title: Bonding of Hg(II) to reduced organic sulfur in humic acid as affected by S/Hg ratio.
abstract: Organic matter is an important sorbent of heavy metals in soils and sediments. The heterogeneity of organic matter, including the presence of various reactive O-, N-, and S-bearing ligands, makes it difficult to precisely characterize the nature of metal-ligand binding sites. The objective of this research was to characterize the extent and nature of Hg(II) bonding with reduced 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 reduced 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 (EXAFS) spectroscopy at the Hg LIII 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 Hg(II) to reduced organic S sites and indicated that multiple sulfur ligands were coordinated with Hg2+ ions at high S/Hg ratios, which corresponded to low levels of complexed Hg(II).
authors.affiliation: Department of Soil Science, Box 7619, and Department of Physics, Box 7518, North Carolina State University, Raleigh, North Carolina 27695, USA. dean_hesterberg@ncsu.edu
authors.forename: D
authors.initials: D
authors.lastname: Hesterberg
authors.forename: J W
authors.initials: JW
authors.lastname: Chou
authors.forename: K J
authors.initials: KJ
authors.lastname: Hutchison
authors.forename: D E
authors.initials: DE
authors.lastname: Sayers
mesh_terms: D000042:Absorption; D055598:Chemical Phenomena; D002627:Chemistry, Physical; D006812:Humic Substances / Q000737:chemistry*; D008024:Ligands; D008628:Mercury / Q000737:chemistry*; D009930:Organic Chemicals; D012989:Soil Pollutants / Q000032:analysis*; D013455:Sulfur / Q000737:chemistry*; D014874:Water Pollutants, Chemical / Q000032:analysis
chemical_list: D006812:Humic Substances; D008024:Ligands; D009930:Organic Chemicals; D012989:Soil Pollutants; D014874:Water Pollutants, Chemical; D013455:Sulfur; D008628:Mercury