The ecological fate of mercury in aquatic systems depends, in large part, on dissolved organic matter (DOM) concentration, the concentrations of inorganic ligands, especially sulfide, and the presence of sulfate-reducing bacteria that convert Hg2+ into methylmercury, a highly toxic form of mercury that is readily bioaccumulated. Recent research shows that Hg(II) binds to DOM more strongly (KDOM 1023 L kg-1) than previously thought under environmentally relevant conditions. Strong binding of mercury by DOM, which is controlled by a small fraction of the DOM containing reactive thiol functional groups, is second only to sulfide when compared to other ligands of geochemical significance. In addition to strong binding of Hg(II) by thiol-like moieties associated with DOM, strong DOM-Hg interactions are apparent from studies of the effects of DOM on the dissolution and precipitation of relatively-insoluble cinnabar (HgS). Organic matter enhances HgS dissolution through surface reactions favored by DOM rich in aromatic moieties. Precipitation of metacinnabar (HgS) is inhibited by low concentrations (=3 mg C L-1) of DOM by prevention of the aggregation of nanocolloidal mercuric sulfide. Interactions of HgS with DOM can influence the geochemistry and bioavailability of Hg in aquatic environments by maintaining higher dissolved total Hg concentrations than predicted by current speciation models. In this proposal, Joe Ryan (University of Colorado at Boulder), Kathryn Nagy (University of Illinois at Chicago), and George Aiken (U.S. Geological Survey, Boulder, Colorado) outline a plan to investigate more challenging aspects of Hg-organic matter interactions. Similar interactions with selected metals (from soft to hard) will be studied. Better definition of these interactions is required for the improvement of metal speciation models and increased understanding of the factors controlling metal cycling in aquatic systems. Specific objectives of this research are to (1) quantify the effects of organic matter, metal nature, and sulfide concentration on metal binding by dissolved and solid organic matter at environmentally relevant concentrations, (2) elucidate the contributions to metal binding by sulfur, nitrogen, and oxygen functional groups in organic matter, (3) examine the effect of organic matter nature on the inhibition of metal sulfide precipitation, with an emphasis on mercuric sulfide, and (4) probe the mechanism of DOM enhancement of metal-sulfide dissolution. Major products of this research will be the determination of organic matter binding constants for selected toxic metals at environmentally relevant concentrations and the assessment of colloidal stabilization as a process contributing to the occurrence of dissolved metals in aquatic systems. Broader impacts of the proposed research include dissemination of the research results at national conferences and in peer-reviewed journals in addition to the following special activities. The co-PIs and their research assistants will collaborate with Dr. Edward Tipping, developer of a widely used geochemical equilibrium model (WHAM), to incorporate new DOM-metal binding constants into metal speciation calculations and with Dr. Alain Manceau to characterize competitive binding of metals with organic matter used in bioremediation schemes. The co-PIs will develop a graduate class on the interactions of organic matter with contaminants. The information resulting from this study will be directly applicable to the effective management of aquatic ecosystems, and has important implications for ecosystem restoration programs. Dr. Aiken will continue to participate in U.S. Geological Survey efforts to make science accessible for environmental regulators managing the Florida Everglades and the San Joaquin-Sacramento River delta in California.
