Dissolved natural organic matter (DOM) is a complex matrix of organic matter that is ubiquitous in natural aquatic environments. So far, substantial research has been conducted on the DOM adsorption on Mn oxides as well as the reduction processes of Mn oxides by DOM. However, little is known about the oxidative roles of DOM in oxidizing Mn2+(aq) to Mn(III/IV) oxide solids. Sunlight-driven processes can initiate the degradation of DOM accompanied by the formation of photochemically produced reactive intermediates, including excited triplet state DOM (3DOM*), hydroxyl radical (•OH), superoxide radical (O2•−), hydrogen peroxide (H2O2), and singlet oxygen (1O2). Further, in the presence of halide ions, reactive halogen species can be generated by reactions between 3DOM* and halide ions, and by reactions between •OH and halide ions. In this study, we found that the solution pH controlled the oxidation of Mn2+(aq) to Mn oxide solids during photolysis of DOM. Among the reactive oxygen species, Mn2+(aq) was found to be oxidized to Mn oxide solids mainly by O2•−. The DOM with different quantities of aromatic functional groups affected its oxidative capability. With the addition of bromide ions (Br−), Mn2+(aq) oxidation was promoted further by formation Br radicals, which can also oxidize Mn2+(aq) to Mn oxide solids. These findings can help us better understand the oxidative role of DOM in the formation of Mn oxide solids in organic-rich surface water. In addition, this study assists in comprehending the impacts of the photolytic reactions between DOM and halide ions and their resulting reactive oxygen and halogen species on the oxidation and reduction processes of other transition metal oxides in the environment.
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Photolytic reaction of dissolved organic matter and bromide ions promote the formation of manganese oxides
Manganese (Mn) oxide solids widely exist in nature, serving as both electron donors and acceptors for a variety of redox reactions. Previous studies have highlighted the adsorption of dissolved organic matter (DOM) on Mn oxides, as well as the reduction of Mn oxides by DOM. Here, we show the underappreciated roles of photolytic reactions of DOM in Mn2+(aq) oxidation and its consequential formation of Mn oxide solids. During the photolysis of DOM, reactive intermediates including excited triplet state DOM (3DOM*), hydroxyl radical (•OH), superoxide radical (O2•−), hydrogen peroxide (H2O2), and singlet oxygen (1O2) can be generated. Among them, we found that O2•− was responsible for Mn oxidation. In addition, in the presence of bromide ions (Br−), the photolytic reactions between DOM and Br− formed reactive bromide radicals and facilitated the oxidation of Mn2+(aq) to Mn oxide solids. Moreover, the composition of DOM affected its oxidative capability. When DOM contained more aromatic functional groups, we observed more oxidation of Mn2+ to Mn oxides. These new findings advance our knowledge of natural Mn2+ oxidation and Mn(III/IV) oxide formation, as well as the hitherto overlooked oxidative role of DOM in the oxidation of metal ions in surface water under sunlight illumination.
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- Award ID(s):
- 1905077
- NSF-PAR ID:
- 10352819
- Date Published:
- Journal Name:
- American Chemical Society National Meeting
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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