Study On The Surface Grafting Of PIP/TMC Thin-film Composite Membrane With Hydrophilic Monomers And Properties Of The Modified Membranes

Polyamide thin-film composite nanofiltration(NF) membrane, which separates solutes of interest mainly through a combination of steric hindrance and Donnan effect, has become widely used in the fields of purification and softening of water,treatment and reclamation of wastewater, separation and concentration of industrial processing fluids for its key distinguishing characteristics of low rejection to monovalent ions, high rejection to multivalent ions and higher water permeability.However, membrane fouling is the major obstacle for the extensive application of nanofiltration, which usually leads to performance degradation and thus a shortened lifespan.Therefore, it is of great importance to design and develop novel nanofiltration membranes with special surface properties that are less prone to fouling.In this study, hydrophilic monomers of triethanolamine(TEOA), diethanolamine(DEA) and glycerol(GLy) were grafted onto the surface of the nascent membrane to modulate the structure and property of the polypiperazine-amide thin-film composite(TFC) membrane through esterification reaction or amidation reaction. The membrane properties including surface chemical structure, morphology, hydrophilicity and charge of surface was characterized by ATR-FTIR, XPS, SEM, AFM, surface contact angle measurement and Zeta potential measurement. Membrane solute rejection performance was determined through cross-flow permeation tests with the model solutes of inorganic salts, anionic dyes and polyethylene glycol(PEG). Finally, the anti-fouling property of the virgin membrane and modified membranes were evaluated through fouling experiments with the model foulants of bovine serum albumin(BSA), sodium alginate(SA) and cationic polyacrylamide(CPAM),respectively. The conclusions obtained from the experimental results were as follows:(1) Surface grafting of hydrophilic monomer could be performaed with the nascent PIP/TMC membrane through the unreacted acyl chloride groups that remain on membrane surface. The results of FTIR and XPS analyses indicated that TEOA or GLy was grafted on membrane surface through esterification reaction between the hydroxyl group and the unreacted acyl chloride group, while DEA was graftedthrough the amidation reaction. Membrane surface became less negatively charged and more hydrophilic after modification. SEM and AFM analyses revealed that the modification performed in this work has no evident influence on the morphology of the TFC membrane. Both of the thickness of the skin layer and the surface roughness remained nearly constant after modification. All the virgin and modified membranes T exhibited a molecular weight cut off of around 300 Da, indicating that the surface grafting has nearly no impact on the cross-linking degree of the active layer.(2) Parametric study showed that the separation performance of modified membrane was largely affected by the modification conditions such as monomer concentration, modification time and acid absorbent concentration. With the increase of TEOA concentration and/or modifiction time, the flux of the modified membrane increased firstly and then remained constant. Since, the prolongation of reaction time would promote the hydrolysis of the acyl chloride group, therefore, it is a more rational way to control the modification process by adjusting the concentration of modifying monomer.The optimal conditions for the modification of the nascent PIP/TMC TFC membrane using were as follows: TEA=0.30 wt%, SDS=0.05 wt%;modification time=2 min, Modifier(TEOA, DEA and GLy)= 0.035 mol/L.(3) The water permeability of PIP/TMC nanofiltration membrane could be effectively improved through surface grafting of small molecular hydrophilic substance. Under 0.5 MPa and 25.0 ℃, membrane flux increased from 80.6 L/(m2·h)of the virgin membrane to 93.5, 91.9 and 90.5 L/(m2·h) of modified membranes NF-TEOA-0.035, NF-DEA-0.035 and NF-Gly-0.035, respectively, while the Na2SO4 remained almost a constant value around 98.0%. All the virgin and modified membrane could effectively remove dyes from aqueous soltuon, showing the dye removal order of Methyl blue>Congo red>Sunset yellow>Alizarin Yellow R>Neutral red. The modified membranes exhibited a higher water flux compared with the virgin membrane.(4) Fouling experiments illustrated that the surface grafting by small molecular hydrophilic substance could effectively enhance the anti-fouling property of PIP/TMC nanofiltration membrane. All the three types of modified membranes exhibited animproved fouling resistance to the model foulants of BSA, SA and CPAM, showing lower flux decline ratios compared with the virgin membrane under the same conditions. Moreover, all the model foulants adsorbed and deposited on the surface of modified NF membrane could be readily washed off by simple hydraulic washing,indicating that all the modified membranes possessed an improved cleaning efficiency compared with the virgin membrane.