Study On Structure Control And Separation Performance Of Polyamide Thin-film Composite Nanofiltration Membrane Based On Polyethylene Membrane

The separation performance of nanofiltration（NF）is between ultrafiltration and reverse osmosis.Nanofiltration has been one of the most vital components in the areas of membrane separation technology due to its low power consumption,high efficiency separation which is able to achieve effective separation of most multivalent ions and low molecular weight organic molecules.So far,most of the available nanofiltration membranes are polyamide film composite（TFC）membranes,which are widely used in water treatment due to their high water flux and high selectivity.The TFC nanofiltration membranes are usually composed of a thin and dense separation layer and a porous ultrafiltration support layer.The support layer is mostly prepared by phase transformation and its thickness is usually more than 100 microns,which reduces the stacking density of the membrane module and hinders the passage of water.Commercial polyethylene（PE）lithium-ion battery separator is considered to be the most promising candidate as the substrate layer,due to its ultra-thin thickness,high mechanical strength,and excellent solvent resistance.Ultra-thin thickness can reduce the concentration polarization of the TFC membranes,improve the packing density of the NF membrane module,and reduce the transport paths of water.In this thesis,commercial PE membranes were used as the support layer,and the PE membranes were hydrophilized followed by interfacial polymerization to prepare the high-performance polyamide nanofiltration membranes.The researches and conclusions could be summarized as follows:（1）Polyethylene（PE）membrane with a thickness of 7μm was hydrophilized by polydopamine and polyethyleneimine（PDA/PEI）,and then directly initiated interfacial polymerization（IP）with trimesoyl chloride（TMC）,forming the dense and negatively charged polyamide-polyester selective layer.With the reaction time of PDA/PEI increases,the selective layer becomes denser,leading to the efficient removal of anionic dyes at high water flux.The water flux of the optimal membrane of M3-5 is 61.0 L m^-2 h^-1 at 1 bar,and the removal efficiency is 91.4%for an anionic dye eosin Y（EY,M_w=691.9 Da）.But removal efficiency for natural VB₁₂（M_w=1355.4 Da）was only 7.4%.This indicates that the adsorption of molecules plays a key role in the decolorization of anionic dyes during the filtration process.Meanwhile,M3-5showed good selectivity for the dye/salt mixture.（2）The PE membrane was hydrophilized by low-temperature plasma treatment,and the hydrophilicity of the PE membrane was significantly improved.Then the dense and defect-free polyamide separation layer was fabricated via the interfacial polymerization method using an aqueous-phase monomer of piperazine（PIP）and oil-phase monomer of TMC.The experimental results show that the prepared polyamide nanofiltration membrane of PPE-9-30has a rejection rate of 89.2%for Mg SO₄ at 2 bar with a water flux of 5.3 L m^-2 h^-1 bar^-1.The aqueous phase additive of taurine（TI）can improve the rejection rate of divalent sulfate in the polyamide separation layer,and the rejection rate of PPE-9-31 for Mg SO₄ increases from 89.2%to 95.1%,showing very excellent separation performance.（3）The PA-TFC NF membrane was prepared by the interfacial polymerization of a new aqueous-phase monomer of N-aminoethylpiperazine（AEP）on the PE membrane,which was hydrophilized by plasma treatment,and was able to achieve effective retention of divalent inorganic salts.The results indicated that the optimal membrane of PPE-9-AC-T could effectively reject inorganic salts of divalent cationic through pore sieving and charge exclusion.The optimal membrane exhibits a Mg SO₄ rejection of 94.5%and a Mg Cl₂ rejection of 91.5%。PPE-9-AC-T also showed good performance stability in the long-term stability tests of NF.