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

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 1.62.6 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.