## 3 Results and Discussion
### Reactive Oxygen Species During PMS Activation
To de  
Figure 8: (**a,b**) High-performance liquid chromatography/electrospray ionization tandem mass spectrometry full-scan chromatogram and the molecular ion mass spectra of PMSO and PMSO2 before and after the catalytic reaction. termine whether high-valent iron oxides were surface-complexed or free in solution, potassium dichromate (K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\)) was used as an electron scavenger. Minimal inhibition of SMX removal (Figure 9a) indicated that the degradation mechanism likely relied on direct electron transfer from the catalyst surface. The structural formula of LDH, [M\({}_{1X}\)\({}^{2+}\)M\({}_{X}\)\({}^{3+}\)(OH)\({}_{2}\)(A\({}_{n}\)\({}^{-}\))\({}_{x}\)/n]\({}^{+}\)\(\cdot\)mH\({}_{2}\)O, suggests that M(III) sites within the layered structure act as catalytic active centers. In conclusion, FeCo-LDH activates PMS by forming high-valent iron-oxo species, which are crucial intermediates for SMX degradation. Chronoamperometric studies confirmed this electron transfer mechanism. No current response was observed upon adding SMX alone, but introducing PMS induced a sharp current increase, signifying electron transfer from surface M\({}^{3+}\) sites to PMS. Subsequently, SMX addition reversed the current direction, indicating electron transfer from SMX to the catalyst, thus driving its degradation [60]. The variation of zeta potential in acidic, neutral, and alkaline solutions is influenced by both the pH of the solution and the charge characteristics of the colloidal particle surface. Under acidic conditions, i.e., a low pH environment, there is a higher concentration of H\({}^{+}\) in solution, which promotes protonation of surface groups. For example, metal oxides usually have positively charged surfaces with positive zeta potentials when hydroxyl groups (-OH) are present on the surface [61]. Organic substances such as humic acids and fulvic acids are usually negatively charged under neutral to alkaline conditions. This is because their functional groups such as carboxyl and phenolic hydroxyl groups are deprotonated under neutral or alkaline conditions, resulting in the formation of a negative charge [62]. In contrast, in neutral solutions, the concentrations of hydrogen and hydroxide ions are relatively balanced, and the particle surface charge is typically more stable at this pH [63]. We measured the \(\zeta\)-potential of the catalyst at different pH values (Figure 9d) and found that the \(\zeta\)-potential remained around 0 mV in the pH range of 3-9, indicating that the charge on the particle surface remains stable over a wide pH range. This phenomenon is likely due  
Figure 9: (**a**) Effect of SCN\({}^{-}\), 1, 10-phenanthroline and K\({}_{2}\)Cr\({}_{2}\)O\({}_{7}\) on the degradation of SMX by FeCo-LDH/PMS system; (**b**,**c**) Continuous injection of SMX and PMS current reactions on catalysts’ working electrodes; (**d**) Zeta potential of catalyst at different pH. to cobalt hydroxide on the surface of the material, which neutralizes a significant number of hydrogen ions in the solution, driving the solution's pH toward neutrality. Furthermore, based on the unique structure of layered double hydroxides (LDHs), the octahedral sites can readily accommodate Co(II), Co(III), Fe(II), and Fe(III). This structure allows Fe and Co species to reversibly undergo redox reactions while maintaining the overall structure, in addition to neutralizing the pH and generating reactive species [64]. The degradation products of sulfamethoxazole (SMX) at different pH values were examined using LC-MS (Figure S10). The main degradation intermediates identified were TP-187, TP-129, TP-99, and TP-158. Among these, TP-99 and TP-158 were formed through the cleavage of sulfonamide (S-N) bonds (Figure 10). Previous studies have shown that the S-N bond in SMX is particularly susceptible to attack by free radical species such as SO4\({}^{-}\)-, O2\({}^{-}\)-, etc. [65; 66].