Methylmercury (MeHg) is a bioaccumulative neurotoxin produced in anoxic soils and sediments, but we possess incomplete understanding of pathways contributing to MeHg degradation in such environments. Recent ab initio and experimental evidence suggests that MeHg reacts with inorganic sulfide via a bis(methylmercuric) sulfide intermediate to produce dimethylmercury (dMeHg) and metacinnabar (HgS(s)). To elucidate this process, we examined the effect of varying concentrations of sulfide ([H2S]T) and MeHg on the extent of MeHg transformation at circumneutral pH by monitoring MeHg concentrations over time using direct ethylation isotope dilution gas chromatography inductively coupled mass spectrometry. Additionally, we sampled the headspace of MeHg/sulfide solutions to monitor a volatile Hg species believed to be dMeHg. Timecourses for MeHg displayed two phases: initial loss of MeHg occurring within minutes (hypothesized to be formation of (CH3Hg)2S)) followed by slow loss occurring on a timescale of days (breakdown of (CH3Hg)2S into dMeHg and HgS). In reactions with fixed initial MeHg concentration (2.5 nM), MeHg transformation increased with increasing sulfide concentration up to 200 µM and then decreased with additional sulfide. In experiments with fixed [H2S]T, rates of MeHg loss and dMeHg production increased with increasing initial MeHg concentration. In an initial rates analysis, the rate of dMeHg production showed an approximate order of 1.67 for the dependence on [MeHg]. Over a 25 day period, up to 0.01% of added MeHg (2.5 to 75 nM) converted to dMeHg in the presence of ~50 µM sulfide, an efficiency similar to that recently reported by Jonsson et al. Follow-up experiments will attempt to confirm the bis(methylmercuric) sulfide intermediate, as part of ongoing efforts to understand MeHg degradation in the environment.

Sulfide-induced abiotic methylmercury transformation

253rd National Meeting (2017)

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