Transformation of dissolved nitrogen under UV/H2O2: A review on composition, behavior and potential risk

N-containing pollutants, which pose a serious threat to the ecological environment and human health, are attracting increasing attention. Ultraviolet/hydrogen peroxide (UV/H2O2) process, a commonly used advanced treatment process, shows promising application prospects in the degradation of dissolved nitrogen (DN). This review first introduces the changes in the composition of DN in conventional treatment process and UV/H2O2 process from a macroscopic perspective, and summarizes influencing factors and degradation efficiency. The review also presents the transformation pathways of DN in UV/H2O2 in a microscopic perspective, mainly as follows: 1) Dissolved organic nitrogen (DON) transformation in UV/H2O2. The complete degradation behaviors (mineralization) of common N-containing substances are summarized, as well as the incomplete degradation process of common N-containing functional groups (-NH2, -NO2, -CN) and common Nitrogen containing pharmaceutical and personal care product (N-PPCPs) (sulfonamides and quinolones). 2) Dissolved inorganic nitrogen (DIN) transformation in UV/H2O2. The reaction schemes among stable forms of DIN (NH3-N, NO2-N, NO3-N) are concluded. Reactive nitrogen species (RNS) generated in the process also join the reaction. Finally, possible risk of the products is noted, including toxic transformation products (TPs) and disinfection. This review can provide a theoretical basis and technical guidance for the application of UV/H2O2 technology, especially in the degradation of N-containing pollutants.