| Abstract
| - The chemical speciation of inorganic mercury (Hg) is to agreat extent controlling biologically mediated processes,such as mercury methylation, in soils, sediments, and surfacewaters. Of utmost importance are complexation reactionswith functional groups of natural organic matter (NOM),indirectly determining concentrations of bioavailable,inorganic Hg species. Two previous extended X-ray absorptionfine structure (EXAFS) spectroscopic studies haverevealed that reduced organic sulfur (S) and oxygen/nitrogen (O/N) groups are involved in the complexation of Hg(II) to humic substances extracted from organic soils. Inthis work, covering intact organic soils and extending tomuch lower concentrations of Hg than before, we show thatHg is complexed by two reduced organic S groups(likely thiols) at a distance of 2.33 Å in a linear configuration.Furthermore, a third reduced S (likely an organic sulfide)was indicated to contribute with a weaker secondshell attraction at a distance of 2.92−3.08 Å. When all high-affinity S sites, corresponding to 20−30% of total reducedorganic S, were saturated, a structure involving onecarbonyl-O or amino-N at 2.07 Å and one carboxyl-O at2.84 Å in the first shell, and two second shell C atoms atan average distance of 3.14 Å, gave the best fit todata. Similar results were obtained for humic acid extractedfrom an organic wetland soil. We conclude that modelsthat are in current use to describe the biogeochemistry ofmercury and to calculate thermodynamic processesneed to include a two-coordinated complexation of Hg(II)to reduced organic sulfur groups in NOM in soils andwaters.
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