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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 0.4M: CH3Hg+ + HCO3- ...CH3HgHCO3, log10K = 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; 2CH3Hg+ +H2O...(CH3Hg)2OH+ +H+, log10K = -2.15; CH3Hg+ +Cl-...CH3HgCl, log10K = 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+ +l -...CH3Hgl, log10K = 8.85 (20°C corr. Schwartzenbach and Schellenberg, 1965); and CH3Hg+ +SO4 ²_...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+ +CO3 ²-...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 CH3Hg+ +CH3HgS¹ -..(CH3Hg)2S, log10K = 16.32 (Schwartzenbach and Schellenberg, 1965; Rabenstein et al., 1978; and Erni, 1981). [ABSTRACT FROM AUTHOR]

The sequestering capacity of synthetic and naturally occurring polycarboxylate ligands towards monomethylmercury( II) was evaluated by stability quantitative data on the interaction of CH3Hg+ with different molecular weight synthetic polyacrylates (2 and 20 kDa average M.wt) and alginate (70-100 kDa) extracted from brown algae Macrocystis pyrifera. The influence of ionic medium was evaluated by measurements on the CH3Hg+-polyacrylate systems in NaNO3 medium at different ionic strengths (0.10, 0.25, 0.50 and 0.75 mol -1), and a Debye-Hückel type equation was used for the dependence of complex formation constants on ionic strength. Measurements on the CH3Hg+-alginate system were carried out at l=0.10 mol L-1 in NaNO3 medium. By using the stability data, the sequestering capacity of both ligands towards monomethylmercury(II) was determined at different pH values. Results obtained show that the binding ability of polyacrylic ligands (PAA) is stronger than the alginate (AA), following the trend PAA (20 kDa) > PAA (2 kDa) > AA. [ABSTRACT FROM AUTHOR]