Bonding of Hg(II) to reduced organic, sulfur in humic acid as affected by S/Hg ratio

Organic matter is an important sorbent of heavy metals in soils and sedimen ts. The heterogeneity of organic matter, including the presence of various reactive O-, Ni, and S-bearing ligands, makes it difficult to precisely cha racterize the nature of metal-ligand binding sites. The objective of this r esearch was to characterize the extent and nature of Hg(ll) bonding with re duced organic S in soil organic matter. Sulfur-rich humic acid (0.7 +/- 0.1 mol of S kg(-1)) was extracted from samples of surface! soil from a marine wetland. Synchrotron X-ray absorption near-edge structure (XANES) analysis at the S K edge indicated that 70 +/- 3 mol % of the organic S was in a re duced oxidation state. Aqueous solutions containing 2 mmol of Hg kg(-1), 0. 1 M NaNO3, and humic acid added at various S/Hg molar ratios at pH 5.60 +/- 0.02 were characterized using extended X-ray absorption fine structure (EX AFS) spectroscopy at the Hg L-III edge. Spectral fitting showed that as the total S/Hg ratio increased from 0.6 to 5.6 (reduced S/Hg of 0.4-4.0), the fraction of Hg-S bonding relative to Hg-O (or Hg-N) bonding increased from 0.4 to 0.9. Results demonstrated preferential bonding of Ha(ll)to reduced o rganic S sites and indicated that multiple sulfur ligands were coordinated with Hg2+ ions at high S/Hg ratios, which corresponded to low levels of com plexed Hg(ll).