The photocatalytic degradation of organic pollutants using TiO2 nanoparticles under UV irradiation involves a series of reactions that ultimately lead to the breakdown and mineralization of the pollutants. The mechanism can be described in the following steps:1. Activation of TiO2 nanoparticles: When TiO2 nanoparticles are exposed to UV light with a wavelength below 387 nm, they absorb the photons and become excited. This excitation leads to the promotion of an electron e- from the valence band VB to the conduction band CB , leaving behind a positive hole h+ in the valence band. This process creates electron-hole pairs, which are the primary reactive species in the photocatalytic process.2. Formation of reactive oxygen species ROS : The photogenerated electrons and holes can react with the surrounding water and dissolved oxygen molecules. The holes in the valence band can oxidize water molecules, producing hydroxyl radicals OH :h+ + H2O OH + H+The electrons in the conduction band can reduce dissolved oxygen molecules, generating superoxide radicals O2- :e- + O2 O2-These reactive oxygen species ROS are highly reactive and play a crucial role in the degradation of organic pollutants.3. Degradation of organic pollutants: The ROS, particularly hydroxyl radicals, can react with the organic pollutants, initiating a series of oxidation reactions that lead to the breakdown of the pollutant molecules. These reactions can involve hydrogen abstraction, addition to double bonds, or electron transfer, among other processes. The ultimate goal is the mineralization of the pollutants, converting them into harmless products such as CO2, H2O, and inorganic ions.4. Termination: The photogenerated electrons and holes can also recombine, releasing their energy as heat or light luminescence . This process is generally undesirable, as it reduces the efficiency of the photocatalytic process. However, the use of TiO2 nanoparticles with a high surface area and appropriate crystalline structure can help minimize electron-hole recombination and improve the overall photocatalytic performance.In summary, the mechanism of photocatalytic degradation of organic pollutants using TiO2 nanoparticles under UV irradiation involves the generation of electron-hole pairs upon light absorption, the formation of reactive oxygen species, and the subsequent oxidation and breakdown of the pollutants into harmless products.