Sulfide controls on mercury speciation and bioavailability to methylating bacteria in sediment pore waters

A chemical equilibrium model for Hg complexation in sediments with sulfidic pore waters is presented. The purpose of the model was to explain observed relationships between pore water sulfide, dissolved inorganic Hg (Hg-D), and bulk methylmercury(MeHg) in surficial sediments of two biogeochemically different ecosystems, the Florida Everglades and Patuxent River, MD. The model was constructed to test the hypothesis that the availability of Hg for methylation in sediments is a function of the concentration of neutral dissolved Hg complexes rather than Hg2+ or total Hg-D. The model included interaction of mercury with solids containing one or two sulfide groups, and it was able to reproduce observed Hg-D and bulk MeHg trends in the two ecosystems. The model is consistent with HgS0 as the dominant neutral Hg complex and the form of Hg accumulated by methylating bacteria in sulfidic pore waters. The model-estimated decline in HgS0 with increasing sulfide was consistent with the observed decline in bulk sediments MeHg. Since bacterial Hg uptake rate is one of the factors affecting methylation rate, Hg complexation models such as the one presented are helpful in understanding the factors that control MeHg production and accumulation in aquatic ecosystems.