Insights into the\nPhotochemical Mechanism of Goethite:\nRoles of Different Types of Surface Hydroxyl Groups in Reactive Oxygen\nSpecies Generation and Fe(III) Reduction

The surface photochemical activity of goethite, which\noccurs widely\nin surface soils and sediments, plays a crucial role in the environmental\ntransformation of various pollutants and natural organic matter. This\nstudy systemically investigated the mechanism of different types of\nsurface hydroxyl groups on goethite in generating reactive oxygen\nspecies (ROSs) and Fe(III) reduction under sunlight irradiation. Surface\nhydroxyl groups were found to induce photoreductive dissolution of\nFe(III) at the goethite–water interface to produce Fe<sup>2+</sup><sub>(aq)</sub>, while promoting the production of ROSs. Substitution\nof the surface hydroxyl groups on goethite by fluoride significantly\ninhibited the photochemical activity of goethite, demonstrating their\nimportant role in photochemical activation of goethite. The results\nshowed that the surface hydroxyl groups (especially the terminating\nhydroxyl groups, FeOH) led to the formation of Fe(III)-hydroxyl\ncomplexes via ligand–metal charge transfer on the goethite\nsurface upon photoexcitation, facilitating the production of Fe<sup>2+</sup><sub>(aq)</sub> and •OH. The bridging hydroxyl groups\n(Fe<sub>2</sub>OH) were shown to mainly catalyze the production\nof H<sub>2</sub>O<sub>2</sub>, leading to the subsequent light-driven\nFenton reaction to produce •OH. These findings provide important\ninsights into the activation of molecular oxygen on the goethite surface\ndriven by sunlight in the environment, and the corresponding degradation\nof anthropogenic and natural organic compounds caused by the generated\nROSs.