Study On The Preparation And Performance Of High Performance Thin-Film Composite Nanofiltration Membrane With Cellulose Nanofiber Interlayer

One of the key pursuits in the development of pressure-driven membrane technology is flux.In the case of the same operating pressure,a higher permeation means higher efficiency and lower energy consumption,which can reduce the operation cost effectively.Compared with reverse osmosis（RO）membranes,nanofiltration（NF）membranes,which work in large dependence of pore size and Donnan exclusion,show great advantages including higher flux and better multivalence ion selectivity while keep a molecular weight cut-off between 200-1000Da.These characteristics make NF membranes possess great superiorities in seawater desalination,wastewater treatment,food industry and so on.At present,the NF membrane is mainly composed of porous support layer at the bottom and ultra-thin separation layer on the surface which synthesized from the polymerization of diamine in the water phase and acyl chloride in the oil phase.The performance of the NF membrane is determined by the ultra-thin separation layer,whose quality and thickness associate with the interface polymerization process.The commonly used porous support layer mainly adopts commercialized NF membrane such as polyethersulfone ultrafiltration membrane and polyvinylidene fluoride ultrafiltration membrane.These porous support layers have poor hydrophilicity and uneven pore size distribution,making it difficult to obtain uniform water/oil interface and good interface reaction.Thus,the structure of ultra-thin separation layer is poorly controlled and the separation performance is not distinguished.In recent years,more and more attention has been paid to the development of a new type of porous supporting layer or the improvement of the existing supporting layer by introducing an interlayer,which is becoming a research hotspot in this field.Ideally,the bottom or middle layer should have excellent hydrophilicity,flat and uniform surface structure,and can provide the highest porosity and appropriate effective pore size.In this thesis,we propose to use natural polymer bacterial cellulose nanofiber as interlayer to prepare thin-film-composite nanofiltration membrane.Bacterial cellulose nanofibers have an outgoing hydrophilic property and low environmental toxicity.Especially,bacterial cellulose nanofibers can form a porous network with a pore size at nanometer scale,which can provide a better interface for the reaction of diamine and acyl chloride,subsequently optimizing the structure,improving the quality of active layer,and finally obtaining thin-film composite nanofiltration membrane with higher separation performance.The main contents of this thesis are as follows:1.Preparation of bacterial cellulose nanofibers（BCNs）.The bulk bacterial cellulose fiber was used as raw material.Firstly,the impurities,amorphous area and cellulose fiber with low crystallinity were removed after chemical oxidation,and then the oxidized bacterial cellulose was dispersed in deionized water through ultrasonic to prepare uniform and transparent cellulose nanofiber dispersion.Finally,the nanofiber dispersion was deposited on the hydrophilic polytetrafluoroethylene（PTFE）microfiltration membrane to form composite substrate.The substrate has advantages of good hydrophilicity,smooth surface,high porosity and uniform pore size distribution.2.Preparation of thin film composite nanofiltration（TFC NF）membrane with cellulose nanofibers interlayer.The BCNs/PTFE composite membrane was soaked in piperazine aqueous solution,then the excess solution on the surface of the substrate was removed,after that the substrate was immersed in the oil solution of acyl chloride.the Thin film composite nanofiltration membrane with polyamide separation layer was obtained by interfacial polymerization at the two-phase interface.Because of its good hydrophilic property,the water layer of BCNs interlayer can be strengthened.The strengthened hydration layer provides a continuous and even interface for interface polymerization of PIP and TMC,which improve the quality of PA separation layer.Meanwhile,the reaction heat accumulation causes Marangoni convection,leading to crumples of the separation layer and increases the active area of the separation layer.Therefore,the flux of nanofiltration membrane can be greatly increased while the rejection of divalent ions is not changed.At last,the rejection rate of the TFC NF for Na₂SO₄ is about 99%and the performance is around 40 Lh^-1m^-2bar^-1.3.Preparation of Ca²⁺doped TFC NF membrane.Ca Cl₂ was introduced into the polymerization system to further improve the separation performance of TFC NF membrane with cellulose nanofibers interlayer.Ca²⁺can combine with C=O in TMC molecule to increase the hydrolysis rate of TMC,so that the crosslinking degree of polyamide layer decreases,thus the separation layer becomes looser and the pore diameter of separation layer becomes larger.With BCNs working as interlayer,the flux of the TFC NF membrane with Ca²⁺dropping is significantly improved compared with the TFC NF membrane without Ca Cl₂.The flux of Ca²⁺doped TFC NF membrane achieved 74.4 Lh^-1m^-2bar^-1 and the rejection of Na₂SO₄ is 93.1%.