High performance ultrafiltration membrane based on modified chitosan coating and electrospun nanofibrous PVDF scaffolds

Journal of Membrane Science 394–395 (2012) 209–217

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High performance ultrafiltration membrane based on modified chitosan coating and electrospun nanofibrous PVDF scaffolds Zhiguo Zhao, Jianfen Zheng ∗∗ , Mingji Wang, Haiyuan Zhang, Charles C. Han ∗ Beijing National Laboratory for Molecular Sciences, Joint Laboratory of Polymer Science and Materials, State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e

i n f o

Article history: Received 22 September 2011 Received in revised form 30 November 2011 Accepted 28 December 2011 Available online 3 January 2012 Keywords: Chitosan Nanofibers Modification Ultrafiltration

a b s t r a c t Liquids filtration technology as one of the paths to sustainable water use is getting more and more attention. A new type of high flux ultrafiltration (UF) or nanofiltration medium based on electrospun fibrous scaffold and ultrathin top barrier layer was fabricated recently. Based on this new method, the chitosan (CTS) which is one of the best top layer materials due to its hydrophilicity and high waterpermeability was coupled with electrospun polyvinylidene fluoride (PVDF) nanofibers to compose a new type UF membrane. In this work, the chitosan was crosslinked and modified by glutaraldehyde (GA) and terephthaloyl chloride (TPC) to adjust its water resistance and surface properties. The modified membrane was characterized by FTIR, SEM, UV-spectra, static water contact angle analysis and filtration test. The modified membrane gets broader operating environment range and keeps a good flux rate and rejection efficiency in bovine serum albumin (BSA) filtration tests at 0.2 MPa, about 70.5 L/m2 h, rejection efficiency >98% which are higher than that of 57.1 L/m2 h, rejection efficiency∼98% of the commercial UF membranes, while the fouling of the membrane was kept at a very low level. This work may provide a practical possibility to the water filtration industry. © 2012 Elsevier B.V. All rights reserved.

1. Introduction The worldwide problems associated with the shortage of clean, fresh water have attracted more and more attention. Liquid filtration technique was discovered in 19th century, just as diffusion, dialysis and osmosis [1,2], and became well developed in the 20th century. Liquid filtration membrane such as micro-, ultraand nano-filtration membranes and even reverse osmosis membranes were invented and their markets spread rapidly throughout the world. As one of the most important technical methods for potable water purification and biomaterials concentration, ultrafiltration played an important role and took a significant portion of worldwide market. Sol–gel liquid filtration was the first generation ultrafiltration membrane [1], and with the great development of engineering plastics, new types of ultrafiltration membranes were invented, mostly as asymmetric membranes at present, such as polysulfone, polyvinylidene fluoride and polyacrylonitrile

Abbreviations: UF, ultrafiltration; CTS, chitosan; PVDF, polyvinylidene fluoride; GA, glutaraldehyde; TPC, terephthaloyl chloride; BSA, bovine serum albumin; NFCM, nanofibrous composite membrane. ∗ Corresponding author. Tel.: +86 10 82618089; fax: +86 10 62521519. ∗∗ Corresponding author. Tel.: +86 10 62550093; fax: +86 10 62521519. E-mail addresses: [email protected] (J. Zheng), [email protected] (C.C. Han). 0376-7388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.12.043

membranes [3–11]. Currently, much effort is being devoted to improve the membrane performance in terms of flux rate, antifouling property, chemical resistance, etc. [2,12–15]. Meanwhile, the non-woven fiber industry has made a rapid development recently. Traditionally, wet-laid, melt-blown and spun-bonded non-woven articles were well and vastly produced. In addition, the electro-spinning as a new non-woven technique that creates sub-micron to nano scale fibers through an electrically charged jet of polymer solution/melt, has gained greatly increasing appeal [16–19]. Compared with the traditional non-woven articles (such as wetlaid and melt-blown non-woven article), electrospun fibers have much smaller diameter, usually 10–100 times thinner than the former. Consequently, the electrospun nanofibrous articles possess some attractive attributes, such as high porosity, wide pore size range, interconnected open pore structure, a large surface area per unit volume, high flexibility, good biocompatibility and biodegradability (for fibers of biomaterials) and good modifiability. With these advantages, the electrospun nanofibers have been demonstrated in many applications, such as optical and chemical sensors [20–22], photovoltaics cells [23], protective textiles [24–26], wound dressing and as scaffolds in tissue engineering [27–31], immobilized enzymes and catalyst system [32], and filtration materials [33–36]. The nanofibrous membranes were first successfully used in air filtrations, and then directly used as liquid macro-filtration

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media [34]. In 2005, the electrospun nanofibers and liquid filtration were organically tied together by Ben Chu’s group [37]. In their work, composite membranes were prepared with electrospun PAN nanofibers as the scaffold and chitosan as the top ultrathin barrier layer. For the water–oil solution as the feeding liquid, the nanofibrous composite membranes obtained an excellent flux rate at high rejection efficiency of >99%. With their precursor work, a lot of excellent works were carried out, such as crosslinking PVA and cellulose acetate on the scaffolds of PAN electrospun nanofibers [38,39], and surface polymerization of polyamide on the electrospun nanofibrous scaffolds of PAN [40]. Chitosan, poly[␤-(1,4)-2-amino-2-deoxy-d-glucose], has good water permeability and antifouling abilities. However, its high swelling property in water makes it imperfect in some filtration environment. Protein filtration for example, the original chitosan coating barrier on the electrospun nanofibers gives less efficiency compared with that for oil–water filtration. So, the modification of the chitosan is necessary to enhance its scope of application. The modification of chitosan has been studied by many scientific groups; glutaraldehyde (GA), lactic acid, maleic anhydride, etc. were used as the modification regent [41–44]. In this work, GA was used to moderate bulk-crosslink firstly, and subsequently terephthaloyl chloride (TPC) was used to crosslink the surface of the chitosan barrier. When the CTS was crosslinked and the amino group was reacted, the thin film of chitosan can achieve distinct advantages for water filtration: first, dramatic water swelling decreased, so it could stand high pressure in filtration tests and keep film morphology well. Second, the electric charge was normalized, and thus reduced the interaction between chitosan and protein, so the protein adsorption became low. Third, the range of pH environment for chitosan was expanded: the membrane could function well at a high efficiency, in both acidic and basic environment.

2. Experimental 2.1. Materials and preparation Polyvinylidene fluoride (PVDF) with a weight average molecular weight (Mw) of about 37 × 104 g/mol and terephthaloyl chloride (TPC) were purchased from Aldrich Chemicals. Chitosan (high viscosity, Mw ∼ 40 × 104 g/mol) with about 80% deacetylate was purchased from Fluke Aldrich Chemical. No-woven PET article was supplied by Guocheng CO. (Wuxi, China). Glutaraldehyde (GA), triethylamine, N,N-dimethyl formamide (DMF), acetic acid, 1,2,4,5-benzenetetracarboxylic anhydride, tetrahydrofuran (THF), and bovine serum albumin (BSA) were purchased from Beijing Chem. Co. (Beijing, China). And other reagents involved in this work with no special descriptions were used directly without further purification. Chitosan was necessarily purified before use by the following procedure. A certain amount of chitosan sample was dissolved in dilute acetic acid (1%, v/v) to form 1 wt% chitosan solution and the solution was filtered using a sintered glass filter to remove insoluble substance. Subsequently, the solution was casted into a large glass Petri dish and left in the fume-hood for about 2–3 days to dry. The dried chitosan film was soaked in sodium hydroxide solution (1 mol/L) for several hours to remove the residual acetic acid. Then the double distilled water was used to wash the film several times until it was totally neutralized. The neutralized film was freezedried at last. The purified chitosan sample was dissolved in dilute acetic acid solution (1%, v/v) to prepare the casting solution at the concentration ranging from 0.6 to 1.0 mg/ml. Triethylamine needs to be redistilled to remove the first and the second amine which will have fierce side reaction with the chloride

groups. The original triethylamine reagent was mixed with a certain amount, about 5 g/100 ml, of 1,2,4,5benzenetetracarboxylic anhydride with a mild stirring, and redistilled at 90 ◦ C gently. The distillation was stored in conical beaker at cool and dry place to avoid the humidity and the sunlight. 2.2. Electrospinning PVDF could be Electrospun easily and efficiently based on some previous work in our group [45]. PVDF was dissolved in mixed solution of N,N-dimethyl formamide (DMF) and acetone, at different concentration range from 14 wt% to 24 wt%, with the ratio of DMF/acetone at 3/7, 4/6, 5/5, 6/4 and 7/3 at 50 ◦ C with gentle stirring for 24 h or a sufficiently long period of time to form the homogenous state. The PVDF solution was directly electrospun onto the surface of non-woven PET micro-filter substrate at 15–18 kV through an electrospinning device. The tip-to-collector distance was 15 cm and the electrospinning temperature including polymer solution and the environment was controlled at 50 ± 2 ◦ C. The solution was fed at a rate of 50 ␮l/min from a 5 ml syringe with a capillary tip which has an inner diameter of 0.3 mm. With the change of the mixed solvent and the concentration, the morphology and the diameter of the PVDF nanofibers was well controlled. To make the nanofibrous article smoother and denser, gentle hot-pressing process was executed, at 100 ◦ C and 5000 Pa for 2 h. The amount of PVDF nanofibers spun per unit area was about 1.2–1.5 mg/cm2 . 2.3. Chitosan coating and modification 2.3.1. Initial chitosan coating The composite fibrous support containing PVDF nanofibers and non-woven PET substrate was placed onto the plate of a stepped coaterR [46]. Certain amount of the chitosan casting solution, the concentration of which ranged from 0.6 wt% to 1.0 wt%, was applied into the space between casting-knife and composite fibrous support. Specific casting speed, 5 mm/s, and constant temperature of 25 ◦ C was applied. The three-tier composite membrane was dried for at least 24 h at 50 ◦ C in the blast drying oven. The dried membrane was then washed with double distilled water several times to make sure it completely neutralized and dried the second time under ambient condition. 2.3.2. Chitosan crosslinking with GA The amount of monomers in the chitosan solution was first calculated, and then different amount of GA was added into the casting solution. The R represents the ratio of the amount of crosslinking functional groups in the added GA reagent and the chitosan solution according to the Schiff Base Imine Functionality [42]. And then the mixture was stirred and dispersed under ultrasonic for 5 min to form homogenous solution. Coating process of the three-tier composite membrane with cross-linking reagent (GA) was the same as the initial chitosan coating. The three-tier composite membrane was dried at 60 ◦ C in the drying oven and then 100 ◦ C for additional 3 h to stabilize the crosslinking between chitosan and GA. 2.3.3. Chitosan modification with TPC The experiment was carried out in the dry box (relative humidity <30%). THF, 20 ml at 0 ◦ C, was filled into 100 ml beaker. The former three-tier original CTS composite membrane with the size of 4 cm × 4 cm in square was immersed into the THF solvent. Then 2 ml of cold triethylamine was added into the beaker to form an alkaline environment. And then, 2 ml TPC solution, 5 wt% in THF solvent, at 0 ◦ C, was slowly added into the beaker drop by drop under a mild stirring. The whole beaker was sealed up and kept for 72 h at 0 ◦ C. The modified membrane was washed with THF, 1 M NaOH and 1 M

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HCl subsequently for 3 times, and then double distilled water for several times and dried in ambient condition. 2.3.4. Chitosan co-modification with GA and TPC First the chitosan was crosslinked with GA to form a bulk crosslinked chitosan, and then the surface of the chitosan was modified with TPC. The whole process was carried out precisely in accordance to the separate process mentioned above.

The viscosity of the chitosan casting solution was measured with SNB-1 digital viscosity meter (NiRun Co., Shanghai, China) at a constant temperature of 25 ◦ C. The morphologies of the electrospun fibers and the composited membranes were observed using scanning electron microscope (SEM, JEOL JSM-6700F, Japan), at an accelerating voltage of 5 kV. Each sample was sputter-coated with platinum for analysis. The virgin chitosan and the modified chitosan were tested by FTIR-ATR mode on an infrared spectrometer (IR, BRUKER TENSOR27, Germany) in the range of 4000–600 cm−1 . The hydrophilicity performance of the membranes was evaluated on static water contact angle measurement instrument (PowerEach Co., Shanghai, China). The water uptake property was determined gravimetrically. The completely neutralized and dried chitosan and GA crosslinked chitosan dense films, about 20 ␮m thick, were weighted directly on the electronic balance (METTLER TOLEDO AL204). Then the samples were introduced into beakers containing 20 ml of swelling medium and shaken at 25 ◦ C. Every 30 min the films were removed from the medium, blotted off the water on the surface and immediately weighted. This procedure was repeated until the films reached constant weight (equilibrium water uptake). The water uptake (Wu ) of the films was calculated according to the following equation: Wc − W0 × 100, W0

where W0 and Wc are the weights of the samples in the dry and swollen states, respectively. 2.5. Filtration performance test Bovine serum albumin was separately dissolved in four types of solvent: ultra-pure water and three different buffers, pHs 3, 6, and 9 (1% acetate acid, phosphate buffered saline and borax salt dissolved in ultra-pure water). Also these four different solvents were prepared as feed solution for ultrafiltration process. A series of BSA aqueous solution was detected by ultraviolet–visible (UV) spectroscopy at a wavelength of 280 nm. Standard UV absorption curve of BSA was plotted by varying concentrations of the initial BSA feed solutions, by which the concentration of filtrate solution was determined. The rejection percentage was calculated by using the following equation: Rejection (%) =

Cf − Cp Cf

filtration data involved in this work with no special descriptions were all obtained under the same conditions. Three membrane specimens were tested and the final results were averaged. The commercial UF membrane (Sepro UF, PES10) was tested for comparison. 3. Result and discussion 3.1. Electrospinning of PVDF

2.4. Characterization

Wu (%) =

211

× 100,

where Cf and Cp represent the BSA concentration of the feed solution and that of the filtrate solution, respectively. The three-tier composite membranes were tested on an ultrafiltration cup at a constant pressure of 0.2 MPa, an effective filtration area of 13.4 cm2 , a constant temperature of 25 ◦ C and a stirring rate of 300 rpm. A 10 min pre-compacted process with pure water filtration at 0.2 MPa was carried out for each membrane. The BSA concentration of 1 g/L was used, with different types of solvents, including pure water, pH 7 buffer, pH 9 buffer and 1%HAc aqueous solution (pure water as blank test was operated as well). And the

PVDF nanofibers were successfully fabricated in the laboratory. Fig. 1 showed the morphology of the electrospun PVDF nanofibers. The series of SEM photographs demonstrated that with the increase of the concentration of the PVDF, the diameter of the fiber increases; meanwhile, with the increase of the ratio of acetone in the mixed solvent, the diameter of the fiber increase greatly as well. Ignore some usual factors in the process, for example, conductivity, electric charge property and surface energy, the dominating factor in this process was that, with the speed-up of the solvent evaporation rate, the solidification rate of the polymer was speeded up as well, as a result, the diameter of the fibers increased. The more uniform and thinner the fibers are, the higher operating pressure the top layer could withstand for filtration process. Therefore, the fabrication condition were chosen at the 16 wt% polymer concentration with the ratio of DMF/acetone = 7/3 and the average diameter of the PVDF nanofibers is about 170 nm. 3.2. Preparation of three-tier composite membrane Viscosity of the casting solution was one of the key parameters for the casting process. The thickness of the top layer of CTS ranges from 200 nm to 800 nm with the change of the concentration of the casting solution from 0.6% to 1.0% (shown in Table 1). At the concentration of 0.5 wt%, a flawless CTS layer can hardly be obtained. Fig. 2 showed the morphology of the three-tier composite membranes with a concentration of casting solution of 0.6 wt%. The SEM images of CTS and modified CTS composite membrane showed little difference. The smooth top layer surface was kept well after modification and the thickness of the casting CTS layer was uniform. The thickness of the top barrier layer is one of the most important parameter for the final performance. With the prerequisite of enough mechanical strength, the thinner the top layer is the better the performance will be. 3.3. Modification of CTS with GA and TPC The natural polysaccharides–chitosan, has a lot of amine groups which is quite reactive with other groups, for example acyl halides, aldehyde, peroxide and epoxy. The mechanism of the reactions in this work between GA&CTS and TPC&CTS is shown in Figs. 3 and 4. The former is based on Schiff Base Imine Functionality or Michaeltype Adducts with Terminal Aldehydes and the latter is based on electrophilic substitution reaction. Crosslinking chitosan with GA is a general method which has been studied by many research groups [41,42]. Fig. 5 showed the results of FTIR-ATR spectra of the virgin CTS samples, the samples after crosslinking with GA (CTS-GA) and the samples after crosslinking with TPC (CTS-TPC). The virgin CTS spectrum showed the majority of the function groups: the superposition peak of OH and NH2 band at 3400–3200 cm−1 , C H stretching band at 2870–2920 cm−1 , the bridge oxygen stretching band at 1160 cm−1 , and the C O stretching bands at 1070 cm−1 and 1030 cm−1 . The chitosan used in this work has an average degree of deacetylation of 80%, so that, the peaks at 1640 cm−1 and 1550 cm−1 were formed by the co-contribution of the residual acetamide groups bands which

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Fig. 1. The morphology of the electrospun PVDF nanofibers obtained from different PVDF concentrations and different solvent compositions: (A) 14 wt%, 5/5; (B) 18 wt%, 5/5; (C) 20 wt%, 5/5; (D) 24 wt%, 5/5; (E) 16 wt%, 7/3; (F) 16 wt%, 6/4; (G) 16 wt%, 4/6; (H) 16 wt%, 3/7*. *The ratio of DMF/acetone by v/v.

Table 1 The viscosity of the chitosan casting solution with different concentration and the thickness of corresponding casting chitosan layer. Concentration

0.5%

0.6%

0.7%

0.8%

0.9%

1.0%

Viscosity (mPa s) Thickness of casting layer (nm)

115 –

188 200 ± 35

224 320 ± 45

338 450 ± 60

450 600 ± 80

571 800 ± 150

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213

Fig. 2. The morphology of the composite membrane. (A) CTS layer; (B) GA crosslinked CTS layer (CTS-GA); (C) TPC modified CTS layer (CTS-TPC); (D) the cross section of the CTS coating three-tier composite membrane. The concentration of the casting solution is 0.6 wt% (inset: enlarged image of the top layer).

are located from 1660 cm−1 to 1500 cm−1 , and the NH2 bending band which is broad and centred near 1585 cm−1 . As a result of crosslinking reaction, significant changes were observed in the FTIR-ATR spectrum of the crosslinked samples versus the virgin CTS. The new peak at 1720 cm−1 and 1450 cm−1 indicated the presence of a carbonyl groups and CH2 groups, respectively. And at the carbonyl-amide region, because of the depletion of the NH2 , the corresponding broad band which is centred near 1585 cm−1 , together with the appearance of the new peak for C N, which can be located anywhere from 1620 cm−1 to 1660 cm−1 according to the literature [41,42], the peak at 1640 cm−1 was strengthened and meanwhile the peak at 1550 cm−1 was weakened. Modification of CTS with TPC is a novel method to modify the surface of the CTS ultra-thin barrier. At the alkaline condition, electrophilic reagents, acyl chloride was attacked by the nucleophilic reagent, amino groups, and formed the carbonyl-amide groups. The rise at 1590 cm−1 and 1450 cm−1 indicated the introduction of the benzene ring whose peaks are located at 1600 cm−1 ,1580 cm−1 , and 1450 cm−1 all of which became stronger especially while the ring is connected with the C O group due to the conjugation effect. Also the peak at 870 cm−1 of C H band in benzene ring as the corroborative evidence could be observed at the fingerprint region. The change of the spectrum band from 1700 cm−1 to 1500 cm−1 was influenced by two simultaneous factors. One was the disappearance of the NH2 , which influence a broad range centred near 1585 cm−1 , to reduce the absorption. While the other one was the appearance of aromatic amide, whose peaks located at 1650 cm−1 and 1550 cm−1 , to raise the absorption. And the former was the dominant factor, so the peaks were weakened. Furthermore, no peaks were obtained at the range from 1700 cm−1 to 1800 cm−1 , which indicated that neither ester groups were generated nor residual acyl halides retained.

The four types of CTS: virgin CTS, GA crosslinked CTS, TPC modified CTS and GA, TPC co-modified CTS were subjected to the static water contact angle measurement. The results in Fig. 6 showed that, even though the amine groups were reacted, the hydroxyl group survived and kept the barrier layer with excellent hydrophilic nature which is very important for anti-fouling property of the ultrafiltration membranes. 3.4. The filtration performance of the composite membranes Fig. 7 showed the performance of virgin CTS coated composite membranes. Take CTS-1% for example, the low BSA rejection exposed the low effectiveness of the virgin CTS barrier. Due to the penetration of BSA into the barrier, the water flux became extremely lowered. The thicker the CTS barrier was, the higher BSA rejection and lower water flux were obtained. The influence of the pH condition of the feed solution was also evaluated. Table 2 showed the different water flux and BSA rejection of the CTS-1% and CTS-0.6% composite membrane. At pHs 9 and 7, no obvious change was obtained, while at pH 3 (1% acetic acid), at the very beginning of the filtration test, the water flux became higher and the BSA rejection get lower oppositely. After a period of filtration, about 4–5 h, the CTS barrier layer was totally destroyed, and the BSA solution passed through the membrane directly. The reason is that, in the acetous condition, the CTS became more swollen; the potential gaps for water flow through became more possible and thus caused to the poor performance. As time goes on the CTS layer was gradually dissolved, and destroyed at last. For the purpose of obtaining high flux and BSA rejection efficiency and also obtaining acid resistance simultaneously, a modified process was carried out. First, CTS was crosslinked with GA. The water uptake property of the CTS casting films varied with the crosslinking ratio (R), as shown

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Fig. 3. Chitosan crosslinked with GA: Schiff Base Imine Functionality (left) and/or Michael-type Adducts with Terminal Aldehydes (right).

Fig. 4. The amidation of the CTS with TPC.

Fig. 5. FTIR-ATR spectrum of virgin CTS, GA crosslinked CTS (CTS-GA) and TPC modified CTS (CTS-TPC).

Z. Zhao et al. / Journal of Membrane Science 394–395 (2012) 209–217

215

Fig. 6. Static water contact angle measurement to membranes of virgin, GA crosslinked, TPC modified, and GA, TPC co-modified CTS.

Fig. 7. Filtration test of virgin CTS coated NFCMs.

Fig. 8. Filtration test of GA crosslinked CTS-0.6% coated NFCMs: R = 0, R = 1/10 and R = 1/3.

in Table 3. The results indicated that, the CTS obtained less water swelling property and good acid resistance after GA crosslinking. Meanwhile, the performance of the GA modified CTS-0.6% coated nanofibrous composite membrane (NFCM) was shown in Fig. 8. The membrane CTS-0.6% has the thinnest barrier layer and the highest flux for water filtration, so it is the best choice for the modification. When the crosslink ratio R ≥ 1, the crosslinked CTS barrier layer became too fragile to work, so there is no data for R ≥ 1. The results showed that, after the crosslinking, the membranes get a little increase in rejection efficiency of BSA, and decrease in flux rate simultaneously. The filtration test showed the consistent results with the water uptake test. The reason is obviously that, because of the form of crosslinked network, the CTS barrier layer became denser and less water swelling. At the ratio R = 1, the crosslinked CTS barrier becomes too fragile to work under pressure, and meanwhile at the ratio R = 1/10, the effect of crosslinking was not enough. So the middle case at the ratio R = 1/3 was selected. Second, the CTS barrier was modified by TPC. Long modification time, about 72 h, was taken to make sure the surface modification

reaction completed at a high ratio. The CTS-0.6% was the main choice because it has visibly high flux. The filtration performance of TPC modified CTS coated NFCM was evaluated at the same time. After the surface modification, the BSA became more difficult to pass through the barrier layer, so the high rejection efficiency was obtained. The results of water flux rate at 71.4 L/m2 h, and BSA rejection at 98% (shown in Table 4), was very satisfiable. The modification reagent GA was added into the solution of CTS to form homogeneous solution. With the water solvent evaporating, both the CTS and GA became concentrated, and at last the CTS was crosslinked with GA at the bulk level. While the modification regent TPC was added into THF environment with chitosan membrane immersed. Chitosan solid surface was wettable with THF and after the reaction between CTS and TPC, the top surface of CTS barrier became denser. The former modification reaction mainly occurred in the bulk of chitosan, while the latter occurred at the top surface. So the filtration results shown in Table 4 were different.

Table 2 PH influence of the CTS-1% and CTS-0.6% membranes. Sample

H2 O

BSA in pure water 2

CTS-1% CTS-0.6%

2

BSA in pH 9 buffer 2

BSA in pH 7 buffer 2

BSA in 1%HAc buffer

Flux (L/m h)

Rej ratio (%)

Flux (L/m h)

Rej ratio (%)

Flux (L/m h)

Rej ratio (%)

Flux (L/m h)

Rej ratio (%)

Flux (L/m2 h)

Rej ratio (%)

20.0 ± 1.5 204.8 ± 8.2

– –

14.8 ± 0.8 128.6 ± 6.4

85.2 ± 1.0 19.2 ± 3.5

13.8 ± 0.7 119 ± 5.9

86.4 ± 0.9 20.4 ± 2.7

13.8 ± 0.7 123.8 ± 6.1

85.7 ± 0.9 20.1 ± 2.7

16.2 ± 0.9 161.9 ± 8.0

80.3 ± 1.1 12.7 ± 3.7

Table 3 The water uptake of dense CTS films and CTS-GA films: R = 1/10, R = 1/3, and R = 1/1. Sample

CTS

CTS-GA 1/10

CTS-GA1/3

CTS-GA1/1

Wu in deionized water (%) Wu in 1%HAc buffer (%)

157 ± 11 Dissolved

115 ± 9 126 ± 7

67 ± 7 71 ± 5

31 ± 3 32 ± 3

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Table 4 Filtration test of the CTS-0.6%, CTS-0.6%-GA, CTS-0.6%-TPC and CTS-0.6%-GA-TPC. Sample

H2 O Flux (L/m2 h)

CTS-0.6% CTS-0.6%-GA CTS-0.6%-TPC CTS-0.6%-GA-TPC

204.8 181.0 101.0 72.7

± ± ± ±

8.2 8.0 4.4 3.6

BSA in pure water

BSA in pH 9 buffer

BSA in pH 7 buffer

BSA in 1%HAc buffer

Rej ratio (%)

Flux (L/m2 h)

Flux (L/m2 h)

Flux (L/m2 h)

Flux (L/m2 h)

– –

128.6 114.3 71.4 70.5

± ± ± ±

6.4 5.7 3.5 3.4

Rej ratio (%) 19.2 33.4 98.5 98.9

± ± ± ±

3.5 2.1 0.2 0.2

119.0 109.5 69.3 68.6

To evaluate the acid resistance property directly, the CTS and GA crosslinked CTS coated membranes were immersed into 1%HAc solution, and subjected to ultrasound treatment for at least 1 h, and the CTS-GA membrane kept the original morphology well while virgin CTS coated membranes was soon totally destroyed. Also the TPC modified CTS coated membranes were immersed into 1%HAc solution, and subjected to the same ultrasound treatment. Relied only on the top surface crosslinking, the membranes could not withstand the treatment for more than 5 min. The most important advantage for GA crosslinking is the acquisition of acid resistance property for the CTS barrier layer which makes the membrane to withstand the operation under the acid environment. While the most important advantage for TPC modification is the increase of surface crosslinking density and greatly increased BSA rejection efficiency. So with the combination of the two steps of modification, high performance membrane with both acid resistance and high BSA rejection efficiency was obtained. The final performance of the GA and TPC co-modified CTS coated NFCM is 70.5 L/m2 h at the BSA rejection of 98.9%, compared with the commercial UF membrane (Sepro UF, PES10), 57.1 L/m2 h, 98% BSA rejection, the performance is enhanced for more than 20%. Long period performance of the antifouling property is also evaluated. The results were shown in Fig. 9. Take three types of membrane: CTS-1% coated membrane, GA-TPC co-modified CTS0.6% coated membrane and the commercial membrane, of which the BSA rejection efficiency of both the latter two are all over than 98%. After 24 h operation, the efficiency of CTS-0.6%-GA-TPC NFCM decrease from 70.5 to 65.6 L/m2 h, while virgin CTS-1% from 14.8 to 8.32 L/m2 h and commercial UF from 57.1 to 37.8 L/m2 h. The result revealed that the modified CTS-0.6%-GA-TPC membrane showed very good antifouling property: after working 24 h, the flux decreased less than 7%.

Fig. 9. The performance of each membrane for long period of operation.

± ± ± ±

5.9 5.4 3.1 3.2

Rej ratio (%) 20.4 36.7 98.8 99.1

± ± ± ±

2.7 1.9 0.1 0.1

123.8 109.5 70.5 70.0

± ± ± ±

6.1 5.3 3.2 3.4

Rej ratio (%) 20.1 35.4 98.7 99.3

± ± ± ±

2.7 2.3 0.2 0.1

161.9 114.3 73.0 71.0

± ± ± ±

8.0 5.8 3.67 3.5

Rej ratio (%) 12.7 30.8 96.7 98.7

± ± ± ±

3.7 2.1 0.2 0.2

4. Conclusion The CTS coated, based on nanofibrous PVDF scaffold composite ultrafiltration membrane was successfully fabricated and a novel method to modify by GA and TPC was successfully carried out. With the two steps modification, the top barrier layer of chitosan on the scaffolds of electrospun PVDF nanofibers provided an excellent performance in the water-protein liquid filtration. The thin-film composite nanofibrous ultrafiltration membrane gave a result of 70.5 L/m2 h of flux, and 98.9% of BSA rejection, at 0.2 MPa. Compared with the commercial UF membrane (Sepro UF, PES10), 57.1 L/m2 h and 98%, the filtration efficiency was enhanced about 25% at a very high rejection efficiency level. And the membrane showed a very good antifouling property that after 24 h of operation, the flux decreased for less than 7%. This work may provide a new direction for filtration applications. Acknowledgements This work was financially supported by Main Direction Program of Knowledge Innovation of Chinese Academy of Sciences (KJCX2-YW-H19) and National Natural Science Foundation of China (20904060). References [1] J.D. Ferry, Ultrafilter membranes and ultrafiltration, Chem. Rev. 18 (1936) 373–455. [2] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marinas, A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301–310. [3] T.A. Tweddle, O. Kutowy, W.L. Thayer, S. Sourirajan, Polysulfone ultrafiltration membranes, Ind. Eng. Chem. Prod. Res. Dev. 22 (1983) 320–326. [4] P. Radovanovic, S.W. Thiel, S.T. Hwang, Formation of asymmetric polysulfone membranes by immersion precipitation. 1. Modeling mass-transport during gelation, J. Membr. Sci. 65 (1992) 213–229. [5] P. Radovanovic, S.W. Thiel, S.T. Hwang, Formation of asymmetric polysulfone membranes by immersion precipitation. 2. The effects of casting solution and gelation bath compositions on membrane-structure and skin formation, J. Membr. Sci. 65 (1992) 231–246. [6] I. Pinnau, W.J. Koros, Structures and gas separation properties of asymmetric polysulfone membranes made by dry, wet, and dry wet phase inversion, J. Appl. Polym. Sci. 43 (1991) 1491–1502. [7] S. Munari, A. Bottino, G. Capannelli, Casting and performance of polyvinylidene fluoride based membranes, J. Membr. Sci. 16 (1983) 181–193. [8] A. Bottino, G. Cameraroda, G. Capannelli, S. Munari, The formation of microporous polyvinylidene difluoride membranes by phase-separation, J. Membr. Sci. 57 (1991) 1–20. [9] D.L. Wang, K. Li, W.K. Teo, Preparation and characterization of polyvinylidene fluoride (PVDF) hollow fiber membranes, J. Membr. Sci. 163 (1999) 211–220. [10] Y. Maeda, M. Tsuyumoto, H. Karakane, H. Tsugaya, Separation of water–ethanol mixture by pervaporation through hydrolyzed polyacrylonitrile hollow fiber membranes, Polym. J. 23 (1991) 501–511. [11] I.C. Kim, H.G. Yun, K.H. Lee, Preparation of asymmetric polyacrylonitrile membrane with small pore size by phase inversion and post-treatment process, J. Membr. Sci. 199 (2002) 75–84. [12] J.J. Qin, F.S. Wong, Y. Li, Y.T. Liu, A high flux ultrafiltration membrane spun from PSU/PVP (K90)/DMF/1,2-propanediol, J. Membr. Sci. 211 (2003) 139–147. [13] A. Asatekin, S. Kang, M. Elimelech, A.M. Mayes, Anti-fouling ultrafiltration membranes containing polyacrylonitrile-graft-poly (ethylene oxide) comb copolymer additives, J. Membr. Sci. 298 (2007) 136–146. [14] Y.H. Zhao, B.K. Zhu, L. Kong, Y.Y. Xu, Improving hydrophilicity and protein resistance of poly(vinylidene fluoride) membranes by blending with amphiphilic hyperbranched-star polymer, Langmuir 23 (2007) 5779–5786.

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