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)/HO) 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)/HO 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 (Cl) was discovered to play a crucial role in BA degradation, which was largely overlooked in previous reports. Although the reaction rate of Cl with BA (k = 2.0 × 10 M s) 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)/HO 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.