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Enhancing the permeability of TFC membranes based on incorporating polyamide matrix into MWCNTs framework
Applied Surface Science 496 (2019) 143680
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Enhancing the permeability of TFC membranes based on incorporating polyamide matrix into MWCNTs framework Hao Suna,b, Dan Lia,b, Bing Liua,b, Jie Yaoa,b,c,
T
⁎
a
School of Environment, Harbin Institute of Technology, Harbin 150090, PR China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China c National Engineering Center of Urban Water Resources, 202 Hehai Road, Harbin 150090, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Reverse osmosis Carbon nanotubes framework High water flux Antifouling
Reverse osmosis (RO) technology can get rid of small molecular contaminants and salts with a very high efficiency, so that it has exhibited promising potential for addressing the emerging problem of a clean water shortage. In this paper, one new thin film composite (TFC) membrane with a novel MWCNT/polyamide (MWCNT/PA) composite rejection layer had been manufactured via interfacial polymerization (IP) method under negative pressure. The MWCNTs framework was first fabricated on surface of PVDF substrate and then the polyamide matrix was filled into the framework by IP process between the two monomers (M-phenylenediamine and trimesoyl chloride) under negative pressure. The novel rejection layer obviously enhanced the water flux and antifouling performance without harming the desalination rate in RO process. The improved performances can be attributed to the larger amount of hydrophilic MWCNTs which were embedded into the rejection layer and worked as water channels. Meanwhile, this research also revealed that enhancing the density of water channels in polyamide rejection layer and the hydrophilicity of membrane surface could effectively improve the TFC membrane performance.
1. Introduction Reverse osmosis, a popular technique for water treatment, has been drawing an extensive attention for its high-efficiency in getting rid of small molecule inorganic salts and organic compounds, which holds great promise in addressing the emerging problem of a clean water shortage [1–3]. However, the high energy consumption has weighed heavily against the overall performance before the RO technology can make significant progress in desalination market. In some degree, this can be mostly attributed to the poor RO membrane permeability [4]. Besides, previous reports have found that energy consumption would be reduced 15% in seawater desalination process and 46% in brackish water, when the membrane permeability raises three times [5]. Therefore, synthesizing a high-flux RO membrane is still significant for further reduction of energy consumption in RO processes. Carbon nanotubes, which own nanoscale diameters and slick surfaces at an atomic level, can provide specific channels to transport water molecules efficiently. This character gives a possibility to fabricate high-flux RO membranes based on carbon nanotubes. For now, much effort has been devoted to developing pure carbon nanotubes or carbon nanotube/polymer composite membranes to further enhance
⁎
membrane permeability for water desalination. Holt et al. fabricated novel aligned carbon nanotube membranes on a silicon chip using both SWCNTs and MWCNTs, which firstly provided an experimental evidence to demonstrate the enormous potential of carbon nanotubes as water channels in membrane [6]. In their research, a vertically aligned array of carbon nanotubes was prepared on surface of a silicon chip by the Chemical Vapor Deposition (CVD) method. Then the aligned array of carbon nanotubes was conformally encapsulated with a chemical vapor-deposited Si3N4 matrix. However, the microelectromechanical systems used in this membrane fabrication process seem to be quite complex and high cost and are unsuitable for large-scale promotion. Zhao's group dispersed MWCNTs in MPD aqueous solution firstly and then incorporated carboxy-functionalized MWCNTs into the polyamide rejection layer based on the interfacial polymerization (IP) process between MPD aqueous solution and TMC organic solution. The flux of the resulting membrane increased from 0.93 L m−2 h−1 bar−1 to 1.75 L m−2 h−1 bar−1 without sacrifice of the salt rejection performance [7]. Nonetheless, MWCNTs seem difficult to be well dispersed in polyamide rejection layer and limited the increase of the membrane flux. Similarly, Wu's group reported a novel TFC membrane by incorporated MWCNTs intermediate layer into the middle of support and
Corresponding author at: School of Environment, Harbin Institute of Technology, Harbin 150090, PR China. E-mail address: [email protected] (J. Yao).
https://doi.org/10.1016/j.apsusc.2019.143680 Received 25 March 2019; Received in revised form 23 July 2019; Accepted 13 August 2019 Available online 14 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 496 (2019) 143680
H. Sun, et al.
Fig. 1. Schematic of the fabrication process for MWCNT/PA-TFC membranes.
2. Experimental
Table 1 The MWCNTs content in different MWCNT/PA-TFC membranes. Samples
C-1
C-2
C-3
C-4
C-5
C-6
Volume of MWCNTs suspension (mL) MWCNTs mass density (g/m2)
0 0.00
3 0.48
6 0.96
9 1.43
12 1.91
16 2.39
2.1. Materials Polyvinylidene fluoride (PVDF) was bought from 3F New Material Co., Ltd., Shanghai. Polyvinylpyrrolidone (PVP) and bovine serum albumin (BSA, Mw = 66.4 kDa) were purchased from Bodi Chemical Co., Ltd., Tianjin. n-Hexane (95%), N,N-dimethylacetamide (DMAc, 99%) and sodium chloride (NaCl, 99%) were bought from Fuyu Fine Chemical Co., Ltd., Tianjin. M-phenylenediamine (MPD, 99%), calcium bicarbonate (Ca(HCO3)2, 98%) and trimesoyl chloride (TMC, 98%), were purchased from Aladdin, China. Multi-walled carbon nanotubes (MWCNTs, outside diameter 8–15 nm and length 15 μm) were procured from Chengdu Institute of Organic Chemistry Co., Ltd., Chinese Academy of Sciences. All the above materials were utilized as received. 2.2. Preparation of PVDF substrates The PVDF substrates were prepared by nonsolvent induced phase separation method. Briefly, the casting solution containing 14% PVDF, 1% PVP and 85% DMAc was agitated at 75 °C, defoamed and then refrigerated to 25 °C. Then, the 100 μm thick polymer solution was spread on an undefiled glass pane utilizing an Elcometer 3600 Doctor Blade Film Applicator. Next, the PVDF substrate was formed through immersing the glass pane with the earlier PVDF membrane into the coagulant tank smoothly. After that, the solidified PVDF substrate was soaked into deionized water (DIW) to wash off the residual solvent and then preserved in another DIW bath.
Fig. 2. Schematic of the cross-flow test unit (1 feed tank; 2 permeate tank; 3 pump; 4 values; 5 pressure gauges; 6 flow gauge; 7 cross-flow cell).
rejection layer [8]. Whereas, the MWCNTs layer is on the outside of the polyamide rejection layer and could not enhance the membrane permeability significantly. All of the above studies have shown a great prospect for the MWCNTs applied in TFC membranes. In this paper, a novel TFC RO membrane had been manufactured via incorporating a novel MWCNT/PA composite rejection layer on the PVDF substrate. Different from previous studies about MWCNT/PA composite membranes, the MWCNTs in this paper act as a framework and the polyamide matrix was filled into the framework via IP process under negative pressure. Therefore, the larger amount of MWCNTs can be embedded into the novel rejection layer to work as water channels. This method provides higher density water channels for water transport and is expected to obviously enhance the permeability without damaging the selectivity of TFC membranes.
2.3. Preparation of MWCNT/PA-TFC membranes The TFC membranes were fabricated through forming a rejection layer on the PVDF substrates via IP method as shown in Fig. 1 [7,9]. Briefly, MWCNTs framework was first fabricated on the PVDF substrate surface through vacuum filtration of a 0.15 g/L MWCNTs suspension. Then, the substrates were first immersed into the aqueous solution contained 2 wt% MPD that lasted for 5 min and then placed horizontally at 25 °C to clear away the superfluous aqueous solution. Next, the polyamide rejection layer was formed through gently pouring an n2
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Fig. 3. SEM images of PA-TFC (C-1) and MWCNT/PA-TFC (C-6) membrane.
2.4. Membrane characterization
hexane solution contained 0.1 wt% TMC on PVDF substrate surfaces to react with the residue MPD for 3 min under negative pressure. After that, the resulting TFC membranes were post-treated in a thermostat at 60 °C for 30 min, then reserved in DIW. Eventually, the manufactured TFC membranes based on different MWCNTs contents were marked as C-6, C-5, C-4, C-3, C-2 and C-1 respectively and the mass density of MWCNTs framework in each membrane was calculated and shown in Table 1.
The FTIR (Fourier transform infrared) spectroscopy of MWCNT/PATFC membranes was acquired via a FTIR spectrophotometer at 2000–550 cm−1 to analyze the surface chemical structure. To investigate the dispersion of MWCNT/PA rejection layer, EDX mappings of C, N and O (three major elements of polyamide materials) on the membrane surfaces were carried out via an Oxford Inca Energy 250× EDX apparatus. The SEM (scanning electron microscope) pictures of the TFC membranes were conducted by a SEM apparatus. The tidy crosssections were acquired through freezing and fracturing the dry 3
Applied Surface Science 496 (2019) 143680
H. Sun, et al.
utilizing a home-made cross-flow filtration apparatus. The effective area of the filtration apparatus was 28.26 cm2. A 2 g/L sodium chloride feed solution was applied to test membrane water permeance. Pressurenormalized water permeance was calculated from Eq. (1):
J=
ΔV A × ΔP × Δt
(1)
where A means the effective membrane area (m ), ΔV means the permeate volume (L), ΔP means the operating pressure (bar) and Δt means the testing time (h). Membrane salt rejection was acquired via an intelligent conductivity meter and was calculated through Eq. (2): 2
Cf − Cp ⎞ R = ⎜⎛ ⎟ × 100% ⎝ Cf ⎠
(2)
where Cf means the salt concentration in feed solution, and Cp means the salt concentration in permeate solution. The 2 g/L sodium chloride feed solution has a conductivity of the typically 3.8 mS/cm. 3. Results and discussions
Fig. 4. FTIR/ATR spectra of MWCNT/PA-TFC membrane.
3.1. Membrane characterization
membrane in liquid nitrogen. And then the membrane conductivity was rendered before observation through sputtering-coating a thin layer of platinum onto the membrane surfaces. The membrane surface roughness was characterized using a Park Systems XE-100 AFM within an area of 25 μm2. The contact angles of TFC membranes were acquired by viewing a 1 μL droplet on the TFC membranes through a contact angle analyzer. The images of the static contact angles for each sample were recorded and averaged.
The surface and cross-section morphologies of both PA-TFC membrane (C-1) and MWCNT/PA-TFC membrane (C-6) were analyzed by SEM, as shown in Fig. 3. It could be seen from the membrane surfaces that the typical ridge-valley structure without any defects had been detected in all samples illustrating the successful formation of polyamide rejection layer [10]. Besides, the as-prepared MWCNT/PA-TFC membrane owns more surface crumples than the PA-TFC membrane and the more surface crumples usually means the larger surface area. As is well known that the large surface area was favorable for the enhanced permeability [11]. The more crumples formation might be attributed to the acceleration of the IP process, due to that the sufficient amount of MPD aqueous solution had been filled into the hydrophilic
2.5. Permeation property measurements As shown in Fig. 2, the permeability of each MWCNT/PA-TFC membrane was evaluated at a feed pressure of 15 bar respectively,
Fig. 5. EDX map scanning spectra of MWCNT/PA-TFC membranes. 4
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Fig. 6. AFM images of MWCNT/PA-TFC membranes.
outside the MWCNTs [12]; the other one was the gap inside the MWCNTs [6]. The water channels formed in MWCNT/PA composite rejection layer was favorable for the increase of membrane permeability. Furthermore, the thickness of the rejection layer (300 nm) is larger than the general polyamide rejection layer (100–150 nm), suggesting the higher desalination rate. As shown in Fig. 4, all the samples show the polyamide typical peaks at 1663, 1609 and 1540 cm−1. Specifically, the peak at 1663 cm−1 represents C]O and CeN stretching (amide I band) [13,14]; the peak at 1609 cm−1 represents NeH deformation vibration and C]C ring stretching vibration (aromatic amide) [15]; the peak at 1541 cm−1 represents the NeH in-plane bending and NeC stretching (amide II band) [13]. Meanwhile, the characteristic peaks of α-phases PVDF at 762 and 976 cm−1 and β-phases PVDF at 840, 1276 and 1403 cm−1 could also be seen in Fig. 4 [16–18]. Obviously, these peaks further suggest the successful formation of fully aromatic polyamide layer on PVDF substrate. Furthermore, it is worth noting that the polyamide characteristic peaks are strengthened gradually with the increase of MWCNTs mass density in MWCNT/PA composite rejection layer. This phenomenon was consistent with the increase of the rejection layer thickness as shown in the SEM images, suggesting that the addition of MWCNTs would enhance the IP process between MPD and TMC solutions. Besides, the characteristic peaks of carboxyl group at 1640 cm−1 could also be seen in Fig. 4, confirming the carboxyl group grafted on the MWCNTs [19]. EDX map scanning spectra in Fig. 5 displays the dispersion of C, N and O (three major elements of polyamide matrix) on membrane surfaces. Compared with the C-1 membranes, the C-6 membrane has a more uniform and denser distribution for the three major elements on membrane surfaces. Therefore, it could be deduced that the C-6 membrane owns more uniform and denser MWCNT/PA rejection layer. This observed phenomenon was consistent with above-mentioned SEM images in Fig. 3 (C-6 a and C-1 a), suggesting a higher desalination efficiency for MWCNT/PA composite rejection layer. Surface morphologies of the MWCNT/PA-TFC membranes were also investigated by AFM, and the observed images could be seen in Fig. 6. Surface roughness parameters of the membranes are shown in Table 2. The results of Table 2 and Fig. 6 indicated that the original PA-TFC
Table 2 Surface roughness parameters of MWCNT/PA-TFC membranes. Sample
C-1
C-2
C-3
C-4
C-5
C-6
Raa (nm) Rqb (nm)
51.311 65.136
25.461 32.221
28.566 36.018
31.950 40.331
39.814 51.431
41.246 53.118
a b
Rq: root mean square of z date. Ra: average roughness.
Fig. 7. Water contact angle of MWCNT/PA-TFC membrane.
MWCNTs framework before the IP process [11]. It could also be seen in the cross-section SEM images that the as-prepared MWCNT/PA-TFC membrane has a unique MWCNT/PA composite rejection layer owing a thickness of 300 nm. This MWCNTs framework was built up through layer-by-layer deposition and then polyamide matrix was deeply filled into the MWCNTs framework after the IP process under negative pressure. The MWCNTs framework embedded in the polyamide matrix could form two kinds of water channels: the one was the gap between the MWCNTs framework and the polyamide matrix and was formed 5
Applied Surface Science 496 (2019) 143680
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increasing the amount of MWCNTs in rejection layer would lead to the more crumples formation and increase membrane surface roughness gradually. It can be attributed to the accelerated IP process resulting from the more MPD aqueous solution in MWCNTs framework. The water contact angle can be applied to evaluate the membrane hydrophilicity [20,21]. Besides, the smaller the contact angle, the higher the substrate hydrophilicity [22]. The contact angles of MWCNT/PA-TFC membranes with different MWCNTs content were shown in Fig. 7. The original PA-TFC membrane owned the largest contact angle of 91.2°. In contrast, the MWCNT/PA-TFC membranes possessed relative smaller contact angles. Obviously, as the MWCNTs content increased in composite rejection layer, the contact angle decreased gradually, and then reached a relative stable parallel. The decreased contact angles may be the consequence of the stronger surface composition hydrophilicity and the larger surface roughness (see in Fig. 6 and Table 2). To be more specific, the construction of hydrophilic MWCNT/PA rejection layer made the contact angle decreased from 91.2° to 64.6°. And then the increased surface roughness (from 32.221 nm to 53.118 nm) made the contact angle decreased from 64.6° to 55.5° for the well-known law that the larger surface roughness will lead to the less surface contact angle if the contact angle less than 90°. This hydrophilic membrane surface would be beneficial to lighten the membrane fouling and enhance the membrane flux.
Fig. 8. The separation performances of MWCNT/PA-TFC membrane.
3.2. Separation performance of the MWCNT/PA-TFC membrane The permeability and selectivity performances were evaluated at a feed pressure of 5 bar utilizing the home-made cross-flow filtration apparatus and the results were recorded in Fig. 8. Notably, all MWCNT/ PA-TFC membranes owned higher water flux and better salt rejection rate than the original PA-TFC membrane. The improved permeability of the membrane might be attributed to several properties of the MWCNT/ PA composite rejection layer. First and foremost, the better permeability could be put down to the additional water channels both inside and outside the MWCNTs framework in MWCNT/PA rejection layer [6,12]. Second, as shown in Fig. 6, the higher surface roughness meant a larger surface area of contact with the saline solution. Last, it could be observed in Fig. 7 that the construction of hydrophilic MWCNTs framework made the contact angle sharply decreased from 91.2° to 64.6°, which would also be helpful to enhance the water flux [23]. The improved selectivity performances can be ascribed to the larger thickness of MWCNT/PA rejection layer (300 nm for C-6) than the general polyamide rejection layer (100–150 nm for C-1), as shown in Fig. 3. In this paper, BSA and Ca(HCO3)2 were applied as targeted compounds to investigate the antifouling performance of MWCNT/PA-TFC membrane and the results were shown in Fig. 9(a) and (b). In above images, these MWCNT/PA-TFC membranes had shown relative better antifouling properties than the original TFC membrane. The improved antifouling performance of MWCNT/PA-TFC membrane could be attributed to the several properties of MWCNT/PA rejection layer. First and foremost, the novel rejection layer owns the better hydrophilic surface, which was advantageous to the antifouling performance for both organic and inorganic compounds [24–26]. Second, the novel rejection layer owned more negative surface charge of the membrane, enhancing the electrostatic exclusion with BAS owning negative surface charge [27]. Last, the MWCNT/PA-TFC membrane showed lesser surface roughness than the original PA-TFC membrane, which was benefit to the decrease of colloidal fouling and the membrane cleaning [28].
Fig. 9. Antifouling performance of MWCNT/PA-TFC membrane: (a) BSA solution; and (b) Ca(HCO3)2 solution.
membrane (C-1) owned the roughest surface with a Ra of 51.311 nm. The average surface roughness of the MWCNT/PA-TFC membranes is 25.461, 28.566, 31.950, 39.814 and 41.264 nm for C-2, C-3⋯C-6, respectively. Obviously, all of the MWCNT/PA-TFC membranes were apt to have more smooth surfaces than the original PA-TFC membrane. It can be put down to the more uniform distribution of MPD aqueous solution in MWCNTs framework before IP process. Nevertheless, further
4. Conclusions In summary, the novel TFC RO membranes were manufactured by incorporating novel MWCNT/PA composite rejection layers upon PVDF substrate surfaces. Contrasting with the original TFC RO membrane, the MWCNT/PA-TFC membranes own better permeability and antifouling performance without harming membrane desalination rate. These 6
Applied Surface Science 496 (2019) 143680
H. Sun, et al.
improved performances can be attributed to the larger amount of hydrophilic MWCNTs embedded into the rejection layer to work as water channels. This feature provides higher density water channels and more hydrophilic membrane surface for water transport during RO process.
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7
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Enhancing the permeability of TFC membranes based on incorporating polyamide matrix into MWCNTs framework Hao Suna,b, Dan Lia,b, Bing Liua,b, Jie Yaoa,b,c,
T
⁎
a
School of Environment, Harbin Institute of Technology, Harbin 150090, PR China State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, PR China c National Engineering Center of Urban Water Resources, 202 Hehai Road, Harbin 150090, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Reverse osmosis Carbon nanotubes framework High water flux Antifouling
Reverse osmosis (RO) technology can get rid of small molecular contaminants and salts with a very high efficiency, so that it has exhibited promising potential for addressing the emerging problem of a clean water shortage. In this paper, one new thin film composite (TFC) membrane with a novel MWCNT/polyamide (MWCNT/PA) composite rejection layer had been manufactured via interfacial polymerization (IP) method under negative pressure. The MWCNTs framework was first fabricated on surface of PVDF substrate and then the polyamide matrix was filled into the framework by IP process between the two monomers (M-phenylenediamine and trimesoyl chloride) under negative pressure. The novel rejection layer obviously enhanced the water flux and antifouling performance without harming the desalination rate in RO process. The improved performances can be attributed to the larger amount of hydrophilic MWCNTs which were embedded into the rejection layer and worked as water channels. Meanwhile, this research also revealed that enhancing the density of water channels in polyamide rejection layer and the hydrophilicity of membrane surface could effectively improve the TFC membrane performance.
1. Introduction Reverse osmosis, a popular technique for water treatment, has been drawing an extensive attention for its high-efficiency in getting rid of small molecule inorganic salts and organic compounds, which holds great promise in addressing the emerging problem of a clean water shortage [1–3]. However, the high energy consumption has weighed heavily against the overall performance before the RO technology can make significant progress in desalination market. In some degree, this can be mostly attributed to the poor RO membrane permeability [4]. Besides, previous reports have found that energy consumption would be reduced 15% in seawater desalination process and 46% in brackish water, when the membrane permeability raises three times [5]. Therefore, synthesizing a high-flux RO membrane is still significant for further reduction of energy consumption in RO processes. Carbon nanotubes, which own nanoscale diameters and slick surfaces at an atomic level, can provide specific channels to transport water molecules efficiently. This character gives a possibility to fabricate high-flux RO membranes based on carbon nanotubes. For now, much effort has been devoted to developing pure carbon nanotubes or carbon nanotube/polymer composite membranes to further enhance
⁎
membrane permeability for water desalination. Holt et al. fabricated novel aligned carbon nanotube membranes on a silicon chip using both SWCNTs and MWCNTs, which firstly provided an experimental evidence to demonstrate the enormous potential of carbon nanotubes as water channels in membrane [6]. In their research, a vertically aligned array of carbon nanotubes was prepared on surface of a silicon chip by the Chemical Vapor Deposition (CVD) method. Then the aligned array of carbon nanotubes was conformally encapsulated with a chemical vapor-deposited Si3N4 matrix. However, the microelectromechanical systems used in this membrane fabrication process seem to be quite complex and high cost and are unsuitable for large-scale promotion. Zhao's group dispersed MWCNTs in MPD aqueous solution firstly and then incorporated carboxy-functionalized MWCNTs into the polyamide rejection layer based on the interfacial polymerization (IP) process between MPD aqueous solution and TMC organic solution. The flux of the resulting membrane increased from 0.93 L m−2 h−1 bar−1 to 1.75 L m−2 h−1 bar−1 without sacrifice of the salt rejection performance [7]. Nonetheless, MWCNTs seem difficult to be well dispersed in polyamide rejection layer and limited the increase of the membrane flux. Similarly, Wu's group reported a novel TFC membrane by incorporated MWCNTs intermediate layer into the middle of support and
Corresponding author at: School of Environment, Harbin Institute of Technology, Harbin 150090, PR China. E-mail address: [email protected] (J. Yao).
https://doi.org/10.1016/j.apsusc.2019.143680 Received 25 March 2019; Received in revised form 23 July 2019; Accepted 13 August 2019 Available online 14 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 496 (2019) 143680
H. Sun, et al.
Fig. 1. Schematic of the fabrication process for MWCNT/PA-TFC membranes.
2. Experimental
Table 1 The MWCNTs content in different MWCNT/PA-TFC membranes. Samples
C-1
C-2
C-3
C-4
C-5
C-6
Volume of MWCNTs suspension (mL) MWCNTs mass density (g/m2)
0 0.00
3 0.48
6 0.96
9 1.43
12 1.91
16 2.39
2.1. Materials Polyvinylidene fluoride (PVDF) was bought from 3F New Material Co., Ltd., Shanghai. Polyvinylpyrrolidone (PVP) and bovine serum albumin (BSA, Mw = 66.4 kDa) were purchased from Bodi Chemical Co., Ltd., Tianjin. n-Hexane (95%), N,N-dimethylacetamide (DMAc, 99%) and sodium chloride (NaCl, 99%) were bought from Fuyu Fine Chemical Co., Ltd., Tianjin. M-phenylenediamine (MPD, 99%), calcium bicarbonate (Ca(HCO3)2, 98%) and trimesoyl chloride (TMC, 98%), were purchased from Aladdin, China. Multi-walled carbon nanotubes (MWCNTs, outside diameter 8–15 nm and length 15 μm) were procured from Chengdu Institute of Organic Chemistry Co., Ltd., Chinese Academy of Sciences. All the above materials were utilized as received. 2.2. Preparation of PVDF substrates The PVDF substrates were prepared by nonsolvent induced phase separation method. Briefly, the casting solution containing 14% PVDF, 1% PVP and 85% DMAc was agitated at 75 °C, defoamed and then refrigerated to 25 °C. Then, the 100 μm thick polymer solution was spread on an undefiled glass pane utilizing an Elcometer 3600 Doctor Blade Film Applicator. Next, the PVDF substrate was formed through immersing the glass pane with the earlier PVDF membrane into the coagulant tank smoothly. After that, the solidified PVDF substrate was soaked into deionized water (DIW) to wash off the residual solvent and then preserved in another DIW bath.
Fig. 2. Schematic of the cross-flow test unit (1 feed tank; 2 permeate tank; 3 pump; 4 values; 5 pressure gauges; 6 flow gauge; 7 cross-flow cell).
rejection layer [8]. Whereas, the MWCNTs layer is on the outside of the polyamide rejection layer and could not enhance the membrane permeability significantly. All of the above studies have shown a great prospect for the MWCNTs applied in TFC membranes. In this paper, a novel TFC RO membrane had been manufactured via incorporating a novel MWCNT/PA composite rejection layer on the PVDF substrate. Different from previous studies about MWCNT/PA composite membranes, the MWCNTs in this paper act as a framework and the polyamide matrix was filled into the framework via IP process under negative pressure. Therefore, the larger amount of MWCNTs can be embedded into the novel rejection layer to work as water channels. This method provides higher density water channels for water transport and is expected to obviously enhance the permeability without damaging the selectivity of TFC membranes.
2.3. Preparation of MWCNT/PA-TFC membranes The TFC membranes were fabricated through forming a rejection layer on the PVDF substrates via IP method as shown in Fig. 1 [7,9]. Briefly, MWCNTs framework was first fabricated on the PVDF substrate surface through vacuum filtration of a 0.15 g/L MWCNTs suspension. Then, the substrates were first immersed into the aqueous solution contained 2 wt% MPD that lasted for 5 min and then placed horizontally at 25 °C to clear away the superfluous aqueous solution. Next, the polyamide rejection layer was formed through gently pouring an n2
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Fig. 3. SEM images of PA-TFC (C-1) and MWCNT/PA-TFC (C-6) membrane.
2.4. Membrane characterization
hexane solution contained 0.1 wt% TMC on PVDF substrate surfaces to react with the residue MPD for 3 min under negative pressure. After that, the resulting TFC membranes were post-treated in a thermostat at 60 °C for 30 min, then reserved in DIW. Eventually, the manufactured TFC membranes based on different MWCNTs contents were marked as C-6, C-5, C-4, C-3, C-2 and C-1 respectively and the mass density of MWCNTs framework in each membrane was calculated and shown in Table 1.
The FTIR (Fourier transform infrared) spectroscopy of MWCNT/PATFC membranes was acquired via a FTIR spectrophotometer at 2000–550 cm−1 to analyze the surface chemical structure. To investigate the dispersion of MWCNT/PA rejection layer, EDX mappings of C, N and O (three major elements of polyamide materials) on the membrane surfaces were carried out via an Oxford Inca Energy 250× EDX apparatus. The SEM (scanning electron microscope) pictures of the TFC membranes were conducted by a SEM apparatus. The tidy crosssections were acquired through freezing and fracturing the dry 3
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utilizing a home-made cross-flow filtration apparatus. The effective area of the filtration apparatus was 28.26 cm2. A 2 g/L sodium chloride feed solution was applied to test membrane water permeance. Pressurenormalized water permeance was calculated from Eq. (1):
J=
ΔV A × ΔP × Δt
(1)
where A means the effective membrane area (m ), ΔV means the permeate volume (L), ΔP means the operating pressure (bar) and Δt means the testing time (h). Membrane salt rejection was acquired via an intelligent conductivity meter and was calculated through Eq. (2): 2
Cf − Cp ⎞ R = ⎜⎛ ⎟ × 100% ⎝ Cf ⎠
(2)
where Cf means the salt concentration in feed solution, and Cp means the salt concentration in permeate solution. The 2 g/L sodium chloride feed solution has a conductivity of the typically 3.8 mS/cm. 3. Results and discussions
Fig. 4. FTIR/ATR spectra of MWCNT/PA-TFC membrane.
3.1. Membrane characterization
membrane in liquid nitrogen. And then the membrane conductivity was rendered before observation through sputtering-coating a thin layer of platinum onto the membrane surfaces. The membrane surface roughness was characterized using a Park Systems XE-100 AFM within an area of 25 μm2. The contact angles of TFC membranes were acquired by viewing a 1 μL droplet on the TFC membranes through a contact angle analyzer. The images of the static contact angles for each sample were recorded and averaged.
The surface and cross-section morphologies of both PA-TFC membrane (C-1) and MWCNT/PA-TFC membrane (C-6) were analyzed by SEM, as shown in Fig. 3. It could be seen from the membrane surfaces that the typical ridge-valley structure without any defects had been detected in all samples illustrating the successful formation of polyamide rejection layer [10]. Besides, the as-prepared MWCNT/PA-TFC membrane owns more surface crumples than the PA-TFC membrane and the more surface crumples usually means the larger surface area. As is well known that the large surface area was favorable for the enhanced permeability [11]. The more crumples formation might be attributed to the acceleration of the IP process, due to that the sufficient amount of MPD aqueous solution had been filled into the hydrophilic
2.5. Permeation property measurements As shown in Fig. 2, the permeability of each MWCNT/PA-TFC membrane was evaluated at a feed pressure of 15 bar respectively,
Fig. 5. EDX map scanning spectra of MWCNT/PA-TFC membranes. 4
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Fig. 6. AFM images of MWCNT/PA-TFC membranes.
outside the MWCNTs [12]; the other one was the gap inside the MWCNTs [6]. The water channels formed in MWCNT/PA composite rejection layer was favorable for the increase of membrane permeability. Furthermore, the thickness of the rejection layer (300 nm) is larger than the general polyamide rejection layer (100–150 nm), suggesting the higher desalination rate. As shown in Fig. 4, all the samples show the polyamide typical peaks at 1663, 1609 and 1540 cm−1. Specifically, the peak at 1663 cm−1 represents C]O and CeN stretching (amide I band) [13,14]; the peak at 1609 cm−1 represents NeH deformation vibration and C]C ring stretching vibration (aromatic amide) [15]; the peak at 1541 cm−1 represents the NeH in-plane bending and NeC stretching (amide II band) [13]. Meanwhile, the characteristic peaks of α-phases PVDF at 762 and 976 cm−1 and β-phases PVDF at 840, 1276 and 1403 cm−1 could also be seen in Fig. 4 [16–18]. Obviously, these peaks further suggest the successful formation of fully aromatic polyamide layer on PVDF substrate. Furthermore, it is worth noting that the polyamide characteristic peaks are strengthened gradually with the increase of MWCNTs mass density in MWCNT/PA composite rejection layer. This phenomenon was consistent with the increase of the rejection layer thickness as shown in the SEM images, suggesting that the addition of MWCNTs would enhance the IP process between MPD and TMC solutions. Besides, the characteristic peaks of carboxyl group at 1640 cm−1 could also be seen in Fig. 4, confirming the carboxyl group grafted on the MWCNTs [19]. EDX map scanning spectra in Fig. 5 displays the dispersion of C, N and O (three major elements of polyamide matrix) on membrane surfaces. Compared with the C-1 membranes, the C-6 membrane has a more uniform and denser distribution for the three major elements on membrane surfaces. Therefore, it could be deduced that the C-6 membrane owns more uniform and denser MWCNT/PA rejection layer. This observed phenomenon was consistent with above-mentioned SEM images in Fig. 3 (C-6 a and C-1 a), suggesting a higher desalination efficiency for MWCNT/PA composite rejection layer. Surface morphologies of the MWCNT/PA-TFC membranes were also investigated by AFM, and the observed images could be seen in Fig. 6. Surface roughness parameters of the membranes are shown in Table 2. The results of Table 2 and Fig. 6 indicated that the original PA-TFC
Table 2 Surface roughness parameters of MWCNT/PA-TFC membranes. Sample
C-1
C-2
C-3
C-4
C-5
C-6
Raa (nm) Rqb (nm)
51.311 65.136
25.461 32.221
28.566 36.018
31.950 40.331
39.814 51.431
41.246 53.118
a b
Rq: root mean square of z date. Ra: average roughness.
Fig. 7. Water contact angle of MWCNT/PA-TFC membrane.
MWCNTs framework before the IP process [11]. It could also be seen in the cross-section SEM images that the as-prepared MWCNT/PA-TFC membrane has a unique MWCNT/PA composite rejection layer owing a thickness of 300 nm. This MWCNTs framework was built up through layer-by-layer deposition and then polyamide matrix was deeply filled into the MWCNTs framework after the IP process under negative pressure. The MWCNTs framework embedded in the polyamide matrix could form two kinds of water channels: the one was the gap between the MWCNTs framework and the polyamide matrix and was formed 5
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increasing the amount of MWCNTs in rejection layer would lead to the more crumples formation and increase membrane surface roughness gradually. It can be attributed to the accelerated IP process resulting from the more MPD aqueous solution in MWCNTs framework. The water contact angle can be applied to evaluate the membrane hydrophilicity [20,21]. Besides, the smaller the contact angle, the higher the substrate hydrophilicity [22]. The contact angles of MWCNT/PA-TFC membranes with different MWCNTs content were shown in Fig. 7. The original PA-TFC membrane owned the largest contact angle of 91.2°. In contrast, the MWCNT/PA-TFC membranes possessed relative smaller contact angles. Obviously, as the MWCNTs content increased in composite rejection layer, the contact angle decreased gradually, and then reached a relative stable parallel. The decreased contact angles may be the consequence of the stronger surface composition hydrophilicity and the larger surface roughness (see in Fig. 6 and Table 2). To be more specific, the construction of hydrophilic MWCNT/PA rejection layer made the contact angle decreased from 91.2° to 64.6°. And then the increased surface roughness (from 32.221 nm to 53.118 nm) made the contact angle decreased from 64.6° to 55.5° for the well-known law that the larger surface roughness will lead to the less surface contact angle if the contact angle less than 90°. This hydrophilic membrane surface would be beneficial to lighten the membrane fouling and enhance the membrane flux.
Fig. 8. The separation performances of MWCNT/PA-TFC membrane.
3.2. Separation performance of the MWCNT/PA-TFC membrane The permeability and selectivity performances were evaluated at a feed pressure of 5 bar utilizing the home-made cross-flow filtration apparatus and the results were recorded in Fig. 8. Notably, all MWCNT/ PA-TFC membranes owned higher water flux and better salt rejection rate than the original PA-TFC membrane. The improved permeability of the membrane might be attributed to several properties of the MWCNT/ PA composite rejection layer. First and foremost, the better permeability could be put down to the additional water channels both inside and outside the MWCNTs framework in MWCNT/PA rejection layer [6,12]. Second, as shown in Fig. 6, the higher surface roughness meant a larger surface area of contact with the saline solution. Last, it could be observed in Fig. 7 that the construction of hydrophilic MWCNTs framework made the contact angle sharply decreased from 91.2° to 64.6°, which would also be helpful to enhance the water flux [23]. The improved selectivity performances can be ascribed to the larger thickness of MWCNT/PA rejection layer (300 nm for C-6) than the general polyamide rejection layer (100–150 nm for C-1), as shown in Fig. 3. In this paper, BSA and Ca(HCO3)2 were applied as targeted compounds to investigate the antifouling performance of MWCNT/PA-TFC membrane and the results were shown in Fig. 9(a) and (b). In above images, these MWCNT/PA-TFC membranes had shown relative better antifouling properties than the original TFC membrane. The improved antifouling performance of MWCNT/PA-TFC membrane could be attributed to the several properties of MWCNT/PA rejection layer. First and foremost, the novel rejection layer owns the better hydrophilic surface, which was advantageous to the antifouling performance for both organic and inorganic compounds [24–26]. Second, the novel rejection layer owned more negative surface charge of the membrane, enhancing the electrostatic exclusion with BAS owning negative surface charge [27]. Last, the MWCNT/PA-TFC membrane showed lesser surface roughness than the original PA-TFC membrane, which was benefit to the decrease of colloidal fouling and the membrane cleaning [28].
Fig. 9. Antifouling performance of MWCNT/PA-TFC membrane: (a) BSA solution; and (b) Ca(HCO3)2 solution.
membrane (C-1) owned the roughest surface with a Ra of 51.311 nm. The average surface roughness of the MWCNT/PA-TFC membranes is 25.461, 28.566, 31.950, 39.814 and 41.264 nm for C-2, C-3⋯C-6, respectively. Obviously, all of the MWCNT/PA-TFC membranes were apt to have more smooth surfaces than the original PA-TFC membrane. It can be put down to the more uniform distribution of MPD aqueous solution in MWCNTs framework before IP process. Nevertheless, further
4. Conclusions In summary, the novel TFC RO membranes were manufactured by incorporating novel MWCNT/PA composite rejection layers upon PVDF substrate surfaces. Contrasting with the original TFC RO membrane, the MWCNT/PA-TFC membranes own better permeability and antifouling performance without harming membrane desalination rate. These 6
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improved performances can be attributed to the larger amount of hydrophilic MWCNTs embedded into the rejection layer to work as water channels. This feature provides higher density water channels and more hydrophilic membrane surface for water transport during RO process.
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