Manganese (hydr)oxides are ubiquitous in aquatic and terrestrial environments. Previously, naturally abundant Mn oxides were thought to be formed mainly through a biotically mediated process that it could accelerate the rate of Mn(II) oxidation. On the other hand, abiotic inorganic Mn oxidation has been considered a minor contributor, owing to its slow kinetics. Recently, however, we reported the photochemical abiotic inorganic oxidation of Mn2+(aq) to δ-MnO2 in the presence of nitrate at a rate comparable to that of biotic processes. Expanding on that work, this study investigated how Co2+ affects photochemical Mn2+ oxidation and is incorporated into newly formed Mn oxides. Using X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy with energy dispersive X-ray analysis, we observed the co-oxidation of Mn2+ and Co2+ and the formation of CoIII-MnIVO2. The initial Co2+ concentration affected the crystalline phases, oxidation states, and oxygen vacancies of Mn oxides. Our findings provide an improved understanding of the effect of Co2+ on Mn2+ oxidation in nature. These natural effects will augment current process and property information about Co-doped Mn oxides that are useful in engineered applications, such as catalysis of oxygen evolution reactions. Thus, this study helps design environmentally-friendly processes for Mn oxide materials synthesis.

Photochemically-induced co-oxidation of Mn2+ and Co2+ and the formation of CoIII-incorporated MnIV oxides

ACS Spring Meeting 2021

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