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

WashU Affiliated Authors: Zhenwei Gao, Jing Liu, Charlie Skurie, Yaguang Zhu, Young-Shin Jun (Dept. of Energy, Environmental and Chemical Engineering)

Abstract: 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.

Citation or DOI: https://doi.org/10.1016/j.watres.2022.118831