An assessment of thermodynamic reaction constants for simulating aqueous environmental monomethylmercury speciation

Abstract

Monomethylmercury (CH3Hg+) is both the most ecologically significant and the least well characterized species of mercury in environmental settings. Our understanding of the environmental speciation behavior of this compound is limited both as the result of lesser available laboratory data (when compared to inorganic mercury) as well as the uncertainties associated with our understanding of the properties of environmental ligands. A careful examination and synthesis of data reported in the technical literature led to the following findings: (1) a 25°C, zero ionic strength bicarbonate ion complexation constant estimate is remarkably close to an earlier reported value at 0.4 M: CH3Hg+ + HCO3- ⇔ CH3HgHCO3, log 10K = 2.6 (±0.22, 1 SD), (2) three 25°C zero ionic strength reaction constants reported by DeRobertis et al. (1998) were confirmed to within ∼±0.1 log10K units: CH3Hg+ + OH- ⇔ CH3HgOH, log10K = 9.47; 2CH 3Hg+ + H2O ⇔ (CH3Hg) 2OH+ + H+, log10K = -2.15; CH 3Hg+ + Cl- ⇔ CH3HgCl, log 10K = 5.45, (3) "best estimate" literature complexation constants corrected to zero ionic strength include: CH3Hg+ + F- ⇔ CH3HgF, log10K = 1.75 (20°C corr. Schwartzenbach and Schellenberg, 1965); CH3Hg+ + Br- ⇔ CH3HgBr, log10K = 6.87 (20°C corr. Schwartzenbach and Schellenberg, 1965); CH3Hg+ + I- ⇔ CH3HgI, log10K = 8.85 (20°C corr. Schwartzenbach and Schellenberg, 1965); and CH3Hg+ + SO42- ⇔ CH3HgSO4-, log10K = 2.64 (25°C, DeRobertis et al., 1998), (4) literature reported values for simulating monomethylmercury complexation with the carbonate ion may be too low: CH3Hg+ + CO32- ⇔ CH3HgCO3-, log10K = 6.1 (Rabenstein et al., 1976; Erni, 1981), and (5) "best estimate" constants for simulating methyl mercury complexation with reduced environmental sulfur species include: CH3Hg+ + S2- ⇔ CH3HgS-, log10K = 21.1; CH3Hg + + SH- ⇔ CH3HgSH, log10K = 14.5 (H+ + SH- ⇔ H2S, log10K = 6.88; Dyrssen and Wedborg, 1991); CH3Hg+ + RS- ⇔ CH3HgSR, log10K = 16.5 (H+ + RS- ⇔ RSH, log10K = 9.96; Qian et al., 2002); and CH 3Hg+ + CH3HgS1- ⇔ (CH 3Hg)2S, log10K = 16.32 (Schwartzenbach and Schellenberg, 1965; Rabenstein et al., 1978; and Erni, 1981).