Sulfate pollution in freshwater systems can stimulate microorganisms responsible for converting inorganic mercury (Hg) into methylmercury (MeHg), a neurotoxin that bioaccumulates in humans and wildlife. The development of remediation technologies for limiting sulfate concentrations and its transport into Hg-methylation hotspots is thus critical to prevent MeHg formation and its environmental impacts. We recently developed an electrolysis-based passive treatment system for in situ biological treatment of sulfate-impacted watersheds. The system demonstrated effective sulfate reduction, but it unknown whether sulfidogenic conditions resulting from electrolysis will inadvertently alter the microbial and chemical processes controlling the fate of Hg. In this study, a series of Hg isotope-spiked incubations were performed with sediment from a historically sulfate-impacted stream to evaluate the effects of electrolysis and active carbon amendments on sulfur geochemistry and Hg methylation and MeHg demethylation activity (K_m and K_d). Spiking concentrations of ¹⁹⁸Hg (220 ng/g) and Me²⁰⁴Hg (3 ng/g) were selected to match the background concentrations in the sediment. Short- (24h) and long-term (7d) K_m and K_d values were measured along with changes in total Hg, MeHg, sulfate, reduced sulfur products, and microbial functional genes associated with sulfate reduction and Hg methylation. Sulfate concentrations during treatment were reduced by up to 2000 mg/L (52% reduction) and resulted primarily in the formation of acid-volatile sulfides, chromium-reducible sulfur, and elemental sulfur. Moreover, sulfate reduction was enhanced by raising the applied electrical potential and activated carbon increased the amount of elemental sulfur formed. Results from the spiked incubations will be used to determine (1) whether bioelectrochemical treatment influences Hg methylating microbes affecting MeHg production and demethylation, (2) if activated carbon amendments immobilize inorganic Hg and inhibit Hg methylation, and (3) how the form of sulfur influences Hg dynamics either by fueling sulfate reducing microorganisms that perform Hg methylation or sequestering free Hg (e.g., as HgS_(s)) and limiting Hg methylation. This study will improve our understanding of Hg methylation and MeHg demethylation during active sulfidogenic activity in freshwater sediment to improve the design, configuration, and implementation of biological treatment technologies for sulfate-impacted ecosystems.

Matthew Berens

Natural Resources Research Institute

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