Selective Oxidation of Sulfoxide to Sulfone in Chloride/Peroxymonosulfate Systems: Reassessing a Diagnostic Probe for High-Valent Metal-Oxo Identification

The selective oxidation of sulfoxides to sulfones, particularly using methyl phenyl sulfoxide (PMSO), has been widely adopted for over a decade as a diagnostic method for high-valent metal-oxo (HVMO) species in advanced oxidation processes (AOPs). Here, we demonstrate that this transformation proceeds efficiently via non-HVMO pathways in metal-free systems, challenging the probe’s specificity. Using the chloride/peroxymonosulfate (Cl−/PMS) system as a model, PMSO is converted to methyl phenyl sulfone (PMSO2) with high selectivity (83.5%−98.7%) across pH 3.5−11. Kinetic, isotopic, and computational analyses reveal a pH-dependent mechanistic switch for the high-yield transformation of PMSO to PMSO2. Under acidic conditions, Cl2/HOCl generated from the two-electron oxidation of Cl− by HSO5− through nucleophilic addition mediate oxygen-atom transfer from water via a chlorosulfonium cation intermediate, with Cl2 dominating PMSO selective oxidation (kapp = 1.58 × 106 M−1 s−1; 47.91 M−1 s−1 for HOCl). Under alkaline conditions, nucleophilic attack by SO52− formed via PMS hydrolysis becomes predominant (16.38 M−1 s−1). The reaction kinetics are linearly dependent on Cl− concentration, and similar selective oxidation is observed in other metal-free AOPs (e.g., base/PMS, HCO3−/PMS, and Cl−/peracetic acid). Our findings necessitate reassessing the sulfoxide probe’s specificity for HVMO identification in PMS-based systems containing chloride or alkalinity, and underscore the imperative for rigorous mechanistic validation in water treatment.