ABIOTIC DEMETHYLATION OF METHYLMERCURY IN SEDIMENTS

Monomethylmercury (CH₃Hg) is of human and wild life concern due to its neurotoxic properties and ability to biomagnify in aquatic food webs. Marine sediments are important sites for net formation of CH₃Hg which is controlled by methylation and demethylation rates of the mercury (Hg) forms. Whereas research strongly supports biotic processes to be the main driver of methylation, both biotic and abiotic processes have been shown to be responsible for CH₃Hg demethylation. Knowledge about the potential abiotic demethylation pathways in sedimentary systems is scant and at present, only two processes in the absence of light have been shown to occur. One process involves demethylation by dihydrogen sulfide (H₂S) and the other by selenoaminoacids. Similar mechanisms involving the formation of di-monomethylmercury sulfide (CH₃Hg)₂S or di-monomethylmercury selenide (CH₃Hg)₂Se as an intermediate, with subsequent decomposition to dimethylmercury ((CH₃)₂Hg) and particulate HgS(s) or HgSe(s) as end products, have been suggested for both processes. Disordered Mackinawite (FeS_m) is the first product formed from precipitation of sulfide (S^2-) and divalent iron (Fe^II) in sedimentary systems, and has in laboratory studies been shown to limit net formation of CH₃Hg in sediments. FeS_m is highly reactive towards inorganic mercury (Hg^II) and CH₃Hg and currently known reactions include; adsorption on FeS(s), reduction of Hg^II to elemental mercury (Hg⁰), and the formation of metacinnabar (β-HgS(s)). These reaction have been assumed to reduce the availability of inorganic Hg in sediment and thus the rate of methylation. Possible interactions of CH₃Hg and Mackinawite, leading to its decomposition, have, however, not been considered. Here, we show that monomethylmercury is degraded on the surface of mackinawite forming (CH₃)₂Hg and elemental Hg (Hg⁰) as volatile end products. We have further studied the effect of aging mackinawite, the variation of the CH₃Hg:FeS_m ratio, and the presence of DOC on the rate of (CH₃)₂Hg formation. These results and there implications will be discussed in terms of the sources and sinks for CH₃Hg in marine sediments.