X-ray Absorption Spectroscopic Evidence for the Complexation of Hg(II) by Reduced Sulfur in Soil Humic Substances

Abstract

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 humic substances 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 Å, respectively, was found in the first coordination shell of Hg(II) complexed by humic acid. Model calculations show that a second coordination sphere could contain one carbon atom and a second sulfur atom at 2.78 and 2.93 Å, 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 O-containing ligand such as carboxyl and phenol ligands rather than H2O molecule is bound to the Hg(II). The involvement of oxygen 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.

Stoichiometry of mercury-thiol complexes on bacterial cell envelopes

“…330 9 Published literature suggests that a Hg-S bond distance of 2.49 Å is representative of the Hg-S3 331 complex, but could possibly include small components of Hg-S2 and Hg-S4 complexes as well 332 (Manceau and Nagy, 2008;Warner and Jalilehvand, 2016). In summary, the fitting parameters 333 for the solution standards reported in this study are in good agreement with previously published 334 values (Xia et al, 1999; Qian et al, 2002; Skyllberg et al, 2006;Mishra et al, 2011; Thomas et 335 al., 2016). 336…”

“…In aquatic systems, Hg solubility is high under oxygen-rich 71 acidic conditions but it is significantly inhibited under anoxic sulfide-rich waters (Martell and 72 Smith, 1974). The Hg-sulfide complexes are among the strongest complexes of all known Hg 73 inorganic and organic complexes in the aquatic environment (Carty and Malone, 1979 (Xia et al, 1999;Haitzer et al, 2002;Khwaja et al, 2006; 81 Skyllberg et al, 2005; Skylberg 2008;Nagy et al, 2011;Hesterberg et al, 2001). Because 82 dissolved organic matter (DOM) is the main source of reduced cysteine residues in natural 83…”

Mercury concentrations and pools in four Sierra Nevada forest sites, and relationships to organic carbon and nitrogen

“…A combined linear regression showed a slope of 15.2 µg Hg g −1 N and indicates that soil N contents alone explained a full 92% of the Hg variability observed across all sites and soil horizons. The affinity of Hg to organic matter has been attributed to complexation of Hg with reduced S and O or N groups abundantly present in organic molecules (Schuster, 1991;Skyllberg et al, 2000;Xia et al, 1999). Our results showing strong relationships between Hg and N in soils and similar relationships in litter (see above) suggest that nitrogen groups may be key ligands for retention of Hg in terrestrial ecosystem compartments.…”

Bioavailability and mobility of mercury in selected soil profiles

“…The content of Hg NaOH forms was significantly, positively correlated with cation exchange capacity (r = 0.66; p \ 0.05, Table 3), which is linked with high sorption capacity of organic matter (Gabriel and Williamson 2004) and high affinity of Hg to functional groups containing sulphur (Kabata-Pendias and Pendias 2000; Skyllberg et al 2003;Xia et al 1998) proved in their research that from 50 to 70 % of the total sulphur content in soils was included in the functional groups (mainly thiol) of organic matter, which bind metallic mercury as well as alkyl compounds very easily.…”