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.

This study demonstrates the capability of noble metal nanoparticles immobilized on Al2O3 or TiO2 support to effectively activate peroxymonosulfate (PMS) and degrade select organic compounds in water. The noble metals outperformed a benchmark PMS activator such as Co(2+) (water-soluble) for PMS activation and organic compound degradation at acidic pH and showed the comparable activation capacity at neutral pH. The efficiency was found to depend on the type of noble metal (following the order of Pd > Pt ≈ Au ≫ Ag), the amount of noble metal deposited onto the support, solution pH, and the type of target organic substrate. In contrast to common PMS-activated oxidation processes that involve sulfate radical as a main oxidant, the organic compound degradation kinetics were not affected by sulfate radical scavengers and exhibited substrate dependency that resembled the PMS activated by carbon nanotubes. The results presented herein suggest that noble metals can mediate electron transfer from organic compounds to PMS to achieve persulfate-driven oxidation, rather than through reductive conversion of PMS to reactive sulfate radical.

This study elucidates the mechanism behind persulfate activation by exploring the role of various oxyanions (e.g., peroxymonosulfate, periodate, and peracetate) in two activation systems utilizing iron nanoparticle (nFe) as the reducing agent and single-wall carbon nanotubes (CNTs) as electron transfer mediators. Since the tested oxyanions serve as both electron acceptors and radical precursors in most cases, oxidative degradation of organics was achievable through one-electron reduction of oxyanions on nFe (leading to radical-induced oxidation) and electron transfer mediation from organics to oxyanions on CNTs (leading to oxidative decomposition involving no radical formation). A distinction between degradative reaction mechanisms of the nFe/oxyanion and CNT/oxyanion systems was made in terms of the oxyanion consumption efficacy, radical scavenging effect, and EPR spectral analysis. Statistical study of substrate-specificity and product distribution implied that the reaction route induced on nFe varies depending on the oxyanion (i.e., oxyanion-derived radical), whereas the similar reaction pathway initiates organic oxidation in the CNT/oxyanion system irrespective of the oxyanion type. Chronoamperometric measurements further confirmed electron transfer from organics to oxyanions in the presence of CNTs, which was not observed when applying nFe instead.

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This study is the first to demonstrate the capability of Cl − to markedly accelerate organic oxidation using thermally activated peroxymonosulfate (PMS) under acidic conditions. The treatment efficiency gain allowed heat-activated PMS to surpass heat-activated peroxydisulfate (PDS). During thermal PMS activation at excess Cl − , accelerated oxidation of 4-chlorophenol (susceptible to oxidation by hypochlorous acid (HOCl)) was observed along with significant degradation of benzoic acid and ClO 3 − occurrence, which involved oxidants with low substrate specificity. This indicated that heat facilitated HOCl formation via nucleophilic Cl − addition to PMS and enabled free chlorine conversion into less selective oxidizing radicals. HOCl acted as a key intermediate in the major oxidant transition based on temperaturedependent variation in HOCl concentration profiles, kinetically retarded organic oxidation upon NH 4 + addition, and enabled rapid organic oxidation in heated PMS/HOCl mixtures. Chlorine atom that formed via the one-electron oxidation of Cl − by the sulfate radical served as the primary oxidant and was involved in hydroxyl radical production. This was corroborated by the quenching effects of alcohols and bicarbonates, reactivity toward multiple organics, and electron paramagnetic resonance spectral features. PMS outperformed PDS in degrading benzoic acid during thermal activation operated in reverse osmosis concentrate, which was in conflict with the wellestablished superiority of heat-activated PDS.