Bacterial Reduction of Toxic Hexavalent Chromium and its Application to Biological Treatment of Waste Waters

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Hexavalent chromiun (chromate : CrO_4^<2->) is toxic and mutagenic for most organisms. Waste waters containing toxic chromate are generated in many industrial processes. We have isolated a chromate-reducing bacterium, identified as Enterobacter cloacae, from activated sludge. Reduced chromium (Cr^<3+>), which is much less toxic than chromate, readily formed insoluble chromium hydroxides at a neutral pH. The ability of this bacterium to reduce chromate was examined for detoxification and removal of toxic chromium from waste waters.The important features of the reduction kinetics were : (1) the rate of reduction was proportional to cell density ; (2) high concentrations of chromate inhibited the reduction ; (3) the effect of temperature followed the Arrhenius equation over the temperature range of 10-30C ; (4) the reduction occurred at pH6.5 and 8.5 ; (5) the reduction rate was dependent on the amount of added carbon and energy sources ; (6) the reduction of chromate was sensitive to oxygen stress, but when released from the oxygen stress, it readily resumed.E. cloacae strain HOl could reduce toxic chromate in a contaminated industrial effluent. Carbon and energy sources were required for the reduction of chromate. Chromate reduction was inhibited to some extent by other metal ions including Cu^<2+>, Mn^<2+> and Zn^<2+>.This method requires neither high energy nor chemical reagents. It can be said that there is a potential for establishing a cost effective method to remove toxic chromate from contaminated waters.