A review of factors affecting the formation and roles of primary and secondary reactive species in UV₂₅₄-based advanced treatment processes

The presence of organic micropollutants (OMPs) in water has been threatening human health and aquatic ecosystems worldwide. Ultraviolet-based advanced treatment processes (UV-ATPs) are one of the most effective and promising technologies to transform OMPs in water; therefore, an increasing number of emerging UV-ATPs are proposed. However, appropriate selection of UV-ATPs for practical applications is challenging because each UV-ATP generates different types and concentrations of reactive species (RSs) that may not be sufficient to degrade specific types of OMPs. Furthermore, the concentrations and types of RSs are highly influenced by anions and dissolved organic matter (DOM) coexisting in real waters, making systematic understandings of their interfering mechanisms difficult. To identify and address the knowledge gaps, this review provides a comparison of the generations and variations of various types of RSs in different UV-ATPs. These analyses not only prove the importance of water matrices on formation and consumption of primary and secondary RSs under different conditions, but also highlight the non-negligible roles of optical properties and reactivities of DOM and anions. For example, different UV-ATPs may be applicable to different target OMPs under different conditions; and the concentrations and roles of secondary RSs may outperform those of primary RSs in OMP degradation for real applications. With continuous progress and outstanding achievements in the UV-ATPs, it is hoped that the findings and conclusions of this review could facilitate further research and application of UV-ATPs.