Ferryl species-independent PMSO₂ formation: Multiple oxygen atom transfer pathways in Fe²⁺-based peroxide activation

Methyl phenyl sulfoxide (PMSO) is widely used to identify ferryl-oxo species ([Fe^IV=O]²⁺) in Fenton and Fenton-like advanced oxidation processes (AOPs) through the formation of methyl phenyl sulfone (PMSO₂). However, chemical probe-based detection inherently requires high selectivity toward target intermediates, whereas whether PMSO₂ is formed only by [Fe^IV=O]²⁺ in Fenton and Fenton-like AOPs has hitherto been overlooked, potentially leading to misinterpretation of reaction mechanisms. Herein, the involvement of [Fe^IV=O]²⁺ in Fe²⁺-activated peroxymonosulfate (PMS), peroxydisulfate (PDS), and peracetic acid (PAA) AOPs was carefully investigated, while the mechanism of PMSO₂ formation and the reliability of the PMSO probe method were comprehensively evaluated. H₂¹⁸O isotope-labeling confirmed that solvent-water-derived Fe-O species (such as [Fe(H₂O)₆]^2+/3+ and their hydrolysis intermediates) did not serve as oxygen donors for PMSO₂ formation. Instead, all the transferred oxygen atoms were shown to originate from the corresponding peroxides, particularly from the distal oxygen atoms with unsaturated electronic structures. Notably, PMSO₂ was readily generated in various iron-free peroxide activation systems, and molecular orbital of high-valent iron complexes combined with thermodynamic analysis confirmed a thermodynamically unfavorable conversion of [(H₂O)₅-Fe^IV=O]²⁺ from [(H₂O)₆-Feᴵᴵ]²⁺ with a Gibbs free energy of 230.9 kJ/mol. These findings challenged the selectivity and reliability of PMSO as a ferryl-oxo probe and demonstrated that PMSO₂ formation alone could not be regarded as definitive evidence for [Fe^IV=O]²⁺ involvement. Accordingly, the widespread occurrence of ferryl-oxo species in Fenton and Fenton-like AOPs should be critically reassessed.