Study On The Surface Grafting Of The Polyamide Composite Nanofiltration Membrane With Silane Coupling Agent And The Properties Of The Modified Membrane

Nanofiltration membrane has been widely used in the fields of the removal of organic compounds from surface water,the removal of hardness from ground water,partial desalination of water,separation and concentration of dye solutions for its advantages of high efficiency and low energy consumption.However,fouling is the key factor that limits the application of nanofiltration membrane.Therefore,it is still of great importance to improve the antifouling property of the nanofiltration membrane.In this study,3-aminopropyltriethoxysilane(APTES)was used as the modifying agent and grafted onto the surface of the nascent poly(piperazinamide)nanofiltration membrane through the reaction between APTES and the residual acyl chloride groups on membrane surface.The effects of surface modification on the surface properties,separation performance and anti-fouling properties of the composite membranes were investigated.The surface morphology properties and chemical structure properties of the membrane were characterized by the ATR-FTIR,XPS,AFM,FESEM,and the measurements of surface zeta potential and water contact angle.The molecular weight cut-off and rejection performance to different inorganic salts and organic dyes of the modified membrane were systematically studied through cross-flow permeation tests.Model foulants such as humic acid(HA),cationic surfactants dodecyl trimethyl ammonium bromide(DTAB)and Bovine serum albumin(BSA)were employed to investigate the effects of modification on membrane antifouling property through cross-flow fouling experiments.The results obtained from the experiments were as the follows:(1)The results of XPS and ATR-FTIR analyses revealed that APTES could be grafted onto the surface of the nascent poly(piperazine-amide)composite membrane successfully through amidation or esterification between the acyl chloride and the amino or silicon hydroxyl.The modification was found to make the membrane denser,showing the decreases of molecular weight cut-off from about 290 to 260 Da and membrane pore size from 0.88 to 0.84 nm.AFM analysis revealed that both the rawand modified membranes have a RMS roughness of lower than 20.0 nm,and modification has little influence on membrane surface morphological structure.The surface of the membrane became more hydrophilic and less negatively charged after modification,showing a decline in both surface water contact angle and zeta potential.(2)The property and separation performance of the modified membrane were largely affected by the concentration and reaction time of APTES.Increasing the concentration of APTES and prolonging the reaction time could promote the grafting reaction to a certain extent,since the amidation reaction or the esterification reaction was competitive with the hydrolysis reaction of the acid chloride.In order to get the composite membrane which has best comprehensive separation performance,the optimum modifying conditions were as follows: APTES concentration=0.4 wt% and the reaction time=4 min,under which,the ratios of the reactions of amidation,esterification and hydrolysis of the residual acyl chloride groups were 13.2%,17.6%and 69.2%,respectively.(3)When tested with 500 ppm Na2SO4 under 0.5 MPa and 25±1?,the flux of the composite membrane increased from 67.97 to 73.32 LMH and the salt rejection decreased from 97.83% to 96.08% after modification,showing a trade-off effect between flux and rejection.The rejections of the modified membrane to different inorganic salts followed the order of Na2SO4>MgSO4>NaCl>MgCl2,which agrees with the typical rejection order of the negatively charged NF membrane.The rejection of the modified membrane to different dyes followed the order of Crystal violet ?Rhodamine B?Cresol red?Methylene blue?Neutral red,and both the rejection and flux of the modified membrane were higher than those of the raw membrane.After10.0 h of diafiltration of dye aqueous solution,the salt content was reduced to 5.53%and 0.62% by the raw and modified membranes,respectively.Compared with the raw membrane,the flux of diafiltration was increased by 7.0 %,resulting in a declined operation time.(4)Grafting of APTES on membrane surface could improve the anti-fouling properties of the composite membrane.The flux decline rates of the modified membrane to DTAB,HA and BSA aqueous solutions were lower than those of the rawmembrane.The steady-state flux reduction ratios of the modified membranes to DTAB,HA and BSA aqueous solutions were 29.7%,25.8% and 26.8%,respectively,while for the raw membrane,they were 39.6%,28.0% and 33.7%,respectively.The modified membrane also showed a better physical cleaning efficiency compared with the raw membrane.The flux recovery ratio(FRR)of the DTAB-fouled membrane increased from 90.6% of the raw membrane to 96.0% of the modified membrane,the FRR of HA-fouled membrane ascended from 88.0% to 93.0%,while the FRR of the BSA-fouled membrane improved from 82.5% to 91.3%.