Distribution coefficients (K(OC)) for Hg2+ binding by IHSS Pahokee peat humic acid (PHA) and humic acids separated from O-horizons and peats in a northern temperate forest were determined using a competitive ligand-exchange method. All measurements were made at low ratios of added Hg2+ to reduced S. The commonly used chelating agents, EGTA and DTPA, were found to be ineffective competitive ligands; thus, we used DL-penicillamine, a synthetic amino acid with a thiol group. Calculated free [Hg2+] at equilibrium is very low, ranging from 10(-26.4) at pH 1.9 to 10(-36.9) at pH 5.8. Corresponding log Koc values ranged from 22.6 to 32.8. The slope of the plot of pH versus log K(OC) was 2.68, suggesting that two or more protons are released when each Hg2+ is bound. This is consistent with binding of Hg2+ to bidentate thiol sites with some participation of a third weak-acid group, presumably a thiol. The 1:2 stoichiometry is consistent with X-ray spectroscopy data for Hg2+ bound to HA and with other pH-dependency results showing release of two protons with the binding of each Hg2+. Our K(OC) values are much greater than indicated by the data from most previous studies.

A competitive-ligand, equilibrium-dialysis technique using bromide measured methylmercury (MeHg(+)) binding to Suwannee River fulvic acid (SRFA) and NOM from a lake and a bog in Minnesota. Distribution coefficients (K(OC)) and stability constants (K') varied only slightly over a range of [Br(-)] and ratios of MeHg(+) to reduced sulfur, S(re), the putative NOM binding site. For SRFA at pH 3.0, K(OC) ranged from 10(7.7) to 10(8.2) and K' ranged from 10(15.5) to 10(16.0) over MeHg(+):S(re) ratios from 1:1220 to 1:12 200 (well below S(re) saturation). The importance of pH depends on the calculation model for binding constants. Over pH 2.98-7.62, K(OC) had little pH dependence (slope = 0.2; r(2) = 0.4; range 10(7.7)-10(9.1)), but K' calculated using thiol ligands with pK(a) = 9.96 had an inverse relationship (slope = -0.8; r(2) = 0.9; range 10(15.6)-10(12.3)). A pH-independent model was obtained only with thiol pK(a) < or = approximately 4. The mean K'(4) for SRFA (K' with thiol pK(a) = 4.2) was 10(9.8) (range 10(9.11)-10(10.27)) and small slope (0.02). Similar values were found for Spring Lake NOM; bog S2 NOM had values one-tenth as large. These constants are generally similar to published values; differences reflect variations in methods, pH, types of NOM, and calculation models.

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