Singlet oxygen in the removal of organic pollutants: An updated review on the degradation pathways based on mass spectrometry and DFT calculations.

The degradation of pollutants by a non-radical pathway involving singlet oxygen (O) is highly relevant in advanced oxidation processes. Photosensitizers, modified photocatalysts, and activated persulfates can generate highly selective O in the medium. The selective reaction of O with organic pollutants results in the evolution of different intermediate products. While these products can be identified using mass spectrometry (MS) techniques, predicting a proper degradation mechanism in a O-based process is still challenging. Earlier studies utilized MS techniques in the identification of intermediate products and the mechanism was proposed with the support of theoretical calculations. Although some reviews have been reported on the generation of O and its environmental applications, a proper review of the degradation mechanism by O is not yet available. Hence, we reviewed the possible degradation pathways of organic contaminants in O-mediated oxidation with the support of density functional theory (DFT). The Fukui function (FF, f, f, and f), HOMO-LUMO energies, and Gibbs free energies 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 O. Since environmental matrices contain several contaminants, it is difficult to experiment with all 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.