HR: 14:00h
AN: B13F-02 INVITED    [Abstracts]
TI: Chemical Speciation and Mobilization of Mercury and Methyl Mercury at Sub-anoxic Conditions in Wetlands and Sediments with Special Emphasis on Organic Thiols
AU: * Skyllberg, U
EM: ulf.skyllberg@sek.slu.se
AF: Swedish University of Agricultural Sciences, Department of Forest Ecology and Management, Umea, S-90183, Sweden
AB: A correct description of the chemical speciation of Hg and MeHg is a prerequisite for understanding biogeochemical processes like retention-mobilization, methylation-demethylation and bioaccumulation. Binding affinity experiments and spectroscopic studies have clearly established that both inorganic Hg and methyl mercury (MeHg) are strongly complexed by reduced organic functional groups (thiols, RSH) in natural organic matter (NOM). Stability constants for Hg and MeHg associated to NOM macromolecules (humic substances) have been shown to be of a similar magnitude as Hg-thiol and MeHg-thiol associations in well-defined organic molecules like cysteine. As a consequence of this strong bonding, complexes with NOM will highly dominate the speciation of Hg and MeHg, both in the solid phase and in pore water, under oxidized conditions. This also holds for environments with concentrations of halides (Cl) in the mM range. Under anoxic conditions in soils and sediments, inorganic mono-, bi- and polysulfides will form complexes with Hg. At concentrations of dissolved inorganic sulfides exceeding 10 mikroM, Hg-sulfide complexes will dominate the speciation and the contribution from organic Hg-thiols will be less than 1%. However, at slightly reduced (sub-anoxic) conditions with inorganic sulfides in the 0.1-2 mikroM range, complexes with thiols, bi- and polysulfides will all contribute to the solubility of Hg. Crucial is whether elemental sulphur is formed or not. With elemental S present, Hg-polysulfides will outcompete all other species, including neutral Hg-sulfides available for methylating bacteria. Thus, as a basis for selection of relevant chemical speciation models, independent spectroscopic information about solid phases like S₈⁰(s) and HgS(s) (using sulfur XANES, Hg EXAFS etc) is crucial. The identity of some proposed Hg-sulfides and their stability constants (e.g. HOHgSH⁰ and HgS₅OH^- ), are still uncertain and their influence on the chemical speciation will be discussed. The importance of relevant models for the chemical speciation of MeHg is discussed in the context that changes in MeHg solubility in many studies erroneously are interpreted as a consequence of methylation-demethylation and transport processes.
DE: 0409 Bioavailability: chemical speciation and complexation
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting