Binding of mercury(II) to aquatic humic substances: Influence of pH and source of humic substances

Conditional distribution coefficients (K_DOM‘) for Hg(II) binding to seven dissolved organic matter (DOM) isolates were measured at environmentally relevant ratios of Hg(II) to DOM. The results show that K_DOM‘ values for different types of samples (humic acids, fulvic acids, hydrophobic acids) isolated from diverse aquatic environments were all within 1 order of magnitude (10^22.5±1.0−10^23.5±1.0 L kg^-1), suggesting similar Hg(II) binding environments, presumably involving thiol groups, for the different isolates. K_DOM‘ values decreased at low pHs (4) compared to values at pH 7, indicating proton competition for the strong Hg(II) binding sites. Chemical modeling of Hg(II)−DOM binding at different pH values was consistent with bidentate binding of Hg(II) by one thiol group (pK_a = 10.3) and one other group (pK_a = 6.3) in the DOM, which is in agreement with recent results on the structure of Hg(II)−DOM bonds obtained by extended X-ray absorption fine structure spectroscopy (EXAFS).