Construction And Regulation Of Polyamide Layer For The Nanofiltration Membranes

Membrane separation technology,as an energy-saving and environmentally friendly water treatment technology,has shown wide development prospects in coping with the water crisis.Nanofiltration（NF）membranes occupy a special position in membrane separation technology due to their structure and physicochemical properties.Currently,thin film composite（TFC）nanofiltration membranes with thin polyamide（PA）layers synthesized via interfacial polymerization（IP）have become an important membrane that dominates the market.The structures of TFC membrane are mainly composed of an underlying substrate and a dense selective layer,among which the physical and chemical properties of the PA selective layer play key roles in the membrane performance.This unique composite structure exhibits a great flexibility in tailoring the substrate and selective layer independently to optimize the separation performance of the TFC membrane.However,the highly crosslinked PA selective layer derived from the instant IP reaction inevitably generated the issue of“trade-off”effect,which hampers the further development and application of TFC membranes.Therefore,a host of researchers are committed to improving the performance of nanofiltration membranes by solving this problem.Several studies have shown that introducing hydrophilic materials with abundant hydrophilic groups,such as hydroxyl,carboxyl,amino,sulfonic acid and etc,into the IP process can effectively regulate the structure and properties of the PA layer,thereby tuning the membrane performance.Herein,this work selected inorganic hydrophilic materials（Si O₂）and organic hydrophilic materials（lecithin）respectively from the perspective of material design,and combined relevant regulatory measures based on the interfacial polymerization method to design and construct the structure and properties of the PA separation layer,thereby preparing PA-TFC nanofiltration membranes with high permeability selectivity.The specific research content and main conclusions are as follows:（1）Inorganic hydrophilic material（SiO₂）was used as the modified material.The precursor tetraethyl orthosilicate（TEOS）and TMC were dissolved into organic solution for reacting with PIP under alkaline conditions by the method of combing the IP and in-situ sol-gel,thereby forming the active selective layer with the organic-inorganic network characteristics of polyamide and Si O₂ interpenetrating.The Si O₂ nanoparticles formation process inside PA layer formation were adjusted by changing the concentration of Na OH,thereby regulating the structure and properties of the membrane.The results exhibited that the in-situ generated Si O₂nanoparticles were uniformly distributed within the PA layer.The surface hydrophilicity of the membrane was greatly enhanced due to the presence of abundant Si-OH groups,as well as the modified membrane exhibited a rougher surface morphology.The separation performance test results presented that the TFN-3 membrane under optimal conditions with 0.05 wt%Na OH displayed a high pure water permeance to70.69 L m^-2 h^-1 bar^-1 and over 99%rejection rates against dyes for CR,G250,and RB19,while maintaining lower than 7.5%salts rejection for Na Cl and Na2SO4.This alleviated the limitations of“trade-off”effect between permeability and selectivity to some content.Meanwhile,the optimal membrane presented an excellent long-term operational stability for 72 hours,and exhibited a good dye/salt separation performance after 7 cycles of operation in a CR/Na Cl mixed solution.Therefore,the membrane prepared by the combination of in-situ sol-gel and IP process is expected to realize the treatment and wide application of dye wastewater.（2）Organic hydrophilic material（lecithin）was used as the modified material.Soybean lecithin（SL）was applied as an additive of aqueous phase to tune the interfacial polymerization（IP）process to achieve thin-film composite（TFC）NF membrane with enhanced perm-selectivity for dye/salt separation.The structure and properties of the PA layer are regulated by changing the concentration of SL addition to obtain a TFC membrane with optimal separation performance.The results revealed that the distribution position and diffusion rate of the PIP monomer can be affected through the electrostatic attraction and hydrogen bonding between the polar head of SL with the PIP monomer,as well as the generated physical barriers of spontaneously formed liposome structure of SL in aqueous phase solution.With the increase of SL concentration in aqueous solution,modified membranes possessed a thinner selective layers,rougher surfaces,and higher hydrophilicity,which greatly improved the transport of water molecules.The separation performance test results exhibited that the TFC membrane prepared by this method effectively alleviated the issue of the“trade-off”effect.At optimal conditions with 0.05 wt%SL decoration,TFC membrane displayed a high pure water permeance to 43.8L m^-2 h^-1 bar^-1 with over 99.0%dye rejections,while presented good separation stability.In addition,the SL modified membrane maintained a relatively low chloride salt rejection rate（17.5%for Na Cl and 7.2%for Mg Cl₂）and a satisfactory dyes/salts screening ability in the filtration experiment of dye/salt mixture solutions.