Kinetic Study of Cr(VI) Reduction in an Indigenous Mixed Culture of Bacteria in the Presence of As(III)

Abstract

Organic compounds can serve as electron donors during Cr(VI) reduction by live microbial cultures. However, Cr(VI) reduction with concomitant oxidation of metalloids such as arsenic has not been studied. In this study, an indigenous mixed culture of bacteria collected from the Brits Wastewater Treatment Plant (North West Province, South Africa) was used to biologically reduce Cr(VI) to Cr(III) while utilising electrons derived from the oxidation of As(III) to As(V). Both processes, i.e., reduction of Cr(VI) oxidation of As(III), are desirable since both the hexavalent form of chromium and the trivalent state of arsenic are acutely toxic at high concentrations and carcinogenic under low and subchronic conditions. In experiments conducted under aerobic conditions, near complete reduction of Cr(VI) was achieved under initial Cr(VI) concentrations up to 70 mg/L and a fixed As(III) concentration of 20 mg/L. However, Cr(VI) reduction was inhibited at Cr(VI) concentrations equal to and above 100 mg/L. Further experiments conducted at Cr(VI) concentration of 70 mg/L and varying As(III) concentration from 5-70 mg/L showed that Cr(VI) reduction rate increased with increasing As(III) concentration from 5-40 mg/L. However, 20% drop in Cr(VI) reduction efficiency was observed at concentrations higher than 40 mg/L. A similar trend of Cr(VI) reduction was observed in a continuous reactor operated under an optimum pH of 7.0±0.2, and dissolved oxygen concentration (DO = 0.6±0.1 mg/L) during steady state operation. These results show that arsenic served as an electron donor for the beneficial reduction of Cr(VI) to Cr(III) using a mixed culture of Cr(VI) reducing bacteria.