Preparation Of Porous Magnetic Chitosan Microspheres As An Efficient Fenton-like Catalyst And Its Degradation To Tetracycline Antibiotics

In recent years,antibiotics have been widely used in the pharmaceutical industry due to their broad-spectrum resistance characteristics.In particular,tetracycline antibiotics have become the second most used antibiotics due to their huge production and usage world-widely.Fenton technology is a common and effective method to remove antibiotics in wastewater.However,it also has many fatal disadvantages,such as iron sludge,high iron consumption,acidic p H and so on.Therefore,it is particularly important to explore a novel Fenton-like catalyst with high activity,good stability and wide range of p H applications.In this study,tetracycline hydrochloride（TC）was targetted as the model pollutant.Porous magnetic chitosan microspheres with millimeter size（3–4 mm）were synthesized by using chitosan bio-based materials to support Fe₃O₄ NPs.The efficiency and mechanism of Fenton-like degradation of tetracycline antibiotics were studied,and a Fenton-like reactor suitable for porous magnetic chitosan microspheres was preliminarily designed.The main content and discussion are as follows:1)Fe₃O₄-Cs microspheres were synthesized by in-situ co-precipitation method and two-step method,and the catalytic activity of the two microspheres to tetracycline hydrochloride was compared.The results showed that in-situ co-precipitation microspheres had significantly better activity.The reason may be that Fe₃O₄ NPs in the in-situ co-precipitation microspheres had a smaller particle size（5.97 nm）and could be evenly dispersed on the surface of the microspheres,thus resulting in more active sites,while the Fe₃O₄ NPs in the two-step microspheres had a particle size of180.37 nm and some of the active sites were lost by chitosan inclusion in the synthesis process.2)Many techniques,including XRD,FT-IR,SEM,TEM,XPS,TG,BET and other techniques were used to characterized Fe₃O₄-Cs microspheres and confirmed that the porous structure of the microspheres,strong stability,large specific surface area,morphology and structure.The reaction conditions were optimized with the removal rate of tetracycline hydrochloride as the index.The results showed that more than96.0%tetracycline hydrochloride could be removed within 20 min under the optimal reaction conditions.The high degradability of the other tetracycline antibiotics:oxytetracycline（OTC）and doxycycline（DOTC）further confirmed its3)universality.The reaction kinetics analysis of different degradation processes showed that the system conformed to the pseudo first-order kinetics model.4)Six recycling experiments were carried out on the catalyst,which proved that the catalyst has high stability and reusability.Four degradation paths were preliminarily simulated by HPLC-MS.The results showed that the main degradation mode of TC was ring opening reaction and functional group separation.Through free radical capture experiments and electron paramagnetic resonance capture experiments,it was proved that hydroxyl radical was the main active group in this system,and superoxide radical also contributed a small part.A Fenton-like reactor suitable for Fe₃O₄-Cs microspheres was designed for the large spherical（3–4 mm）structure of the catalyst,and a schematic diagram of industrial mass production was designed.In short,Fe₃O₄NPs distributed on the surface of microspheres uniformly by in-situ co-precipitation method,iron groups on Fe₃O₄ NPs chelated with amino groups on chitosan chain strongly,and the large specific surface area promoted the strong catalytic activity and high stability of Fe₃O₄-Cs microspheres together.