Binding of mercury(II) to dissolved organic matter: The role of the mercury-to-DOM concentration ratio

The binding of Hg(II) to dissolved organic matter (DOM; hydrophobic acids isolated from the Florida Everglades by XAD-8 resin) was measured at a wide range of Hg-to-DOM concentration ratios using an equilibrium dialysis ligand exchange method. Conditional distribution coefficients (K-DOM') determined by this method were strongly affected by the Hg/DOM concentration ratio. At Hg/DOM ratios below approximately 1 mug of Hg/mg of DOM, we observed very strong interactions (K-DOM' = 10(23.2+/-1.0) L kg(-1) at pH = 7.0 and I = 0.1), indicative of mercury-thiol bonds. Hg/DOM ratios above approximately 10 mug of Hg/mg of DOM, as used in most studies that have determined Hg-DOM binding constants, gave much lower K-DOM' values (10(10.7+/-1.0) L kg(-1) at pH = 4.9-5.6 and I = 0.1), consistent with Hg binding mainly to oxygen functional groups. These results suggest that the binding of Hg to DOM under natural conditions (very low Hg/DOM ratios) is controlled by a small fraction of DOM molecules containing a reactive thiol functional group. Therefore, Hg/DOM distribution coefficients used for modeling the biogeochemical behavior of Hg in natural systems need to be determined at low Hg/DOM ratios.