Enterobacter cloacae strain (HOI) capable of reducing hexavalent chromium (chromate) was isolated from activated sludge. This bacterium was resistant to chromate under both aerobic and anaerobic conditions. Only the anaerobic culture of the E. cloacae isolate showed chromate reduction. In the anaerobic culture, yellow turned white with chromate and the turbidity increased as the reduction proceeded, suggesting that insoluble chromium hydroxide was formed. E. cloacae is likely to utilize toxic chropiate as an electron acceptor anaerobically because (i) the anaerobic growth of E. cloacae HO1 accompanied the decrease of toxic chromate in culture medium, (ii) the chromate-reducing activity was rapidly inhibited by oxygen, and (iii) the reduction occurred more rapidly in glycerolor acetate-grown cells than in glucose-grown cells. The chromate reduction in E. cloacae HO1 was observed at pH 6.0 to 8.5 (optimum pH, 7.0) and at 10 to 40°C (optimum, 30°C).
A method for removal of toxic hexavalent chromium (chromate: CrO2-4) was developed by use of dialysis-sac cultures of a chromate-reducing strain of Enterobacter cloacae (HO1). E. cloacae strain HO1 cells were put in dialysis (semipermeable membrane) sacs, and the sacs were submerged in water containing toxic CrO2-4. The dialysis sacs allowed CrO2-4 to diffuse into the culture, and CrO2-4 was reduced anaerobically in the dialysis sacs by the E. cloacae cells. Because reduced chromium readily formed insoluble chromium hydroxides in the dialysis sacs, the greater part of reduced chromium was unable to diffuse out through the semipermeable membrane. Thus the dialysis culture of E. cloacae strain HO1 could successfully remove toxic chromium from the surrounding water. If the initial concentration of CrO2-4 was less than 4 mM (208 ppm as chromium), about 90% of the total chromium could be removed from water by the described method.
Hexavalent chromium (chromate: CrO.+-) is toxic and mutagenic for most organisms.'.* In humans, chromate causes irritation and corrosion of skin and respiratory tract, and is believed to be responsible for lung carcinoma. Chromate is also hazardous to fauna and flora in natural aquatic ecosyst e m~.~ Wastewaters containing toxic chromate are generated in many industrial processes including chrome leather tanning, chromium plating, metal cleaning and processing, wood preservation, and alloy p r e p a r a t i~n .~ These wastewaters must be treated before discharged to natural environments. Recently, we have isolated a chromate-reducing bacterium, identified as Enterobacter cloacae, which is resistant to high levels of chromate (10 mM) under both aerobic and anaerobic conditions.6 During anaerobic growth, the E. cloacar isolate reduces chromate to trivalent chromium (Cr3') which is subsequently precipitated in the culture medium. Trivalent chromium is much less toxic than chromate. I Therefore, the ability of this bacterium to reduce chromate may be useful not only for detoxification, but also for removal of toxic chromium from wastewaters.Since there are no reports on the biological removal of aqueous chromium species, excepting simple microbial ads~r p t i o n ,~ the present communication demonstrates the potential of the E. cloacae chromate-reducing strain to remove toxic chromate from wastewaters.
MATERIALS AND METHODS
Bacterial StrainEnterobacter cloacae strain HO 1 was isolated from an activated sludge sample from a municipal wastewater treatment plant. The characteristics of E . cloacae H 0 1 have been previously described.6
Resistance to toxic hexavalent chromium (chromate: CrO42−_ in Enterobacter cloacae strain HO1, isolated from an activated sludge sample, was investigated under aerobic and anaerobic conditions. Decreased uptake of 51CrO42− in E. cloacae strain HO1 was observed under aerobic conditions, when compared with a standard laboratory E. cloacae strain (IAM 1624). Under anaerobic conditions E. cloacae strain HO1 was able to reduce hexavalent chromium to the less toxic trivalent form. When E. clocacae strain HO1 was grown with nitrate anaerobically, the cells were observed to lose simultaneously their chromate‐reducing ability and chromate‐resistance under anaerobic conditions.
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