SOLUBILITY OF CINNABAR (RED HGS) AND IMPLICATIONS FOR MERCURY SPECIATION IN SULFIDIC WATERS

New experiments have been conducted to determine the speciation of dissolved mercury (Hg) over wide pH (1-12) and sulfide concentration ranges (0.5-30 mM) and in the presence of elemental sulfur (S degrees) or Hg degrees, conditions that encompass those of near-bottom and pore waters of sediments. Samples containing synthetic red mercuric sulfide (HgS, cinnabar), buffer solution, aliquots of bisulfide (HS-) solution, and, in special cases, S degrees or Hg degrees were prepared anaerobically and allowed to equilibrate for several months. Filtered samples were analyzed for pH, total sulfide (Sigma S2-), and total mercury [Hg](tot). Plots of [Hg](tot) values vs. pH at varying Sigma S2- verified the formation of three previously known mercury-sulfide complexes (HgS2Hnn-2) and revealed that a new Hg2SOH+ complex is important at low pH and low Sigma S2-. Our constants for ionic strength (I) 0.7 and 25 degrees C are as follows: K-1 = 10(-5.76(+0.71,) (-1.02)) for HgScinn + H2S <-> HgS2H2 degrees; K-2 = 10(-4.82(+0.72,) (-1.10)) for HgScinn + HS- <-> HgS2H-; K-3 = 10(-13.41(+0.76,) (-0.93)) for HgScinn + HS- <-> HgS22- + H+; K-4 = 10(-8.36(+0.71,) (0.93)) for 2HgS(cinn) + H+ + H2O <-> Hg2SOH+ + H2S. With decreasing pH, below 1, Hg solubility decreased sharply, indicating the formation of a new solid phase, inferred to be corderoite (Hg3S2Cl2). From our solubility data, we calculated the free energy of formation (Delta G(f) degrees) of Hg3S2Cl2 to be -396 (+3, -11) kJ/mol. In experiments where excess S degrees(s) was present, a new mercury-polysulfide dimer was identified; its formation constant is K-5 = 10(-1.99(+0.69,) (-1.27)) for 2HgS(cinn) + 2HS(-) + nS degrees <-> (Hg2S4S)-S-II degrees(n)H(2)(2-). Data from experiments where Hg degrees(aq) was added confirmed the reversibility of HgS dissolution. An application of our mercury-sulfide speciation model to a natural anoxic basin, Saanich Met, British Columbia, is discussed.