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 (S⁰) or Hg⁰, 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⁻¹) solution, and, in special cases, S⁰ or Hg⁰ 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 25⁰ C are as follows: K₁=10^{−5.76(+0.71, −1.02)} for HgS_cinn+H₂S ↔ HgS₂H₂ ⁰; K₂=10^{−4.82(+0.72, −1.10)} for HgS_cinn+HS⁻ ↔ HgS₂H⁻; K₃=10^{−13.41(+0.76, −0.93)} for HgS_cinn+HS⁻ ↔ HgS₂ ²⁻+H⁺; K₄=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 (ΔG_f ^o) of Hg₃S₂Cl₂ to be −396 (+3, −11) kJ/mol. In experiments where excess S⁰(s) was present, a new mercury-polysulfide dimer was identified; its formation constant is K₅=10^{−1.99(+0.69, −1.27)} for 2HgS_cinn+2HS⁻ + nS⁰ ↔ Hg₃S₄ ^IIS_n ^oH₂ ²⁻. Data from experiments where Hg⁰(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.