Insights into the oxidation of organic contaminants by Co(II) activated peracetic acid: The overlooked role of high-valent cobalt-oxo species

The combination of Co(II) and peracetic acid (PAA) is a promising advanced oxidation process for the abatement of refractory organic contaminants, and acetylperoxy (CH3CO3•) and acetoxyl (CH3CO2•) radicals are generally recognized as the dominant and selective intermediate oxidants. However, the role of high-valent cobalt-oxo species [Co(IV)] have been overlooked. Herein, we confirmed that Co(II)/PAA reaction enables the generation of Co(IV) at acidic conditions based on multiple lines of evidences, including methyl phenyl sulfoxide (PMSO)-based probe experiments, 18O isotope-labeling technique, and in situ Raman spectroscopy. In-depth investigation reveals that the PAA oxidation mechanism is strongly pH dependent. The elevation of solution pH could induce major oxidants converting from Co(IV) to oxygen-centered radicals (i.e., CH3CO3• and CH3CO2•). The presence of H2O2 competitively consumes both Co(IV) and reactive radicals generated from Co(II)/PAA process, and thus, leading to an undesirably decline in catalytic performance. Additionally, as a highly reactive and selective oxidant, Co(IV) reacts readily with organic substances bearing electron-rich groups, and efficiently attenuating their biological toxicity. Our findings enrich the fundamental understanding of Co(II) and PAA reaction and will be useful for the application of Co(IV)-mediated processes.