Unraveling Different Reaction Characteristics of Alkoxy Radicals in a Co(II)-Activated Peracetic Acid System Based on Dynamic Analysis of Electron Distribution

Peracetic acid (PAA)-based advanced oxidation processes (AOPs) have shown broad application prospects in organic wastewater treatment. Alkoxy radicals including CH₃COO^• and CH₃COOO^• are primary reactive species in PAA-AOP systems; however, their reaction mechanism on attacking organic pollutants still remains controversial. In this study, a Co(II)/PAA homogeneous AOP system at neutral pH was constructed to generate these two alkoxy radicals, and their different reaction mechanisms with a typical emerging contaminant (sulfacetamide) were explored. Dynamic electron distribution analysis was applied to deeply reveal the radical-meditated reaction mechanism based on molecular orbital analysis. Results indicate that hydrogen atom abstraction is the most favorable route for both CH₃COO^• and CH₃COOO^• attacking sulfacetamide. However, both radicals cannot react with sulfacetamide via the radical adduct formation route. Interestingly, the single-electron transfer reaction is only favorable for CH₃COO^• due to its lower E_SUMO. In comparison, CH₃COOO^• can react with sulfacetamide via a similar radical self-sacrificing bimolecular nucleophilic substitution (S_N2) route owing to its high E_SOMO and easy escape of unpaired electrons from n orbitals of O atoms in the peroxy bond. These findings can significantly improve the knowledge of reactivity of CH₃COO^• and CH₃COOO^• on attacking organic pollutants at the molecular orbital level.