Binding Strength of Methylmercury to Aquatic NOM

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.