BIOLOGICAL REDUCTION OF CHROMIUM BY ESCHERICHIA-COLI

Toxic hexavalent chromium, Cr(VI), was reduced by an Escherichia coli strain, ATCC 33456, under both aerobic and anaerobic conditions by using a variety of electron donors including glucose, acetate, propionate, glycerol and glycine. In addition to electron-donor type, the initial cell concentration, Cr(VI) concentration, pH, and temperature had profound effects on the rate of Cr(VI) reduction. Cr(VI) reduction was not significantly affected by the presence of moderate level of phenolic compounds. The concentration levels that caused a 50% reduction in the rate of Cr(VI) reduction (IC50) were 986, 1,526, and 5,263 mg/L for p-cresol, 2-chlorophenol, and phenol, respectively, in anaerobic cultures with higher IC50 values noted in aerobic cultures. Cr(VI) reduction by this strain was more susceptible to heavy metals (Zn2+ and Cu2+) than the phenolic compounds. The reduction of Cr(VI) by E. coli ATCC 33456 was enzymatic, and was not affected by redox potential of the culture media and other possible electron acceptors such as sulfate and nitrate.