Selective oxidation of organic pollutants by unactivated and carbonate-activated peroxymonosulfate (PMS): Mechanism, kinetics, and transformation pathway

Although the activation mechanisms of peroxymonosulfate (PMS) by various homogeneous and heterogeneous catalysts have been reported, the chemistry of PMS in catalyst-free systems and its interaction with background oxygenated anions remains poorly understood. In this study, the unactivated PMS and carbonate-activated PMS (CO₃^2–/PMS) systems for removal of various organic pollutant (including oxytetracycline (OTC), metronidazole (MNZ), methylene blue (MB), and acid orange G (OG)) were investigated. The results showed that 74.5 %, 90.21 %, 1.22 %, and 2.25 % of OTC, MB, OG, and MNZ were removed, respectively within 180 min in the unactivated PMS system due to the formation of ·OH and ¹O₂. 95.88 %, 100 %, 100 %, and 6.09 % of OTC, MB, OG, and MNZ were removed, respectively in the CO₃^2–/PMS system, which is primary due to the generation of ¹O₂. The removal efficiencies of these four pollutants were significantly improved under alkaline conditions. In addition, the TOC removal rates were 21.88 % for MB and 26.53 % for OTC within 180 min in the CO₃^2–/PMS system. The presence of Cl⁻ and SO₄²⁻ can greatly enhance the OTC removal efficiency both in the unactivated and CO₃^2–/PMS systems. Through the high performance liquid chromatography-mass spectrometry (HPLC-MS) analysis, OTC degradation products in the unactivated PMS and CO₃^2–/PMS systems were identified, revealing six different reaction mechanisms, including hydroxylation (+16 Da), decarbonylation (−28 Da), demethylation (−14 Da), secondary alcohol oxidation (−2 Da), deamination (−15 Da), and dehydration (−18 Da). This study provides insight into the reaction mechanism of catalyst-free PMS systems and may promote the application of unactivated PMS and CO₃^2–/PMS systems for the remediation of organic contaminated water.