Magnetic Fe3O4–N-doped carbon sphere composite for tetracycline degradation by enhancing catalytic activity for peroxymonosulfate: A dominant non-radical mechanism

Abstract The design of sustainable, effective and recyclable hybrid catalysts for advanced oxidation processes is highly significant for remediation of the water environment. In this study, we synthesized magnetic Fe3O4–N-doped carbon sphere composite catalysts (Fe3O4-NCS-x) for efficient removal of tetracycline by activating peroxymonosulfate (PMS). The Fe3O4-NCS-x composite was obtained by facile hydrothermal treatment of chitosan-iron complexes followed by pyrolysis. The unique structure of N-doped carbon spheres embedded in Fe3O4 nanoparticles intensified the electron transport, consequently improving the catalytic activity via a synergistic effect. Factors influencing the catalytic activity of the Fe3O4-NCS-2 were systematically investigated. High degradation efficiency of TC—97.1% within 1 h—was achieved in this Fe3O4-NCS-2/PMS system under the optimum conditions (C0 = 20 mg L−1, catalyst dosage 0.2 g L−1, PMS concentration 2.4 mM, native pH and 25 °C). The inhibitory effect of anions in the water matrix decreased in the order Cl− > NO3− > SO42− > CH3COO− > HCO3−. The obtained results from the competitive quenching tests and electron paramagnetic resonance measurements demonstrated that singlet oxygen (1O2), a non-radical species, plays a major role in TC degradation. It is estimated that 1O2 and hydroxyl radicals (·OH) contributed ∼65.2% and ∼24.2% to TC degradation in the Fe3O4-NCS-2/PMS system, respectively. The M-H hysteresis loop of Fe3O4-NCS-2 revealed that its saturation moment is 56 emu g−1. Magnetic responsive behavior and consecutive runs confirmed that Fe3O4-NCS-2 possesses remarkable separation performance and desirable reusability. This novel magnetic Fe3O4-NCS-2 composite activator for PMS promises great potential in TC degradation.