Preparation And Characterization Of TpPa/Polyamide Nanofiltration Membranes

The pore size of nanofiltration membrane is between 0.5-2 nm,and the removal rate of organic small molecules and inorganic salt ions with molecular weight above 200 Da is high,but most of them have trade off phenomenon,that is,large flux with low removal rate,which largely affects the separation efficiency.In this experiment,a COFs material TpPa with different structure were incorporated into membrane for nanofiltration.Two methods were used to prepare nanofiltration membrane containing TpPa in this experiment:（1）A series of naofiltration membranes were directly prepared by the interfacial polymerization of 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde（Tp）and p-phenylenediamine（Pa）on the surface of polyacrylonitrile membrane（2）TpPa Porous materials were prepared first and then incorporated into the active separation layer generated by interfacial polymerization of triptylamine（2-aminoethyl）amine（TAEA）and trimesoyl chloride（TMC）.The TpPa prepared by hydrothermal method showed spherical petal structure with high crystallinity,while the other prepared by using calcium carbonate as template at room temperature showed hollow structure.The chemical composition,morphology and properties of the materials were characterized by scanning electron microscope（SEM）,transmission electron microscope（TEM）,atomic force microscope（AFM）,infrared spectrometer,TG,specific surface area tester,membrane evaluator,UV visible photometer and contact angle tester etc.The influences of monomer concentration,composition and TpPa addition on the membrane separation performance was investigated.The anti-protein adsorption of each kind of optimized membranes and their application in bisphenol A removal were also investigated.The results showed that:（1）The nanofiltration membrane prepared by interfacial polymerization of 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde（Tp）and p-phenylenediamine（Pa）was relatively loose,and can be used for removing large dyes.When the concentration of 2,4,6-trihydroxy-1,3,5-benzenetrialdehyde（Tp）and p-phenylenediamine（Pa）were 0.05 wt%and 0.5 wt%respectively,the interfacial reaction was 20 s,and the heat treatment time was 10 min,the prepared membrane showed rejection rate larger than 98%for alkaline blue 26,direct yellow4,Direct Black 38 and acid red 92,but for the orange yellow G（452）was only about 9%.Increasing the monomer concentration could improve the rejection rate of the dye.Meanwhile,it should be noted that adding a few of TMC into the oil solution,the removal rate of orange G could be increased obviously.When the concentration of TMC,Tp and Pa were 0.0075 wt%,0.0425 wt%and 0.5 wt%respectively,the rejection rate for orange yellow G reached 98%.（2）The spherical petal TpPa with high crystallinity can be synthesized by hydrothermal method.The separation mechanism of nanofiltration is mainly based on size sieving and Donnan effect.The introduction of TpPa not only affects the pore size of polyamide separation layer,but also affects the surface charge distribution.Therefore,the removal rate of Na Cl,Na₂SO₄,Mg Cl₂,Mg SO₄ of the TpPa/polyamide membranes changed with the amount of TpPa.However,the removal order of the four inorganic salts remained nearly unchanged,Mg SO₄≥Na₂SO₄>Mg Cl₂>Na Cl,while the removal rate of Mg SO₄ was higher than that of Na₂SO₄.It is necessary to point out that the addition of COFs TpPa did improve the membrane flux.Compared with the membrane without COF,the membrane prepared by adding 10 mg TpPa remained the removal rate about 82%for Mg Cl₂,Na₂SO₄ and Mg SO₄,and the membrane flux increased nearly 1 time higher.（3）The hollow structure H-TpPa can be successfully prepared by using calcium carbonate as template at room temperature.Compared with the TpPa synthesized by hydrothermal method,the crystallinity of the film decreased significantly.When the material was combined with the polyamide separation layer,and the more the TpPa,the higher the flux was.However,compared with the TpPa synthesized by hydrothermal method,the optimization performance of the membrane was limited,which may be related to the agglomeration of calcium carbonate template,which needs further optimization.In addition,the experiments also indicated that the membranes with optimal separation performance showed well anti-protein adsorption properties.And when they were applied for removing bisphenol A,the removal rate of bisphenol A is about 95%when the concentration of bisphenol A ranged from 5 to 50 mg/l.