## 3 Investigating Methodologies for Assessing PMS Activation with Copper-Based Catalysts
### Radical Quenching  
When examining the activation of PMS, radical quenching is a crucial methodology used to understand the behavior and reactivity of radical species formed during the reaction. Basically, quenching agents are added at the beginning of the process to intercept and neutralize the generated radicals. If these radicals are present in the system, they will react with the quenching agents and will not participate in the degradation process. In this way, the involvement of the radicals in the degradation can be determined, as clearly shown in Figure 6.  
The detection of the active species is governed by the reaction rate constant (Kobs) between the active species and the quenching agent (Table 3). Considering the radical mechanisms, methanol (MeOH) or ethanol (EtOH) is commonly used as a radical quencher to inhibit the contributions of both \(HO^{\bullet}\) and \(SO_{4}^{\bullet-}\) because of their reactivity with \(SO_{4}^{\bullet-}\)[154; 155]. Apparently, EtOH might be a more suitable quencher than MeOH to assess the relative contribution of \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\), as MeOH exhibits lower reactivity with \(SO_{4}^{\bullet-}\). Additionally, [PERSON] and [PERSON] [156], in their literature review on the application and activation of PMS, also analyze this methodology for the identification of radicals. They explain that the difference in degradation efficiencies in the presence of MeOH or EtOH and TBA reflects the function of sulfate radicals. EtOH is commonly used  
Figure 6: Example of (**a**) degradation spectra when quenching agents are added to determine the presence of the species and the effect they have on (**b**) the kinetic constants of the degradation process. Adapted from [148] copyright 2023 with permission from Elsevier (Amsterdam, The Netherlands). because, in various studies on contaminant degradation, the addition of EtOH showed a significant radical-scavenging effect, while the addition of TBA did not result in a significant quenching effect. This is because the \(\alpha\)-hydrogen of EtOH can interact with \(SO_{4}^{\bullet-}\) to generate \(\bullet\) CHOH, which is capable of decomposing PMS [157]. TBA is usually employed as a radical scavenger to suppress the contribution of \(HO^{\bullet}\) because it reacts faster with \(HO^{\bullet}\) than with \(SO_{4}^{\bullet-}\)[158; 159]. Other chemical scavengers used such as furfuryl alcohol (FFA) [160; 161], sodium azide (\(NaN_{3}\)) [162], and tryptophan [163; 164] are typically adopted to recognize \({}^{1}O_{2}\). Nevertheless, it is important to mention that FFA can undergo a reaction with \(HO^{\bullet}\), and \(NaN_{3}\) can participate in a reaction with both \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\). Therefore, hindrance by FFA or \(NaN_{3}\) can arise from their reactions not only with \({}^{1}O_{2}\) but also with \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\). Thus, the identified hindrance impact of FFA or \(NaN_{3}\) is more significant than that of MeOH or EtOH but does not imply that \({}^{1}O_{2}\) is the predominant reactive species. Other chemical scavengers that react well with \(HO^{\bullet}\) and \(SO_{4}^{\bullet-}\) are employed, such as phenol [165], and potassium iodide is employed to pinpoint \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\) on the surface of materials. Para-benzquinone (p-BQ) is utilized to recognize \(O_{2}^{\bullet-}\)[166] and anisole [167]. In general, if a chemical scavenger is stable, does not interfere with the catalysts, and exhibits significantly higher reactivity with \(HO^{\bullet}\) and \(SO_{4}^{\bullet-}\) in comparison with other oxidants present within the system, it can be utilized to remove or scavenge both \(HO^{\bullet}\) and \(SO_{4}^{\bullet-}\) species. Overall, the combination of EPR/ESR and quenching experiments can effectively elucidate the existing active species, providing robust support for the activation of PMS.