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Cr(VI)-reducing bacteria are widespread and Cr(VI) reduction occurs under both aerobic and anaerobic conditions. Under aerobic conditions, both NADH and endogenous cell reserves may serve as the electron donor for Cr(VI) reduction. Under anaerobic conditions, electron transport systems containing cytochromes appear to be involved in Cr(VI) reduction. High cell densities are necessary to obtain a significant rate of Cr(VI) reduction. Cr(VI) reduction by bacteria may be inhibited by Cr(VI), oxygen, heavy metals, and phenolic compounds. The optimum pH and temperature observed for Cr(VI) reduction generally coincide with the optimal growth conditions of cells. The optimum redox potential for Cr(VI) reduction has not yet been established.

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Chromium reduction by Escherichia coli ATCC 33456 quantitatively transferred hexavalent chromium, Cr(VI), to trivalent chromium, Cr(m). The reduced chromium was predominantly present in the external medium. Supernatant fluids of cell extract, obtained by centrifugation at 12,000 and 150,000 x g, showed almost the same Cr(VI) reduction activity, indicating that Cr(VI) reduction by E. coli ATCC 33456 was a largely soluble reductase activity. In studies with respiratory inhibitors, no inhibitory effects on aerobic and anaerobic Cr(VI) reduction were demonstrated by addition of cyanide, azide, and rotenone into both intact cell cultures and supernatant fluids of E. coli ATCC 33456. Although cytochromes b and d were identified in the membrane fraction of cell extracts, Cr(VI) was not reduced by the membrane fraction alone. The cytochrome difference spectra analysis also indicated that these cytochromes of the respiratory chain require the presence of the soluble Cr(VI) reductase to mediate electron transport to Cr(VI). Stimulation of Cr(VI) reduction by an uncoupler, 2,4-dinitrophenol, indicated that the respiratory-chain-linked electron transport to Cr(VI) was limited by the rate of dissipation of the proton motive force.

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The potential for fixed-film biorectors to reduce Cr(VI) was demonstrated using a Cr(VI)-reducing species, Bacillus sp. A bench-scale, packed-bed bioreactor was operated to steady-state conditions under a range of influent Cr(VI) concentrations (10−200 mg/L) and hydraulic detention times (6−24 h, clean bed) with near complete removal of Cr(VI). The steady-state Cr(VI) reduction efficiency was not affected by the influent Cr(VI) concentration or hydraulic detention time. Chromium mass balance analysis revealed that nearly all the Cr(VI) fed to the bioreactor was accounted for in the effluent as Cr(VI) and Cr(III). Total cell mass in the bioreactor decreased with increasing Cr(VI) loading rate, but it stabilized after a loading limit was reached (1016 mg of Cr(VI)/L·day) when operated under 24 h hydraulic detention time. The bioreactor showed strong resilience by recovering from Cr(VI) overloading through reduction in influent Cr(VI) concentration.

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