The strong binding of CH{sub}3Hg{sup}+ to natural organic matter (NOM) in soils and waters determines the speciation of CH{sub}3Hg under aerobic conditions and indirectly its bioavailability and rates of demethylation. In lab experiments, halides (Cl, Br, I) were used as competing ligands to determine the strength of CH{sub}3Hg{sup}+ binding to solid-phase soil organic carbon (SOC)and to dissolved soil organic carbon (DOC) as a function of time, pH, and concentration of halide. Experiments were conducted with native concentrations of CH{sub}3Hg (1.7-9.8 ng g{sup}(-1)) in organic soils, and equilibrium concentrations of CH{sub}3Hg were determined by species-specific-isotope-dilution (SSID) gas-chromatography-induced-coupled-plasma-mass-spectrometry (GC-ICP-MS). A simple model (RS{sup}- + CH{sub}3Hg{sup}+ = CH3HgSR; log K{sub}(CH{sub}3HgSR)) was used to simulate the binding to SOC and DOC, in which the binding sites (RSH) were independently determined by X-ray absorption near-edge structure (XANES) spectroscopy. The pk{sub}a values of RSH groups were fixed at 8.50 and 9.95, reflecting the two major thiol groups in proteins. Log K{sub}(CH{sub}3HgSR) values determined for SOC and DOC were similar, showing a range of 15.6-17.1 for all experiments covering a pH range of 2.0-5.1. Despite large differences in affinities between Cl, Br, and I for CH{sub}3Hg{sup}+, determined constants were independent of type and concentration of halide used in the experiments (log K{sub}(CH{sub}3HgSR)) = 16.1-16.7 at pH 3.5-3.6). Even if our log K{sub}(CH{sub}3HgSR) values were conditional in that they decreased with pH above 3.5, they were in fair agreement with stability constants determined for the association between CH{sub}3Hg{sup}+ and thiol groups in well-defined organic molecules (log K{sub}1 = 15.7-17.5). Speciation calculations based on our results show that, in absence of substantial concentrations of inorganic sulfides, neutral chloro-complexes (CH{sub}3HgCI) and free CH{sub}3Hg{sup}+ reach concentrations on the order of 10{sup}(-17)-10{sup}(-18) M at pH 5 in soil solutions with 3 × 10{sup}(-5) M of chloride.
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