Iron oxide nanominerals are generated by photocatalytic oxidation of Fe2+aq, and Fe2+aq promotes the transformation of the photochemically synthesized schwertmannite.

Ferrihydrite commonly occurs in soils and sediments, especially in acid mine drainage (AMD). Solar irradiation may affect Fe­(II)-catalyzed transformation of metastable ferrihydrite to more stable iron oxides on AMD surface. We investigated the Fe­(II)-catalyzed transformation process and mechanism of ferrihydrite under light irradiation. In nitrogen atmosphere, Fe2+ aq could be oxidized to goethite and lepidocrocite by hydroxyl radical (OH•), superoxide radical (O2 •–) and hole (hvb +) generated from ferrihydrite under ultraviolet (UV) irradiation (300–400 nm) at pH 6.0, and O2 •– and hvb + were mainly responsible for Fe2+ aq oxidation. In addition, the ligand-to-metal charge-transfer (LMCT) process between Fe­(II) and ferrihydrite could be promoted by UV irradiation. Goethite proportion increased with increasing Fe2+ aq concentration. Both visible (vis) and solar irradiation could also lead to the oxidation of Fe2+ aq to goethite and lepidocrocite, and the proportion of lepidocrocite increased with increasing light intensity. Fe2+ aq was photochemically oxidized to schwertmannite at pH 3.0 and 4.5, and the oxidation rate was higher than that under dark conditions in air. The photochemical oxidation rate of Fe2+ aq decreased in the presence of humic acid. This study facilitates a better understanding of the formation and transformation of iron oxides in natural environments and ancient Earth.

As important components with excellent oxidation and adsorption activity in soils and sediments, manganese oxides affect the transportation and fate of nutrients and pollutants in natural environments. In this work, birnessite was formed by photocatalytic oxidation of Mn in the presence of nitrate under solar irradiation. The effects of concentrations and species of interlayer cations (Na, Mg, and K) on birnessite crystal structure and micromorphology were investigated. The roles of adsorbed Mn and pH in the transformation of the photosynthetic birnessite were further studied. The results indicated that Mn was oxidized to birnessite by superoxide radicals (O) generated from the photolysis of NO under UV irradiation. The particle size and thickness of birnessite decreased with increasing cation concentration. The birnessite showed a plate-like morphology in the presence of K, while exhibited a rumpled sheet-like morphology when Na or Mg was used. The different micromorphologies of birnessites could be ascribed to the position of cations in the interlayer. The adsorbed Mn and high pH facilitated the reduction of birnessite to low-valence manganese oxides including hausmannite, feitknechtite, and manganite. This study suggests that interlayer cations and Mn play essential roles in the photochemical formation and transformation of birnessite in aqueous environments.

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