Catalytic ozonation of NH4+-N in wastewater over composite metal oxide catalyst

Large amounts of water containing-ammonium nitrogen (NH₄⁺-N) have attracted increasing attention. Catalytic ozonation technology, involving the generation of hydroxyl radical (·OH) with strong oxidation ability, was originally utilized to degrade organic-containing wastewater. In this paper, Ce/MnO_x composite metal oxide catalysts prepared with different preparation conditions were used to degrade wastewater containing inorganic pollutant (NH₄⁺-N). The as-prepared catalyst features were characterized using X-ray diffraction (XRD), Brunauer–Emmett–Teller method (BET), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and H₂-temperature programmed reduction (H₂-TPR) techniques. The results show that the catalyst, prepared by conditions with precipitant Na₂CO₃ and Ce/Mn molar ratio 1:2 calcined at 400 °C for 3 h in pH 11.0, displays the optimal performance, with the removal rate of NH₄⁺-N and selectivity to gaseous nitrogen, 88.14 wt% and 53.67 wt%, respectively. The effects of several operating factors including solution pH, initial NH₄⁺-N concentrations and scavengers were evaluated. In addition, XRD patterns of catalyst with the best performance and the comparative study on decontamination of NH₄⁺-N by various processes (O₃, catalyst and catalyst/O₃) show that the primary metal oxides are CeO₂ and MnO₂ in Ce/MnO_x composite metal oxide catalysts, which have a synergistic effect on the catalytic ozonation of NH₄⁺-N, and the new phase MnO₂ plays a great role. After 5 consecutive use cycles, the degradation efficiency is declined slightly, and can still achieve better than 70 wt% over 1 h reaction. Additionally, the application of catalytic ozonation for actual wastewater on the removal rate of NH₄⁺-N was investigated. Possible mechanism and degradation pathway of NH₄⁺-N were also proposed. In a word, the application of CeO₂–MnO₂ composite metal oxide catalysts in catalytic ozonation can be regarded as an effective, feasible and promising method for the treatment of NH₄⁺-N.