Why Dissolved Organic Matter Enhances Photodegradation of Methylmercury

Methylmercury (MeHg) is known to degrade photochemically, but it remains unclear what roles naturally dissolved organic matter (DOM) and complexing organic ligands play in MeHg photodegradation. Here we investigate the rates and mechanisms of MeHg photodegradation using DOM with varying oxidation states and origins as well as organic ligands with known molecular structures. All DOM and organic ligands increased the rate of MeHg photodegradation under solar irradiation, but the first-order rate constants varied depending on the oxidation state of DOM and the type and concentration of the ligands. Reduced DOM photochemically degraded MeHg 3 times faster than oxidized DOM. Compounds containing both thiol and aromatic moieties within the same molecule (e.g., thiosalicylate and reduced DOM) increased MeHg photodegradation rates far more than those containing only aromatics or thiols (e.g., salicylate or glutathione, or their combinations). The mechanism is attributed in part to strong binding between MeHg and thiolates that resulted in direct energy transfer from excited triplet state of the aromatics to break the Hg-C bond in MeHg. Our results suggest that, among other factors, the synergistic effects of thiol and aromatics in DOM greatly enhance MeHg photodegradation.