Sulfate radical (SO 4•− ) is widely recognized as the predominant species generated from the cobalt(II)-activated peroxymonosulfate (PMS) process. However, in this study, it was surprisingly found that methyl phenyl sulfoxide (PMSO) was readily oxidized to the corresponding sulfone (PMSO 2 ) with a transformation ratio of ∼100% under acidic conditions, which strongly implied the generation of highvalent cobalt-oxo species [Co(IV)] instead of SO 4•− in the Co(II)/PMS process. Scavenging experiments using methanol (MeOH), tert-butyl alcohol, and dimethyl sulfoxide further suggested the negligible role of SO 4•− and hydroxyl radical ( • OH) but favored the generation of Co(IV). By employing 18 O isotope-labeling technique, the formation of Co(IV) was conclusively verified and the oxygen atom exchange reaction between Co(IV) and H 2 O was revealed. Density functional theory calculation determined that the formation of Co(IV) was thermodynamically favorable than that of SO 4•− and • OH in the Co(II)/PMS process. The generated Co(IV) species was indicated to be highly reactive due to the existence of oxo-wall and capable of oxidizing the organic pollutant that is rather recalcitrant to SO 4•− attack, for example, nitrobenzene. Additionally, the degradation intermediates of sulfamethoxazole (SMX) in the Co(II)/PMS process under acidic conditions were identified to further understand the interaction between Co(IV) and the representative contaminant. The developed kinetic model successfully simulated PMSO loss, PMSO 2 production, SMX degradation, and/or PMS decomposition under varying conditions, which further supported the proposed mechanism. This study might shed new light on the Co(II)/PMS process.• OH-mediated pathways (eqs e and f). 15−17

Potassium periodate (PI, KIO 4 ) was readily activated by Fe(II) under acidic conditions, resulting in the enhanced abatement of organic contaminants in 2 min, with the decay ratios of the selected pollutants even outnumbered those in the Fe(II)/ peroxymonosulfate and Fe(II)/peroxydisulfate processes under identical conditions. Both 18 O isotope labeling techniques using methyl phenyl sulfoxide (PMSO) as the substrate and Xray absorption near-edge structure spectroscopy provided conclusive evidences for the generation of high-valent iron−oxo species (Fe(IV)) in the Fe(II)/PI process. Density functional theory calculations determined that the reaction of Fe(II) with PI followed the formation of a hydrogen bonding complex between Fe(H 2 O) 62+ and IO 4 (H 2 O) − , ligand exchange, and oxygen atom transfer, consequently generating Fe(IV) species. More interestingly, the unexpected detection of 18 O-labeled hydroxylated PMSO not only favored the simultaneous generation of • OH but also demonstrated that • OH was indirectly produced through the self-decay of Fe(IV) to form H 2 O 2 and the subsequent Fenton reaction. In addition, IO 4− was not transformed into the undesired iodine species (i.e., HOI, I 2 , and I 3 − ) but was converted to nontoxic iodate (IO 3 − ). This study proposed an efficient and environmental friendly process for the rapid removal of emerging contaminants and enriched the understandings on the evolution mechanism of • OH in Fe(IV)-mediated processes.

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