Unveiling the role of non-metal oxidants in PMSO oxidation: Mechanistic insights across pH regimes

The selective oxidation of methyl phenyl sulfoxide (PMSO) to methyl phenyl sulfone (PMSO 2 ) is widely used to probe high-valent metal-oxo species in advanced oxidation processes (AOPs). However, its specificity remains uncertain. This study systematically investigates PMSO oxidation by sodium hypochlorite (NaClO), sodium periodate (PI), and peroxymonosulfate (PMS) under acidic (pH = 3.0 ± 0.2), neutral (pH = 7.2 ± 0.2), and alkaline (pH = 9.2 ± 0.2) conditions. Batch experiments and kinetic simulations using Kintecus V6.80 revealed that NaClO exhibited the highest reactivity across all pH values, with second-order rate constants of 125, 25, and 2.15 M −1  s −1 under acidic, neutral, and alkaline conditions, respectively. PI was reactive only in acidic media (7.5 M −1  s −1 ), while PMS showed comparable oxidation rates under acidic and alkaline conditions (1.25 and 1.28 M −1  s −1 ), but negligible activity at neutral pH. H 2 18 O labeling confirmed 18 O incorporation into PMSO 2 even without metal catalysts, supporting oxygen atom transfer (OAT) as the dominant mechanism. These results demonstrate that PMSO oxidation and 18 O labeling are not exclusive indicators of high-valent metal species and should therefore be interpreted with caution. The study underscores the need for more selective and reliable approaches to identify transition-metal-based oxidants in AOPs.