Advanced oxidation processes AOPs are a group of chemical treatment methods designed to remove organic and inorganic materials in water and air by generating highly reactive species, primarily hydroxyl radicals OH . These radicals are non-selective and can oxidize a wide range of pollutants, making AOPs an attractive option for the degradation of emerging pollutants in the environment. Photochemical reactions, which involve the absorption of light to initiate chemical reactions, are an essential component of many AOPs.Some of the most common AOPs utilizing photochemical reactions include:1. Photocatalysis: This process involves the use of a photocatalyst, typically titanium dioxide TiO2 , which is activated by ultraviolet UV light to generate hydroxyl radicals. Photocatalysis is effective in degrading various pollutants, including pharmaceuticals, personal care products, and pesticides. The key reaction mechanism involves the generation of electron-hole pairs in the photocatalyst, which then react with water and oxygen to produce hydroxyl radicals.2. Photo-Fenton: This process involves the combination of UV light, hydrogen peroxide H2O2 , and ferrous iron Fe2+ to generate hydroxyl radicals. Photo-Fenton is particularly effective in degrading recalcitrant organic pollutants, such as polycyclic aromatic hydrocarbons PAHs and endocrine-disrupting compounds EDCs . The reaction mechanism involves the reduction of Fe3+ to Fe2+ by UV light, followed by the reaction of Fe2+ with H2O2 to produce hydroxyl radicals.3. UV/H2O2: This process involves the direct photolysis of hydrogen peroxide under UV light to generate hydroxyl radicals. This method is effective in degrading a wide range of pollutants, including pharmaceuticals, pesticides, and industrial chemicals. The key reaction mechanism involves the homolytic cleavage of the O-O bond in H2O2 by UV light, resulting in the formation of hydroxyl radicals.4. Ozone-based AOPs: These processes involve the use of ozone O3 in combination with UV light or H2O2 to generate hydroxyl radicals. Ozone-based AOPs are effective in degrading various pollutants, including pharmaceuticals, EDCs, and taste and odor compounds. The reaction mechanisms involve the photolysis of ozone by UV light or the reaction of ozone with H2O2 to produce hydroxyl radicals.The effectiveness of different AOPs in degrading pollutants depends on several factors, including the type of pollutant, the environmental matrix, and the specific AOP used. In general, AOPs are more effective in degrading pollutants with high reaction rate constants with hydroxyl radicals, such as EDCs and PAHs. However, some pollutants, such as certain pharmaceuticals, may be more resistant to degradation by AOPs due to their chemical structure or the presence of scavenging species in the environmental matrix.In conclusion, advanced oxidation processes utilizing photochemical reactions offer a promising approach for the efficient degradation of emerging pollutants in the environment. By generating highly reactive hydroxyl radicals, AOPs can effectively degrade a wide range of pollutants, including pharmaceuticals, personal care products, and industrial chemicals. The choice of the most suitable AOP depends on the specific pollutant and environmental matrix, as well as the availability of resources and infrastructure for implementing the technology.