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

Highlights: • First report on removal of Hg^{2 +} by Escherichia coli via HgSe, HgCl and Hg⁰. • Superoxide is involved in reducing selenite to selenide to immobilize Hg^{2 +}. • The versatile E. coli is a promising candidate for treatment of Hg^{2 +} wastewater. 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 Hg^{2 +} by a selenite reducing Escherichia coli was assessed. E. coli can reduce Hg^{2 +} to Hg⁺ and Hg⁰ and selenite to selenide at the same time. This makes a multiple-pathway mechanisms for removal of Hg^{2 +} from water in addition to biosorption. It was found that when the original Hg^{2 +} concentration was 40 μg L⁻¹, 93.2 ± 2.8% of Hg^{2 +} 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 Hg^{2 +} to form HgSe precipitates. On the other hand Hg^{2 +} is successively reduced to Hg⁺, which forms solid HgCl, and Hg⁰. This is the report on bacterially transformation of Hg^{2 +} 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 Hg^{2 +} and selenite.