Photochemical formation of hydroxyl radicals catalyzed by montmorillonite

In this work, the photooxidation of benzene and the formation of phenol in aqueous suspensions of the iron-rich montmorillonite under irradiation of a 250 W metal halide lamp (λ ⩾ 365 nm) were investigated. We confirmed that hydroxyl radicals (OH) were produced by illuminating montmorillonite and was responsible for the photooxidation of benzene in aqueous suspensions of montmorillonite. Low pH value facilitated the formation of hydroxyl radicals in the pH range of 2.0–10.0. The OH concentration increased with increasing the concentration of montmorillonite in aqueous solutions in the range of 0–20.0 g l−1. Higher concentration like 25.0 g l−1 montmorillonite inhibited the OH production. Iron, predominantly free iron in the clays, is believed to be one of the most important factors determining OH formation. Structural irons in montmorillonite have contributions to OH formation, especially in the presence of carboxylate ions. The formation of OH from montmorillonite under irradiation of near UV and visible light indicates that clays might play important role not only in transfer through adsorption but also in transformation through oxidation of organic compounds on the surface of clay particles in air, water, soil or even top sediments.