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

摘要

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 (β-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 μg 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 β-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 forrnthis reaction (log K= -16.1- -15.4) supports a strongrnbonding of Hg(II) to organic thiols, corresponding to a log β_2 for the formation of Hg(SR)_2 on the order of 42 or greater.