W4 AM Environmental Fate of Mercury
Wednesday, 16 November 2005: 8:00 AM - 11:40 AM in Ballroom 4

(MIL-1117-641743) Interaction of mercury with dissolved organic matter under sulfidic conditions.

Miller, C¹, Mason, R¹, Gilmour, C², ¹ University of Maryland Center of Environmental Science, Chesapeake Biological Laboratory, Solomons, MD, USA² Smithsonian Environmental Research Center, Edgewater, MD, USA

ABSTRACT- In most environments, thermodyamic models predict mercury (Hg) is complexed with dissolved organic matter (DOM) but under sulfidic conditions, inorganic Hg-sulfide complexes dominate the Hg speciation. The complexation of Hg with sulfide has been correlated with the bacterial formation of methylmercury (MeHg), the form of Hg with bioaccumalates in organisms. The diffusive uptake of neutrally charged Hg-sulfide complexes by sulfate reducing bacteria, a group of bacteria that have been linked to Hg methylation, is proposed to be a controlling factor in Hg methylation. While the importance of Hg-sulfide complexation to Hg methylation has been demonstrated, the abundance of these complexes has only been predicted using thermodynamic models. Using octanol-water partitioning, it was determined that thermodynamic models overestimate the abundance of neutrally charged Hg-sulfide complexes in sulfidic porewater samples. Subsequent laboratory studies, using octanol-water partitioning and ultrafiltration, indicated that the presence of DOM reduced the formation of inorganic Hg-sulfide complexes, likely due to the formation of complexes between the Hg, sulfide and DOM. Such complexes are not included in current thermodynamic models. Thioglycolate and EDTA were used as model DOM ligands to elucidate the interaction occurring between Hg, sulfide and organic compounds. The complexation of Hg with DOM under oxic conditions has been shown to be controlled by thiol groups on DOM, but it does not appear that thiol functional groups are the only functional groups on organic molecules which can facilitate the formation of Hg, sulfide and DOM complexes. This demonstrated interaction of Hg with DOM under sulfidic conditions changes the current understanding of Hg speciation. Since Hg-sulfide speciation can control Hg methylation, the interaction of Hg with DOM in sulfidic environments could impact the formation of MeHg and could account for the differences found in the degree of Hg methylation across ecosystems.

Key words: Hg complexation, sulfidic, DOM, octanol-water partitioning