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.

Fe­(II)-catalyzed ferrihydrite transformation under anoxic conditions has been intensively studied, while such mechanisms are insufficient to be applied in oxic environments with depleted Fe­(II). Here, we investigated expanded pathways of sunlight-driven ferrihydrite transformation in the presence of dissolved oxygen, without initial addition of dissolved Fe­(II). We found that sunlight significantly facilitated the transformation of ferrihydrite to goethite compared to that under dark conditions. Redox active species (hole–electron pairs, reactive radicals, and Fe­(II)) were produced from the ferrihydrite interface via the photoinduced electron transfer processes. Experiments with systematically varied wet chemistry conditions probed the relative contributions of three pathways for the production of hydroxyl radicals: (1) oxidation of water (5.0%); (2) reduction of dissolved oxygen (40.9%); and (3) photolysis of Fe­(III)-hydroxyl complexes (54.1%). Results also showed superoxide radicals as the main oxidant for Fe­(II) reoxidation under acidic conditions, thus promoting the ferrihydrite transformation. The presence of inorganic ions (chloride, sulfate, and nitrate) did not only affect the hydrolysis and precipitation of Fe­(III) but also the generation of radicals via photoinduced charge transfer reactions. The involvement of redox active species and the accompanying mineral transformations would exert a profound effect on the fate of multivalent elements and organic contaminants in aquatic environments.

The formation and transformation of schwertmannite affect the migration, conversion, and toxicity of chromium (Cr) in soils and sediments. Schwertmannite could be obtained from the oxidation of Fe­(II) by hydroxyl radicals (OH•) and superoxide radicals (O2 •–) generated from the photolysis of NO3 – under acidic and sulfate-rich conditions. As one of the most abundant components, montmorillonite is widely distributed in soils and sediments. However, the effect of montmorillonite on the photochemical formation process and corresponding Cr adsorption behaviors of schwertmannite remains elusive. This study indicates that schwertmannite could be formed on the montmorillonite surface during the photocatalytic oxidation of dissolved Fe­(II). The formation rate, particle size, and crystallinity degree of schwertmannite formed on montmorillonite surface increased with increasing FeSO4 concentration (1.0–5.0 mmol L–1). The presence of montmorillonite led to a decrease in the particle size of schwertmannite. When the initial concentration of Fe­(II) was 5.0 mmol L–1, the specific surface area of schwertmannite–montmorillonite aggregates reached 243.3 m2 g–1, which was remarkably larger than that of single-phase schwertmannite (24.6 m2 g–1) and montmorillonite (138.1 m2 g–1). The schwertmannite–montmorillonite aggregates showed a higher adsorption capacity for Cr­(VI) (97.4 mg Cr g–1 Fe) than single-phase schwertmannite (72.9 mg Cr g–1 Fe). This work reveals the possible formation pathway and Cr adsorption behavior of schwertmannite on the surface of montmorillonite in waters and soils.