Unraveling the Overlooked Involvement of High-Valent Cobalt-Oxo Species Generated from the Cobalt(II)-Activated Peroxymonosulfate Process.

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 high-valent 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.