## 3 Results and Discussion
### Reactive Oxygen Species During PMS Activation  
To confirm the reactive species in the FeCo-LDH/PMS system, scavenging experiments were conducted to evaluate the roles of both radical and non-radical species (Figure 7a). For conventional radicals such as \(\bullet\)OH, SO\({}_{4}\bullet^{-}\), and O\({}_{2}\bullet^{-}\), quenchers including tert-butanol (TBA), methanol (MeOH), and p-benzoquinone (p-BQ) were used. Additionally, TBA and ethanol (EtOH) helped differentiate the high-valent contributions of \(\bullet\)OH and SO\({}_{4}\bullet^{-}\)[49]. For non-radical species, dimethyl sulfoxide (DMSO) and furfuryl alcohol (FFA) were employed as specific quenchers. DMSO selectively quenches iron oxides that degrade SMX [50], while FFA assesses the contribution of singlet oxygen (\({}^{1}\)O\({}_{2}\)). The quenching effects revealed that free radical inhibitors reduced degradation rates modestly (1.7%, 3.5%, and 18.1% for TBA, MeOH, and p-BQ, respectively), while non-radical inhibitors had a more pronounced impact: FFA reduced the rate by 42.3%, and DMSO nearly halted the reaction. These results indicate that high-valent iron oxides were the primary reactive species, accompanied by smaller contributions from \({}^{1}\)O\({}_{2}\) and O\({}_{2}\bullet^{-}\). Figure 6: Metal ion leaching rates across five reactions. Figure 7: (**a**) Effect of free radical scavengers on SMX degradation in FeCo-LDH/PMS system; (**b**–d) EPR mapping in different catalytic systems. Electron spin resonance spectroscopy (ESR) measurements were carried out using 5,5-dimethyl-1-pyrrolidine-N-oxide (DMPO) [51] and 2,2,6,6-tetramethyl-4-piperidone (TEMP) [52] as spin trapping agents to qualitatively and semi-quantitatively assess radical and non-radical species in the FeCo-LDH/PMS reaction system. In aqueous solutions, no DMPO-OH or DMPO-SO4- signals were observed. In contrast, upon addition of FeCo-LDH and PMS, a seven-line spectrum with an intensity pattern of 1:2:1:2:2:1:2:1 was observed, identified as 5,5-dimethylpyrrolidone-2(oxy)-(1) (DMPX). This indicates the presence of strong oxidizing intermediates in the system. These intermediates are most likely high-valent iron species such as Fe(IV) or Fe(V), which are known to play a role in oxidation processes involving iron-based catalysts [53]. In order to further verify the presence of high-valent iron species, we will conduct further quenching experiments on them in the following. Moreover, peaks related to DMPO-O2- in ethanol confirmed the involvement of free radicals in the reaction [54; 55]. TEMP-captured signals of 1O2 revealed higher intensities in the FeCo-LDH/PMS system compared to PMS alone, further supporting the presence of 1O2 during activation [56]. High-valent iron oxides exist in multiple forms, including Fe(V)=O, Fe(IV)-OH, and Fe(IV)=O [57]. The formation of PMSO2 from PMSO via oxygen atom transfer by Fe(IV)=O was used to detect its presence [58]. HPLC-MS chromatograms of standard PMSO and oxidized PMSO in the FeCo-LDH/PMS system (Figure 8a,b) showed selective conversion to PMSO2, with a prominent peak (m/z = 157.03) at 3.13 min. This confirmed Fe(IV)=O as a key intermediate. The molar ratio of PMSO2 formed to PMSO consumed exceeded 85.3% throughout the reaction (Figure S9), indicating that high-valent iron oxides predominantly drove SMX degradation, while 1O2 and O2- contributed through secondary pathways. Further investigation revealed electron transfer between Fe(III) and Co(II), with \(\equiv\)Fe(IV)=O derived directly from Fe(III) rather than via Fe(II) [59]. Quenching experiments using 1,10-phenanthroline and KSCN confirmed this mechanism. While 1,10-phenanthroline had minimal impact on SMX degradation, KSCN significantly inhibited the reaction at higher concentrations, suggesting that SCN- strongly coordinates with Fe(III), preventing the formation of Fe(IV)=O. These findings highlight the central role of Fe(III) active sites in FeCo-LDH during the catalytic process.