Applied Surface Science 256 (2010) 1657–1663
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Preparation and characterization of asymmetric polyethersulfone and thin-film composite polyamide nanofiltration membranes for water softening Ahmad Rahimpour a,*, Mohsen Jahanshahi a, Narmin Mortazavian a, Sayed Siavash Madaeni b, Yaghob Mansourpanah c a b c
Nanobiotechnology Research Laboratory, Faculty of Chemical Engineering, Babol University of Technology, Shariati Street, Babol, Iran Membrane Research Center, Department of Chemical Engineering, Razi University, Kermanshah, Iran Department of Chemistry, Faculty of Science, Lorestan University, Khorramabad, Iran
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 April 2009 Received in revised form 21 September 2009 Accepted 26 September 2009 Available online 6 October 2009
In this research, two types of nanofiltration membranes were prepared and evaluated for water softening. Their nanofiltration performance was evaluated by cross-flow filtration using NaCl (1 g/l) and MgSO4 (1 g/l) solution at 5 and 10 bar, 25 8C and 10 l/min. The morphological studies were performed with SEM and AFM instruments. In addition, the hydrophilicity of membranes was examined by contact angle measurements. In the first type, asymmetric polyethersulfone (PES) nanofiltration membranes were prepared using phase inversion induced by immersion precipitation technique. Different components such as polyvinylpyrrolidone (PVP), polyethyleneglycole (PEG), acrylic acid and Triton X-100 were used as additive in the PES casting solution, which lead to the formation of new asymmetric nanofiltration membranes. Two concentrations of PES (20 and 25 wt%) and two different non-solvents (pure water and mixture of water (80 vol.%) and IPA (20 vol.%)) were used for preparing asymmetric nanofiltration membranes. The morphological studies showed that the membranes prepared with nonsolvent containing 20 vol.% IPA have smoother surface and smaller pores in surface and sub-layer compared to membranes prepared with pure water as non-solvent. The flux was decreased when higher polymer concentration and mixture of water and IPA were employed for membrane formation. However, NaCl and MgSO4 rejections were improved. In the second type, thin-film composite polyamide nanofiltration membrane was fabricated using interfacial polymerization of 1,3-phenylenediamine (PDA) with trimesoyl chloride (TMC). A rough and dense film was formed on the PES support membrane by interfacial polymerization. The water permeability of composite membrane was 7 and 21 kg m2 h1 at 5 and 10 bar, respectively. Moreover, the rejection to the MgSO4 as divalent salt (85 and 90%) was high compared to the NaCl as monovalent salt (64 and 67%). ß 2009 Elsevier B.V. All rights reserved.
Keywords: Membrane Nanofiltration Composite Preparation Water softening
1. Introduction Nanofiltration is a kind of pressure-driven process between reverse osmosis and ultrafiltration processes. Because of its advantages such as low operating pressures, high fluxes, high retentions of multivalent salts, low investment and operation costs, this technology is rapidly developed for different applications such as water softening, color removal, chemical oxygen demand (COD) and biological oxygen demand (BOD) reduction [1– 3]. Most of the commercial nanofiltration membranes are thin-film composite (TFC) membrane, such as NF series made by Filmtec Corporation, NTR series by the Nitto Denko Company, UTC series by Toray Industries, Desal series by Desalination Systems, and ATF
* Corresponding author. Tel.: +98 912 8093155; fax: +98 111 3220342. E-mail address:
[email protected] (A. Rahimpour). 0169-4332/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2009.09.089
series by Advanced Membrane Technology Inc. [4,5]. Nanofiltration membranes are generally fabricated using interfacial polymerization [6–10], which is generally employed for composite membrane preparation. In this technique, a thin film is introduced by forming an ultra-thin dense layer on a porous substrate. In interfacial polymerization method, reactive monomers are dissolved in two immiscible phases and the polymerization of the reactive monomers occurs on the surface of the porous support membrane [11–13]. On the other hand, polymerization reaction eventuates at the interface of the two liquids that are immiscible. The key to the interfacial polymerization method is to select the right partition coefficient of the reactants in the two-phase solution and to set the appropriate diffusion speed of the reactants to achieve the ideal degree of densification of the ultra-thin layer. The main advantage of TFC membrane is that the membrane properties can be controlled by optimizing the characteristics of the thin selective layer and support. The thin selective layer can be
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optimized for solute rejection and permeability by controlling the polymerization such as coating time, monomers concentration and curing temperature. Various supports with different porosity, strength and solvent resistance can be selected for preparing TFC membrane. The polysulfone or polyethersulfone ultrafiltration membranes are generally used as support for making thin-film composite nanofiltration membrane. Some of the commercial high-pressure nanofiltration membranes are asymmetric membranes without composite structure, such as NP010 and NP030 made by Nadir Corporation. The material of this membrane is polyethersulfone. Another common technique, which has been adopted for the development of asymmetric and polymeric RO and NF membranes is the phase inversion induced by immersion precipitation method [14,15]. This process results in a membrane with a dense top-layer and a porous sublayer containing macrovoids, pores and micro-pores [16]. In this technique, a homogeneous polymer solution containing polymer and solvent with appropriate additives is cast on a polyester or polyethylene/polypropylene non-woven fabric and immersed in a coagulation bath (in some cases after a short period of solvent evaporation). The diffusive exchange of solvent and non-solvent introduces liquid–liquid phase separation, i.e. the formation of a polymer-rich and a polymer-lean phase in the casting solution lowers the Gibbs free energy of mixing. The morphology and performance of membranes strongly depend on the thermodynamics as well as kinetics of the phase inversion process [17]. The present work describes the development of thin-film composite and new low-pressure asymmetric nanofiltration membranes prepared through interfacial polymerization and immersion precipitation techniques, respectively. Two types of nanofiltration membranes were developed: (i) thin-film polyamide composite nanofiltration membrane which was prepared through interfacial polymerization of 1,3-phenylenediamine (PDA) and trimesoyl chloride (TMC) and (ii) asymmetric polyethersulfone nanofiltration membranes which prepared through immersion precipitation. The performances of the membranes were evaluated by separation of NaCl and MgSO4 salt solutions. In addition, the surfaces and cross-sections of the membranes were investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM) and contact angle measurements. 2. Experimental 2.1. Materials Polyethersulfone (PES ultrason E6020P with Mw = 58,000 g/mol) and dimethylacetamide (DMAc) were supplied by BASF Company. Polyvinylpyrrolidone (PVP) with 25,000 g/mol molecular weight, acrylic acid (AA), trimesoyl chloride (TMC), 1,3-phenylenediamine (PDA), Triton X-100, polyethyleneglycole (PEG 600), sodium chloride (NaCl), magnesium sulfate (MgSO4) and n-hexane were obtained from Merck. 2-propanol (IPA) was obtained from Minko Company. Distilled water was used throughout this study. 2.2. Preparation of asymmetric PES ultrafiltration and nanofiltration membranes Asymmetric PES ultrafiltration and nanofiltration membranes were prepared via phase inversion induced by immersion precipitation [18,19] using casting solutions containing PES, polyvinylpyrrolidone (PVP), polyethyleneglycole (PEG), acrylic acid (AA) and Triton X-100 in DMAc as solvent. For preparing the homogenous solution, all components were simultaneously blended with DMAc and then mixed by mechanical stirrer for 12 h. The solution was sprinkled and cast using self-made casting knife with 75 mm thickness on polyester non-woven fabric. This was
immediately moved to the non-solvent bath for immersion at room temperature without any evaporation. The non-solvents were water and mixture of water (80 vol.%) and IPA (20 vol.%). After primarily phase separation and membrane formation, the membranes were stored in water/IPA for 24 h to guarantee the complete phase separation. As the final stage, membranes were sandwiched between two sheets of filter papers for 24 h at room temperature for drying. 2.3. Preparation of thin-film composite nanofiltration membranes The composite nanofiltration membrane was prepared by interfacial polymerization between 1,3-phenylenediamine (PDA) with trimesoyl chloride (TMC). For preparing the polyamide composite membrane, the PES ultrafiltration membranes were immersed for 3 min in aqueous solutions of PDA (1 wt%). The surface was rolled with a soft rubber roller to remove excess solution and eliminate any tiny bubbles from the membrane surface. The saturated membrane was then immersed in 0.2 wt% of organic solution of TMC for around 40 s, which resulted in the formation of a thin-film polyamide on the top of the PES support membrane. Finally, the obtained composite membranes were heat-cured at 80 8C for around 3 min. 2.4. Contact angle measurements The hydrophilicities of the prepared membranes were analyzed to examine the variations in wetting characteristics of the membranes using contact angle measurements. The contact angles between water and surfaces of membranes were obtained using a contact angle-measuring instrument (G10, KRUSS, Germany) for evaluating of the membrane hydrophilicity. De-ionized water was used as the probe liquid in all measurements. To minimize the experimental error, the contact angle was measured at five random locations for each sample and the average value was reported. 2.5. Scanning electron microscopy (SEM) A scanning electron microscope directly provides the visual information of the top surface as well as cross-sectional morphology of the membrane. Cambridge Scanning Electron Microscope (SEM, CamScan MV2300) was employed in this work. The membranes were cut into small pieces and cleaned with filter paper. The pieces were immersed in liquid nitrogen for 60–90 s and were frozen. Frozen fragments of the membranes were broken and kept in air for drying. The dried samples were gold sputtered for producing electric conductivity. After sputtering with gold, they were viewed with the microscope at 10 kV. 2.6. Atomic force microscopy (AFM) Atomic force microscopy was used to analyze the surface morphology and roughness of the prepared membranes. The AFM device was DualScopeTM scanning probe-optical microscope (DME model C-21, Denmark). Small squares of the prepared membranes (approximately 1 cm2) were cut and glued on glass substrate. The membrane surfaces were imaged in a scan size of 5 mm 5 mm and 10 mm 10 mm. 2.7. Flux and retention Asymmetric and thin-film composite nanofiltration membranes were characterized by measuring the pure water flux, salt solution flux and salt rejection. A salt solution containing NaCl and MgSO4 with 2000 ppm (1000 ppm or 1 g/l NaCl and 1000 ppm or 1 g/l MgSO4) was used as feed solution. The nanofiltration
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Table 1 Water permeability and salt rejections of different asymmetric nanofiltration membranes. Membrane
NF11 NF12 NF31 NF32
Water flux (kg m2 h1)
Salt rejections (5 bar)
5 bar
10 bar
NaCl
MgSO4
39 2.0 30 2.0 100 5.0 82 4.0
72 4.0 63 3.0 240 12.0 211 11.0
48 1.0 43 1.0 49 1.0 38 1.0
80 2.0 68 1.0 80 2.0 77 2.0
NF11: 25 wt% PES concentration and pure water as non-solvent; NF12: 25 wt% PES concentration and mixture of water (80 vol.%) and IPA (20 vol.%) as non-solvent; NF31: 20 wt% PES concentration and pure water as non-solvent; NF32: 20 wt% PES concentration and mixture of water (80 vol.%) and IPA (20 vol.%) as non-solvent.
Fig. 1. Cross-flow filtration system.
experiments were conducted using a laboratory-scale cross-flow filtration system as shown in Fig. 1. The cross-flow cell houses flat sheet membrane pieces with an effective area of 24 cm2. A centrifugal pump was used to pass the feed solution through the membrane surface. The valve located at the end of the cell was used to pressurize the feed solutions and to control the feed pressure. The feed flow rate was 10 l min1. The membranes were pre-compressed with pure water at 5 bar for 15 min. Then, the pure water and salt solution fluxes were evaluated at trans-
membrane pressures up to 5 and 10 bar and room temperature (around 25 8C). The concentrations of NaCl and MgSO4 in permeate were analyzed by atomic absorption. The presented data for permeability and salt rejections of membranes in this study are the average of two measurements conducted with standard deviation of about 5 and 2%, respectively. 3. Results and discussions Asymmetric PES membrane was fabricated with phase inversion induced by immersion precipitation technique. Polyamide thin-film composite membrane was prepared by interfacial polymerization of PDA with TMC. The nanofiltration performance
Fig. 2. Cross-sectional SEM images of different nanofiltration membranes: (a) 20 wt% of PES and water as non-solvent, (b) 20 wt% of PES and mixture of water (80 vol.%) and IPA (20 vol.%) as non-solvent, (c) 25 wt% of PES and water as non-solvent, and (d) 25 wt% of PES and mixture of water (80 vol.%) and IPA (20 vol.%) as non-solvent.
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of the membranes was evaluated as well as the surface and sublayer morphology, surface hydrophilicity and roughness. 3.1. Asymmetric PES nanofiltration membranes For preparing the asymmetric nanofiltration membranes, different components such as acrylic acid, Triton X-100, PVP and PEG were added to the polymer solution. Acrylic acid and Triton X100 are hydrophile and viscose components. The purpose of adding these two components was to obtain hydrophile surface with smaller pores as these components increase the viscosity of polymer casting solution. PEG is known to influence the pore formation in the immersion precipitation process due to strong hydration and large excluded volume in aqueous systems. For preventing the strong effect of PEG on pore size, low concentration of PEG was added to the casting solution. PVP is also hydrophilic, but less than PEG; therefore, it has an influence on the membrane pore structure. However it may also be entrapped in the PES matrix leading to hydrophilization of the pore surface. It is well known that the addition of PVP in the polymer solution should be controlled in order to form the best morphological membrane structure. In addition, the membrane properties such as water
Fig. 3. Surface SEM images of different nanofiltration membranes: (a) 20 wt% of PES and water as non-solvent and (b) 20 wt% of PES and mixture of water (80 vol.%) and IPA (20 vol.%) as non-solvent.
permeability and hydrophilicity were found to be affected by the residual PVP in the pore or dissolution of PVP during membrane formation and treatment process. Thus, 1 wt% of PVP was used in the casting solution. In this study, two concentrations of PES (20 and 25 wt%) and two different non-solvents (pure water and mixture of water (80 vol.%) and IPA (20 vol.%)) were employed. Accordingly four different PES nanofiltration membranes were obtained. The water permeabilities and salt rejection of different asymmetric nanofiltration membranes are presented in Table 1. As expected, flux is increased with an increase in trans-membrane pressure (TMP). This behavior is described by Spiegler–Kedem Model [20]: J v ¼ L p ðDP sDpÞ
(1)
where Jv is the water flux, Lp is the pure water permeability, DP is the trans-membrane pressure (TMP), s is the reflection factor of the membrane, and Dp is the osmosis pressure. The water permeabilities of nanofiltration membranes prepared with 25 wt% of polymer are low compared to the membranes prepared with 20 wt% of polymer. When the polymer concentration in the casting dope is increased from 20 to 25 wt%, the membranes obtained by coagulation in the same conditions show
Fig. 4. 3-Dimension AFM images of (a) NF31 and (b) NF32 asymmetric membranes. NF31: 20 wt% PES concentration and pure water as non-solvent; NF32: 20 wt% PES concentration and mixture of water (80 vol.%) and IPA (20 vol.%) as non-solvent.
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denser morphology with smaller and fewer macrovoids in the sublayer (see Fig. 2). This change in membrane morphology with increasing the polymer concentration is anticipated. The more concentrated polymer solution leads to higher polymer concentration at the binodal phase separation point (hence a denser structure), as well as lower possibility of solvent extraction from the surrounding polymer solution to the polymer-lean phase during the formation of the macrovoids. The pore structure of the skin formed on the face in contact with water is also changed in the same way, i.e. the porosity and the pore size is decreased with increasing the polymer concentration in the casting dope. The NaCl and MgSO4 rejections of asymmetric nanofiltration membranes were improved with an increase in PES concentration in the casting solution. With the increase in the polymer content in the dope solution, the membrane pore size is decreased leading to a decline in the permeation rate and an enhancement in the retention capacity. Increasing alcohol (IPA) composition in the coagulation bath from 0 to 20% (see Table 1) causes a decline in water permeability for the asymmetric nanofiltration membranes from 39 to 30 kg m2 h1 at 25 wt% of PES concentration and from 100 to 82 kg m2 h1 at 20 wt% of PES concentration. Moreover, the NaCl and MgSO4 rejections were increased when the mixture of water and IPA was employed as non-solvent in the gelation media. This
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Table 2 Surface roughness and hydrophilicity of NF31 (pure water as non-solvent) and NF32 (mixture of water and IPA as non-solvent) asymmetric PES membranes. Membrane
NF31 NF32
Roughness
Contact angles (8)
Sa (nm)
Sq (nm)
Sz (nm)
6.53 3.68
8.33 4.72
57.9 31.5
54.3 50.7
phenomenon can be explained by the fact that, if IPA is used as non-solvent, the binodal region in the ternary phase diagram significantly shrinks [21]. This signifies that more non-solvent (IPA) is needed to diffuse into the polymer solution before this solution reaches the binodal region and phase separation can occur. This results in the formation of a membrane with a dense top and sub-layer (Fig. 2) with small pores on the surface (Fig. 3). This morphology explains the water flux decline and salt rejection improvement. Fig. 4 represents the AFM images of the surfaces of NF31 (pure water as non-solvent) and NF32 (mixture of water and IPA as nonsolvent) asymmetric PES membranes over a scan area of 5 mm 5 mm. The surface seems to be rougher when pure water was used as non-solvent for preparing the membranes. Moreover, the surface roughness parameters of the membranes which are
Fig. 5. Surface and cross-sectional SEM images of PES support (a-1 and a-2) and polyamide composite (b-1 and b-2) membrane.
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expressed in terms of the mean roughness (Sa), the root mean square of the Z data (Sq) and the mean difference between the highest peaks and lowest valleys (Sz) were calculated from AFM images by SPM DME software and are presented in Table 2. Moreover, the contact angles of NF31 and NF32 membranes that indicate the membrane surface hydrophilicity are presented in Table 2. The NF32 membrane demonstrates a flat and smooth surface with higher hydrophilicity compared to the NF31 membrane. The contact angles indicate that both NF31 and NF32 membranes have hydrophile surfaces. The surface roughness is known to impact the contact angle or wettability of a membrane surface. When the polymer type of the membrane surface is identical, a direct correlation between the surface roughness and contact angle of membrane surface are observed. It means that the membrane with lower contact angles (higher hydrophilicity) has smooth surface (lower roughness) and the membrane with higher contact angles (lower hydrophilicity) has rough surface. The surface properties of membranes such as high hydrophilicity and low roughness are dominant factors to reduce the fouling of the membrane. 3.2. Thin-film composite polyamide nanofiltration membrane The SEM micrographs of the cross-section, top surface and the top skin layer of the PES support membrane and composite polyamide nanofiltration membrane are presented in Fig. 5. The SEM image of the membrane surface (b-1) shows a rough film totally covers the surface of the PES support (a-1). The crosssection image of the membrane (b-2) proves the establishments of a film on the top of the PES support (a-2). These observations indicate that the PDA–TMC film is formed on the surface of PES ultrafiltration membrane. The surfaces of PES support and composite membranes were characterized using atomic force microscopy. Fig. 6 indicates the AFM images of the surfaces of PES support and polyamide membranes over a scan area of 10 mm 10 mm. The surface seems to be rougher with polymerization of PDA and TMC onto the PES membrane surface. The surface roughness parameters obtained from AFM images and hydrophilicity of PES support and polyamide membranes are shown in Table 3. The obtained results for surface roughness parameters confirm that a rough surface was introduced onto the PES membrane surface by polymerization of PDA and TMC. The contact angle of the polyamide composite membrane is lower than the PES support (41.58 versus 59.88), i.e. a more hydrophilic membrane is established. Although the PES support has smoother surface compared to composite polyamide membrane, the strong hydrophilic polar amide functional groups introduced on the PES membrane is responsible for the higher hydrophilicity of composite polyamide membrane. The results of the pure water flux, salt solution flux and rejection of NaCl and MgSO4 salts at two operating pressures of 5 and 10 bar are presented in Fig. 7. The pure water flux of PES support ultrafiltration membrane employed for preparing nanofiltration membrane was 450 kg m2 h1. The pronounced decrease in water permeability indicates that a dense film was formed on the top surface of the support. This provides higher retention for NaCl and MgSO4 solutions using composite polyamide membrane. The rejections to NaCl and MgSO4 were 64 and 85% (at 5 bar) and 67 and 90% (at 10 bar), respectively. The lower rejection for monovalent salt is a vital characteristic of nanofiltration membrane with numerous applications including pretreatment before reverse osmosis process. In general, the prepared thin-film composite polyamide membrane provides capability for water softening. The mechanical property of the produced thin-film on the top surface of PES support and the performance stability are vital
Fig. 6. Surface AFM images of (a) PES support and (b) thin-film polyamide composite membrane.
parameters. Accordingly, a long-term trial was carried out at 5 bar and 25 8C with pure water. The fluxes of the prepared composite nanofiltration membrane during 12 h of filtration are presented in Fig. 8. It can be seen that the composite membrane exhibits a good long-term stability. A comparison between performances of asymmetric PES nanofiltration membranes (Table 1) and thin-film composite polyamide nanofiltration membrane (Fig. 7) indicates that, the water permeability of asymmetric PES membranes is higher than the composite polyamide nanofiltration membrane. On the other hand, the composite membrane shows higher NaCl and MgSO4 rejections compared to asymmetric membranes. Table 3 Surface roughness and hydrophilicity of PES support and polyamide composite nanofiltration membrane. Membrane
PES membrane Composite membrane
Roughness
Contact angles (8)
Sa (nm)
Sq (nm)
Sz (nm)
10.1 45.0
12.9 57.9
110 393
59.8 41.5
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flux was decreased when the higher polymer concentration and mixture of water and IPA were used for membrane preparation. However, the rejections of NaCl and MgSO4 were increased. The results indicate that PES nanofiltration membrane with higher performance was prepared by adding the new additives in the casting solution. (ii) Thin-film composite polyamide nanofiltration membrane was prepared using interfacial polymerization of PDA with TMC. The SEM and AFM images show that the rough and dense film is formed on the PES support by interfacial polymerization. The composite membrane exhibited water permeability of 7 and 21 kg m2 h1 for salt solution containing NaCl (1 g/l) and MgSO4 (1 g/l) at 5 and 10 bar, respectively. The rejection to the divalent salt MgSO4 (85 and 90%) was high compared to the monovalent salt NaCl (64 and 67%). In summary, the thin-film composite polyamide membrane provided higher capability for water softening. Fig. 7. Performance of polyamide nanofiltration membrane at 5 and 10 bar.
References
Fig. 8. Long-term stability of the composite membrane (5 bar and 25 8C using pure water as the feed).
4. Conclusions Two types of nanofiltration membranes were prepared in the current study: (i) Asymmetric PES nanofiltration membranes using phase inversion induced by immersion precipitation technique. Different components such as PVP, PEG, acrylic acid and Triton X-100 were used as additives. The addition of these components in the PES casting solution leads to the formation of new asymmetric nanofiltration membrane. Two concentrations of PES (20 and 25 wt%) and two different non-solvents (pure water and mixture of water (80 vol.%) and IPA (20 vol.%)) were employed for preparing asymmetric nanofiltration membranes. The SEM and AFM studies demonstrated that the membranes prepared with non-solvent containing 20 vol.% IPA have smoother surface and smaller pores compared to membranes prepared with pure water as non-solvent. The
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