| Abstract
| - The strong binding of CH3Hg+ to natural organic matter(NOM) in soils and waters determines the speciation ofCH3Hg under aerobic conditions and indirectly itsbioavailability and rates of demethylation. In lab experiments,halides (Cl, Br, I) were used as competing ligands todetermine the strength of CH3Hg+ binding to solid-phasesoil organic carbon (SOC) and to dissolved soil organic carbon(DOC) as a function of time, pH, and concentration ofhalide. Experiments were conducted with native concentrations of CH3Hg (1.7−9.8 ng g-1) in organic soils, andequilibrium concentrations of CH3Hg were determined byspecies-specific-isotope-dilution (SSID) gas-chromatography-induced-coupled-plasma-mass-spectrometry (GC−ICP-MS). A simple model (RS- + CH3Hg+ = CH3HgSR;log KCH3HgSR) was used to simulate the binding to SOC andDOC, in which the binding sites (RSH) were independentlydetermined by X-ray absorption near-edge structure (XANES)spectroscopy. The pKa values of RSH groups were fixedat 8.50 and 9.95, reflecting the two major thiol groups inproteins. Log KCH3HgSR values determined for SOC andDOC were similar, showing a range of 15.6−17.1 for allexperiments covering a pH range of 2.0−5.1. Despite largedifferences in affinities between Cl, Br, and I for CH3Hg+,determined constants were independent of type andconcentration of halide used in the experiments (log KCH3HgSR= 16.1−16.7 at pH 3.5−3.6). Even if our log KCH3HgSRvalues were conditional in that they decreased with pHabove 3.5, they were in fair agreement with stability constantsdetermined for the association between CH3Hg+ andthiol groups in well-defined organic molecules (log K1 = 15.7−17.5). Speciation calculations based on our results showthat, in absence of substantial concentrations of inorganicsulfides, neutral chloro-complexes (CH3HgCl) and freeCH3Hg+ reach concentrations on the order of 10-17−10-18M at pH 5 in soil solutions with 3 × 10-5 M of chloride.
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