Analysis of Hg(II) complexed by a soil humic acid (HA) using synchrotron-based X-ray absorption spectroscopy (XAS) revealed the importance of reduced sulfur functional groups (thiol (R-SH) and disulfide (R-SS-R)/disulfane (R-SSH)) in humicsubstances in the complexation of Hg(II). A two-coordinate binding environment with one oxygen atom and one sulfur atom at distances of 2.02 and 2.38 A, respectively, was found in the first coordination shell of Hg(II) complexed by humic acid. Modelcalculations show that a second coordination sphere could contain one carbon atom and a second sulfur atom at 2.78 and 2.93 A, respectively. This suggests that in addition to thiol S, disulfide/disulfane S may be involved with the complexation of Hg(II)in soil organic matter. The appearance of carbon atom in the second coordination shell suggests that one 0-containing ligand such as carboxyl and phenol ligands rather than H{sub}20 molecule is bound to the Hg(II). The involvement of oxygen ligand inaddition to the reduced S ligands in the complexation of Hg(II) is due to the low density of reduced S ligands in humic substances. The XAS results from this experiment provided direct molecular level evidence for the preference of reduced S functionalgroups over oxygen ligands by Hg(II) in the complexation with humic substances.
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