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Magnetite and green rust have been shown to reduce aqueous Hg^II to Hg⁰. 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 μM) and low (5 μM) 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 Hg⁰ 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 Hg⁰ in 2 d. These results suggest that Hg^II binding to carboxyl groups …