Nonphotochemical generation of Singlet oxygen in pollutant remediation: A mini-review on the degradation pathway based on mass spectrometry and DFT calculations

Abstract The degradation of pollutants by a non-radical pathway involving singlet oxygen (1O2) is a hot topic in persulfate-based advanced water treatment processes (PS-AOPs). The selective reaction of 1O2 with organic pollutants resulting the evolution of different intermediate products. While these products are identified by mass spectrometry (MS) techniques, predicting a proper degradation mechanism in 1O2 -based process is still challenging. Hence, we review the possible degradation pathways of organic contaminants in 1O2-mediated oxidation with the support of density functional theory (DFT). The Fukui function (FF, f-, f+, and f0), and HOMO–LUMO energy obtained using DFT were used to identify the active site in the molecule and the degradation mechanism, respectively. Electrophilic addition, outer sphere type single electron transfer (SET), and addition to the hetero atoms are the key mechanisms involved in the degradation of organic contaminants by 1O2. Since the environmental matrices contain several contaminants, it is difficult to experiment with all the contaminants to identify their intermediate products. Therefore, the DFT studies are useful for predicting the intermediate compounds during the oxidative removal of the contaminants, especially for complex composition wastewater.