The biogeochemistry of mercury (Hg) is very complex, tightly linked to the activity of bacteria and to sulfur and iron geochemistry. In focus are processes resulting in a net production of the toxic and bioaccumulating methyl mercury (MeHg) molecule. Chemical speciation analysis requires a multidisciplinary approach. Mercury LIII-edge EXAFS has, in combination with sulfur K-edge XANES and binding affinity studies, provided evidence that Hg and MeHg are complexed by organic thiol groups (RSH) under oxidized conditions in soils and sediments. EXAFS studies have furthermore shown that Hg forms soluble complexes with inorganic sulfides and solid phases like metacinnabar (β-HgS) and possibly cinnabar (α-HgS) under sub- and anoxic conditions. Mercury may also adsorb to FeS surfaces or form HgFeS(s) coprecipitates. Under highly contaminated conditions, such as in mine tailings and at industrial sites, Hg may in addition be adsorbed to iron oxyhydroxides and/or form solid phases like, for example, HgO(s) and HgCl2(s).
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