Multiple-pathway remediation of mercury contamination by a versatile selenite-reducing bacterium

Mercury contamination is a global concern because of its high toxicity, persistence, bioaccumulative nature, long distance transport and wide distribution in the environment. In this study, the efficiency and multiple-pathway remediation mechanisms of Hg²⁺ by a selenite reducing Escherichia coli was assessed. E. coli can reduce Hg²⁺ to Hg⁺ and Hg⁰ and selenite to selenide at the same time. This makes a multiple-pathway mechanisms for removal of Hg²⁺ from water in addition to biosorption. It was found that when the original Hg²⁺ concentration was 40μgL^-1, 93.2±2.8% of Hg²⁺ was removed from solution by E. coli. Of the total Hg removed, it was found that 3.3±0.1% was adsorbed to the bacterium, 2.0±0.5% was bioaccumulated, and 7.3±0.6% was volatilized into the ambient environment, and most (80.6±5.7%) Hg was removed as HgSe and HgCl precipitates and Hg⁰. On one hand, selenite is reduced to selenide and the latter further reacts with Hg²⁺ to form HgSe precipitates. On the other hand Hg²⁺ is successively reduced to Hg⁺, which forms solid HgCl, and Hg⁰. This is the report on bacterially transformation of Hg²⁺ to HgSe, HgCl and Hg⁰ via multiple pathways. It is suggested that E. coli or other selenite reducing microorganisms are promising candidates for mercury bioremediation of contaminated wastewaters, as well as simultaneous removal of Hg²⁺ and selenite.