Revealing the Generation of High-Valent Cobalt Species and Chlorine Dioxide in the CoO-Activated Chlorite Process: Insight into the Proton Enhancement Effect.

A CoO-activated chlorite (CoO/chlorite) process was developed to enable the simultaneous generation of high-valent cobalt species [Co(IV)] and ClO for efficient oxidation of organic contaminants. The formation of Co(IV) in the CoO/chlorite process was demonstrated through phenylmethyl sulfoxide (PMSO) probe and O-isotope-labeling tests. Both experiments and theoretical calculations revealed that chlorite activation involved oxygen atom transfer (OAT) during Co(IV) formation and proton-coupled electron transfer (PCET) in the Co(IV)-mediated ClO generation. Protons not only promoted the generation of Co(IV) and ClO by lowering the energy barrier but also strengthened the resistance of the CoO/chlorite process to coexisting anions, which we termed a proton enhancement effect. Although both Co(IV) and ClO exhibited direct oxidation of contaminants, their contributions varied with pH changes. When pH increased from 3 to 5, the deprotonation of contaminants facilitated the electrophilic attack of ClO, while as pH increased from 5 to 8, Co(IV) gradually became the main contributor to contaminant degradation owing to its higher stability than ClO. Moreover, ClO was transformed into nontoxic Cl rather than ClO after the reaction, thus greatly reducing possible environmental risks. This work described a Co(IV)-involved chlorite activation process for efficient removal of organic contaminants, and a proton enhancement mechanism was revealed.