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Manganese (Mn) oxides have excellent oxidation and adsorption capabilities that can affect geochemical element cycling and the fate of pollutants in the environment. Naturally existing Mn oxides are believed to be formed mainly by oxidation of Mn2+(aq) mediated by fungi or bacteria, while abiotic Mn oxidation has been considered a minor contribution due to its slow kinetics. However, in a recent study, we discovered abiotic inorganic oxidation of Mn2+(aq) to δ-MnO2 by superoxide radicals generated from nitrate photolysis at a rate comparable to that of biotic processes. In the current study, we investigated the effects of abundant halide ions (such as Cl−) on photochemically-driven oxidation of Mn2+(aq). Halide ions are abundant in the environment and many engineered systems, including seawater and blackish water, as well as effluent water from desalination and unconventional oil and gas recovery. Halide ions can participate in photochemical Mn2+ oxidation reactions by forming additional radicals. We found that in the presence of halide ions, the oxidation rates of Mn2+ and quantities of formed Mn oxides were both increased. In addition, high concentrations of halide ions greatly changed the ionic strengths of the systems, affecting the crystallinity of the resulting Mn oxide. Our findings highlight unexpected impacts of halogen ions on solid formations in surface water rich in halides.
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Effects of Sodium and Magnesium Ions on the Photochemically Induced Heterogeneous Formation of Manganese Oxides and Their Structural Evolution
Manganese (Mn) oxides are abundant in aquatic and terrestrial environments, where they play significant roles in redox cycling and biological metabolisms. We recently observed that Mn oxides were homogenously formed during the abiotic oxidation of Mn2+(aq) to Mn(IV) by O2•− via nitrate photolysis, at a rate comparable to that of biotic Mn oxides formation. On the other hand, for the heterogeneous formation of Mn oxides, the presence of a substrate can alter the required thermodynamic driving force, which may affect their crystalline phases and further influence the oxidative capability of redox cycling in environmental systems. However, little is known about the photochemically-induced heterogeneous formation of Mn oxides on substrates. In this study, we investigated the heterogeneous formation of Mn oxides on a quartz substrate in the presence of two environmentally abundant cations, Na+ and Mg2+. In contrast to homogeneously generated Mn oxides, the heterogeneously formed Mn oxides displayed earlier crystalline phase evolutions and morphological changes over time. Additionally, the coexistence of Na+ and Mg2+ ions greatly affected the initial crystalline phase and the phase evolution, as well as the surface morphologies of the Mn oxides. These discoveries contribute to our understanding of how various Mn oxides form in nature and provide insight into the processes involved in manufacturing specific Mn oxide crystalline structures for engineering applications.
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- Award ID(s):
- 1905077
- PAR ID:
- 10482878
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- The Journal of Physical Chemistry C
- Volume:
- 127
- Issue:
- 26
- ISSN:
- 1932-7447
- Page Range / eLocation ID:
- 12558 to 12567
- Subject(s) / Keyword(s):
- Heterogenous nucleation Mn oxides tunnel structure Mg Na
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Manganese (Mn) oxides have excellent oxidation and adsorption capabilities that can affect geochemical element cycling and the fate of pollutants in the environment. Naturally existing Mn oxides are believed to be formed mainly by oxidation of Mn2+(aq) mediated by fungi or bacteria, while abiotic Mn oxidation has been considered a minor contribution due to its slow kinetics. However, in a recent study, we discovered abiotic inorganic oxidation of Mn2+(aq) to δ-MnO2 by superoxide radicals generated from nitrate photolysis at a rate comparable to that of biotic processes. In the current study, we investigated the effects of abundant halide ions (such as Cl−) on photochemically-driven oxidation of Mn2+(aq). Halide ions are abundant in the environment and many engineered systems, including seawater and blackish water, as well as effluent water from desalination and unconventional oil and gas recovery. Halide ions can participate in photochemical Mn2+ oxidation reactions by forming additional radicals. We found that in the presence of halide ions, the oxidation rates of Mn2+ and quantities of formed Mn oxides were both increased. In addition, high concentrations of halide ions greatly changed the ionic strengths of the systems, affecting the crystallinity of the resulting Mn oxide. Our findings highlight unexpected impacts of halogen ions on solid formations in surface water rich in halides.more » « less
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.jpcc.3c00644
