Study On Surface Modification And Pollution Resistance Of PVDF-CTFE Membran

Facing the wastewater system with complex composition and high pollution levels,membrane fouling has always been an urgent problem to be solved.There is an urgent need to develop an inexpensive and eco-friendly polymer membrane with excellent anti-fouling and easy-cleaning properties.As a membrane-forming material with excellent comprehensive properties,PVDF has been widely used and studied in the field of membranes.However,its application and development are severely restricted by the problems of difficult modification and a high degree of side reaction.Herein,PVDF-CTFE was used as the membrane material,and the following aspects were studied:1)In this paper,PVDF-CTFE membranes were positively charged under mild conditions by using small molecular polyamines.Firstly,the reaction mechanism was analyzed by the XPS method,and then the reaction conditions were investigated and optimized.The results showed that the degree of in-situ grafting substitution reaction based on the C-Cl active sites was high,and the positive-charged effect of the modified membrane was excellent,the water contact angle decreased by 23.9%,and the mechanical properties were well maintained.After 7 cycles,the flux recovery rate of ultrafiltration cationic polyacrylamide aqueous solution was still above 97.5%,and the flux decay rate was kept below 20%,showing excellent resistance to positive-charged pollutants.2)Herein,a PVDF-CTFE membrane with cleaning dual p H-responsiveness was successfully fabricated via a facile and eco-friendly method.The isoelectric point of zeta potential of the modified membrane appeared at p H=4.31.It exhibited positive-charged and the water uptake rate reached 427%under acidic cleaning conditions,while it showed negative-charged with the water intake rate reaching 492%under alkaline cleaning conditions,showing an excellent dual p H-responsiveness.Meanwhile,the modified membrane displayed an excellent oil-fouling resistance in the treatment of hexadecane/water mixed emulsion,with the water flux decay rate（FDR）only 17.5%after 7 cycles of operation,far lower than that of the original membrane 76.2%,and the water flux recovery rate（FRR）remained above 99.7%after alkali cleaning.Notably,the membrane M-D-IA also had excellent dual p H-responsiveness when treating typical polluted wastewater HA/Ca CO₃ feed solution.The FRR of the modified membrane after washing with alkali and acid solution in turn was as high as 98.3%,which is much higher than that of the original membrane（38.6%）,indicating that the membrane not only possessed a significant resistance to organic/inorganic pollution but also had an outstanding dual p H-responsiveness during the cleaning process.Moreover,the long-term storage experiment was carried out to investigate the influence of bacterial growth on membrane performance,and it was found that the water flux of the membrane was maintained at 850 L m^-2 h^-1 during 30 days of storage,displaying an excellent resistance to biological pollution.3)Based on the concept of fluorine-free,non-toxic,environmental-friendly and low-cost,the discontinuous silicon-island structures were constructed by introducing Si O₂ nanoparticles on the super-hydrophilic layer of the PVDF-CTFE membrane modified by itaconic acid,and then hydrophobically treated with polydimethylsiloxane（PDMS）.The experimental results exhibited that the surface of the modified membrane reached the hydrophobic and underwater super-oleophobic state with the water contact angle（WCA）and the underwater oil contact angle（OCA）were 124°and 151°,respectively,and the water flux is still maintained at this time.Moreover,the water flux decay rate（FDR）remained below 19.6%when the modified membrane treated soybean oil/water emulsion for 6 cycles,which was far lower than that of the original membrane by 93.9%,presenting an excellent anti-fouling performance.Additionally,the water flux recovery rate（FRR）was over 99.3%after alkali cleaning,indicating the modified membrane possessed excellent cleaning p H-responsiveness.The above analysis exhibited that the synergistic effect of hydrophilic micro-domains and oleophobic micro-domains of the modified membrane surface not only slows down the attachment of hydrophobic pollutants but also promotes the separation of hydrophobic pollutants during chemical cleaning,which greatly improves its anti-fouling and easy-cleaning performance.