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 (HgS) 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) + H 2 S ⇄ Hg(SH) 2 0, p K = 5.36 ± 0.10; HgS(cinn) + SH - ⇄ HgS(SH) -, p K = 5.34 ± 0.30; and HgS(cinn) + 2SH - ⇆ HgS 2 2 - + H 2 S, p K = 7.14 ± 0.16 (all at I = 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) ⇄ Hg(S n )SH -, p K = 3.97 ± 0.17; n could not be determined but is probably 4−6. In near-neutral, sulfidic natural waters, previously unknown Hg(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.