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

Abstract

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.