Biological reduction of chromium(VI) under anaerobic conditions

Cr(VI) causes major environmental pollution problems. This study was carried out to investigate an innovative biological process for treatment of Cr(VI) wastes. The investigation was conducted in both continuous flow and batch reactor systems.; A Cr(VI)-reducing culture was enriched in an anaerobic chemostat fed with a synthetic medium containing Cr(VI) and acetate. With an influent concentration of 26 mg Cr(VI)/L, a nearly complete removal of Cr(VI) was achieved at the dilution rates of 0.15 and 0.32 day{dollar}sp{lcub}-1{rcub}{dollar} at 20{dollar}spcirc{dollar}C and at 35{dollar}spcirc{dollar}C, respectively. The optimum Cr(VI) loading rate was found to be 5 mg Cr(VI)/L{dollar}cdot{dollar}day or 0.02 mg Cr(VI)/mg VSS{dollar}cdot{dollar}day at 20{dollar}spcirc{dollar}C. Comparison of the performance of the systems with and without Cr(VI) addition indicated that 26 mg Cr(VI)/L in the influent did not affect the biomass yield and carbon consumption.; Microbial Cr(VI) reduction was dependant upon the availability of organic carbon, type of substrates, and the biomass concentration. The optimum pH and temperature for Cr(VI) reduction were found to be 7.3 and 32{dollar}spcirc{dollar}C. Cr(VI) reduction was found to be sensitive to metal cations which may coexist with Cr(VI) in Cr(VI) industrial wastes.; Both nitrate and sulfate inhibited Cr(VI) reduction. Reduction of nitrate but not sulfate was observed along with a higher acetate-degrading rate, which indicated that the enriched culture might be denitrifiers but not sulfate-reducing bacteria. Cr(VI) reduction did not occur with the aerobically-growing culture; however, once anaerobic conditions were resumed, Cr(VI) reduction was immediately restored.; A reaction mechanism was proposed with the assumptions of the tri-complex formation. With the determined kinetic parameters, the model predicted well different Cr(VI) concentrations for data obtained in the present study as well as reported in literature.; Overall, the results of this study revealed a definite Cr(VI)-reducing capacity of the enriched anaerobic culture and provided the essential information of environmental and operational conditions for the effective biological treatment of Cr(VI) wastes.