Photooxidation of Divalent Manganese by Surface Hydroxyl Groups on Titanium Dioxide via the Ligand-To-Metal Charge...

The redox reactions of manganese are of great importance for understanding the biogeochemical cycling of the Earth. Although the biological oxidation of Mn(II) is believed to be the primary pathway involved in the oxidation of Mn(II), the widespread manganese oxide coatings on the surface of minerals indicate that the photocatalytic oxidation of Mn(II) by natural semiconductors also contributes to the formation of manganese oxide. In this study, the modified TiO₂ (commercial P25) with abundant surface hydroxyl groups exhibited high photocatalytic reactivity in the abiotic oxidation of Mn(II). Kinetic results show that an increased hydroxyl density on the P25 surface significantly accelerates the oxidation of Mn(II) to Mn(III,IV) oxides. After 150 min of reaction at pH 8.5 under light irradiation, the average oxidation state of manganese reached 3.03 with an initial Mn(II) concentration of 0.5 mM. Characterizations indicate that the P25-2 surface forms Ti-O-Mn complexes with Mn(II). Compared to pure P25 (3.07 eV), P25-0.5 (3.05 eV), P25-1 (2.99 eV), and P25-2 (2.97 eV) samples exhibit a reduced bandgap, suggesting the contribution of the ligand-to-metal charge transfer pathway to the photocatalytic oxidation of Mn(II). Moreover, the increased surface hydroxyl density in P25 enhances hydrophilicity, improves surface adsorption capacity for manganese, and shifts the valence band upward. These effects collectively promote the ligand-to-metal charge transfer pathway and enhance the Mn(II) oxidation efficiency. This study provides new insights into the influence of surface hydroxyl density on the ligand-to-metal charge transfer pathway and the abiotic photocatalytic oxidation process of Mn(II).