Solar Irradiation Induced Transformation of Ferrihydrite in the Presence of Aqueous Fe²⁺

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 (h_vb⁺) generated from ferrihydrite under ultraviolet (UV) irradiation (300–400 nm) at pH 6.0, and O₂^•– and h_vb⁺ 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.