Reaction of the Peroxyacetyl Radical with Dissolved Organic Matter: Kinetics, Mechanism, and Impacts on Contaminant Degradation

The peroxyacetyl radical CH3C(O)O2•, PAR is a highly selective oxidant that plays a critical role in peracetic acid (PAA)-based advanced oxidation processes (AOPs). The kinetic and mechanisms of PAR reactions with dissolved organic matter (DOM), a major sink and reactive matrix in natural waters, have been poorly understood. In this study, we quantitatively characterized the reactivity of PAR toward diverse DOM isolates using a competitive kinetic method. The second-order rate constants between PAR and DOM isolates (kPAR,DOM) were determined, ranging from (4.0 ± 0.2) × 101 to (4.0 ± 0.2) × 102 L mgC-1 s-1 at pH 7.3. Our results demonstrated that phenolic moieties are the dominant PAR-reactive sites in DOM. The measured kPAR,DOM values correlated strongly with bulk DOM properties, including the phenolic content, electron-donating capacity (EDC), and optical E2/E3, enabling empirical prediction of DOM reactivity toward PAR. Beyond quenching, DOM exhibited dual effects on PAR-mediated contaminant oxidation. Anilines or phenols with an electron-drawing group (e.g., -CN and -Cl) were inhibited through back reduction of their radical cations by DOM, while those with an electron-donating group (e.g., -CH3) were promoted through secondary oxidation by reactive DOM-derived intermediates (RIsDOM). These findings establish the first comprehensive kinetic framework for PAR-DOM reactions in aquatic environments.