Photochemical reactions of dissolved organic matter and bromide ions facilitate abiotic formation of manganese oxide solids

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.