceeded as long as Cr(VI) was present, ceased when Cr(VI) was completely removed, and continued once Cr(VI) was added. Furthermore, the observed benzoate removal, linearly correlated with Cr(VI) consumed, and the stoichiometric ratio were in good agreement to the theoretical ratio for the complete oxidation of benzoate to carbon dioxide coupled to the reduction of Cr(VI) to Cr(III). The addition of nitrate up to a concentration of 5.0 mM did not inhibit Cr(VI) reduction but enhanced benzoate degradation in the enrichments. This process may provide the opportunity for in-situ biotreatment of Cr(VI) in subsurface environments.