New Insights into the Combination of Permanganate and Hydrogen Peroxide as a Novel Oxidation Process for Enhanced Removal of Organic Contaminants

Hydrogen peroxide (H₂O₂) has recently been reported as a novel activator for enhancing permanganate (Mn(VII)) oxidation, where the removal of trace organic contaminants (TrOCs) was mainly ascribed to the contribution of Mn(VI). This study reassessed the performance and mechanism of the Mn(VII)/H₂O₂ process for TrOC removal and identified the role of diverse potential reactive species (i.e., intermediate manganese species and radicals). The maximum removal efficiency of organics by the Mn(VII)/H₂O₂ process was achieved at a [H₂O₂]/[Mn(VII)] ratio of 1, and acidic pH conditions were conducive to the reactions. UV–vis and XPS spectra suggested the in situ formation of MnO₂ in the Mn(VII)/H₂O₂ process. Degradation of structurally diverse TrOCs by the Mn(VII)/H₂O₂ process and Mn(VII)/MnO₂ process exhibited similar selectivity and pH-dependence, implying that the in situ-formed MnO₂ should play an important role in catalyzing Mn(VII) oxidation toward TrOCs in the Mn(VII)/H₂O₂ process. In addition, the degradation kinetics of TrOCs in the Mn(VII)/H₂O₂ process were generally higher in phosphate buffer than those in borate buffer, which was attributed to the contribution of Mn(III)–phosphate complexes. The formation of Mn(III) in the presence of ligands (e.g., phosphate, pyrophosphate) was proved via UV–vis spectra and ligand concentration experiments. Based on results from the methyl phenyl sulfoxide (PMSO) probe and metal ion effect experiments, the possible involvement of high-valent manganese species (i.e., Mn(V)/Mn(VI)) was ruled out. Moreover, the results of radical quenching experiments indicated the negligible contribution of O₂^•– and ^•OH. Findings in this study advance the mechanistic understanding of a novel oxidation process by combining Mn(VII) with H₂O₂ for environmental decontamination.