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
(KDOM‘) determined by this method were strongly affected
by the Hg/DOM concentration ratio. At Hg/DOM ratios
below approximately 1 μg of Hg/mg of DOM, we observed
very strong interactions (KDOM‘ = 1023.2±1.0 L kg-1 at
pH = 7.0 and I = 0.1), indicative of mercury−thiol bonds.
Hg/DOM ratios above approximately 10 μg of Hg/mg of
DOM, as used in most studies that have determined Hg−DOM binding constants, gave much lower KDOM‘ values
(1010.7±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.