Chloride-mediated enhancement in Cu(II)-catalyzed Fenton-like reaction: The overlooked reactive chlorine species

The practical application of Cu(II)-catalyzed Fenton-like reaction (Cu(II)/H₂O₂) exhibits a low efficiency in the degradation of refractory compounds of wastewater. The impact of chloride ions (Cl⁻) on Fenton-like reactions have been investigated, but the influence mechanism is still unclear. Herein, the presence of Cl⁻ (5 mM) significantly accelerated the degradation of benzoic acid (BA) under neutral conditions. The degradation of BA follows pseudo-first-order kinetics, with a degradation rate 7.3 times higher than the Cu(II)/H₂O₂ system. Multiple evidences strongly demonstrated that this reaction enables the production of reactive chlorine species (RCS) rather than HO^• and high-valent copper (Cu(III)). The kinetic model revealed that Cl⁻ could shift reactive species from the key intermediate (Cu(III)-chloro complexes) to RCS. Dichlorine radicals (Cl₂^•-) was discovered to play a crucial role in BA degradation, which was largely overlooked in previous reports. Although the reaction rate of Cl₂^•- with BA (k = 2.0 × 10⁶ M⁻¹ s⁻¹) is lower than that of other species, its concentration is 10 orders of magnitude higher than that of Cu(III) and HO^•. Furthermore, the exceptional efficacy of the Cu(II)/H₂O₂ system in BA degradation was observed in saline aquatic environments. This work sheds light on the previously unrecognized role of the metal-chloro complexes in production the RCS and water purification.