# Pilot-Scale Fenton-like System for Wastewater Treatment Using Iron Mud Carbon Catalyst
## 3 Results and Discussion
### Catalytic Performance Test Using Sequential Batch Experiments
#### 3.2.2 Optimization of the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) System for COD Removal  
Figure 5 shows the effects of different pH values, total oxidant concentration, and the ratio of H\({}_{2}\)O\({}_{2}\) to Fe(VI) on COD removal in the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system under pilot-scale conditions. The results indicate that after 2.5 h of reaction, there are significant differences in COD removal rates under different pH values (4, 6, and 8) (Figure 5a). Specifically, when pH = 4, the COD removal rate in the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system is only 38%. This is because in an acidic environment, Fe(VI) undergoes acid-catalyzed decomposition, generating Fe(III) and O\({}_{2}\), resulting in a decrease in oxidant concentration and accelerating the decomposition of H\({}_{2}\)O\({}_{2}\), which inhibits the generation of free radicals. When the system is in a slightly neutral environment with pH = 6, the COD removal rate reached its maximum, with the highest value being 57%. When pH increases to 8, the COD removal rate  
Figure 4: (**a**) The adsorption effect of FSC and (**b**) removal efficiency of COD in different reaction systems. The reaction conditions: A: FSC/Fe(VI) system, [Fe(VI)] = 0.9 mmol-L\({}^{-1}\), C\({}_{\text{FSC}}\) = 0.5 g-L\({}^{-1}\), pH = 6, Time = 2.5 h; B: FSC/H\({}_{2}\)O\({}_{2}\) system, [H\({}_{2}\)O\({}_{2}\)] = 0.9 mmol-L\({}^{-1}\), C\({}_{\text{FSC}}\) = 0.5 g-L\({}^{-1}\), pH = 6, Time = 2.5 h; C: Fe(VI)/H\({}_{2}\)O\({}_{2}\) system, [H\({}_{2}\)O\({}_{2}\)] = 0.8 mmol-L\({}^{-1}\), [Fe(VI)] = 0.1 mmol-L\({}^{-1}\), pH = 6, Time = 2.5 h; D: FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system, [H\({}_{2}\)O\({}_{2}\)] = 0.8 mmol-L\({}^{-1}\), [Fe(VI)] = 0.1 mmol-L\({}^{-1}\), C\({}_{\text{FSC}}\) = 0.5 g-L\({}^{-1}\), pH = 6, Time = 2.5 h.  
in the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system is only 13%. The reason is that in an alkaline environment, the protonation degree of organic molecules decreases, weakening their electron transfer efficiency with Fe(VI). In addition, Fe(IV) may be generated in the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system, which is beneficial for the degradation of organic pollutants. As shown in Figure S1, the available Fe(IV) content decreases at pH 4 and 8 compared to pH 6 [27], which further proves that the reaction system at pH 6 promotes COD degradation. The effect of the H\({}_{2}\)O\({}_{2}\) and Fe(VI) concentration ratio on COD removal rate is shown in Figure 5b. As the concentration ratio of H\({}_{2}\)O\({}_{2}\) and Fe(VI) increases from 1:1 to 8:1, the COD removal rate increases from 15.59% to 57%. At a 16:1 H\({}_{2}\)O\({}_{2}\)/Fe(VI) molar ratio, COD removal decreased sharply from 57% to 11.86%. The reduction correlates with H\({}_{2}\)O\({}_{2}\) acting as a reductant. Excess H\({}_{2}\)O\({}_{2}\) intensifies Fe(VI) decomposition through enhanced Fe(IV)/Fe(V) formation. Reaction conditions are optimized to enhance the oxidative activity of Fe(VI) and promote the generation of its reactive intermediates [28]. Figure 5c shows the effect of total oxidant concentration on COD removal rate. When the concentration of total oxidant is 0.563 and 0.844 mmol\(\cdot\)L\({}^{-1}\), the COD removal rate is similar at 25.00% and 28.57%, respectively. The removal rate reaches the highest (57.01%) when the total oxidant concentration is 0.9 mmol\(\cdot\)L\({}^{-1}\). When the concentration of total oxidant is 1.35 M, the removal rate drops to 8.20%, which is due to the rapid reaction of a too high concentration of H\({}_{2}\)O\({}_{2}\) with Fe(VI) to produce O\({}_{2}\) and Fe(II), resulting in a decrease in the concentration of active species [28]. Therefore, when pH = 6, H\({}_{2}\)O\({}_{2}\)/Fe(VI) = 8:1, and with a total concentration of 0.9 mmol\(\cdot\)L\({}^{-1}\), COD can be effectively removed, and excessive consumption of oxidants can be avoided, providing the best reaction conditions for the pilot scale.