Review of characteristics, generation pathways and detection methods of singlet oxygen generated in advanced oxidation processes (AOPs)

Advanced oxidation processes (AOPs) have been widely used in contaminants decomposition to reduce their harm. Especially, the singlet oxygen (1O2)-mediated AOPs have been considered a promising decontamination method due to their good ability to resist water matrix in actual wastewater treatment. However, there are still some doubts about the important role of 1O2 in wastewater treatment. Therefore, it is necessary to comprehensively summarize the characteristics, detection methods, and generation pathways of 1O2 based on the latest research. In this review, various characteristics of 1O2 are comprehensively summarized, including its molecular structure, orbital energy, lifetime in solvents, reaction characteristics and reaction pathways. Next, the generation pathways of 1O2 including catalytic ozonation, hydrogen peroxide, persulfate, periodate, photocatalysis, electrochemical, and peracetic acid activation systems are summarized. Additionally, 1O2 can be detected by various methods such as fluorescence probes, chemiluminescence probes, quenching tests, electron paramagnetic resonance, deuterium oxide experiments, and product analysis. The combination of multiple detection methods can improve accuracy of 1O2 detection. Finally, the existing deficiencies and improvements in mechanism exploration, detection means and practical application of 1O2 are also contained. It is expected that this review can provide new inspiration for subsequent research on 1O2 -mediated AOPs.