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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 (<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. 

The methylation of mercury is known to depend on the chemical forms of mercury (Hg) present in the environment and the methylating bacterial activity. In sulfidic sediments, under conditions of supersaturation with respect to metacinnabar, recent research has shown that mercury precipitates as β-HgS(s) nanoparticles (β-HgS(s) nano ). Few studies have examined the precipitation of β-HgS(s) nano in the presence of marine dissolved organic matter (DOM). In this work, we used dynamic light scattering (DLS) coupled with UV-Vis spectroscopy and transmission electron microscopy (TEM) to investigate the formation and fate of β-HgS(s) nano formed in association with marine DOM extracted from the east and west of Long Island Sound, and at the shelf break of the North Atlantic Ocean, as well as with low molecular weight thiols. We found that while the β-HgS(s) nano formed in the presence of oceanic DOM doubled in size after 5 weeks, those forming in solutions with coastal DOM did not grow over time. In addition, when the Hg II  : DOM ratio was varied, β-HgS(s) nano only rapidly aggregated at high ratios (>41 μmol Hg II per mg C) where the concentration of thiol groups was determined to be substantially low relative to Hg II . This suggests that functional groups other than thiols could be involved in the stabilization of β-HgS(s) nano . Furthermore, we showed that β-HgS(s) nano forming under anoxic conditions remained stable and could therefore persist in the environment sufficiently to impact the methylation potential. Exposure of β-HgS(s) nano to sunlit and oxic environments, however, caused rapid aggregation and sedimentation of the nanoparticles, suggesting that photo-induced changes or oxidation of organic matter adsorbed on the surface of β-HgS(s) nano affected their stability in surface waters.