Research On Preparation And Separation Property Of Functionalized Halloysite Nanotubes/Polyvinylidene Fluoride Blend Membranes

Polyvinylidene fluoride?PVDF?polymer has gradually become one of the ideal materials for membrane separation and widely used in basic research and practical applications,due to its excellent chemical and physical characteristics.However,owing to the low surface energy of the PVDF material,the hydrophilicity of PVDF membrane is very poor,which makes it easy to be polluted by contaminants in the water during its practical application,and thus will decrease the separation performance of membrane,shorten the self life of membrane and increase the costs of water treatment eventually.Hence,it is necessary to enhance the hydrophilicity of PVDF membrane to improve its permeability and anti-fouling property in membrane based separation process.In recent years,it has been a hot topic for membrane separation,that researchers investigate the organic/inorganic composite membrane by blending inorganic nanoparticles?NPs?into PVDF casting solution.Several literature reports indicated that the blending modification of NPs including titanium dioxide?TiO₂?,aluminium oxide?Al2O3?and so on could help to improve the microstructure of PVDF membrane dramatically,enhance the hydrophilicity,permeability and anti-fouling property.However,there are not much reports on the modification of polymer membrane blending by halloysite nanotubes?HNTs?.And to the best of our knowledge,investigation on HNTs/PVDF blend membrane has not been reported so far.Therefore,a systematical investigation on the effects of functionalized HNTs on the structure and performances of PVDF membrane was carried out for the first time.Different types of organic/inorganic blend ultrafiltration membranes and blend nanofiltration membranes were prepared and their performances in the oil/water separation and dye removal from wastewater were tested in detail,respectively.In this article,different contents of pure HNTs were blended into PVDF casting solution and the HNTs/PVDF blend membranes were prepared by non-solvent phase inversion method?NIPS?so as to investigate the changes of membranes structure and performances.The pore size and porosity of membrane were calculated by the gravimetric method and Guerout-Elford-Ferry equation.The changes in surface morphology,cross-sectional structure and roughness were investiged by scanning electron microscope?SEM?and atomic force microscope?AFM?,respectively.The hydrophilicity of membrane surface was characterized by the static contact angle measurement instrument.The dead-end filtration device was used to investigate the permeation flux,rejection ratio and anti-fouling property of the membrane.The results indicated that the blending of HNTs helped to enhance the pore size and porosity of the membrane,but the overall structure of the membrane was not destroyed by HNTs.With increasing HNTs concentrations,the contact angle of PVDF membrane was formed to decrease indicating the enhancement in hydrophilicity.Pure water flux?PWF?,bovine serum albumin?BSA?and anti-fouling property of membrane were also formed to be improved.However,aggregation behavior was observed in presence of excess HNTs content resulting deterioration in membrane performances.In summary,the structure and performances of PVDF membranes reached the best at optimized HNTs content?3%based on PVDF weight?.TiO₂-HNTs hybrid materials were synthesized by loading TiO₂ on the surface of HNTs,and then the novel TiO₂-HNTs/PVDF blend ultrafiltration membranes were prepared after adding TiO₂-HNTs into PVDF casting solution.Compared to HNTs/PVDF and pure PVDF membranes,TiO₂-HNTs/PVDF membrane showed a larger pore size and a rougher surface with the same contents of additives.The PWF reached as high as 354.4 L�m-2�h-1 at the pressure of 0.1 MPa,and the other two types of membranes were 192.7 and 97.1 L�m-2�h-1,respectively.TiO₂-HNTs/PVDF membranes still showed an excellent anti-fouling property.However,a higher pore size caused the decrease in BSA rejection,which was just 69.8%.Moreover,in order to improve the dispersion of HNTs in the PVDF membrane,HNTs were functionally modified with coupling agent,3-aminopropyltriethoxysilane?APTES?.Then a series of novel APTES-HNTs/PVDF blend membranes were prepared by NIPS,and their performances in oil-water separation were investigated.The composition,surface morphology and properties of HNTs before and after modification were analyzed by Fourier transform infrared spectroscopy?FTIR?,X-ray photoelectric spectroscopy?XPS?,transmission electron microscope?TEM?and thermogravimetric analysis?TGA?.Besides,the static dispersion test,energy dispersive spectroscopy?EDS?were used to investige the dispersion of APTES-HNTs in solution and in membrane matrix.The experimental results revealed that the micro-topography,hydrophilicity,permeability and anti-fouling property of APTES-HNTs/PVDF blend ultrafiltration membrane was further improved owing to the enhancement of HNTs dispersion,and the optimized blending concentration was 3%.In oil-water separation experiments,the oil rejection of APTES-HNTs/PVDF blend ultrafiltration membranes for four types of oil-water emulsions were more than 90%,which was slightly higher than that of pure PVDF ultrafiltration membrane.Moreover,the hydrophilic APTES-HNTs contributed to preventing the aggregation of oil droplets on the membrane surface and the blockage of membrane pores,and thus improved the overall oil anti-fouling property of the membrane.After three cycles of fouling and cleaning,the flux recovery rate?FRR?of blend ultrafiltration membrane reached 82.9%.In the practical operation process of oil-water separation,the higher pressure and stirring rate helped to prevent the accumulation of oil and increased the flux of the membrane.Through changing the contents of polymer in casting solution,APTES-HNTs/PVDF nanofiltration membranes were prepared via NIPS method,and the separation performances for dye and heavy metal ions from wastewater were investiged.Similarly,the hydrophilicity and anti-fouling property of nanofiltration membranes were improved significantly after blending APTES-HNTs into PVDF membrane.However,unlike ultrafiltration membranes,the pore size and porosity of nanofiltration membrane did not change.In the process of dye separation,3%APTES-HNTs/PVDF nanofiltration membrane presented a 94.9%rejection ratio of Congo red from aqueous solution,whereas the pure PVDF nanofiltration membrane was only 65.5%.For the removal of heavy metal ions,the pore size of pure PVDF nanofiltration membrane was much bigger than the size of heavy metal ions resulting removal ratio values from 10%to 20%.When heavy metal ions passed through the blend nanofiltration membrane,a large number of heavy metal ions could be adsorbed on APTES-HNTs by chelation,thus the heavy metal ions were removed to much higher extent.After adding 3%contents of NPs,the removal ratio of Cu²⁺,Cd²⁺and Cr6+in the solution by membrane were 47.9%,44.2%and 52.3%,respectively.The novel APTES-HNTs/PVDF blended nanofiltration membrane could separate pollutants with different scales from wastewater via different separation mechanism,which exhibited versatility and presented high value for the practical applications.