Preparation,Characterization And Application Of UiO-66-NH₂/PVDF Composite Membranes

The rapid development of industrialization promotes economic growth and the progress of human society,but it also leads to the increasingly serious consumption of water resources and pollution of water environment.The treatment of wastewater containing dyes and heavy metals and desalination of brine by membrane separation technology are one of the effective methods to protect water bodies and alleviate the shortage of water resources.In this work,different types of UiO-66-NH₂composite membranes were prepared by direct blending method,in-situ deposition self-assembly process and interfacial polymerization with polyvinylidene fluoride（PVDF）as the base membrane.The intrinsic structure,hydrophilicity,permeability and retention properties of the membranes were gradually improved,and their applications in the separation of various pollutants were studied.In this paper,UiO-66-NH₂was blended into PVDF,and the traditional dry preparation method of mixed matrix membranes（MMMs）was optimized through the proposed wet blending process.A series of PVDF-based UiO-66-NH₂MMMs were prepared by nonsolvent induced phase separation（NIPS）method,and the different methods were compared.The crystal structure of UiO-66-NH₂was characterized by XRD,and the chemical composition and microstructure of the membranes were characterized by fourier transform infrared（FTIR）spectroscopy,scanning electron microscopy（SEM）.The results show that the prepared UiO-66-NH₂has good crystal structure and nano-sized particle size,and was successfully blended into PVDF.The abundant hydrophilic amino functional groups and porous structure of UiO-66-NH₂effectively improved the surface hydrophilicity and permeability of the membrane.In addition,when the mass fraction of filler was the same,the M_Wwhich was prepared by wet blending improved the dispersion uniformity of filler and reduced the generation of defect-free structure by avoiding agglomeration.By comparison,M_W10（M_Wwith filler mass fraction of 10%）membrane has the best comprehensive performance and finger hole structure.The dye separation performance of different membranes was tested by Rhodamine（Rh）and Congo red（CR）.The adsorption performance of the membrane for dyes was proportional to the amount of UiO-66-NH₂.At the same time,the adsorption capacity of all membranes for Rh was greater than that of CR,which was related to the better adsorption performance of UiO-66-NH₂filler for Rh.In the long-term interception,the retention rates of Rh and CR of M_W10 were 91.40%and99.20%at 5 minutes,and decreased to 33.09%and 91.33%at 60 minutes,respectively.This indicates that the MMMs has excellent separation performance for both dyes,but it has better stable long-term retention ability for CR,which is related to the different molecular size and retention mechanism of the two dyes.The results showed that the dye separation performance of the PVDF membrane can be significantly improved by blending UiO-66-NH₂,and the improved wet blending process is more worthy of consideration for the preparation of MMMs.As the low utilization rate of UiO-66-NH₂in MMMs,MOFs was considered to load on the surface layer of the membrane.Amphiphilic PVDF-g-PAA（Pg A）was obtained by grafting acrylic acid onto PVDF,and which was prepared into substrate membrane by NIPS.Then,with the assistance of tannic acid（TA）,two different UiO-66-NH₂composite membranes were in-situ prepared by stepwise deposition or co-deposition.The chemical composition,morphology and surface properties of the membranes were characterized by FTIR,SEM and water contact angle test equipment.After PAA modification,the increased carboxyl active sites on the membrane surface are conducive to further modification and coordination assembly.TA can not only be used for anchoring adhesion of UiO-66-NH₂,but also significantly improved the size screening effect of surface hydrophilicity and pore size through hydrophilic functional groups,while UiO-66-NH₂used its inherent porous structure to improve the membrane water permeability.The prepared composite membrane was used to the adsorption and interception of Mo（VI）ions to evaluate the removal effect of heavy metal ions pollutants.The amino group of UiO-66-NH₂could effectively adsorb and separate Mo（VI）by protonation,while the pore structure provided a rich space for adsorption,and the optimum p H of this process is 4.Compared with the co-deposition method,step-by-step deposition can better realize the synergistic enhancement of the two on the membrane separation performance.Further,the TFN membrane with highly selective PA layer was constructed by interfacial polymerization of PIP and TMC,which improved the structural stability of the membrane and enhanced the separating ability of the membrane.The PA top layer can not only improved the structural stability of the membrane,but also used its dense structure to regulate the pore size of the membrane to improve the selectivity of size effect.The chemical composition,morphology,structure,roughness and surface hydrophilicity of the membrane were characterized by FTIR,SEM,AFM and water contact angle test equipment.Through the reasonable design of TA and UiO-66-NH₂embedding layer,the PA layer with thinner polymer layer thickness and smoother surface can be obtained,and the hydrophilicity and permeability of the membrane were improved.The contact angle decreases from 87.6°of TFC to 66.1°～72.7°,and the permeability of the membrane increases by 1.84 times at most.Single heavy metal aqueous solution and heavy metal ion aqueous solution containing different inorganic salts were used as simulated wastewater.Through heavy metal separation,inorganic salt nanofiltration and salt-containing heavy metal separation experiments,it can be seen that the membrane has excellent nanofiltration performance and heavy metal treatment ability.