The photochemical degradation of pollutants in the environment can be enhanced using advanced oxidation processes AOPs by generating highly reactive and non-selective oxidizing species, such as hydroxyl radicals OH , which can effectively break down a wide range of organic contaminants. AOPs can be applied to various environmental matrices, including water, air, and soil, to remove persistent and toxic pollutants.There are several AOPs that can be employed to enhance photochemical degradation of pollutants, including:1. Photocatalysis: In this process, a photocatalyst e.g., titanium dioxide, TiO2 is activated by ultraviolet UV or visible light, generating electron-hole pairs. These pairs react with water and oxygen molecules to produce hydroxyl radicals, which then oxidize the pollutants.2. Photo-Fenton process: The Fenton reaction involves the generation of hydroxyl radicals through the reaction of ferrous ions Fe2+ with hydrogen peroxide H2O2 . In the photo-Fenton process, UV or visible light is used to regenerate ferrous ions from ferric ions Fe3+ , enhancing the production of hydroxyl radicals and improving the degradation efficiency.3. UV/H2O2 process: In this process, UV light directly photolyzes hydrogen peroxide to generate hydroxyl radicals. The presence of UV light also enhances the reaction rate of hydroxyl radicals with pollutants.4. Ozone-based AOPs: Ozone O3 can be used in combination with UV light O3/UV or hydrogen peroxide O3/H2O2 to generate hydroxyl radicals. Ozone itself is also a strong oxidant and can directly react with some pollutants.5. Sonophotocatalysis: This process combines photocatalysis with ultrasound, which generates hydroxyl radicals through the sonochemical degradation of water molecules. The synergistic effect of both processes enhances the overall degradation efficiency.The mechanisms of AOPs generally involve the generation of highly reactive oxidizing species, such as hydroxyl radicals, which can initiate the degradation of pollutants through various pathways, including:1. Hydrogen abstraction: Hydroxyl radicals can abstract hydrogen atoms from the pollutant molecules, forming water and a radical species that can undergo further reactions.2. Addition reactions: Hydroxyl radicals can add to double bonds or aromatic rings in pollutant molecules, forming new radical species that can undergo further reactions.3. Electron transfer: Hydroxyl radicals can transfer an electron to the pollutant molecule, forming a radical anion that can undergo further reactions.4. Direct oxidation: Some AOPs, such as ozone-based processes, can directly oxidize pollutant molecules without the involvement of hydroxyl radicals.These reactions ultimately lead to the mineralization of pollutants into harmless products, such as carbon dioxide, water, and inorganic ions, or the formation of less toxic and more biodegradable intermediates that can be further degraded by natural processes in the environment.