X-ray absorption spectroscopic evidence for the complexation of Hg(II) by reduced sulfur in soil humic substances

Analysis of Hg(II) complexed by a soil humic acid (HA) using synchrotron-ba sed 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 humic substances in the complexation of Hg(II). A two-coordinate b inding 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 o f Hg(II) complexed by humic acid. Model calculations show that a second coo rdination 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, d isulfide/disulfane S may be involved with the complexation of Hg(II) in soi l organic matter. The appearance of carbon atom in the second coordination shell suggests that one O-containing ligand such as carboxyl and phenol lig ands rather than H2O molecule is bound to the Hg(II). The involvement of ox ygen ligand in addition 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 functional groups over oxygen ligands by Hg (II) in the complexation with humic substances.