Competition between Disordered Iron Sulfide and Natural Organic Matter Associated Thiols for Mercury(II)—An EXAFS Study

Abstract

Knowledge about the chemical speciation of Hg(II) is a prerequisite for a proper understanding of biogeochemical processes in control of the transformation of Hg(II) into toxic and bioaccumulating monomethyl mercury. Of critical importance are structures and the stability of Hg(II)-complexes with inorganic and organic sulfur ligands in aqueous and solid phases of soils and sediments. On the basis of Hg L(III)-edge EXAFS experiments, we report Hg(II) to form a four-coordinated metacinnabar [beta-HgS(s)] phase when reacted with disordered FeS(s) (mackinawite), at pH 9.0 and a Hg(II) to FeS(s) molar ratio of 0.002-0.012. When Hg(II) (1000-20,000 microg Hg g(-1)) was added to mixtures of <5 days of aged FeS(s) (2-20%) and an organic soil at pH 5.7-6.1, a mixture of Hg(II) coordinated with two organic thiols [Hg(SR)(2)] and Hg(II) coordinated with four inorganic sulfides in a metacinnabar-like phase was formed. Surface complex formation between Hg(II) and FeS(s), or substitution of Hg(II) for Fe(II) in FeS(s), was not observed. Quantities of beta-HgS(s) and Hg(SR)(2) formed (as determined by EXAFS) were in fair agreement with theoretical thermodynamic calculations, as described by the reaction: Hg(SR)(2) + FeS(s) = HgS(s) + Fe(2+) + 2RS(-). The calculated stability constant for this reaction (log K = -16.1 - -15.4) supports a strong bonding of Hg(II) to organic thiols, corresponding to a log beta(2) for the formation of Hg(SR)(2) on the order of 42 or greater.

Chemical Speciation of Mercury in Soil and Sediment

“…In contrast to reactions with pyrite, Hg has been shown to react with and dissolve FeS m during the formation of β-HgS(s) (Jeong et al, 2007(Jeong et al, , 2010Skyllberg and Drott, 2010). In another study, a four-coordinated HgS compound (in agreement with β-HgS(s) formation) was formed under partly oxidized conditions, whereas at strictly reducing conditions a distorted surface complex was suggested to form Wolfenden et al (2005).…”

“…Therefore, the time of FeS m ageing could be important for whether reactions with Hg will lead to surface complex formation, dissolution-precipitation reactions, or a mixture of these processes. In the study by Skyllberg and Drott (2010), FeS m was allowed to age for 8 h, and no significant surface complexation was observed by Hg L IIIedge EXAFS at Hg to FeS m molar ratios of 0.002-0.012. In contrast, surface complexation of Hg onto FeS m aged for three days was observed at a Hg to FeS m molar ratio less than 0.05 in an adsorption experiment (Jeong et al, 2007).…”

“…The binding of Hg to thiol groups in NOM is well studied, but what will happen in the presence of FeS m ? In the only existing study, it was shown that FeS m aged for five days was dissolved by Hg in the presence of NOM under the formation of Hg(SR) 2 complexes and β-HgS(s) (Skyllberg and Drott, 2010) at pH 5.7-6.1. Hg(SR) 2 complexes and β-HgS(s) (as determined by Hg EXAFS) coexisted after 30 h of equilibration.…”

Mercury (II) Adsorption on Three Contrasting Chinese Soils Treated with Two Sources of Dissolved Organic Matter: II. Spectroscopic Characterization

“…And the much weaker O/N sites made the significant contribution. Thus, all R-SH groups available for complexation have been saturated with Hg (Xia et al, 1999;Hesterberg et al, 2001;Haitzer et al, 2002;Khwaja et al, 2006;Skyllberg et al, 2006) (either as two-coordinated complexes involving only R-SH or mixed R-SH and O/N ligation) (Skyllberg and Drott, 2010). However, the signals at 2600-2500 cm -1 to determine R-SH groups were not detected by FTIR spectroscopy.…”

Study on the Leaching of Mercuric Oxide with Thiosulfate Solutions

“…The aqueous speciation and coordination of Hg have been well-documented [30][31][32][33][34][35][36]. The oxidation states of Hg in aqueous systems are 0, +1, and +2, of which Hg (2+) is the most stable in typical aerated water.…”