TRACKING THE METHYLATION OF STABLE-ISOTOPE-LABELED INORGANIC FORMS OF MERCURY AND THE EFFECT OF ACTIVATED CARBON ON THEIR BIOAVAILABILITIES

One of the major challenges in evaluating the risk potential of soils and sediments contaminated with mercury (Hg) is the lack of simple, reliable, environmentally-relevant methods to predict methylmercury (MeHg) formation. In anoxic soils and sediments, Hg is transformed to MeHg via certain types of bacteria carrying Hg-methylating genes (hcgA and B). The primary objectives of this study were to 1) compare the bioavailability of different types of inorganic Hg that occur naturally in sediments and 2) understand how activated carbon amendments might affect Hg methylation potential. A selective leaching test using glutathione, a thiol-based metal-chelator produced by bacteria, was used to indicate Hg bioavailability. We hypothesized that glutathione-leached Hg and hgcA abundance would correlate with methylmercury production, and that the addition of activated carbon will decrease both Hg bioavailability and methylation rate.

For these experiments, we aimed to strike a balance between simplified, controlled conditions and environmental relevance. Sediment and water collected from an estuary provided the basis for the microcosms. Four stable-isotope-labeled forms of inorganic Hg – dissolved Hg²⁺, nanoparticulate and microparticulate HgS, and Hg sorbed to nanocrystalline FeS (mackinawite) – were spiked together at twice the background Hg concentration. Samples of water and sediment slurries were collected periodically over 14 days and analyzed for total Hg, MeHg, glutathione-leachable Hg, hgcA gene abundance, and other important water chemistry parameters. Total Hg or MeHg analyzers were connected to an ICP-MS, which allowed us to track the behavior of each isotopically-labled Hg form.

The results show that glutathione-leached Hg decreased in the order Hg²⁺ ≅ nano-HgS > Hg-FeS > micro-HgS. The addition of activated carbon decreased both the leachable Hg concentration and MeHg production. hgcA gene abundance, its relationship to MeHg production, and the evaluation of glutathione as a measure of bioavailability are currently in progress. Broader impacts of this work include improving our ability to predict the methylation potential of Hg-contaminated sediments, which is necessary for risk assessment.