Oxygen adsorption and activation control the photochemical activity of common iron oxyhydroxide polymorphs in mediating oxytetracycline degradation under visible light

The natural minerals with semiconducting properties possess photochemical activity through generating reactive oxygen species (ROSs) and affect the fate of adsorbed organic pollutants. Iron oxyhydroxides occur in different polymorphic structures under various geological and climatic conditions in natural environment. However, the difference in their photoactivity has not been well understood. This work elucidates the mechanism of light-induced generation of ROSs by common iron oxyhydroxide polymorphs, including goethite (α-FeOOH), akaganeite (β-FeOOH), lepidocrocite (γ-FeOOH), and feroxyhyte (δ-FeOOH), and the degradation of oxytetracycline (OTC) mediated by them. Under visible light irradiation, all these iron oxyhydroxide polymorphs generated superoxide radicals (O₂^-•), hydroxyl radicals (•OH), and H₂O₂. Among the ROSs, O₂^-•, whose concentrations in the photochemical systems of α-, β-, γ- and δ-FeOOH reached 4.25 × 10^-4 ± 5.67 × 10^-5, 2.29 × 10^-4 ± 2.55 × 10^-5, 6.2 × 10^-4 ± 8.5 × 10^-5 and 7.65 × 10^-4 ± 5.67 × 10^-5 μmol/L, respectively, played a dominant role in the degradation of OTC. The corresponding rate of OTC degradation in these photochemical systems were 0.054 ± 0.010, 0.015 ± 0.001, 0.338 ± 0.073, and 0.404 ± 0.016 min^-1, respectively. A good linear relationship was observed between the steady-state concentrations of O₂^-• and OTC degradation rates in the photochemical systems of iron oxyhydroxide polymorphs. O₂^-• was primarily generated by the electron reduction of O₂ on the conduction bands (CBs) of iron oxyhydroxide polymorphs, which was controlled by the surface adsorption and activation of O₂. H₂O₂ and surface Fe²⁺ on iron oxyhydroxide polymorphs synergistically contributed to •OH generation through Fenton reaction. The generation of surface Fe²⁺ was determined by the geometrical configuration, crystallinity, and surface oxygen vacancies of iron oxyhydroxide polymorphs. These findings demonstrate the photochemical generation of ROSs in the presence of major iron oxyhydroxide polymorphs was primarily controlled by O₂ adsorption and activation, leading to different activity in mediating the degradation of adsorbed organic pollutants.