MEASUREMENT OF COMPLEXATION OF METHYLMERCURY(II) COMPOUNDS BY FRESH-WATER HUMIC SUBSTANCES USING EQUILIBRIUM DIALYSIS

Humic and fulvic acids from lakes in Ontario, Canada, have been isolat ed and been examined for their ability to bind methylmercury(II). Equi librium dialysis has been applied to differentiate between bound and u nbound fractions of MeHg(II). Losses of methylmercury(II) from solutio ns 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 mo lecular weight cutoff of 500. Dialysis equilibrium for MeHg(II) is rea ched within this period of time. For each humic material, two classes of binding sites could be detected. Log K values of conditional stabil ity 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. Calcul ation of equilibrium distribution shows that methylmercury(II)-HS comp lexes predominate in oxidized freshwaters with low sulfide concentrati ons. In anoxic waters, CH3HgSH should prevail. The CH3HgCl complex is tao weak to compete with either of the latter complexes.