Environ. Sci. Technol. 2001, 35, 2741-2745
Bonding of Hg(II) to Reduced Organic Sulfur in Humic Acid As Affected by S/Hg Ratio DEAN HESTERBERG,* JEFF W. CHOU, KIMBERLY J. HUTCHISON, AND DALE E. SAYERS Department of Soil Science, Box 7619, and Department of Physics, Box 7518, North Carolina State University, Raleigh, North Carolina 27695
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 nearedge 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).
Introduction Organic matter is one of the more important soil components responsible for binding metal cations, both in the solid phase and as soluble complexes (1-3). Humic acid, an extracted form of natural organic matter rich in chemically reactive functional groups (4), tends to have a decreasing affinity for heavy metals with increasing metal loading (2, 5). This phenomenon has been ascribed to preferential binding of metal cations to high-affinity sites at lower levels of complexed metal (2). Consequently, some models for describing metal binding with humic acid account for a distribution of sites of various affinities for metal cations (5-7). The most abundant reactive functional groups (ligand sites) in soil humic acid are carboxylic and phenolic acids (4, 8), with mean concentrations of 3.6 and 3.9 mol kg-1, respectively (8). Although N- and S-bearing ligands are less abundant, they may form high-affinity bonds with heavy metal cations (1, 8, 9). For example, electron paramagnetic resonance studies on fulvic acid, another extracted form of * Corresponding author (Department of Soil Science) telephone: (919) 513-3035; fax (919) 515-2167; e-mail:
[email protected]. 10.1021/es001960o CCC: $20.00 Published on Web 05/31/2001
2001 American Chemical Society
organic matter, gave evidence for coordination of Cu(II) to organic N (9). Organic N concentrations in soil and marine humic acids range from 0.6 to 4.5 mol of N kg-1 (4, 10), with a significant fraction being metal-reactive free amino components (4). Soil humic acids contain 0.15-0.4 mol of S kg-1 (8), and humic acids from marine and brackish lake sediments contain between
