Chloride ions accelerated the removal of naproxen in Cu(II)/peracetic acid system: Promotion of copper cycling through the formation of copper-chloride complexes.

It is well acknowledged that the reduction of Cu(II) to Cu(I) is the primary rate-limiting step in the Cu(II)/peracetic acid (PAA) system, significantly hindering its broader application. In this study, we found that chloride ion (Cl-) could complex with copper ion to form copper-chloride complexes (Cu(II)-Cln), promoting the reduction of Cu(II) to Cu(I) by coexisting H2O2 (kCu(II)-Cln/H2O2 = ∼70 M-1 s-1vs kCu(II)/H2O2 = <1 M-1 s-1), ultimately enhancing the oxidative capability of the Cu(II)/PAA system. The Cl-/Cu(II)/PAA system exhibited rapid removal of naproxen (NPX) across a wide pH range of 7.5-9.0 and achieved a 139.5-fold improvement in NPX removal compared to the Cu(II)/PAA system at pH 8. Results of the quenching experiments, UV-Vis spectroscopy, and electron paramagnetic resonance indicated that acetylperoxy radicals (CH3C(O)OO•), Cu(III) and hydroxyl radicals (•OH) participated in the removal of NPX in the Cl-/Cu(II)/PAA system, while CH3C(O)OO• was identified as the primary reactive species responsible for NPX removal. A significant reduction in acute toxicity was achieved by the Cl-/Cu(II)/PAA treatment, concurrent with the proposal of three potential NPX degradation pathways. In addition, the Cl-/Cu(II)/PAA system was remarkably resistant to common water matrices. In summary, this study presents a simple yet viable strategy to enhance the oxidizing capacity of the Cu(II)/PAA system across a wide pH range, demonstrating its potential applicability in treating high-salinity wastewater.