A novel nanofiltration membrane

Journal of Membrane Science269(2006)84–93A novel nanofiltration membrane prepared with PAMAMand TMC by in situ interfacial polymerization onPEK-C ultrafiltration membraneLi Lianchao a,∗,Wang Baoguo a,Tan Huimin b,Chen Tianlu c,Xu Jiping da Department of Chemical Engineering,Tsinghua University,Beijing100084,Chinab School of Chemical Engineering and Material,Beijing Institute of Technology,Beijing100081,Chinac Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun133022,Chinad School of Chemistry and Chemical Technology,Shanghai Jiaotong University,Shanghai200030,ChinaReceived9October2004;received in revised form31May2005;accepted9June2005Available online19July2005AbstractA novel nanofiltration membrane(PEK-C NF membrane)prepared with poly(amidoamine)(PAMAM)dendrimer and trimesoyl chloride (TMC)by in situ interfacial polymerization on phenolphthalein poly(ether ether ketone)(PEK-C)ultrafiltration membrane was investigated. FTIR–ATR,XPS,SEM and contact angle of the water were employed to characterize the chemical and physical changes of resulting membranes.Theflux and rejection of resulting membranes increased with increasing concentrations of PAMAM and/or generation numbers of PAMAM for NaCl concentration.Both rejection andflux decrease with an increase of MgSO4concentration.The salts rejections order of resulting membrane is MgCl2>MgSO4>NaCl>Na2SO4,which shows that resulting membranes were all positively charged NF membrane. Influences of operating temperature onflux and rejection of different solutes show that resulting membrane is thermostable at higher temperature (e.g.80◦C).The PEK-C NF membrane is especially suitable for separating cationic solutes from others by increasing operating temperature. The pH in the feed increase from2to9resulted in over two-fold increase in theflux for NaCl solution.Therefore the PEK-C NF membrane is also particularly suitable for treating acidic feed.The studied membrane possesses maximum operating pressure lower than2.0MPa and excellent stability during1080h(1.5months)of continuousfiltration at30◦C.©2005Elsevier B.V.All rights reserved.Keywords:PAMAM;PEK-C;Nanofiltration membrane;Interfacial polymerization1.IntroductionNanofiltration(NF)is increasingly gaining attention in many separation and treatment processes such as water softening,color removal,chemical oxygen demand(COD) reduction and separation of medicines[1–3].NF membranes are often negatively charged,displaying separation charac-teristics in the intermediate range between reverse osmosis (RO)and ultrafiltration(UF).NF membranes generally have a thin skin layer enabling higherfluxes and lower operating pressures than RO membranes and are able to ∗Corresponding author.Tel.:+861062795275.E-mail address:lc-li@(L.Lianchao).reject small organic molecules having molecular weights as low as200–500Da such as many medicines(streptomycin, penicillin etc.).Moreover,NF membranes are also able to reject ions,especially bivalent ions,due to the Donnan effect [4]stemming from the membrane charge[5,6].There are many methods for the preparation of com-posite membranes for NF including vapor deposition[7], plasma-initiated polymerization[8–11],photo-initiated polymerization[12],the radiation polymerization[13], the dip coating process[14],interfacial polymerization [15,16],electron beam irradiation[17],atom transfer radical polymerization[18],resin-filled chelating[19],and in situ amines cross-linking[20].Interfacial polymerization, however,is still a key method to produce commercial NF0376-7388/$–see front matter©2005Elsevier B.V.All rights reserved. doi:10.1016/j.memsci.2005.06.021L.Lianchao et al./Journal of Membrane Science269(2006)84–9385Fig.1.Structure of PAMAM G1.0dendrimer. membranes such as the NF series(Filmtec Corporation),the NTR series(Nitto Denko Company),the UTC series(Toray Industries)and so forth.Although they have been used in manyfields successfully,it should be an important promising work for membrane manufacturing to develop a novel NF membrane with special properties using high performance materials.Dendrimers developed as novel high performance materi-als over the past decade have received a great deal of atten-tion because they have numerous terminal functional groups and a highly branched structure.The most-studied den-drimer is probably the so-called polyamidoamine(PAMAM) dendrimer,which has a global shape containing much higher amino group densities on the surface than conven-tional macromolecules,as is schematically shown in Fig.1. These dendrimers are typically synthesized by the repeti-tive Micheal addition and aminolysis[21].The high density of their terminal groups provides a large number of reac-tive sites[22]for potential application as nanoscale cata-lysts,micelle mimics,drug delivery agents,chemical sensors,high-performance polymers,and adhesives[23,24].In recent years,some studies have appeared in the literature describ-ing the applications of PAMAM in the membrane separation field,including PAMAM supported UF to improve separation performance of UF[25],PAMAM dendrimer-induced cross-linking modification of polyimide membranes to enhance CO2permeability[26],using PAMAM as liquid membrane to improve separation performance of CO2from gas mix-tures[27–29],and grafting PAMAM and poly(acrylic acid) onto Au-coated Si wafer to prepare pH-switchable modified-electrode for probing both cationic and anionic redox-active molecules[30].Up to now,few researchers prepared NF membranes with dendrimers,Willem and co-workers pre-sented their primary study on NF membrane formation with poly(propyl imine)(PPI)dendrimers polymerized with TMC [31].However,Preparation of PPI dendrimer suffered from the shortcoming of material cost,higher pressure operation and low yield.On the contrast,PAMAM can be synthesized under normal pressure at room temperature,and possesses the similarity to PPI in its chemical structure and physical ing PAMAM to develop a novel NF membrane with special properties is expected to be a promising method in the future(Fig.2).Poly(ether ether ketone)containing the cardo group of phenolphthalein(PEK-C)developed by ours,has super ther-mal stability(T g up to228◦C),chemical resistance(only resolved in DMF or DMAC or DMSO)and excellent mem-brane formation property,the PEK-C membrane has showed good selectivity and permeationflux for both gases and liq-uids[32–35].PEK-C was expected to play important parts in the development of NF membrane.The objective of this study focuses on developing novel NF membrane from PAMAM and TMC by in situ interfacial polymerization process,using PEK-C as substrate membrane,and investigating its desali-nation properties.Several surface analytical techniques were used to characterize the chemical and physical changesin Fig.2.Scheme of PEK-C synthesis.86L.Lianchao et al./Journal of Membrane Science269(2006)84–93 the resulting membranes.Additionally,resulting PEK-C NFmembranes performance were characterized by measuringsome soluteflux and/or rejection.Influences of operatingpressure and temperature and pH on soluteflux and/or rejec-tion were also investigated.2.Experimental2.1.Synthesis of PAMAM and PEK-C materials2.1.1.Synthesis of PAMAMPoly(amidoamine)dendrimers were synthesized bymeans of the procedure described by Tomalia et al.[22]whereethylenediamine was used as a nitrogen core,and a repeatedstepwise reaction was conducted including Michael additionand aminolysis.Repetition of these sequences was used tomake successively higher generations.The structure of thePAMAM G1.0dendrimer is shown in Fig.1.The reactionprocess was as follows:NH2CH2CH2NH2+4CH2CHCOOCH3Micheal addition−→Step1Aminolysis−→Step2PAMAM G0Step1−→Step2PAMAM G1Step1−→Step2PAMAM G2where all reagent was purchased from Beijing Fine Chemi-cal Industry Co.and purified before use,PAMAM was further purified before use according to the method described else-where[38].2.1.2.Synthesis of PEK-C materialsPhenolphthalein poly(ether ether ketone)(PEK-C)with high molecular weight were synthesized via solution nucle-ophilic polycondensation reaction of phenolphthalein and 4,4 -difluorobenzophenone in the presence of potassium car-bonate.The process of synthesis was shown in Fig.2.In a 500mL three-necked round bottomflask,10.61g(50mmol) of4,4-difluorobenzophenone,15.91g(50mmol)of phe-nolphthalein and7.59g of potassium carbonate were dis-solved in a mixture of100mL dimethyl sulfoxide(DMSO) and50mL toluene.The mixture was refluxed for5h under a constant stream of nitrogen.After the water was essentially removed from the reaction mixture by azeotropic distillation, the temperature was raised to175◦C to distill out the toluene. The solution was kept at this temperature for6h,and then the reaction mixture was poured into a great deal of ethanol to precipitate the polymer.White Solid PEK-C was obtained, Mn=12,653,yield91%.DMSO,phenolphthalein and potassium carbonate pur-chased from Beijing Chemical Reagent Company were purified or dried before use;4,4 -difluorobenzophenone was purchased from Aldrich and other reagents and solvents were obtained commercially and used without further purification.2.2.Preparation of microporous PEK-C UF support membranePEK-C casting solution was prepared by dissolving 15wt.%PEK-C in N,N-dimethylformamide(DMF)at 50–70◦C with continuous stirring.The resultant polymer solution was cast on a glass plate and gelled in a water bath consisting of2%ethanol.After10min of gelation,the resulting PEK-C membrane was removed from the gela-tion bath and washed thoroughly with deionized water to remove all DMF.The membrane was then subsequently employed as a support medium for the NF membrane.Molec-ular weight cut-off of the resulting UF membrane is50,000. The entire casting operation process was kept at a tempera-ture of25–30◦C with a relative humidity of30–35%.The PEK-C/PEG/DMF solution consisted of15wt.%PEK-C, 15wt.%PEG(M W=300)as pore-former and70wt.%DMF. In all experiments,deionized water(produced by RO)was used.N,N-dimethylformamide(DMF)(A.R.grade),PEG (M W=300)and ethanol were obtained commercially and used without further purification.2.3.Fabrication of the PEK-C NF membranesThe PEK-C NF membrane was fabricated to form an active skin layer containing PAMAM units over the microporous PEK-C UF membrane by adopting the in situ interfacial polymerization technique.A typical PEK-C NF membrane was prepared by allowing the plate sheet PEK-C UF substrate membranes to be submerged in a 11.2×10−5mol/L aqueous solution of PAMAM G0,to which0.01wt.%of sodium dodecylsulfate(SDS)had been added as a surfactant.The excess solution was allowed to drip off the substrate membranes.They were then immersed for60s at room temperature in a10.5×10−5mol/L solution of TMC in hexane.This resulted in the formation of a thin polyamidefilm on the microporous surface of the PEK-C UF membrane.The composite membranes obtained then underwent heat treatment in an oven for10min at80◦C to attain the desired stability of the formed structure.PAMAM were synthesized by ourselves,and was purified by the other nanofiltration membrane before use[36];TMC was purchased from Aldrich and other reagents and solvents were obtained commercially and used without further purifi-cation.2.4.Membrane performance evaluationThe PEK-C NF membranes were evaluated for permeate flux and rejection on a cross-flow NF test cell with an active membrane area of24cm2.Testing was done with NaCl salts solution of1000mg L−1at a pressure of0.6MPa.Circular membrane samples were placed in the test cell with the active skin layer facing the incoming feed.The membrane sample was supported on a porous stainless steel sintered disc with a rubber O-ring around it.The membrane feed solution sideL.Lianchao et al./Journal of Membrane Science 269(2006)84–9387was stirred.Before testing F and R ,deionized water from a reservoir was pressurized through the membranes for at least 20min at 0.8MPa in order to stabilize membranes.Then the flux was repeatedly measured by collection of the filtrate for given periods until a stable value of the permeation flux was observed.Values of three membranes were averaged.The filtration characteristics including permeation flux (F )and rejection (R )were determined under a pressure difference of 0.6MPa at ambient temperature.The permeation flux was calculated as Eq.(1):F =V At(1)where V is the total volume of the solution permeated dur-ing the experiment,A the effective membrane area,and t the operation time.The rejection,R ,was calculated using Eq.(2):R =1−C p C f(2)where C p and C f are the concentration of the permeate and the feed,respectively.Salt concentrations in the feed and permeate were determined by Electrical Conductiv-ity DDS-11A (Shanghai Leichi Instrument,China).Sac-charose (M W =342)and safranine T (M W =351)concen-trations in the feed and permeate were measured using a UV Spectrophotometer-751(Shanghai Angles Instrument,China).Sodium chloride,sodium sulfate,magnesium chloride,magnesium sulfate,safranine T and saccharose were obtained commercially as reagent grade chemicals and used as received.2.5.Analytical technique for characterization of active layerFTIR-ATR spectroscopy (FTIR-ATR)spectra were recorded using a Nicolet IR 560spectrometer with a hor-izontal ATR accessory equipped with a ZnSe crystal.For evaluation,32scans were taken at a resolution of 4cm −1.X-ray photoelectron spectroscopy (XPS)analysis was performed using a Perkin-Elmer Phi-5300X-ray photoelec-tron spectrometer with a monochromatized Al K ␣(15kV ,250mA)X-ray source.Vacuum during analyses was con-ducted at 10.10–10.9Pa.The take off angle between the analyzer and the sample surface was 45.For calibration the carbon C 1s peak was used (285eV).A JEOL JSM 6301F scanning electron microscope (SEM)operated at 20kV was used to analyze the porous membrane for the PEK-C NF membrane.Contact angles of membranes were determined on a level surface with water for 1min in air at 25◦C using a JY-82Contact Angle Analyzer (Chengde Experimental Equipment,China).Fig.3.FTIR-ATR of PEK-C NF membrane (a)and PEK-C UF membrane (b).3.Results and discussion 3.1.FITR-ATR studyIn order to obtain information about the structural changes of PEK-C NF membrane resulting from in situ interfacial polymerization of PAMAM and TMC,FTIR spectra of the surface region of PEK-C UF and PEK-C NF membranes were recorded using the ATR technique,as shown in pared with the PEK-C UF membrane,the IR spectra of the NF membranes exhibited evident absorbance signals at 2846,2920(C H)and around 3292cm −1(νN H).The similar spectra for the PEK-C UF membrane were not observed.Though the PEK-C UF membrane had a signal at 1648cm −1assigned to aryl carbonyl in PEK-C,the PEK-C NF membrane had a stronger signal at 1648cm −1assigned to carbonyl in the amido bond.This suggested that the active skin layer of the PEK-C NF membrane was polyamide containing amido bonds and amino and alkyl groups after in situ interfacial polymerization of PAMAM and TMC.3.2.XPS analysisThe chemical changes that occurred in the PEK-C NF membranes as a result of in situ interfacial polymerization of PAMAM with TMC were further ascertained by XPS anal-ysis.The XPS survey scans for the investigated samples are shown in pared with virgin PEK-C UF membrane,a nitrogen-containing functionality peak was observed,and the oxygen-containing functional peak was also higher than the PEK-C UF membrane.That can be attributed to active skin layer of the PEK-C NF membrane containing amino groups and amide bonds after interfacial polymerization of PAMAM with TMC.88L.Lianchao et al./Journal of Membrane Science269(2006)84–93Fig.4.XPS survey scans of PEK-C UF membrane(a)and PEK-C NF mem-branes(b).3.3.SEM imageThe active skin layer and cross-section morphology of the PEK-C NF membrane are shown in SEM patterns (Fig.5).In Fig.5,photo(a)was cross-section in NF PEK-C membrane,photo(b)was the active skin layer of the NF PEK-C membrane which located to the bottom in photo (a).And photo(b)was enlarged region which located to active skin layer in photo(a),photo(c)was also enlarged region which located to the top in photo(a),photo(d)was enlarged region which located to top in photo(c).The active skin layer in NF PEK-C membrane(Fig.5a and b)was an uniform,smooth and dense layer,no pore structure could be found on the active layer,so there was no porefilling found in the UF membranes.In fact,the pore sizes on the active skin layer of the NF membranes were usually in the magnitude of nanometers.It is often difficult to observe the pore structure of NF membrane surfaces using SEM.On the other hand, the cross-section photos(Fig.5a,c and d)showed that the PEK-C UF membrane was a good microporous membrane because it had regularfinger-like pores and sponge-like pores in the walls of pores(Fig.5c and d)enabling goodflux.3.4.Effects of concentrations of PAMAM and TMC on F and R of the PEK-C NF membranePerformance of the PEK-C NF membranes prepared under various concentrations and generation numbers of PAMAM are shown in Table1.Flux and rejection of the PEK-C NF membrane increased with increasing concentrations of PAMAM or generation numbers of PAMAM,except the R of NF1.This was ascribed to the fact that amino groups in the active skin layer increased with increasing concentrations of PAMAM and/or generation numbers of PAMAM;further-more these amino groups could improve thehydrophilicity Fig.5.SEM of PEK-C NF membrane(a)survey(b)active layer(c)microporous substrate(d)microporous wall in substrate.L.Lianchao et al./Journal of Membrane Science269(2006)84–9389 Table1Effects of concentration of PAMAM and TMC on F and R of the PEK-C NF membraneMembrane Generation numbers of PAMAM Concentration of reagents NF membrane propertiesPAMAM(×10−5mol/L)TMC(×10−5mol/L)F(L m−2h−1)R(%)NF1G0(4)a 4.7(18.8)b10.5(31.5)c39.243.7NF2G0(4)11.2(44.8)10.5(31.5)37.536.2NF3G0(4)16.8(67.2)10.5(31.5)51.257.8NF4G0(4)31.7(126.8)10.5(31.5)49.758.2NF5G1(8) 6.6(52.8)10.5(31.5)45.648.1NF6G1(8)10.9(87.2)10.5(31.5)51.852.4NF7G1(8)15.2(121.6)10.5(31.5)61.269.5NF8G1(8)32.1(256.8)10.5(31.5)59.568.8NF9G2(16)15.9(254.4)10.5(31.5)68.272.5 Operating pressure:0.6MPa;NaCl concentration:1000mg L−1.a Denotes number of amino group per molecule.b Denotes total number of amino group.c Denotes total number of acryl chloride group.of the PEK-C NF membrane so as to attain a higherflux. On the other hand,these amino groups and/or tertiary amino groups in water were able to be changed into RH3N+and R3HN+because of the combination of the lone electron pair on N and water[37].As the concentration of amino groups on membrane surface increase,RH3N+and R3HN+on the membrane surface increase,the repulsion between mem-brane with ions in feed is intensified,result in rejection of salts for this membrane increase.However,as concentration of PAMAM is more than three-fold of TMC concentration, effect of PAMAM concentration onflux and rejection is not obvious.Flux and rejection of NF3are similar to NF4,or NF7to NF8,as shown in Table1.Table1shows that the R of NF1was higher than NF2.This was mainly because TMC act as cross-linking point during interfacial polymerization,and the more cross-linking points, the more dense the cross-linked structure of the active layer. Therefore,the R of NF membrane decreased with decreasing PAMAM concentration.Different separation performances of the NF membranes could be obtained by changing the concentrations and generation numbers of PAMAM and/or TMC.3.5.Effects of different membranes on contact angle of waterThe contact angles of the PEK-C UF membranes and the PEK-C NF membranes using interfacial polymerization of PAMAM and TMC are listed in Table2.The contact angles of the NF membranes were lower than those of the PEK-C Table2The contact angles of NF membrane and UF membrane for water Membrane Contact angleθ(◦)UF membrane51NF142NF337NF633NF930UF membrane.This indicates an increase in membrane sur-face hydrophilicity due to the introduction of NH2groups to the membrane active layer,and hydrophilicity increased with increasing PAMAM concentration and/or the number of PAMAM generations.3.6.Rejection of different salts for PEK-C NF membranesThe rejection for different salts and their solutionflux was compared for NF2and NF5and NF9as shown in Table3.The salts rejections order of these three mem-branes is MgCl2>MgSO4>NaCl>Na2SO4.which is simi-lar to the sequence developed by Runhong Du[38],whereas this order is opposite to the one developed by Yu[39,40], i.e.Na2SO4>NaCl>MgSO4>MgCl2,where the negatively charged NF membranes was used.This sequence shows that PEK-C NF membranes used were all positively charged nanofiltration membrane.This order can be explained by Donnan exclusion[41,42].A positively charged nanofil-tration membranes have higher rejections for high-valence cations and low-valence anions than low-valence cations and high-valence anions.These results demonstrate that the rejec-tion to salts for a NF membrane is not only related with the pore size of the membrane,but also largely depends on the electrostatic action between the membrane and ions in solu-tion.Therefore,the separation property to salts of positively charged NF membranes will be very different from negatively charged NF membranes.The rejection of NaCl for NF9is Table3Performances of NF PEK-C membranes for different salts solutionSalts NF2(G0),R(%)NF5(G1),R(%)NF9(G2),R(%) MgCl277.284.295.7MgSO467.672.385.1NaCl36.248.172.5Na2SO438.247.658.2Driven pressure:0.6MPa;concentration of Na2SO4,MgSO4and NaCl and MgCl2:1000mg L−1,respectively.90L.Lianchao et al./Journal of Membrane Science269(2006)84–93Fig.6.The effect of MgSO4concentration on rejection andflux. 72.5%and the rejection of MgSO4is85.1%,showing that the NF9membrane is an excellent NF membrane.3.7.Effect of MgSO4concentration on R and F for NF7 membraneThe effect of MgSO4concentration on rejection and solu-tionflux for NF7under operating pressure of0.6MPa is shown in Fig.6.MgSO4concentration effects greatly on its rejection andflux.Both rejection andflux decrease with an increase of MgSO4concentration.The sequence can be explained by Donnan exclusion[41].As the ionic concen-tration increases,the electrostatic repulsion effect of the electrolytic membrane on the ions decreases,resulting in a decrease in rejection.Otherwise,the osmotic pressure increases with increasing the ionic concentration,the actual driving force of the system( P− π)is also reduced,result-ing in a decrease in waterflux.3.8.Effect of operation pressure on R and F of NF6 membraneEffect of operating pressure on the permeateflux and rejection of MgSO4(1000mg L−1)was investigated using NF6membrane.As can be seen in Fig.7,the permeateflux increases linearly with the operation pressure,and the change ratio is77.3L m−2h−1MPa−1.The permeateflux is in direct proportion to the operation transmembrane pressure which is the difference of operating pressure and osmoticpressure.Fig.7.Effect of operating pressure on R and F. Compared with the operating pressure,the osmotic pressure is so low that it can be neglected for the low concentration of salts.Therefore,the operating pressure is approximate to the transmembrane pressure.Other explanations based on equa-tion was appeared in previous literatures[42,43].Fig.7also shows that with operating pressure increasing in a certain range,the salt rejection of NF membrane is improved.This sequence is explained by Eq.(3):F=B C S(3)where B is the salt permeation coefficient, C S is salt the concentration difference on the two sides of the membrane, respectively.As P increases,waterflux increases, C S increase too,these effects combine to result in a gradual increase in salt rejection.These influencing trends are similar to sequences in previous reports[44,45].3.9.Effect of operating temperature on R and F of different solute using NF7membraneThe influence of operating temperature on rejections and solutionfluxes of MgSO4,sucrose and safranine T was determined using NF7membrane.Table4shows a trends of rejection for MgSO4decreased from82.4to72.6%, but the solutionfluxes increased two-fold with increase of the operation temperature.NF7membrane maintained 95.8–92.9%rejection of safranine T even though the solution flux increased nearly2.5-fold.In addition,molecular weight of safranine T is similar to sucrose,but the rejections ofTable4Effect of operating temperature on R and F of different solutes for NF7Temperature(◦C)MgSO4(M W=120)Sucrose(M W=342)Safranine T(M W=351)F(L m−2h−1)R(%)F(L m−2h−1)R(%)F(L m−2h−1)R(%) 2557.482.433.890.127.895.8 5078.678.558.684.546.594.1 80118.572.673.577.969.392.9Driven pressure:0.6MPa,Na2SO4concentration:1000mg L−1and safranine T and sucrose concentration:1000mg L−1,respectively.L.Lianchao et al./Journal of Membrane Science269(2006)84–9391Fig.8.Effect of pH in feed on F and R for NF3. safranine T dye were distinctly higher than sucrose.This ascribes to that electrostatic repulsion between the positively charged NF7membrane and cationic safranine T.When the temperature was returned back to25◦C,results similar to the original values were obtained.When the tem-perature was increased again,similar values were obtained again.This shows that the PEK-C NF membrane is ther-mostable at or below80◦C and gives reproducibleflux and rejection characteristics when changed temperatures repeat-edly.Table4shows that NF7membrane is especially suitable for separating cationic solutes from others by increasing oper-ating temperature.3.10.Effect of pH in feed on the PEK-C NF membraneflux of NaCl solutionFig.8shows the effect of pH in the feed on solutionflux for the NF3membrane(pH in the solution was adjusted from 2to9using sodium hydroxide and hydrochloric acid.The pressure applied wasfixed at0.6MPa.The feed solution is 1000mg L−1NaCl solution).Interestingly,it was found that the PEK-C NF membranes,when the pH is lowered(pH2,3 and5),exhibit higherfluxes,compared with theflux at pH7 or9using the same membranes,especially,theflux(at pH2) is higher than two-fold of theflux(at pH7).This implies that amino groups(NH2)groups and/or tertiary amino groups on the membrane surface interact with ions in the feed.As the pH in feed is decreased,amino groups on membrane surface changed into RH3N+,R3HN+[37].The electrostatic repul-sion between charged membrane and H+in the feed decreased due to more NH2groups on the surface of the membrane transferring to NH3+,resulting in enlarged pore surface. In contrast,increasing the pH in the feed,NH2groups on the surface of the membrane were not able to transfer into NH3+,and the electrostatic repulsion between membrane and ions increases,resulting in the shrinkage of pores on the membrane surface.This results are similar to NF mem-brane made from PPI dendrimer and TMC[31].Therefore the PEK-C NF membrane prepared with PAMAM and TMCby Fig.9.Effect of operating pressure onflux of NF1membrane at30◦C,when a solution containing1000mg L−1NaCl.interfacial polymerization is particularly suitable for treating acidic feeds.3.11.Stability-pressure of the PEK-C NF membrane forflux and rejection of NaCl solutionPrior tofiltration,deionized water from a reservoir was pressurized through the membranes for30min at1.2MPa in order to stabilize membranes.Thenfluxes were mea-sured separately at0.3,0.6,0.9,1.2,1.5,1.8and2.0MPa with1000mg L−1NaCl solution,as showed in Fig.9.Flux increases almost linearly with operating pressures from0.3to 1.5MPa.Flux decrease markedly when operating pressure up to1.8MPa.A significant increase in theflux of the resulting NF membrane at2.0MPafiltration indicated deterioration of the selective surface layer.That showed that maximum operating pressure for the studied membrane lower than 2.0MPa.The long-term test was carried out at operating pressure of0.6MPa with1000mg L−1NaCl solution after deionized water from a reservoir was pressurized through the mem-branes for30min at0.8MPa in order to stabilize membranes. Between periodical feed and permeate collection,additional flux and rejection measurements were carried out to check membrane performance.Thefluxes and rejections of the studied NF1membrane during the1080h(1.5months)of filtration were shown in Fig.10.The measuredfluxes of the studied NF membranes was33.6–39.4L m−2h−1at0.6MPa. The studied membranes showed slight difference influx dur-ing1080h(1.5months)of operation.And there is no big change in NaCl retention for the studied NF membranes dur-ing1080hfiltration period at0.6MPa,the overall retention of the SEPK-C NF1membrane reaching43.5–46.5%.This shows that the studied membrane maintained permeability and their selectivity after over1.5months of operation at 30◦C,which showed excellent stability during1080h(1.5 months)of operation.。