Is Singlet Oxygen an Important Oxidant in Advanced Oxidation Processes?

Advanced Oxidation Processes (AOPs) have emerged as promising solutions for degrading persistent contaminants, including naturally occurring toxins and emerging pollutants. While AOPs have traditionally relied on the hydroxyl radical (•OH) generation, recent focus has shifted to developing AOPs that facilitate the generation of alternative reactive species such as singlet oxygen (¹O₂) due to their selective reactivity and reduced interference from water matrix components as compared to •OH. Apart from ¹O₂, superoxide (O₂•-) is also recognized as a dominant reactive species in many AOPs, both as a direct oxidant and as a precursor of ¹O₂. This review critically examines the reactivity of ¹O₂ and O₂•- to determine the feasibility of the oxidation of various common organic contaminants by these reactive species. Furthermore, the common pitfalls associated with the ¹O₂ and O₂•- quantification methods, which often lead to misinterpretations, are also discussed. By addressing these methodological challenges and providing a framework for improved assessment, this review aims to prevent misinterpretations of reactive species' roles and support the development of more effective and scientifically robust advanced oxidation strategies for water treatment.