Identification and Transformation of Reactive Species in Peracetic Acid Catalytic Activation Systems Using In Situ Electron Paramagnetic Resonance

Peracetic acid (PAA)-based advanced oxidation processes (AOPs) have attracted significant attention in the water decontamination field due to the production of various reactive species, but accurate identification of such diverse species with different characteristics is challenging. Herein, typical PAA catalytic activation systems were constructed, including metal ion-induced homogeneous and material-induced heterogeneous catalytic activation systems. The formation of ^•OH, alkyl radicals (^•CH₃), alkoxy radicals (CH₃C(═O)OO^• and CH₃C(═O)O^•), and nonradical species (¹O₂) was systematically studied and detected by in situ electron paramagnetic resonance (EPR). In the homogeneous catalytic activation systems, the concentrations and types of generated radicals were dominated by the extranuclear electron configuration in the metal ions, while in the heterogeneous systems, carbon material showed lower efficiency in radical formation but directly produced ¹O₂ through double electron transfer. The activation performance of the photocatalyst was dominated by the reduction ability of photogenerated electrons and the oxidation ability of photogenerated holes. Furthermore, an EPR-based method for quantification of ¹O₂ was proposed by using 2,2,6,6-tetramethylpiperidine (TEMP) or its derivatives, confirming that ¹O₂ was transformed from the self-decomposition of PAA or the Russell reaction of complexed CH₃C(═O)OO^•. This study can greatly advance the understanding of the formation and transformation behaviors of reactive species in PAA-AOP systems.