title: 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.
authors.affiliation: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, SE-90183 Umeå, Sweden. ulf.skyllberg@sek.slu.se
authors.forename: Ulf
authors.initials: U
authors.lastname: Skyllberg
authors.forename: Andreas
authors.initials: A
authors.lastname: Drott
mesh_terms: D007501:Iron / Q000737:chemistry*; D008628:Mercury / Q000737:chemistry*; D009930:Organic Chemicals / Q000737:chemistry*; D012987:Soil; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013440:Sulfides / Q000737:chemistry*
chemical_list: D009930:Organic Chemicals; D012987:Soil; D013438:Sulfhydryl Compounds; D013440:Sulfides; D007501:Iron; D008628:Mercury