ZIFs@MIL-101/urea-derived magnetic FeCo nitrogen-rich carbon as an effective and stable catalyst for peroxymonosulfate activation

High efficiency and stability are key issues in heterogeneous metal catalytic materials/peroxymonosulfate (PMS) systems during wastewater degradation. However, catalytic materials that are difficult to recover and metal leaching can cause secondary contamination of water bodies. Herein, we designed the magnetic FeCo nitrogen-rich carbon (FeCo-NC) obtained by layer-by-layer encapsulation, followed by urea encapsulation of in situ grown MOF-in-MOF and carbonization. The CoFe₂O₄ crystalline nature of FeCo-NC was confirmed by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and selected angle electron diffraction (SAED) results, which is responsible for the magnetic properties of FeCo-NC. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) present that the material's morphology transforms from dodecahedral ZIF-67 to spindle-shaped ZIF-67@MIL-101 and then to octahedral FeCo-NC. Compared with FeCo-C and ZIF-67@MIL-101, the removal efficiency of FeCo-NC was maintained at 78.6 % after 10 consecutive cycles, with only a 17 % loss of raw material. The Fe and Co leaching amounts in the FeCo-NC/PMS system were approximately 0.6 and 0.1 mg/L, respectively, much lower than China's Fe and Co emission standards. FeCo-NC characterization and TC degradation results demonstrated that magnetic CoFe₂O₄ nanoparticles formed on the surface or inside the FeCo-NC could effectively inhibit metal (Fe and Co) loss and be easily regenerated in multiple degradation cycles. The mechanism and degradation routes in the FeCo-NC/PMS/ TC system, including the radical/nonradical pathways, Fe/Co valence change, and electron transfer, were further investigated by electron paramagnetic resonance experiments, electrochemical analyses, in situ Raman spectroscopy, and HPLC–MS experiments.