Experimental Study on the Factors Affecting Hexavalent Chromium Bioreduction by Bacillus cereus

Chromium through natural processes and human activities enters the air, soil and water. Chromium-resistant bacteria are capable of reducing toxic Cr(VI) to less toxic Cr(III). In this work, batch studies were conducted to evaluate the effect of environmental factors on the rate of Cr(VI) reduction from synthetic wastewater of metal plating industry by Bacillus cereus under aerobic conditions. The effect of different inoculum volumes (5, 10, 15 and 20 mL), pH (5, 7 and 9), temperatures (20, 30 and 40 °C) and initial concentrations of Cr(VI) (10, 50, 100 and 200 mg/L) for the best performance of chromium removal were investigated during 72 h of cultivation by Bacillus cereus. Complete reduction of Cr(VI) by Bacillus cereus was achieved after 48 h of incubation under optimized conditions of pH 9, inoculum volume of 5 mL, initial chromium concentration of 50 mg/L, and temperature of 40 °C. The results showed the highest rate of reduction at the lowest Cr(VI) concentration (1.04×10-2 h-1.10 mg/L) and the lowest at the highest Cr(VI) concentration (0.55×10-4 h-1.200 mg/L). Atomic absorption spectroscopy analyses under optimized conditions showed the concentration of Cr(III) in the culture supernatant was 49 mg/L after 48 h. The presence of almost all the reduced Cr(III) in the supernatant revealed Cr(VI)-reductase in Bacillus cereus is mainly associated with the soluble fraction of the enzyme. High Cr(VI) concentration resistance and high Cr(VI) reducing ability of Bacillus cereus make it a suitable candidate for bioremediation.