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

• Published in: Environmental science & technology 2020-12, Vol.54 (24), p.16231-16239
• Main Authors: Zong, Yang, Guan, Xiaohong, Xu, Jun, Feng, Yong, Mao, Yunfeng, Xu, Longqian, Chu, Huaqiang, Wu, Deli
• Format: Article
• Language: English
• Subjects: Butanol; Cobalt; Contaminants; Degradation; Density functional theory; Dimethyl sulfoxide; Hydroxyl Radical; Hydroxyl radicals; Intermediates; Nitrobenzene; Oxidation; Oxidation-Reduction; Peroxides; Pollutants; Scavenging; Species; Sulfamethoxazole; Sulfoxides; Treatment and Resource Recovery
• ISSN: 0013-936X; 1520-5851
• DOI: 10.1021/acs.est.0c06808

Sulfate radical (SO4 •–) 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 (PMSO2) with a transformation ratio of ∼100% under acidic conditions, which strongly implied the generation of high-valent cobalt-oxo species [Co­(IV)] instead of SO4 •– in the Co­(II)/PMS process. Scavenging experiments using methanol (MeOH), tert-butyl alcohol, and dimethyl sulfoxide further suggested the negligible role of SO4 •– and hydroxyl radical (•OH) but favored the generation of Co­(IV). By employing 18O isotope-labeling technique, the formation of Co­(IV) was conclusively verified and the oxygen atom exchange reaction between Co­(IV) and H2O was revealed. Density functional theory calculation determined that the formation of Co­(IV) was thermodynamically favorable than that of SO4 •– 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 SO4 •– 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, PMSO2 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.