Identifying\nthe Nonradical Mechanism in the Peroxymonosulfate\nActivation Process: Singlet Oxygenation Versus Mediated Electron Transfer

Abstract

Select\npersulfate activation processes were demonstrated to initiate\noxidation not reliant on sulfate radicals, although the underlying\nmechanism has yet to be identified. This study explored singlet oxygenation\nand mediated electron transfer as plausible nonradical mechanisms\nfor organic degradation by carbon nanotube (CNT)-activated peroxymonosulfate\n(PMS). The degradation of furfuryl alcohol (FFA) as a singlet oxygen\n(¹O₂) indicator and the kinetic retardation\nof FFA oxidation in the presence of l-histidine and azide\nas ¹O₂ quenchers apparently supported a role\nof ¹O₂ in the CNT/PMS system. However, the ¹O₂ scavenging effect was ascribed to a rapid PMS\ndepletion by l-histidine and azide. A comparison of CNT/PMS\nand photoexcited Rose Bengal (RB) excluded the possibility of singlet\noxygenation during heterogeneous persulfate activation. In contrast\nto the case of excited RB, solvent exchange (H₂O to D₂O) did not enhance FFA degradation by CNT/PMS and the pH-\nand substrate-dependent reactivity of CNT/PMS did not reflect the\nselective nature of ¹O₂. Alternatively, concomitant\nPMS reduction and trichlorophenol oxidation were achieved when PMS\nand trichlorophenol were physically separated in two chambers using\na conductive vertically aligned CNT membrane. This result suggested\nthat CNT-mediated electron transfer from organics to persulfate was\nprimarily responsible for the nonradical degradative route.