Synthesis Of Magnetic Aluminum-based Metal Organic Frameworks And Their Adsorption Performance For Tetracyclines

Antibiotics are a new class of pollutants and the extensive use of antibiotics in human and veterinary medicines worldwide has resulted in significant amounts of these compounds in wastewater.Most antibiotics are difficult for the digestive tracts of humans and animals to completely degrade,which has led to their discharge and accumulation in both surface and coastal waters through human and animal excretions.The accumulation of residual antibiotics in water can promote the evolution and development of drug-resistant bacteria,and eventually enter the human body with the food chain,causing harm to human health.Therefore,it is urgent to find an effective way to remove it from water.Adsorption is considered to be a promising method due to its excellent removal efficiency,low cost and the lack of toxic by-products.The successful synthesis of a water-stable adsorbent with a high adsorption capacity for antibiotics has been a major focus of adsorption research.In this study,a novel magnetic adsorbent,Fe₃O₄@MIL-68（Al）,was prepared and used to explore its TCs adsorption performance.The adsorbent was separated easily by external magnets and still exhibited high adsorption capacity and good stability for TCs solution.Most importantly,it is save costs and improve the recycling performance of materials and easy to implement industrial applications.Therefore,Fe₃O₄@MIL-68（Al）were suggested to be an excellent adsorbent and show great potential for the removal of TCs from aqueous solutions.This study started from the preparation of magnetic aluminum-based metal organic framework adsorbents with good stability and liquid phase separation performance,regarded the removal of the representative antibiotic TCs pollutants in water as the application goal,explored the preparation methods and techniques of Fe₃O₄@MIL-68（Al）,and then obtained magnetic aluminum-based metal organic framework adsorbent with good adsorption properties,separation and regeneration performance and structure stability.The magnetic materials were thoroughly characterized by multiple techniques.Based on the characterization results,the optimal loading of magnetic Fe₃O₄nanoparticles was determined.Then,minocycline（MC）and tetracycline（TC）were chosen as adsorbates,and used to investigate the adsorption behavior of MC and TC onto Fe₃O₄@MIL-68（Al）adsorbent,and the adsorption research mainly included influencing factors,adsorption model and adsorption characteristics.The main research contents and conclusions are as follows:（1）XRD results show that the characteristic peaks of Fe₃O₄@MIL-68（Al）contains the characteristic peaks of Fe₃O₄and MIL-68（Al）,and the diffraction peaks are similar to MIL-68（Al）,indicating that the addition of Fe₃O₄nanoparticles does not damage the original structure of MIL-68（Al）.FTIR results showed that compared with MIL-68（Al）,the peak at3706 cm^-1almost completely disappeared,likely because the O-H onμ₂-hydroxo groups reacted with the carboxyl groups on Fe₃O₄during the reaction,which weakened the O-H signal.SEM results showed that Fe₃O₄@MIL-68（Al）retained rod-like crystals and the average particle size decreased as the Fe₃O₄content increased.N₂adsorption-desorption results showed that the mesopore volumes increased in the Fe₃O₄@MIL-68（Al）composites after the introduction of Fe₃O₄and the optimal Fe₃O₄loading was 5%.VSM results showed that the saturation magnetization value was 15.82 emu·g^-1,which was sufficient to use an external magnetic field to separate the composite from an aqueous solution.（2）The amount of adsorption increased with the increase of initial concentration;the adsorption equilibrium time for MIL-68（Al）was 22 h,while just 3 h was enough for Fe₃O₄@MIL-68（Al）composites and it also increased the equilibrium adsorption capacity;the magnetic composites showed high MC and TC adsorption amounts and good stability within a broad p H range;the ionic strength conditions has an adsorption process minimal inhibitory effect;the equilibrium adsorption amount of MC and TC increased with the temperature.（3）Adsorption kinetics experiments showed that the pseudo-second-order model more suitably describe the adsorption process of MC and TC onto Fe₃O₄@MIL-68（Al）composites.This suggests that the rate-limiting step may be chemisorption.Compared with MIL-68（Al）,Fe₃O₄@MIL-68（Al）has greater adsorption strength and faster adsorption rate for TC and MC.Adsorption isotherm experiments showed that the adsorption isotherms conformed to the Freundlich equation.This suggests that the surface of adsorbent is heterogeneous and a large number of adsorption sites were present on the surface of adsorbents.Fe₃O₄@MIL-68（Al）showed a maximum MC and TC adsorption of 248.05 mg·g^-1and 179.02 mg·g^-1at 25℃(C_o=60 mg·L^-1).（4）The adsorption characteristics results exhibit that Fe₃O₄@MIL-68（Al）samples had good water stability and regenerability,and maintained about 85%of the initial saturated adsorption capacity after 3 cycles of regeneration.Al-N and Fe-N covalent bond,hydrogen bond andπ-πstacking together contributed to MC and TC adsorption on Fe₃O₄@MIL-68（Al）.