Inorganic speciation of mercury in sulfidic waters: The importance of zero-valent sulfur

Chemical speciation governs the biologic behavior of Hg. Because sulfide-producing microoganisms accomplish methylation, Hg speciation in sulfidic habitats is of scientific interest. Here, speciation is determined from solubilities of cinnabar (HSS) only and of cinnabar + sulfur in 0.7 M KCl medium at 298 K over the ranges, pH 1-12, and total dissolved sulfide, 10(-3)-10(-1) M. Without dissolved zero-valent sulfur (S-0), solubilities can be explained by the following: HgS(cinn) + H2S reversible arrow HS(SH)(2)(0), pK = 5.36 +/- 0.10; HgS(cinn) + SH- reversible arrow HSS(SH)-, pK = 5.34 +/- 0.30; and HgS(cinn) + 2SH(-) reversible arrow HgS22- + H2S, PK = 7.14 +/- 0.16 (all at l = 0.7 M). Unlike ZnS and CdS, HgS solubility is increased by S-0, which promotes formation of bidentate polysulfide ligands. Additional Hg solubility in S-0 saturated solutions up to pH 9.5 can be explained by: HgS(cinn) + SH- + (n - 1)S-0(rhom) reversible arrow HS(S-n)SH-, pK= 3.97 +/- 0.17; n could not be determined but is probably 4-6. In near-neutral, sulfidic natural waters, previously unknown HS(S-n)SH- might commonly exceed other inorganic Hg(II) species because dissolved S-0 occurs widely, especially near redox fronts where it is generated by biotic and abiotic oxidation of sulfide.