- Email: [email protected]
Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes
DES-11534; No of Pages 8 Desalination xxx (2012) xxx–xxx
Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes Jun Xiang a, Zongli Xie b, Manh Hoang b, Kaisong Zhang a,⁎ a b
Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China CSIRO Materials Science and Engineering, Private Bag 33, Clayton, Vic. 3169, Australia
H I G H L I G H T S ► The effect of the types of amine salt surfactants on nanofiltration performance was investigated. ► Amine salt surfactants play an important role in the performance of TFC poly(piperazine–amide) NF membranes. ► Amine salt containing larger steric configuration cationic amine group resulted in better performance.
a r t i c l e
i n f o
Article history: Received 29 June 2012 Received in revised form 19 October 2012 Accepted 30 October 2012 Available online xxxx Keywords: Amine salt surfactants Interfacial polymerization Thin film composite(TFC) membrane Poly(piperazine-amide) Nanofiltration
a b s t r a c t The role of amine salt surfactants on the performance of thin film composite nanofiltration (TFC NF) membranes is crucial for the development of advanced NF membranes, but the information is still not enough. This paper presents a systematic investigation of the effect of the types of amine salt surfactants on nanofiltration performance. Four types of amine salts with different molecular structures, molecular weights (MW) and charges, including tetraethylammonium chloride (TEAC), tetrabutylammonium bromide (TBAB), camphorsulfonic acid triethylamine salt (CAS-TEA), and 1-butyl-3-methylimidazolium chloride (BMMIC), were incorporated into piperazine (PIP) solution during interfacial polymerization(IP) with trimesoyl chloride (TMC). The formed thin film composite (TFC) piperazine–amide polymer was supported on a polyethersulfone (PES) ultrafiltration membrane (UF). The membrane surfaces were characterized by scanning electronic microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS). The flux and NaCl rejection were also evaluated. The addition of amine salt containing larger steric configuration cationic amine group in aqueous solution, resulted in a TFC membrane with better performance, and the amine salt would act as a surfactant and not adhere to the polymer layer at the end of polymerization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanofiltration (NF) has been widely used in the water industry for drinking water purification, wastewater treatment and reuse as it can remove turbidity, microorganisms and hardness, as well as a fraction of the dissolved salts at low operating pressure with high water flux [1]. Most of the commonly used commercial NF membranes are thin film composite (TFC) polyamide membranes with a thin film layer fabricated onto a microporous substrate via interfacial polymerization (IP) technique [2,3]. By employing IP technique, the properties of both top selective layer and bottom porous substrate can be independently controlled and optimized to achieve desired water permeability and selectivity while offering excellent mechanical strength [4]. The crosslinked aromatic polyamide is generally formed by interfacial polymerization of two monomers, a polyfunctional amine (e.g., m-phenylenediamine) in aqueous phase and a polyfunctional acid chloride (e.g. trimesoyl ⁎ Corresponding author. Tel./fax: +86 592 6190782. E-mail address: [email protected] (K. Zhang).
chloride) in organic solvent. Since water and the hydrocarbon solvent are immiscible, polymerization reaction takes place at the water/ hydrocarbon interface [5,6]. In the early development of the TFC membrane (1960s–1980s), most studies focused on researching the IP reaction mechanism and finding a suitable material for the top barrier layer. Since the 1980s, the focus shifted to improve the properties of TFC membranes with respect to permeability and selectivity [3,7,8]. As membrane performance is mainly determined by the structure and property of the top thin film layer, efforts have been devoted to the preparation of the ultrathin barrier layer to fabricate membranes with high salt rejection properties. The IP process parameters, such as type of monomer, monomer concentration, surfactant, reaction time, temperature, aqueous solution pH, additives for cross-linker structure or pore-induced, as well as the fabrication process and post-treatment affect the membrane performance. In a recent review by Kang et al. [9], the monomers in the IP process were examined. Lau et al. [4] also investigated monomers with different molecular weights and functional groups. These studies revealed that functional monomers play an important role in the extent of cross-linking and different monomer
0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.10.038
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
2
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
structures lead to different configurations of polymer. I1 Juhn Roh and co-workers found that increasing acid chloride concentration resulted in thicker polyamide (PA) film with lower flux, however changing the diamine concentration did not remarkably affect the membrane performance [10]. The curing conditions in the IP process such as reaction time, curing temperature and the type of organic solvent used were also systematically studied [11,12]. It was found that a higher diffusivity of functional amine reactant could lead to higher amount of amine dissolving in the organic solvent, obtaining higher water flux, thickness, and roughness, but less cross-linking. To increase the flux and salt rejection, more amine needs to react with the acyl halide groups at the interface but not to diffuse into the bulk organic phase. To obtain a membrane exhibiting both high water flux and selectivity, a higher curing temperature is generally needed to remove organic solvent, but with the disadvantage of increasing roughness. In addition, LiBr, an inorganic salt drew researchers' interest [13], when added in aqueous solution, it could produce high property NF membranes, and by controlling the concentration of LiBr in aqueous phase solution, the morphology of membranes could be adjusted. Previous researches mainly focused on selection of monomers, monomer ratios, reaction conditions and post-treatment. Other factors such as amine salt surfactant and support membrane structure and properties, especially the role of amine salt surfactant, were not frequently reported. However, the addition of surfactants is generally recommended as a necessary component for IP process as it is believed that the surfactant assists the monomer in water phase moving into organic phase, thus improving polymerization efficiency and consequently properties of formed TFC membranes [4]. In early patents [14–17] amine salts, known as phase-transfer catalysts (PTC), had been investigated in detail, including concentration, types, structure, etc., however the working mechanism was still unclear [11,12,18,19], and less information was reported about the roles of amine salts on the properties of NF membrane in IP process. In this study, the effect of amine salts promoting IP process was systematically investigated by incorporating different types of amine salts into aqueous phase solutions. The relationship between the amine salts and the properties of the membrane was discussed. 2. Experimental 2.1. Materials Table 1 lists chemicals used in this study. Except for lithium chloride (LiCl) which was heat treated to remove moisture, all other chemicals were used as received without further purification. Deionized water (resistivity18 MΩ·cm −1 at 25 °C) was used for preparing aqueous solution.
Table 1 Materials used in this work. Material
CAS number
Source
Lithium chloride (≥99%) Polyehersulfone (E6020) Piperrazine (99%) Trimesic acid trichloride (98%) Tetraethylammonium chloride (≥99%) Tetrabutylammonium bromide (98%) Camphorsulfonic acid (99%) 1-Butyl-3-methylimidazolium chloride (≥98%) Triethylamine (≥99%)
7447-41-8 25667-42-9 110-85-0 4422-95-1
Sigma-Aldrich Inc., St. Louis, MO, USA BASF (China) Co. Ltd. Guangzhou Sigma-Aldrich Inc., St. Louis, MO, USA Sigma-Aldrich Inc., St. Louis, MO, USA
56-34-8
Sigma-Aldrich Inc., St. Louis, MO, USA
1643-19-2
Sigma-Aldrich Inc., St. Louis, MO, USA
Sodium dodecyl sulfate (99%) N-hexane (99%) Sodium phosphate (96%) Polyvinylpyrrolidone, K30
151-21-3 110-54-3 7601-54-9 9003-29-8
N, N-dimethylacetamide (≥99%) MgSO4 (≥99%)
127-19-5
NaCl (≥99%)
7467-14-5
3144-16-9 Sigma-Aldrich Inc., St. Louis, MO, USA 79917-90-1 Sigma-Aldrich Inc., St. Louis, MO, USA 121-44-8
7487-88-9
Merck Pty Limited, Kilsyth, Victoria, Australia Sigma-Aldrich Inc., St. Louis, MO, USA Sigma-Aldrich Inc., St. Louis, MO, USA Sigma-Aldrich Inc., St. Louis, MO, USA Sinopharm Chemical Reagent Co., Ltd., China Jinshan Jingwei Chemical Co., Ltd., Shanghai,China Sinopharm Chemical Reagent Co., Ltd., China Sinopharm Chemical Reagent Co., Ltd., China
2.3. Synthesis of TFC NF membranes TFC membranes were prepared using traditional IP approach described elsewhere [2,20–22]. Membrane fabrication was conducted using two 8 mm deep polytetrafluoroethylene (PTFE) circular rings, with outside and inner diameter of 140 mm and 100 mm respectively. Four aqueous solutions were prepared, namely 1.6 wt.% PIP, 0.2 mol/L amine salt (where, CAS-TEA was generated by reacting CAS with TEA in a weight ratio of 2.32:1), 0.15 wt.% sodium dodecyl sulfate (SLS) and 0.5 wt.% sodium phosphate, while the organic phase solution contained 0.35 wt.% TMC, using N-hexane as solvent. Four different types of amine salts including tetraethylammonium chloride (TEAC), tetrabutylammonium bromide (TBAB), camphorsulfonic acid triethylamine salt (CAS-TEA), 1-butyl-3-methylimidazolium chloride Table 2 The chemical structures of amine salts. Name
Chemical structure
Structure of cationic amine group
MW (cationic amine group)
CAS-TEA
101.9
TEAC
130.2
TBAB
242.37
BMMIC
139.17
2.2. Preparation of microporous PES membrane substrate Microporous PES membrane was used as the supporting substrate of the polyamide thin film layer. It was fabricated via phase inversion by the following procedure: First, PES cast solution was prepared by dissolving 19 wt.% PES, 3.5 wt.% Polyvinylpyrrolidone (K30), and 3 wt.% LiCl in DMAC with a ratio of 74.5 wt.% at 80 °C. After stirring for 12 h, the resulting homogeneous solution was casted onto a non-woven fabric (thickness 100–110 μm) using a casting knife, followed by dipping the membrane into a deionized water bath for immediate phase inversion. The thickness of wet film was controlled at ~ 250 μm. After 30 min in a gelation media, the membrane was taken out and washed thoroughly using distilled water. The entire membrane fabrication process was carried out at room temperature and at the relative humidity of 30%. Resulting microporous membrane with a molecular weight cut off (MWCO) of between 30 k and 50 k·Dalton was stored in a refrigerator at 4 °C prior to use.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
(BMIMCL) were used in the study with the conditions listed in Table 2. Except for using different amine salt, a typical procedure for synthesizing poly(piperazine-amide) membranes is described below. First, the non-woven fabric side of PES membrane substrate was taped onto one of the PTFE rings, and then clamped to the other PTFE ring, so that it was sandwiched in between 2 PTFE rings. PIP solutions were poured onto the dense side of the PES substrate, membrane was immersed for 1 min, then the excess PIP solution was removed using a soft rubber roller and dried for 2 min vertically at room temperature. The TMC solution was poured over the membrane, kept contact with TMC solution for 20 s, to allow in situ formation of a TFC layer over
3
the surface of PES support. Subsequently the fabricated membrane was cured for 3 min in an air-circulation oven at 50 °C. Finally the membrane was thoroughly washed and stored in deionized water until the membrane performance was evaluated. To ensure reproducibility, five samples were prepared for each membrane. 2.4. Membrane testing Performance of fabricated TFC NF membranes were evaluated by the water flux and salt rejection using a cross-flow PTFE cell CF042
Fig. 1. SEM micrograph of different membranes. (a) PES membrane without TFC layer, (b) TFC membrane with CAS added into aqueous solution, (c) TFC membrane with TEAC added into aqueous solution, (d) TFC membrane with TBAB added into aqueous solution, (e) TFC membrane with BMMIC added into aqueous solution.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
4
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
}
Fig. 1 (continued).
(Sterlitech Co.) at 25 °C and 0.35 MPa. The feed solutions containing 500 ppm NaCl and 2000 ppm MgSO4 were tested separately. The water flux was calculated by measuring the weight of permeate solution passing through the membrane over time (h) and over the exposed
membrane area (m2). Salt rejections NaCl and MgSO4 were calculated respectively, from the conductivities of the feed and the permeate measured with a digital conductivity meter (sensION + EC5 Portable Conductivity Meter, Hach Co.).
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
5
}
Fig. 1 (continued).
2.5. Membrane characterization
3. Results and discussion
2.5.1. ATR-FTIR The functional structure of TFC membrane samples was analyzed using a Thermo Scientific Nicolet 6700 iTR spectrometer. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) was performed on membrane samples from 600 to 4000 cm−1 wavelength with an 8 cm−1 resolution.
3.1. Membrane morphologies
2.5.2. SEM The surface morphology of the TFC membrane samples was imaged using a Philips XL30 scanning electronic microscope (SEM). Before testing, all the membrane samples were dried in vacuum oven at 80 °Cfor more than 48 h. The samples were sputter coated with a 10–20 nm thin layer of iridium under vacuum. 2.5.3. XPS X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Ultra DLD spectrometer (Kratos Analytical Inc., Manchester, UK), with a monochromatized AlKa source at a power of 180 W (15 kV× 12 mA), a hemispherical analyzer operating in the fixed analyzer transmission mode and the standard aperture (analysis area: 0.3 mm× 0.7 mm). The total pressure in the main vacuum chamber during analysis was less than 10–8 mbar. Survey spectra were acquired at a pass energy of 160 eV. To obtain more detailed information about chemical structure, oxidation states, etc., high resolution spectra were recorded from individual peaks at 40 eV pass energy (yielding a typical peak width for polymers of 1.0–1.1 eV). Data processing was performed using CasaXPS processing software version 2.3.15 (Casa Software Ltd., Teignmouth, UK). All elements present were identified from survey spectra. Binding energies were referenced to the aliphatic hydrocarbon peak at 285.0 eV. Other details of test condition and the equipment information of software were given elsewhere [23,24].
The surface morphologies of NF membranes were monitored by FESEM and are presented in Fig. 1a. The pure PES membrane substrate shows a smooth surface. However, the poly(piperazine-amide) layer polymerized onto the PES substrate resulted in a thin film layer with rougher surface. When CAS-TEA, TEAC, TBAB and BMMIC were added to the PIP solutions respectively, the common “ridge-and-valley” surface morphologies of all the polymerized membranes was shown in Fig. 1b–e. It is similar to the previous reports [25–27]. As shown in Fig.1, it is interesting to note that as the roughness of TFC layers increases with the MW of cationic amine group increasing. The total thickness of TFC membranes (including the thickness of rough surface) are distributed in a relatively narrow range of 500–850 nm. CAS-TEA resulted in the thinnest membrane, followed by TEAC, TBAB. It should be noted that the CAS-TEA molecule possesses two organic groups, one containing a super-hydrophilic bond of − SO3H, the other one with an amine functional bond. The former may have played a role as a surfactant in the formation of a smooth surface of the TFC membrane. 3.2. FTIR and XPS analysis ATR-FTIR and XPS analyses were used to characterize the physiochemical structure of the TFC layer and to understand catalytic effect of the four amine salts. XPS is a surface sensitive technique and measures the element concentration (except H) and chemical binding information for the top 1–10 nm depth of the surface region [28–30], while ATR-FTIR offers much deeper penetration (from b200 nm to > 1 um). Fig. 2 shows the ATR-FTIR spectra of PES membrane and four TFC NF membranes with different amino salts.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
6
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
ATR-FTIR spectra feature information of the TFC layer in the range of 4000–2600 cm −1, while PES and part of TFC layer in a lower range (2000–600 cm −1) [31], The peak at 1426 cm −1 (R-CH2-R) and 1664 cm −1 (-C-N) were assigned to PVP K30. Spectra of TFC membranes with CAS-TEA, TEAC, TBAB, BMMIC added into aqueous solution are presented in Figs. 2b and 3. FTIR spectra of these membranes show two collective features: 1. Fig. 3 shows the peak resolve between 1520 cm −1–1750 cm −1, the peak at 1664 cm −1 for PVP that belongs to PES was overlapped by the peak of 1626 cm −1 corresponding to strong amideIband signal for poly(piperazine-amide). No peak standing for amine salts was detected in TFC composite membranes, indicating amine salts could be easily washed away and not be integrated into the poly(piperazine-amide) layer. This confirms that amine salt functions as a catalyst in the aqueous solution during the IP process, consistent with previous reports [19]. 2. Fig. 2(a) shows that there is a broad and strong signal observed at the wavelength of 3442 cm−1 which was assigned to the carboxylic acid group or hydroxyl group on the surface of the composite membranes compared to pure PES membrane. This implied that acyl groups' hydroxylations would make the fabricated NF membranes more hydrophilic if the carboxylic acid groups were in excess.
Table 3 shows the results of XPS analysis for C, O, and N contained on the surfaces of the nanocomposite membranes. The bond of O\C_O represents −COOH [2], N\C_O stands for piperazine-amide, the concentration of C (−C_O, N\C_O) and C (O\C_O) were assigned to C (piperazine-amide) and C (−COOH). The presence of cross-linker in TFC layer can be analyzed by comparing the data of C (−COOH) to the sum of C (−COOH) and C (piperazine-amide). Previous researchers [28,29] have used the following method to estimate the cross-linking degree (η):
η¼
V CON V CON þ V COOH
where VCON and VCOOH stand for the concentration of C (−C_O, N\C_O) and C (O\C_O) respectively and data were taken from XPS measurement. TBAB shows the highest extent of cross-linker in the TFC layer, CAS-TEA and BMMIC show a similar extent. Containing a relatively long cationic amine group, BMMIC obtained both efficacies of surfactant and catalyst as that of CAS-TEA. It could be seen from the morphology of BMMIC, with two types of morphologies as shown in Fig. 1a–e.
Fig. 2. FTIR feature of PES support membrane and TFC membranes fabricated by four types of amine salt. (a) PES support membrane; (b) four TFC NF membranes FTIR feature compared with PES support membrane.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
7
Fig. 3. FTIR peak resolve between 1520 cm−1 and 1750 cm−1 for TFC NF membranes.
3.3. Membrane testing The membrane performance including selectively and flux was evaluated by using the feed solutions of 500 ppm NaCl and 2000 ppm MgSO4 respectively. The MgSO4 rejection remained at high level of 96.4± 1.8%, regardless of the type of amine salts. However, the significant differences were observed for NaCl rejection as shown in Fig. 4. TBAB shows the highest rate and CAS-TEA shows the lowest rate. For the permeate flux, BMMIC shows the highest. Compared with the three symmetric amine salts, the water flux increased with the MW of cationic amine groups. The lowest crosslinking extent shown by XPS suggests that CAS-TEA makes an ion pair with the PIP molecule and transfers it into the organic solution thus increasing the diffusivity of functional reactants.
Table 3 Surface composition: atomic concentrations relative to that of carbon (atomic ratios X/C).
C (CHx, aliphatic or aromatic) C (C\O, C\N) C (C\O, N\C_O) C (O\C_O) O N
CAS-TEA
BMMIC
TBAB
TEAC
0.484 0.329 0.132 0.028 0.176 0.148
0.499 0.277 0.122 0.026 0.169 0.132
0.501 0.304 0.123 0.013 0.161 0.130
0.484 0.285 0.141 0.022 0.176 0.148
The molecular structure of the tested amine salts are shown in Table 2. The three dimensional structures show that the cationic amine group of CAS-TEA is a planar configuration despite the ethyl group. TEAC has a tridimensional structure, and its molecular size is much larger than that of the TEA group. The amine salts, when used to catalyze the IP reaction, increase cross-linking in the layer and narrow down the pore of TFC layer due to their large steric configuration interfering in the membrane formation. BMMIC, a large asymmetric molecule with a cationic amine group, has a similar MW to TEA and with a tridimensional configuration [13,19], which was also thought to be a phase transfer catalyst. By improving the transfer mechanism and increasing the bond length due to increased MW, the role of the amine salt may change to that of a surfactant, increasing the membrane water flux while decreasing salt rejection. 4. Conclusions A series of TFC membranes were fabricated by IP process, using amine salts (CAS-TEA, TEAC, TBAB, BMMIC) with different molecular structures and molecular weights (MW) as surfactant. The properties of the membranes were systematically characterized. The results indicated that these amine salts noticeably affected the membrane properties. The extent of cross-linking was increased with increasing molecular weight of cationic amine groups. As MW of amine group increases, the amine solubility in aqueous solution reduces. An optimal type of amine salt in IP process amine and its role in IP process should be considered for further advanced NF membrane development. Acknowledgments The authors would like to thank the Knowledge Innovation Program of CAS (no. KZCX2-YW-452) and the Program for Key International S&T Cooperation Projects, MOST (2009DFB90120), CSIRO Water for Healthy Country National Research Flagship for financial support. Dr Thomas, Dr James Mardel and Mr Mark Greaves from CSIRO are greatly acknowledged for their assistance in XPS, FTIR and SEM analysis. Jun Xiang would like to acknowledge the scholarship from the Chinese Scholar Council. References
Fig. 4. Four TFC membrane properties with different types of amine salt added in aqueous phase.
[1] N. Hilal, H. Al-Zoubi, N.A. Darwish, A.W. Mohamma, M. Abu Arabi, A comprehensive review of nanofiltration membranes: treatment, pretreatment, modelling, and atomic force microscopy, Desalination 170 (2004) 281–308. [2] R.J. Petersen, Composite reverse osmosis and nanofiltration membranes, J. Membr. Sci. 83 (1993) 81–150.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
8
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
[3] P.W. Morgan, Condensation polymers: by interfacial and solution Methods, Angew. Chem. (1965) 561. [4] W.J. Lau, A.F. Ismail, N. Misdan, M.A. Kassim, A recent progress in thin film composite membrane: a review, Desalination 287 (2012) 190–199. [5] W. Xie, G.M. Geise, B.D. Freeman, H.-S. Lee, G. Byun, J.E. McGrath, Polyamide interfacial composite membranes prepared from m-phenylene diamine, trimesoyl chloride and a new disulfonated diamine, J. Membr. Sci. 403–404 (2012) 152–161. [6] J.E. Cadotte, R.J. Petersen, R.E. Larson, E.E. Erickson, A new thin-film composite seawater reverse osmosis membrane, Desalination 32 (1980) 25–31. [7] K.P. Lee, T.C. Arnot, D. Mattia, A review of reverse osmosis membrane materials for desalination—development to date and future potential, J. Membr. Sci. 370 (2011) 1–22. [8] A. Kumano, N. Fujiwara, Cellulose triacetate membranes for reverse osmosis, in: Advanced Membrane Technology and Applications, John Wiley & Sons, Inc., 2008, pp. 21–46. [9] G.-D. Kang, Y.-M. Cao, Development of antifouling reverse osmosis membranes for water treatment: a review, Water Res. 46 (2012) 584–600. [10] I.J. Roh, J.-J. Kim, S.Y. Park, Mechanical properties and reverse osmosis performance of interfacially polymerized polyamide thin films, J. Membr. Sci. 197 (2002) 199–210. [11] A.K. Ghosh, B.-H. Jeong, X. Huang, E.M.V. Hoek, Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties, J. Membr. Sci. 311 (2008) 34–45. [12] H. Zou, Y. Jin, J. Yang, H. Dai, X. Yu, J. Xu, Synthesis and characterization of thin film composite reverse osmosis membranes via novel interfacial polymerization approach, Sep. Purif. Technol. 72 (2010) 256–262. [13] B. Tang, C. Zou, P. Wu, Study on a novel polyester composite nanofiltration membrane by interfacial polymerization. II. The role of lithium bromide in the performance and formation of composite membrane, J. Membr. Sci. 365 (2010) 276–285. [14] A.K. Agarwal, Composite nanofiltration and reverse osmosis membranes and method for producing the same, in, Google Patents, 2004. [15] J.Y. Koo, Y.S. Yoon, N. Kim, J.E. Kim, Composite polyamide reverse osmosis membrane and method of producing the same, in, Google Patents, 2002. [16] J.E. Tomaschke, Interfacially polymerized, bipiperidine-polyamide membranes for reverse osmosis and/or nanofiltration and process for making the same, in, Google Patents, 2002. [17] J.E. Tomaschke, I.E. Ary, Interfacially synthesized reverse osmosis membranes and processes for preparing the same, in, Google Patents, 1993. [18] J. Jegal, S.G. Min, K.-H. Lee, Factors affecting the interfacial polymerization of polyamide active layers for the formation of polyamide composite membranes, J. Appl. Polym. Sci. 86 (2002) 2781–2787.
[19] L. Yung, H. Ma, X. Wang, K. Yoon, R. Wang, B.S. Hsiao, B. Chu, Fabrication of thin-film nanofibrous composite membranes by interfacial polymerization using ionic liquids as additives, J. Membr. Sci. 365 (2010) 52–58. [20] J.E. Tomaschke, Interfacially synthesized reverse osmosis membranes and processes for preparing the same, in, Google Patents, 1993. [21] H.S. Lee, S.J. Im, J.H. Kim, H.J. Kim, J.P. Kim, B.R. Min, Polyamide thin-film nanofiltration membranes containing TiO2 nanoparticles, Desalination 219 (2008) 48–56. [22] M. Fathizadeh, A. Aroujalian, A. Raisi, Effect of added NaX nano-zeolite into polyamide as a top thin layer of membrane on water flux and salt rejection in a reverse osmosis process, J. Membr. Sci. 375 (2011) 88–95. [23] C.Y. Tang, Y.-N. Kwon, J.O. Leckie, Probing the nano- and micro-scales of reverse osmosis membranes—a comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS, TEM, ATR-FTIR, and streaming potential measurements, J. Membr. Sci. 287 (2007) 146–156. [24] H. Thissen, T. Gengenbach, R. du Toit, D.F. Sweeney, P. Kingshott, H.J. Griesser, L. Meagher, Clinical observations of biofouling on PEO coated silicone hydrogel contact lenses, Biomaterials 31 (2010) 5510–5519. [25] M. Jahanshahi, A. Rahimpour, M. Peyravi, Developing thin film composite poly(piperazine-amide) and poly(vinyl-alcohol) nanofiltration membranes, Desalination 257 (2010) 129–136. [26] N.K. Saha, S.V. Joshi, Performance evaluation of thin film composite polyamide nanofiltration membrane with variation in monomer type, J. Membr. Sci. 342 (2009) 60–69. [27] L. Li, S. Zhang, X. Zhang, Preparation and characterization of poly(piperazineamide) composite nanofiltration membrane by interfacial polymerization of 3,3′,5,5′biphenyl tetraacyl chloride and piperazine, J. Membr. Sci. 335 (2009) 133–139. [28] C.Y. Tang, Y.-N. Kwon, J.O. Leckie, Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: II. Membrane physiochemical properties and their dependence on polyamide and coating layers, Desalination 242 (2009) 168–182. [29] C.Y. Tang, Y.-N. Kwon, J.O. Leckie, Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry, Desalination 242 (2009) 149–167. [30] W.-C. Chao, S.-H. Huang, Q. An, D.-J. Liaw, Y.-C. Huang, K.-R. Lee, J.-Y. Lai, Novel interfacially-polymerized polyamide thin-film composite membranes: studies on characterization, pervaporation, and positron annihilation spectroscopy, Polymer 52 (2011) 2414–2421. [31] T. Uemura, M. Kurihara, High performance semipermeable composite membrane and process for producing the same, in, Google Patents, 1985.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
Contents lists available at SciVerse ScienceDirect
Desalination journal homepage: www.elsevier.com/locate/desal
Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes Jun Xiang a, Zongli Xie b, Manh Hoang b, Kaisong Zhang a,⁎ a b
Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China CSIRO Materials Science and Engineering, Private Bag 33, Clayton, Vic. 3169, Australia
H I G H L I G H T S ► The effect of the types of amine salt surfactants on nanofiltration performance was investigated. ► Amine salt surfactants play an important role in the performance of TFC poly(piperazine–amide) NF membranes. ► Amine salt containing larger steric configuration cationic amine group resulted in better performance.
a r t i c l e
i n f o
Article history: Received 29 June 2012 Received in revised form 19 October 2012 Accepted 30 October 2012 Available online xxxx Keywords: Amine salt surfactants Interfacial polymerization Thin film composite(TFC) membrane Poly(piperazine-amide) Nanofiltration
a b s t r a c t The role of amine salt surfactants on the performance of thin film composite nanofiltration (TFC NF) membranes is crucial for the development of advanced NF membranes, but the information is still not enough. This paper presents a systematic investigation of the effect of the types of amine salt surfactants on nanofiltration performance. Four types of amine salts with different molecular structures, molecular weights (MW) and charges, including tetraethylammonium chloride (TEAC), tetrabutylammonium bromide (TBAB), camphorsulfonic acid triethylamine salt (CAS-TEA), and 1-butyl-3-methylimidazolium chloride (BMMIC), were incorporated into piperazine (PIP) solution during interfacial polymerization(IP) with trimesoyl chloride (TMC). The formed thin film composite (TFC) piperazine–amide polymer was supported on a polyethersulfone (PES) ultrafiltration membrane (UF). The membrane surfaces were characterized by scanning electronic microscopy (SEM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), and X-ray photoelectron spectroscopy (XPS). The flux and NaCl rejection were also evaluated. The addition of amine salt containing larger steric configuration cationic amine group in aqueous solution, resulted in a TFC membrane with better performance, and the amine salt would act as a surfactant and not adhere to the polymer layer at the end of polymerization. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Nanofiltration (NF) has been widely used in the water industry for drinking water purification, wastewater treatment and reuse as it can remove turbidity, microorganisms and hardness, as well as a fraction of the dissolved salts at low operating pressure with high water flux [1]. Most of the commonly used commercial NF membranes are thin film composite (TFC) polyamide membranes with a thin film layer fabricated onto a microporous substrate via interfacial polymerization (IP) technique [2,3]. By employing IP technique, the properties of both top selective layer and bottom porous substrate can be independently controlled and optimized to achieve desired water permeability and selectivity while offering excellent mechanical strength [4]. The crosslinked aromatic polyamide is generally formed by interfacial polymerization of two monomers, a polyfunctional amine (e.g., m-phenylenediamine) in aqueous phase and a polyfunctional acid chloride (e.g. trimesoyl ⁎ Corresponding author. Tel./fax: +86 592 6190782. E-mail address: [email protected] (K. Zhang).
chloride) in organic solvent. Since water and the hydrocarbon solvent are immiscible, polymerization reaction takes place at the water/ hydrocarbon interface [5,6]. In the early development of the TFC membrane (1960s–1980s), most studies focused on researching the IP reaction mechanism and finding a suitable material for the top barrier layer. Since the 1980s, the focus shifted to improve the properties of TFC membranes with respect to permeability and selectivity [3,7,8]. As membrane performance is mainly determined by the structure and property of the top thin film layer, efforts have been devoted to the preparation of the ultrathin barrier layer to fabricate membranes with high salt rejection properties. The IP process parameters, such as type of monomer, monomer concentration, surfactant, reaction time, temperature, aqueous solution pH, additives for cross-linker structure or pore-induced, as well as the fabrication process and post-treatment affect the membrane performance. In a recent review by Kang et al. [9], the monomers in the IP process were examined. Lau et al. [4] also investigated monomers with different molecular weights and functional groups. These studies revealed that functional monomers play an important role in the extent of cross-linking and different monomer
0011-9164/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.desal.2012.10.038
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
2
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
structures lead to different configurations of polymer. I1 Juhn Roh and co-workers found that increasing acid chloride concentration resulted in thicker polyamide (PA) film with lower flux, however changing the diamine concentration did not remarkably affect the membrane performance [10]. The curing conditions in the IP process such as reaction time, curing temperature and the type of organic solvent used were also systematically studied [11,12]. It was found that a higher diffusivity of functional amine reactant could lead to higher amount of amine dissolving in the organic solvent, obtaining higher water flux, thickness, and roughness, but less cross-linking. To increase the flux and salt rejection, more amine needs to react with the acyl halide groups at the interface but not to diffuse into the bulk organic phase. To obtain a membrane exhibiting both high water flux and selectivity, a higher curing temperature is generally needed to remove organic solvent, but with the disadvantage of increasing roughness. In addition, LiBr, an inorganic salt drew researchers' interest [13], when added in aqueous solution, it could produce high property NF membranes, and by controlling the concentration of LiBr in aqueous phase solution, the morphology of membranes could be adjusted. Previous researches mainly focused on selection of monomers, monomer ratios, reaction conditions and post-treatment. Other factors such as amine salt surfactant and support membrane structure and properties, especially the role of amine salt surfactant, were not frequently reported. However, the addition of surfactants is generally recommended as a necessary component for IP process as it is believed that the surfactant assists the monomer in water phase moving into organic phase, thus improving polymerization efficiency and consequently properties of formed TFC membranes [4]. In early patents [14–17] amine salts, known as phase-transfer catalysts (PTC), had been investigated in detail, including concentration, types, structure, etc., however the working mechanism was still unclear [11,12,18,19], and less information was reported about the roles of amine salts on the properties of NF membrane in IP process. In this study, the effect of amine salts promoting IP process was systematically investigated by incorporating different types of amine salts into aqueous phase solutions. The relationship between the amine salts and the properties of the membrane was discussed. 2. Experimental 2.1. Materials Table 1 lists chemicals used in this study. Except for lithium chloride (LiCl) which was heat treated to remove moisture, all other chemicals were used as received without further purification. Deionized water (resistivity18 MΩ·cm −1 at 25 °C) was used for preparing aqueous solution.
Table 1 Materials used in this work. Material
CAS number
Source
Lithium chloride (≥99%) Polyehersulfone (E6020) Piperrazine (99%) Trimesic acid trichloride (98%) Tetraethylammonium chloride (≥99%) Tetrabutylammonium bromide (98%) Camphorsulfonic acid (99%) 1-Butyl-3-methylimidazolium chloride (≥98%) Triethylamine (≥99%)
7447-41-8 25667-42-9 110-85-0 4422-95-1
Sigma-Aldrich Inc., St. Louis, MO, USA BASF (China) Co. Ltd. Guangzhou Sigma-Aldrich Inc., St. Louis, MO, USA Sigma-Aldrich Inc., St. Louis, MO, USA
56-34-8
Sigma-Aldrich Inc., St. Louis, MO, USA
1643-19-2
Sigma-Aldrich Inc., St. Louis, MO, USA
Sodium dodecyl sulfate (99%) N-hexane (99%) Sodium phosphate (96%) Polyvinylpyrrolidone, K30
151-21-3 110-54-3 7601-54-9 9003-29-8
N, N-dimethylacetamide (≥99%) MgSO4 (≥99%)
127-19-5
NaCl (≥99%)
7467-14-5
3144-16-9 Sigma-Aldrich Inc., St. Louis, MO, USA 79917-90-1 Sigma-Aldrich Inc., St. Louis, MO, USA 121-44-8
7487-88-9
Merck Pty Limited, Kilsyth, Victoria, Australia Sigma-Aldrich Inc., St. Louis, MO, USA Sigma-Aldrich Inc., St. Louis, MO, USA Sigma-Aldrich Inc., St. Louis, MO, USA Sinopharm Chemical Reagent Co., Ltd., China Jinshan Jingwei Chemical Co., Ltd., Shanghai,China Sinopharm Chemical Reagent Co., Ltd., China Sinopharm Chemical Reagent Co., Ltd., China
2.3. Synthesis of TFC NF membranes TFC membranes were prepared using traditional IP approach described elsewhere [2,20–22]. Membrane fabrication was conducted using two 8 mm deep polytetrafluoroethylene (PTFE) circular rings, with outside and inner diameter of 140 mm and 100 mm respectively. Four aqueous solutions were prepared, namely 1.6 wt.% PIP, 0.2 mol/L amine salt (where, CAS-TEA was generated by reacting CAS with TEA in a weight ratio of 2.32:1), 0.15 wt.% sodium dodecyl sulfate (SLS) and 0.5 wt.% sodium phosphate, while the organic phase solution contained 0.35 wt.% TMC, using N-hexane as solvent. Four different types of amine salts including tetraethylammonium chloride (TEAC), tetrabutylammonium bromide (TBAB), camphorsulfonic acid triethylamine salt (CAS-TEA), 1-butyl-3-methylimidazolium chloride Table 2 The chemical structures of amine salts. Name
Chemical structure
Structure of cationic amine group
MW (cationic amine group)
CAS-TEA
101.9
TEAC
130.2
TBAB
242.37
BMMIC
139.17
2.2. Preparation of microporous PES membrane substrate Microporous PES membrane was used as the supporting substrate of the polyamide thin film layer. It was fabricated via phase inversion by the following procedure: First, PES cast solution was prepared by dissolving 19 wt.% PES, 3.5 wt.% Polyvinylpyrrolidone (K30), and 3 wt.% LiCl in DMAC with a ratio of 74.5 wt.% at 80 °C. After stirring for 12 h, the resulting homogeneous solution was casted onto a non-woven fabric (thickness 100–110 μm) using a casting knife, followed by dipping the membrane into a deionized water bath for immediate phase inversion. The thickness of wet film was controlled at ~ 250 μm. After 30 min in a gelation media, the membrane was taken out and washed thoroughly using distilled water. The entire membrane fabrication process was carried out at room temperature and at the relative humidity of 30%. Resulting microporous membrane with a molecular weight cut off (MWCO) of between 30 k and 50 k·Dalton was stored in a refrigerator at 4 °C prior to use.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
(BMIMCL) were used in the study with the conditions listed in Table 2. Except for using different amine salt, a typical procedure for synthesizing poly(piperazine-amide) membranes is described below. First, the non-woven fabric side of PES membrane substrate was taped onto one of the PTFE rings, and then clamped to the other PTFE ring, so that it was sandwiched in between 2 PTFE rings. PIP solutions were poured onto the dense side of the PES substrate, membrane was immersed for 1 min, then the excess PIP solution was removed using a soft rubber roller and dried for 2 min vertically at room temperature. The TMC solution was poured over the membrane, kept contact with TMC solution for 20 s, to allow in situ formation of a TFC layer over
3
the surface of PES support. Subsequently the fabricated membrane was cured for 3 min in an air-circulation oven at 50 °C. Finally the membrane was thoroughly washed and stored in deionized water until the membrane performance was evaluated. To ensure reproducibility, five samples were prepared for each membrane. 2.4. Membrane testing Performance of fabricated TFC NF membranes were evaluated by the water flux and salt rejection using a cross-flow PTFE cell CF042
Fig. 1. SEM micrograph of different membranes. (a) PES membrane without TFC layer, (b) TFC membrane with CAS added into aqueous solution, (c) TFC membrane with TEAC added into aqueous solution, (d) TFC membrane with TBAB added into aqueous solution, (e) TFC membrane with BMMIC added into aqueous solution.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
4
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
}
Fig. 1 (continued).
(Sterlitech Co.) at 25 °C and 0.35 MPa. The feed solutions containing 500 ppm NaCl and 2000 ppm MgSO4 were tested separately. The water flux was calculated by measuring the weight of permeate solution passing through the membrane over time (h) and over the exposed
membrane area (m2). Salt rejections NaCl and MgSO4 were calculated respectively, from the conductivities of the feed and the permeate measured with a digital conductivity meter (sensION + EC5 Portable Conductivity Meter, Hach Co.).
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
5
}
Fig. 1 (continued).
2.5. Membrane characterization
3. Results and discussion
2.5.1. ATR-FTIR The functional structure of TFC membrane samples was analyzed using a Thermo Scientific Nicolet 6700 iTR spectrometer. Attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) was performed on membrane samples from 600 to 4000 cm−1 wavelength with an 8 cm−1 resolution.
3.1. Membrane morphologies
2.5.2. SEM The surface morphology of the TFC membrane samples was imaged using a Philips XL30 scanning electronic microscope (SEM). Before testing, all the membrane samples were dried in vacuum oven at 80 °Cfor more than 48 h. The samples were sputter coated with a 10–20 nm thin layer of iridium under vacuum. 2.5.3. XPS X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Ultra DLD spectrometer (Kratos Analytical Inc., Manchester, UK), with a monochromatized AlKa source at a power of 180 W (15 kV× 12 mA), a hemispherical analyzer operating in the fixed analyzer transmission mode and the standard aperture (analysis area: 0.3 mm× 0.7 mm). The total pressure in the main vacuum chamber during analysis was less than 10–8 mbar. Survey spectra were acquired at a pass energy of 160 eV. To obtain more detailed information about chemical structure, oxidation states, etc., high resolution spectra were recorded from individual peaks at 40 eV pass energy (yielding a typical peak width for polymers of 1.0–1.1 eV). Data processing was performed using CasaXPS processing software version 2.3.15 (Casa Software Ltd., Teignmouth, UK). All elements present were identified from survey spectra. Binding energies were referenced to the aliphatic hydrocarbon peak at 285.0 eV. Other details of test condition and the equipment information of software were given elsewhere [23,24].
The surface morphologies of NF membranes were monitored by FESEM and are presented in Fig. 1a. The pure PES membrane substrate shows a smooth surface. However, the poly(piperazine-amide) layer polymerized onto the PES substrate resulted in a thin film layer with rougher surface. When CAS-TEA, TEAC, TBAB and BMMIC were added to the PIP solutions respectively, the common “ridge-and-valley” surface morphologies of all the polymerized membranes was shown in Fig. 1b–e. It is similar to the previous reports [25–27]. As shown in Fig.1, it is interesting to note that as the roughness of TFC layers increases with the MW of cationic amine group increasing. The total thickness of TFC membranes (including the thickness of rough surface) are distributed in a relatively narrow range of 500–850 nm. CAS-TEA resulted in the thinnest membrane, followed by TEAC, TBAB. It should be noted that the CAS-TEA molecule possesses two organic groups, one containing a super-hydrophilic bond of − SO3H, the other one with an amine functional bond. The former may have played a role as a surfactant in the formation of a smooth surface of the TFC membrane. 3.2. FTIR and XPS analysis ATR-FTIR and XPS analyses were used to characterize the physiochemical structure of the TFC layer and to understand catalytic effect of the four amine salts. XPS is a surface sensitive technique and measures the element concentration (except H) and chemical binding information for the top 1–10 nm depth of the surface region [28–30], while ATR-FTIR offers much deeper penetration (from b200 nm to > 1 um). Fig. 2 shows the ATR-FTIR spectra of PES membrane and four TFC NF membranes with different amino salts.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
6
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
ATR-FTIR spectra feature information of the TFC layer in the range of 4000–2600 cm −1, while PES and part of TFC layer in a lower range (2000–600 cm −1) [31], The peak at 1426 cm −1 (R-CH2-R) and 1664 cm −1 (-C-N) were assigned to PVP K30. Spectra of TFC membranes with CAS-TEA, TEAC, TBAB, BMMIC added into aqueous solution are presented in Figs. 2b and 3. FTIR spectra of these membranes show two collective features: 1. Fig. 3 shows the peak resolve between 1520 cm −1–1750 cm −1, the peak at 1664 cm −1 for PVP that belongs to PES was overlapped by the peak of 1626 cm −1 corresponding to strong amideIband signal for poly(piperazine-amide). No peak standing for amine salts was detected in TFC composite membranes, indicating amine salts could be easily washed away and not be integrated into the poly(piperazine-amide) layer. This confirms that amine salt functions as a catalyst in the aqueous solution during the IP process, consistent with previous reports [19]. 2. Fig. 2(a) shows that there is a broad and strong signal observed at the wavelength of 3442 cm−1 which was assigned to the carboxylic acid group or hydroxyl group on the surface of the composite membranes compared to pure PES membrane. This implied that acyl groups' hydroxylations would make the fabricated NF membranes more hydrophilic if the carboxylic acid groups were in excess.
Table 3 shows the results of XPS analysis for C, O, and N contained on the surfaces of the nanocomposite membranes. The bond of O\C_O represents −COOH [2], N\C_O stands for piperazine-amide, the concentration of C (−C_O, N\C_O) and C (O\C_O) were assigned to C (piperazine-amide) and C (−COOH). The presence of cross-linker in TFC layer can be analyzed by comparing the data of C (−COOH) to the sum of C (−COOH) and C (piperazine-amide). Previous researchers [28,29] have used the following method to estimate the cross-linking degree (η):
η¼
V CON V CON þ V COOH
where VCON and VCOOH stand for the concentration of C (−C_O, N\C_O) and C (O\C_O) respectively and data were taken from XPS measurement. TBAB shows the highest extent of cross-linker in the TFC layer, CAS-TEA and BMMIC show a similar extent. Containing a relatively long cationic amine group, BMMIC obtained both efficacies of surfactant and catalyst as that of CAS-TEA. It could be seen from the morphology of BMMIC, with two types of morphologies as shown in Fig. 1a–e.
Fig. 2. FTIR feature of PES support membrane and TFC membranes fabricated by four types of amine salt. (a) PES support membrane; (b) four TFC NF membranes FTIR feature compared with PES support membrane.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
7
Fig. 3. FTIR peak resolve between 1520 cm−1 and 1750 cm−1 for TFC NF membranes.
3.3. Membrane testing The membrane performance including selectively and flux was evaluated by using the feed solutions of 500 ppm NaCl and 2000 ppm MgSO4 respectively. The MgSO4 rejection remained at high level of 96.4± 1.8%, regardless of the type of amine salts. However, the significant differences were observed for NaCl rejection as shown in Fig. 4. TBAB shows the highest rate and CAS-TEA shows the lowest rate. For the permeate flux, BMMIC shows the highest. Compared with the three symmetric amine salts, the water flux increased with the MW of cationic amine groups. The lowest crosslinking extent shown by XPS suggests that CAS-TEA makes an ion pair with the PIP molecule and transfers it into the organic solution thus increasing the diffusivity of functional reactants.
Table 3 Surface composition: atomic concentrations relative to that of carbon (atomic ratios X/C).
C (CHx, aliphatic or aromatic) C (C\O, C\N) C (C\O, N\C_O) C (O\C_O) O N
CAS-TEA
BMMIC
TBAB
TEAC
0.484 0.329 0.132 0.028 0.176 0.148
0.499 0.277 0.122 0.026 0.169 0.132
0.501 0.304 0.123 0.013 0.161 0.130
0.484 0.285 0.141 0.022 0.176 0.148
The molecular structure of the tested amine salts are shown in Table 2. The three dimensional structures show that the cationic amine group of CAS-TEA is a planar configuration despite the ethyl group. TEAC has a tridimensional structure, and its molecular size is much larger than that of the TEA group. The amine salts, when used to catalyze the IP reaction, increase cross-linking in the layer and narrow down the pore of TFC layer due to their large steric configuration interfering in the membrane formation. BMMIC, a large asymmetric molecule with a cationic amine group, has a similar MW to TEA and with a tridimensional configuration [13,19], which was also thought to be a phase transfer catalyst. By improving the transfer mechanism and increasing the bond length due to increased MW, the role of the amine salt may change to that of a surfactant, increasing the membrane water flux while decreasing salt rejection. 4. Conclusions A series of TFC membranes were fabricated by IP process, using amine salts (CAS-TEA, TEAC, TBAB, BMMIC) with different molecular structures and molecular weights (MW) as surfactant. The properties of the membranes were systematically characterized. The results indicated that these amine salts noticeably affected the membrane properties. The extent of cross-linking was increased with increasing molecular weight of cationic amine groups. As MW of amine group increases, the amine solubility in aqueous solution reduces. An optimal type of amine salt in IP process amine and its role in IP process should be considered for further advanced NF membrane development. Acknowledgments The authors would like to thank the Knowledge Innovation Program of CAS (no. KZCX2-YW-452) and the Program for Key International S&T Cooperation Projects, MOST (2009DFB90120), CSIRO Water for Healthy Country National Research Flagship for financial support. Dr Thomas, Dr James Mardel and Mr Mark Greaves from CSIRO are greatly acknowledged for their assistance in XPS, FTIR and SEM analysis. Jun Xiang would like to acknowledge the scholarship from the Chinese Scholar Council. References
Fig. 4. Four TFC membrane properties with different types of amine salt added in aqueous phase.
[1] N. Hilal, H. Al-Zoubi, N.A. Darwish, A.W. Mohamma, M. Abu Arabi, A comprehensive review of nanofiltration membranes: treatment, pretreatment, modelling, and atomic force microscopy, Desalination 170 (2004) 281–308. [2] R.J. Petersen, Composite reverse osmosis and nanofiltration membranes, J. Membr. Sci. 83 (1993) 81–150.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038
8
J. Xiang et al. / Desalination xxx (2012) xxx–xxx
[3] P.W. Morgan, Condensation polymers: by interfacial and solution Methods, Angew. Chem. (1965) 561. [4] W.J. Lau, A.F. Ismail, N. Misdan, M.A. Kassim, A recent progress in thin film composite membrane: a review, Desalination 287 (2012) 190–199. [5] W. Xie, G.M. Geise, B.D. Freeman, H.-S. Lee, G. Byun, J.E. McGrath, Polyamide interfacial composite membranes prepared from m-phenylene diamine, trimesoyl chloride and a new disulfonated diamine, J. Membr. Sci. 403–404 (2012) 152–161. [6] J.E. Cadotte, R.J. Petersen, R.E. Larson, E.E. Erickson, A new thin-film composite seawater reverse osmosis membrane, Desalination 32 (1980) 25–31. [7] K.P. Lee, T.C. Arnot, D. Mattia, A review of reverse osmosis membrane materials for desalination—development to date and future potential, J. Membr. Sci. 370 (2011) 1–22. [8] A. Kumano, N. Fujiwara, Cellulose triacetate membranes for reverse osmosis, in: Advanced Membrane Technology and Applications, John Wiley & Sons, Inc., 2008, pp. 21–46. [9] G.-D. Kang, Y.-M. Cao, Development of antifouling reverse osmosis membranes for water treatment: a review, Water Res. 46 (2012) 584–600. [10] I.J. Roh, J.-J. Kim, S.Y. Park, Mechanical properties and reverse osmosis performance of interfacially polymerized polyamide thin films, J. Membr. Sci. 197 (2002) 199–210. [11] A.K. Ghosh, B.-H. Jeong, X. Huang, E.M.V. Hoek, Impacts of reaction and curing conditions on polyamide composite reverse osmosis membrane properties, J. Membr. Sci. 311 (2008) 34–45. [12] H. Zou, Y. Jin, J. Yang, H. Dai, X. Yu, J. Xu, Synthesis and characterization of thin film composite reverse osmosis membranes via novel interfacial polymerization approach, Sep. Purif. Technol. 72 (2010) 256–262. [13] B. Tang, C. Zou, P. Wu, Study on a novel polyester composite nanofiltration membrane by interfacial polymerization. II. The role of lithium bromide in the performance and formation of composite membrane, J. Membr. Sci. 365 (2010) 276–285. [14] A.K. Agarwal, Composite nanofiltration and reverse osmosis membranes and method for producing the same, in, Google Patents, 2004. [15] J.Y. Koo, Y.S. Yoon, N. Kim, J.E. Kim, Composite polyamide reverse osmosis membrane and method of producing the same, in, Google Patents, 2002. [16] J.E. Tomaschke, Interfacially polymerized, bipiperidine-polyamide membranes for reverse osmosis and/or nanofiltration and process for making the same, in, Google Patents, 2002. [17] J.E. Tomaschke, I.E. Ary, Interfacially synthesized reverse osmosis membranes and processes for preparing the same, in, Google Patents, 1993. [18] J. Jegal, S.G. Min, K.-H. Lee, Factors affecting the interfacial polymerization of polyamide active layers for the formation of polyamide composite membranes, J. Appl. Polym. Sci. 86 (2002) 2781–2787.
[19] L. Yung, H. Ma, X. Wang, K. Yoon, R. Wang, B.S. Hsiao, B. Chu, Fabrication of thin-film nanofibrous composite membranes by interfacial polymerization using ionic liquids as additives, J. Membr. Sci. 365 (2010) 52–58. [20] J.E. Tomaschke, Interfacially synthesized reverse osmosis membranes and processes for preparing the same, in, Google Patents, 1993. [21] H.S. Lee, S.J. Im, J.H. Kim, H.J. Kim, J.P. Kim, B.R. Min, Polyamide thin-film nanofiltration membranes containing TiO2 nanoparticles, Desalination 219 (2008) 48–56. [22] M. Fathizadeh, A. Aroujalian, A. Raisi, Effect of added NaX nano-zeolite into polyamide as a top thin layer of membrane on water flux and salt rejection in a reverse osmosis process, J. Membr. Sci. 375 (2011) 88–95. [23] C.Y. Tang, Y.-N. Kwon, J.O. Leckie, Probing the nano- and micro-scales of reverse osmosis membranes—a comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS, TEM, ATR-FTIR, and streaming potential measurements, J. Membr. Sci. 287 (2007) 146–156. [24] H. Thissen, T. Gengenbach, R. du Toit, D.F. Sweeney, P. Kingshott, H.J. Griesser, L. Meagher, Clinical observations of biofouling on PEO coated silicone hydrogel contact lenses, Biomaterials 31 (2010) 5510–5519. [25] M. Jahanshahi, A. Rahimpour, M. Peyravi, Developing thin film composite poly(piperazine-amide) and poly(vinyl-alcohol) nanofiltration membranes, Desalination 257 (2010) 129–136. [26] N.K. Saha, S.V. Joshi, Performance evaluation of thin film composite polyamide nanofiltration membrane with variation in monomer type, J. Membr. Sci. 342 (2009) 60–69. [27] L. Li, S. Zhang, X. Zhang, Preparation and characterization of poly(piperazineamide) composite nanofiltration membrane by interfacial polymerization of 3,3′,5,5′biphenyl tetraacyl chloride and piperazine, J. Membr. Sci. 335 (2009) 133–139. [28] C.Y. Tang, Y.-N. Kwon, J.O. Leckie, Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: II. Membrane physiochemical properties and their dependence on polyamide and coating layers, Desalination 242 (2009) 168–182. [29] C.Y. Tang, Y.-N. Kwon, J.O. Leckie, Effect of membrane chemistry and coating layer on physiochemical properties of thin film composite polyamide RO and NF membranes: I. FTIR and XPS characterization of polyamide and coating layer chemistry, Desalination 242 (2009) 149–167. [30] W.-C. Chao, S.-H. Huang, Q. An, D.-J. Liaw, Y.-C. Huang, K.-R. Lee, J.-Y. Lai, Novel interfacially-polymerized polyamide thin-film composite membranes: studies on characterization, pervaporation, and positron annihilation spectroscopy, Polymer 52 (2011) 2414–2421. [31] T. Uemura, M. Kurihara, High performance semipermeable composite membrane and process for producing the same, in, Google Patents, 1985.
Please cite this article as: J. Xiang, et al., Effect of amine salt surfactants on the performance of thin film composite poly(piperazine-amide) nanofiltration membranes, Desalination (2012), http://dx.doi.org/10.1016/j.desal.2012.10.038