Regulation Of The Selective Layer Structure Of TFC Membranes For Forward Osmosis

The osmotically driven forward osmosis（FO）technology has shown promising application in various fields owing to its potential low energy consumption.However,the lack of high performance FO membrane restricts its practical application in the industry.The aromatic polyamide film composite（TFC）membrane prepared by interfacial polymerization of m-phenylenediamine（MPD）and trimesoyl chloride（TMC）is currently the mainstream type of FO membrane,which however generally suffers from the limited water flux and poor fouling resistance due to the highly crosslinked,rough and relatively hydrophobic selective layer.Regulating the microstructure and physicochemical properties of the selective layer can be effective to overcome the above drawbacks.Therefore,this thesis is mainly focused on tailoring the selective layer properties of TFC membrane via employing suitable monomers for the interfacial polymerization or functional materials for the post surface modification,thereby improving the separation and antifouling performance of TFC membrane.Firstly,an aliphatic diamine N-[3-（trimethoxysilyl）propyl]ethylenediamine（NPED）with the siloxane group is mixed with MPD for the interfacial polymerization.The high solubility of NPED in the organic phase benefits for the interfacial polymerization and the siloxane group can be hydrolyzed into hydrophilic Si-OH groups to improve the membrane hydrophilicity.It can be found that the incorporation of NPED increase the hydrophilic,fractional free volume and reduces the roughness of the selective layer,due to the present soft NPED/TMC segment and NPED hydrolysis.As a result,TFC membrane exhibits the enhanced water flux and antifouling properties but the sacrificed selectivity.To avoid the trade-off between the permeability and selectivity of TFC membrane prepared with the above ordinary aliphatic hydrophilic monomer,a pore-containing compound,the cyclodextrin functionalized with ethylenediamine（CD-EDA）,is then employed as the aqueous monomer to fabricate a novel nanofiltration-like TFC membrane.The mechanism of interfacial polymerization with CD-EDA,and the effects of fabrication conditions as well as the draw solutes on the FO performance of the resulted TFC membrane are studied.In addition,the effects of the CD cavity size and EDA substitution on the physicochemical properties,pore structures,FO performances and fouling resistance of TFC membranes are investigated with three EDA-substituted CDs（α,β,γ-CD-EDA）.Owing to the selective CD cavity and plentiful hydrophilic hydroxyl groups,TFC_CD-EDA membranes exhibit the high water flux,low reverse salt flux and good antifouling ability with the suitable draw solution employed,and therefore show the superiorty in FO application treated with relatively large feed solutes such as the protein enrichment.Employing functional monomers for the interfacial polymerization as above is a convenient and effective approach to regulate the properties of TFC membrane,but the involved significant changes in the structure of the selective layer may result in the permeability-selectivity trade-off or limit the applicability of the resulted TFC membranes.Alternatively,the surface modification can alter the surface properties of TFC membrane with the bulk structure of the original selective layer well-remained.Therefore,the surface of the conventional polyamide TFC membrane is further modified by the layer-by-layer assembly of hydrophilic phytic acid（PA）and antibacterial metal ions(Ag⁺and Cu²⁺).The effects of PA concentration and PA-Ag/Cu assembly layer are studied to optimize the membrane properties,and the water flux and organic/biofouling resistance of TFC membranes can be improved greatly without impairing the selectivity with the deposition of the hydrophilic and antibacterial PA-Ag/Cu complexes.