BPA transfer rate increase using molecular imprinted polyethersulfone hollow fiber membrane

Available online at Journal of Membrane Science310(2008)38–43BPA transfer rate increase using molecular imprintedpolyethersulfone hollowfiber membraneChangsheng Zhao∗,Binyu Yu,Bosi Qian,Qiang Wei,Kaiguang Yang,Aimin ZhangCollege of Polymer Science and Engineering,State Key Laboratory of Polymer Materials Engineering,Sichuan University,Chengdu,Sichuan610065,ChinaReceived28August2007;received in revised form19October2007;accepted19October2007Available online4November2007AbstractBisphenol A(BPA)imprinted polyethersulfone(PES)hollowfiber membrane was spun using a dry–wet spinning method,the membrane was then prepared as afilter with an effective area of200cm2.The hollowfiberfilter was employed to study the BPA transport behavior.The transport ability of the prepared hollowfiber membrane was measured using100␮mol/l BPA aqueous solutions at aflowflux of50and75ml/min,respectively.The BPA transfer rate increased for the imprinted hollowfiber membranes due to the larger amount of binding sites,comparing with the non-imprinted one.In the present study,hollowfiber membrane and the molecular imprinting technique were combined for advanced separation and the data suggested that small molecules could transfer in the direction opposite to the concentration gradient due to different pH.©2007Elsevier B.V.All rights reserved.Keywords:Molecular imprinted polymer;Hollowfiber membrane;Transport;Bisphenol A;Polyethersulfone1.IntroductionMolecular imprinting technology plays an important role in constructing molecule recognition sites into some polymer matrix[1].The current opinion about molecular imprinting is a technology in which specific recognition sites are formed in a polymer matrix by the synthesis in the presence of the tem-plate molecule,or‘imprinting’molecule which results in the formation of specific recognition cavities complementary to the template in shape and chemical functionality[2].It has been shown that molecular imprinted polymers(MIPs)possess high selectivity and sensitivity for low molecular mass compounds, and have been widely used in medical and technologicalfield such as:chromatography[3],adsorbents to environmental estro-gens[4],filtration or adsorption biomolecules[5],etc.For the molecular imprinting methods,two general approaches are usually employed:non-covalent and covalent.∗Corresponding author.Tel.:+862885400453;fax:+862885405402.E-mail addresses:zhaochsh70@,zhaochsh70@ (C.Zhao).Recently,an easy non-covalent approach was developed with the use of phase inversion imprinting technology,which was a com-mon method to prepare membranes;and dibenzofuran imprinted polysulfoneflat membranes were prepared[4].Further investi-gations on the molecular recognition by MIP were performed withflat membranes,based on the selectivity,permeability,and binding[5,6].However,the relationship between the transporta-tion and the adsorption of the recognized molecules has not been reported.A hollowfiber membrane is superior to aflat sheet membrane because of its high surface area/volume ratio.Therefore,the solute transportation through a hollowfiber of MIP attracts much interest.The aim of this study is to combine the hollowfiber membrane and the molecular imprinting technology,and thus to investigate the solute transportation behavior of MIP hollow fiber membrane.PES is a well-known polymeric material.The PES and PES-based membranes show outstanding oxidative,thermal,and hydrolytic stabilities as well as good mechanical andfilm-forming properties.In our earlier study,PES hollowfiber membrane was prepared using phase transition technique,and used for plasma separation[7]and scaffolds for artificial lacrimal0376-7388/$–see front matter©2007Elsevier B.V.All rights reserved. doi:10.1016/j.memsci.2007.10.042C.Zhao et al./Journal of Membrane Science 310(2008)38–4339glands in vitro [8].Also,BPA-imprinted PES particles were prepared for the binding and recognition of BPA [9].In the present study,we prepared BPA-imprinted PES hollow fiber membrane using phase transition technique and dry–wet spinning method based on our previous work.The BPA trans-portation through the hollow fiber membranes and the BPA adsorption to the membranes were systemically investigated.2.Experimental 2.1.MaterialsAll the reagents used in the experiments were of reagent grade unless otherwise described.PES (Ultrason E 6020P)was pur-chased from BASF chemical company (Germany).BPA was purchased from Shanghai Chemical Reagent Co.Ltd.(China).Dimethyl acetamide (DMAC)was obtained from Chengdu Chemical Reagent Co.Ltd.(China).Distilled water passed through ion-exchange columns was used throughout the studies.2.2.Preparation of imprinted PES hollow fiber membranes and filtersPES and BPA were dissolved in DMAC with a concentra-tion of 24and 5wt%,respectively.Then the resultant polymer solution was degassed.Dry/wet-spinning technique,described elsewhere [10],was used to fabricate BPA-imprinted PES hol-low fiber membranes.The membranes were incubated in water for over 24h to elute the residual DMAC from the membranes,and the extracted water was changed every 5h.Finally,extrac-tion of the template molecules from the solidified membrane was carried out in ethanol.FT-IR and UV–vis spectrophotometer (Shanghai 752)were used to ensure the complete extraction of the template molecules BPA.The resultant fibers were immersed in a tank containing 50wt%glycerol solution for at least 6h to keep the pore morphology,and then dried in air at room tempera-ture.Simultaneously,the non-imprinted hollow fiber membrane was prepared under the same conditions without adding template molecules.The PES hollow fiber membrane filter was prepared,employing epoxy resin as the potting material,with an effective area of about 200cm 2.Fig.1.Diagram for the filtration experiments.2.3.Scanning electron micrograph (SEM)and Fourier transform infrared (FT-IR)For SEM observation,the membrane samples were dried at room temperature.Then the membranes were quenched by liq-uid nitrogen gas,cut with a single-edged razor blade,attached to the sample supports and coated with a gold layer.A scan-ning electron microscope (JSM-5900LV ,JEOL)was used for the morphology observation of the hollow fiber cross-sections.To prepare the FT-IR sample,the imprinted and non-imprinted membranes were dissolved in chloroform,respec-tively and cast on the glass plates with about 1␮m thickness,and then the cast polymer solution was dried to remove the chloro-form.The FT-IR spectra of the membrane were measured with FT-IR Nicolet560(Nicol American).2.4.Transport and ultrafiltration experimentsThe BPA aqueous solution (400ml)was introduced into the filter by a blood pump with a controlled flow rate,as shown in Fig.1.The initial concentration was 100␮mol/l with pH 6.3for the internal side of the hollow fiber membrane;while the initial concentration for the outside of the membrane was 120␮mol/l with pH 13.0adjusted by NaOH solution.The fil-tration rate was controlled by the pressure of the outlet of the filter.The samples (both for the lumen side and shell side)were collected every 5min,and the concentration of BPA was deter-mined with an UV–vis spectrophotometer at the wavelength of 276nm.The effect of the pH value on the BPA absorption has beenconsidered.Fig.2.SEM photographs of the cross-section of the imprinted membrane.V oltage:20kV;magnification:(a)80×,(b)1000×,and (c)20,000×.40 C.Zhao et al./Journal of Membrane Science 310(2008)38–43Fig.3.Bisphenol A (BPA)concentration changes for the internal side and outside of the membrane.(a)The imprinted membrane;(b)the non-imprinted membrane;data are expressed as the mean ±S.D.of three independent measurements.Q I and Q F are the inlet flow rate and filtration rate,respectively.3.Results and discussion 3.1.SEM observationThe prepared PES polymer membranes were opaque in appearance and satisfactorily strong for a high-pressure application.The cross-sections of the imprinted hollow fiber membrane are shown in Fig.2.As shown in the total cross-section,Fig.2(a),the thickness of the hollow fiber membrane was about 180␮m,and the diameter was about 1000␮m.A skin layer was found on the side of the membrane wall,under which was finger-like structure as shown in Fig.2(b),which was magnified 1000×.In this case,many small pores were observed inside the membranes as shown in Fig.2(c),which was magnified 20,000×.The SEM shows that the morphology and porous structure of the imprinted and non-imprinted are similar to each other.The PES transformation from polymer solution to the solid state occurred quickly in poor water solvent and followed by instantaneous demixing of PES precipitation [11].Because of the poor solubility of BPA in water,the BPA remained in the solid PES mem-branes,when the solvent exchange between DMAc and water proceeded.Thus,after the extraction of BPA from the solid membranes,imprinting sites of the template were formed.Furthermore,it was clearly observed that the finger-like structure was inter-dicted in the middle of the membrane,as shown in Fig.2(b).This was also caused by the exchange between DMAc and water during the membrane for-mation.The PES hollow fiber membrane was spun by the dry–wet spinning method.Exchange between DMAc and water occurred rapidly from the inter-nal side of the nascent hollow fiber membrane when the polymer solution was extruded through the spinneret.After the nascent fiber immersed in the coag-ulation bath,the exchange began from the outside of the membrane.Thus,a porous wall formed in the middle of the hollow fiber membrane.By manipu-lating the membrane wall thickness and the air gap,the porous wall could be adjusted [12].With the increase of the extrusion rate of the polymer solution,the wall thickness increased.When the internal coagulant injection rate or the take-up rate increased,the wall thickness decreased.The membrane wall thickness decreased when the air gap increased.3.2.BPA concentration changes for the two sides of the membranesMembrane systems can be managed either through dead-end filtration or through cross-flow filtration.When dead-end filtration takes place,all the matter enters the membrane surface and pressed through the membrane.Some solids and components will stay in the feed solution while water flows through the membrane.For flat sheet membranes,dead-end filtration is usually applied,asmentioned above for the imprinted membranes [4–6].When cross-flow filtration takes place,feed water is recycled,and the feed water flow is parallel to the membrane.Only a small part of the feed water is used for permeate production,most of the feed will be left in the module.In this case,hollow fiber membrane is always used.In this study,as described in Section 2.4,it was cross-flow filtration.The PES hollow fiber membrane in this study belongs to ultrafiltration mem-brane.Ultrafiltration is a pressure-driven process usually employed to remove solvent (typically water)and small solutes from large macromolecules.Since mass transport is dominated by convection,the rate of mass transport for both the product and the small solutes is proportional to the filtrate flux and the cor-responding solute sieving coefficients (Sc),where Sc is equal to the ratio of the solute concentration in the filtrate to that in the bulk (feed)solution [13].Here,the Sc is the observed sieving coefficient;the actual sieving coefficients can be calculated from the experimental data for the observed sieving coeffi-cients using a stagnant film model [14].For cross-flow filtration,the observed sieving coefficients were about 0.73and 0.58,respectively for the imprinted and non-imprinted PES membranes.The BPA concentration changes for the two sides of the membrane wall of the imprinted and non-imprinted hollow fiber were investigated using cross-flow filtration system at two flow rates (inlet flow rates Q I are 50and 75ml/min,respectively;and the ultrafiltration rates Q F are 1.0and 1.2ml/min,respectively).As shown in Fig.3,it was clearly observed that the BPA concentration increased at the higher concentration side,and decreased at the lower concentration side.These results indicated that BPA transferred opposite to the concentration gra-dient.Furthermore,for the imprinted hollow fiber membrane,the concentration increased quickly to a large value at the higher concentration side.Additionally,the ultrafiltration rate also had an effect on the BPA transport.With the increase of the filtration rate (Q F )and the inlet flow rate (Q I ),the BPA concentration increased quickly at the higher concentration side,though a large amount of solvent (water)permeated into the higher concentration compartment.These results indicated that a relatively larger amount of BPA transferred through the membrane compared to the solvent.3.3.BPA transport and adsorptionAlthough BPA molecules transfer through both the imprinted and non-imprinted membranes,it should be noted that their transport ability is different.Fig.4shows the transferred amount and the adsorbed amount by the mem-branes.The transferred amounts increased with time for both the imprinted and non-imprinted membranes;the adsorbed amounts into the membranes increased with time,and then reached an equilibrium value after 10min.A relatively stable ratio was found comparing the adsorption amount (or the transport amount)for the imprinted and non-imprinted membranes.The ratio is the recognition coef-ficient,which is commonly used in molecular imprinting technique [15–17],C.Zhao et al./Journal of Membrane Science 310(2008)38–4341Fig.4.Bisphenol A (BPA)transferred amount through the membranes and the adsorbed amount into the membranes.Q I =75ml/min;Q F =1.2ml/min;data are expressed as the mean ±S.D.of three independent measurements.and the recognition coefficient was about 1.8for the PES membranes.The BPA transferred and adsorbed amounts for the imprinted PES membranes were higher than that for the non-imprinted ones.These results indicated that there existed a correlation between the transport and the adsorption,and the adsorption might play an important role on the transport performance.For further understanding the BPA transfer through the membranes,the effects of pH value and ethanol on the BPA adsorption were investigated at static condition.In the experiments,the hollow fiber membranes were cut to 1–2mm in length,then about 30mg of the short fiber membranes were immersed in 6ml BPA solution (150␮mol)with different pH value and ethanol content,respectively,and incubated for 24h.And then the BPA adsorbed amount was calculated,data are shown in Fig.5As shown in Fig.5(a),there was no change in the BPA adsorbed amounts when the pH value increased from 0.5to 9;however,when the pH value was larger than 9,the BPA adsorbed amount decreased.When the pH value was larger than 12,almost no BPA was adsorbed to both the imprinted and non-imprinted PES membranes.The p K a value of BPA ranged from 9.59to 11.30[18].At the pH value larger than the p K a value,the proton of –OH in the BPA molecules is deprotonated,thus negatively charged BPA molecules formed,the negatively charged BPA molecules were soluble in water and affected the binding to the membranes.The low adsorption at high pH value might be the main reason that led to the BPA molecules transferred through the membranes.Also,the BPA adsorbed amount decreased with the increase of the ethanol content in theBPAFig.6.Scheme of bisphenol A (BPA)transfer from low concentration to high concentration.solution,as shown in Fig.5(b).This was also caused by the high solubility of BPA in high ethanol concentration solution.3.4.BPA transport mechanismThe transfer of BPA molecules by the molecular imprinted membrane in the direction opposite to the concentration gradient is shown in Fig.6.The density of the bound BPA to the membrane in the lower BPA concentration side is larger than that in the higher concentration side due to the lower sol-ubility and stronger interaction.Although the binding sites are settled in the membranes,BPA transport along with the solution flow through the membrane (ultrafiltration)by binding site gradient via binding/desorption from the lower concentration to the higher concentration side.At the higher concentration side,the bound BPA molecules desorbed from the sites due to the high solubility and the weak interaction at the high pH value.For the non-imprinted membrane,the BPA transferred due to the hydrophobic interaction,while for the imprinted membrane,the BPA transfer rate increased because of the larger amount of binding sites.The transport pathways in a polymer membrane can be either the free vol-ume between polymer chains,the solvent fraction of a swollen polymer gel or connected pores in a solid polymer.In fact,Ulbricht [19]has concluded that for molecular imprinted membranes as a consequence of the binding selec-tivity obtained by imprinting for a substance.The transport is facilitated via binding/desorption to neighbored MIP sites.In our study,there was no specific interaction for the non-imprinted membrane;BPA was adsorbed to the mem-brane due to the hydrophobic interaction.For the imprinted membrane,the BPA adsorption was caused by specific binding sites and hydrophobic interaction.Thus,the BPA transfer rate increased due to the larger amount of binding sites for the imprinted hollow fibermembrane.Fig.5.Effect of pH value and ethanol content in the solution on the bisphenol A (BPA)adsorption at static condition.BPA solution:6ml,150␮mol and for 24h;( )imprinted and ( )non-imprinted membranes.42 C.Zhao et al./Journal of Membrane Science310(2008)38–43Fig.7.FT-IR spectra of the non-imprinted polyethersulfone(PES)membrane (a)and imprinted PES membranes(b).The different pH of the BPA solutions was a critical factor for BPA trans-fer from low concentration to high concentration,since the pH value had great effect on the BPA adsorption.When the pH value for the high concentration side decreased to about9,no concentration increase was observed,though the BPA molecules may also be transferred through the membranes due to the convec-tion.Furthermore when the concentration for the high concentration side was very large,also no concentration increase(for the high concentration side)was observed.To demonstrate the effects of the solubility and the interaction on the BPA transfer,aqueous BPA solution was placed in the lower concentration com-partment,while ethanol/water mixed BPA solution was placed in the higher concentration compartment.When the ratio of the ethanol to water is small,the BPA molecules were adsorbed into the membrane from two sides at the begin-ning,and the BPA concentration decreased.With the increase of the ratio,it is easy to observe that the BPA molecules transferred in the direction opposite to the concentration gradient,and the BPA concentration increased in the higher concentration compartment.The results could be easily explained by the BPA adsorption in ethanol/water mixed BPA solution as shown in Fig.5(b).3.5.Interaction between PES and BPAThe BPA-imprinted PES membranes showed high binding of BPA in contrast to the non-imprinted membranes,and the BPA transfer rate increased through the imprinted hollowfiber membranes.It is proved that the morphology and porous structure of the imprinted and non-imprinted are similar to each other.It is the recognition sites that lead to the transfer difference between the imprinted and non-imprinted membranes.To study the recognition sites and BPA interaction, FT-IR analysis was carried out.The difference in FT-IR spectra of the non-imprinted(a)and the imprinted PES(b)are shown in Fig.7.In the spectra of PES (a),the bands at1241cm−1corresponded to the S(O)2asymmetric stretching, and1149and1103cm−1to the symmetric stretching[4].Also,C–O–C stretch-ing of the main chain of the PES appeared near1010cm−1.Aromatic bending peaks at719,700,and554cm−1were assigned to the out-of-plane CH bending [20].A difference between the spectrum(a)and(b)was apparently observed in the PES bands.The1241cm−1S(O)2of the PES peak shifted to1238cm−1 for the imprinted PES.From this change,it can be deduced that the electronic state of the S(O)2in the PES is influenced by the attached BPA.This result suggested that the S(O)2in the PES molecules can interact with the–OH in the BPA molecules through hydrogen bonding according to a report[21].When the PES and BPA were dissolved in the DMAc together,the interaction began. This interaction seems solidified in the initial membranes.After the BPA was extracted from the solidified membranes,this interaction is to be remained inthe Fig.8.Schematic of the hydrogen bond interaction between polyethersulfone and bisphenol A.imprinted polymers,which is verified by the red shift of S(O)2in the Fig.7. Through this process,the recognition sites formed and had special binding to BPA[22,23].The diagram of the hydrogen bond interaction between PES and BPA is shown in Fig.8.4.ConclusionThe transfer of BPA molecules can be regarded as the BPA bound to the sites in the lower concentration compartment and released in the higher concentration compartment.Thus,the binding sites are needed.For the binding,the BPA molecules should interact with the binding sites;which means that at the lower concentration side,they should have strong interaction. Therefore,they should have weak interaction and good solu-bility at the higher concentration side,where desorption could happen.The process needs energy–pressure,which leads to the filtration and the“diffusion”.Moreover the concentration gra-dient also affects the BPA transfer.Here we demonstrated that small molecules could transfer from low concentration to high concentration,and the transfer rate increased for the imprinted PES hollowfiber membranes due to the large amount of binding sites.The imprinted membrane has potential use for separation, purification,and chromatography.AcknowledgementsThis work wasfinancially supported by the National Natural Science Foundation of China(nos.5040328,50673064)and the Project-sponsored by state Education Ministry of China(no. 2004527-16-14and NCET-05-0791).The authors thank Prof. Makoto Takeishi for many useful discussions and comments. 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