Enhanced desalination of polyamide thin film ...

Desalination 409 (2017) 163–170

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Enhanced desalination of polyamide thin film nanocomposite incorporated with acid treated multiwalled carbon nanotube-titania nanotube hybrid I. Wan Azelee, P.S. Goh ⁎, W.J. Lau, A.F. Ismail, M. Rezaei-DashtArzhandi, K.C. Wong, M.N. Subramaniam Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, 81310 Skudai, Johor, Malaysia

H I G H L I G H T S • • • •

Multiwalled carbon nanotubes-titania nanotube hybrid was synthesized by hydrothermal method. Surface charge of as-synthesized MWCNT-TNT was modified by acid treatment. TFN was fabricated by incorporating MWCNT-TNT hybrid and applied for RO desalination. TFN composite membrane exhibited higher flux and salt rejection.

a r t i c l e

i n f o

Article history: Received 4 September 2016 Accepted 23 January 2017 Available online 1 February 2017 Keywords: Water desalination Thin film nanocomposite membrane Reverse osmosis Acid treated MWCNT-TNT

a b s t r a c t Polyamide (PA) thin film nanocomposite (TFN) membrane incorporated with multiwalled carbon nanotubes-titania nanotube (MWCNT-TNT) hybrid was successfully fabricated. The hybrid was introduced to the PA selective layer during the interfacial polymerization (IP) of trimesoyl chloride (TMC) and m-phenylenediamine (MPD) monomers over porous commercial polysulfone (PS) ultrafiltration support. The resultant TFN was characterized and applied for desalination. The results revealed that the acid treated MWCNT-TNT, which act as filler in the PA membrane, improved the surface properties of the membrane in term of surface charge, surface roughness and contact angle. Consequently, the water permeability increased significantly without compromising the salt rejection performance. The highest water permeability of 0.74 L/m2 h bar was achieved for the TFN membrane containing 0.05 wt% acid treated MWCNT-TNT, which is approximately 57.45% than that of the neat PA membrane. The NaCl and Na2SO4 rejection of this membrane was 97.97% and 98.07%, respectively that is almost similar to the neat membrane. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Currently, providing clean water from limited water resources with only 0.8% fresh water of the total earth's water [1] for human consumption, agriculture, energy production and industries is one of the most important concerns faced by human society. Desalination has been identified as one of the key pillars of current and future water systems to ensure sustainable supplies of safe and clean drinking water [2]. Thin film composite (TFC) reverse osmosis (RO) membrane with excellent water flux and rejection has been commonly fabricated for desalination [3–5]. Recently, the advancement in nanotechnology opens the doors to fabricate thin film nanocomposite (TFN) membranes. The incorporation of nanomaterials in the support membrane and/or dense thin polyamide (PA) layer has been evidenced to heighten the ⁎ Corresponding author. E-mail address: [email protected] (P.S. Goh).

http://dx.doi.org/10.1016/j.desal.2017.01.029 0011-9164/© 2017 Elsevier B.V. All rights reserved.

performance of the RO membranes and consequently reduced the desalination cost and environmental impacts [6]. Several studies have revealed that the proper selection of these fillers can significantly increase the flux, stability and rejection of the TFN membranes [7,8]. Carbon nanotube (CNTs) and titanate nanotube (TNTs) are some commonly used nanomaterials for the fabrication of TFN membrane desalination. CNTs is one of the fullerene-based nanomaterials which has been widely applied to strengthen and modify membrane surface chemistry [9]. CNTs in PA layer provide an additional water path for water molecules to flow through the nanochannel while the formation of ordered hydrogen bonds increases the rate of water transport [10,11]. The functionalization of CNTs with –COOH and –OH groups can further improve their surface hydrophilicity [12]. The capability of CNTs to reject certain dissolved ions based on their structural properties is highly desired for membrane desalination application [13]. Work conducted by Amini et al. (2013) [14] showed that TFN membrane embedded with 0.05 wt% functionalized MWCNTs exhibited nearly 14% water

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I. Wan Azelee et al. / Desalination 409 (2017) 163–170

Fig. 1. TEM images of MWCNT-TNT hybrid (a) and a-MWCNT-TNT hybrid (b). Agglomeration of MWCNTs in a-MWCNT-TNT hybrid is reduced (yellow circle).

permeability improvement (3.6 L/m2 h) and 28% increase in NaCl rejection (89.3%) compared to that of neat TFC membrane On the other hand, TNTs with high hydrophilicity, the abundant surface functional group as well as high pore volume and the specific surface area has also offers unique properties for desalination application [8,15]. Moreover, the tubular structure of TNTs is found to be less adhesive compared to CNTs, resulted in lower agglomeration [16,17]. Emadzadeh et al. (2015) [18] observed two times improvement in water flux without sacrificing salt rejection when 0.05 wt% of functionalized TNTs was embedded into the PA layer of TFN membrane. Inspired by the desired properties demonstrated by these nanomaterials, the objective of this study is to develop a highly permeable and selective membrane comprising a thin film layer containing a hybrid of MWCNT-TNT. The MWCNT-TNT hybrid was modified with acid to facilitate the nanomaterial dispersion and alter the surface charge of the resultant TFN membrane. As a single nanomaterial is incapable of offering all beneficial features at a time, it is anticipated that the hybrid can render desired functionalities in terms of surface hydrophilicity, charges and roughness. Additionally, the existence of pore channels of both fillers in this hybrid would also provide additional water channels to enhance the water permeability [19]. The physicochemical characteristics of the hybrid and TFN membrane were characterized, followed by performance evaluation in term of pure water permeability and NaCl and Na2SO4 salt rejection. To the best of our knowledge, this is the first attempt to prepare and incorporate modified MWCNT-TNT hybrid nanoparticles into TFN RO membranes for desalination. Hence, this study can shed insights into the possible potential of the surface modified hybrid fillers to fulfill the requirements of the desalination process.

Fig. 2. XRD patterns for MWCNT-TNT (a) and a-MWCNT-TNT (b).

2. Experimental 2.1. Materials A commercial ultrafiltration PS support membrane (PS35, Nanostone Flat Sheet Membrane) was purchased from Sterlitech Corporation. 1,3,5-benzenetricarboxylic acid chloride (TMC) was obtained from Acros Organic. N-hexane was supplied from Fisher Chemical. 1,3phenylenediamine (MPD), hydrochloric acid (HCl, fuming 37%), sulfuric acid (H2SO4, 95–97%), nitric acid (HNO3, 65%) sodium hydroxide (NaOH) and sodium chloride (NaCl) were purchased by Merck. Titanium dioxide nanoparticles (TiO2) was obtained from Degussa P25 Evonik, multiwalled carbon nanotube (MWCNTs, 98% carbon basis, OD × ID × L: 10 ± 1 nm × 4.5 ± 0.5 nm × 3–6 μm) was obtained from Aldrich. Sodium sulphate (Na2SO4) was obtained from Riedel-de Haën. TNTs was self-synthesized from the TiO2 nanoparticles. 2.2. Synthesis of MWCNT-TNT and a-MWCNT-TNT hybrid MWCNT-TNT hybrid was synthesized by hydrothermal method. 1 g of MWCNTs was added into 100 ml of H2SO4:HNO3 (each 3 M) mixed acids' solution with volume ratio of 3:1 (v/v) [20] at 80 °C for 24 h to introduce carboxylic acid functional groups onto the MWCNTs surface. The dispersion solution was collected by filtration and was rinsed with RO water until the result reached neutral before left to dry overnight at 60 °C. Next, 3 g of oxidized MWCNTs and TiO2 in the ratio of 1:5 (w/w) were stirred in 100 ml aqueous solutions of 10 M NaOH and the suspension was transferred into an autoclave coated by Teflon and heated in an oven at 120 °C for 24 h. After the heating process, the autoclave was allowed to cool down to room temperature before dark grey

Fig. 3. ATR-FTIR spectra for MWCNT-TNT (a) and a-MWCNT-TNT (b). It was observed that the peak at 669 cm−1 disappeared for a-MWCNT-TNT due to less entangled nanotube structure.


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2.3. Preparation of PA selective layer

Fig. 4. Proposed structure for a-MWCNT-TNT (Cl = green, H = white). The adsorption of HCl molecule on MWCNT-TNT provide H groups which responsible for the positively charged surface

The PA layer to prepare TFN RO membrane was formed via interfacial polymerization (IP) process on the surface of the PS35 support membrane. The PS35 support was kept in RO water for at least 12 h prior to IP process. The filler was dispersed in the organic phase during IP process to minimize the loss in PA layer [19]. Moreover, acid modification on the filler improved the diamine diffusion towards the IP zone during PA layer formation. To start the IP process, sufficient amount of 2% (w/v) MPD aqueous solution was poured on the PS35 support, which was held horizontally for 1 min to allow the penetration of MPD solution into the pores of the support. The excess MPD solution was then drained off from the support surface and its residual droplets were gently removed by using a soft rubber roller. Then, sufficient amount of 0.1% (w/v) TMC solution in n-hexane with 0.05% (w/v) MWCNT-TNT was poured onto the support surface and was drained off after 50 s contact time. Later, the synthesized TFN membrane was dried at ambient temperature for 1 min and further drying was conducted in the oven at 65 °C for 5 min. Finally, these membranes were stored in RO water container until they were tested. In addition, the TiO2, MWCNTs and self-synthesized TNTs were also individually incorporated into the PA layer of the membranes for comparison towards revealing the possible potentiality of the synthesized hybrid nanoparticles. 2.4. Characterization of nanomaterials and membranes

powder was collected by filtration. The powder was rinsed with RO water and treated with 0.1 M HCl solution until pH 3–4. Later the powder was rinsed back with RO water until the pH of the solution became close to 7. After that, it was left to dry overnight at 60 °C to complete remove water. The acid treatment of MWCNT-TNT was performed to modify the surface charge. In this work, the surface modification of MWCNT-TNT was performed using HCl solution. In a method adapted from Mudunkotuwa and Grassian (2010) [21], a 0.2 g of MWCNT-TNT was shaken in a solution of pH 2.0 containing 0.05 N HCl solutions. All the solutions were prepared with 0.03 M NaCl solutions as a buffer. The final product was dried at 60 °C for overnight and denoted as a-MWCNT-TNT.

Transmission electron microscopy (TEM, HT 7700, Hitachi, Japan) was used to analyze the morphological structure of MWCNT-TNT and a-MWCNT-TNT. The crystallinity of the nanomaterials was characterized by X-ray diffraction (XRD, Bruker D8 Advance D8-02/01-378). The functional groups of nanomaterials and membranes were identified using the attenuated total reflectance Fourier transmission infrared spectroscopy (ATR-FTIR, Thermo Nicolet Avatar 360). Zeta potential test (Malvern Zetasizer Nano ZS) was used to measure the charge on nanomaterials and the fabricated membranes. Thermal properties of the membranes were evaluated using a thermogravimetric analyzer (TGA, Mettler Toledo). The morphology membranes were conducted using field emission scanning electronic microscope (FESEM, TM3000, HITACHI). Atomic force microscopy (AFM) (SII Nano Technology SPA

Fig. 5. ATR-FTIR of TFC (a), MWCNT-TNT/TFN (b) and a-MWCNT-TNT/TFN (c). Adsorbed HCl resulting in peaks' broadening for a-MWCNT-TNT/TFN.

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Fig. 6. TGA analysis of TFC (a), MWCNT-TNT/TFN (b), a-MWCNT-TNT/TFN (c).

300 HV using dynamic force contact mode (DFM)) was used to determine the surface roughness (Ra) of the thin film membranes while the hydrophilicity measurement of the membranes were performed using a contact angle goniometer (OCA 15Pro, Dataphysics). 2.5. Performance study of TFC and TFN membranes Permeability and selectivity of TFC and TFN membrane were conducted using RO experimental setup with dead-end mode at an applied pressure of 15 bar. The permeation cell was designed to have a total effective membrane area of 14.6 cm2. The water flux (J) and water permeability (A) of membrane were evaluated using Eqs. (1) and (2) as follows:

J¼

ΔV Am Δt

ð1Þ

A¼

J ΔP

ð2Þ

where Am is the effective membrane area and ΔV, Δt and ΔP are permeate volume, time and transmembrane pressure difference, respectively. Both feed NaCl and Na2SO4 concentration was 2000 ppm. The salt rejection, R (%) was calculated based on the following equation:

R¼

Cp 100 1− Cf

ð3Þ

where Cp and Cf are the salt concentration (ppm) in the permeate and feed solution, respectively.

Fig. 7. FESEM images of cross section (a), top surface of TFC (b), MWCNT-TNT/TFN (c) and a-MWCNT-TNT/TFN (d). Yellow arrows show more visible “leaf-like” structure on a-MWCNTTNT/TFN membrane.

I. Wan Azelee et al. / Desalination 409 (2017) 163–170 Table 1 Surface charge properties of TFC and TFN membranes.

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Table 2 Contact angle of TFC and TFN membranes.

Membrane

Surface zeta potential (mV)

Membrane

Contact angle (°)

TFC MWCNT-TNT/TFN a-MWCNT-TNT/TFN

−15.9 −21.1 −27.0

TFC MWCNT-TNT/TFN a-MWCNT-TNT/TFN

76.20 ± 4.34 72.12 ± 6.04 69.66 ± 4.79

3. Results and discussion 3.1. Characterization and analysis of synthesized hybrid nanomaterials TEM images of the synthesized MWCNT-TNT hybrid and acid treated material (a-MWCNT-TNT) are shown in Fig. 1. The hybrid consists of cylindrical tubes of open-ended MWCNTs and TNTs with multilayer walls of the nanotubes [8,22], in the form of mixed individual MWCNTs and TNTs. Based on TEM images, the MWCNTs and TNTs possessed an outer diameter in the range of 16–20 nm and 71–141 nm, respectively. The MWCNTs in a-MWCNT-TNT were found to be less entangled after the acid treatment as shown by the yellow circle in Fig. 1a. The surface protonation that occurred after the acid treatment has resulted in high repulsive forces within the molecular structure of MWCNTs which increasing double layer forces, resulting in less aggregation as related to Derjaguin, Landau, Verwey, and Overbeek theory (DLVO theory) [21]. According to DLVO theory, the sum of van der Waals attraction and the formation electrical double layer forces plays an important role in the stability of the nanomaterials. The X-ray diffractometer peaks of MWCNT-TNT and a-MWCNT-TNT nanotubes are shown in Fig. 2. Both samples exhibited almost similar XRD patterns, suggesting that the surface modification by acid treatment does not significantly alter the crystalline structure of the MWCNT-TNT. The diffractometer peaks for X-ray analysis shown at 2theta 11.0°, 25.6° and 48.7° indicates a large number of a hydroxyl group (OH) at the outer structure of TNTs [18]. The peak at 25.6° might also be correlated to superimposing of TNTs over that of CNTs [23]. ATR-FTIR spectra of MWCNT-TNT and a-MWCNT/TNT nanotubes can be observed in Fig. 3. The FTIR spectrum of MWCNT-TNT shows a peak at 669 cm−1 corresponding to Ti\\O\\Ti vibration besides those exhibited for carbonyl and unsaturated carbon [23]. The peak disappeared after the acid treatment may be possibly resulted from the less entanglement of nanotubes as verified in the TEM image (Fig. 1). The band in the region of the asymmetric carboxylate stretching mode at 1366 cm− 1 was due to the interactions between carboxylate oxygen groups of CNTs with the titanium ion [24], which initiated the Ti\\O\\C bond formation. The zeta potential test was performed on MWCNT-TNT and aMWCNT-TNT to identify their surface charge alterations by acid treatment. The results revealed that the negative surface charge of MWCNT-TNT (− 8.95 mV) changed to a positive charge of 26.60 mV after acid treatment. Similarly, Mudunkotawa and Grassian (2010) [21] detected surface charge changes of TiO2 when they treated the material with the organic citric acid. However, the positive charge value of

the treated TiO2 was lower than the acid treated MWCNT-TNT that most likely is due to the lower electronegativity of oxygen compared to chlorine group. The possible demonstration of the treatment of the MWCNT-TNT by HCl is shown in Fig. 4. It indicates positive charge on the surface of MWCNT-TNT is mainly attributed to the H group of HCl [21]. 3.2. Characterizations of TFN membranes' properties 3.2.1. ATR-FTIR and TGA analysis The results of ATR-FTIR which shows molecular bonding and interaction of TFN membranes with MWCNT-TNT are demonstrated in Fig. 5. The peaks at the particular wavenumber of 1151 cm−1 (symmetric O_S_O stretching), 1242 cm− 1 (asymmetric C\\O\\C stretching), 1293 cm−1 (asymmetric O_S_O stretching), 1409 cm−1 (C_C aromatic ring stretching) and 1503 cm−1 (CH3\\C\\CH3 stretching), corresponding to the specific functional groups of the support membrane made of PS polymer [25,26]. The peaks at 1585 cm−1 and 1552 cm−1 correspond to the C\\N stretching and C_O stretching, respectively which indicates the successful of PA layer formation [27–29]. However, the broadening of the peaks at 1585 cm−1, 1505 cm−1 and 1151 cm−1 of a-MWCNT-TNT/TFN in comparison with those of MWCNT-TNT/TFN suggests that new bond has formed during the IP process due to the presence of adsorbed HCl molecules at MWCNT-TNT. The TGA analysis of TFN membrane (see Fig. 6) revealed a successful modification of the membrane surface upon the incorporation of the acid treated MWCNT-TNT. The a-MWCNT-TNT/TFN membrane decomposed at temperature earlier than that of TFC and MWCNTTNT/TFN membrane. A relatively early decomposition can be detected at 450 °C, which is most likely due to the condensation of remaining hydroxyl molecule on the nanotube [30]. It was suggested that the hydroxyl group at the MWCNT-TNT experienced competition with the adsorbed HCl molecules to bond with the free –COCl groups of TMC which are in accordance with ATR-FTIR analysis (Fig. 5), since there are still remaining –COCl groups of the TMC after the reaction with MPD during IP process [18]. 3.2.2. FESEM, AFM and contact angle analysis The morphology of TFC and TFN membranes was analyzed by FESEM images as shown in Fig. 7. The cross section FESEM images exhibited a typical morphology of an interfacial polymerized PA membrane layers consisted of thin film, PS support layer and a non-woven fabric layer (Fig. 7a). Since the cross section of all the fabricated membranes looks almost similar, one of them was chosen as a representative of all the membranes. Based on the surface FESEM images, both TFC and TFN

Fig. 8. Three dimensional AFM images for TFC (a), MWCNT-TNT/TFN (b) and a-MWCNT-TNT/TFN (c) membranes.

168


Fig. 9. Water permeability and salt rejection of TFC and TFN membranes (test conditions: 15 bar, 25 °C, 2000 ppm aqueous solution of NaCl and Na2SO4).

membranes with PA layer have a rough surface with a nodular structure [8]. However, more visible “leaf-like” structure can be observed for the modified membrane by a-MWCNT-TNT hybrid in Fig. 7d (shown by yellow arrows), which was confirmed in later AFM section. It can be reasonably explained that the less entangled MWCNTs in a-MWCNT-TNT hybrid (Fig. 1b) have facilitated good dispersion of the nanomaterials in PA matrix. Besides that, based on the membrane surface charges (see Table 1), it can be deduced that the positively charged aMWCNT-TNT has established better interactions with the PA layer that is negatively charged, hence the filler can be better incorporated within the PA linkages. The top surface morphological structure of TFC and TFN membrane were examined by AFM images (Fig. 8). From the figures, it is observed that both TFC and TFN membrane have ridge and valley structure PA layer, which distributed throughout the plane [31]. It can also be detected that the embedment of a-MWCNT-TNT in the PA layer of TFN membrane has created higher ridges, as stated previously on the visible “leaflike” structure in the FESEM section (see Fig. 7d), indicating an increment in the surface average roughness (Ra). The surface hydrophilicity of the membrane is evaluated through contact angle measurement. The results are presented in Table 2. The contact angles of the membrane surfaces impregnated with MWCNTTNT/TFN and a-MWCNT-TNT/TFN were lower than the TFC. The contact angle reduced from 76.20 ± 4.34 for TFC to 72.12 ± 6.04 and 69.66 ± 4.79 for MWCNT-TNT/TFN and a-MWCNT-TNT/TFN, respectively. This is most likely related to the hydrophilic functional groups of the added fillers existed on the membrane surface as well as the increased membrane surface roughness. Emadzadeh et al. [18] had incorporated the

hydrophilic functionalized TNTs in the PA layer of TFN membrane and detected that the contact angle of the membrane decreases due to the hydrophilic nature of TNTs as well as increasing the surface roughness. 3.3. RO performance evaluation Fig. 9 shows the water permeability and salts rejection of the TFC and TFN membranes. The TFN membranes incorporated with MWCNT-TNT hybrid and acid modified fillers exhibited higher permeability than the TFC. In particular, the TFN membrane with embedded a-MWCNT-TNT in the PA thin layer showed higher water permeability than the TFN membrane containing unmodified MWCNT-TNT. The water permeability of a-MWCNT-TNT/TFN membrane was 0.74 L/m2 h bar, which was approximately 57% and 19% higher than the TFC and MWCNT-TNT/TFN membranes, respectively. The improvement of water permeability of MWCNT-TNT/TFN membrane was most likely ascribed to the decreasing membrane surface contact angle (see Table 2) and increasing membrane surface roughness [32] (see Fig. 8). The spongy structure of the support membrane may also result in high tortuosity [33], hence lower water permeability. Zhao et al. (2014) [34] indicated that the existence of microvoids and a good water channel of CNTs in the TFN membrane favors the water permeation, however, the salt rejection is slightly affected. Moreover, the presence of the abundant hydroxyl groups on TNTs enhanced the hydrophilic properties [8] of the TFN membrane leading to a preferential flow of water molecules [19]. The NaCl rejection of all the synthesized membranes was above 97% and the incorporation of nanoparticles into PA layer had not significant

Fig. 10. Schematic of bonding formation between PA and a-MWCNT-TNT. The deprotonated of H group from the adsorbed HCl resulted in negative surface charge.


169

Table 3 Comparison of the a-MWCNT-TNT/TFN membrane properties with commercial and in-house made membranes. Membrane

Water permeabilityb (L/m2 h bar)

Salt rejection (%) NaCl

Contact angle (°)

References

CTA-HW (commercial)a CTA-W (commercial)a CTA-NW (commercial)a HTI–NW (commercial)a HTI–ES (commercial)a MWNT/TFN a-MWCNT-TNT/TFN

1.19 0.33 0.46 0.48 0.54 1.75 0.74

78.50c 81.90c 92.40c ~92.0d ~92.0d 90.00e 97.97f

63.00 73.00 64.00 – – – 69.66

[33] [33] [33] [36] [36] [34] This work

a b c d e f

± 3.00 ± 2.00 ± 2.00

± 4.79

All the commercial membrane were manufactured by Hydration Technology Innovations (HTI). Water permeability were measured using dead end RO cell and RO water as feed solution. Were measured in cross flow RO at an applied pressure of 3.75 bar and 1271 ppm NaCl as feed solution. Were measured in cross flow RO at an applied pressure of 2.5 bar and 1271 ppm NaCl as feed solution. Was measured in cross flow RO at an applied pressure of 16 bar and 2000 ppm NaCl as feed solution. Salt rejection were measured using dead end RO cell at an applied pressure of 15 bar and 2000 ppm of both NaCl and Na2SO4 solution.

effect on their rejection. Similarly, the membranes possessed high Na2SO4 rejection (N 96%) without considerable decline by embedding fillers. Meanwhile, the better water permeability and salt rejection of the a-MWCNT-TNT/TFN membrane was most likely due to the enhanced hydrophilicity and high negative charge on the surface of the membrane (see Table 1). In fact, the adsorbed group (⋯Cl\\H) on the surface of MWCNT-TNT reacted with acid chloride groups of PA as illustrated in Fig. 10. It led to the deprotonation of hydrogen from the adsorbed group (⋯Cl\\H) [8]. Therefore, the negative charge value of a-MWCNT-TNT/TFN membrane surface increased as confirmed in zeta potential test and consequently enhanced the salt rejection through the electrostatic interaction towards the negatively charged solutes [35]. To reveal the potential of the hybrid nanoparticles, the TFN membranes containing TiO2, MWCNTs and TNTs nanoparticles were prepared, tested and the results in terms of water permeability and salt rejection were compared with TFN membranes embedded with hybrid fillers. The results demonstrated in Fig. 9 showed higher both water permeability and salt rejection of the membranes with hybrid nanoparticles compared to the TFN membranes which are incorporated with the individual nanoparticles. This confirmed the high potentiality of the synthesized hybrid nanoparticles for water desalination. As the TFN membrane with a-MWCNT-TNT/TFN exhibited the considerable water permeability and salt rejection among the fabricated membranes, its performance was compared with some in-house made and commercial membranes (see Table 3). An in-house carboxyl-functionalized MWCNTs TFN membrane was prepared by Zhao et al. [34] and showed higher water permeability than a-MWCNT-TNT/TFN. However, the mentioned membrane, MWCNT/TFN, suffered from the “tradeoff” between permeability and selectivity which could not be maintained and the salt rejection of the membrane reduced significantly compared to the TFC in contrary to the a-MWCNT-TNT/TFN membrane in this work. The a-MWCNT-TNT/TFN membrane exhibited significantly higher NaCl rejection than all the commercial membranes, while maintaining high water permeability. In particular, the CTA-HW commercial membrane has higher water permeability than a-MWCNT-TNT/TFN and other commercial membranes which is probably due to highly porous support layer containing numerous large finger-like pores as detected from its SEM images. Also, it can be related to the slightly porous PA thin layer of this membrane since the salt rejection was very low of only 78.50%. 4. Conclusions In this study, the hybrid MWCNT-TNT, which was acid treated by HCl (a-MWCNT-TNT), was synthesized and embedded into the PA layer of TFN membrane. The characterization of the prepared aMWCNT-TNT and PA of TFN membrane by different instruments suggested that the desired interaction has occurred during the IP process, which in turn favored the formation of defect-free and highly

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