Manganese (hydr)oxides are abundant in aquatic and terrestrial environments. The multiple oxidation states of Mn and the high reduction potential of Mn oxides make them highly redox active. Mn oxides can also scavenge heavy metals in nature, and are of interest in engineered systems, such as in batteries and water oxidation catalysis. The roles of bacteria and fungi in acceleration of Mn2+(aq) oxidation to Mn(IV) have been considered dominant for the oxidative pathway from Mn2+(aq) to MnIVO2 in natural systems. In a recent study, 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. In this study, we expanded our investigation to study how cations affect photochemical Mn2+ oxidation, and how species such as Na+, Mg2+, and Co3+ are often incorporated in natural Mn oxide minerals. We found that the crystalline structure of Mn oxides was greatly affected by the different oxidation states of the cations and their incorporation mechanisms. In addition, pH affected the aqueous species of the cation by creating complexation species, resulting in different Mn oxide crystalline phases. This study provides new insights into the mechanism of Mn oxides formation and suggests a new, environmentally friendly pathway (under neutral pH and ambient pressure and temperature conditions) for Mn oxide materials synthesis.

Effects of cations on abiotic photochemical oxidation of aqueous Mn2+

ACS Fall 2020 Virtual Meeting

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