Catalytic oxidation of methylene blue by using Ni-Fe bimetallic catalyst/NaClO system: Performance, kinetics, mechanism, and DFT calculations

Abstract

Nickel-iron bimetallic catalysts (Ni-Fe/Al2O3) with activated alumina as the carrier were prepared by impregnation pyrolysis, and the structure and morphology of the catalysts were analyzed by characterization via X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, programmed temperature rise reduction, and X-ray photoelectron spectroscopy. The prepared Ni-Fe/Al2O3 was used to catalyze sodium hypochlorite (NaClO) for oxidative degradation of methylene blue (MB). The effects of the dosage of iron nitrate in the preparation process and the catalyst dosage, effective chlorine concentration, pH, temperature, initial MB concentration, and coexisting ions on the MB removal effect in the oxidative degradation experiment were investigated. Then, the oxidative degradation mechanism and practical application prospects of the system were examined. Results showed that the Ni and Fe nitrate mass ratio of 5:1 was favorable for the dispersion of Ni elements in the catalyst preparation process. When the temperature was 45 °C, the initial concentration of MB was 50 mg/L, the dosage of Ni-Fe/Al2O3 was 20 g/L, the initial effective chlorine concentration was 1.76%, and the initial pH was 4.0. The removal rate of MB could reach 98.98% within 40 min, and the degradation process was in accordance with quasi primary kinetics, with reaction rate constant of 0.0873 min−1. The experimental result of free radical quenching and electron paramagnetic resonance indicated that the addition of Ni-Fe/Al2O3 induced the decomposition of NaClO to produce a large amount of singlet oxygen (1O2), 1O2 and HOCl worked together to oxidatively degrade MB. Ni3+/Ni2+ redox cycles play an important role in activating HOCl to produce active species. The common inorganic anions (Cl−, NO3−, and SO42−) did not inhibit the removal of MB significantly, whereas humic acid showed a more significant inhibitory effect only at concentrations greater than 20 mg/L. In the five recycling experiments, the catalysts showed low metal leaching rate and high MB removal rate. Ni-Fe/Al2O3 was reusable, and it showed excellent application potential.