Generation and identification of ¹O₂ in catalysts/peroxymonosulfate systems for water purification

Catalysts for peroxymonosulfate (PMS) activation are appealing in the purification of organic wastewater. Singlet oxygen (¹O₂) is widely recognized as a crucial reactive species for degrading organic contaminants in catalysts/PMS systems due to its adamant resistance to inorganic anions, high selectivity, and broad pH applicability. With the rapid growth of studies on ¹O₂ in catalysts/PMS systems, it becomes necessary to provide a comprehensive review of its current state. This review highlights recent advancements concerning ¹O₂ in catalysts/PMS systems, with a primary focus on generation pathways and identification methods. The generation pathways of ¹O₂ are summarized based on whether (distinguished by the geometric structures of metal species) or not (distinguished by the active sites) the metal element is included in the catalysts. Furthermore, this review thoroughly discusses the influence of metal valence states and metal species with different geometric structures on ¹O₂ generation. Various potential strategies are explored to regulate the generation of ¹O₂ from the perspective of catalyst design. Identification methods of ¹O₂ primarily include electron paramagnetic resonance (EPR), quenching experiments, reaction in D₂O solution, and chemical probe tests in catalysts/PMS systems. The principles and applications of these methods are presented comprehensively along with their applicability, possible disagreements, and corresponding solutions. Besides, an identifying procedure on the combination of main identification methods is provided to evaluate the role of ¹O₂ in catalysts/PMS systems. Lastly, several perspectives for further studies are proposed to facilitate developments of ¹O₂ in catalysts/PMS systems.