Mechanistic insights into PMSO oxidation by PMS: Reliability of PMSO as a high-valent metal probe in PMS-based processes

Transition metal-activated peroxymonosulfate (PMS) processes are extensively studied for pollutant degradation, with phenyl methyl sulfoxide (PMSO) commonly used as a probe for detecting high-valent metal species. However, recent studies have attributed PMSO oxidation to reactive oxygen species generated from PMS self-decomposition, raising concerns about the probe’s specificity. This study systematically evaluated PMSO oxidation under varying pH conditions to reassess its reliability. The results showed that the PMSO degradation and methyl phenyl sulfone (PMSO2) formation increased significantly with rising pH. This pH-dependency, intrinsic to the PMS/PMSO system, indicates potential interference from PMS itself. Multiple analytical approaches indicate that 1O2 and O2•- generated from PMS self-decomposition make negligible contributions, whereas PMS itself is the primary oxidant converting PMSO to PMSO2. Kinetic studies revealed that SO52– primarily drives PMSO oxidation via nucleophilic attack, with a rate constant of 10.44 M–1s–1, far exceeding that of HSO5– (0.037 M–1 s–1). Theoretical calculations further confirmed that PMSO is relatively resistant to nucleophilic attack by SO52–, making it a comparatively reliable sulfoxide probe in acidic and neutral PMS systems. These findings highlight the limitation of PMSO as a high-valent metal probe under alkaline conditions and offer guidance for probe selection in future studies.