Distinct pathways for superoxide radical generation induced by Mn and Cu-based catalysts in electro-Fenton like process

Abstract

Superoxide radicals (·O₂⁻) has been regarded as one of the reactive oxygen species (ROS) for the elimination of complex contaminants via electro-Fenton like (EF-like) technology. However, the generation path of ·O₂⁻ is diverse, and the influence of the physicochemical properties of metals on the mechanism of ·O₂⁻ conversion is significant in the EF-like treatment of wastewater. Herein, metals (M = Mn, Cu) loaded zeolitic imidazolate frameworks catalytic materials (M–NC) were prepared for sulfamethoxazole (SMX) removal to analyze the effect of metals on the pathways of ·O₂⁻ generation. The removal kinetic rate of SMX by Cu–NC was 1.32 times higher than that of Mn–NC. Quenching experiments demonstrated that ·O₂⁻ is the most important oxidizing species to achieve SMX removal. The RRDE measurements and quantitative experiment on the concentration of H₂O₂ experiments indicated that Mn–NC was more inclined to generate ROS through activation of H₂O₂ and Cu–NC through other ways. Therefore, the transformation pathways of ·O₂⁻ in different catalytic systems were thoroughly analyzed. Electron paramagnetic resonance test and reactive oxygen species quenching experiments indicated that the pathway for ·O₂⁻ production of Mn–NC was O₂ → H₂O₂ → ·O₂⁻, and that of Cu–NC was O₂ → ·O₂⁻. The strategy of using Mn and Cu-based catalysts to investigate the mechanism of the ·O₂⁻ generation pathway provided a way to efficiently utilize the conversion of ·O₂⁻.