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 ([FeIV=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 [FeIV=O]2+ in Fenton and Fenton-like AOPs has hitherto been overlooked, potentially leading to misinterpretation of reaction mechanisms. Herein, the involvement of [FeIV=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-FeIV=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 [FeIV=O]2+ involvement. Accordingly, the widespread occurrence of ferryl-oxo species in Fenton and Fenton-like AOPs should be critically reassessed.