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Monomethylmercury (CH 3 Hg) is a neurotoxic pollutant that biomagnifies in aquatic food webs. In sediments, the production of CH 3 Hg depends on the bacterial activity of mercury (Hg) methylating bacteria and the amount of bioavailable inorganic divalent mercury (Hg II ). Biotic and abiotic reduction of Hg II  to elemental mercury (Hg 0 ) may limit the pool of Hg II  available for methylation in sediments, and thus the amount of CH 3 Hg produced. Knowledge about the transformation of Hg II  is therefore primordial to the understanding of the production of toxic and bioaccumulative CH 3 Hg. Here, we examined the reduction of Hg II  by sulfidic minerals (FeS (s)  and CdS (s) ) in the presence of dissolved iron and dissolved organic matter (DOM) using low, environmentally relevant concentrations of Hg and ratio of Hg II :FeS (s) . Our results show that the reduction of Hg II  by Mackinawite (FeS (s) ) was lower (&amp;lt;15% of the Hg II  was reduced after 24 h) than when Hg II  was reacted with DOM or dissolved iron. We did not observe any formation of Hg 0  when Hg II  was reacted with CdS (s)  (experiments done under both acidic and basic conditions for up to four days). While reactions in solution were favorable under the experimental conditions, Hg was rapidly removed from solution by co-precipitation. Thermodynamic calculations suggest that in the presence of FeS (s) , reduction of the precipitated Hg II  is surface catalyzed and likely involves S −II  as the electron donor. The lack of reaction with CdS may be due to its stronger M-S bond relative to FeS, and the lower concentrations of sulfide in solution. We conclude that the reaction of Hg with FeS (s)  proceeds  via  a different mechanism from that of Hg with DOM or dissolved iron, and that it is not a major environmental pathway for the formation of Hg 0  in anoxic environments.

Abiotic Reduction of Mercury(II) in the Presence of Sulfidic Mineral Suspensions