Measurement of complexation of methylmercury(II) compounds by freshwater humic substances using equilibrium dialysis

Humic and fulvic acids from lakes in Ontario, Canada, have been isolated and been examined for their ability to bind methylmercury(II). Equilibrium dialysis has been applied to differentiate between bound and unbound fractions of MeHg(II). Losses of methylmercury(II) from solutions containing 0.5-100 ng/L MeHg(II) and 10 mg/L humic substances (HS) are negligible over a 24-h time interval when using acid cleaned glass beakers, membrane closures, and cellulose ester membranes having a molecular weight cutoff of 500. Dialysis equilibrium for MeHg(II) is reached within this period of time. For each humic material, two classes of binding sites could be detected. Log K values of conditional stability constants for the strong and the weaker binding sites were in the range of 13.02-14.48 and 12.15-13.07, respectively. Binding capacities have been calculated as 0.2-13 of ng MeHg(II)/mg of HS for the strong sites and 1.2-72 ng of MeHg(II)/mg of HS for the weaker sites. Calculation of equilibrium distribution shows that methylmercury(II)-HS complexes predominate in oxidized freshwaters with low sulfide concentrations. In anoxic waters, CH3HgSH should prevail. The CH3HgCl complex is tao weak to compete with either of the latter complexes.