Structure And Properties Tailoring Of Thin Film Composite Forward Osmosis Membranes

Fresh water shortage is becoming one considerable global crisis with population bloom and social development.Forward osmosis（FO）process has received intensive attention in recent years due to its great potential as a sustainable solution to seawater desalination and wastewater reuse.Thin-film composite（TFC）FO membranes are the state-of-the-art FO membranes with superior perm-selectivity.However,conventional TFC FO membranes exhibit self-limiting water fluxes due to the presence of severe internal concentration polarization（ICP）induced by the porous supports.Therefore,the rational design and fabrication of porous supports is a key to minimize the ICP problem and thus to improve the performance of FO membranes.（1）Fabrication of PES-CNT-PA TFC FO membranes and optimization of their structure and properties.We proposed a novel strategy for fabricating high-performance FO membrane（PES-CNT-PA）with a three-layer structure by forming a polyamide（PA）selective layer via interfacial polymerization on the top of an interconnected porous carbon nanotube（CNT）network membrane,which was the interlayer coated on a polyethersulfone（PES）substrate via vacuum filtration and spray coating.Results showed that in the AL-DS（e.g.,active layer facing draw solution）orientation using DI water as feed solution and 1 mol/L NaCl as draw solution,the optimal PES-CNT-PA-3 membrane exhibited a water flux of 37.9 L m^-2 h^-1（LMH）,which was about 5.5times higher than that of PES-PA membrane without CNT interlayer.The reverse salt flux of the PES-CNT-PA-3 membrane was lower than 0.5 g m^-2 h^-1（gMH）,which was better than that of the reported TFC FO membranes.The results showed that the structure of the PA could be optimized by tailoring the CNT interlayer.Besides,we also investigated and discussed the impacts of the CNT interlayer on the structural and transport properties of the PES substrate and the PA layer of the fabricated membrane.（2）Fabrication of CNT-PA TFC FO membranes and optimization of their structure and properties.For further optimizing the membrane structure and fabricating high-performance TFC FO membrane with ideal support,the CNT-PA membranes with two-layer structure were first achieved upon the removal of PES support.The optimal CNT-PA-3 membrane dramatically improved their water flux by 4-fold to over 150 LMH in the AL-DS orientation compared with their corresponding PES-CNT-PA membranes with the PES supports.Such high water flux is almost the highest value reported so far under same measurement conditions.The water permeability coefficient（A）of the CNT-PA-3 membrane is 9.22±1.14 LMH bar^-1,which is the highest A-value among the reported FO membranes.The structure parameter（S）of the CNT-PA-3 membrane is 38μm,which is much lower than most of reported results.The fabrication of such a TFC membrane with a highly porous ultrathin CNT support could result in a dramatical increase of effective surface area and permeability of PA layer for water permeation,greatly decreased water transport resistance and structure parameter（S）and thereby rendering a significant decrease in ICP effect.（3）Fabrication of ultrathin PA single membranes and optimization of their structure and properties.To investigate the impact of the structure of PA on transport properties and the ICP of PA layer from TFC FO membranes,we fabricated the PA single membrane through interfacial polymerization on polysulfone ultrafiltration membrane,and then obtained free-standing PA film upon the removal of polysulfone support through solvent dissolving.Results showed that there was a great difference in structure and cross-linking degree between the top and bottom surface of the PA membrane.A rough PA film with structure parameter（S）of 24μm is beneficial to large effective surface area for water transport.However,the hollow leaf-like structures serve as unstirred diffusive boundary layer resulting in a sever ICP at the AL-FS mode.The ICP of PA membrane can be significantly alleviated by simply controling the morphologies and structures of the PA film.The structure parameter（S）of the optimal PA is 14.2μm.For the first time,we revealed the ICP effect of PA film prepared by an in-situ detaching method.By tailoring the morphology and structure of PA film,the ICP effect could be alleviated as expected.The revised concentration polarization model given in this work provided new insights on the ICP effect.Above all,three different FO membranes（PES-CNT-PA,CN-PA and PA film）were developed in this work.With a detailed study of the structural and transport properties of the fabricated FO membranes,water flux,salt rejection and ICP were evaluated to determine the membrane perm-selectivity.Our findings provide new insights,inventive methods and novel materials on the fabrication and optimization of TFC FO membranes and promote the extensive applications of the FO process in desalination and wastewater treatment.