Competitive ligand exchange reveals time dependant changes in the reactivity of Hg-dissolved organic matter complexes

Mercury interactions with dissolved organic matter (DOM) are important in aquatic environments but the kinetics of Hg binding to and repartitioning within the DOM remain poorly understood. We examined changes in Hg-DOM complexes using glutathione (GSH) titrations, coupled with stannous-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 present as Hg-DOM complexes did not react to GSH addition. This unreactive Hg fraction increased with time from 13% at 1 h to 74% after 48 h of equilibration with a Suwannee River DOM. In East Fork Poplar Creek water in Oak Ridge, Tennessee, similar to 58% of the DOM-complexed Hg was unreactive with GSH 1 h after the sample was collected. This time-dependent increase in unreactive Hg suggests that Hg forms stronger complexes with DOM over time. Alternatively the DOM-complexed Hg may become more sterically protected from the ligand exchange reactions, as the binding environment changes within the DOM over time. These results have important implications to understanding Hg transformations in the natural environment, particularly in contaminated aquatic systems due to non-equilibrium interactions between Hg and DOM.