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.