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1. Reactive halogen radicals in saline water promote photochemically-assisted formation of manganese oxide nanosheets

   https://doi.org/10.1039/D2EN00410K  

   Gao, Zhenwei; Skurie, Charlie; Jun, Young-Shin (October 2022, Environmental Science: Nano)

   Halide ions are naturally abundant in oceans and estuaries. Large amounts of highly saline efflux are also made and discharged to surface water from desalination processes and from unconventional oil and gas recovery. These highly concentrated halides can generate reactive halogen radicals. However, the redox reactions of halogen radicals with heavy metals or transition metals have received little attention. Here, we report undiscovered fast oxidation of manganese ions (Mn2+) by reactive halogen radicals. Hydroxyl radicals (˙OH) are produced by nitrate photolysis. While ˙OH radicals play a limited role in the direct oxidation of Mn2+, ˙OH can react with halide ions to generate reactive halogen radicals to oxidize Mn2+. In addition, more Mn2+ was oxidized by bromide (Br) radicals generated from 1 mM Br− than by chloride (Cl) radicals generated from 500 mM Cl−. In the presence of Br radicals, the abiotic oxidation rate of Mn2+ to Mn(IV)O2 nanosheets is greatly promoted, showing a 62% increase in Mn2+ (aq) oxidation within 6 h of reaction. This study advances our understanding of natural Mn2+ oxidation processes and highlights unexpected impacts of reactive halogen radicals on redox activities with heavy metals and corresponding nanoscale solid mineral formation in brine. This work suggests a new, environmentally-friendly, and facile pathway for synthesizing MnO2 nanosheets. 

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2. Effects of halide ions on photochemically-induced abiotic formation of Mn oxides

   Gao, Zhenwei; Skurie, Charlie; Jun, Young-Shin (August 2020, American Chemical Society National Meeting)

   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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3. Effects of halide ions on photochemically-induced abiotic formation of Mn oxides

   Gao, Zhenwei; Skurie, Charlie; Jun, Young-Shin (August 2020, American Chemical Society National Meeting)

   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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4. Photochemically-induced formation of manganese oxide nanoparticles by reactive halogen radicals in briny water

   Gao, Zhenwei; Skurie, Charlie; Liu, Jing; Jun, Young-Shin (August 2022, The 2022 Fall ACS National Meeting & Exposition)

   In meeting rapidly growing demands for energy and clean water, engineered systems such as unconventional oil and gas recovery and desalination processes produce large amounts of briny water. In the environment, these highly concentrated halides can be oxidized and transformed to reactive halogen radicals, whose roles in the degradation and transformation of organic pollutants have been studied. However, redox reactions between halogen radicals and heavy metal ions are still poorly understood. In this work, we found that aqueous manganese ions (Mn2+) could be oxidized to Mn oxide solids by reactive halogen radicals generated from reactions between halide ions and hydroxyl radicals or between halide ions and triplet state dissolved organic matter. In particular, more Mn2+ was oxidized by Br radicals generated from bromide ion (Br−) than by Cl radicals generated from chloride ion (Cl−), even though the concentrations of Br− in surface waters are much lower than Cl− concentrations. In addition, the highly concentrated halides greatly increased the ionic strength of the solution, affecting Mn2+ oxidation kinetics and the crystallinity and oxidation state of the newly formed Mn oxides. These newly discovered pathways involving Mn2+(aq) and reactive halogen radicals aid in understanding the generation of abiotic Mn oxide solids and forecasting their redox activities. Moreover, this work emphasizes the critical need for a better knowledge of the roles of reactive halogen radicals in inorganic redox reactions. 

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5. Photolytic reaction of dissolved organic matter and bromide ions promote the formation of manganese oxides

   Gao, Zhenwei; Liu, Jing; Skurie, Charlie; Zhu, Yaguang; Jun, Young-Shin (March 2022, American Chemical Society National Meeting)

   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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