Biological removal of toxic chromium using an Enterobacter cloacae strain that reduces chromate under anaerobic conditions

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.6During 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.