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Novel composite membranes prepared by interfacial polymerization on polypropylene fiber supports pretreated by ozone-induced polymerization
Desalination 294 (2012) 36–43
Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Novel composite membranes prepared by interfacial polymerization on polypropylene fiber supports pretreated by ozone-induced polymerization Kai Pan, Peng Fang, Bing Cao ⁎ College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China Key Laboratory of carbon fiber and functional polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing, 100029, China
a r t i c l e
i n f o
Article history: Received 25 October 2011 Received in revised form 28 February 2012 Accepted 1 March 2012 Available online 6 April 2012 Keywords: Interfacial polymerization Hydrophilicity Ozone treatment Nanofiltration membrane
a b s t r a c t Interfacial polymerization (IP) is a powerful technique for the fabrication of thin film composite (TFC) membranes. In this work, porous polypropylene fiber films were used as a support to fabricate TFC nanofiltration membrane using IP technique. Firstly, the surface of polypropylene fiber film was hydrophilized by introducing peroxide onto the membrane surface using ozone treatment followed by grafting acrylamide. And then interfacial polymerization could be successfully proceeding on the hydrophilized PP membrane. The monomeric system chosen for IP was m-phenylenediamine (MPDA) and trimesoyl chloride (TMC). The ozone treatment time was investigated, and in the range of 5–10 min ozone treatment time, the PP fiber membrane had an available amount of peroxides for grafting while keeping the mechanical strength. The characteristics of the original membrane, grafted membrane and TFC membrane were studied with various analytical methods, such as SEM and FTIR-ATR. Meanwhile, the nanofiltration performance was evaluated with aqueous solutions of Na2SO4 (2 g/L) and Fast Green FCF (MW 808.84, 10 mg/L). The rejection rate of 60% and 90% were achieved for Na2SO4 and Fast Green FCF, respectively, at a transmembrane pressure of 0.5 MPa. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a new separation method, membrane technology is developing rapidly in recent decades. Its application area includes water purification, chemical industry, biological engineering, medicine, foods and so on [1–3]. According to different molecular weight, chemical structure and physical properties of the separation material, membranes can be divided into reverse osmosis (RO) membrane, nanofiltration (NF) membrane, ultrafiltration (UF) membrane and microfiltration (MF) membrane. Nanofiltration (NF) membranes, which exhibit separation characteristics in the intermediate range between reverse osmosis and ultrafiltration, are gaining significant attention because of advantages such as low operation pressure, high permeate flux, and high retention of multivalent ion salts. And composite nanofiltration membranes are generally fabricated using interfacial polymerization (IP) [4,5]. The IP technique is based on a polymerization reaction which takes place at the interface between two immiscible phases, for example, an aqueous phase and a hexane phase. Each phase is a solution containing a dissolved monomer. For example, diamine and diacyl chloride can be used as monomers in IP, and polyamide is the reaction ⁎ Corresponding author at: College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China. Tel.: + 86 10 64413857. E-mail addresses: [email protected] (K. Pan), [email protected] (B. Cao). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.03.007
product which forms a selective layer on the support (see Scheme 1). The nature of the solvents and monomers as well as monomer concentrations, reaction time and heat curing defines the porosity, pore size and thickness of the selective layer [6–9]. Hydrophilic supports have a distinct advantage compared to hydrophobic ones, for example, hydrophilic support can make aqueous solution spread on the surface to benefit the dispersing of amine monomers, which achieves goals of interfacial polymerization; moreover, hydrophilic membrane can reduce fouling due to pore plugging with biological macromolecules. Supports currently used for IP are mostly polysulfone (PSf) [10], polyethersulfone (PES) [11] and polyacrylonitrile (PAN) [12]. Polypropylene (PP) membrane has been widely used due to the well-controlled porosity, high stability, and low cost of the raw material. However, PP membranes are relative hydrophobic and non-wettable with water, the potential application in preparation of composite membrane with IP is limited, and so PP must be conducted with hydrophilic modification firstly. The chemical modification of polymeric surfaces such as graft polymerization has been extensively studied. In graft polymerization, functional groups were first introduced to the polymeric substrate by UV irradiation [13], plasma [14–16], or ozone [17] and then decomposed and initiated for polymerization. In this study, the oxidation of PP membranes, which can trigger acrylamide (AM) monomer grafting on the surface, was done by exposing ozone gas for predetermined time. The effects of ozone treatment time on the tensile strength of the membrane and the degree of graft
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reactor was removed by oxygen gas purging, and then the membrane was placed under vacuum for 30 min to make sure the physically absorbed ozone molecules were removed.
Scheme 1. Structural formula showing the chemistry of the film composite membrane.
polymerization were examined. Then the polyamide functional layer was formed on the surface of hydrophilic PP support by the reactive monomer system of m-phenylenediamine and trimesoyl chloride. The Na2SO4 solution and Fast Green FCF solution were used to characterize the permeate property of fabricated TFC membranes. 2. Experimental 2.1. Materials Polypropylene microporous membranes made by melt-spinning method were purchased from Zetao industrial Co., Ltd. (China) and used as a substrate for this study; acrylamide (98%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used after recrystallization in acetone; FeSO4·7H2O (99%) was purchased from Chemical Reagent (China) and used directly; acetone (99%), ethanol (99.7%), MPDA (99.55%), Na3PO4 (98%) and n-hexane (97.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used without any further purification; TMC (99.0%) was purchased from Alfa Aesar (China) and used directly; De-ionized water was used as the solvent of the aqueous phase. 2.2. Membrane surface modification and polymerization 2.2.1. PP membrane surface ozone treatment Firstly, the PP membrane was put into acetone for 12 h and then dried in vacuum oven for 6 h. Ozone was generated by an ozone generator (CF-G-2-50 g, Guolin, China) using pure oxygen gas as illustrated in Fig. 1. The ozone concentration and the oxygen gas flow rate were adjusted to about 96 mg/L and 1 L/min. And then ozone went through the ozone reactor which contains the PP membrane under room temperature. After ozone treatment for a given time, residual ozone in the
Fig. 1. Experimental setup for ozonation. 1. Oxygen tank; 2. Ozone generator; 3. Flow meter; 4. Ozonation reactor; 5. Membrane.
2.2.2. PP membrane surface grafting with AM The ozone-treated membrane was soaked in ethanol for 10 min in order to wet the surface, and then placed in AM solution immediately after wetting. The grafting reaction was initiated by the addition of FeSO4·7H2O and proceeded as shown in Fig. 2. Graft polymerization was allowed to proceed in the jar at 50 °C with magnetic mixing for 5 h. The jar was filled with N2 during the whole grafting polymerization process. The overall concentrations of AM solution and the catalyst (FeSO4·7H2O) were 10 wt/v% and 0.01 wt/v% respectively. When the polymerization was finished, the membrane was soaked in methanol for 12 h and washed by deionized water several times to remove residual monomers and homopolymers. Then the grafted membranes were stored in deionized water before used. 2.2.3. Interfacial polymerization on grafted PP membrane The polyamide active skin layer was synthesized using interfacial polymerization technique. Firstly, the PP membrane previously modified by ozone and grafted with AM was soaked in an aqueous solution of MPDA for 15 min and then positioned vertically to drain the excess solution. Subsequently, the PP membrane soaked with the amine aqueous solution of m-phenylenediamine (2 wt/v%) reacted with a hexane solution of trimesoyl chloride (0.4wt/v%). The interfacial polymerization time was 1 min. The modified membranes were placed in an oven at 70 °C, heat treated for about 20 min to attain the desired stable structures. Finally, the polyamide thin-film composite membrane was kept under ambient air conditions for further use. 2.3. Characterization 2.3.1. FTIR-ATR characterization Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR) spectrometer (Spectrum RX-I, Perkin-Elmer, USA) was used to detect functional groups of grafted membranes. 2.3.2. Water contact angle measurement The hydrophilic of the grafted membrane surface was tested by water contact angle measurements instrument (JC2000C, Shanghai Zhongchen Co., China) at room temperature. Before the samples were tested, they were fully dried. Five different locations were chosen for each value and record every point after 15 s stable. 2.3.3. Scanning electron microscopy (SEM) The surface morphology of the membranes was investigated by scanning electron microscopy (SEM) on Hitachis-4700 SEM (Hitachi, Japan). The membranes were mounted on the sample studs by means of double-sided adhesive tapes. A thin layer of palladium was sputtered onto the membrane surface prior to the SEM measurement. The cross section of sample was obtained by breaking the sample in liquid nitrogen. The measurements were performed at an accelerating voltage of 20 kV. 2.3.4. Determination of peroxide groups on ozone treated membrane The amount of peroxide groups produced on the membrane by the ozone treatment was determined using the DPPH (2,2-diphenyl-1(2,4,6-trinitrophenyl)-hydrazyl) method. Different concentration of DPPH solution was determined by the absorbance at the wavelength of 517 nm using a UV–vis spectrophotometer. Standard curve of DPPH was obtained for further test. One equivalent of hydroperoxide reacts with two equivalents of DPPH. The PP membrane treated for a given period of time by ozone was put into 50 ml NMP solution with a certain amount of DPPH. The sample solutions prepared were refluxed with stirring and N2 purging at 60 °C for 24 h. Finally the remaining
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Fig. 2. The preparation process of the PP-g-PAM membrane via surface copolymerization.
DPPH concentration was determined according to the standard curve. The amount of peroxide groups is defined as follow: D¼
ðC0 −CÞV 2M DPPH S
where C0 and C are DPPH concentration in original solution and reacted solution, V represented the volume of solution, S is surface area of membrane. 2.3.5. Mechanical properties Mechanical properties of the original PP membrane and ozonized PP membrane were tested by material experimental machine (Instron1185, USA) under the constant ambient temperature (temperature, 25 °C; humidity, 50± 10%RH). 2.3.6. Performance of the polyamide thin-film composite membrane The permeate flux and the rejection are evaluated by standard permeation tests. The filtration apparatus is a stainless steel dead-end stirred cell (HP4750 Stirred Cell, Sterlitech Corp., USA) resting on a magnetic stir plate. The filtration cell has a capacity of 0.350 L and effective membrane area of 14.6 cm2. All tests are conducted at room temperature (25 °C). Nanofiltration of Na2SO4 was investigated using TFC membrane at a transmembrane pressure of up to 0.5 MPa. The salt concentration of the feed was 2 g/L. The concentration of Na2SO4 in the permeate was analyzed by conductivity meter (DDS-11A). The salt rejection of composite membrane (R) is defined as follow: κp Rð% Þ ¼ 1− 100 κf where Κp is the conductivity of the product (μS/cm) and Κf is the conductivity of the feed (μS/cm). Nanofiltration of TFC membrane was studied using Fast Green FCF (MW 808.84) solution at a transmembrane pressure up to 0.5 MPa. Deionized water was used as a solvent. Solute concentration in the feed solution was 100 mg/L. The concentration of Fast Green FCF in the permeate solution was analyzed by UV-spectrophotometer (TU-1810) at 632 nm. The permeation flux (F, L/m 2h) calculated from the flux at a unit time on a unit membrane area may be expressed as: F ¼ Q =ðS tÞ where Q is the total volume of the permeated product (L); S is the effective membrane area (m 2); and t is the time to obtain Q liter product (h).
in Fig. 3. The figure represents the relationship of peroxide concentration and ozone treatment time. The longer the membrane was contacted by ozone gas, the more the peroxide group was generated. However, the excessive exposure in the ozone could also make C\C link interrupted, the PP membrane became mechanically fragile as a result of the chain of polypropylene broken by ozone. As shown in Fig. 4, as the PP membrane was treated by ozone, the tensile strength had little change at first; however, the tensile strength decreased sharply when ozone treatment time is longer than 7 min. Simultaneously, the elongation at break steadily decreased with the increasing of ozone treatment time. It was thought that 15 min ozone treatment was too long to prepare the composite membrane because the membrane could be easily broken in pressure filtration process, so we did not adopt 15 min ozone treatment in this study. 3.2. Water contact angle measurement of the PAM-grafted PP membranes To evaluate the hydrophilicity of PAM grafted membranes, the water contact angles on membrane surface were measured, the results of which are shown in Fig. 5. The original PP membrane had a water contact angle of 113°. As can be seen, by increasing the ozone treatment time from 0 to 10 min, the contact angle of membrane surface decreased slowly from 113° to 65° after grafted PAM at the same condition. It revealed that the hydrophilicity was enhanced through the grafting of hydrophilic PAM on the surface of hydrophobic PP membrane. 3.3. FTIR-ATR characterization of the original and modified membranes Fig. 6 shows the FTIR-ATR spectra of the origin (a), ozonized (b), grafted (c) and polyamide coated (d) PP membranes. As shown in Fig. 6(b), when PP membrane is contacted by ozone gas for a period of time, additional absorption is observed at 1720 cm − 1, which is attributed to the C_O introduced onto the PP membrane. It indicated that the peroxide groups are introduced on the surface of membrane by oxidation. Compared to Fig. 6(b), (c) has a stronger peak at 1650 cm− 1, which belongs to the C_O stretching vibration of PAM; this proved that PAM was grafted onto PP chains. Fig. 6(d) is FTIR-ATR spectra of the polyamide coated PP membrane. Compared with Fig. 6(b) and (c), significant differences can be found. The peaks at 1650 cm− 1 and 1560 cm− 1 corresponding to C_O and \NH2 are enhanced apparently. Moreover, a new peak at 1600 cm− 1 which belongs to the C\C stretching vibration of benzene rings of mphenylenediamine (MPDA) can be seen. Therefore, it is apparent that the PP membranes were coated with the polyamide material. 3.4. Morphology of composite membranes
3. Results and discussions 3.1. The influence of mechanical properties on PP membranes for ozone treatment The hydrogen atoms of methyl on polypropylene are easier to be taken off under ozone environment. At the mean time, peroxides are produced on the molecular chain. The peroxide concentration on the membrane surface was measured and the results are shown
The PAM grafted membrane surface which becomes more hydrophilic so that aqueous solution can spread on the surface. Also, the amino groups which can react with compounds with acyl chloride groups (like TMC) were introduced onto the surfaces of PP-PAM membranes. Fig. 7 shows the SEM micrographs of membrane surface and cross-section of PP and MPDA-TMC/PP composite membrane. Comparing Fig. 7(a) with Fig. 7(b), (c), and (d), we can find that the fiber structure of PP membrane was covered by polyamide functional
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Fig. 3. Peroxide concentration on the membrane surface as ozone treatment time.
layer, and that the functional layer became more complete as the ozone treatment time increased. The cross-section images show that the thickness of polyamide functional layer was about 0.5–1 μm and it was very dense; but the thickness showed no apparent change when ozone treatment time changes. The results also show that the interfacial polymerization could be easily carried out on PAM grafted PP membranes. The PAM chains were not only used to increase the hydrophilic of PP membrane surface but also reacted with acyl chloride groups to form the separation layer of composite membrane using interfacial polymerization. 3.5. Separation performances of the composite membrane Na2SO4 and Fast green separation tests were carried out to find out the characterization of composite membranes. Fast green has a small
molecule with the molecular weight of 808.84. From Table 1 and Table 2 we can conclude that the fluxes and rejection rates of composite membranes ozonized 1 min, 3 min and 15 min were unavailable. Too short ozone treatment time, such as 1 min and 3 min, the grafted PAM chains were also short, the reactive amine groups on the PP membrane surface and the hydrophilic of the PP membrane surface were not enough for interfacial polymerization. However, 15 min ozone treatment was too long to keep the mechanical strength of PP membrane. Fig. 8 compares the flux of composite membranes prepared by different ozone treatment time for Na2SO4 and Fast green. On the other hand, Fig. 9 shows rejection rates of different composite membranes. We can clearly see that the flux of Fast green solution is lower than that of Na2SO4 solution. However, the rejection rate of Fast green is much higher than that of Na2SO4 in each ozone treatment conditions. Besides, the composite membranes showed a decrease in permeation
Fig. 4. Effect of ozone treatment time on tensile strength and elongation at break.
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K. Pan et al. / Desalination 294 (2012) 36–43
Fig. 5. Static water contact angle of PAM-grafted PP with different ozone treatment time (AM concentration: 10 wt/v%; reaction time: 5 h; reaction temperature: 50 °C).
flux with the increase of the ozone treatment time. Meanwhile the both Na2SO4 and Fast green rejection for the composite membranes increased with the increase of the ozone treatment time. The results were mainly decided by thicknesses of the polyamide functional layers. The polyamide grew gradually around the fibers, forming functional layer and covered the pores. The pores of support were formed by PP fibers overlapped together, so if the diameter of fibers was larger, the pores would be bigger. Bigger pores are more difficult for the polyamide to cover, and the functional layer at the center of the pores was very thin. In addition, because the polyamide layer grew closely around the fibers, the surface of the functional layer was
rough, thus the charge repulsion effect was weak. These may be the reasons that the composite membrane did not exhibit high rejection for Na2SO4. On the other hand, the Fast Green was a small molecular organic matter, and the sieving effect was the main factor, so the rejection was higher. Furthermore, the rejection rate of composite membrane increased gradually with longer ozone treatment times. Because the longer ozone treatment can generate more radical on the membrane, the graft polymerization might be caused perfectly. The modified membrane which had better hydrophilicity adsorbed more amine during interfacial polymerization. This is benefit to form an impeccable functional layer. However, the grafted membrane would still be hydrophobic
Fig. 6. The FTIR-ATR spectra of original, ozonized, grafted and composite PP membrane. (a) original PP membrane; (b) ozonized for 7 min; (c) ozonized for 7 min and grafted for 5 h with 10% AM solution; (d) composite PP membrane with interfacial polymerization.
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Fig. 7. The SEM images for the surfaces of (a) original PP membrane, and PP-PA membranes prepared under ozone treatment time of (b) 5 min, (c) 7 min, and (d) 9 min, and for the cross-section of (e) original PP membrane, and PP-PA membranes prepared under ozone treatment time of (f) 5 min, (g) 7 min, and (h) 9 min.
when the membrane had exposed to ozone for less than 3 min, so interfacial polymerization could not be carried out on the grafted membrane which was ozone-treated for 1 min and 3 min. Well, when ozone treatment time is 15 min, the PP membrane became very fragile, so interfacial polymerization did not process either. In summary, nanofiltration membrane can be prepared using this method. 4. Conclusions We showed a new thin film composite membrane prepared by interfacial polymerization on polypropylene fiber supports. With the longer ozone treatment time, the grafted membranes had better hydrophilic properties. In our work, the ozone treatment time could not exceed 10 min, otherwise the membranes would be damaged. MPDA and TMC were used as monomers for interfacial polymerization.
Analysis of the functional layer was carried out by means of FTIR-ATR and SEM, which showed that the functional layer was able to completely cover the membrane surface. The composite membranes had rejection rate of 65.47% and 92.72% for Na2SO4 and Fast green respectively at an operating pressure of 0.5 MPa in room temperature. From the separation performance, it was considered that the polyamide composite membrane could be the candidate for the nanofiltration process.
Acknowledgment The project is supported by the National Natural Science Foundation of China (20804002), and the Major Project of Science and Technology Research from the Ministry of Education of China (308003).
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Fig. 8. Flux of the composite membranes for Na2SO4 and Fast Green.
Fig. 9. Rejection rate of the composite membranes for Na2SO4 and Fast Green.
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[7] In-Chul Kim, Kew-Ho Lee, Tae-Moon Tak, Preparation and characterization of integrally skinned uncharged polyetherimide asymmetric nanofiltration membrane, J. Membr. Sci. 183 (2001) 235–247. [8] Shu-Hsien Huang, Chi-Lan Li, Chien-Chieh Hu, H.A. Tsai, Kueir-Rarn Lee, Juin-Yih Lai, Polyamide thin-film composite membranes prepared by interfacial polymerization for pervaporation separation, Desalination 200 (2006) 387–389. [9] A. Prakash Rao, S.V. Joshi, J.J. Trivedi, C.V. Devmurari, V.J. Shah, Structure–performance correlation of polyamide thin film composite membranes: effect of coating conditions on film formation, J. Membr. Sci. 211 (2003) 13–24. [10] Yujun Song, Patricia Sun, Laurence L. Henry, Benhui Sun, Mechanisms of structure and performance controlled thin film composite membrane formation via interfacial polymerization process, J. Membr. Sci. 251 (2005) 67–79. [11] Ahmad Rahimpour, Mohsen Jahanshahi, Narmin Mortazavian, Sayed Siavash Madaeni, Yaghob Mansourpanah, Preparation and characterization of asymmetric polyethersulfone and thin-film composite polyamide nanofiltration membranes for water softening, J. Appl. Surf. Sci. 256 (2010) 1657–1663. [12] Chi-Lan Li, Shu-Hsien Huang, Der-Jang Liaw, Kueir-Rarn Lee, Juin-Yih Lai, Interfacial polymerized thin-film composite membranes for pervaporation separation of aqueous isopropanol solution, Sep. Purif. Technol. 62 (2005) 694–701. [13] Hai-Yin Yu, Zhi-Kang Xua, Hao Lei, Meng-Xin Hu, Qian Yang, Photoinduced graft polymerization of acrylamide on polypropylene microporous membranes for the
K. Pan et al. / Desalination 294 (2012) 36–43 improvement of antifouling characteristics in a submerged membrane-bioreactor, Sep. Purif. Technol. 53 (2007) 119–125. [14] Hai-Yin Yu, Meng-Xin Hu, Zhi-Kang Xu, Jian-Li Wang, Shu-Yuan Wang, Surface modification of polypropylene microporous membranes to improve their antifouling property in MBR: NH3 plasma treatment, Sep. Purif. Technol. 45 (2005) 8–15. [15] Hai-Yin Yu, Ya-Jie Xie, Meng-Xin Hu, Jian-Li Wang, Shu-Yuan Wang, Zhi-Kang Xu, Surface modification of polypropylene microporous membrane to improve its
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antifouling property in MBR: CO2 plasma treatment, J. Membr. Sci. 254 (2005) 219–227. [16] Hai-Yin Yu, Meng-Xin Hu, Zhi-Kang Xu, Improvement of surface properties of polypropylene hollow fiber microporous membranes by plasma-induced tethering of sugar moieties, Plasma Processes Polym. 2 (2005) 627–632. [17] Young Wang, Jae-Hong Kim, Kwang-Ho Choo, Yoon-Sik Lee, Chung-Hak Lee, Hydrophilic modification of polypropylene microfiltration membranes by ozoneinduced graft polymerization, J. Membr. Sci. 169 (2000) 269–276.
Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Novel composite membranes prepared by interfacial polymerization on polypropylene fiber supports pretreated by ozone-induced polymerization Kai Pan, Peng Fang, Bing Cao ⁎ College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China Key Laboratory of carbon fiber and functional polymers, Ministry of Education, Beijing University of Chemical Technology, Beijing, 100029, China
a r t i c l e
i n f o
Article history: Received 25 October 2011 Received in revised form 28 February 2012 Accepted 1 March 2012 Available online 6 April 2012 Keywords: Interfacial polymerization Hydrophilicity Ozone treatment Nanofiltration membrane
a b s t r a c t Interfacial polymerization (IP) is a powerful technique for the fabrication of thin film composite (TFC) membranes. In this work, porous polypropylene fiber films were used as a support to fabricate TFC nanofiltration membrane using IP technique. Firstly, the surface of polypropylene fiber film was hydrophilized by introducing peroxide onto the membrane surface using ozone treatment followed by grafting acrylamide. And then interfacial polymerization could be successfully proceeding on the hydrophilized PP membrane. The monomeric system chosen for IP was m-phenylenediamine (MPDA) and trimesoyl chloride (TMC). The ozone treatment time was investigated, and in the range of 5–10 min ozone treatment time, the PP fiber membrane had an available amount of peroxides for grafting while keeping the mechanical strength. The characteristics of the original membrane, grafted membrane and TFC membrane were studied with various analytical methods, such as SEM and FTIR-ATR. Meanwhile, the nanofiltration performance was evaluated with aqueous solutions of Na2SO4 (2 g/L) and Fast Green FCF (MW 808.84, 10 mg/L). The rejection rate of 60% and 90% were achieved for Na2SO4 and Fast Green FCF, respectively, at a transmembrane pressure of 0.5 MPa. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a new separation method, membrane technology is developing rapidly in recent decades. Its application area includes water purification, chemical industry, biological engineering, medicine, foods and so on [1–3]. According to different molecular weight, chemical structure and physical properties of the separation material, membranes can be divided into reverse osmosis (RO) membrane, nanofiltration (NF) membrane, ultrafiltration (UF) membrane and microfiltration (MF) membrane. Nanofiltration (NF) membranes, which exhibit separation characteristics in the intermediate range between reverse osmosis and ultrafiltration, are gaining significant attention because of advantages such as low operation pressure, high permeate flux, and high retention of multivalent ion salts. And composite nanofiltration membranes are generally fabricated using interfacial polymerization (IP) [4,5]. The IP technique is based on a polymerization reaction which takes place at the interface between two immiscible phases, for example, an aqueous phase and a hexane phase. Each phase is a solution containing a dissolved monomer. For example, diamine and diacyl chloride can be used as monomers in IP, and polyamide is the reaction ⁎ Corresponding author at: College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing, 100029, China. Tel.: + 86 10 64413857. E-mail addresses: [email protected] (K. Pan), [email protected] (B. Cao). 0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2012.03.007
product which forms a selective layer on the support (see Scheme 1). The nature of the solvents and monomers as well as monomer concentrations, reaction time and heat curing defines the porosity, pore size and thickness of the selective layer [6–9]. Hydrophilic supports have a distinct advantage compared to hydrophobic ones, for example, hydrophilic support can make aqueous solution spread on the surface to benefit the dispersing of amine monomers, which achieves goals of interfacial polymerization; moreover, hydrophilic membrane can reduce fouling due to pore plugging with biological macromolecules. Supports currently used for IP are mostly polysulfone (PSf) [10], polyethersulfone (PES) [11] and polyacrylonitrile (PAN) [12]. Polypropylene (PP) membrane has been widely used due to the well-controlled porosity, high stability, and low cost of the raw material. However, PP membranes are relative hydrophobic and non-wettable with water, the potential application in preparation of composite membrane with IP is limited, and so PP must be conducted with hydrophilic modification firstly. The chemical modification of polymeric surfaces such as graft polymerization has been extensively studied. In graft polymerization, functional groups were first introduced to the polymeric substrate by UV irradiation [13], plasma [14–16], or ozone [17] and then decomposed and initiated for polymerization. In this study, the oxidation of PP membranes, which can trigger acrylamide (AM) monomer grafting on the surface, was done by exposing ozone gas for predetermined time. The effects of ozone treatment time on the tensile strength of the membrane and the degree of graft
K. Pan et al. / Desalination 294 (2012) 36–43
37
reactor was removed by oxygen gas purging, and then the membrane was placed under vacuum for 30 min to make sure the physically absorbed ozone molecules were removed.
Scheme 1. Structural formula showing the chemistry of the film composite membrane.
polymerization were examined. Then the polyamide functional layer was formed on the surface of hydrophilic PP support by the reactive monomer system of m-phenylenediamine and trimesoyl chloride. The Na2SO4 solution and Fast Green FCF solution were used to characterize the permeate property of fabricated TFC membranes. 2. Experimental 2.1. Materials Polypropylene microporous membranes made by melt-spinning method were purchased from Zetao industrial Co., Ltd. (China) and used as a substrate for this study; acrylamide (98%) was purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used after recrystallization in acetone; FeSO4·7H2O (99%) was purchased from Chemical Reagent (China) and used directly; acetone (99%), ethanol (99.7%), MPDA (99.55%), Na3PO4 (98%) and n-hexane (97.5%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China) and used without any further purification; TMC (99.0%) was purchased from Alfa Aesar (China) and used directly; De-ionized water was used as the solvent of the aqueous phase. 2.2. Membrane surface modification and polymerization 2.2.1. PP membrane surface ozone treatment Firstly, the PP membrane was put into acetone for 12 h and then dried in vacuum oven for 6 h. Ozone was generated by an ozone generator (CF-G-2-50 g, Guolin, China) using pure oxygen gas as illustrated in Fig. 1. The ozone concentration and the oxygen gas flow rate were adjusted to about 96 mg/L and 1 L/min. And then ozone went through the ozone reactor which contains the PP membrane under room temperature. After ozone treatment for a given time, residual ozone in the
Fig. 1. Experimental setup for ozonation. 1. Oxygen tank; 2. Ozone generator; 3. Flow meter; 4. Ozonation reactor; 5. Membrane.
2.2.2. PP membrane surface grafting with AM The ozone-treated membrane was soaked in ethanol for 10 min in order to wet the surface, and then placed in AM solution immediately after wetting. The grafting reaction was initiated by the addition of FeSO4·7H2O and proceeded as shown in Fig. 2. Graft polymerization was allowed to proceed in the jar at 50 °C with magnetic mixing for 5 h. The jar was filled with N2 during the whole grafting polymerization process. The overall concentrations of AM solution and the catalyst (FeSO4·7H2O) were 10 wt/v% and 0.01 wt/v% respectively. When the polymerization was finished, the membrane was soaked in methanol for 12 h and washed by deionized water several times to remove residual monomers and homopolymers. Then the grafted membranes were stored in deionized water before used. 2.2.3. Interfacial polymerization on grafted PP membrane The polyamide active skin layer was synthesized using interfacial polymerization technique. Firstly, the PP membrane previously modified by ozone and grafted with AM was soaked in an aqueous solution of MPDA for 15 min and then positioned vertically to drain the excess solution. Subsequently, the PP membrane soaked with the amine aqueous solution of m-phenylenediamine (2 wt/v%) reacted with a hexane solution of trimesoyl chloride (0.4wt/v%). The interfacial polymerization time was 1 min. The modified membranes were placed in an oven at 70 °C, heat treated for about 20 min to attain the desired stable structures. Finally, the polyamide thin-film composite membrane was kept under ambient air conditions for further use. 2.3. Characterization 2.3.1. FTIR-ATR characterization Fourier transform infrared spectroscopy in attenuated total reflection mode (FTIR-ATR) spectrometer (Spectrum RX-I, Perkin-Elmer, USA) was used to detect functional groups of grafted membranes. 2.3.2. Water contact angle measurement The hydrophilic of the grafted membrane surface was tested by water contact angle measurements instrument (JC2000C, Shanghai Zhongchen Co., China) at room temperature. Before the samples were tested, they were fully dried. Five different locations were chosen for each value and record every point after 15 s stable. 2.3.3. Scanning electron microscopy (SEM) The surface morphology of the membranes was investigated by scanning electron microscopy (SEM) on Hitachis-4700 SEM (Hitachi, Japan). The membranes were mounted on the sample studs by means of double-sided adhesive tapes. A thin layer of palladium was sputtered onto the membrane surface prior to the SEM measurement. The cross section of sample was obtained by breaking the sample in liquid nitrogen. The measurements were performed at an accelerating voltage of 20 kV. 2.3.4. Determination of peroxide groups on ozone treated membrane The amount of peroxide groups produced on the membrane by the ozone treatment was determined using the DPPH (2,2-diphenyl-1(2,4,6-trinitrophenyl)-hydrazyl) method. Different concentration of DPPH solution was determined by the absorbance at the wavelength of 517 nm using a UV–vis spectrophotometer. Standard curve of DPPH was obtained for further test. One equivalent of hydroperoxide reacts with two equivalents of DPPH. The PP membrane treated for a given period of time by ozone was put into 50 ml NMP solution with a certain amount of DPPH. The sample solutions prepared were refluxed with stirring and N2 purging at 60 °C for 24 h. Finally the remaining
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Fig. 2. The preparation process of the PP-g-PAM membrane via surface copolymerization.
DPPH concentration was determined according to the standard curve. The amount of peroxide groups is defined as follow: D¼
ðC0 −CÞV 2M DPPH S
where C0 and C are DPPH concentration in original solution and reacted solution, V represented the volume of solution, S is surface area of membrane. 2.3.5. Mechanical properties Mechanical properties of the original PP membrane and ozonized PP membrane were tested by material experimental machine (Instron1185, USA) under the constant ambient temperature (temperature, 25 °C; humidity, 50± 10%RH). 2.3.6. Performance of the polyamide thin-film composite membrane The permeate flux and the rejection are evaluated by standard permeation tests. The filtration apparatus is a stainless steel dead-end stirred cell (HP4750 Stirred Cell, Sterlitech Corp., USA) resting on a magnetic stir plate. The filtration cell has a capacity of 0.350 L and effective membrane area of 14.6 cm2. All tests are conducted at room temperature (25 °C). Nanofiltration of Na2SO4 was investigated using TFC membrane at a transmembrane pressure of up to 0.5 MPa. The salt concentration of the feed was 2 g/L. The concentration of Na2SO4 in the permeate was analyzed by conductivity meter (DDS-11A). The salt rejection of composite membrane (R) is defined as follow: κp Rð% Þ ¼ 1− 100 κf where Κp is the conductivity of the product (μS/cm) and Κf is the conductivity of the feed (μS/cm). Nanofiltration of TFC membrane was studied using Fast Green FCF (MW 808.84) solution at a transmembrane pressure up to 0.5 MPa. Deionized water was used as a solvent. Solute concentration in the feed solution was 100 mg/L. The concentration of Fast Green FCF in the permeate solution was analyzed by UV-spectrophotometer (TU-1810) at 632 nm. The permeation flux (F, L/m 2h) calculated from the flux at a unit time on a unit membrane area may be expressed as: F ¼ Q =ðS tÞ where Q is the total volume of the permeated product (L); S is the effective membrane area (m 2); and t is the time to obtain Q liter product (h).
in Fig. 3. The figure represents the relationship of peroxide concentration and ozone treatment time. The longer the membrane was contacted by ozone gas, the more the peroxide group was generated. However, the excessive exposure in the ozone could also make C\C link interrupted, the PP membrane became mechanically fragile as a result of the chain of polypropylene broken by ozone. As shown in Fig. 4, as the PP membrane was treated by ozone, the tensile strength had little change at first; however, the tensile strength decreased sharply when ozone treatment time is longer than 7 min. Simultaneously, the elongation at break steadily decreased with the increasing of ozone treatment time. It was thought that 15 min ozone treatment was too long to prepare the composite membrane because the membrane could be easily broken in pressure filtration process, so we did not adopt 15 min ozone treatment in this study. 3.2. Water contact angle measurement of the PAM-grafted PP membranes To evaluate the hydrophilicity of PAM grafted membranes, the water contact angles on membrane surface were measured, the results of which are shown in Fig. 5. The original PP membrane had a water contact angle of 113°. As can be seen, by increasing the ozone treatment time from 0 to 10 min, the contact angle of membrane surface decreased slowly from 113° to 65° after grafted PAM at the same condition. It revealed that the hydrophilicity was enhanced through the grafting of hydrophilic PAM on the surface of hydrophobic PP membrane. 3.3. FTIR-ATR characterization of the original and modified membranes Fig. 6 shows the FTIR-ATR spectra of the origin (a), ozonized (b), grafted (c) and polyamide coated (d) PP membranes. As shown in Fig. 6(b), when PP membrane is contacted by ozone gas for a period of time, additional absorption is observed at 1720 cm − 1, which is attributed to the C_O introduced onto the PP membrane. It indicated that the peroxide groups are introduced on the surface of membrane by oxidation. Compared to Fig. 6(b), (c) has a stronger peak at 1650 cm− 1, which belongs to the C_O stretching vibration of PAM; this proved that PAM was grafted onto PP chains. Fig. 6(d) is FTIR-ATR spectra of the polyamide coated PP membrane. Compared with Fig. 6(b) and (c), significant differences can be found. The peaks at 1650 cm− 1 and 1560 cm− 1 corresponding to C_O and \NH2 are enhanced apparently. Moreover, a new peak at 1600 cm− 1 which belongs to the C\C stretching vibration of benzene rings of mphenylenediamine (MPDA) can be seen. Therefore, it is apparent that the PP membranes were coated with the polyamide material. 3.4. Morphology of composite membranes
3. Results and discussions 3.1. The influence of mechanical properties on PP membranes for ozone treatment The hydrogen atoms of methyl on polypropylene are easier to be taken off under ozone environment. At the mean time, peroxides are produced on the molecular chain. The peroxide concentration on the membrane surface was measured and the results are shown
The PAM grafted membrane surface which becomes more hydrophilic so that aqueous solution can spread on the surface. Also, the amino groups which can react with compounds with acyl chloride groups (like TMC) were introduced onto the surfaces of PP-PAM membranes. Fig. 7 shows the SEM micrographs of membrane surface and cross-section of PP and MPDA-TMC/PP composite membrane. Comparing Fig. 7(a) with Fig. 7(b), (c), and (d), we can find that the fiber structure of PP membrane was covered by polyamide functional
K. Pan et al. / Desalination 294 (2012) 36–43
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Fig. 3. Peroxide concentration on the membrane surface as ozone treatment time.
layer, and that the functional layer became more complete as the ozone treatment time increased. The cross-section images show that the thickness of polyamide functional layer was about 0.5–1 μm and it was very dense; but the thickness showed no apparent change when ozone treatment time changes. The results also show that the interfacial polymerization could be easily carried out on PAM grafted PP membranes. The PAM chains were not only used to increase the hydrophilic of PP membrane surface but also reacted with acyl chloride groups to form the separation layer of composite membrane using interfacial polymerization. 3.5. Separation performances of the composite membrane Na2SO4 and Fast green separation tests were carried out to find out the characterization of composite membranes. Fast green has a small
molecule with the molecular weight of 808.84. From Table 1 and Table 2 we can conclude that the fluxes and rejection rates of composite membranes ozonized 1 min, 3 min and 15 min were unavailable. Too short ozone treatment time, such as 1 min and 3 min, the grafted PAM chains were also short, the reactive amine groups on the PP membrane surface and the hydrophilic of the PP membrane surface were not enough for interfacial polymerization. However, 15 min ozone treatment was too long to keep the mechanical strength of PP membrane. Fig. 8 compares the flux of composite membranes prepared by different ozone treatment time for Na2SO4 and Fast green. On the other hand, Fig. 9 shows rejection rates of different composite membranes. We can clearly see that the flux of Fast green solution is lower than that of Na2SO4 solution. However, the rejection rate of Fast green is much higher than that of Na2SO4 in each ozone treatment conditions. Besides, the composite membranes showed a decrease in permeation
Fig. 4. Effect of ozone treatment time on tensile strength and elongation at break.
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Fig. 5. Static water contact angle of PAM-grafted PP with different ozone treatment time (AM concentration: 10 wt/v%; reaction time: 5 h; reaction temperature: 50 °C).
flux with the increase of the ozone treatment time. Meanwhile the both Na2SO4 and Fast green rejection for the composite membranes increased with the increase of the ozone treatment time. The results were mainly decided by thicknesses of the polyamide functional layers. The polyamide grew gradually around the fibers, forming functional layer and covered the pores. The pores of support were formed by PP fibers overlapped together, so if the diameter of fibers was larger, the pores would be bigger. Bigger pores are more difficult for the polyamide to cover, and the functional layer at the center of the pores was very thin. In addition, because the polyamide layer grew closely around the fibers, the surface of the functional layer was
rough, thus the charge repulsion effect was weak. These may be the reasons that the composite membrane did not exhibit high rejection for Na2SO4. On the other hand, the Fast Green was a small molecular organic matter, and the sieving effect was the main factor, so the rejection was higher. Furthermore, the rejection rate of composite membrane increased gradually with longer ozone treatment times. Because the longer ozone treatment can generate more radical on the membrane, the graft polymerization might be caused perfectly. The modified membrane which had better hydrophilicity adsorbed more amine during interfacial polymerization. This is benefit to form an impeccable functional layer. However, the grafted membrane would still be hydrophobic
Fig. 6. The FTIR-ATR spectra of original, ozonized, grafted and composite PP membrane. (a) original PP membrane; (b) ozonized for 7 min; (c) ozonized for 7 min and grafted for 5 h with 10% AM solution; (d) composite PP membrane with interfacial polymerization.
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Fig. 7. The SEM images for the surfaces of (a) original PP membrane, and PP-PA membranes prepared under ozone treatment time of (b) 5 min, (c) 7 min, and (d) 9 min, and for the cross-section of (e) original PP membrane, and PP-PA membranes prepared under ozone treatment time of (f) 5 min, (g) 7 min, and (h) 9 min.
when the membrane had exposed to ozone for less than 3 min, so interfacial polymerization could not be carried out on the grafted membrane which was ozone-treated for 1 min and 3 min. Well, when ozone treatment time is 15 min, the PP membrane became very fragile, so interfacial polymerization did not process either. In summary, nanofiltration membrane can be prepared using this method. 4. Conclusions We showed a new thin film composite membrane prepared by interfacial polymerization on polypropylene fiber supports. With the longer ozone treatment time, the grafted membranes had better hydrophilic properties. In our work, the ozone treatment time could not exceed 10 min, otherwise the membranes would be damaged. MPDA and TMC were used as monomers for interfacial polymerization.
Analysis of the functional layer was carried out by means of FTIR-ATR and SEM, which showed that the functional layer was able to completely cover the membrane surface. The composite membranes had rejection rate of 65.47% and 92.72% for Na2SO4 and Fast green respectively at an operating pressure of 0.5 MPa in room temperature. From the separation performance, it was considered that the polyamide composite membrane could be the candidate for the nanofiltration process.
Acknowledgment The project is supported by the National Natural Science Foundation of China (20804002), and the Major Project of Science and Technology Research from the Ministry of Education of China (308003).
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Fig. 8. Flux of the composite membranes for Na2SO4 and Fast Green.
Fig. 9. Rejection rate of the composite membranes for Na2SO4 and Fast Green.
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