Complexation of Mercury(II) in Soil Organic Matter:  EXAFS Evidence for Linear Two-Coordination with Reduced Sulfur Groups

Abstract

The chemical speciation of inorganic mercury (Hg) is to a great extent controlling biologically mediated processes, such as mercury methylation, in soils, sediments, and surface waters. Of utmost importance are complexation reactions with functional groups of natural organic matter (NOM), indirectly determining concentrations of bioavailable, inorganic Hg species. Two previous extended X-ray absorption fine structure (EXAFS) spectroscopic studies have revealed that reduced organic sulfur (S) and oxygen/ nitrogen (O/N) groups are involved in the complexation of Hg(II) to humic substances extracted from organic soils. In this work, covering intact organic soils and extending to much lower concentrations of Hg than before, we show that Hg is complexed by two reduced organic S groups (likely thiols) at a distance of 2.33 A in a linear configuration. Furthermore, a third reduced S (likely an organic sulfide) was indicated to contribute with a weaker second shell attraction at a distance of 2.92-3.08 A. When all high-affinity S sites, corresponding to 20-30% of total reduced organic S, were saturated, a structure involving one carbonyl-O or amino-N at 2.07 A and one carboxyl-O at 2.84 A in the first shell, and two second shell C atoms at an average distance of 3.14 A, gave the best fit to data. Similar results were obtained for humic acid extracted from an organic wetland soil. We conclude that models that are in current use to describe the biogeochemistry of mercury and to calculate thermodynamic processes need to include a two-coordinated complexation of Hg(II) to reduced organic sulfur groups in NOM in soils and waters.

Mercury speciation in a tropical soil association; Consequence of gold mining on Hg distribution in French Guiana

“…This is consistent with previous reports of organics, and more especially of reduced sulfur groups, being the most effective sorbents for Hg in acidic environments (pH < ~4.5-5.0), compared to iron oxides and clay minerals (Andersson, 1979;Schuster, 1991;Roulet and Lucotte, 1995;Skyllberg et al, 2000;Skyllberg et al, 2006;Manceau and Nagy, 2008 values. This observation is consistent with the strong affinity of Hg(II) for gibbsite (Kim et al, 2004a;Kim et al, 2004b;Weerasooriya et al, 2007).…”

“…In particular, the relative proportions of soil components responsible for Hg retention [clay minerals, Fe-, Al-, and Mn-(oxyhydr)oxides, organic matter] vary with the geochemical conditions which in turn depend on the topographic position along the soil association (Boulet et al, 1993;do Valle et al, 2005;Fritsch et al, 2005;Fritsch et al, 2006). Hg has a high affinity for soil organic matter, especially for reduced sulfur groups such as thiol ligands (Schuster, 1991;Skyllberg et al, 2000;Khwaja et al, 2006;Skyllberg et al, 2006). High Hg levels can also be associated to Fe-(oxyhydr)oxides.…”

Establishing geochemical baselines in forest soils for environmental risk assessment in the Montague and Goldenville gold districts, Nova Scotia, Canada

“…B-horizon during pedogenesis (Boyle and Jonasson 1973;SCWG 1998). The strong enrichment of Hg in humus results from the atmospheric deposition of Hg from local and distal sources, as well as gaseous Hg flux from underlying soils, and the subsequent bonding of Hg to thiol groups in soil organic matter (Grigal 2003;Skyllberg et al 2006). As expected, the concentrations of both As and Hg are lowest in the Ae horizon at most sample sites, reflecting the leaching of soluble constituents during soil formation and their transport to deeper horizons.…”

Anthropogenic mercury sequestration in different soil types on the southeast coast of China

“…The significant correlation between Hg concentration and SOM, humic acids, CEC, and amorphous iron oxides suggested that chelated with humic substance and specifically adsorbed to the hydrous ferric oxides (HFO) might be the dominant mechanisms of Hg sequestration in soils of the YRD and the PRD. Mercury exhibits a great affinity for organic matter in soils by forming complexes with reduced sulfur groups (Skyllberg et al 2006; Feo (mg/kg) Fig. 3 Linear regression between the measured Hg concentrations and topsoil properties.…”

“…Besides being affected by anthropogenic sources, accumulation of Hg in soil also depends on soil properties such as soil organic matter (SOM) content, pH, and cation exchange capacity (CEC) (Huang et al 2011;Smith-Downey et al 2010;Yang et al 2007). It has been well known that Hg has a high affinity for organic matter in soil and sediment (Ravichandran 2004;Skyllberg et al 2006;Wallschlager et al 1998). Mercury therefore tends to be retained in soils although losses do occur by leaching and volatilization (Grigal 2002;Scholtz et al 2003).…”