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Synthesis of highly stable UiO-66-NH2 membranes with high ions rejection for seawater desalination
Microporous and Mesoporous Materials 252 (2017) 207e213
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Synthesis of highly stable UiO-66-NH2 membranes with high ions rejection for seawater desalination Linlin Wan, Chen Zhou, Kai Xu, Bo Feng, Aisheng Huang* Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, CAS, 1219 Zhongguan Road, 315201 Ningbo, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2017 Received in revised form 25 May 2017 Accepted 13 June 2017 Available online 15 June 2017
Compact and phase-pure UiO-66-NH2 membranes were synthesized on the 3-aminopropy-ltriethoxysilane (APTES) modified macroporous Al2O3 tubes through a repeated synthesis strategy. APTES acts as a molecular linker for anchoring the metal ions onto the support surface to promote the nucleation and the crystallization of UiO-66-NH2 membrane. Therefore, well-intergrown UiO-66-NH2 membranes could be prepared through a repeated synthesis method on the APTES-modified macroporous Al2O3 tubes. The developed UiO-66-NH2 membranes were evaluated for seawater desalination by pervaporation. It is found that the UiO-66-NH2 membranes show high desalination performances attributing to the narrow pore size which is exactly in between the size of water molecules and hydrated ions. With increasing the feed temperature from 318 to 363 K, the water fluxes increase from 1.5 to 12.1 kg m2 h1, with ions rejections of above 99.7%. Further, the UiO-66-NH2 membranes display high stability for a long time in seawater desalination, which is very promising for seawater desalination. © 2017 Elsevier Inc. All rights reserved.
Keywords: UiO-66-NH2 membranes Metal-organic frameworks membranes Covalent modification Seawater desalination Pervaporation
1. Introduction Water scarcity has increasingly become a global issue due to the increase of water demand and aggravation of water pollution [1]. Therefore, the development of renewable freshwater resources such as seawater desalination has attracted much attention to resolve the problem of water supply [2e4]. The membrane-based RO is considered to be one of the most promising technologies due to its low energy consumption and ease of operation [5e7]. However, the present polymeric RO membranes usually suffer from biofouling, oxidation, metal oxide fouling, abrasion and mineral scaling because of their low stability [8,9]. Inorganic membranes, such as zeolite membranes [10e17], carbon membrane [18] and graphene oxide (GO) membrane [19e22], are expected to break through these material-based limitations for desalination due to their high thermal and chemical stability. However, it is still highly desired to develop novel molecular sieving membranes that can be facilely prepared and activated, and thus effectively used for seawater desalination. Microporous metal-organic frameworks (MOFs) have drawn much attention for the fabrication of membranes/films due to their
* Corresponding author. E-mail address: [email protected] (A. Huang). http://dx.doi.org/10.1016/j.micromeso.2017.06.025 1387-1811/© 2017 Elsevier Inc. All rights reserved.
highly diversified pore structures and pore sizes as well as specific adsorption affinities. In particular, no SDAs are used in the synthesis of MOF membranes, thus MOF membranes can be easily activated easily under mild conditions (e.g. vacuum or heating with activation temperatures below 423 K). Therefore, MOFs have emerged as a novel type of microporous materials for the fabrication of molecular sieve membranes [24e28]. Especially, zeolitic imidazolate framework (ZIF) membranes have attracted intense interest for gas separation due to their exceptionally thermal and chemical stability [29e41]. Recently, we have reported the synthesis of ZIF membranes for seawater desalination [42]. Attributing to the small aperture size as well as high stability in seawater, the developed ZIF membranes display high separation performances for seawater desalination. Attributing to the high hydrothermal and chemical stability of UiO-66 [43], especially its high resistance towards humidity, UiO66 has been drawn much interest for many practical applications in dye separation and seawater desalination [43e45]. Further, the BDC linker can by modified by introducing functional groups like -NH2, -Br, -F, -OH, -COOH, -CH3 to obtain derivatives of UiO-66 with special adsorption properties [46]. Among these derivatives, UiO66-NH2 is of special interest for gas adsorption and separation [47,48]. UiO-66-NH2 has a same topological structure with UiO-66, consisting of highly symmetric Zr6(OH)4O4 clusters and 2-amino1,4-benzenedi-carboxalate (NH2-BDC) linkers (Fig. 1). As reported
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outside diameter, 9 mm inside diameter, 75 mm length, ca. 1.0 mm pore size, 30% porosity) were used as supports. 2.2. Synthesis of UiO-66-NH2 membranes
Fig. 1. Topological structure of UiO-66-NH2 consisting of Zr6(OH)4O4 clusters and NH2BDC linkers.
previously [49], UiO-66-NH2 shows permanent microporosity with narrow pores size (about 0.52 ± 0.02 nm), which is located between the size of water molecules (0.26 nm) and hydrated ions [50] (e.g. Naþ 0.72 nm, Kþ 0.66 nm, Ca2þ 0.82 nm, Mg2þ 0.86 nm, Cl 0.66 nm, Table 1). It can be expected that UiO-66-NH2 membranes can show high ion rejections in seawater desalination by ionic sieving. To the best of our knowledge, however, no studies on UiO66-NH2 membranes have been reported for seawater desalination. In the present work, we report the synthesis of high stable UiO-66NH2 membranes for seawater desalination by using 3aminopropyltriethoxysilane (APTES) as a covalent linker between the UiO-66-NH2 membrane and the porous a-Al2O3 support [51,52]. 2. Experimental
All chemicals were obtained commercially and used without further purification: zirconium (IV) chloride (ZrCl4, Sigma, 99.5% metals basis), 2-aminoterephthalic acid (2-NH2-BDC, Alfa Aesar, 99%), N, N-dimethylformamide (DMF, Aladdin, >99.9% GC), 3aminopropyltriethoxysilane (APTES, Aladdin, 98%). Porous aAl2O3 tubes (Jiexi Lishun Technology Co., Guangdong, China: 12 mm Table 1 Ion concentrations in the feed and permeate as well as ion rejection of the UiO-66NH2 membrane prepared on APTES-modified a-Al2O3 tube for desalination of 3.5 wt % seawater at 348 K.
þ
Na Kþ Mg2þ Ca2þ NHþ 4 F Cl NO 3 a b
Hydrated diametera (nm)
0.72 0.66 0.86 0.82 0.66 0.70 0.66 0.68
2.3. Characterizations of UiO-66-NH2 membranes The morphology and thickness of the UiO-66-NH2 membrane were characterized by field emission scanning electron microscopy (FESEM) on an S-4800 (Hitachi) with a cold field emission gun operating at 4 kV and 10 mA. X-ray diffraction (XRD) was applied to confirm the phase structure of the UiO-66-NH2 membranes at room temperature under ambient pressure, which was recorded on a Bruker D8 Advance operating at 40 kV and 40 mA with a Cu Ka1 radiation source (l ¼ 0.154056 nm) at a stepwise increase of 2 $s1 in the Bragg angle (2q) range from 5 to 50 . 2.4. Seawater desalination measurement through the UiO-66-NH2 membranes by pervaporation
2.1. Materials
Main ions in feed
The UiO-66-NH2 membranes were prepared on non-modified or APTES-modified a-Al2O3 tubes according to the procedure as reported in elsewhere [45] with minor modification. Before membrane synthesis, the a-Al2O3 tubes were modified with APTES (0.2 mM in 10 mL toluene) at 383 K for 1 h under argon, leading to an APTES monolayer deposited on the surface of a-Al2O3 supports [51, 52]. In order to prepare UiO-66-NH2 membrane on the nonmodified or APTES-modified a-Al2O3 tube, a synthesis solution with the molar ratio of 1 ZrCl4: 1 NH2-BDC: 1 H2O:500 DMF was prepared by mixing ZrCl4, NH2-BDC, H2O and DMF under vigorous stirring, and stirring was continued to produce a clear and homogenous solution. The non-modified or APTES-modified a-Al2O3 tubes were vertically placed in a Teflon autoclave, and then the synthesis solution was poured into the autoclave. After in-situ growth for 24 h at 393 K, the UiO-66-NH2 membranes were grown in-situ on the outside of the APTES-modified Al2O3 tubes. Afterwards, the as-synthesized membrane was washed with DMF and dried at room temperature for characterization and seawater desalination permeation measurement. In order to prepare denser UiO-66-NH2 membranes, the above process of hydrothermal synthesis was repeated.
Ion concentration (ppm) Feed
Permeate
14061.00 609.50 894.20 141.30 223.30 3.57 27765.00 3527.40
8.05 n.a.b 0.39 n.a. n.a. n.a. 3.02 5.64
Hydrated diameters are adopted from Ref. [50]. Below the detection limit of equipment.
Rejection (%)
99.94 100.00 99.96 100.00 100.00 100.00 99.98 99.84
The integrity of the as-synthesized UiO-66-NH2 membrane is confirmed by measurement of single gas permeation by a soap-film flow-meter. For the single gas permeation, the UiO-66-NH2 membrane prepared on the APTES-modified a-Al2O3 tube was sealed in a permeation tubular module, and the permeation of the single gas CO2, N2 was tested at 298 K and 1 bar across the membrane. The separation performance of the UiO-66-NH2 membrane was evaluated for seawater desalination by pervaporation [19,20]. The seawater was prepared by dissolving sea salts in deionized water with concentrations ranging from 2 to 10 wt%. Unless mentioned, the conventional concentration of the seawater is 3.5 wt%, and the details of the corresponding ion concentration in the 3.5 wt% seawater was listed in Table 1. The supported UiO-66-NH2 membranes were sealed in a permeation module with silicone O-ring, and the seawater preheated to 318e363 K was fed to the membrane side. The whole membrane module was heated in a thermostatic water bath together with the feed solution. The permeate side of the membrane was evacuated with a vacuum pump. Two freezing traps with liquid N2 cooling were used to collect the permeation at every time interval. Continuous stirring and recirculation of the feed solution was utilized to prevent the concentration polarization on the contact membrane side. The water flux has been determined by the amount of water collected in the two cooling traps. For the
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determination of ion rejection, the permeate side of the membrane (i.e. the support side, since the UiO-66-NH2 layer was facing to the feed seawater) was washed periodically after certain times with deionized water to collect possibly permeated salts which have been deposited as crystals on the top or inside of the porous alumina supports [12]. The performance of seawater desalination was assessed in terms of water flux (F) and total ion rejection (Ri). The water flux, F (kg$m2$h1), was calculated by
F¼
W
Dt$A
(1)
where W is the permeate mass (kg), Dt is collecting time (h), A is the membrane area (m2). The total ion rejection was defined as the ratio of the ion conductivity in the permeate to that in the feed, and given by
Ri ¼
Sif Sip 100% Sif
(2)
where Sif and SiP are the ion conductivities in the feed and permeate, respectively. Both Sif and SiP were analyzed after water dilution by 100 times. The ions conductivities of the feed and the permeate were measured by conductivity meter (DDSJ-308A, Shanghai REX Instrument Factory). The ion rejection for every ion was defined as the ratio of the ion concentration in the permeate to that in the feed, and given by
Ri ¼
Cif Cip 100% Cif
(3)
where Cif and CiP are the ion concentration in the feed and permeate, respectively. Both Cif and CiP were analyzed after water dilution by 100 times. The concentrations of cations (Naþ, Kþ, Ca2þ, Mg2þ, NHþ 4 ) and anions (F , Cl , and NO3 ) in the feed and the permeate were analyzed by ion chromatography (Dionex Corporation, USA) with an AS-19 column and ASRS 4 mm suppressor. 3. Results and discussions 3.1. Synthesis and characterizations of UiO-66-NH2 membranes Since the organic linker NH2-BDC of UiO-66-NH2 is difficult to form covalent bonds with the hydroxy groups on the supports surface, the heterogeneous nucleation of UiO-66-NH2 membrane on the alumina supports surface is very poor. Therefore, coating of the supports with seed crystals to provide heterogeneous nucleation centers for membrane MOF growth is usually indispensable [33,35]. Indeed, in our first attempt to synthesize a UiO-66-NH2 membrane on the non-modified a-Al2O3 tubes, we find that there is no dense UiO-66-NH2 membrane can be formed on the nonmodified a-Al2O3 tube. Therefore, we try to synthesize UiO-66NH2 membranes on the APTES-modified a-Al2O3 tubes. Fig. 2 shows the FESEM images of the UiO-66-NH2 membrane prepared on the APTES-modified a-Al2O3 tube at 393 K for 24 h. It can be seen that the support surface has been covered with sub-micron UiO66-NH2 crystals and form a thin layer with a thickness of about 0.4 mm after one-step synthesis (Fig. 2a and b), but observable intercrystalline defects are still observed in the UiO-66-NH2 membrane layer, indicating that the heterogeneous nucleation and the growth of a dense UiO-66-NH2 membrane is more challenging on the coarse and macroporous a-Al2O3 tubes. In order to prepare a dense UiO-66-NH2 membrane, a repeated synthesis is developed to promote the heterogeneous nucleation
209
and the growth of the UiO-66-NH2 layer. Fig. 2c shows the FESEM image of the UiO-66-NH2 membrane prepared on the APTESmodified a-Al2O3 tubes after a two-step synthesis. It can be seen that after repeated synthesis at 393 K for 24 h, the surface of the aAl2O3 tubes is completely covered by compact octahedral UiO-66NH2 crystals, and no visible cracks, pinholes or other defects are observed in the membrane layer. From the cross-section view shown in Fig. 2d, it can be seen that the membrane is well intergrown with a thickness of about 1.0 mm. Since the support pores are partially filled with UiO-66-NH2 crystals, resulting in an unclear border line between the external UiO-66-NH2 layer and the alumina support. The density of the as-synthesized UiO-66-NH2 membrane is also confirmed by measurement of single gas permeation with a soap-film flow-meter. The as-synthesized UiO66-NH2 membrane shows CO2 and N2 permeance of 2.73 107 and 1.47 108 mol m2 s1$Pa1, respectively. Thus, an ideal separation factor (determined as the ratio of the single component permeance) of CO2/N2 is 18.5, which by far exceed the corresponding Knudsen coefficients (1.8), suggesting that the UiO-66NH2 membrane prepared on the APTES-modified a-Al2O3 tube displays a high gas separation selectivity. The formation of a phasepure UiO-66-NH2 membrane with a high degree of crystallinity was confirmed by XRD (Fig. 3), which indicates that all peaks match well with those of UiO-66-NH2 powder besides the a-Al2O3 signals from the support. It is worth to note that APTES-modification of the a-Al2O3 tube is indispensable for the formation a dense UiO-66NH2 membrane. As shown in Fig. S1, no continuous UiO-66-NH2 membrane can be formed on the non-modified macroporous Al2O3 tube after two-step synthesis, and observable gaps can be observed between the UiO-66-NH2 membrane and Al2O3 support. 3.2. Desalination performance of the UiO-66-NH2 membrane by pervaporation Pervaporation has been widely studied for the separation of organics/water mixtures due to its low energy consumption and ease of operation [53]. Recently, pervaporation is also recommended to be a promising technique for seawater desalination [15e17]. Fig. 4 shows the water flux and ion rejection of the UiO-66NH2 membrane prepared on APTES-modified a-Al2O3 tube as a function of the operation temperature for desalination of 3.5 wt% seawater by pervaporation. It can be seen that all the ion rejections are over 99.7% in the temperature range of 318e363 K, indicating that the UiO-66-NH2 membrane displays high desalination perþ 2þ 2þ formance. In fact, the ion rejection of Naþ, NHþ 4 , K , Mg , Ca , Cl, NO3, F is 99.94, 100.00, 100.00, 99.96, 100.00, 99.98, 99.84, 100%, respectively (Table 1). These results are in good agreement with the previous report that the UiO-66 membrane is very effective for seawater desalination [45]. Since there are observable gaps between the UiO-66-NH2 membrane and Al2O3 support, the UiO-66NH2 membrane prepared on the non-modified Al2O3 tube show low ion rejections. At 348 K, the ion rejection is only 90%, and the ion rejection of the UiO-66-NH2 membrane sharply decreases to 44% when the feed temperature increases to 363 K (Fig. S2). When the feed temperature increased from 318 to 363 K, the water flux increased from 1.5 to 12.1 kg m2 h1, and the ion rejection of the UiO-66-NH2 membrane keeps almost unchanged (Fig. 4). The increase of the water flux with increasing temperature is attributed to the increase of the driving force of water permeation. According to Arrhenius law, when the feed temperature increases, the water vapor pressure on the feed side exponentially increases, while the vapor pressure on the permeate side keeps constant, then leading to an enhancement of the driving force across the membrane. Further, when the temperature increases, thermal motion of water molecules in solutions is drastic, thus the
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Fig. 2. FESEM images of the UiO-66-NH2 membrane prepared on APTES-modified macroporous Al2O3 substrate after one-step synthesis (a, b), and two-step synthesis (c, d). (a, c) top view, and (b, d) cross-section view. The circles in (a) indicate the inter-crystal defects in the membrane layer.
Fig. 4. The water flux and ion rejection of UiO-66-NH2 membrane prepared on APTESmodified a-Al2O3 tube for desalination of 3.5 wt% seawater by pervaporation as a function of the operation temperature. Fig. 3. XRD patterns of the simulated UiO-66-NH2 (a), UiO-66-NH2 powder collected from membrane synthesis solution (b), and UiO-66-NH2 membranes prepared on APTES-modified a-Al2O3 tube (c). Peaks marked with asterisk show the a-Al2O3 tube.
diffusion rate of water molecules through the membrane is also accelerated. Consequently, the water flux of the UiO-66-NH2 membrane increase with increasing of temperature. Fig. 5 shows an Arrhenius type plot with ln (flux) versus 1/T for water permeation through the UiO-66-NH2 membranes. According to an Arrhenius relationship, as shown in Equation (4), the linear trend in the plot suggests that water permeation through the UiO66-NH2 membranes is an activated diffusion process. Based on the slope of Arrhenius plot, the activation energy of the water permeation (Ea) through the UiO-66-NH2 membrane is calculated about 46.8 kJ mol1. Furthermore, the positive value of the activation energy indicates that the water flux increases with increasing the feed temperature.
Ji ¼ Ai $exp$ Ep;i RT
(4)
where Ji is the permeate flux of the membrane, Ai is the preexponential factor, R is the gas constant, T is the absolute temperature and Ep,i is the apparent activation energy for diffusion and the heat of sorption. Benefiting from the existence of hydrophilic adsorption sites (hydroxyl groups and amino groups) in the framework, both the water flux and ion rejection of the UiO-66-NH2 membrane is rather high. At 348 K, a relatively high water flux of about 7.5 kg m2 h1 and ion rejection of 99.8% can be obtained for desalination of 3.5 wt % seawater. Comparing with the reports of zeolite membranes for seawater desalination [15,16,54], the as-synthesized UiO-66-NH2 membrane developed in the present work shows higher water flux due to its high hydrophilicity and large pore size, while maintaining
L. Wan et al. / Microporous and Mesoporous Materials 252 (2017) 207e213
Fig. 5. The Arrhenius plot for the water flux and the feed temperature through the UiO-66-NH2 membrane prepared on APTES-modified a-Al2O3 tube for desalination of 3.5 wt% seawater.
high ion rejection. These results suggest that UiO-66-NH2 membrane is an attracting candidate for production of pure water through seawater desalination. Fig. 6 shows the water flux and ion rejection of the UiO-66-NH2 membrane prepared on APTES-modified a-Al2O3 tube as a function of the feed concentration at 348 K. It can be seen that the water flux decreases with increasing the feed concentration, i.e., the water flux decreases from 8.0 to 4.9 kg m2 h1 when the feed concentration increases from 2 to 10 wt%, while the ion rejection rate still maintains high (over 99.7%) even for desalination of 10 wt% seawater. With increasing the feed concentration, the concentration polarization takes place at the interface between the feed solution and the membrane [55]. Thus, more ionic clusters are formed and the electrostatic interaction between the ions and the polarized water become stronger. Therefore, on one hand, the driving forces (effective pressure to overcome the osmotic pressure) for water permeation decreases, leading to the reduction of water flux [56]. On the other hand, the deposition of salts on the outer surface of the membrane becomes more severe since more salts are recrystallized from the mixtures. Consequently, the channels for water passing are blocked and thus causing the decrease of water flux.
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Fig. 7. Water flux and ion rejection of the UiO-66-NH2 membrane prepared on APTESmodified a-Al2O3 tube for desalination of 3.5 wt% seawater for 120 h at 348 K by pervaporation.
3.3. Stability evaluation of UiO-66-NH2 membrane for seawater desalination As reported previously [57], UiO-66-NH2 show high chemical and hydrothermal stability. The studied on the stability of UiO-66NH2 membrane was evaluated for desalination of 3.5 wt% seawater at 348 K for 120 h. As shown in Fig. 7, both the water flux (7.5 kg m2 h1) and ion rejection (over 99.7%) are almost unchanged after 120 h. With further increasing the testing time, the water flux begins to decrease slightly. As the testing time is extended to 120 h, the water flux decreases to 6.7 kg m2 h1, while the ion rejection still maintains above 99.7%. The slight decrease of the water flux after 60 h might be attributed to the following reasons [15]: (1) the feed concentration increases with increasing time on stream, leading to the decrease of water flux, which is in agreement with Fig. 6; (2) the salts will be accumulated on the surface of the membrane with increasing testing time, which will block the channels and/or pores of the UiO-66-NH2 membrane, and thus increasing the resistance of mass transferring. In good agreement with the high desalination performances of the UiO-66-NH2 membrane for a long time measurement, there are no remarkable differences in membrane morphology after the measurement of seawater desalination at 348 K for 120 h, and the UiO-66-NH2 membrane is still well intergrown without any visible cracks, pinholes or other defects (Fig. 8). A typical XRD pattern (Fig. S3) shows that the high crystallinity of the UiO-66-NH2 membrane remains unchanged after the measurement of seawater desalination. All the XRD peaks of the UiO-66-NH2 membrane match well with those of the as-synthesized UiO-66-NH2 membrane, indicating that the unusual thermal and chemical stability of UiO-66-NH2 membrane allows it being used for seawater desalination under relative harsh conditions. Further, in complete accordance with the high stability of the UiO-66-NH2 membrane, no notable changes in XRD patterns of the UiO-66-NH2 powder collected from the membrane synthesis can be observed after 200 h treatment with 3.5 wt% seawater (Fig. S4), further confirming the high stability of UiO-66-NH2 in contact with seawater. 4. Conclusions
Fig. 6. Water flux and ion rejection of the UiO-66-NH2 membrane prepared on APTESmodified a-Al2O3 tube for seawater desalination as a function of the feed concentration at 348 K.
Attributing to their highly diversified pore structures and pore sizes as well as specific adsorption affinities, MOFs membranes have been widely studied in gas separation. Considering the narrow
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Fig. 8. Top view (a) and cross-section (b) FESEM images of the UiO-66-NH2 membrane prepared on the APTES-modified a-Al2O3 tube after desalination of 3.5 wt% seawater for 120 h at 348 K.
pore size of the MOF which is exactly in between the size of water molecules and hydrated ions, MOFs membranes are also potentially promising in liquid separation and seawater desalination. In this work, we have prepared thin, phase-pure and well intergrown UiO66-NH2 membranes for seawater desalination on the APTESmodified macroporous a-Al2O3 tubes. Attributing to the relatively moderate pore size and the hydrophilic adsorption sites in the framework, the developed UiO-66-NH2 membranes display high separation performances for seawater desalination. At 348 K for desalination of 3.5 wt% seawater, the water flux through the UiO66-NH2 membrane is 7.5 kg m2 h1, with ion rejections of over 99.7%. Further, the UiO-66-NH2 membranes can keep an excellent separation performance during a long-term measurement at 348 K for 120 h due to its high chemical stability. Acknowledgements This work is supported by the National Natural Science Foundation of China (21606246), Ningbo Science and Technology Innovation Team (2014B81004), and Ningbo Municipal Natural Science Foundation (2015A610055). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2017.06.025. References [1] R.F. Service, Desalination freshens up, Science 311 (2006) 1088e1090. ~ as, [2] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marin A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301e310. [3] Q. Schiermeier, Purification with a pinch of salt, Nature 452 (2008) 260e261. [4] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712e717. [5] M.G. Marcovecchio, S.F. Mussati, P.A. Aguirre, N.J. Scenna, Optimization of hybrid desalination processes including multi stage flash and reverse osmosis systems, Desalination 182 (2005) 111e122. [6] A.D. Khawaji, I.K. Kutubkhanah, J.-M. Wie, Advances in seawater desalination technologies, Desalination 221 (2008) 47e69. [7] W. Zhu, L. Gora, A.W.C. van den Berg, F. Kapteijn, J.C. Jansen, J.A. Moulijn, Water vapour separation from permanent gases by a zeolite-4A membrane, J. Membr. Sci. 253 (2005) 57e66. [8] J. Glater, S.K. Hong, M. Elimelech, The search for a chloring-resistant reverseosmosis membrane, Desalination 95 (1994) 325e345. [9] H. El-Saied, A.H. Basta, B.N. Barsoum, M.M. Elberry, Cellulose membranes for reverse osmosis Part I. RO cellulose acetate membranes including a composite with polypropylene, Desalination 159 (2003) 171e181. [10] L. Li, J. Dong, T.M. Nenof, R. Lee, Desalination by reverse osmosis using MFI zeolite membranes, J. Membr. Sci. 243 (2004) 401e404. [11] L. Li, N. Liu, B. McPherson, R. Lee, Influence of counter ions on the reverse osmosis through MFI zeolite membranes: implications for produced water desalination, Desalination 228 (2008) 217e225.
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Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso
Synthesis of highly stable UiO-66-NH2 membranes with high ions rejection for seawater desalination Linlin Wan, Chen Zhou, Kai Xu, Bo Feng, Aisheng Huang* Institute of New Energy Technology, Ningbo Institute of Materials Technology and Engineering, CAS, 1219 Zhongguan Road, 315201 Ningbo, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 5 April 2017 Received in revised form 25 May 2017 Accepted 13 June 2017 Available online 15 June 2017
Compact and phase-pure UiO-66-NH2 membranes were synthesized on the 3-aminopropy-ltriethoxysilane (APTES) modified macroporous Al2O3 tubes through a repeated synthesis strategy. APTES acts as a molecular linker for anchoring the metal ions onto the support surface to promote the nucleation and the crystallization of UiO-66-NH2 membrane. Therefore, well-intergrown UiO-66-NH2 membranes could be prepared through a repeated synthesis method on the APTES-modified macroporous Al2O3 tubes. The developed UiO-66-NH2 membranes were evaluated for seawater desalination by pervaporation. It is found that the UiO-66-NH2 membranes show high desalination performances attributing to the narrow pore size which is exactly in between the size of water molecules and hydrated ions. With increasing the feed temperature from 318 to 363 K, the water fluxes increase from 1.5 to 12.1 kg m2 h1, with ions rejections of above 99.7%. Further, the UiO-66-NH2 membranes display high stability for a long time in seawater desalination, which is very promising for seawater desalination. © 2017 Elsevier Inc. All rights reserved.
Keywords: UiO-66-NH2 membranes Metal-organic frameworks membranes Covalent modification Seawater desalination Pervaporation
1. Introduction Water scarcity has increasingly become a global issue due to the increase of water demand and aggravation of water pollution [1]. Therefore, the development of renewable freshwater resources such as seawater desalination has attracted much attention to resolve the problem of water supply [2e4]. The membrane-based RO is considered to be one of the most promising technologies due to its low energy consumption and ease of operation [5e7]. However, the present polymeric RO membranes usually suffer from biofouling, oxidation, metal oxide fouling, abrasion and mineral scaling because of their low stability [8,9]. Inorganic membranes, such as zeolite membranes [10e17], carbon membrane [18] and graphene oxide (GO) membrane [19e22], are expected to break through these material-based limitations for desalination due to their high thermal and chemical stability. However, it is still highly desired to develop novel molecular sieving membranes that can be facilely prepared and activated, and thus effectively used for seawater desalination. Microporous metal-organic frameworks (MOFs) have drawn much attention for the fabrication of membranes/films due to their
* Corresponding author. E-mail address: [email protected] (A. Huang). http://dx.doi.org/10.1016/j.micromeso.2017.06.025 1387-1811/© 2017 Elsevier Inc. All rights reserved.
highly diversified pore structures and pore sizes as well as specific adsorption affinities. In particular, no SDAs are used in the synthesis of MOF membranes, thus MOF membranes can be easily activated easily under mild conditions (e.g. vacuum or heating with activation temperatures below 423 K). Therefore, MOFs have emerged as a novel type of microporous materials for the fabrication of molecular sieve membranes [24e28]. Especially, zeolitic imidazolate framework (ZIF) membranes have attracted intense interest for gas separation due to their exceptionally thermal and chemical stability [29e41]. Recently, we have reported the synthesis of ZIF membranes for seawater desalination [42]. Attributing to the small aperture size as well as high stability in seawater, the developed ZIF membranes display high separation performances for seawater desalination. Attributing to the high hydrothermal and chemical stability of UiO-66 [43], especially its high resistance towards humidity, UiO66 has been drawn much interest for many practical applications in dye separation and seawater desalination [43e45]. Further, the BDC linker can by modified by introducing functional groups like -NH2, -Br, -F, -OH, -COOH, -CH3 to obtain derivatives of UiO-66 with special adsorption properties [46]. Among these derivatives, UiO66-NH2 is of special interest for gas adsorption and separation [47,48]. UiO-66-NH2 has a same topological structure with UiO-66, consisting of highly symmetric Zr6(OH)4O4 clusters and 2-amino1,4-benzenedi-carboxalate (NH2-BDC) linkers (Fig. 1). As reported
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outside diameter, 9 mm inside diameter, 75 mm length, ca. 1.0 mm pore size, 30% porosity) were used as supports. 2.2. Synthesis of UiO-66-NH2 membranes
Fig. 1. Topological structure of UiO-66-NH2 consisting of Zr6(OH)4O4 clusters and NH2BDC linkers.
previously [49], UiO-66-NH2 shows permanent microporosity with narrow pores size (about 0.52 ± 0.02 nm), which is located between the size of water molecules (0.26 nm) and hydrated ions [50] (e.g. Naþ 0.72 nm, Kþ 0.66 nm, Ca2þ 0.82 nm, Mg2þ 0.86 nm, Cl 0.66 nm, Table 1). It can be expected that UiO-66-NH2 membranes can show high ion rejections in seawater desalination by ionic sieving. To the best of our knowledge, however, no studies on UiO66-NH2 membranes have been reported for seawater desalination. In the present work, we report the synthesis of high stable UiO-66NH2 membranes for seawater desalination by using 3aminopropyltriethoxysilane (APTES) as a covalent linker between the UiO-66-NH2 membrane and the porous a-Al2O3 support [51,52]. 2. Experimental
All chemicals were obtained commercially and used without further purification: zirconium (IV) chloride (ZrCl4, Sigma, 99.5% metals basis), 2-aminoterephthalic acid (2-NH2-BDC, Alfa Aesar, 99%), N, N-dimethylformamide (DMF, Aladdin, >99.9% GC), 3aminopropyltriethoxysilane (APTES, Aladdin, 98%). Porous aAl2O3 tubes (Jiexi Lishun Technology Co., Guangdong, China: 12 mm Table 1 Ion concentrations in the feed and permeate as well as ion rejection of the UiO-66NH2 membrane prepared on APTES-modified a-Al2O3 tube for desalination of 3.5 wt % seawater at 348 K.
þ
Na Kþ Mg2þ Ca2þ NHþ 4 F Cl NO 3 a b
Hydrated diametera (nm)
0.72 0.66 0.86 0.82 0.66 0.70 0.66 0.68
2.3. Characterizations of UiO-66-NH2 membranes The morphology and thickness of the UiO-66-NH2 membrane were characterized by field emission scanning electron microscopy (FESEM) on an S-4800 (Hitachi) with a cold field emission gun operating at 4 kV and 10 mA. X-ray diffraction (XRD) was applied to confirm the phase structure of the UiO-66-NH2 membranes at room temperature under ambient pressure, which was recorded on a Bruker D8 Advance operating at 40 kV and 40 mA with a Cu Ka1 radiation source (l ¼ 0.154056 nm) at a stepwise increase of 2 $s1 in the Bragg angle (2q) range from 5 to 50 . 2.4. Seawater desalination measurement through the UiO-66-NH2 membranes by pervaporation
2.1. Materials
Main ions in feed
The UiO-66-NH2 membranes were prepared on non-modified or APTES-modified a-Al2O3 tubes according to the procedure as reported in elsewhere [45] with minor modification. Before membrane synthesis, the a-Al2O3 tubes were modified with APTES (0.2 mM in 10 mL toluene) at 383 K for 1 h under argon, leading to an APTES monolayer deposited on the surface of a-Al2O3 supports [51, 52]. In order to prepare UiO-66-NH2 membrane on the nonmodified or APTES-modified a-Al2O3 tube, a synthesis solution with the molar ratio of 1 ZrCl4: 1 NH2-BDC: 1 H2O:500 DMF was prepared by mixing ZrCl4, NH2-BDC, H2O and DMF under vigorous stirring, and stirring was continued to produce a clear and homogenous solution. The non-modified or APTES-modified a-Al2O3 tubes were vertically placed in a Teflon autoclave, and then the synthesis solution was poured into the autoclave. After in-situ growth for 24 h at 393 K, the UiO-66-NH2 membranes were grown in-situ on the outside of the APTES-modified Al2O3 tubes. Afterwards, the as-synthesized membrane was washed with DMF and dried at room temperature for characterization and seawater desalination permeation measurement. In order to prepare denser UiO-66-NH2 membranes, the above process of hydrothermal synthesis was repeated.
Ion concentration (ppm) Feed
Permeate
14061.00 609.50 894.20 141.30 223.30 3.57 27765.00 3527.40
8.05 n.a.b 0.39 n.a. n.a. n.a. 3.02 5.64
Hydrated diameters are adopted from Ref. [50]. Below the detection limit of equipment.
Rejection (%)
99.94 100.00 99.96 100.00 100.00 100.00 99.98 99.84
The integrity of the as-synthesized UiO-66-NH2 membrane is confirmed by measurement of single gas permeation by a soap-film flow-meter. For the single gas permeation, the UiO-66-NH2 membrane prepared on the APTES-modified a-Al2O3 tube was sealed in a permeation tubular module, and the permeation of the single gas CO2, N2 was tested at 298 K and 1 bar across the membrane. The separation performance of the UiO-66-NH2 membrane was evaluated for seawater desalination by pervaporation [19,20]. The seawater was prepared by dissolving sea salts in deionized water with concentrations ranging from 2 to 10 wt%. Unless mentioned, the conventional concentration of the seawater is 3.5 wt%, and the details of the corresponding ion concentration in the 3.5 wt% seawater was listed in Table 1. The supported UiO-66-NH2 membranes were sealed in a permeation module with silicone O-ring, and the seawater preheated to 318e363 K was fed to the membrane side. The whole membrane module was heated in a thermostatic water bath together with the feed solution. The permeate side of the membrane was evacuated with a vacuum pump. Two freezing traps with liquid N2 cooling were used to collect the permeation at every time interval. Continuous stirring and recirculation of the feed solution was utilized to prevent the concentration polarization on the contact membrane side. The water flux has been determined by the amount of water collected in the two cooling traps. For the
L. Wan et al. / Microporous and Mesoporous Materials 252 (2017) 207e213
determination of ion rejection, the permeate side of the membrane (i.e. the support side, since the UiO-66-NH2 layer was facing to the feed seawater) was washed periodically after certain times with deionized water to collect possibly permeated salts which have been deposited as crystals on the top or inside of the porous alumina supports [12]. The performance of seawater desalination was assessed in terms of water flux (F) and total ion rejection (Ri). The water flux, F (kg$m2$h1), was calculated by
F¼
W
Dt$A
(1)
where W is the permeate mass (kg), Dt is collecting time (h), A is the membrane area (m2). The total ion rejection was defined as the ratio of the ion conductivity in the permeate to that in the feed, and given by
Ri ¼
Sif Sip 100% Sif
(2)
where Sif and SiP are the ion conductivities in the feed and permeate, respectively. Both Sif and SiP were analyzed after water dilution by 100 times. The ions conductivities of the feed and the permeate were measured by conductivity meter (DDSJ-308A, Shanghai REX Instrument Factory). The ion rejection for every ion was defined as the ratio of the ion concentration in the permeate to that in the feed, and given by
Ri ¼
Cif Cip 100% Cif
(3)
where Cif and CiP are the ion concentration in the feed and permeate, respectively. Both Cif and CiP were analyzed after water dilution by 100 times. The concentrations of cations (Naþ, Kþ, Ca2þ, Mg2þ, NHþ 4 ) and anions (F , Cl , and NO3 ) in the feed and the permeate were analyzed by ion chromatography (Dionex Corporation, USA) with an AS-19 column and ASRS 4 mm suppressor. 3. Results and discussions 3.1. Synthesis and characterizations of UiO-66-NH2 membranes Since the organic linker NH2-BDC of UiO-66-NH2 is difficult to form covalent bonds with the hydroxy groups on the supports surface, the heterogeneous nucleation of UiO-66-NH2 membrane on the alumina supports surface is very poor. Therefore, coating of the supports with seed crystals to provide heterogeneous nucleation centers for membrane MOF growth is usually indispensable [33,35]. Indeed, in our first attempt to synthesize a UiO-66-NH2 membrane on the non-modified a-Al2O3 tubes, we find that there is no dense UiO-66-NH2 membrane can be formed on the nonmodified a-Al2O3 tube. Therefore, we try to synthesize UiO-66NH2 membranes on the APTES-modified a-Al2O3 tubes. Fig. 2 shows the FESEM images of the UiO-66-NH2 membrane prepared on the APTES-modified a-Al2O3 tube at 393 K for 24 h. It can be seen that the support surface has been covered with sub-micron UiO66-NH2 crystals and form a thin layer with a thickness of about 0.4 mm after one-step synthesis (Fig. 2a and b), but observable intercrystalline defects are still observed in the UiO-66-NH2 membrane layer, indicating that the heterogeneous nucleation and the growth of a dense UiO-66-NH2 membrane is more challenging on the coarse and macroporous a-Al2O3 tubes. In order to prepare a dense UiO-66-NH2 membrane, a repeated synthesis is developed to promote the heterogeneous nucleation
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and the growth of the UiO-66-NH2 layer. Fig. 2c shows the FESEM image of the UiO-66-NH2 membrane prepared on the APTESmodified a-Al2O3 tubes after a two-step synthesis. It can be seen that after repeated synthesis at 393 K for 24 h, the surface of the aAl2O3 tubes is completely covered by compact octahedral UiO-66NH2 crystals, and no visible cracks, pinholes or other defects are observed in the membrane layer. From the cross-section view shown in Fig. 2d, it can be seen that the membrane is well intergrown with a thickness of about 1.0 mm. Since the support pores are partially filled with UiO-66-NH2 crystals, resulting in an unclear border line between the external UiO-66-NH2 layer and the alumina support. The density of the as-synthesized UiO-66-NH2 membrane is also confirmed by measurement of single gas permeation with a soap-film flow-meter. The as-synthesized UiO66-NH2 membrane shows CO2 and N2 permeance of 2.73 107 and 1.47 108 mol m2 s1$Pa1, respectively. Thus, an ideal separation factor (determined as the ratio of the single component permeance) of CO2/N2 is 18.5, which by far exceed the corresponding Knudsen coefficients (1.8), suggesting that the UiO-66NH2 membrane prepared on the APTES-modified a-Al2O3 tube displays a high gas separation selectivity. The formation of a phasepure UiO-66-NH2 membrane with a high degree of crystallinity was confirmed by XRD (Fig. 3), which indicates that all peaks match well with those of UiO-66-NH2 powder besides the a-Al2O3 signals from the support. It is worth to note that APTES-modification of the a-Al2O3 tube is indispensable for the formation a dense UiO-66NH2 membrane. As shown in Fig. S1, no continuous UiO-66-NH2 membrane can be formed on the non-modified macroporous Al2O3 tube after two-step synthesis, and observable gaps can be observed between the UiO-66-NH2 membrane and Al2O3 support. 3.2. Desalination performance of the UiO-66-NH2 membrane by pervaporation Pervaporation has been widely studied for the separation of organics/water mixtures due to its low energy consumption and ease of operation [53]. Recently, pervaporation is also recommended to be a promising technique for seawater desalination [15e17]. Fig. 4 shows the water flux and ion rejection of the UiO-66NH2 membrane prepared on APTES-modified a-Al2O3 tube as a function of the operation temperature for desalination of 3.5 wt% seawater by pervaporation. It can be seen that all the ion rejections are over 99.7% in the temperature range of 318e363 K, indicating that the UiO-66-NH2 membrane displays high desalination perþ 2þ 2þ formance. In fact, the ion rejection of Naþ, NHþ 4 , K , Mg , Ca , Cl, NO3, F is 99.94, 100.00, 100.00, 99.96, 100.00, 99.98, 99.84, 100%, respectively (Table 1). These results are in good agreement with the previous report that the UiO-66 membrane is very effective for seawater desalination [45]. Since there are observable gaps between the UiO-66-NH2 membrane and Al2O3 support, the UiO-66NH2 membrane prepared on the non-modified Al2O3 tube show low ion rejections. At 348 K, the ion rejection is only 90%, and the ion rejection of the UiO-66-NH2 membrane sharply decreases to 44% when the feed temperature increases to 363 K (Fig. S2). When the feed temperature increased from 318 to 363 K, the water flux increased from 1.5 to 12.1 kg m2 h1, and the ion rejection of the UiO-66-NH2 membrane keeps almost unchanged (Fig. 4). The increase of the water flux with increasing temperature is attributed to the increase of the driving force of water permeation. According to Arrhenius law, when the feed temperature increases, the water vapor pressure on the feed side exponentially increases, while the vapor pressure on the permeate side keeps constant, then leading to an enhancement of the driving force across the membrane. Further, when the temperature increases, thermal motion of water molecules in solutions is drastic, thus the
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Fig. 2. FESEM images of the UiO-66-NH2 membrane prepared on APTES-modified macroporous Al2O3 substrate after one-step synthesis (a, b), and two-step synthesis (c, d). (a, c) top view, and (b, d) cross-section view. The circles in (a) indicate the inter-crystal defects in the membrane layer.
Fig. 4. The water flux and ion rejection of UiO-66-NH2 membrane prepared on APTESmodified a-Al2O3 tube for desalination of 3.5 wt% seawater by pervaporation as a function of the operation temperature. Fig. 3. XRD patterns of the simulated UiO-66-NH2 (a), UiO-66-NH2 powder collected from membrane synthesis solution (b), and UiO-66-NH2 membranes prepared on APTES-modified a-Al2O3 tube (c). Peaks marked with asterisk show the a-Al2O3 tube.
diffusion rate of water molecules through the membrane is also accelerated. Consequently, the water flux of the UiO-66-NH2 membrane increase with increasing of temperature. Fig. 5 shows an Arrhenius type plot with ln (flux) versus 1/T for water permeation through the UiO-66-NH2 membranes. According to an Arrhenius relationship, as shown in Equation (4), the linear trend in the plot suggests that water permeation through the UiO66-NH2 membranes is an activated diffusion process. Based on the slope of Arrhenius plot, the activation energy of the water permeation (Ea) through the UiO-66-NH2 membrane is calculated about 46.8 kJ mol1. Furthermore, the positive value of the activation energy indicates that the water flux increases with increasing the feed temperature.
Ji ¼ Ai $exp$ Ep;i RT
(4)
where Ji is the permeate flux of the membrane, Ai is the preexponential factor, R is the gas constant, T is the absolute temperature and Ep,i is the apparent activation energy for diffusion and the heat of sorption. Benefiting from the existence of hydrophilic adsorption sites (hydroxyl groups and amino groups) in the framework, both the water flux and ion rejection of the UiO-66-NH2 membrane is rather high. At 348 K, a relatively high water flux of about 7.5 kg m2 h1 and ion rejection of 99.8% can be obtained for desalination of 3.5 wt % seawater. Comparing with the reports of zeolite membranes for seawater desalination [15,16,54], the as-synthesized UiO-66-NH2 membrane developed in the present work shows higher water flux due to its high hydrophilicity and large pore size, while maintaining
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Fig. 5. The Arrhenius plot for the water flux and the feed temperature through the UiO-66-NH2 membrane prepared on APTES-modified a-Al2O3 tube for desalination of 3.5 wt% seawater.
high ion rejection. These results suggest that UiO-66-NH2 membrane is an attracting candidate for production of pure water through seawater desalination. Fig. 6 shows the water flux and ion rejection of the UiO-66-NH2 membrane prepared on APTES-modified a-Al2O3 tube as a function of the feed concentration at 348 K. It can be seen that the water flux decreases with increasing the feed concentration, i.e., the water flux decreases from 8.0 to 4.9 kg m2 h1 when the feed concentration increases from 2 to 10 wt%, while the ion rejection rate still maintains high (over 99.7%) even for desalination of 10 wt% seawater. With increasing the feed concentration, the concentration polarization takes place at the interface between the feed solution and the membrane [55]. Thus, more ionic clusters are formed and the electrostatic interaction between the ions and the polarized water become stronger. Therefore, on one hand, the driving forces (effective pressure to overcome the osmotic pressure) for water permeation decreases, leading to the reduction of water flux [56]. On the other hand, the deposition of salts on the outer surface of the membrane becomes more severe since more salts are recrystallized from the mixtures. Consequently, the channels for water passing are blocked and thus causing the decrease of water flux.
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Fig. 7. Water flux and ion rejection of the UiO-66-NH2 membrane prepared on APTESmodified a-Al2O3 tube for desalination of 3.5 wt% seawater for 120 h at 348 K by pervaporation.
3.3. Stability evaluation of UiO-66-NH2 membrane for seawater desalination As reported previously [57], UiO-66-NH2 show high chemical and hydrothermal stability. The studied on the stability of UiO-66NH2 membrane was evaluated for desalination of 3.5 wt% seawater at 348 K for 120 h. As shown in Fig. 7, both the water flux (7.5 kg m2 h1) and ion rejection (over 99.7%) are almost unchanged after 120 h. With further increasing the testing time, the water flux begins to decrease slightly. As the testing time is extended to 120 h, the water flux decreases to 6.7 kg m2 h1, while the ion rejection still maintains above 99.7%. The slight decrease of the water flux after 60 h might be attributed to the following reasons [15]: (1) the feed concentration increases with increasing time on stream, leading to the decrease of water flux, which is in agreement with Fig. 6; (2) the salts will be accumulated on the surface of the membrane with increasing testing time, which will block the channels and/or pores of the UiO-66-NH2 membrane, and thus increasing the resistance of mass transferring. In good agreement with the high desalination performances of the UiO-66-NH2 membrane for a long time measurement, there are no remarkable differences in membrane morphology after the measurement of seawater desalination at 348 K for 120 h, and the UiO-66-NH2 membrane is still well intergrown without any visible cracks, pinholes or other defects (Fig. 8). A typical XRD pattern (Fig. S3) shows that the high crystallinity of the UiO-66-NH2 membrane remains unchanged after the measurement of seawater desalination. All the XRD peaks of the UiO-66-NH2 membrane match well with those of the as-synthesized UiO-66-NH2 membrane, indicating that the unusual thermal and chemical stability of UiO-66-NH2 membrane allows it being used for seawater desalination under relative harsh conditions. Further, in complete accordance with the high stability of the UiO-66-NH2 membrane, no notable changes in XRD patterns of the UiO-66-NH2 powder collected from the membrane synthesis can be observed after 200 h treatment with 3.5 wt% seawater (Fig. S4), further confirming the high stability of UiO-66-NH2 in contact with seawater. 4. Conclusions
Fig. 6. Water flux and ion rejection of the UiO-66-NH2 membrane prepared on APTESmodified a-Al2O3 tube for seawater desalination as a function of the feed concentration at 348 K.
Attributing to their highly diversified pore structures and pore sizes as well as specific adsorption affinities, MOFs membranes have been widely studied in gas separation. Considering the narrow
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Fig. 8. Top view (a) and cross-section (b) FESEM images of the UiO-66-NH2 membrane prepared on the APTES-modified a-Al2O3 tube after desalination of 3.5 wt% seawater for 120 h at 348 K.
pore size of the MOF which is exactly in between the size of water molecules and hydrated ions, MOFs membranes are also potentially promising in liquid separation and seawater desalination. In this work, we have prepared thin, phase-pure and well intergrown UiO66-NH2 membranes for seawater desalination on the APTESmodified macroporous a-Al2O3 tubes. Attributing to the relatively moderate pore size and the hydrophilic adsorption sites in the framework, the developed UiO-66-NH2 membranes display high separation performances for seawater desalination. At 348 K for desalination of 3.5 wt% seawater, the water flux through the UiO66-NH2 membrane is 7.5 kg m2 h1, with ion rejections of over 99.7%. Further, the UiO-66-NH2 membranes can keep an excellent separation performance during a long-term measurement at 348 K for 120 h due to its high chemical stability. Acknowledgements This work is supported by the National Natural Science Foundation of China (21606246), Ningbo Science and Technology Innovation Team (2014B81004), and Ningbo Municipal Natural Science Foundation (2015A610055). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.micromeso.2017.06.025. References [1] R.F. Service, Desalination freshens up, Science 311 (2006) 1088e1090. ~ as, [2] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Marin A.M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301e310. [3] Q. Schiermeier, Purification with a pinch of salt, Nature 452 (2008) 260e261. [4] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, Science 333 (2011) 712e717. [5] M.G. Marcovecchio, S.F. Mussati, P.A. Aguirre, N.J. Scenna, Optimization of hybrid desalination processes including multi stage flash and reverse osmosis systems, Desalination 182 (2005) 111e122. [6] A.D. Khawaji, I.K. Kutubkhanah, J.-M. Wie, Advances in seawater desalination technologies, Desalination 221 (2008) 47e69. [7] W. Zhu, L. Gora, A.W.C. van den Berg, F. Kapteijn, J.C. Jansen, J.A. Moulijn, Water vapour separation from permanent gases by a zeolite-4A membrane, J. Membr. Sci. 253 (2005) 57e66. [8] J. Glater, S.K. Hong, M. Elimelech, The search for a chloring-resistant reverseosmosis membrane, Desalination 95 (1994) 325e345. [9] H. El-Saied, A.H. Basta, B.N. Barsoum, M.M. Elberry, Cellulose membranes for reverse osmosis Part I. RO cellulose acetate membranes including a composite with polypropylene, Desalination 159 (2003) 171e181. [10] L. Li, J. Dong, T.M. Nenof, R. Lee, Desalination by reverse osmosis using MFI zeolite membranes, J. Membr. Sci. 243 (2004) 401e404. [11] L. Li, N. Liu, B. McPherson, R. Lee, Influence of counter ions on the reverse osmosis through MFI zeolite membranes: implications for produced water desalination, Desalination 228 (2008) 217e225.
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