Complexation of mercury(II) in soil organic matter: EXAFS evidence for linear two-coordination with reduced sulfur groups

The chemical speciation of inorganic mercury (Hg) is to a great extent controlling biologically mediated processes, such as mercury methylation, in soils, sediments, and surface waters. Of utmost importance are complexation reactions with functional groups of natural organic matter (NOM), indirectly determining concentrations of bioavailable, inorganic Hg species. Two previous extended X-ray absorption fine structure (EXAFS) spectroscopic studies have revealed 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. In this work, covering intact organic soils and extending to much lower concentrations of Hg than before, we show that Hg is complexed by two reduced organic S groups (likely thiols) at a distance of 2.33 angstrom in a linear configuration. Furthermore, a third reduced S (likely an organic sulfide) was indicated to contribute with a weaker second shell attraction at a distance of 2.92-3.08 angstrom. When all high-affinity S sites, corresponding to 20-30% of total reduced organic S, were saturated, a structure involving one carbonyl-O or amino-N at 2.07 angstrom and one carboxyl-O at 2.84 angstrom in the first shell, and two second shell C atoms at an average distance of 3.14 angstrom, gave the best fit to data. Similar results were obtained for humic acid extracted from an organic wetland soil. We conclude that models that are in current use to describe the biogeochemistry of mercury and to calculate thermodynamic processes need to include a two-coordinated complexation of Hg(II) to reduced organic sulfur groups in NOM in soils and waters.