Peroxymonosulfate activation by cobalt(II) for degradation of organic contaminants via high-valent cobalt-oxo and radical species

The reaction between Co(II) and PMS is an appealing advanced oxidation process (AOP), where multiple reactive oxidizing species (ROS) including high-valent cobalt-oxo [Co(IV)], sulfate radical (SO4•−), and hydroxy radical (•OH) are intertwined together for degrading pollutants. However, the relative contribution of various ROS and the influences of nontarget matrix constituents, on the degradation process are still unclear and yet to be answered. In this study, we confirmed the generation Co(IV) as dominant intermediate oxidant at acid medium by using methyl phenyl sulfoxide (PMSO) as a probe compound. Using chemical scavenging methods, the role of SO4•− and •OH was also identified, and the major ROS were converted from Co(IV) to radical species with the increase of PMS/Co(II) molar ratio as well as pH value. In addition, we found that their contributions to the abatement of organic contaminants are highly dependent on both their available amount and substrate-specific reactivity. Generally, organic substrates with low ionization potential (IP) are prone to react with Co(IV). More interestingly, in contrast to radical-based oxidation, Co(IV) exhibited the great resistance to humic acid (HA) and background ions. This study might shed new light on the PMS activation by cobalt(II) for degradation of organic contaminants.