Solar Irradiation-Induced Oxidation of Dissolved Mn(II) on the Ferrihydrite Surface under Oxic Conditions

Photochemical oxidation of Mn(II) facilitates the formation of natural manganese oxides due to the generation of reactive oxygen species. Natural iron oxides exhibit photochemical activity and can generate reactive oxygen species under solar irradiation. However, the mechanism for Mn(II) photochemical oxidation in the presence of ferrihydrite and dissolved oxygen remains unclear. This study investigated the formation mechanism of manganese oxides on the ferrihydrite surface under air-saturated conditions and further evaluated the effects of pH and initial Mn(II) concentration on the reaction process. The results indicated that solar irradiation promotes Mn(II) oxidation in the presence of ferrihydrite. Superoxide radicals generated from the ferrihydrite photochemical reaction contributed much to Mn(II) oxidation. Mn(II) absorbed on the ferrihydrite surface was oxidized to feitknechtite and manganite, and dissolved Mn(II) tended to be oxidized to birnessite. An increase in pH promoted the adsorption of dissolved Mn(II) by ferrihydrite, facilitating the formation of feitknechtite and manganite on the ferrihydrite surface. As pH increased from 6.0 to 8.0, the relative contents of feitknechtite and manganite increased from 18.2% and 2.9% to 67.0% and 14.8%, respectively. Overall, this study elucidates the potential mechanism for the photochemical formation of natural iron–manganese binary oxides in supergene environments.