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Research article2003Peer reviewed

Bonding of ppb levels of methyl mercury to reduced sulfur groups in soil organic matter

Karlsson T, Skyllberg U

Abstract

The strong binding of CH3Hg+ to natural organic matter (NOM) in soils and waters determines the speciation of CH3Hg under aerobic conditions and indirectly its bioavailability and rates of demethylation. In lab experiments, halides (Cl, Br, 1) were used as competing ligands to determine the strength of CH3Hg+ 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 CH3Hg (1.7-9.8 ng g(-1)) in organic soils, and equilibrium concentrations of CH3Hg were determined by species-specific-isotope-dilution (SSID) gas-chromatographyinduced-coupled-plasma-mass-spectrometry (GC-ICP-MS). A simple model (RS- + CH3Hg+ = CH3HgSR; log K-CH3HgSR) 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(a) values of RSH groups were fixed at 8.50 and 9.95, reflecting the two major thiol groups in proteins. Log K-CH3HgSR 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 CH3Hg+, determined constants were independent of type and concentration of halide used in the experiments (log KCH3HgSR = 16.1-16.7 at pH 3.5-3.6). Even if our log KCH3HgSR 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 CH3Hg+ and thiol groups in well-defined organic molecules (log K-1 = 15.71-17.5). Speciation calculations based on our results show that, in absence of substantial concentrations of inorganic sulfides, neutral chloro-complexes (CH3HgCl) and free CH3Hg+ reach concentrations on the order of 10(-17)-10(-18) M at pH 5 in soil solutions with 3 x 10(-5) M of chloride

Published in

Environmental Science and Technology
2003, Volume: 37, number: 21, pages: 4912-4918 Publisher: AMER CHEMICAL SOC

      SLU Authors

    • Karlsson, Torbjörn

      • Department of Forest Ecology, Swedish University of Agricultural Sciences

      • Skyllberg, Ulf

        • Department of Forest Ecology, Swedish University of Agricultural Sciences

      UKÄ Subject classification

      Environmental Sciences related to Agriculture and Land-use
      Fish and Aquacultural Science

      Publication identifier

      DOI: https://doi.org/10.1021/es034302n

      Permanent link to this page (URI)

      https://res.slu.se/id/publ/72