Oxidation of Dissolved Elemental Mercury by Thiol Compounds under Anoxic Conditions

Mercuric ion, Hg2+, forms strong complexes with thiolate compounds that commonly dominate Hg(II) speciation in natural freshwater. However, reactions between dissolved aqueous elemental mercury (Hg(0)(aq)) and organic ligands in general, and thiol compounds in particular, are not well studied although these reactions likely affect Hg speciation and cycling in the environment. In this study, we compared the reaction rates between Hg(0)(aq) and a number of selected organic ligands with varying molecular structures and sulfur (S) oxidation states in dark, anoxic conditions to assess the role of these ligands in Hg(0)(aq) oxidation. Significant Hg(0)(aq) oxidation was observed with all thiols but not with ligands containing no S. Compounds with oxidized S (e.g., disulfide) exhibited little or no reactivity toward Hg(0)(aq) either at pH 7. The rate and extent of Hg(0)(aq) oxidation varied greatly depending on the chemical and structural properties of thiols, thiol/Hg ratios, and the presence or absence of electron acceptors. Smaller aliphatic thiols and higher thiol/Hg ratios resulted in higher Hg(0)(aq) oxidation rates than larger aromatic thiols at lower thiol/Hg ratios. The addition of electron acceptors (e.g., humic acid) also led to substantially increased Hg(0)(aq) oxidation. Our results suggest that thiol-induced oxidation of Hg(0)(aq) is important under anoxic conditions and can affect Hg redox transformation and bioavailability for microbial methylation.