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The evaluation of thin film composite membrane composed of an electrospun polyacrylonitrile nanofibrous mid-layer for separating oil–water mixture
Desalination 359 (2015) 14–21
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The evaluation of thin film composite membrane composed of an electrospun polyacrylonitrile nanofibrous mid-layer for separating oil–water mixture Mehrdad Khamforoush ⁎, Omid Pirouzram, Tahmasb Hatami Department of Chemical Engineering, Faculty of Engineering, University of Kurdistan, Sanandaj 66177, Iran
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Increasing the CA leads to a gradual and then sharp climb in pure water flux. • Spongy structure of the PS membrane is strongly affected by its CA concentration. • TFC has higher flux and lower rejection than PS asymmetric membrane. • Mid-layer thickness has slight effect on the flux of TFC membrane.
a r t i c l e
i n f o
Article history: Received 13 September 2014 Received in revised form 12 December 2014 Accepted 14 December 2014 Available online xxxx Keywords: Thin film composite membrane Ultrafiltration PSF PAN Electrospinning
a b s t r a c t Conventional ultrafiltration (UF) membranes, which are fabricated by phase inversion method, have relatively low amount of flux rate. To overcome this limitation, a new type of thin film composite (TFC) membrane was manufactured and tested in the current study. This membrane has composed of a nonwoven Polyester (PET) support, an electrospun polyacrylonitrile (PAN) nanofibrous mid-layer, and a Polysulfone (PSF) composite coating top layer. In order to investigate the reliability of the TFC membrane, its performance for separating an oil/water emulsion was compared with an asymmetric UF membrane. The UF membrane was fabricated by phase inversion method from a PSF solution with Polyvinylpyrrolidone (PVP) as additive and 1 to 2% cellulose acetate (CA) as hydrophilic agent. The efficiencies of flux and rejection of both types of membranes were accurately determined and compared at various operating pressures. In general, the pure water fluxes of TFC membrane showed rises of 20 to 160% in comparison with PSF asymmetric membrane. The results also indicated that with the application of TFC membrane, the efficiency of rejection was averagely picked up by 2%. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (M. Khamforoush).
http://dx.doi.org/10.1016/j.desal.2014.12.016 0011-9164/© 2014 Elsevier B.V. All rights reserved.
Nowadays, due to the industrial development and successive droughts around the world, access to the freshwater supply is really difficult. Therefore, development of more effective methods for water treatment has attracted a lot of interests [1,2]. Among various water
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treatment methods, purification of water/oil emulsions, nanofiltration (NF) and ultrafiltration (UF) have gained considerable attention due to their lower energy demand, operational facility, and high separation efficiency [2]. Notably, UF membrane has a lot of applications in wastewater treatment and food industries [3,4]. From various kinds of UF membrane, polysulfone (PSF) ultrafiltration membranes have been widely used not only in ultrafiltration but also in the support layer of composite membranes. The applications of PSF UF are mainly due to its special characters like excellent heat-aging resistance, chemical stability, favorable mechanical property, and wide pH range value [5,6]. However, the main disadvantages of PSF UF membranes are their low permeability and serious fouling, which lead to limited applications and short longevity [7–9]. In order to overcome these limitations and improve the membrane performance, some additives like polyvinylpyrrolidone (PVP) [10–12], polyethylene glycol (PEG) [13,14], TiO2 nanoparticles [15,16], carbon nanotubes [17,18], and polyaniline (PANi) [19,20] have been used during the membrane preparation process [21]. Conventional polymeric UF membranes are generally manufactured with phase inversion method [22,23]. Although these kinds of UF membranes are effective in the removal of oily microemulsions, as mentioned, they are restricted with a couple of drawbacks namely low flux rate and surface fouling [24,25]. Fouling is an unavoidable outcome of pore blockage that is caused by solute adsorption on the membrane surface. Recently, TFC membrane has been shown to be very effective for the reduction of fouling in ultrafiltration [26]. In a relevant research, Yoon et al. [27] demonstrated a new kind of thin film nanofibrous composite (TFNC) membrane which was contained a cross linked PVA barrier layer and an electrospun PAN nanofibrous scaffold. Due to the high flux amount and low fouling traits, this type of membrane is very appropriate for ultrafiltration applications such as the separation of oil and water emulsion. In another research effort which was accomplished by Yoon et al. [28], surface polymerization of polyamides at various ratios of piperaz and bipiperidine was carried out on both PAN nanofibrous scaffold and PAN UF membrane supports [28]. It was reported that TFNC membranes with PAN nanofibrous scaffold had significantly higher degree of surface porosity and pore inter-connection than TFC membranes with PAN UF support. They also claimed that the increase of flux in TFNC membranes was possibly due to the low hydraulic resistance of nanofibrous support, large open pore structure, and the interface region between nanofiber and IFP matrix. Another interesting research in this field was performed by Nunes et al. [29] and Pinnau and Freeman [30,31]. They used commercially hydrophilic polyetherb-polyamide (Pebax) copolymers as a top layer to prevent fouling in several numbers of UF membranes. Akthakul et al. [32] synthesized a novel class of amphiphilic self-organizing comb copolymer, which could be used as porous membranes as well as anti-fouling coating. In addition, Arnold et al. [33] and DiGiano et al. [34] developed a new type of hydrophilic/fluorinated copolymer that was used as nonporous coating materials on porous membranes and could experienced microphase separation. According to what have been mentioned, it can be concluded that TFC membrane is more applicable than UF membrane. In brief, TFC membranes are composed of three separate layers including a support, a mid-layer, and a coating. The support layer is made up of non-woven microfibrous substrate with high mechanical strength, the mid-layer is fabricated from hydrophilic nanofibers, and the coating layer is fabricated from a thin nonporous hydrophilic layer. It must be mentioned that the flux rate in TFC membrane in comparison with conventional UF membrane can be raised several times just by controlling the thickness of coating layer and selecting more hydrophilic polymer. Furthermore, it is possible to hinder fouling phenomena by creation a 3-D porous structure within the mid-layer. For this purpose, the well-known technique of electrospinning can be applied to generate ultrafine fibers with diameters ranging from few nanometers to several micrometers [35,36].
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The main novelty of this study was fabricating a new type of polysulfone TFC membrane for the separation of water–oil emulsion. For this purpose, a commercial non-woven microfibrous layer (Novatex 2413N) was selected as the support. Meanwhile, PAN was electrospun into precursor form to fabricate carbon nanofibers for the manufacturing of mid-layer. In addition, a combination of polysulfone (PSF) and polyvinylpyrrolidone (PVP) solution was used for the generation of coating layer. To evaluate the effectiveness of the TFC membrane, a conventional UF membrane was also fabricated using the phase inversion method. The performances of these two membranes in the terms of flux rate and rejection percent were precisely evaluated for the separating of water/oil emulsion. 2. Experimental 2.1. Materials PAN was purchased from Polyacryle IRAN Company, but PSF, cellulose acetate, and PVP were purchased from Sigma-Aldrich. Average molecular weight of PAN, PSF, cellulose acetate, and PVP were approximately 100,000, 35,000, 50,000, and 40,000 g/mol, respectively. Additionally, N,N-dimethylformamide (DMF, Merck) was used as solvent. In order to construct the membrane support, nonwoven polyethylene terephthalate (PET) substrate (Novatexx 2413) was purchased from Freudenberg Company. 2.2. Fabrication of asymmetric membrane PSF and PVP flat membranes were prepared by phase inversion via immersion precipitation, which was described in detail as follows. First of all, PSF (18 wt.%) and PVP (4 wt.%) [37] were dissolved in DMF at 60 °C. Then, the solution was stirred for sufficiently long period of time, about 5 h, to become completely homogenous. After that, the homogeneous polymer solution was kept unchanged to remove the whole immerse bubbles. Subsequently, the solution was cast using a handcasting knife with 110 μm thickness on a glass plate substrate. Then, the membranes were kept to the atmosphere for about 60 s for the aim of complete solvent evaporation. After that, it was immersed in the non-solvent bath at the room temperature. Following that, the prepared membranes were washed and stored in water for at least 1 day to leach out the residual solvents and additives. Finally, the membranes were placed between two sheets of filter paper for 24 h at room temperature to dry them completely. For the improvement of membrane performance, 1 to 3% of CA was added to the primary solution. The UF asymmetric membranes, made up of CA with different concentrations, are listed in Table 1. 2.3. Fabrication of three-layer composite membrane To fabricate a three-layer composite membrane, first of all the nonwoven PET substrate was coated with a layer of DMF solution to enhance its adhesion with electrospun PAN nanofibers. In detail, PAN solutions with the concentration of 12 wt.% and 8 wt.% were electrospun directly onto the surface-coated PET non-woven substrate at 15 kV. The PAN solution flow rate was 10–20 ml/h. The distance between the collector (PET substrate) and the spinneret was 10–18 cm. In the electrospinning setup, a rotating drum with an approximate
Table 1 Recipes to fabricate different asymmetric membranes by phase inversion method. Membrane
% PS
% PVP
% CA
UF1 UF2 UF3 UF4
18 18 18 18
4 4 4 4
0 1 2 3
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diameter of 6 cm and rotating speed of 300 rpm was employed to collect the deposited nanofibers. It was apparently visible that nanofibers were suitably prepared a strong mid-layer support that increased the mechanical resistance of membrane at high operating pressures. The coating top layer was applied onto the fibrous composite support. This support contained an electrospun PAN scaffold as well as a non-woven polyester (PET) substrate. To enhance the flux rate of fabricated membrane, small amount of CA with the concentration of 1 wt.% and 2 wt.%, as a hydrophilic agent, was mixed with the casting polymer solution. The composite membrane was immediately immersed in the water bath at room temperature for 24 h. 2.4. Characterization In this paper, the feed solution was prepared by mixing vegetable oil (400 ppm) in distilled water. In order to test the filtration performance of membranes, a cross-flow filtration cell (active filtration area: 0.0024 m2) was used. The whole experiments were run at the ambient temperature of 25–30 °C. The morphologies of both types of asymmetric and TFC membranes were accurately examined by scanning electron microscopy. In addition, both flux rate and rejection of TFC membrane were evaluated and the acquired results were compared with those obtained for asymmetric membrane. For reliability, the measurements of flux rate were repeated three times for both types of membranes. The rejection percent of the membranes was calculated using the following formula: C f −C p 100 ð1Þ Rejectionð%Þ ¼ Cf where Cf and Cp stand for the oil concentration in the feed solution and permeate, respectively. The oil concentration of both initial feed solution and permeate was determined by ultraviolet–visible (UV) spectroscopy (Varian Cary 50) at a wavelength of 220 nm. 3. Results and discussion 3.1. Evaluation of asymmetric membrane performance According to Gomez and Lin [37] research, polysulfone (18 wt.%) solution in DMF with PVP (4 wt.%) resulted in the best membrane performance among other concentrations. Therefore, this concentration of solution was applied in the current study to fabricate the asymmetric membrane. The cross-flow results of an asymmetric membrane, which was fabricated by this solution, are represented in Fig. 1. As can be seen from this figure, water–oil and, in particular, pure water flux
Fig. 2. Pure water flux of asymmetric membranes with the concentration of 0–3% CA.
increased gradually with increasing pressure. Obviously, raising the operating pressure leaded to the higher pressure drop so as to higher flux could pass through the membrane. For this reason, there was a gradual decline in the rejection at higher pressures. It is also observed that the flux of oil–water emulsion was much lower than pure water flux. This was mainly due to the presence of large oil particles in the feed mixture which were accumulated on the surface of membrane and blocked its pores. As mentioned before, for improving the performance of membrane, CA with the concentration of 1–3 wt.% was added to the primary solution. The obtained results for pure water flux are shown in Fig. 2. According to this figure, pure water flux rose steadily with increasing CA concentration and then climbed sharply. The pure water flux of membrane with 1 to 2 wt.% CA was approximately 30–75% higher than the CA-free membrane. Interestingly, the change of pure water flux for membrane with 3 wt.% CA was 40% at 4 bar and 240% at 1 bar. The sharp increase in the water flux of membrane with 3 wt.% CA may be explained as follows. With higher amount of CA, the pore size of support layer in asymmetric membrane increases dramatically (according to Fig. 4). In addition, it seems that increasing the CA concentration up to 2% not only decrease the pore size in the active layer, but also gradually reduce the mechanical resistance of the membrane. In particular, the active layer, at 3% CA concentration, is approximately irresistible against pressure and this consequently leads to produce big pore size in this layer and remarkable change in flux rate. In Fig. 3, the oil–water flux and rejection of asymmetric membranes are shown with various values of CA concentration. Referring to this figure, oil–water flux, up to the pressure of 2 bars, has approximately the same trend for all asymmetric membranes. The fluxes of all membranes
Fig. 1. Cross-flow results of asymmetric membrane fabricated by the solution of PSF (18%) and PVP (4%).
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Fig. 3. Oil–water flux and rejection results of asymmetric membranes with different values of CA.
increased linearly except for UF1 that its variation was rather erratic. This trend can be justified by the presence of hydrophilic agent in UF2 to UF4. Moreover, as shown in Fig. 3, the rejection of asymmetric membranes fell slowly with increasing the operating pressure. However,
there was substantially lower reduction for membranes with 1 and 2 wt.% CA than the others. This figure also indicates that by adding 1 and 2 wt.% CA to the membrane, the separation of oil–water emulsion increased from 1 to 40% depending on the applied pressure. To put it in another way, the effect of CA concentration is also clearly revealed
Fig. 4. SEM images for the cross section of asymmetric membranes with a) 0%, b) 1%, and c) 2% CA concentration.
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Fig. 5. A comparison between the performances of TFC membrane made by polymeric solution of PSF (18%) and PVP (4%) as top layer with that of UF1 asymmetric membrane.
according to the SEM images in Fig. 4. The fluxes and rejections trends were justified in the last paragraph prior to Fig. 2.
between the rejections of these two membranes is on account of their active layers.
To evaluate the performance of TFC membranes, non-woven substrate was used as the supporter. The principal reason for developing electrospinning-based membrane is the considerable increase in output flux at approximately unchanged value of rejection. According to the literature studies [27,28], PAN solutions with the concentration of 12 wt.% in DMF has been efficiently used for nanofiber electrospinning. Thus, in the current study, a PAN solution with this common concentration was directly electrospun onto the surface-coated non-woven substrate, and a thin layer of PSF solution on nanofibers was coated. The performances of TFC membrane, which was made up of polymeric solution of 18% PSF and 4% PVP as the top layer, as well as UF1 asymmetric membrane are depicted in Fig. 5. According to this figure, there were rises of 20–160% in the pure water fluxes of TFC in comparison with PSF asymmetric membrane. These improvements were chiefly due to the reduction of top layer thickness of TFC membrane as well as the reinforcement of membrane structure using the support layers. This figure also signifies that the oil–water flux of TFC membrane was considerably higher than that of PSF asymmetric membrane. About the trend of rejection, its curve for UF1 decreased gradually with pressure, but the curve of TFC membrane fell and then leveled off. Unquestionably, the obvious difference
3.2.1. Effect of CA on performance of TFC membrane In order to improve the performance of TFC membranes, the earliest traces of CA were added to the primary polymer solution (18 wt.% PSF and 4 wt.% PVP) to fabricate the active layer of TFC membrane. Due to the adverse results of asymmetric membranes in the presence of 3 wt.% CA, this particular percent was ignored from the test run. Thus, CA concentrations with the values of 1 wt.% and 2 wt.% were implemented in this section. In Fig. 6, the oil–water flux of both TFC and asymmetric membranes is represented. As observed, the three-tier composite membranes, PSF/PAN/PET, exhibits clearly higher flux than asymmetric PSF UF-membrane. These improvements were due to the considerable reduction of top layer thickness of TFC membrane together with the reinforcement of membrane structure using support layers. This is also argued by this figure that oil–water flux rate of the whole TFC membranes experienced a very similar trend. As previously mentioned, a very good reason for this similar trend was the presence of oil particles in the oil water mixture that prevent the passage of water through the active layer. The filtration efficiencies of both membranes are compared in Fig. 7 at various operating pressures. As illustrated, the filtration efficiencies were conspicuously dropped with increasing pressure. Moreover, the presence of CA increased the rejection efficiency of TFC and asymmetric membrane by 5–30%. Although the thickness of TFC top layer was less than the thickness of asymmetric membrane, the rejection efficiency of TFC membrane was noticeably improved.
Fig. 6. The oil–water flux of TFC membranes compared to asymmetric membranes with 0–2% CA.
Fig. 7. The rejection of TFC membranes compared to asymmetric membranes with 0–2% CA.
3.2. Three-tier composite membrane for ultrafiltration
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Fig. 8. SEM image for reducing the top layer of TFC membrane. Fig. 10. SEM image for cross section of TFC membrane with 5 h electrospinning in mid-layer.
SEM image for reduction the top layer of TFC membranes is shown in Fig. 8 that provides a couple of conclusions. The first one is the significant increase of the flux of TFC in comparison with asymmetric membrane. The second one is the positive effect of CA for the separation of oil–water mixture. 3.2.2. Effect of mid-layer thickness on the performance of TFC membrane Hitherto, the duration of electrospinning of PAN nanofibers to fabricate TFC membranes was only 3 h. In order to evaluate the effect of mid-layer thickness, the eletrospinning time was increased to 5 h. To achieve a better performance, 1% CA casting solution was used for forming the active layer. Fig. 9 illustrates the effect of electrospinning time on the membrane flux and rejection over the operating pressure of 1 to 4 bars. As expected, the oil–water flux of TFC membrane with 5 h electrospinning was 2–5% higher than the others. From theoretical point of view, with more electrospinning time, the mid-layer thickness raises correspondingly. As a hand-casting knife with fixed thickness of 110 μm was used for fabricating the active layer, the summation of active and mid layer was always constant. As a consequence, active layer on the nanofibers becomes thinner, which leads to higher flux rate. On the other side, the rejection of TFC membrane declined dramatically with raising the electrospinning time. In fact, as mentioned before, the reason of rejection drop is that with rising the electrospinning time, the active layer thickness reduces, and this consequently leads to passing too many oil particles through the membrane. The SEM image of
Fig. 10 gives information about the cross section area of TFC membrane with 5 h electrospinning in mid-layer. 3.2.3. Effect of nanofiber diameter on TFC membrane performance In this section, nanofibers with various diameters were regularly applied in the mid-layer to investigate the effect of nanofiber diameter on TFC membrane performance. For fabricating this membrane, first of all, PAN solution with the concentration of 12 wt.% was directly electrospun onto the PET non-woven substrate. Then, the solution of 8 wt.% PAN was electrospun onto this layer of nanofibers. The SEM images of electrospun nanofibers with 12 and 8 wt.% of polyacrylonitrile (PAN) solution together with their corresponding diameter distributions are shown in Fig. 11. As can be seen, mean, standard deviation, and the range of fiber diameters were determined in these SEM pictures. Based on this figure, the average fiber diameter for the case of 12 wt.% was more than that of 8 wt.%. Lastly, a thin layer of casting solution (PSF 18%, PVP 4% and CA 1%) was formed onto this mid-layer. It should be noted that operating conditions for fabricating this membrane were the same as TFC membrane. In Fig. 12, the performances of TFC membrane made up of 12 and 8% PAN nanofibers are compared with that of 12% PAN nanofibers. One of the most striking features of this figure is that TFC membrane made up of 12 and 8% PAN nanofibers had as much oil–water fluxes as TFC membrane made up of 12% PAN nanofibers.
Fig. 9. Performance evaluation of TFC membrane made by 5 h and 3 h electrospinning of nanofibers in mid-layer.
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Fig. 11. SEM image of electrospun nanofibers with a) 12% and b) 8 wt.% of PAN solution.
This figure also records higher rejection for TFC membrane made up of 12 and 8% PAN nanofibers. 3.2.4. Evaluation of fouling problem of TFC and asymmetric membranes The obtained results in previous sections have proved the superiority of TFC over asymmetric membranes in term of flux and rejection performance. There is no doubt against the fact that fabricating a good membrane depends on many factors rather than just flux and rejection. One of these prominent key factors is membrane fouling during the separation process. To evaluate this factor, the TFC and asymmetric membranes were examined at 2 bars. Fig. 13 shows the trend of flux and rejection of three aforementioned membranes during 8 h. It is apparent from this figure that output fluxes of the membranes dropped slowly, but their rejection picked up moderately. The reduction of the flux was mainly due to the fact that the vast majority of large oil particles caused pore blockage as well as concentration polarization on the membrane surface. On the other side, many of oil particles were not able to pass through the blocked pores which consequently led to the increment of rejection with time. Another important conclusion of this figure is that the average percent of flux reduction was approximately 60% for asymmetric membranes and 50% for TFC membrane. These results were primarily due to the effect of nanofibers in the reduction of membrane fouling. Although it is true that fouling typically occurred in the active
layer, the effect of sublayer on the separation performance should not be ignored. In fact, due to the three-dimensional porosity of nanofiber substrate in TFC membrane, pore blockage in TFC is lower than UF membrane. 4. Conclusions In this paper, the fabrication of a new kind of TFC membrane was investigated. In addition, the performance of this membrane was evaluated by measuring the flux rate and rejection efficiency for the separation of an oil/water emulsion at various operating conditions. The main conclusions are presented as follows: 1) By increasing the CA concentration, a gradual and then sharp climb in pure water flux was noticeable. The maximum change of pure water flux in the presence of 1–2% CA and 3% CA was 75% and 240%, respectively. 2) The spongy structure of the PSF membrane was strongly affected by the CA concentration. It was observed that adding 1–2% CA led to a substantial increase in the rejection. 3) By comparison TFC with PSF asymmetric membrane, a considerable increase in pure water flux, between 20 and 160%, and a downward trend in rejection were noticeable.
Fig. 12. The performance evaluation of TFC membrane made by 8% and 12% PAN nanofibers.
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Fig. 13. The variations of flux and rejection for UF1, UF2, and TFC membranes during 8 h function.
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Desalination journal homepage: www.elsevier.com/locate/desal
The evaluation of thin film composite membrane composed of an electrospun polyacrylonitrile nanofibrous mid-layer for separating oil–water mixture Mehrdad Khamforoush ⁎, Omid Pirouzram, Tahmasb Hatami Department of Chemical Engineering, Faculty of Engineering, University of Kurdistan, Sanandaj 66177, Iran
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Increasing the CA leads to a gradual and then sharp climb in pure water flux. • Spongy structure of the PS membrane is strongly affected by its CA concentration. • TFC has higher flux and lower rejection than PS asymmetric membrane. • Mid-layer thickness has slight effect on the flux of TFC membrane.
a r t i c l e
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Article history: Received 13 September 2014 Received in revised form 12 December 2014 Accepted 14 December 2014 Available online xxxx Keywords: Thin film composite membrane Ultrafiltration PSF PAN Electrospinning
a b s t r a c t Conventional ultrafiltration (UF) membranes, which are fabricated by phase inversion method, have relatively low amount of flux rate. To overcome this limitation, a new type of thin film composite (TFC) membrane was manufactured and tested in the current study. This membrane has composed of a nonwoven Polyester (PET) support, an electrospun polyacrylonitrile (PAN) nanofibrous mid-layer, and a Polysulfone (PSF) composite coating top layer. In order to investigate the reliability of the TFC membrane, its performance for separating an oil/water emulsion was compared with an asymmetric UF membrane. The UF membrane was fabricated by phase inversion method from a PSF solution with Polyvinylpyrrolidone (PVP) as additive and 1 to 2% cellulose acetate (CA) as hydrophilic agent. The efficiencies of flux and rejection of both types of membranes were accurately determined and compared at various operating pressures. In general, the pure water fluxes of TFC membrane showed rises of 20 to 160% in comparison with PSF asymmetric membrane. The results also indicated that with the application of TFC membrane, the efficiency of rejection was averagely picked up by 2%. © 2014 Elsevier B.V. All rights reserved.
1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (M. Khamforoush).
http://dx.doi.org/10.1016/j.desal.2014.12.016 0011-9164/© 2014 Elsevier B.V. All rights reserved.
Nowadays, due to the industrial development and successive droughts around the world, access to the freshwater supply is really difficult. Therefore, development of more effective methods for water treatment has attracted a lot of interests [1,2]. Among various water
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treatment methods, purification of water/oil emulsions, nanofiltration (NF) and ultrafiltration (UF) have gained considerable attention due to their lower energy demand, operational facility, and high separation efficiency [2]. Notably, UF membrane has a lot of applications in wastewater treatment and food industries [3,4]. From various kinds of UF membrane, polysulfone (PSF) ultrafiltration membranes have been widely used not only in ultrafiltration but also in the support layer of composite membranes. The applications of PSF UF are mainly due to its special characters like excellent heat-aging resistance, chemical stability, favorable mechanical property, and wide pH range value [5,6]. However, the main disadvantages of PSF UF membranes are their low permeability and serious fouling, which lead to limited applications and short longevity [7–9]. In order to overcome these limitations and improve the membrane performance, some additives like polyvinylpyrrolidone (PVP) [10–12], polyethylene glycol (PEG) [13,14], TiO2 nanoparticles [15,16], carbon nanotubes [17,18], and polyaniline (PANi) [19,20] have been used during the membrane preparation process [21]. Conventional polymeric UF membranes are generally manufactured with phase inversion method [22,23]. Although these kinds of UF membranes are effective in the removal of oily microemulsions, as mentioned, they are restricted with a couple of drawbacks namely low flux rate and surface fouling [24,25]. Fouling is an unavoidable outcome of pore blockage that is caused by solute adsorption on the membrane surface. Recently, TFC membrane has been shown to be very effective for the reduction of fouling in ultrafiltration [26]. In a relevant research, Yoon et al. [27] demonstrated a new kind of thin film nanofibrous composite (TFNC) membrane which was contained a cross linked PVA barrier layer and an electrospun PAN nanofibrous scaffold. Due to the high flux amount and low fouling traits, this type of membrane is very appropriate for ultrafiltration applications such as the separation of oil and water emulsion. In another research effort which was accomplished by Yoon et al. [28], surface polymerization of polyamides at various ratios of piperaz and bipiperidine was carried out on both PAN nanofibrous scaffold and PAN UF membrane supports [28]. It was reported that TFNC membranes with PAN nanofibrous scaffold had significantly higher degree of surface porosity and pore inter-connection than TFC membranes with PAN UF support. They also claimed that the increase of flux in TFNC membranes was possibly due to the low hydraulic resistance of nanofibrous support, large open pore structure, and the interface region between nanofiber and IFP matrix. Another interesting research in this field was performed by Nunes et al. [29] and Pinnau and Freeman [30,31]. They used commercially hydrophilic polyetherb-polyamide (Pebax) copolymers as a top layer to prevent fouling in several numbers of UF membranes. Akthakul et al. [32] synthesized a novel class of amphiphilic self-organizing comb copolymer, which could be used as porous membranes as well as anti-fouling coating. In addition, Arnold et al. [33] and DiGiano et al. [34] developed a new type of hydrophilic/fluorinated copolymer that was used as nonporous coating materials on porous membranes and could experienced microphase separation. According to what have been mentioned, it can be concluded that TFC membrane is more applicable than UF membrane. In brief, TFC membranes are composed of three separate layers including a support, a mid-layer, and a coating. The support layer is made up of non-woven microfibrous substrate with high mechanical strength, the mid-layer is fabricated from hydrophilic nanofibers, and the coating layer is fabricated from a thin nonporous hydrophilic layer. It must be mentioned that the flux rate in TFC membrane in comparison with conventional UF membrane can be raised several times just by controlling the thickness of coating layer and selecting more hydrophilic polymer. Furthermore, it is possible to hinder fouling phenomena by creation a 3-D porous structure within the mid-layer. For this purpose, the well-known technique of electrospinning can be applied to generate ultrafine fibers with diameters ranging from few nanometers to several micrometers [35,36].
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The main novelty of this study was fabricating a new type of polysulfone TFC membrane for the separation of water–oil emulsion. For this purpose, a commercial non-woven microfibrous layer (Novatex 2413N) was selected as the support. Meanwhile, PAN was electrospun into precursor form to fabricate carbon nanofibers for the manufacturing of mid-layer. In addition, a combination of polysulfone (PSF) and polyvinylpyrrolidone (PVP) solution was used for the generation of coating layer. To evaluate the effectiveness of the TFC membrane, a conventional UF membrane was also fabricated using the phase inversion method. The performances of these two membranes in the terms of flux rate and rejection percent were precisely evaluated for the separating of water/oil emulsion. 2. Experimental 2.1. Materials PAN was purchased from Polyacryle IRAN Company, but PSF, cellulose acetate, and PVP were purchased from Sigma-Aldrich. Average molecular weight of PAN, PSF, cellulose acetate, and PVP were approximately 100,000, 35,000, 50,000, and 40,000 g/mol, respectively. Additionally, N,N-dimethylformamide (DMF, Merck) was used as solvent. In order to construct the membrane support, nonwoven polyethylene terephthalate (PET) substrate (Novatexx 2413) was purchased from Freudenberg Company. 2.2. Fabrication of asymmetric membrane PSF and PVP flat membranes were prepared by phase inversion via immersion precipitation, which was described in detail as follows. First of all, PSF (18 wt.%) and PVP (4 wt.%) [37] were dissolved in DMF at 60 °C. Then, the solution was stirred for sufficiently long period of time, about 5 h, to become completely homogenous. After that, the homogeneous polymer solution was kept unchanged to remove the whole immerse bubbles. Subsequently, the solution was cast using a handcasting knife with 110 μm thickness on a glass plate substrate. Then, the membranes were kept to the atmosphere for about 60 s for the aim of complete solvent evaporation. After that, it was immersed in the non-solvent bath at the room temperature. Following that, the prepared membranes were washed and stored in water for at least 1 day to leach out the residual solvents and additives. Finally, the membranes were placed between two sheets of filter paper for 24 h at room temperature to dry them completely. For the improvement of membrane performance, 1 to 3% of CA was added to the primary solution. The UF asymmetric membranes, made up of CA with different concentrations, are listed in Table 1. 2.3. Fabrication of three-layer composite membrane To fabricate a three-layer composite membrane, first of all the nonwoven PET substrate was coated with a layer of DMF solution to enhance its adhesion with electrospun PAN nanofibers. In detail, PAN solutions with the concentration of 12 wt.% and 8 wt.% were electrospun directly onto the surface-coated PET non-woven substrate at 15 kV. The PAN solution flow rate was 10–20 ml/h. The distance between the collector (PET substrate) and the spinneret was 10–18 cm. In the electrospinning setup, a rotating drum with an approximate
Table 1 Recipes to fabricate different asymmetric membranes by phase inversion method. Membrane
% PS
% PVP
% CA
UF1 UF2 UF3 UF4
18 18 18 18
4 4 4 4
0 1 2 3
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diameter of 6 cm and rotating speed of 300 rpm was employed to collect the deposited nanofibers. It was apparently visible that nanofibers were suitably prepared a strong mid-layer support that increased the mechanical resistance of membrane at high operating pressures. The coating top layer was applied onto the fibrous composite support. This support contained an electrospun PAN scaffold as well as a non-woven polyester (PET) substrate. To enhance the flux rate of fabricated membrane, small amount of CA with the concentration of 1 wt.% and 2 wt.%, as a hydrophilic agent, was mixed with the casting polymer solution. The composite membrane was immediately immersed in the water bath at room temperature for 24 h. 2.4. Characterization In this paper, the feed solution was prepared by mixing vegetable oil (400 ppm) in distilled water. In order to test the filtration performance of membranes, a cross-flow filtration cell (active filtration area: 0.0024 m2) was used. The whole experiments were run at the ambient temperature of 25–30 °C. The morphologies of both types of asymmetric and TFC membranes were accurately examined by scanning electron microscopy. In addition, both flux rate and rejection of TFC membrane were evaluated and the acquired results were compared with those obtained for asymmetric membrane. For reliability, the measurements of flux rate were repeated three times for both types of membranes. The rejection percent of the membranes was calculated using the following formula: C f −C p 100 ð1Þ Rejectionð%Þ ¼ Cf where Cf and Cp stand for the oil concentration in the feed solution and permeate, respectively. The oil concentration of both initial feed solution and permeate was determined by ultraviolet–visible (UV) spectroscopy (Varian Cary 50) at a wavelength of 220 nm. 3. Results and discussion 3.1. Evaluation of asymmetric membrane performance According to Gomez and Lin [37] research, polysulfone (18 wt.%) solution in DMF with PVP (4 wt.%) resulted in the best membrane performance among other concentrations. Therefore, this concentration of solution was applied in the current study to fabricate the asymmetric membrane. The cross-flow results of an asymmetric membrane, which was fabricated by this solution, are represented in Fig. 1. As can be seen from this figure, water–oil and, in particular, pure water flux
Fig. 2. Pure water flux of asymmetric membranes with the concentration of 0–3% CA.
increased gradually with increasing pressure. Obviously, raising the operating pressure leaded to the higher pressure drop so as to higher flux could pass through the membrane. For this reason, there was a gradual decline in the rejection at higher pressures. It is also observed that the flux of oil–water emulsion was much lower than pure water flux. This was mainly due to the presence of large oil particles in the feed mixture which were accumulated on the surface of membrane and blocked its pores. As mentioned before, for improving the performance of membrane, CA with the concentration of 1–3 wt.% was added to the primary solution. The obtained results for pure water flux are shown in Fig. 2. According to this figure, pure water flux rose steadily with increasing CA concentration and then climbed sharply. The pure water flux of membrane with 1 to 2 wt.% CA was approximately 30–75% higher than the CA-free membrane. Interestingly, the change of pure water flux for membrane with 3 wt.% CA was 40% at 4 bar and 240% at 1 bar. The sharp increase in the water flux of membrane with 3 wt.% CA may be explained as follows. With higher amount of CA, the pore size of support layer in asymmetric membrane increases dramatically (according to Fig. 4). In addition, it seems that increasing the CA concentration up to 2% not only decrease the pore size in the active layer, but also gradually reduce the mechanical resistance of the membrane. In particular, the active layer, at 3% CA concentration, is approximately irresistible against pressure and this consequently leads to produce big pore size in this layer and remarkable change in flux rate. In Fig. 3, the oil–water flux and rejection of asymmetric membranes are shown with various values of CA concentration. Referring to this figure, oil–water flux, up to the pressure of 2 bars, has approximately the same trend for all asymmetric membranes. The fluxes of all membranes
Fig. 1. Cross-flow results of asymmetric membrane fabricated by the solution of PSF (18%) and PVP (4%).
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Fig. 3. Oil–water flux and rejection results of asymmetric membranes with different values of CA.
increased linearly except for UF1 that its variation was rather erratic. This trend can be justified by the presence of hydrophilic agent in UF2 to UF4. Moreover, as shown in Fig. 3, the rejection of asymmetric membranes fell slowly with increasing the operating pressure. However,
there was substantially lower reduction for membranes with 1 and 2 wt.% CA than the others. This figure also indicates that by adding 1 and 2 wt.% CA to the membrane, the separation of oil–water emulsion increased from 1 to 40% depending on the applied pressure. To put it in another way, the effect of CA concentration is also clearly revealed
Fig. 4. SEM images for the cross section of asymmetric membranes with a) 0%, b) 1%, and c) 2% CA concentration.
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Fig. 5. A comparison between the performances of TFC membrane made by polymeric solution of PSF (18%) and PVP (4%) as top layer with that of UF1 asymmetric membrane.
according to the SEM images in Fig. 4. The fluxes and rejections trends were justified in the last paragraph prior to Fig. 2.
between the rejections of these two membranes is on account of their active layers.
To evaluate the performance of TFC membranes, non-woven substrate was used as the supporter. The principal reason for developing electrospinning-based membrane is the considerable increase in output flux at approximately unchanged value of rejection. According to the literature studies [27,28], PAN solutions with the concentration of 12 wt.% in DMF has been efficiently used for nanofiber electrospinning. Thus, in the current study, a PAN solution with this common concentration was directly electrospun onto the surface-coated non-woven substrate, and a thin layer of PSF solution on nanofibers was coated. The performances of TFC membrane, which was made up of polymeric solution of 18% PSF and 4% PVP as the top layer, as well as UF1 asymmetric membrane are depicted in Fig. 5. According to this figure, there were rises of 20–160% in the pure water fluxes of TFC in comparison with PSF asymmetric membrane. These improvements were chiefly due to the reduction of top layer thickness of TFC membrane as well as the reinforcement of membrane structure using the support layers. This figure also signifies that the oil–water flux of TFC membrane was considerably higher than that of PSF asymmetric membrane. About the trend of rejection, its curve for UF1 decreased gradually with pressure, but the curve of TFC membrane fell and then leveled off. Unquestionably, the obvious difference
3.2.1. Effect of CA on performance of TFC membrane In order to improve the performance of TFC membranes, the earliest traces of CA were added to the primary polymer solution (18 wt.% PSF and 4 wt.% PVP) to fabricate the active layer of TFC membrane. Due to the adverse results of asymmetric membranes in the presence of 3 wt.% CA, this particular percent was ignored from the test run. Thus, CA concentrations with the values of 1 wt.% and 2 wt.% were implemented in this section. In Fig. 6, the oil–water flux of both TFC and asymmetric membranes is represented. As observed, the three-tier composite membranes, PSF/PAN/PET, exhibits clearly higher flux than asymmetric PSF UF-membrane. These improvements were due to the considerable reduction of top layer thickness of TFC membrane together with the reinforcement of membrane structure using support layers. This is also argued by this figure that oil–water flux rate of the whole TFC membranes experienced a very similar trend. As previously mentioned, a very good reason for this similar trend was the presence of oil particles in the oil water mixture that prevent the passage of water through the active layer. The filtration efficiencies of both membranes are compared in Fig. 7 at various operating pressures. As illustrated, the filtration efficiencies were conspicuously dropped with increasing pressure. Moreover, the presence of CA increased the rejection efficiency of TFC and asymmetric membrane by 5–30%. Although the thickness of TFC top layer was less than the thickness of asymmetric membrane, the rejection efficiency of TFC membrane was noticeably improved.
Fig. 6. The oil–water flux of TFC membranes compared to asymmetric membranes with 0–2% CA.
Fig. 7. The rejection of TFC membranes compared to asymmetric membranes with 0–2% CA.
3.2. Three-tier composite membrane for ultrafiltration
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Fig. 8. SEM image for reducing the top layer of TFC membrane. Fig. 10. SEM image for cross section of TFC membrane with 5 h electrospinning in mid-layer.
SEM image for reduction the top layer of TFC membranes is shown in Fig. 8 that provides a couple of conclusions. The first one is the significant increase of the flux of TFC in comparison with asymmetric membrane. The second one is the positive effect of CA for the separation of oil–water mixture. 3.2.2. Effect of mid-layer thickness on the performance of TFC membrane Hitherto, the duration of electrospinning of PAN nanofibers to fabricate TFC membranes was only 3 h. In order to evaluate the effect of mid-layer thickness, the eletrospinning time was increased to 5 h. To achieve a better performance, 1% CA casting solution was used for forming the active layer. Fig. 9 illustrates the effect of electrospinning time on the membrane flux and rejection over the operating pressure of 1 to 4 bars. As expected, the oil–water flux of TFC membrane with 5 h electrospinning was 2–5% higher than the others. From theoretical point of view, with more electrospinning time, the mid-layer thickness raises correspondingly. As a hand-casting knife with fixed thickness of 110 μm was used for fabricating the active layer, the summation of active and mid layer was always constant. As a consequence, active layer on the nanofibers becomes thinner, which leads to higher flux rate. On the other side, the rejection of TFC membrane declined dramatically with raising the electrospinning time. In fact, as mentioned before, the reason of rejection drop is that with rising the electrospinning time, the active layer thickness reduces, and this consequently leads to passing too many oil particles through the membrane. The SEM image of
Fig. 10 gives information about the cross section area of TFC membrane with 5 h electrospinning in mid-layer. 3.2.3. Effect of nanofiber diameter on TFC membrane performance In this section, nanofibers with various diameters were regularly applied in the mid-layer to investigate the effect of nanofiber diameter on TFC membrane performance. For fabricating this membrane, first of all, PAN solution with the concentration of 12 wt.% was directly electrospun onto the PET non-woven substrate. Then, the solution of 8 wt.% PAN was electrospun onto this layer of nanofibers. The SEM images of electrospun nanofibers with 12 and 8 wt.% of polyacrylonitrile (PAN) solution together with their corresponding diameter distributions are shown in Fig. 11. As can be seen, mean, standard deviation, and the range of fiber diameters were determined in these SEM pictures. Based on this figure, the average fiber diameter for the case of 12 wt.% was more than that of 8 wt.%. Lastly, a thin layer of casting solution (PSF 18%, PVP 4% and CA 1%) was formed onto this mid-layer. It should be noted that operating conditions for fabricating this membrane were the same as TFC membrane. In Fig. 12, the performances of TFC membrane made up of 12 and 8% PAN nanofibers are compared with that of 12% PAN nanofibers. One of the most striking features of this figure is that TFC membrane made up of 12 and 8% PAN nanofibers had as much oil–water fluxes as TFC membrane made up of 12% PAN nanofibers.
Fig. 9. Performance evaluation of TFC membrane made by 5 h and 3 h electrospinning of nanofibers in mid-layer.
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Fig. 11. SEM image of electrospun nanofibers with a) 12% and b) 8 wt.% of PAN solution.
This figure also records higher rejection for TFC membrane made up of 12 and 8% PAN nanofibers. 3.2.4. Evaluation of fouling problem of TFC and asymmetric membranes The obtained results in previous sections have proved the superiority of TFC over asymmetric membranes in term of flux and rejection performance. There is no doubt against the fact that fabricating a good membrane depends on many factors rather than just flux and rejection. One of these prominent key factors is membrane fouling during the separation process. To evaluate this factor, the TFC and asymmetric membranes were examined at 2 bars. Fig. 13 shows the trend of flux and rejection of three aforementioned membranes during 8 h. It is apparent from this figure that output fluxes of the membranes dropped slowly, but their rejection picked up moderately. The reduction of the flux was mainly due to the fact that the vast majority of large oil particles caused pore blockage as well as concentration polarization on the membrane surface. On the other side, many of oil particles were not able to pass through the blocked pores which consequently led to the increment of rejection with time. Another important conclusion of this figure is that the average percent of flux reduction was approximately 60% for asymmetric membranes and 50% for TFC membrane. These results were primarily due to the effect of nanofibers in the reduction of membrane fouling. Although it is true that fouling typically occurred in the active
layer, the effect of sublayer on the separation performance should not be ignored. In fact, due to the three-dimensional porosity of nanofiber substrate in TFC membrane, pore blockage in TFC is lower than UF membrane. 4. Conclusions In this paper, the fabrication of a new kind of TFC membrane was investigated. In addition, the performance of this membrane was evaluated by measuring the flux rate and rejection efficiency for the separation of an oil/water emulsion at various operating conditions. The main conclusions are presented as follows: 1) By increasing the CA concentration, a gradual and then sharp climb in pure water flux was noticeable. The maximum change of pure water flux in the presence of 1–2% CA and 3% CA was 75% and 240%, respectively. 2) The spongy structure of the PSF membrane was strongly affected by the CA concentration. It was observed that adding 1–2% CA led to a substantial increase in the rejection. 3) By comparison TFC with PSF asymmetric membrane, a considerable increase in pure water flux, between 20 and 160%, and a downward trend in rejection were noticeable.
Fig. 12. The performance evaluation of TFC membrane made by 8% and 12% PAN nanofibers.
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Fig. 13. The variations of flux and rejection for UF1, UF2, and TFC membranes during 8 h function.
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