Dissolution Potential of Elemental Mercury in the Presence of Bisulfide and Implications for Mobilization

Liquid elemental mercury (Hg⁰_L) pollution can remain in soils for decades and, over time, will undergo corrosion, a process in which the droplet surface oxidizes soil constituents to form more reactive phases, such as mercury oxide (HgO). While these reactive coatings may enhance Hg migration in the subsurface, little is known about the transformation potential of corroded Hg⁰_L in the presence of reduced inorganic sulfur species to form sparingly soluble HgS particles, a process that enables the long-term sequestration of mercury in soils and generally reduces its mobility and bioavailability. In this study, we investigated the dissolution of corroded Hg⁰_L in the presence of sulfide by quantifying rates of aqueous Hg release from corroded Hg⁰_L droplets under different sulfide concentrations (expressed as the S:Hg molar ratio). For droplets corroded in ambient air, no differences in soluble Hg release were observed among all sulfide exposure levels (S:Hg mole ratios ranging from 10^-4 to 10). However, for droplets oxidized in the presence of a more reactive oxidant (hydrogen peroxide, H₂O₂), we observed a 10- to 25-fold increase in dissolved Hg when the oxidized droplets were exposed to low sulfide concentrations (S:Hg ratios from 10^-4 to 10^-1) relative to droplets exposed to high sulfide concentrations. These results suggest two critical factors that dictate the release of soluble Hg from Hg⁰_L in the presence of sulfide: the extent of surface corrosion of the Hg⁰_L droplet and sufficient sulfide concentration for the formation of HgS solids. The mobilization of Hg⁰_L in porous media, therefore, largely depends on aging conditions in the subsurface and chemical reactivity at the Hg⁰_L droplet interface.