Factors affecting hexavalent chromium reduction in pure cultures of bacteria

Batch studies were conducted to evaluate the effect of environmental factors on the rate of Cr(VI) reduction by two organisms, Bacillus sp. (a new isolate), and Pseudomonas fluorescens LB 300. Both organisms utilized glucose for Cr(VI) reduction under aerobic conditions. Higher Cr(VI) reduction rates were obtained with higher initial cell concentrations, but the specific rate (normalized against initial cell concentration), was higher at lower initial cell concentrations. The rate of Cr(VI) reduction by Bacillus sp. increased with initial Cr(VI) concentrations ranging from 20 to 70 mg/l and decreased at higher concentrations. The rate of Cr(VI) reduction by LB 300 steadily increased with the initial Cr(VI) concentrations over the entire test range of 20–100 mg/l Cr(VI). Further studies with the new isolate, Bacillus sp., showed that neither sulfate nor nitrate affected Cr(VI) reduction. Also no correlations were found between the rate of Cr(VI) reduction and the redox potential of culture media. Experiments with cell-free extracts of Bacillus sp. indicated that soluble type of enzymes were responsible for Cr(VI) reduction in this organism.