Investigation of Hexavalent Chromium Reduction by Aerobic Bacteria in Water and Soil

Hexavalent chromium (Cr(VI)) has been identified as a high priority pollutant by the U.S. Environmental Protection Agency because of its high toxicity and carcinogenicity. Cr(VI) is commonly used in electroplating, pigment, leather processing and mining resulting in widespread Cr(VI) pollution, and a need for the development of cost-effective remediation technologies. Microbial reduction of Cr(VI) to non-toxic trivalent chromium (Cr(III)) is a promising technique. In this study, five bacteria were isolated from the soil of Cr(VI) contaminated plating facility in Putnam, CT. Then were identified tentatively as Leucobacter sp., Serratia sp., Achromobacter sp., Delftia sp. and Pseudomonas fluorescens using the 16S rRNA technique. The biological reduction of aqueous Cr(VI) was initially evaluated in mixed culture of the five bacteria. pH, temperature and initial Cr(VI) concentration had a significant effect on Cr(VI) reduction. The highest Cr(VI) reduction rate was obtained at pH=8, 25-30°C and an initial Cr(VI) concentration less than 50mg/L. Approximately 98% of 40 mg/L Cr(VI) was reduced within 48 hours culture at these conditions. Cr(VI) reduction was shown to correlate with bacterial growth. Bio-adsorption experiment confirmed that the decrease in Cr(VI) not due to bio-adsorption, and bio-reduction was proved by further chromium analysis. Scanning Electron Microscopy was also employed to observe the morphology of the bacteria in cultures with and without Cr(VI). Finally, Pseudomonas fluorescens was used to evaluate Cr(VI) reduction in sterilized and live soil samples. When added at a dosage of 30 mg Cr(VI) per kg dry soil, all Cr(VI) was reduced within one day of curing without the addition of an external carbon source. At an addition of 500 mg/kg Cr(VI), 60% of Cr(VI) was reduced in the sterilized soil within 5 days with 10% (w/w) peptone added, while around 22% Cr(VI) reduction was achieved in the sterilized soil without peptone added. Compared to the sterilized soil, the Cr(VI) reduction rate was slightly increased in the live soil with peptone added