Solar Irradiation Induced Transformation of Ferrihydrite in the Presence of Aqueous Fe.

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, Fe could be oxidized to goethite and lepidocrocite by hydroxyl radical (OH), superoxide radical (O) and hole (h) generated from ferrihydrite under ultraviolet (UV) irradiation (300-400 nm) at pH 6.0, and O and h were mainly responsible for Fe 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 Fe concentration. Both visible (vis) and solar irradiation could also lead to the oxidation of Fe to goethite and lepidocrocite, and the proportion of lepidocrocite increased with increasing light intensity. Fe 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 Fe 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.