Different reaction mechanisms of SO4•-and •OH with organic compound interpreted at molecular orbital level in Co(II)/peroxymonosulfate catalytic activation system

Hydroxyl radical (center dot OH) and sulfate radical (SO4 center dot-) produced in advanced oxidation processes (AOPs) have been widely studied for organic contaminants degradation, however, the different radical characteristics and reaction mechanisms on organics degradation are still needed. In this study, a homogeneous Co(II)/peroxymonosulfate activation system was established for caffeine (CAF) degradation, and pH was controlled to regulate the radicals production. The different attack routes driven by SO4 center dot- and center dot OH were deeply explored by transformation products (TPs) identification and theoretical calculations. Specifically, a method on dynamic electronic structure analysis of reactants (R), transition state (TS) and intermediates (IMs) during reaction was proposed, which was applied to elucidate the underlying mechanism of CAF oxidation by center dot OH and SO4 center dot- at the molecular orbital level. In total, SO4 center dot- is kinetically more likely to attack CAF than center dot OH due to its higher oxidation potential and electrophilicity index. Single electron transfer reaction (SET) is only favorable for SO4 center dot-due to its higher electron affinity than center dot OH, while only center dot OH can react with CAF via hydrogen atom abstraction (HAA) route. Radical adduct formation (RAF) is the most favorable route for both center dot OH and SO4 center dot- attack according to both kinetics and thermodynamics results. These findings can significantly promote the understanding on the degradation mechanism of organic pollutants driven by center dot OH and SO4 center dot- in AOPs.

Automated summary

Hydroxyl radical (center dot OH) and sulfate radical (SO4 center dot-) have been widely studied in advanced oxidation processes (AOPs) for organic pollutants degradation. In this study, a Co(II)/peroxymonosulfate activation system was established to degrade caffeine (CAF), and the different attack routes of SO4 center dot- and center dot OH were explored. It was found that SO4 center dot- is more likely to attack CAF kinetically, while only center dot OH can react via hydrogen atom abstraction (HAA) route. Radical adduct formation (RAF) is the most favorable route for both center dot OH and SO4 center dot- attack. These findings provide insights into the degradation mechanism of organic pollutants in AOPs driven by center dot OH and SO4 center dot-.