# Pilot-Scale Fenton-like System for Wastewater Treatment Using Iron Mud Carbon Catalyst
## 3 Results and Discussion
### Identification of Active Species  
In order to identify the active species that play a major role in the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system, quenching experiments were carried out. COD cannot be determined after the quenching agent is added to the secondary effluent. Therefore, the quenching experiment of methylene blue was carried out in this study. As shown in Figure 7a, when furfuryl alcohol was added, the methylene blue degradation rate was reduced from 91% to 49%. However, the degradation rate did not change significantly after the addition of tertbutanol and trichloromethane. Therefore, the system contains almost no \({}^{\bullet}\)OH and \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\), so \({}^{1}\)O\({}_{2}\) is the main active species in the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system. However, when \({}^{1}\)O\({}_{2}\) in the system is quenched, the degradation rate of methylene blue is still about 50%, so there may be other active species in the system. Furthermore, it was shown that Fe(VI) has oxidative performance at pH = 8 [28]. Similarly, the high-valent iron intermediates (Fe(IV/V)) generated by Fe(VI) activation have been proven to be the dominant active species for the degradation of organic compounds [31]. Therefore, Fe(VI) can be used directly as an oxidant for COD removal.  
Figure 6: (a) Changes in effluent COD and COD removal rate with pH at the pilot scale; (b) removal performance of COD at the pilot scale.  
In order to verify the presence of high-priced metals in the system, PMSO is used as a quenching agent. As shown in Figure 7b, the degradation rate of PMSO reached 47% within 2.5 h, and the yield of PMSO\({}_{2}\) increased by 45%. In the quenching experiment, the decrease in PMSO and the increase in PMSO\({}_{2}\) are relatively obvious, indicating that there are some high-priced metal active species Fe(VI), Fe(V), and (Fe(IV)) in the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system. Figure 7c depicts the concentration profiles of Fe(VI) and Fe(II) over time. During the reaction, Fe(VI) could be converted to other Fe species, with a degradation rate of 80% achieved after 2.5 h. The temporal evolution of Fe(II) concentration was also monitored. The results indicated a rapid initial increase in Fe(II) concentration, reaching 22.61 umol-L\({}^{-1}\), followed by a stabilization phase with an average concentration of 19.76 umol-L\({}^{-1}\). The final effluent contained 21.16 um of Fe\({}^{2+}\) and 19.96 um of Fe(VI), which was the result of both the leaching of iron from the catalyst and the transformation of the oxidant. It was reported that the generation of Fe(II) can be attributed to the reduction in Fe(IV) by H\({}_{2}\)O\({}_{2}\)[28]. Thus, Fe(IV) was proven to exist in the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system via Fe(VI) transformation. Although Fe(II) may react with H\({}_{2}\)O\({}_{2}\) to produce trace amounts of \({}^{\bullet}\)OH, their contribution to the overall reaction is negligible (Figure 7d). Thus, Fe(II) basically does not play a catalytic role. In summary, Fe(IV) and \({}^{1}\)O\({}_{2}\) were the main active species in the FSC/Fe(VI)/H\({}_{2}\)O\({}_{2}\) system. The EPR map can further verify the active species in the system. As shown in Figure 7d, \({}^{1}\)O\({}_{2}\) and Fe(IV) had obvious characteristic peaks, while \({}^{\bullet}\)O\({}_{2}\)\({}^{-}\) has basically no response. In addition, a slight signal peak of \({}^{\bullet}\)OH was also presented.