Preparation of FeMn-ZSM-5/MOR composite molecular sieves: combination of adsorption/oxidation and Fenton-like reaction.

In this paper, FeMn-ZSM-5/MOR(FeMn-Z/M) catalyst was synthesized through in-situ crystallization and wet impregnation method and used as heterogeneous Fenton-like catalyst for degradation of indigo carmine (IC) in aqueous solution. The catalysts were characterized using XRD, BET, SEM, EDS, FT-IR and XPS techniques. The experimental results showed that Fe and Mn were successfully doped onto the ZSM-5/MOR(Z/M) surface. The combination of ZSM-5 and mordenite (MOR) not only enhanced the adsorption capacity of contaminant, but also had more active sites. Degradation experiments and the valence variations indicated that Fe and Mn had a significant synergistic effect. The results of IC degradation experiments under different conditions showed that the optimum conditions (FeMn mass ratio = 1:2, FeMn-Z/M = 25 mg, H₂O₂ = 0.5 ml, pH = 3), the degradation rate of IC reaches 99% in 20 min. Reaction analyses showed that catalyst follow a first-order reaction kinetic. The degradation of IC could be divided into two stages: adsorption/oxidation (After adsorption, oxidative degradation begins) and Fenton-like reaction. Free radicals quenching studies analyzed verified that the dominant free radicals within FeMn-Z/M Fenton-like system were hydroxyl radical (·HO) and hydro-peroxy radical (·HO₂), ·HO₂ was the main free radical in the adsorption/oxidation stage. The possible catalytic mechanism was proposed. Finally, the synthesized FeMn-Z/M catalyst had better performance than other IC removal methods, and had good degradation efficiency and high reusability after multiple runs. [ABSTRACT FROM AUTHOR]