Improved Performance Of Thin-film Composite Membrane With PVDF-based Substrate For Forward Osmosis

In virtue of the unparalleled advantages of low energy consumption and low fouling tendency,forward osmosis?FO?has attracted the worldwide attention and is widely applied in various fields such as the wastewater enrichment,sewage treatment and seawater desalination.However,the lack of high-performance FO membrane confines its further implementation to a large extent.Polymer-based thin-film composite?TFC?membrane has been extensively investigated because of its good separation performance,easy fabrication and excellent chemical stability.Nevertheless,most TFC membranes are encountered by many problems,such as the severe internal concentration polarization?ICP?,poor membrane rejection and therefore the unsatisfactory FO performance,due to the low porosity,high tortuosity of the substrate and the nonuniform polyamide?PA?selective layer.To address above problems,the incorporation of hydrophilic polymers and nanomaterials in polyvinylidene fluoride?PVDF?substrate was performed to improve the overall performance of resultant PVDF-based TFC-FO membranes.Moreover,the effects of various additives on the substrate properties,physicochemical properties of PA layer and the improved performance of as-fabricated TFC-FO membrane are systemically explored and discussed in this study.Firstly,a small amount of hydrophilic perfluorosulfonic acid?PFSA?is incorporated into the PVDF substrate to develop the high-performance TFC membrane for FO.Because of the good hydrophilicity,PFSA not only optimizes the substrate morphology and improves the membrane hydrophilicity,but also favors the uniform formation of PA selective layer,resulting in the elevated water flux and the reduced reverse salt flux of as-prepared TFC-FO membrane.Besides,the excellent compatibility between PVDF and PFSA also contributes to the long-term stability of obtained TFC-FO membranes.The resultant PVDF/PFSA TFC-FO membrane shows a great potential in many applications,for instance,the seawater desalination.In addition,the incorporation of multi-walled carbon nanotubes?MWNTs?with intrinsic permeation channels in substrate can improve the water flux of as-prepared TFC membrane obviously without sacrificing the membrane selectivity,resulting in the decreased specific reverse salt flux?J_S/J_V?.The in-situ decoration of MWNTs using tetraethyl orthosilicate?TEOS?as precursor was conducted,resulting in the improved hydrophilicty and the excellent dispersion of synthesized SiO₂@MWNTs in the PVDF substrate.Therefore,the incorporation of SiO₂@MWNTs in substrate not only contributes to the better formation of PA layer,but also alleviates the ICP of resultant TFC membrane significantly,resulting in the remarkably decreased specific reverse salt flux.Moreover,the parallel study of incorporating SiO₂@MWNTs in hydrophilic substrate?PSf?also demonstrates the universality of this method for developing high-performance TFC-FO membranes.Furthermore,the effect of the alignment of MWNTs on FO performance of resultant TFC-FO membrane is further investigated in the follow-up work.The vertical alignment of MWNTs was achieved by incorporating Fe₃O₄@MWNTs in substrate and appling the external magnetic field during the phase separation process.The results show that the vertical alignment of MWNTs in substrate minimizes the ICP impact effectively and therefore contributes to the elevated water flux of modified TFC-FO membranes.The substrate modification not only contributes to the improved FO performance of resultant TFC-FO membranes,but also has a direct influence on the crosslinking degree and the roughness of PA selective layer.Therefore,the fourth work proposes a feasible way to mitigate the ICP impact and optimize the PA structure of as-fabricated TFC-FO membranes simultaneously by incorporating phosphorylated titanium dioxide?p-TiO₂?in PVDF substrate.Superior water flux and better antifouling properties of the resultant PVDF/p-TiO₂ TFC-FO membrane is achieved,taking advantage of the more compact and smoother PA layer as a result of the confined migration of amine monomer during interfacial polymerization?IP?.Therefore,PVDF/p-TiO₂ TFC-FO membrane with improved water flux and low fouling tendency is beneficial for wastewater treatment and desalination applications,especially for the treatment of brackish water with high fouling tendency or under harsh conditions.Additonally,it is worthwhile to note that the feed solutions with high temperature,comlexed composition and wide pH condition are widely confronted in most FO applications.Therefore,it is necessary to design and develop high-performance TFC-FO membrane with excellent stability to meet the requirements.The incorporation of poly?N-isopropyl acrylamide??PNIPAM?in substrate is developed for the purpose of improving the operation efficiency of FO,especially for concentrating those feed solutions with the relatively high temperature.Taking advantage of the reversible shrinkage of PNIPAM microgels in high temperature,the higher porosity and lower tortuosity of membrane substrate are obtained,therefore mitigating the ICP impact and contributing to the larger water flux significantly.