The Role of Ester Sulfate and Organic Disulfide in Mercury Methylation in Ombrotrophic Peat Soils
Abstract
Peatlands hold large quantities of atmospherically deposited mercury (Hg) and can be significant sources of neurotoxic methylmercury (MeHg) to the surrounding surface waters. Because peatlands are anaerobic, they provide ideal conditions for Hg methylation, so much so that the percentage of peatlands in a watershed is positively correlated with MeHg concentrations in fish. It is known that sulfur is important to the methylation of mercury but the sulfur species in organic soils, such as peatlands, remains unclear in terms of reactants, products, and effect on bioavailability of mercury. This study aims to characterize the interaction of sulfur and mercury within a boreal ombrotrophic peatland. We examined the size, composition, and spatial correlation of sulfur and mercury pools in peat depth profiles. The relationships between sulfur species and mercury were assessed by measuring sulfur speciation using sulfur 1s X-ray absorption near-edge structure (XANES) spectroscopy, measuring total mercury (THg) and MeHg concentrations by cold vapor atomic fluorescence spectrometry (CVAFS) and measuring instantaneous potential methylation and demethylation constants. A significant positive correlation (p < 0.05) between MeHg and organic disulfide (R-S-S-R, mol fraction) with depth was observed and is consistent with these chemical species being products of dissimilatory sulfate reduction (DSR) processes. Peaks of MeHg and organic disulfide were co-located in the zone of water table fluctuation. Conversely, a significant negative correlation between organic monosulfides (R-S-H , R-S-R, mol fraction) with MeHg concentrations was observed and is consistent with the reduction of Hg bioavailability via complexation reactions between Hg and organic monosulfides in peat. Finally, a significant positive correlation between ester sulfate concentrations and instantaneous methylation rate constants was observed and is consistent with ester sulfate being the substrate that drives mercury methylation via DSR. For a peatland system with µM porewater concentrations of sulfate and hydrogen sulfide, our findings indicate that the solid-phase sulfur pools, which have a much greater sulfur concentration range, may be accessible to microbial activity or exchanging with the porewater.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2021
- Bibcode:
- 2021AGUFM.B44C..04P