Binding constants of divalent mercury (Hg2+) in soil humic acids and soil organic matter

Distribution coefficients (K-OC) for Hg2+ binding by IHSS Pahokee peat humic acid (PHA) and humic acids separated from O-horizons and peats in a northern temperate forest were determined using a competitive ligand-exchange method. All measurements were made at low ratios of added Hg2+ to reduced S. The commonly used chelating agents, EGTA and DTPA, were found to be ineffective competitive ligands; thus, we used DL-penicillamine, a synthetic amino acid with a thiol group. Calculated free [Hg2+] at equilibrium is very low, ranging from 10(-26.4) at pH 1.9 to 10(-36.9) at pH 5.8. Corresponding log K-OC values ranged from 22.6 to 32.8. The slope of the plot of pH versus log K-OC was 2.68, suggesting that two or more protons are released when each Hg2+ is bound. This is consistent with binding of Hg2+ to bidentate thiol sites with some participation of a third weak-acid group, presumably a thiol The 1:2 stoichiometry is consistent with X-ray spectroscopy data for Hg2+ bound to HA and with other pH-dependency results showing release of two protons with the binding of each Hg2+. Our K-OC values are much greater than indicated by the data from most previous studies.