Preparation And Characterization Of Cellulose Nanofiltration Membranes With Application In The Depth Treatment Of Drinking Water

At present,domestic materials for the preparation of nanofiltration membranes mainly include cellulose acetate,aromatic polyamides,sulfonated polyether sulfone,polytetrafluoroethylene,polyether sulfone,and ceramics.Such polymers and inorganic membranes have large production costs and energy consumption,difficult to biodegrade,cause environmental pollution,do not meet the requirements for continued development.Combining international trends and domestic foundations for the development of major science and technology infrastructure,focusing on scientific fields such as energy,materials,and engineering technology,we have designed and developed new types of membrane materials and membrane equipment with significant application value,making membrane technology more useful for the national economy.The nanofiltration membrane made of cellulose as raw material has strong hydrophilicity,good biocompatibility,and safety and non-toxicity.Due to the continuous reduction of oil resources reserves,the expansion of high value-added use of natural resources is a strategic need for the development of national renewable resources,and it is also one of the hot areas for the development of the global economy,energy,and new materials.In this dissertation,N-methylmorpholine-N-oxide(NMMO)was used as a solvent to prepare cellulose regeneration membranes from bamboo cellulose,and then through interfacial polymerization,cross-linked coating,spray method and layer self-assembly.In combination,a cellulose composite nanofiltration membrane was further prepared.The preparation conditions,chemical structure,elemental composition,thermal stability,crystallization,microstructure,pore size distribution and separation performance of composite nanofiltration membranes were studied in this thesis.First,an interfacial polymerization reaction(IP)is performed on the surface of a regenerated cellulose membrane by using an amino function of piperazine(PIP)and a carboxyl function of 1,3,5-trimesoyl chloride(TMC)to form a polyamide in situ.Hydrophilic bamboo cellulose composite nanofiltration membrane(IP-NF-BCM)was prepared.The interfacial polymerization reaction was based on the in-situ formation of polyamide on the cellulose membrane,which produces a uniform nanofiltration pore on the surface of the cellulose membrane,thereby forming a composite nanofiltration membrane.The effects of water solubility,organic phase concentration,and reaction time on the structure and properties of cellulose nanofiltration membranes were investigated.Using thermogravimetric analysis(TGA),Fourier transform infrared spectroscopy(FT-IR),field emission scanning electron microscopy(FE-SEM),X-ray diffractometry(XRD),atomic force microscopy(AFM),nuclear magnetic resonance(NMR)The nitrogen absorption of Brunner-Emmet-Teller(BET)characterizes the chemical structure and morphology of the IP-NF-BCM membrane.The water permeability and desalination capacity of bamboo cellulose composite nanofiltration membranes were evaluated using a 500 ppm NaCl salt solution at a pressure of 0.5 MPa.The results showed that the optimal reaction conditions for interfacial polymerization were as follows:PIP was 2 wt%,TMC concentration was 0.15 wt%,reaction time was 3.0 min.At this time,the membrane flux was 15.68 L·m-2·h-1 at a transmembrane pressure difference of 0.5 MPa.For the NaCl rejection rate was 40.12%.The interception rates of five salt ions in cellulose composite nanofiltration membranes were:Na2SO4>MgSO4>NaCl>CaCl2>MgCl2.The average pore size of bamboo cellulose composite nanofiltration membrane is 1.0 nm.Second,bamboo cellulose composite nanofiltration membrane(CL-NF-BCM)was prepared by cross-linking sodium alginate(ALG),sodium carboxymethyl cellulose(CMC)and epichlorohydrin(ECH).The influence of factors such as the concentration of blended solvent,the mass ratio of blends and the concentration of cross-linking agent on the structure and properties of cellulose nanofiltration membranes were studied.The chemical structure,thermal properties,crystallization,surface morphology and average pore size of CL-NF-BCM films were characterized by TGA,FT-IR,FE-SEM,AFM and NMR.The nanofiltration performance of the prepared CL-NF-BCM composite membrane was evaluated with a 500 ppm saline solution under a pressure of 0.5 MPa.The results showed that the optimum conditions for the cross-linking reaction were as follows:the concentration of sodium alginate and carboxymethyl cellulose in the blend solution was 2.0%,the blend ratio was 1:3,and the crosslinking agent was epichlorohydrin.The concentration of epichlorohydrin is 3%.At this time,the water flux of CL-NF-BCM membrane is 17.25 L·m2·h-1 under the transmembrane pressure difference of 0.5 MPa,and the NaCl rejection rate is 48.25%.The interception rates of five salt ions in cellulose composite nanofiltration membranes were as follows:Na2SO4>MgSO4>NaCl>MgCl2>CaCl2.The average pore size of the bamboo cellulose composite nanofiltration membrane(CL-NF-BCM)was 1.1 nm.Then,the surface amino groups were exposed under wet conditions by preparing a chitosan/cellulose membrane,and the carboxymethyl cellulose and chitosan solutions were sequentially applied to the chitosan/cellulose membrane layer by spraying.Layer self-assembly,preparation of hydrophilic bamboo cellulose nanofiltration membrane(LBL-NF-CS/BCM).The carboxyl group of carboxymethyl cellulose is combined with the amino group of chitosan to form a hydrophilic macromolecular complex,and a nanofiltration pore size is constructed on the surface of the chitosan/cellulose membrane.The effects of factors such as the number of assembled layers,chitosan concentration,and carboxymethyl cellulose concentration on the structure and properties of cellulose nanofiltration membranes were studied;CL-NF-BCM was analyzed by TGA,XRD,BET,FE-SEM,AFM and XPS.The thermal stability,surface morphology,crystallization conditions,pore size distribution,chemical structure,and water filtration performance of the BCM film were characterized.The water permeability and desalination capacity of bamboo cellulose membrane composite nanofiltration membranes were evaluated using a 500 ppm saline solution at a pressure of 0.3 MPa.The results showed that the optimal reaction conditions for layer self-assembly:the number of layers assembled was 15,the polycation concentration was 0.75 g/L,and the polyanion concentration was 1.5 g/L.At this time,the membrane flux was 12.08 L·m-2·h-1 at a transmembrane pressure difference of 0.3 MPa,and the NaCl rejection rate was 36.11%.The average pore size of the LBL-NF-CS/BCM membrane is 2.1 nm.Finally,a combination process of polyvinylidene fluoride(PVDF)flat ultrafiltration membrane as the support layer and interfacial polymerized cellulose composite nanofiltration membrane(IP-NF-BCM)as the composite layer was used to perform advanced treatment of the tap water after precipitation from the water plant.The effect of running time on membrane flux,transmembrane pressure difference,ammonia nitrogen,TOC removal,and cleaning and regeneration of the membrane were investigated.The results showed that as the continuous operation time prolonged,the transmembrane pressure difference increased stepwise,and the membrane flux also gradually increased in a stepwise manner,but it was basically stable within a small fluctuation range relative to the operating pressure of the combined membrane.It shows that the combined membrane has good drainage performance and no membrane fouling occurs.With the increase of operating days,the effect of ultrafiltration membrane on removal of ammonia nitrogen is not obvious,and the removal of ammonia nitrogen by the composite membrane is basically above 70%.The removal effect of ultrafiltration membrane on TOC is very limited,and the removal effect of composite membrane on TOC is very small.It is basically maintained at 90%.