Overlooked Role of Coexistent Hydrogen Peroxide in Activated Peracetic Acid by Cu(II) for Enhanced Oxidation of Organic Contaminants

Cu(II)-catalyzed peracetic acid (PAA) processes have shown significant potential to remove contaminants in water treatment. Nevertheless, the role of coexistent H₂O₂ in the transformation from Cu(II) to Cu(I) remained contentious. Herein, with the Cu(II)/PAA process as an example, the respective roles of PAA and H₂O₂ on the Cu(II)/Cu(I) cycling were comprehensively investigated over the pH range of 7.0-10.5. Contrary to previous studies, it was surprisingly found that the coexistent deprotonated H₂O₂ (HO₂^-), instead of PAA, was crucial for accelerating the transformation from Cu(II) to Cu(I) (k_HO2-/Cu(II) = (0.17-1) × 10⁶ M^-1 s^-1, k_PAA/Cu(II) < 2.33 ± 0.3 M^-1 s^-1). Subsequently, the formed Cu(I) preferentially reacted with PAA (k_PAA/Cu(I) = (5.84 ± 0.17) × 10² M^-1 s^-1), rather than H₂O₂ (k_H2O2/Cu(I) = (5.00 ± 0.2) × 10¹ M^-1 s^-1), generating reactive species to oxidize organic contaminants. With naproxen as the target pollutant, the proposed synergistic role of H₂O₂ and PAA was found to be highly dependent on the solution pH with weakly alkaline conditions being more conducive to naproxen degradation. Overall, this study systematically investigated the overlooked but crucial role of coexistent H₂O₂ in the Cu(II)/PAA process, which might provide valuable insights for better understanding the underlying mechanism in Cu-catalyzed PAA processes.