Manganese (Mn) oxide solids are ubiquitous in nature, acting as both electron donors and acceptors in diverse redox reactions in the environment. Reactions of Mn(III/IV) oxides with dissolved natural organic matter (DOM) are commonly described as reductive dissolutions that generate Mn2+(aq). In this study, we investigated the role of photochemical reactions of DOM in Mn2+(aq) oxidation and the resulting formation of Mn oxide solids. During the photolysis of DOM, reactive intermediates can be generated, including excited triplet state DOM (3DOM*), hydroxyl radicals (•OH), superoxide radicals (O2•-), hydrogen peroxide, and singlet oxygen. Among these, we found that O2•- radicals were mainly responsible for Mn oxidation. The solution pH controlled the formation of Mn oxide solids by affecting both Mn2+ oxidation by O2•- during photolysis of DOM and reductive dissolutions of Mn oxide solids by DOM. Further, with the addition of bromide ions (Br-), reactions between 3DOM* and Br-, together with reactions between •OH and Br-, can form reactive bromide radicals. The formed Br radicals also promoted Mn oxide formation. In DOM with more aromatic functional groups, more Mn2+ was oxidized to Mn oxide solids. This enhanced oxidation could be the result of promoted pathways from charge-transfer state DOM (DOM•+/•-) to O2•-. These new observations advance our understanding of natural Mn2+ oxidation and Mn(III/IV) oxide formation and highlight the underappreciated oxidative roles of DOM in the oxidation of metal ions in surface water illuminated by sunlight.
Keywords: Abiotic manganese oxidation; Bromide radicals; Natural organic matter; Photochemistry; Reactive oxygen species.
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