Effectiveness of sorbents to reduce mercury methylation

Experiments were completed to assess the effectiveness of Biochar, ThiolSAMMS^®, SediMite™, and Organoclay-199 to decrease methylmercury (MeHg) production by the known mercury methylator, Desulfovibrio desulfuricans ND132 (ND132), and to decrease aqueous (i.e., 0.2 μM filter passing) MeHg concentrations. Mercury (Hg) was introduced as Hg(II) or a preequilibrated Hg-DOM (dissolved organic matter) complex. Individual experiments tested the effect of sulfate concentration and Hg-sorbent equilibration time (up to 1 year). The sorbents provided little, if any, inhibition of total MeHg production, but decreased the percentage of MeHg that passed a 0.2 μm filter. Total MeHg produced was substantially greater than the calculated equilibrium aqueous inorganic Hg (Hg_i), based on sorption isotherms, implying mercury assumed to be sorbed at the start of the assay was bioaccessible over the 24-h methylation timeframe. MeHg production increased when Hg_i was introduced as an Hg:DOM complex, compared to Hg(II). Some of the increased MeHg production may be attributed to lower Hg_i sorption with DOM present, but a substantial amount of sorbed Hg must have been accessible also. DOM also increased the filter-passing MeHg fraction, stressing the importance of Hg-DOM and MeHg-DOM interactions in experimental investigations. Sulfate concentrations did not affect Hg(II) methylation and could not explain the increased methylation seen with the Hg:DOM complex. Extended equilibration of Hg-sorbent mixtures for up to 1 year before the methylation assays did not affect the amount of MeHg produced for Biochar or SediMite treatments, but significantly lowered MeHg production for ThiolSAMMS. Inclusion of DOM in these experiments presented the sorbents with a more environmentally relevant form of Hg and lowered sorbent effectiveness to decrease MeHg production and sorb produced MeHg.