Enhanced Degradation of Diclofenac by Electro-Activated Permanganate: A Critical Role of In Situ Formed H2O2

Electrochemical technologies and permanganate oxidation are two attractive strategies in water treatment due to their low cost, easy operation, good adaptability, and environmental friendliness. Interestingly, recent studies have demonstrated that the combination of electrolysis and Mn(VII) (E-Mn(VII) process) shows a synergetic effect on organic contaminants degradation. However, the underlying mechanism of the E-Mn(VII) process remains unclear. In this study, the enhanced removal of diclofenac (DCF) by the E-Mn(VII) process was also demonstrated, and the main reactive species were reidentified. It was found that H2O2 was considerably formed via the reduction of O2 on the cathode, and it played a key role in facilitating Mn(VII) transformation into MnO2. Increasing O2 concentration, working potential, and pH value were conducive to H2O2 formation as well as Mn(VII) transformation to MnO2. Higher Mn(VII) dosages did not impact H2O2 formation but also led to faster MnO2 formation. The enhanced formation of MnO2 was favorable for the fast degradation of DCF in the E-Mn(VII) process owing to the catalytic effect of MnO2 on Mn(VII) oxidation. This was confirmed by the observation that (i) the E-Mn(VII) process showed a similar pH-dependent oxidation trend to that of MnO2 catalytic oxidation of Mn(VII) (i.e., the reactivity was much stronger at lower pH); and (ii) the addition of MnO2 prepared ex situ had a significant enhancing effect on DCF oxidation by the E-Mn(VII) process. Moreover, the possible contribution of reactive manganese intermediates such as soluble free Mn(III), Mn(VI), and Mn(V) was ruled out by the results of the probe experiments, UV–visible spectrophotometry, and examining the reactivity of Mn(III) complexes prepared ex situ. Also, the results of radical scavenging experiments indicated the negligible contribution of HO•. Furthermore, for the practical application of the E-Mn(VII) process in water decontamination, suitable operating parameters (e.g., a moderate potential and relatively lower pH) should be determined to avoid the fast loss of Mn(VII) and the electrical energy consumption. Overall, this work brings new insights into the reaction mechanism of the E-Mn(VII) process as well as proposes an efficient, sustainable, and feasible strategy for water and wastewater treatment.