Roles of dissolved organic matter in the speciation of mercury and methylmercury in a contaminated ecosystem in Oak Ridge, Tennessee

Abstract

Environmental context. Mercury (Hg) presents an environmental concern owing to its transformation to the potent neurotoxin methylmercury (CH3Hg+). The environmental factors that control bacterial methylation of mercury are poorly understood, but we know that methylmercury is bioaccumulated and biomagnified in aquatic food webs. We show that, even at low concentrations (~3 mg L–1), natural dissolved organic matter strongly complexes with ionic Hg2+ and CH3Hg+, thereby influencing biological uptake and methylation of Hg in aquatic environments. Abstract. Complexation of the mercuric ion (Hg2+) and methylmercury (CH3Hg+) with organic and inorganic ligands influences mercury transformation and bioaccumulation in aquatic environments. Using aqueous geochemical modelling, we show that natural dissolved organic matter (DOM), even at low concentrations (~3 mg L–1), controls the Hg speciation by forming strong Hg-DOM and CH3Hg-DOM complexes through the reactive sulfur or thiol-like functional groups in DOM in the contaminated East Fork Poplar Creek at Oak Ridge, Tennessee. Concentrations of neutral Hg(OH)2, Hg(OH)Cl, CH3HgCl, and CH3HgOH species are negligible. Of the coexisting metal ions, only Zn2+, at concentrations of 1.6–2.6 × 10–7 M, competes with Hg2+ for binding with DOM, causing decrease in Hg-DOM complexation but having little impact on CH3Hg-DOM complexation. DOM may thus play a dominant role in controlling the transformation, biological uptake, and methylation of Hg in this contaminated ecosystem.

The fate of mercury in Arctic terrestrial and aquatic ecosystems, a review

“…[95,96] There are other interactions between the organic carbon and Hg cycles that are not as well understood. For example, DOM may provide stabilising ligands to maintain Hg in dissolved form, [97] and C-DOM absorbs UV radiation and thus provides an important control on photo-reduction and other photo-initiated processes in surface water. [15,56] In saline waters, organic compounds produced by algae are able to promote oxidation of Hg 0 even under dark conditions.…”

Development of an Enhanced Hydro-geochemical Model to Address Mercury-speciation Fate and Transport in Aquatic Environments

“…The high Hg contamination in East Fork Poplar Creek (EFPC) located in the ORR area at Oak Ridge, TN, has raised concern among researchers due to the accidentally discharged and spilled Hg from the Y-12 Plant into the surrounding environment in the 1950s . Dong et al (2010) reported high concentration of MeHg in EFPC and found that neither total Hg nor dissolved inorganic Hg concentrations correlate with MeHg concentration in EFPC water. This suggests that speciation, mobility, and accumulation of Hg in EFPC are controlled by site aquatic, and geochemical characteristics, and conditions (Dong et al, 2010).…”

“…Dong et al (2010) reported high concentration of MeHg in EFPC and found that neither total Hg nor dissolved inorganic Hg concentrations correlate with MeHg concentration in EFPC water. This suggests that speciation, mobility, and accumulation of Hg in EFPC are controlled by site aquatic, and geochemical characteristics, and conditions (Dong et al, 2010).…”

“…These compounds are released into the environment as biocides, but can also be produced by enzymatic methylation of cations or due to reactions with biogenic methylating agents (Southworth et al, 2001;Ravichandran, 2004;Morel et al, 1993). The high animal tissue affinity of organomercury compounds, allows bioaccumulation and biomagnification of MeHg in aquatic organisms (e.g., fish) and humans to occur easier than other Hg species (Ravichandran, 2004;Dong et al, 2010).…”

“…Hg 0 can be converted to an oxidized form of Hg(II) (Ravichandran, 2004;Dong et al, 2010). Hg(II) is transferred to the watersheds and water bodies through wet and dry deposition (Ravichandran, 2004).…”