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Aiken, George ¹

¹ US Geological Survey

A number of biogeochemical processes that influence the fate, bioavailability and transport of mercury (Hg) in aquatic systems are mediated by the interaction of Hg with dissolved organic matter (DOM). In many cases, the interactions between Hg and DOM in a particular ecosystem or environmental setting are controlled by the chemistry of the DOM itself; however, the mechanisms, reaction pathways and strength of these interactions are poorly defined. In addition to the analytical challenges posed by generally low concentrations of Hg in many aquatic systems, the intrinsic complexity of DOM, which is a heterogeneous mixture of complex molecules of similar, but non-identical structure, presents unique challenges in determining the chemical factors and mechanisms that describe these interactions. For instance, the complex array of molecules that comprise DOM in a given environment contain a variety of potential metal binding sites having different stability constants. These vary in composition of the central heteroatom (O, N, S), the structure surrounding the heteroatom (aliphatic, aromatic, electron withdrawing/donating), and ligand stereochemistry. Recent advancements in analytical methodology designed to explore the nature of Hg-DOM binding interactions and thermodynamics have revealed that, under environmentally relevant conditions, Hg (II) binding is controlled by a small fraction of the DOM containing reactive thiol functional groups and only a small fraction (approximately 2%) of the reduced-S groups is involved with the strongest interactions between Hg and DOM. Other important DOM-Hg interactions are apparent from studies of the effects of DOM on the dissolution and precipitation of relatively-insoluble cinnabar (HgS). The interactions of HgS with DOM are favored by DOM molecules rich in aromatic moieties and are not dependent on the presence of strong binding sites. The variability in the amount and nature of DOC among ecosystems, especially with respect to differences in polarity and aromatic carbon content, are significant factors in controlling DOC reactivity with Hg. Future advances in analytical approaches and experimental design to study those compounds comprising DOM that interact most strongly with Hg are key for understanding the factors controlling Hg-cycling and bioavailability in aquatic systems.