Singlet oxygen (¹O₂) generation mediated by iron-based materials: Mechanisms, activation pathways and applications in wastewater treatment

Singlet oxygen (¹O₂) is regarded as a promising decontamination agent in practical wastewater treatment owing to its strong resistance to water matrix interference. Iron-based materials exhibit high catalytic activity and reaction selectivity in advanced oxidation processes (AOPs) and have become preferred transition metals for mediating ¹O₂ generation. However, the mechanisms underlying iron-based material mediated ¹O₂ generation are complex and vary significantly depending on oxidant types and environmental factors, necessitating a systematic review to guide the design and application of efficient catalytic systems. This study elucidates the technical merits of ¹O₂ in wastewater treatment by systematically analyzing its molecular characteristics, lifetime, orbital energy, reactive properties and oxidative pathways. ¹O₂ generation in different oxidation systems such as ozonation, hydrogen peroxide activation and persulfate activation has been summarized. Key driving mechanisms of ¹O₂ generation mediated by various iron-based materials under different oxidants have been discussed. Enhancement of ¹O₂ generation efficiency by energy (photocatalytic and electrochemical) assisted activation is demonstrated. Finally, challenges and corresponding countermeasures related to reaction mechanisms, mineralization assessment, real-water applicability and recyclability of ¹O₂ systems catalyzed by iron-based materials have been proposed. This review examines the utilization of iron-based catalysts in AOPs, aiming to provide new insights for future research on ¹O₂ generation driven by iron-based materials.