Phase transformation and catalytic performance of metal-doped MgFe₂O₄ prepared from saprolite laterite

Heterogeneous Fenton-like method has attracted considerable attention because of its potential effectiveness in mineralization of organic contaminants in a wide range of reaction medium pH. Spinel ferrites MFe₂O₄ (M=Fe, Zn, Cu, Ni, Mn, Co) as heterogeneous Fenton-like catalysts have been studied extensively due to their good catalytic activity, prominent physical and chemical stability, and excellent magnetic properties, which allow their easy separation from the reaction medium by magnetic field for further circular utilization. Considering the group of ferrites, limited research focused on the utilization of MgFe₂O₄ as heterogeneous Fenton catalytic agent, whereas most of the catalysts are synthesized by pure chemical reagents. In this study, magnetic multi-metal co-doped MgFe₂O₄ heterogeneous Fenton-like catalyst was synthesized from saprolite laterite by acid leaching-hydrothermal calcination method. The effect of calcination temperature on the phase, morphology, specific surface area, and pore size distribution of samples were characterized by X-ray diffraction, scanning electron microscopy, Fourier-transform infrared, Brunauer-Emmett-Teller/Barrett-Joyner-Halenda analysis. The catalytic activity of the as-prepared products as heterogeneous Fenton catalysts for the degradation of Rhodamine B (RhB) solution was also investigated. The results show that the layered double (multi) hydroxides coupled with a portion of magnesium ferrite are synthesized by hydrothermal method, and the cubic crystal MgFe₂O₄ is obtained by decomposition of the layered double (multi) hydroxides after calcination above 300℃. With the increase in calcining temperature, the crystallinity of the products increases, and the particle size becomes larger. The morphology gradually grows to near spheroidal particles, and the dispersion degree gradually increases. Meanwhile, the pore size becomes larger, and the specific surface area is reduced. Calcination of sample H-C500 exhibits the best catalytic activity for the degradation of RhB after 500℃, achieving 97.8% degradation efficiency of 10 mg·L^-1 RhB after 300 min at the reaction conditions of 45℃, pH 6.44, 0.625 g·L^-1 catalyst dosage, and 1.0% (volume fraction) H₂O₂. The total organic carbon (TOC) removal could reach 77.8%. The reused catalyst can still maintain high activity, and after three consecutive degradation cycles, the reduction of degradation efficiency and TOC removal efficiency are less than 3.0% and 5.0%, respectively.