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.