Fabrication And Nanofiltration Performance Of Ultrathin Composite Membranes

Membrane separation technology performs important functions in the separation processes industry including separation,concentration,purification and refinement and continues to grow in significance for solving major problems in waste water treatment,environmental protection and modern energy recovery techniques.Specifically,nanofiltration(NF)membranes with pore diameters ranging from 0.5 to 2 nm are usually suitable for water purification purposes since they exhibit high retention performance for organic materials and can moderately remove inorganic ions.However,the "trade-off"relationship between permeation flux and separation performance is an inherent challenge limiting the industrial application of nanofiltration membrane process.The development of more efficient and cost-effective advanced NF membranes necessitates that the membranes exhibit high permeability in order to minimize processing costs while simultaneously achieving high throughputs.Hence,in an effort to reduce resistance in membrane filtration and enhance the water permeance of the nanofiltration membranes,the strategy in my doctoral research work was to employ an intermediate layer comprising of nanomaterials incorporated over a macroporous support substrate to design and fabricate a variety of high-performance ultrathin nanocomposite nanofiltration membranes for application in fast water purification,dissolved solutes removal and a wide range of molecular separation process.To begin with,a facile approach is presented to prepare ultrafine cellulose nanofibers dispersions.The obtained nanofibers have a diameter of 20±12.5 nm and are homogeneously dispersed in aqueous solution,which was favorable to the fabrication of ultrathin nanoporous membranes on a macroporous support by the vacuum assisted filtration assembly method.The morphology,crystallinity and yield of the obtained cellulose nanofibers were influenced by pre-chemical treatment.The as-fabricated membranes with a controllable membrane thickness show ultrahigh permeation fluxes and good ultrafiltration performances.Typically,the membrane having the thickness of about 500 nm and high porosity of up to 68.5%,displays high water permeabilities of up to 2.75 103 L m-2 h-1 bar-1 and ferritin rejection of 94.3%,signifying potential application for fast size-exclusion/nanoparticles separation.After successfully demonstrating that polymer nanofibers forms ultrathin nanoporous layer on a macroporous support,ultrathin cellulose nanofiber(UCN)membrane was utilized as the intermediate layer to develop ultrathin crosslinked-polyethylenimine(PEI)/UCN composite nanofiltration membrane via the interfacial polymerization approach.The incorporation of the interlayer was anticipated to offer an interconnected nanoporous microstructure that would facilitate improved membrane permeability.This is possible through the creation of a network of tiny directed water channels formed during the growth of the crosslinked-PEI layer within the free spaces of ultrafine cellulose nanofibers and the interfacial gaps formed with the crosslinked polyamide matrix.Subsequently,the conductive transport of water molecules through the interconnected interfaces conduces to high permeation flux.My results revealed that the control of preparation parameters allowed for the fabrication of smooth,hydrophilic and highly permeable nanofiltration membrane.In particular,the crosslinked PEI membrane with a thickness of about 77.4 nm and the cut-off of 824 g mol-1 exhibited high pure water permeability of up to 32.68 L m-2 h-1 bar-1,which is more than 10 times higher than that of previously reported similar nanofiltration membranes.Further long-term separation tests of model synthetic produced water and dye wastewater revealed that the developed nanofiltration membrane offers a potential strategy for fast efficient purification of impaired waters.To illustrate the effect of the mechanical strength of the intermediate layer on the separtion perfomances of composite nanofiltration membrane,we further replaced the cellulose nanofibers with the rigid carbon nanotubes(CNTs)as intermediate layer.Although the cellulose nanofibers nanomaterials convey polymeric compatibility with the crosslinked polyamide selective layer,they are soft and compaction at high pressure filtration could result in the collapse of the membranes pore structure due to the non-rigid polyamide selective layer.Consequently,the continuation of the flux is not sustained during long-term filtration performance.A new asymmetrically structured nanocomposite membrane was constructed through polymer-grafting on carbon nanotube nanomaterials followed by surface inorganic metal functionalization of the selective layer for nanofiltration of heavy metals ions in solution.Crosslinked-PEI was grafted on carboxylated carbon nanotube intermediate layer incorporated on the macroporous cellulose acetate substrate to form the composite membrane.The resulting membrane was inorganically modified via in-situ surface reaction of zinc nitrate with excess ammonia hydroxide to produce the hydrophilic and positively charged membrane.The results showed that the crosslinking enhances the polymer interaction with the carbon nanotube interlayer,which intern endowed mechanical strength and sustains the membrane pore structure during pressure driven filtration.The developed metal surface functionalized positively charged membrane displays outstanding pure water permeability of 16.5±1.3 Lm-2h-1 bar-1 while exhibiting a selective nano filtration performance of bivalent cations.Moreover,the developed membranes exhibited excellent barrier separation capabilities of organic molecules.Furthermore,in order to enhance the interactions between the CNT support layer with polymer matrices,I first demonstrated the preparation of dispersible and stable borate crosslinked polydopamine grafted carbon nanotubes(B-PDA-CNTs)through chemical crosslinking via bioinspired borate chemistry.The excellent water dispersibility and dispersing stability of the obtained B-PDA-CNTs is ascribed to the presence of the borate ions attached on the PDA-CNT surface,which promotes the formation of strong hydrogen bonds with the water molecules in solution.Subsequently,it was possible to fabricate B-PDA-CNT nanoporous membranes with good UF separation performance for protein molecules.In addition,the resultant B-PDA-CNT nanoporous membrane was employed as the intermediate layer to deposit an ultrathin polymer film comprising of an interpenetrating network(IPN)of cross-linked polyvinyl alcohol(PVA)integrated with Zn2+ moieties through coordination with the amino groups of crosslinked PEI to form the composite nanofiltration membrane.The resultant membrane affords good rejection(>85%)of heavy metals ions including Cd,Cu,Zn,Pb and Ni.The positively charged membrane in acidic condition presupposes that the separation process is governed by donnan effect.Meanwhile in comparison to previous reports,a low phosphorus rejection of less than 15%is realized with the application of a relatively low transmembrane pressure suggesting that the developed approach is attractive for low pressure recovery of phosphorus.Taken together,the results from my doctoral research indicate that the appropriate design of nanomaterials incorporated interlayer is conducive to the fabrication of more efficient and cost-effective composite NF membranes with high water flux and good rejection performance of metal ions in water purification,water treatment as well as good application prospect in other fields.