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 homogeneously 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 oxide 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 faster crystalline phase evolutions and morphological changes over time. Additionally, the coexistence of Na⁺ and Mg²⁺ ions greatly affected the initial crystalline phases 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 insights into the processes involved in manufacturing specific Mn oxide crystalline structures for engineering applications.