Structure And Performance Of Functional Skin Layer Of Nanofibrous Composite Nanofiltration Membranes

The scarcity of fresh drinking water has become a big bottleneck restricting economic development and social progress.Membrane separation technology,as a new type of high-efficiency separation technology,has become one of the mainstream technologies of water treatment technology due to its energy-saving and environment-friendly advantages.As one kind of novel membrane separation technology between ultrafiltration（UF）and reverse osmosis（RO）,nanofiltration（NF）has played an important role in desalination of seawater,treatment of industrial wastewater and softening of domestic water recently due to its unique properties:the high water flux and satisfying rejection for divalent ions and most dissolved organic molecules with a low molecular weight（200-1000 Da）.The pore size of commercial nanofiltration membrane is about 1 nm.Composite NF membranes are usually made up of a dense polyamide（PA）separating skin layer and a porous supporting substrate.At present,the thicknesses of PA barrier layer of commercial composite nanofiltration membranes in the market are generally50-200 nm,with high mass transfer resistance and low water flux(approximately 3-12 L m^-2h^-1bar^-1).Interfacial polymerization（IP）is a technology that polymerizes at the interface of immiscible two-phase solution to form a thin and dense separation skin layer.Due to the advantages of simple process and easy control,IP has become the most important method for preparing composite nanofiltration membrane in industry.As the core part of composite membrane,PA separating skin layer directly determines the nanofiltration performance of the composite membrane.Generally speaking,in order to improve the permeate flux of nanofiltration membrane and reduce the operating pressure,the PA skin should meet the following requirements:the separation layer is thin enough,the pore diameter is large enough,the surface roughness is high,and the hydrophilicity is good.Recently,the researchers added synthetic additives with specific structures into the aqueous phase solution during IP to regulate the density of the PA layer and construct wrinkled and hydrophilic PA surfaces.They also introduced the transition layers on the porous supporting mats to reduce the surface pore diameters of the base membranes and regulate mass transfer rates of aqueous phase monomers.However,these methods lacked of specific and systematic analysis of the physical morphology and chemical structure of the PA separating skin layers,and the nanofiltration performance of those prepared composite membranes are still not ideal,which greatly limited the applications of composite nanofiltration membrane in practical productions and lives.In this paper,by introducing cheap additives（such as sodium chloride,glycolic acid）into aqueous phase or changing the adding order of aqueous phase and organic phase（reverse interface polymerization,IP-R）,we analyzed systematically the relationship between the surface structures of PA skin layers and the nanofiltration performances of the composite membranes.To increase the water permeate flux greatly but not sacrificing the rejection property,the pore size of the PA barrier skin layer should be regulated and optimized to the maximum,reducing energy consumption.The main research contents and results are as follows:（1）The PA/PAN nanofibrous composite nanofiltration membrane with specific surface morphology and structure was constructed by interfacial polymerization introducing Na Cl into the PIP aqueous phase solution,which was applied to the high efficiency removal of divalent salt ions.The thickness and density of polyamide separation layer can be regulated by changing the concentration of Na Cl in aqueous solution.Meanwhile,the surface morphology of PA separation layer of composite membrane can be tuned from staggered ridge network structure to Turing structure with dense array convex structure.The low pressure and high flux nanofibrous polyamide composite nanofiltration membrane was successfully prepared.The composite nanofiltration films were characterized by FT-IR,SEM,AFM,XPS and WCA.The mechanism of Na Cl on the properties of aqueous solution and the complexation mechanism of metal ions with acyl chloride and carbonyl groups in polyamide were studied.When the concentration of Na Cl was 20.0 wt%.the permeate flux and Na₂SO₄rejection of the optimized TFNC membrane were129.0 L m^-2h^-1and 99.1%under 0.5 MPa.（2）A small organic molecule（glycolic acid,GLA）was introduced into PIP aqueous solution as an inhibitor during the IP process.Ultra thin polyamide separation layers with different pore sizes were designed and constructed on porous PAN nanofibrous substrate,which was applied to the removal of divalent salts and the efficient separation of anionic dye wastewater.With the introduction of GLA,the thickness and pore size of the separation layer could be controlled accurately,which was suitable for the separation system with different sizes of solutes.In addition,the introduction of carboxyl functional groups could effectively improve the hydrophilicity of the composite membrane,endowing the composite membrane excellent antifouling performance.The fabricated composite membranes were characterized by FT-IR,SEM,XPS and WCA.The experimental results show that under 0.5 MPa operating pressure,when the concentration of GLA was 1.2 wt%,the permeation flux of the membrane NF3 to 2000 ppm Na₂SO₄solution was 142.5L m^-2h^-1,and the rejection rate was 96.4%.When the GLA concentration was 2.0 wt%,the permeation flux of the membrane NF6 to 100 ppm direct red solution was 398.5 L m^-2h^-1,and the rejection rate was 99.2%.（3）TFNC NF membrane was fabricated by low temperature reverse interfacial polymerization（IP-R）,which was applied to the removal of divalent salts.The low temperature reverse interfacial polymerization method could effectively slow down the volatilization rate of the organic phase solution,providing a complete and uniform reaction interface for the following interfacial polymerization process and preparing ultra-thin and defect free PA separation layer.The prepared composite membranes were characterized by FT-IR,SEM,XPS and WCA.The results showed that when the organic phase temperature was-5℃,the surface defects and infiltration problems of PA layer were effectively solved.The permeation flux and Na₂SO₄rejection rate of the optimized TFNC membrane were 66.3 L m^-2h^-1(the flux of the composite membrane prepared by IP-R at 25℃was 43.1 L m^-2h^-1)and 98.2%（the Na₂SO₄rejection of the composite membrane prepared by IP-R at 25℃was 90.2%）.（4）A novel polyamide（PA）thin film nanofibrous composite（TFNC）nanofiltration membrane consisted of electrospun polyacrylonitrile（PAN）nanofibrous substrate,gelatin interlayer and polyamide barrier layer was fabricated by reverse interfacial polymerization（IP-R）,which was applied to the removal of divalent salts.Through the introduction of ultra-fine gelatin nanofibrous layer,the storage and distribution of organic phase solution on the surface of PAN substrate could be effectively controlled,so as to effectively control the interfacial polymerization reaction rate and optimize the compactness of PA layer and change the Turing structure of PA layer surface,endowing the composite membrane with ultra-high permeability and high rejection of divalent salt ions.At the same time,the introduction of gelatin interlayer can effectively improve the adhesion between the interfaces and endow the composite film excellent structural integrity.By changing the deposition time of gelatin layer,the physical morphology and chemical structure of PA separation layer can be controlled.The composite nanofiltration membranes were characterized by FT-IR,SEM,XPS and WCA.The results showed that when the gelatin layer was deposited for 20 min,the thickness of PA separation layer was about 45 nm,and the NF performance of the PA/GE20/PAN composite membrane was the best.The permeation flux and Na₂SO₄rejection rate of the optimized TFNC membrane were 135.6 L m^-2h^-1and 98.1%under0.5 MPa.To sum up,the physical morphologies and chemical structures of the PA separating skin layers could be precisely adjusted by optimizing the process conditions of IP.On the premise of meeting the specific solute rejection efficiency,the permeate fluxes of the composite membranes can be increased greatly by increasing and regulating the pore sizes,surface areas and the thicknesses of the PA layers.The methods by adding additives（Na Cl and GLA）in aqueous phase or changing the adding order of aqueous phase and organic phase could provide a new idea for the fine regulation of the structure of the PA separating skin layer.