Binding of HgII to High-Affinity Sites on Bacteria Inhibits Reduction to Hg0 by Mixed FeII/III Phases

Magnetite and green rust have been shown to reduce aqueous Hg-II to Hg-0. In this study, we tested the ability of magnetite and green rust to reduce Hg-II sorbed to 2 g . L-1 of biomass (Bacillus subtilis), at high (50 mu M) and low (5 mu M) Hg loadings and at pH 6.5 and 5.0. At high Hg:biomass loading, where Hg-II binding to biomass is predominantly through carboxyl functional groups, Hg L-III-edge X-ray absorption spectroscopy showed reduction of Hg-II to Hg-0 by magnetite. Reduction occurred within 2 h and 2 d at pH 6.5 and 5.0, respectively. At low Hg:biomass loading, where Hg-II binds to biomass via sulfhydryl functional groups, Hg-II was not reduced by magnetite at pH 6.5 or 5.0 after 2 months of reaction. Green rust, which is generally a stronger reductant than magnetite, reduced about 20% of the total Hg-II bound to biomass via sulfhydryl groups to Hg-0 in 2 d. These results suggest that He binding to carboxyl groups does not significantly inhibit the reduction of He by magnetite. However, the binding of Hg-II to biomass via sulfhydryl groups severely inhibits the ability of mixed Fe-II/III in phases like magnetite and green rust to reduce Hg-II to Hg-0. The mobility of heavy metal contaminants in aquatic and terrestrial environments is greatly influenced by their speciation, especially their oxidation state. In the case of Hg, reduction of Hg-II to Hg-0 can increase Hg mobility because of the volatility of Hg-0. Since Hg is typically present in aquatic and terrestrial systems at low concentrations, binding of Hg to high-affinity sites on bacteria could have important implications for the potential reduction of Hg-II to Hg-0 and the overall mobility of Hg in biostimulated subsurface environments.