Photochemical Formation Process of Schwertmannite on Montmorillonite and Corresponding Cr(VI) Adsorption Capacity

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.