Mercury Speciation in the Presence of Polysulfides

Environmental mercury methylation appears modulated by sulfide concentrations, possibly via changes in mercury availability to sulfate reducing bacteria, the major agents of this process in aquatic systems. Consequently, there has been much recent interest in quantifying the chemical speciation and lipid solubility of mercury in the presence of sulfide and of polysulfides, which form by reaction of sulfide with elemental sulfur or as intermediates in sulfur cycling. We quantify a large increase in the solubility of cinnabar (HgS_(s)) in the presence of elemental sulfur, particularly at high pH. Based on our data and those of Paquette and Helz, we propose that the complex, Hg(S_x)₂^2-, dominates the speciation of Hg(II) in such waters. At lower sulfide concentrations and at high pH, the data are best fitted by considering also the formation of the species HgS_xOH^-. Octanol−water distribution (D_ow) experiments confirm the charged nature of the dominant mercury−polysulfide complexes and imply the presence of a minor lipophilic polysulfide complex, such as HgS₅. The recently reported decrease in D_ow with increasing sulfide concentration also occurs in the presence of polysulfides.