Further insights into the combination of permanganate and peroxymonosulfate as an advanced oxidation process for destruction of aqueous organic contaminants

Recent studies have reported a novel advanced oxidation process (AOP)by combining permanganate (KMnO₄)and peroxymonosulfate (PMS)for destruction of organic contaminants (i.e., acid orange 7, trichloroethylene, and benzene), where hydroxyl (^•OH)and sulfate radicals (SO₄^•−)are proposed to be generated from PMS activation by amorphous manganese dioxide (MnO₂)formed in situ from KMnO₄ reduction. In this work, appreciable degradation of p-chlorobenzoic acid (p-CBA)was confirmed in KMnO₄/PMS system, while KMnO₄ or PMS alone showed inert reactivity toward p-CBA. Moreover, it was found that pre-synthesized amorphous MnO₂ showed invalid PMS activation for p-CBA degradation, and pre-addition of inorganic or organic reducing agents to promote the formation of amorphous MnO₂ showed negligible influence on p-CBA degradation as well. In these regards, a tentative mechanism for PMS activation by KMnO₄ rather than its product MnO₂ was proposed, involving the substitution of oxo atoms of KMnO₄ by peroxo groups, subsequent reductive generation of peroxomanganese (VI)complexes, and intramolecular disproportionation of these complexes to generate radicals. Efficient degradation of p-CBA was achieved at acid or basic conditions with a maximum rate occurring at pH 3. The coexisting chloride anions showed suppressive effect on p-CBA degradation for scavenging SO₄^•− and ^•OH, while metal ions accelerated the degradation of p-CBA, possibly due to the cation bridging function between negatively-charged MnO₄⁻ and HSO₅⁻. Hydroxylated intermediates of p-CBA were identified in KMnO₄/PMS system. This work improved the fundamental understanding of a new class of AOPs by combining KMnO₄ and PMS for environmental decontamination.