Photochemically Induced Co-oxidation of Aqueous Manganese and Cobalt and the Formation of Cobalt-Incorporated Manganese Oxides

Manganese (Mn) oxides, widely found in aquatic and terrestrial environments, play crucial roles in natural ecosystems and in environmental processes. Previously, it was believed that naturally abundant Mn oxides originated through biotically mediated processes. However, we have revealed the significance of photochemically induced abiotic oxidation of Mn²⁺(aq) to Mn(IV) oxides. This study further elucidates the photochemically induced co-oxidation of aqueous Mn²⁺ and cobalt (Co²⁺), which leads to the predominant formation of Mn(IV)-Co(III) oxide nanosheets. Both pair distribution function analysis and X-ray absorption spectra provide evidence that Co²⁺ is mainly oxidized to Co(III) within the plane of the Mn oxide structure, where it forms double-edge-sharing arrangements. Additionally, the initial concentration of Co²⁺ greatly influences the extent of Co incorporation within the final Mn-Co oxides and Mn oxidation states. Increased Co incorporation correlates with a higher concentration of oxygen vacancies within the Mn oxide structures, which reduces their band gap and significantly influences the reactivity of Mn oxides, governing their ability to participate in pollutant degradation and redox transformations. This study advances our understanding of the mechanism of formation of Co-incorporated Mn oxides in the natural environment and provides insights into their occurrence in the natural environment and their applications in environmental processes.