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1. Manganese Cycling in the Oceans

   https://doi.org/10.1002/9781119300762.wsts0065  

   Tebo, Bradley M. Luther (December 2019, Encyclopedia of Water: Science, Technology, and Society)

   Manganese (Mn) is an essential element for life. Although its concentration is at (sub)nanomolar levels throughout the ocean, it affects the oxygen concentration of the ocean because it is central to the photosynthetic formation of dioxygen, O2, in photosystem center II. Mn inputs into the ocean are from atmospheric transport of particles and their dissolution to form dissolved Mn, and from the flux of dissolved Mn from rivers, sediments and hydrothermal vents. The main removal mechanism is transport of particulate Mn from dust and organic matter to the sediments. The environmental chemistry of manganese centers on its +2, +3 and +4 oxidation states. Most recent data show that Mn(II) is dissolved, that Mn(IV) is particulate MnO2, and that Mn(III) can be particulate or dissolved when bound to organic complexes [denoted as Mn(III)-L]. Mn(II) is oxidized primarily by microbial processes whereas MnO2 is reduced by abiotic and biotic processes. Photochemical processing aids redox cycling in surface waters. In suboxic zones, which are defined as areas with dissolved O2 concentrations below 3 M, both oxidation and reduction processes can occur but usually at different depths. In suboxic zones, dissolved Mn is also released from organic matter during its decomposition and from MnO2 reduction. 

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2. Geometrically flexible synthetic manganese-oxygen and calcium-manganese-oxygen cubane clusters as reactive biomimics of the oxygen evolving complex of photosystem II.

   https://doi.org/10.1021/scimeetings.0c03947  

   Michael J. Zdilla, Shivaiah Vaddypally (March 2020, ACS Spring 2020 National Meeting and Expo)

   The oxygen evolving complex (OEC) of photosystem II is responsible for the four-electron oxidation of water to dioxygen in oxygenic photosynthetic organisms. These organisms use light to drive this thermodynamically uphill process to harvest high-energy electrons from water for cellular energy, generating O2 as a byproduct. While numerous details of the operation of the OEC are well understood, the molecular mechanism of O=O formation remains under debate. Model complex chemistry from our group will be presented wherein we have prepared cluster systems with little or no terminal multidentate ligation in order to generate geometrically flexible and reactive clusters. Two systems will be presented: a series of Ca-Mn-O hemicubane clusters with variable calcium content, and their electrocatalytic activity in activation of water in oxidation reactions, and a Mn cubane cluster with a pendant Mn=O moiety that evolves dioxygen from an oxidation state analagous to the turnover state of the natural system 

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3. Effect of water frustration on water oxidation catalysis in the nanoconfined interlayers of layered manganese oxides birnessite and buserite

   https://doi.org/10.1039/D0TA09635K  

   Bhullar, Ravneet K.; Zdilla, Michael J.; Klein, Michael L.; Remsing, Richard C. (March 2021, Journal of Materials Chemistry A)

   null (Ed.)

   The role of geometric frustration of water molecules in the rate of water oxidation in the nanoconfined interlayer of manganese-oxide layered materials (birnessite, buserite) is examined in a well-controlled experiment. Calcium buserite is prepared, and used in a split-batch synthetic protocol to prepare calcium birnessite, sodium buserite, and sodium birnessite, and partially dehydrated sodium birnessite. Thus, four samples are prepared in which features effecting catalytic efficiency (defect density, average manganese oxidation state) are controlled, and the main difference is the degree of hydration of the interlayer (two layers of water in buserites vs. one layer of water in birnessite). Molecular dynamics simulations predict birnessite samples to exhibit geometric water frustration, which facilitates redox catalysis by lowering the Marcus reorganization energy of electron transfer, while buserite samples exhibit traditional intermolecular hydrogen bonding among the two-layer aqeuous region, leading to slower catalytic behavior akin to redox reactions in bulk water. Water oxdiation activity is investigated using chemical and electrochemical techniques, demonstrating and quantifying the role of water frustration in enhancing catalysis. Calculation and experiment demonstrate dehydrated sodium birnessite to be most effective, and calcium buserite the least effective, with a difference in electrocatlytic overpotential of ∼750 mV and a ∼20-fold difference in turnover number. 

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4. 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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5. 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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