Mechanism and biological toxicity of catalytic degradation of sulfamethoxazole using MgO-Fe₂O₃/acid-modified zeolite

In order to avoid catalyst loss and enhance the treatment efficiency of refractory organic wastewater, MgO-Fe₂O₃/acid-modified zeolite catalyst (MFZ) was prepared by the impregnation method. The structure and composition of the catalyst were analyzed by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), respectively. The effects of ozone concentration, initial solution pH, and catalyst dosage on the removal efficiency and reaction rate of sulfamethoxazole (SMX) were studied, and a reaction kinetics model was established. The degradation pathways and reaction mechanisms of SMX wastewater were explored using catalytic ozonation of MFZ, and the phyto-toxicity of MFZ catalyst leachate was evaluated. The results showed that MFZ retained the highly mesoporous framework structure of natural zeolite, and the specific surface area, pore volume, and pore size were 12.670 m²/g, 0.044 cm³/g, and 4.012 nm, respectively. MgO-Fe₂O₃ was uniformly loaded on the zeolite surface, and the abundant oxygen vacancies and surface hydroxyl groups could promote the decomposition of O₃ to produce ·OH. The degradation of SMX using catalytic ozonation of MFZ followed a second-order reaction kinetics model. The initial pH of the solution had the greatest impact on the removal efficiency and reaction rate of SMX. The highest SMX removal efficiency of 83% and a corresponding reaction rate of 0.131 min⁻¹ were achieved. The degradation process of SMX in the O₃+MFZ system consisted of direct oxidation by O₃ and indirect oxidation by ·OH, and ·OH played a major role. The decomposition of SMX was mainly achieved through amino oxidation and hydroxylation, as well as the breaking of sulfur-nitrogen bonds and nitrogen-oxygen bonds on the heterocyclic ring, resulting in mineralization into small molecules such as carbon dioxide and water. The average root length and germination index of Chinese cabbage cultured in the leachate of MFZ increased by 10.3% and 17.0%, respectively, compared to those in the water, which could promote the germination and growth of Chinese cabbage.