Identifying the Nonradical Mechanism in the Peroxymonosulfate Activation Process: Singlet Oxygenation Versus Mediated Electron Transfer

TLDR

Persulfate activation can trigger oxidation without sulfate radicals, yet the mechanism remains unclear. The study aimed to determine whether singlet oxygenation or mediated electron transfer drives organic degradation in CNT‑activated peroxymonosulfate. Researchers examined singlet oxygenation and electron transfer by testing furfuryl alcohol as a 1O₂ probe, applying quenchers, comparing to Rose Bengal, performing solvent exchange, varying pH and substrate, and using a conductive CNT membrane to physically separate PMS and trichlorophenol. Results showed that while 1O₂ quenchers reduced furfuryl alcohol oxidation, this was mainly due to PMS depletion; Rose Bengal comparison and solvent exchange ruled out singlet oxygenation, and the CNT membrane experiment demonstrated that CNT‑mediated electron transfer from organics to PMS is the dominant nonradical pathway.

Abstract

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