Solubility of cinnabar (red HgS) and implications for mercury speciation in sulfidic waters

Abstract

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°) or Hg°, 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° or Hg° were prepared anaerobically and allowed to equilibrate for several months. Filtered samples were analyzed for pH, total sulfide (ΣS2−), and total mercury [Hg]tot. Plots of [Hg]tot values vs. pH at varying ΣS2− verified the formation of three previously known mercury-sulfide complexes (HgS2H n n−2 ) and revealed that a new Hg2SOH+ complex is important at low pH and low ΣS2−. Our constants for ionic strength (I) 0.7 and 25° C are as follows: K1=10−5.76(+0.71, −102) for HgScinn + H2S ↔ HgS2H2° K2=10−4.82(+0.72, −1.10) for HgScinn + HS− ↔ HgS2H−; K3 =10−13.41(+O.76, −0.93) for HgScinn + HS−. ↔ HgS 2 2− . + H+; K4=10−8.36(+0.71, −0.93) for 2HgScinn + 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 (ΔGf°) of Hg3S2Cl2 to be −396 (+3, −11) kJ/mol. In experiments where excess S°(s) was present, a new mercury-polysulfide dimer was identified; its formation constant is K3=10−1.99(+0 69,−1.27) for 2HgScinn + 2HS− + nS° ↔ Hg2SΠ 4S°nH2 −. Data from experiments where Hg°(aq) was added confirmed the reversibility of HgS dissolution. An application of our mercury-sulfide speciation model to a natural anoxic basin, Saanich Inlet, British Columbia, is discussed.