Competitive ligand exchange reveals mercury reactivity change with dissolved organic matter

Abstract

Abstract Complexation of mercury (Hg) with naturally dissolved organic matter (DOM) is important in freshwater environments but the kinetics of Hg binding to DOM and the repartitioning of Hg within the DOM remain poorly understood. We examined changes in Hg-DOM complexes using glutathione (GSH) titrations, coupled with stannous chloride reducible Hg measurements during Hg equilibration with DOM. In laboratory prepared DOM solutions and in water from a Hg-contaminated creek, a fraction of the Hg that was present in Hg-DOM complexes was not reactive to GSH. This unreactive fraction increased with the reaction time between Hg and DOM. In reactions between Hg and Suwannee River natural organic matter, the unreactive Hg increased from 13% at 1 hour to 74% after 48 hours of equilibration. This time-dependent increase in unreactive Hg suggests that either Hg forms strong complexes with DOM that are unreactive to GSH or the DOM complexed Hg becomes more sterically protected as the binding environment changes within the DOM over time. In the contaminated creek water, 58% of the Hg was present as complexes that were unreactive with GSH, demonstrating that the strength of the Hg-DOM complexes evolves with time. The extent of Hg sequestration inside DOM may provide insight to understanding transformations of Hg in natural aquatic systems.