In-situ encapsulation synthesis of Fe-SAPO-34 for efficient removal of tetracycline via peroxydisulfate activation: Highly dispersed active sites and ultra-low iron leaching

Abstract

The agglomeration and leaching of metal species are critical issues for iron-based catalysts to remove tetracycline (TC) through peroxydisulfate (PDS) activation. In this work, iron-oxide species confined in cha cage of silicon aluminum phosphate zeolite (Fe-SAPO-34) was successfully synthesized by in-situ encapsulation method to address the above challenges. In Fe-SAPO-34/PDS system, approximately 92.6% TC was removed within 150 min at experimental conditions: [pH] = 3, [TC] = 20 mg/L, [Catalyst] = 0.10 g/L, [PDS] = 1 mM. Meanwhile, the biggest degradation rate constant is 0.03689 min−1, which surpasses the counterpart systems. This result originates from the well dispersion of Fe-oxide species in Fe-SAPO-34, providing more active sites for TC degradation. As expected, the Fe-oxide species leaching concentration is ultra-low (0.018 mg/L) in Fe-SAPO-34/PDS system, which is attributed that SAPO-34 has a confinement effect of cha cage and eight-ring pore openings to limit its leaching. The radical quenching tests and electron paramagnetic resonance (EPR) technology verify singlet oxygen (1O2) is the primary reactive oxygen species (ROS) in Fe-SAPO-34/PDS system. Furthermore, two possible pathways for TC degradation are proposed in line with the intermediate identified by liquid chromatography-mass spectrometry (LC-MS).