Binding Constants of Divalent Mercury (Hg²⁺) in Soil Humic Acids and Soil Organic Matter

Abstract

Distribution coefficients (K(OC)) for Hg2+ binding by IHSS Pahokee peat humic acid (PHA) and humic acids separated from O-horizons and peats in a northern temperate forest were determined using a competitive ligand-exchange method. All measurements were made at low ratios of added Hg2+ to reduced S. The commonly used chelating agents, EGTA and DTPA, were found to be ineffective competitive ligands; thus, we used DL-penicillamine, a synthetic amino acid with a thiol group. Calculated free [Hg2+] at equilibrium is very low, ranging from 10(-26.4) at pH 1.9 to 10(-36.9) at pH 5.8. Corresponding log Koc values ranged from 22.6 to 32.8. The slope of the plot of pH versus log K(OC) was 2.68, suggesting that two or more protons are released when each Hg2+ is bound. This is consistent with binding of Hg2+ to bidentate thiol sites with some participation of a third weak-acid group, presumably a thiol. The 1:2 stoichiometry is consistent with X-ray spectroscopy data for Hg2+ bound to HA and with other pH-dependency results showing release of two protons with the binding of each Hg2+. Our K(OC) values are much greater than indicated by the data from most previous studies.

Soil mercury and its response to atmospheric mercury deposition across the northeastern United States

“…The close relationship between Hg and SOM concentrations in this study is consistent with observations that ionic Hg is largely complexed to functional groups of SOM, especially those containing reduced sulfur, in the organic horizon (Khwaja et al 2006). The decomposition of SOM is linked to the transformations of Hg and hence the mass transfer of Hg (Demers et al 2007), which likely affects the redistribution of Hg within soil profiles.…”

Chemical Speciation of Mercury in Soil and Sediment

“…Using this estimate in combination with data from Skyllberg et al (2000), log β 2 values for the reaction Hg 2+ + 2RS − = Hg(SR) 2 were calculated to be in the range 43.3-47.7 for organic soils along an upland peat gradient (Skyllberg, 2008), as well as for extracted and purified humic substances from soils taken along the same gradient (Khwaja et al, 2006). The molar RS tot to Hg ratio was between ∼10 −4 and 10 −5 in the former and between ∼10 −3 and 10 −1 in the latter study.…”

Stoichiometry of mercury-thiol complexes on bacterial cell envelopes

“…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…”