INTERACTIONS BETWEEN DISSOLVED ORGANIC MATTER AND MERCURY IN AQUATIC ENVIRONMENTS

The ecological fate of Hg in aquatic systems is dependent, in large part, on redox state, pH, dissolved organic matter (DOM) concentration, the concentrations of inorganic ligands, especially sulfide, the distribution of Hg+2 between dissolved and particulate phases, and the presence of sulfate-reducing bacteria that convert Hg+2 into MeHg, a highly toxic form of Hg that is readily bioaccumulated. Our research has shown that, under all redox conditions, DOM influences the transport and reactivity of Hg by strong Hg-DOM binding and colloidal stabilization of mercuric sulfide (HgS). Very strong interactions (K_DOM'=10^23.2±0.5 L kg^-1 at pH=7.0 and I=0.1), indicative of Hg-thiol bonds, were observed at Hg/DOM ratios below approximately 1 μg Hg per mg DOM. The reduced sulfur content of DOM (thiophene, organic sulfides and thiols), as determined by X-ray adsorption near edge spectroscopy, was found to be dependent, in part, on abiotic interactions of DOM with sulfide in sulfate reducing environments. Only a small fraction (approximately 2%) of the reduced-S groups was involved with the strongest interactions between Hg and DOM. These results suggest that the binding of Hg to DOM under natural conditions (very low Hg/DOM ratios ranging from 0.01 to 10 ng of Hg/mg of DOM) is controlled by a small fraction of DOM molecules containing reactive thiol functional groups. In the case of fully oxygenated water (sulfide-free), the binding of Hg+2 by DOM should dominate dissolved inorganic mercury speciation. Where measurable total sulfide concentrations are present in surface water and pore water, however, Hg-sulfide complexes predominate. In these cases, common in sulfate reducing environments, DOM interacts strongly with HgS (log Ksp=-52.4) to inhibit aggregation and precipitation of HgS by colloidal stabilization. In experiments designed to define this interaction, precipitation of HgS was strongly inhibited in the presence of low concentrations (<3 mg C/L) of DOM, and, organic matter rich in aromatic moieties was more reactive with HgS than less aromatic fractions. These results suggest that DOM can influence the geochemistry and bioavailability of HgS in aquatic environments by maintaining higher dissolved total Hg concentrations than predicted by speciation models.