Cr6+ Reduction in an Indigenous Mixed Culture of Bacteria in the Presence of As3+

Abstract

The indigenous mixed culture of bacteria collected from a Wastewater Treatment Plant in Brits (North West Province, South Africa) biocatalytically reduced Cr⁶⁺ in the presence of As³⁺. Treatment of these elements before disposal of wastewater is critical since they are both known to be carcinogenic and mutagenic at very low concentrations. Both compounds are known to be acutely toxic under high exposure conditions (Cr⁶⁺>50 mg/L, As³⁺>20 mg/L). In this study, experiments were conducted to evaluate the rate of Cr⁶⁺ reduction under anaerobic conditions in the presence of As³⁺. Near complete Cr⁶⁺ reduction wasachieved under initial Cr⁺⁶ concentrations up to 70 mg/L under an As³⁺ concentration of 20 mg/L. However, after increasing Cr⁶⁺ concentrations to 100 mg/L, an inhibitory effect was observed. Further experiments conducted at Cr⁶⁺ concentration of 70 mg/L and varying As³⁺ concentration from 5-70 mg/L showed that Cr⁶⁺ reduction rate increased with increasing As³⁺ concentration from 5-40 mg/L. However, 20 % drop in Cr⁶⁺ reduction efficiency was observed. These results suggest that the rate of Cr⁶⁺ reduction was hindered by the dual toxicity of the Cr⁶⁺ and As³⁺ in the system. The observed inhibitory effect was attributed to thetoxicity effect of Cr⁶⁺ and As³⁺. Initial evaluation of the bacteria using DNA finger printing showed that cells in the mixed culture comprised predominantly of the Gram-positive species: Staphylococcus sp., Enterobacter sp., and Bacillus sp.