Biocrystal-encased manganese ferrite coupling with peroxydisulfate: Synergistic mechanism of adsorption and catalysis towards tetracycline removal

Biocrystal-encased manganese ferrite (BC@MnFe2O4), an adsorption/catalytic bifunctional catalyst, was synthesized for peroxydisulfate (PDS) activation and tetracycline (TC) removal. Under the optimal conditions (PDS 6.0 mM, pH 7.0, dosage 0.30 g L−1, and TC content 20.0 mg L−1), the TC removal efficiency reached 85.11% and maintained 79.07% after four cycles. Furthermore, inorganic anions (H2PO4− > HCO3− > NO3− > SO42− > Cl−) and natural organic matter (HA) inhibited TC elimination to varying degrees. Compared with the MnFe2O4 nanorods, BC@MnFe2O4 effectively alleviated the side effects caused by particle aggregation and reduced metal ion leaching. Adsorption kinetic and isotherm results exhibited that the adsorption process of TC on MnFe2O4 and BC@MnFe2O4 were better fitted with the pseudo-second-order and Langmuir models. The analyses of quenching experiment, electron paramagnetic resonance, and X-ray photoelectron spectroscopy demonstrated that Fe and Mn sites exposed on the BC@MnFe2O4 served as the catalytic centers for PDS activation, and OH, SO4−, and 1O2 were crucial reactive oxidant species contributing to the rapid TC degradation. Based on the determination of TC degradation intermediates, three possible degradation pathways were proposed, and TC was converted to small molecules via hydroxylation, demethylation, deamidation, dehydration, and other reactions. This study offers novel insights into the synergistic effect of adsorption and catalysis by inorganic-biological hybrid design, and gives inspiration for the construction of high-performance catalysts in organic wastewater treatment.