Aerobic chromium(VI) reduction by chromium-resistant bacteria isolated from activated sludge

A chromium-reducing strain isolated from activated sludge and subsequently identified as Clostridium sp. SS1 by a biomolecular method was studied for its resistance to toxic hexavalent chromium [Cr(VI)] and its ability to reduce Cr(IV). The strain was found to tolerate concentrations of Cr(VI) up to 50 mg/l; however, cell growth was severely affected by the presence of Cr(VI), especially at concentrations of more than 20 mg/l. Clostridium sp. SS1 also effectively reduced toxic Cr(VI) into the comparatively less toxic trivalent form. The optimum pH value for Cr(VI) reduction was 7.0, and the cells reduced Cr(VI) more efficiently at 30°C than at 20 and 40°C. Within the same incubation time, higher amounts of Cr(VI) were reduced at higher initial Cr(VI) concentrations (range 5–40 mg/l). Clostridium sp. SS1 had a higher volumetric [0.82 mg Cr(VI)/l/h] and greater capacity [72 mg Cr(VI)/g biomass] to reduce Cr(VI) at higher initial Cr(VI) concentrations (40 mg/l), suggesting that this strain could be potentially useful for the detoxification of Cr(VI) polluted sites.