Fabrication of a thin film nanocomposite hollow fiber nanofiltration membrane for wastewater treatment

Journal of Membrane Science 488 (2015) 92–102

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Fabrication of a thin film nanocomposite hollow fiber nanofiltration membrane for wastewater treatment Tian-Yin Liu a, Zai-Hao Liu a, Rui-Xin Zhang b, Yao Wang c, Bart Van der Bruggen b, Xiao-Lin Wang a,n a

Beijing Key Laboratory of Membrane Materials and Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China Department of Chemical Engineering, Process Engineering for Sustainable Systems, KU Leuven, Belgium c Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology, Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China b

art ic l e i nf o

a b s t r a c t

Article history: Received 11 January 2015 Received in revised form 2 April 2015 Accepted 11 April 2015 Available online 21 April 2015

A thin film nanocomposite (TFN) hollow fiber membrane containing nanoporous SAPO-34 nanoparticles was prepared on the dual-layer (PES/PVDF) hollow fiber substrate. The nanoparticle exposed TFN membrane (TFN (DOX)) was fabricated via the co-solvent (dioxane) assisted interfacial polymerization process. The TFN(DOX) had a larger nanoporosity and a higher crosslinking degree, which simultaneously led to an increased pure water permeability but a decreased salt permeability than the TFN membrane. The larger nanoporosity of TFN (DOX) membrane was ascribed to the effective exposure of nanoparticles, proved by the less PA coverage and the stable dispersion of nanoparticles in organic solution. Contributed by the exposed nanoparticles, the TFN (DOX) showed superior hydrophilicity and lower streaming potential than the TFN membrane. Compared to the NF-90, the TFN(DOX) membrane simultaneously improved the water flux and the rejections against tris (2-chloroethyl) phosphate, tris(1-chloro-2-propyl) phosphate and tris(1,3-dichloro-2-propyl) phosphate molecules. In addition to the elevated pure water permeability of 20.1 L m
2 h
1 bar
1, the newly developed TFN(DOX) hollow fiber nanofiltration membranes have high rejections to both the multivalent electrolytes and micropollutant molecules, showing the potential applications in industrial wastewater treatment and drinking water purification. & 2015 Elsevier B.V. All rights reserved.

Keywords: Thin film nanocomposite (TFN) membrane Nanofiltration (NF) for micropollutants Hollow fiber membrane

1. Introduction In previous papers in this series, the fabrication of a dual-layer (PES/PVDF) hollow fiber ultrafiltration (UF) membrane was demonstrated with pore size narrowing down from 100 to 10 nm, in order to improve the separation performances (molecular weight cutoffs (MWCOs) of 33–292 kDa) and the mechanical durability (tensile strengths above 10 MPa) [1]. Based on this hollow fiber substrate, a thin film composite (TFC) hollow fiber nanofiltration (NF) membrane with pore size around 1 nm was fabricated to further increase the separation performance with a MWCO of 300 Da and a high water flux (16.6 L m
2 h
1 bar
1) at a low operational pressure, which was designed by the reduced thickness of barrier layer via a co-solvent assisted interfacial polymerization process [2]. Because the wastewater effluents and surface waters are widely considered as drinking-water supplies (indirectly in the case of groundwater recharge), the removal of

n

Corresponding author. Tel.: þ 86 10 62794741. E-mail address: [email protected] (X.-L. Wang).

http://dx.doi.org/10.1016/j.memsci.2015.04.020 0376-7388/& 2015 Elsevier B.V. All rights reserved.

micropollutants, including endocrine disrupting compounds (EDCs), pharmaceuticals and personal care products (PCPs) has raised the substantial concern in the public and in regulatory agencies [3]. The removal of micropollutants is feasible by NF membrane dominated by the close molecular sizes with the MWCO, while using the NF membranes in spiral wound modules suffered from the low water flux because of the organic fouling and difficulty in module cleaning [4]. Considering the high fouling resistance performances, the design of hollow fiber NF membranes with improving permea-selectivity is highly desirable for the treatment of wastewater under low operational pressures [5]. The zeolites improved the nanoporosity and fouling resistance of reverse osmosis (RO) membranes, and resulted in improved permeability and salt rejection especially for seawater desalination [6]. Herein, the selective layer was designed by introducing nanochannels with entrance pores less than 1 nm, in order to improve the water diffusion while maintaining the repulsion against neutral/charged small molecules for wastewater treatment. The thin film nanocomposite (TFN) membrane with nanoporosity attracted much attention because of the enhanced water channels and maintained pore size for NF membrane [7]. Because the

T.-Y. Liu et al. / Journal of Membrane Science 488 (2015) 92–102

separation mechanism of NF is mainly attributed by the size exclusion combined with Donnan exclusion (charge repulsion) and solute-membrane affinity, the incorporation of frameworks with the similar entrance pores but elevated nanoporosity than the bulk membrane provides opportunities to improve permea-selective performances [6]. Provided that the pore sizes of the NF membrane were generally around 0.4–0.5 nm via the IP process assisted by DOX, SAPO-34 nanoparticles were chosen with the precisely matched pore sizes with their eight-membered ring windows (0.4–0.5 nm) [8,9]. This framework was used as entrance pores for water molecules (0.27 nm) and the size exclusive framework against micropollutants with diameters of 0.7–0.9 nm [9]. The nanoparticle was characterized by the hydrophilic framework and high pore volume, indicating the high water adsorption and rapid water diffusion inside the nanostructure, and thus the high water flux [5,9,10]. Possessing the negatively charged framework and narrow pore sizes, SAPO-34 is a good candidate to remove small molecules from water by molecular sieving mechanism and electrostatic repulsion, while applying SAPO34 nanoparticles in nanocomposite membranes was rarely reported. It is desirable to incorporate the nanoporous SAPO-34 framework on the surface of TFN membranes, which improves the filtration performance and the separation capability of the hollow fiber NF membranes with respect to the removal of micropollutants. Generally, the transport of molecules through the NF membranes exhibits a pore-flow character through the nanopores in the membrane [11]. The locale of selective layer lies in the surface of thin film layer, and thus the surface porosity plays an important role in improving the permea-selectivity, which mostly yields low in the order of 10
5–10
6 [12]. Generally, the membrane porosity is improved by depositing nanozeolites with open-end nanopores in the thin film layer, based on the crosslinking in an organic environment [13,14]. It was found that the nanoparticles were fixed in the polymeric network, achieved by controlling the nanoparticle size in the range of the layer thickness [15]. However, the hydrophilic nanoparticles with nanopores submerge below the active surface of the membrane, due to the interfacial segregation of incompatibility [16]. In addition, the thin film layer is occupied by the large voids beneath the functionalizing surface, and the nanopores of submerged nanoparticles attribute ineffectively to water transport [17]. Because the nanoparticles were exposed on the membrane surface, the uniform dispersion nanoparticles in the organic solution and subsequently in the TFN membrane was crucial to avoid the leakage passage of solute molecules via intracrystalline defects [18]. In previous work, IP process assisted by DOX was used to strengthen piperazine diffusivity and solubility in organic phase via forming supramolecular assemblies [19]. Herein, our strategy includes using the cyclic ethers to improve the nanoparticle compatibility in the organic phase by “interfacial bonding” of hydrogen bonds. By using this process, the stable dispersion of nanoparticles in the organic phase ensures the depositing of nanoparticles on the surface of the fabricated nanocomposite thin film (TFN(DOX)), and forms the high surface porosity with selective nanopores. In the present paper, a novel TFN(DOX) hollow fiber membrane was fabricated via the IP process assisted by DOX, in view of removing micropollutants of wastewater. The TFN(DOX) was compared with the TFN membrane in terms of the crosslinking degree and the porosity of the organic–inorganic network. The porosity, water adsorption and streaming potential of the TFN (DOX) were further characterized to verify the effect of nanoporous framework on the surface chemistries. The filtration and rejection performances of the TFN membrane were investigated by increasing the loading concentration, and were fitted with the specific pore volume. The NF separation capability of TFN(DOX) membrane was characterized by studying polarized flame retardants and compared with the NF-90 flat sheet membrane.

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2. Experimental 2.1. Materials Silicon dioxide, aluminum oxide, phosphoric acid (Sinopharm Chemical Reagent China Co., Ltd., China) and deionized (DI) water (laboratory prepared) were taken as the silica, aluminum and phosphorus sources for the synthesis of SAPO-34 framework. Triethylamine and morpholine were used as the organic templates of the SAPO-34 framework. PVDF 6020 and PES 3000P (Solvay Co., Ltd., Belgium) were adopted as the materials of the dual-layer hollow fiber substrate. PVP K30 (BoaiNky Pharmaceuticals Co., Ltd., China) was employed as a hydrophilic additive in the substrate. In order to fabricate the thin film layer, piperazine (PIP) and trimesoyl chloride (TMC) (J&K Scientific Beijing Co., Ltd., China) were used as monomers for the interfacial polymerization (IP) process. Diphenyl carbonate (DPC), N-methyl-2pyrrolidone (NMP) and dimethyl sulfoxide (DMSO) (Sinopharm Chemical Reagent China Co., Ltd., China) were used as the solvent of the casting solution for the inner and outer layers of the duallayer hollow fiber substrate and intermediate-treatment agent between the layers, respectively. DI water and hexane (Sinopharm Chemical Reagent China Co., Ltd., China) were used as the solvents of the aqueous and organic solution for the formation of the thin film layer, respectively. Dioxane (DOX) was incorporated into the organic solutions as the co-solvent additive. Neutral molecules of ethyl alcohol, n-butyl alcohol, glucose, saccharose, raffinose, α-cyclodextrin, β-cyclodextran, and dextran (Mw ¼10, 20, 40 and 100 kDa; Acros Organics Co., Ltd. USA) were employed as model molecules to determine membrane structural parameters. Na2SO4 and NaCl (Beijing Modern Eastern Fine Chemical Co., Ltd., China) were used to characterize the charge properties of the membranes. Tris(2-chloroethyl) phosphate (TCEP), tris(1-chloro-2propyl) phosphate (TCPP) and tris(1, 3-dichloro-2-propyl) phosphate (TDCPP) were used as model micropollutants to characterize the separation capability. 2.2. Fabrication of the hollow fiber NF membranes SAPO-34 cubes were prepared in accordance with the procedure described in the literature [20]. Briefly, TEA and morpholine were incorporated as the organic template to obtain the poreopened zeolite framework after removing the template by calcination. Because the SAPO-34 cubes had a dimension (from 1000 to 10,000 nm) much larger than that of the thin film layer (from 100 to 200 nm), SAPO-34 cubes were post-treated by the procedure of ball milling at 300 rpm for 6 h. The milled powder was dispersed in water by using ultrasonication, and then the large zeolite particles were removed from the solution via a first-stage centrifugation at 2000 rpm for 20 min. The obtained solution was further operated via a second-stage centrifugation at 5000 rpm for 30 min, and the zeolite nanoparticles were collected at the bottom of container [18]. Finally, the obtained nanoparticles were dried at 120 1C before use in the membranes. The TFC and TFC(DOX) hollow fiber membranes were synthesized exactly as described in previous work [1,2]. For fabricating the TFN membranes via the IP process, nanoparticles were first dispersed in hexane by ultrasonication for 30 min, just before the dissolution of 0.1 wt% TMC. Because the solution exhibits poor stability (precipitated within 10 min), the obtained organic solution was immediately used to fabricate the TFN membranes. For fabricating the TFN(DOX) membrane via the IP process assisted by DOX, a given amount of nanoparticles was firstly dispersed in DOX with 30 min ultrasonication at a constant temperature of 25 1C. Then 1 g DOX solution containing dispersed nanoparticles was added into 99 g organic solution of TMC/hexane (TMC: 0.1 wt%). The DOX/hexane solution

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containing dispersed nanoparticles (0.005 0.1 wt%) was maintained stable as characterized by Tyndall's phenomenon, and was stored to fabricate the TFN(DOX) membranes. The solution compositions and the properties of the resultant membranes are shown in Table 1. Having the nanoparticle-dispersed organic solution, the DOX assisted IP (or IP) process was conducted following the procedures: firstly, the dual-layer (PES/PVDF) hollow fiber substrate was consecutively immersed in alcohol and the aqueous solution (PIP: 2 wt%) to obtain saturated monomers in the substrate pores; secondly, the wetted substrate was wiped out by filter paper to remove the excess solution on the surface; then the hollow fiber substrate slowly passed through the nanoparticle-dispersed TMC/DOX/hexane solution (0.1: 1: 98.9 wt %) (or nanoparticle-dispersed TMC/hexane: 0.1: 99.9 wt%) to conduct the DOX assisted IP process (or IP process) for 2 min, followed by 70 1C curing for 8 min. Finally, the membrane with thin film layer on the outside surface was prepared and was stored in the buffer solution (NaHSO3: 500 ppm at 4 1C) for testing. 2.3. Characterization of the nanoparticles and hollow fiber NF membranes The nanoparticle morphologies were observed by a field emission scanning electron microscope (FESEM, 6301F, JEOL Ltd., Japan). The dried nanoparticles were sputtered on to the conducting resin, and were directly observed because of the electrical conductivity. The structures of nanoparticles were further characterized by a transmission electron microscopy (TEM, JEM2010, JEOL Ltd., Japan) at 80 kV. The well-dispersed nanoparticles in the ethanol solution were deposited on the microgrid and oven dried before observation. The crystalline structure of synthesized SAPO34 nanoparticles was evaluated by powder X-ray diffraction, XRD (Bruker AXS D8 diffractometer using Cu Kα radiation). Nitrogen adsorption onto and desorption from the nanoparticles as well as the nanocomposite thin films were carried out on QUADRASORBTM SI (Quantachrome Instruments, Boynton Beach, FL) at 77 K. The specific surface areas and pore size distributions were calculated by the Brunauer–Emmett–Teller (BET) method and density functional theory (DFT) method, respectively. The water sorption of the nanoparticles was characterized by thermogravimetric analysis (TGA and DTG) under air flow in a TGA-DSC thermobalance (STA409PC, Netzsch, Germany). The crosslinking degree and thermal stability of the nanocomposite thin films were characterized by a TGA. Free standing nanocomposite thin films were prepared from the DOX assisted IP (or IP) process to obtain a sufficiently large polyamide (PA) sample (ca. 10 mg). The crosslinking degree of the nanocomposite thin film was identified as the weight loss ratio from 460 to 580 1C, indicating the decomposition of the amide groups in the PA network. Attenuated total reflectance (ATR)-Fourier transform infrared (FTIR) spectroscopy (Nicolet 6700, USA) was used to monitor the functional groups of the nanocomposite thin films. Zinc selenide (ZnSe) was applied as an internal reflection element and each spectrum was captured by 64 averaged scans at a Table 1 Preparation conditions and hydrophilicity, mechanical properties of the TFN and TFN(DOX) membranes. Samples

TFC TFN TFC(DOX) TFN(DOX)

Solution composition

Contact angle (deg)

Nanoparticle loading (wt%)

DOX concentration (wt%)

– 0.05–0.2 – 0.05–0.2

– – 1 1

72 75 717 4–477 6 67 7 3 637 3–417 4

resolution of 4 cm
1. The free-standing samples were fabricated with thickness of 10 μm to exclude the impacts of thickness on the absorption peak intensity. Collected PA segments were sealed and compressed via two steel plates with a PTFE ring of 10 μm. The specific surface areas and pore size distributions of the nanocomposite thin films were calculated by the BET method and DFT method, respectively. The morphology of the TFN membrane was observed by a FESEM at an accelerating voltage of 3 kV. The dried samples were sputter-coated with gold at 10 mA for 5 min. The cross-sectional morphologies of the membranes were further observed by a TEM. The membrane samples embedded in epoxy resin were sliced into ultrathin sections with thickness around 70 nm by a Reichert–Jung Ultramicrotome (701704, Reichert Co., Ltd., USA), and then located on the microgrid for observation. The surface roughness of membranes was probed by a SPM 29500 AFM. A tapping mode was operated on the samples in air at room temperature with the image scanning size of 20 μm  20 μm. The static contact angles of the membranes were measured by a contact angle meter (OCA 20, Dataphysics Co., Ltd., Germany) at 25 1C. Water (1 mL) was carefully dropped onto the dry sample with an automatic piston syringe. The contact angle of each sample was measured at three diverse positions on the sample surface. The tangential streaming potential was measured to calculate the surface streaming zeta (ζ) potential with 0.01 M potassium chloride (KCl, Fisher) used as the electrolyte solution. All measurements were carried out at room temperature (25 1C) and the solution pH was controlled at 5.8 70.2. The streaming potential was obtained by the Helmholtz–Smoluchowski equation. 2.4. Filtration experiments of the hollow fiber NF membranes The hollow fiber NF membranes were evaluated for permeate flux and rejection using a NF module described elsewhere [21]. In short, the filtration experiments were conducted at an operational pressure of 3 bar and a flow velocity of 60 L h
1. Two modules containing four fibers each were tested simultaneously to ensure repeatability. The operational pressure and temperature were kept at 3 bar and 25 1C for obtaining the water fluxes and rejections. Inorganic salt and neutral organic solute concentrations were measured by using a conductivity meter (DDSJ-308A; INESA, China) and a total organic carbon analyzer (TOC-VCPN; Shimadzu, Japan), respectively. Four hollow fiber membranes in one module were evaluated for pure water permeability (PWP) and salt rejection (R¼ 1
cp/cf; cp and cf are the permeate and feed concentrations) to obtain the average results. The molecular weight cut-off (MWCO) of the membrane was defined by the molar mass of a compound rejected for 90% [22]. The hydrodynamic diameter (nm) of dextran was obtained from the literature [11,23]. The solute permeability coefficient (B(¼ Jv(1
R)/ R)) was calculated to characterize the membrane performance. The PWP of the nanoporous membranes was fitted by using the specific surface area. According to Hagen–Poisenille equation, the water flux is related to the surface porosity (Ak) when the pore size is considered constant, because pore size of nanoparticle (0.4–0.5 nm) is slightly narrower than that of the thin film layer (0.53 nm) [2]: LpðTFNÞ AkðTFNÞ ¼ LpðTFC Þ AkðTFC Þ

ð1Þ

where Lp and W represent PWP and weight percentage of nanoparticle in the membrane respectively. Because of the isotropic distribution of nanocavities in SAPO-34 framework, the amount of nanochannels is maintained regardless with the orientation of the nanoparticles. For instance, assuming the nanopores are cylinder in the membrane with the same thickness, the surface porosity is correlated with volume porosity (Vk, m3/m3). Because the zeolite framework (1.4 cm3/g) has similar density with PA network (1.7 cm3/g), the volume porosity is

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substituted by the specific pore volume (Vp, m3/g), obtained from the BET tests [13,24]: AkðTFNÞ W  AkðSAPOÞ þ ð1
W Þ  AkðTFC Þ ¼ AkðTFC Þ V kðTFC Þ W  V pðSAPOÞ þ ð1
W Þ  V pðTFC Þ ¼ V kðTFC Þ

ð2Þ

3. Results and discussion 3.1. Formation and characterization of the hollow fiber NF membranes Because the filtration and separation performances are dominated by the surface of thin film layer, the nanoparticle-exposed TFN membrane was designed in order to obtain water channels with selectivity on the membrane surface (Fig. 1(b) and (d)) [6,25]. The SAPO-34 nanoparticles were used as the model framework, combined with the smooth surfaces and sizes from 50 to 200 nm (Fig. S1). Sub-nanometer pores were observed on the surface of the nanoparticles, which were characterized by the crystal structure and surface pore diameter around 0.4 nm (Fig. S2(a) and (d)). Moreover, the surface morphologies and the chemical functional groups of the nanoparticles have great impacts on the interaction with water molecules [26,27]. The nanoparticle framework was functionalized with hydroxyl groups, indicated by the O
H stretching vibration peak at 3400–3600 cm
1 and Si
OH stretching vibration peak at 950 cm
1 (Fig. S2(b)). In order to build up the water channels, the hydrophilic and nanoporous SAPO-34 nanoparticles are further assembled on the surface of polymeric network (Fig. 1(b) and (d)). The surface morphology of the TFN(DOX) was compared with the TFN membrane. The interfacial gap between the nanoparticles and the PA network was critical to obtain the narrow pore sizes for separation. The surface of the TFN(DOX) membrane shows a “leaflike” morphology (Fig. 3(a)), which has a slightly larger valley than that on the membrane surface of TFC(DOX) membrane (Fig. 3(b)). In contrast, the TFN shows smoother surface morphology, reduced nodular size and more surface defects than TFC membrane (Fig. 3(c) and (d)). The favorable surface morphology of TFN(DOX) membrane was explained by the two reasons: the stable dispersion of nanoparticles without interfacial segregation of hydrophilic nanoparticles

95

towards aqueous solution, achieved by forming supermolecular assemblies with DOX molecules; and the formation of covalent bonds from the nanoparticle surface with the PA network, resulting from the adsorption of amine groups on nanoparticles [2]. Because the nodule arises from the growth of thin film layer in the organic solution, the larger nodular size of the TFN(DOX) than TFN membrane indicated the deeper growth of IP process assisted by DOX and higher crosslinking degree of PA network. The SAPO-34 cubes with dimensions larger than 200 nm were incorporated in TFN and TFN (DOX) membranes. Fig. 3(e) and (f) shows large defects on the surfaces of both membrane samples. Comparing SAPO-34 nanoparticles with SAPO-34 cubes, we propose here the size matched nanoparticles had superior compatibility with polymer network, while the larger particles introduced surface defect and nanogaps. The stability of nanoparticles with increasing loading concentrations, both in the organic phase and at the water–oil interface, was further evaluated in order to verify the underlying mechanism of morphological evolution in the thin film layer. The water–oil interfacial stability during the IP process assisted by DOX is critical for the formation of the nodular morphologies, which was disrupted by the interfacial segregation of nanoparticles during the IP process (Fig. 1(a) and (c)) [28]. The size distribution of nanoparticles in the organic solution was verified by dynamic light scattering method. The nanoparticle dispersion in hexane solution was around 605 nm due to the agglomeration, which was significantly reduced to 229 nm in the DOX/hexane solution, because of the crystalline unit (Fig. S2(c)). The major part of the nanoparticles in the solution was narrower than 200 nm, indicating the homogenous dispersion in the solution. By increasing the loading concentration of nanoparticle in the DOX/hexane solution, the peak intensity of UV absorption gradually strengthens (Fig. 3(a)). Interestingly, the UV peaks originated from the ether group of DOX red shift with incorporating more nanoparticles in the organic solution. This is ascribed to the electron-withdrawing effect of the ether group on DOX via the formation of hydrogen bonds with the hydroxyl group of nanoparticles (Fig. 3(b) and S2(b)). However, the UV adsorption peak of nanoparticles was maintained unchanged in hexane solution with increasing nanoparticle loading, resulting from the severe agglomeration and submersion of nanoparticles as shown in the right-up figure in Fig. 3(a). This result indicated the “interfacial bonding” between the hydroxyl groups of nanoparticles and the ether groups of DOX, shown by the schematic figure of Fig. 1(a and c). The IP process assisted by DOX was found to obtain

Fig. 1. Schematic diagram of the thin film layer formation: (a) the IP process assisted by DOX; (b) the corresponding thin film nanocomposite membrane (TFN (DOX)); (c) the IP process and (d) the corresponding thin film nanocomposite membrane (TFN).

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Fig. 2. FESEM images of membrane surface morphologies: (a) TFC; (b) TFN; (c) TFC(DOX); (d) TFN(DOX); (e) TFN using SAPO-34 cubes and (f) TFN(DOX) using SAPO-34 cubes.

stable dispersion of the hydrophilic nanoparticles in the organic phase, and the TFN(DOX) membrane formed with impregnation of nanoparticles from the organic solution during the thin film growth towards the organic phase. In contrast, the TFN membrane suffered from the interfacial instability and resulted in slower growth of thin film layer because of the hydrolysis of acryl chlorides in the organic solution [29]. For this reason, the desirable surface morphology is obtained by TFN(DOX) membrane without observable defects. The amide peaks of the membranes were further characterized by ATR-FTIR, which reflected the crosslinking density of the polymeric network and the interfacial defects. The vibration peak intensity was determined by the both amide group density and the thickness of the PA network, and thus the free-standing samples with the constant thickness were characterized to obtain the crosslinking density. Fig. 4 illustrates the influence of the

nanoparticles on the crosslinking density of the PA network. The peaks between 1040 and 1080 cm
1 are originated from the stretching vibration of Si
O
Si on the nanozeolite framework [30]. The amide peak intensities of TFN(DOX) are generally unchanged with TFC(DOX) membrane, e.g., 1610 cm
1 (N
H stretching of amide) and 1547 cm
1 (amide II, in-plane N–H bending and C
N stretching vibrations). For comparison, the vibration peaks in the functional group region (from 4000 to 1300 cm
1) of TFN decrease significantly compared to TFC membrane. It was found that the vibration strength of amide groups, indicating the crosslinking degree, was considerably higher in TFN (DOX) than TFN membrane, which could be explained by the much better interfacial bonding of the organic–inorganic interface as well as the elevated crosslinking of the PA network. TGA was used to verify the contribution of the interfacial interaction and intrinsic network density, because the PA network generally forms linear

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The dramatically increased starting temperature indicates that the dense PA network of TFN(DOX) was composed of the much lower ratio of linear backbones, and thus the stronger interfacial interaction in TFN(DOX) membrane dominates the higher crosslinking degree. The effect of thickness of thin film layer was further verified using membranes based on the dual-layer (PES-PVDF) substrates (Fig. 5). The vibration peaks in 1576, 1478, 1317, 1281 and 1130 cm
1 appear for all the samples, indicating aromatic C
C stretching, the doublet from the asymmetric O ¼S¼ O stretching and the symmetric O ¼S¼ O stretching of sulfone groups assigned to the dual-layer (PES/PVDF) substrate (Fig. 5). The peak intensity at 1660, 1601 and 1539 cm
1 showed a significant difference under different IP processes, which were characteristic peaks of C ¼O stretching, N
H bending and N
H stretching vibrations of the PA network [31]. Specifically, the intensity of the PA bands decreases slightly from TFC(DOX) to TFN(DOX) membrane, while it drops considerably from TFC to TFN membrane. Because the crosslinking degree of TFN(DOX) was found to be the same with TFC(DOX) membrane in Fig. 4, the reduced vibration peak of amide in TFN(DOX) than that of TFC(DOX) membrane reflected the slightly thinner thickness of PA network. This was ascribed to the nanoparticles being exposed on the surface and partly immersed in thin film layer. For comparison, the PA network thickness of TFN was much lesser than TFC membrane, as the nanoparticles submerged in the bulky network. The microstructures of the membranes are observed in TEM figures of Fig. 6. The TFN membrane shows covered crystal framework of nanoparticles, and the nanoparticles with sizes around 50 nm were compactly surrounded by the continuous morphology Table 2 TGA results of the TFN and TFN(DOX) membranes. Sample

Fig. 3. UV curves of the nanoparticles/DOX/hexane solution with increasing nanoparticle loading concentration from 0.05 to 0.2 wt%.

TFC TFN TFC(DOX) TFN(DOX)

Nanoparticle loading (wt%)

0 0.1 0 0.1

Decomposition stage Temperature (1C)

wt% loss

495–550 450–530 480–560 485–550

38.9 50.5 36.6 40.4

Fig. 4. ATR-FTIR curves of free standing of TFN(DOX) (IP process assisted by DOX with nanoparticles), TFC(DOX) (IP process assisted by DOX without nanoparticles), TFN (IP process with nanoparticles) and TFC (IP process without nanoparticles) nanocomposite thin films.

chains at interfaces while results in decreased crosslinking points in the network (Table 2). The starting decomposition temperature elevates more than 20 1C for the TFN(DOX) than TFN membrane.

Fig. 5. ATR-FTIR curves of dual-layer (PES-PVDF) hollow fiber substrate, TFC (IP process without nanoparticles), TFN (IP process with nanoparticles), TFC(DOX) (IP process assisted by DOX without nanoparticles) and TFN(DOX) (IP process assisted by DOX with nanoparticles) membranes.

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Fig. 6. TEM figures of (a) the TFN and (b) the TFN(DOX) membranes.

of the thin film layer with thickness around 100 nm (Fig. 6(a)). The thin film layer of TFN(DOX) membrane shows a much thinner “shell” structure in the range from 35 to 50 nm, and macrovoids are observed inside this dense structure. Interestingly, the nanoparticles are observed with sizes larger than 50 nm, and are extruded out of the dense structure. These nanoparticles are penetrated through the “shell” structure, thus building up the water channels from the feed solution to the permeate side of the thin film layer. The nanopores with sizes of 0.4 nm were inferred from the framework structure in Fig. S3 [20]. These narrow pores allow for water permeation (H2O: 0.2 nm) and sieving effect for micropollutants (TCEP: 0.54 nm; TCPP: 0.63 nm; TDCPP: 0.73 nm). In contrast, the nanoparticles are “submerged” in the thin film layer of TFN membrane, and contribute ineffectively on the water transport or separation performance. In a larger scale (10 μm  10 μm), AFM figures show that the surface roughness of the TFN(DOX) membranes increases with nanoparticle concentration, while for the TFN membranes the change is not obvious (Table 3). We consider that the nanoparticles were homogenously exposed on the membrane surface without excess coverage by the PA layer, and led to the gradually increased surface roughness. Thus, the TFN(DOX) membranes with less coverage of polymers generally have higher porosities, which is beneficial for the permeaselectivity further tested by the filtration experiments. To further verify the relative amount of effective nanopores for water transport, the specific surface area of the nanocomposite thin films was characterized by nitrogen adsorption (Fig. 7). The specific surface area of the nanocomposite thin film layer increases in the order: TFC (13 m2/g) o o TFN (63 m2/g)oTFN(DOX) (89 m2/g). The nitrogen molecules diffuse into the polymeric networks through the nanogaps and adsorb into the immersed nanopores of nanoparticles in TFN membrane. Because the TFN(DOX) shows exposed nanopores on the surfaces, the nitrogen molecules diffuse and adsorb into the zeolite framework through these nanoporous entrances. Thus, the increased specific surface area of the TFN(DOX) than the TFN membrane

indicated the larger contact areas with the water molecules in filtration tests and allowed the entrance in the framework. Moreover, the much lower coverage and blockage of the nanoparticles by the PA network contributed additionally to the higher surface area of TFN (DOX) membrane, which was proved by the smaller impregnation depth of the nanoparticles in the thin film layer (Fig. 6). For these reasons, the amount of the effective nano-channels in nanoparticles was considerably larger in the TFN(DOX) than in the TFN membrane, which is favorable for improving the PWP of the resultant membrane. The effect of nanoparticles on the surface morphologies and properties was investigated by the enhancement ratio of water flux in TFN(DOX) than TFC(DOX) membranes. Moreover, the effect of nanoparticle exposure is verified by the comparison between TFN(DOX) and TFN membranes (Fig. 8 and Table 3). The TFN(DOX) membrane shows larger percentage changes in hydrophilicity, surface potential and N2 adsorption volume. The exposed nanoparticles increase the density of “hydrophilic centers” as well as “negative charged centers” on the membrane surface, and accordingly reduce the contact angles as well as the surface potential of TFN(DOX) membranes. However, these “centers” were submerged in the relatively hydrophobic PA network, and thus proved the insignificant changes of contact angles of TFN membranes. Dominated by the hydrophilic frameworks and large pore volume, these frameworks allow the faster water adsorption onto the membrane surface and the faster transport of water molecules in the membrane. Interestingly, the large improvement in intrinsic water permeability as well as the low intrinsic salt permeability of Na2SO4 in TFN(DOX) membrane could be explained by two reasons: the more hydrophilic centers on the membrane surface and greater Columbic repulsive forces originated from the more negative centers, supported by the lower Si/Al ratio of nanoparticles.

Table 3 Surface roughness of the TFN (DOX) and TFN membranes with different nanoparticle loadings. Sample roughness (nm)

TFN(DOX) TFN

Nanoparticle loading (wt%) 0

0.01

0.05

0.1

0.2

737 5 617 10

757 6 547 6

887 2 597 8

1397 8 687 8

223 7 7 797 3

Fig. 7. BET curves of the TFC membrane (IP process without nanoparticles), TFN membrane (IP process with nanoparticles) and TFN(DOX) membrane (IP process assisted by DOX with nanoparticles).

T.-Y. Liu et al. / Journal of Membrane Science 488 (2015) 92–102

3.2. Filtration performances of the hollow fiber NF membranes The size matched nanoparticles were proved to show superior compatibility with polymer network in Fig. 2. Fig. 9 compares the filtration and separation properties of the corresponding membranes. It was found that much higher Na2SO4 rejection of the TFN (DOX) membrane was achieved by using SAPO-34 nanoparticles (over 98%) than using the SAPO-34 large crystals (60-70%) or using the SAPO-34 cubes (20-30%). Fig. 10 compares the PWPs and Na2SO4 rejections of the TFN(DOX) and the TFN membranes with increasing nanoparticle loadings. The PWP of the TFC(DOX) membrane (13.3 L m
2 h
1 bar
1) was larger than that of the TFC membrane (10.8 L m
2 h
1 bar
1) [2]. The TFN(DOX) membranes with nanoparticle loading concentration from 0.05 to 0.2 wt% were found to have 106%, 119%, 144%, 157% and 163% increase in flux over TFC(DOX) membrane. In contrast, the enhancement ratios are larger for TFN(DOX) than for TFN membranes, and the difference becomes greater with higher nanoparticle loadings. Furthermore, the Na2SO4 rejection was nearly unchanged (from 99.3% to 98.9%) in the TFN(DOX) membranes with nanoparticle loading ranging from 0 to 0.1 wt%, which decreased dramatically from 99.1% to 56.3% in the TFN membranes. The elevated preferential flow channels were incorporated into each of the nanocomposite thin film, and the increased number of these channels was responsible for the enhancement of water flow with higher nanoparticle loadings. From the much higher enhancement ratio in TFN(DOX) than TFN membranes, the

99

amount of effective flow channels was further increased due to the higher nanoparticle exposure ratio on the surface. Because the Debye length of Na2SO4 solution determines the distance of electrostatic interaction on the negatively charged pores, the pores with radii larger than the Debye length around 1.5 nm showed inferior rejections against the Na2SO4 solution of 2000 ppm. The prominent higher ion rejection of TFN(DOX) compared to TFN membranes indicates that non-selective holes between the nanoparticles with radii larger than 1.5 nm were eliminated in the “shell” structure. This implies that a dense network was developed at the organic-inorganic interfaces, attributed by the high compatibility of DOX-protected nanoparticles in the PA network [32]. In addition, the larger difference of water/solute permeabilities of the TFN(DOX) membranes proves the preferred penetration of water molecules through the sub-nanometer pores in the inorganic and polymeric networks simultaneously. The enhanced permeability by incorporating the nanoparticles was quantitatively elucidated by comparing the filtration properties with that predicted by Eq. (2). The weight percentage of nanoparticles in the membranes was determined by using TGA, in order to characterize the weight remained after calcination (Fig. S3) [33]. The PWP enhancement ratios of the TFN(DOX) are generally higher than the TFN membranes by filtration results. It is observed that the theoretical PWP enhancement ratio fits well with the experimental

Fig. 10. PWP and Na2SO4 rejections of the TFN(DOX) membrane (IP process assisted by DOX) and the TFN membrane (IP process) and using nanoparticle concentrations from 0.05 to 0.2 wt%. Fig. 8. Percentage changes of water contact angle, porosity, crosslinking degree, intrinsic water and salt permeability of the TFN(DOX) and the TFN membranes using nanoparticle concentrations of 0.1 wt%.

Fig. 9. Water permeabilities and Na2SO4 rejections of the TFN(DOX) membranes using 0.1 wt% SAPO-34 nanoparticles (sizes of 50–200 nm), SAPO-34 large particles (sizes of 200–1000 nm) and SAPO-34 cubes (1000–10,000 nm).

Fig. 11. PWP enhancement ratio of the TFN(DOX) membrane (IP process assisted by DOX) and the TFN membrane (IP process) and the fitting curves of specific surface area, by using nanoparticle concentrations from 0.05 to 0.2 wt%.

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results of TFN(DOX) membrane as nanoparticle loading concentration ranging from 0.05 to 0.1 wt%. In comparison, the theoretical line becomes larger than the experimental results for 0.2 wt% (Fig. 11). In contrast, the TFN membranes showed lowered filtration results than the theoretical line due to the much lower exposure ratio of nanoparticles, which were supported by the TEM and BET characterizations (Figs. 6 and 7). The theoretical line well predicts the results of TFN(DOX) membranes, indicating most of the nanopores of the nanoparticles contribute to the water permeation.

3.3. Separation capability of the hollow fiber NF membranes The NF properties of the TFN(DOX) membranes are further characterized by testing the rejections of inorganic salts (Na2SO4 and NaCl) and neutral organic molecules (Fig. 12). The developed TFN(DOX) membranes exhibit a considerable capability for the rejection of raffinose (from 99.4% to 98.6%), saccharose (from 96.8 to 95.3%) and glucose (from 88.3% to 82.1%) with the nanoparticle loading from 0.05 to 0.1 wt%, while the rejections dropped to around 85% with the nanoparticle loading of 0.2 wt%. For the rejection of inorganic salts, the rejection rate against Na2SO4 was significantly higher than that of NaCl, which was the typical characteristic for a negatively charged NF membrane, and can be explained by the Donnan exclusion effect. Interestingly, the Na2SO4 rejection remained larger than 90% with the nanoparticle loading of 0.2 wt%, which was significantly higher than that of a neutral organic molecule with a similar diameter (SO24
: 0.76 nm; glucose: 0.74 nm). From the streaming-potential measurements in Fig. 7, the TFN(DOX) membrane was strongly negatively charged, thus it rejected the multivalent anions bearing higher co-ion charge more effectively than the monovalent anions due to the stronger electrostatic repulsive interaction between the high valence anions and the membrane surface. However, it must be noted that since the effective pore diameter (0.45 nm) of the TFN (DOX) membranes is comparable to the hydrated diameter of the SO24
ion (0.67 nm), the size exclusion mechanism is also responsible for the high rejection of divalent anions. For this reason, saccharose with larger molecular weight than glucose was highly rejected through the sieving exclusion mechanism. Due to the elevated rejection against both neutral organic molecules and the negatively charged ions, the TFN(DOX)-0.1 wt% membrane was used for the further characterization (Table 4). Owing to the negatively charged surface, the TFN(DOX)-0.1 wt% membrane showed superior rejection against divalent SO24
anions than neutral molecules. It is of interest to examine the capability of the TFN(DOX)-0.1 wt% membrane for the removal of

Fig. 12. Na2SO4, NaCl rejections and water fluxes of the TFN(DOX) membranes using nanoparticle concentrations from 0.05 to 0.2 wt%.

micropollutants, which have polarized structure and molecular weights smaller than the MWCO of the NF membranes (Fig. 13) [3]. Because of the negatively charged surface of TFN(DOX)-0.1 wt%, its was inferred that the micropollutants interacted with the surface via the ion–dipole interaction, and thus improved the affinity of small molecules on the membrane surface, especially on the nanozeolite frameworks. The rejections against the weakly polarized molecules increased in the order: TCEP (96.9%) oTCPP (98.3%) oTDCPP (98.9%), having the molar mass of 285, 328 and 431 Da, respectively. Moreover, the rejections of the micropollutants were slightly larger than the neutral molecules with the similar molecular sizes: saccharose (95.3%) oTCPP (98.3%) and raffinose (98.6%) oTDCPP (98.9%). This proved the contribution of polar-ion interaction forces, which was more prominent by reducing the molecule sizes. The rejection of the TFN(DOX)–0.1 wt% membrane was further compared with the NF-90 flat sheet membrane. The TFN(DOX)–0.1 wt

Table 4 Rejections against neutral molecules of the TFN (DOX) membranes with different nanoparticle loadings. Solute rejection (%)

Raffinose Saccharose Glucose

Nanoparticle loading (wt%) 0

0.05

0.1

0.2

99.5% 98.7% 92.3%

99.4% 96.8% 88.3%

98.6% 95.3% 82.1%

95.2% 90.2% 85.1%

Fig. 13. Water fluxes and micropollutant rejections of the TFN(DOX)–0.1 wt% membrane and the NF-90 flat sheet membrane, by using TCEP, TCPP and TDCPP with molecular weights from 200 to 400 Da.

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% showed as high rejection as the NF-90 membrane against neutral molecules, while it had considerably larger rejections against the polarized organic molecules. Moreover, the water permeabilities of TFN(DOX)-0.1 wt% were significantly higher than that of NF-90 membrane in the micropollutant solutions. The TFN(DOX)-0.1 wt% membrane had much denser negative charged centers, which pushed the electron cloud of the flame retardant molecules, and thereafter interacted with the polarized molecules through ion–dipole interaction force. Moreover, the size of the zeolite framework was slightly narrower than the polymeric network, and further improved the selectivity of TFN(DOX) membrane via size exclusion. For these reasons, the TFN(DOX)–0.1 wt% membrane showed a better rejection of micropollutants as well as a higher water flux resulting from the incorporation of nanoparticles. Remarkably, the novel approach was proposed originating from the conventional TFC structure of NF membranes, e.g., NF-90, which was developed by narrowing down the membrane thickness of TFC(DOX) membrane in previous work, and improving the porosity of TFN(DOX)-0.1 wt% membrane in this work (Fig. 14) [2]. The approach is applicable for the intention of improving filtration performance and reducing energetic consumption, while maintaining the separation capacity of the corresponding membranes, e.g., UF, RO and gas separation membranes. Moreover, because of the highly exposed nanoporosity on the membrane surface, the novel structure of TFN(DOX) membrane built up a concrete base for the further investigations on nano-effects, e.g., rapid water channels and pollutant gatekeepers, through tuning the framework of nanozeolites. In summary, the novel TFN(DOX) membrane has a superior performance, which has the considerable potential in drinking water purification, municipal and industrial wastewater treatment to improve water quality, particularly in terms of removing micropollutants in an energy-efficient approach.

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layer. (2) By increasing the total surface area of the TFN(DOX) membranes through increasing the nanoparticle loading concentration, the PWP was increased proportionally while the pore size was maintained, which were ascribed to the increased porosity by nanoporous zeolites on the surface in a high exposure ratio. (3) When compared with the NF-90, the TFN(DOX) membrane had superior hydrophilicity and lower streaming potential, and showed the superior water flux and separation capability for micro-pollutants. The novel TFN(DOX) hollow fiber membrane had high separation capabilities for polarized micropollutants and a high-flux of 20.1 L m
2 h
1 bar
1, which gave perspectives for application in the integrated treatment of municipal, agricultural and industrial wastewater for drinking water purification. Acknowledgments The authors would like to thank National Key Technologies R&D Program of China (No. 2015BAE06B00), National High Technology Research and Development Program of China (2012AA03A604), National Science Foundation for Young Scientists of China (21406128) and Tsinghua University Initiative Scientific Research Program (20121088039). We also thank Prof. Ji-Ding Li and Dr. Yang Xia (Tsinghua University, Department of Chemical Engineering) for providing help in the BET absorption characterization, and Prof. Yin-Hua Wan (Institute of Process Engineering, Chinese Academy of Science) for providing access to streaming potential measurement and Mrs. YingXu (Zhejiang University, Electron Microscopy Center) for capturing TEM figures. The authors are also grateful to Prof. Huan-Ting Wang (Monash University, Department of Chemical Engineering) and Dr. Yu Cui (Tsinghua University, Department of Chemical Engineering) for providing help in the nanofluidic transport and synthesis of nanomaterial. Appendix A. Supporting information

4. Conclusions A novel TFN(DOX) hollow fiber membrane for NF was fabricated with SAPO-34 nanoparticles via the IP process assisted by DOX. The following conclusions can be drawn from this study: (1) Compared to the TFN, the TFN(DOX) membrane resulted in an improved PWP and smaller pore size, which was ascribed to the formation of a high porosity and the dense thin film layer. The well-dispersion of nanoparticles during the IP process assisted by DOX and the formation of the thinner “shell” structure was found to improve the extrusion of nanoparticles on the surface of the thin film layer, while the interfacial segregation took place during the IP process and led to the submerged nanoparticles in thin film

Fig. 14. A design of novel hollow fiber NF membranes, from NF-90, TFC(DOX) to TFN(DOX)–0.1 wt%.

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.memsci.2015.04.020.

Nomenclature Ak BET B Cf Cp DFT DI DMSO DOX DPC EDCs FESEM H–S IP Jv Mw MWCO NF NMP NIPS PA PES PIP

surface porosity Brunauer–Emmett–Teller solute permeability coefficient the concentration of solute in the feed the concentration of solute in the permeate density functional theory deionized dimethyl sulfoxide dioxane diphenyl carbonate endocrine disrupting compounds field emission scanning electron microscope Helmholtz–Smoluchowski interfacial polymerization volume flux molecular weight molecular weight cutoff nanofiltration N-methyl-2-pyrrolidone non-solvent induced phase separation polyamide poly(ether sulfone) piperazine

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PCPs personal care products PVDF poly(vinyldiene fluoride) PVP poly(vinyl pyrrolidone) PWP (Lp) pure water permeability R rejection RO reverse osmosis TCEP tris(2-chloroethyl) phosphate TCPP tris(1-chloro-2-propyl) phosphate TDCPP tris(1, 3-dichloro-2-propyl) phosphate TEM transmission electron microscopy TMC trimesoyl chloride TIPS thermally induced phase separation TFC thin film composite TFN thin film nanocomposite UF ultrafiltration W weight percentage of nanoparticle membrane

in

the

References [1] T.Y. Liu, R.X. Zhang, Q. Li, B. Van der Bruggen, X.L. Wang, Fabrication of a novel dual-layer (PES/PVDF) hollow fiber ultrafiltration membrane for wastewater treatment, J. Membr. Sci. 472 (15) (2014) 119–132. [2] T.Y. Liu, L.X. Bian, H.G. Yuan, B. Pang, Y.K. Lin, Y. Tong, B. Van der Bruggen, X. L. Wang, Fabrication of a high-flux thin film composite hollow fiber nanofiltration membrane for wastewater treatment, J. Membr. Sci. 478 (2015) 25–36. [3] C. Bellona, J.E. Drewes, Viability of a low-pressure nanofilter in treating recycled water for water reuse applications: a pilot-scale study, Water Res. 41 (17) (2007) 3948–3958. [4] B. Van Der Bruggen, C. Vandecasteele, Removal of pollutants from surface water and groundwater by nanofiltration: overview of possible applications in the drinking water industry, Environ. Pollut. 122 (2003) 435–445. [5] J. de Grooth, D.M. Reurink, J. Ploegmakers, W.M. de Vos, K. Nijmeijer, Charged micropollutant removal with hollow fiber nanofiltration membranes based on polycation/polyzwitterion/polyanion multilayers, Acs Appl. Mater. Interfaces 6 (19) (2014) 17009–17017. [6] B.H. Jeong, E.M.V. Hoek, Y. Yan, A. Subramani, X. Huang, G. Hurwitz, A. K. Ghosh, A. Jawor, Interfacial polymerization of thin film nanocomposites: a new concept for reverse osmosis membranes, J. Membr. Sci. 294 (1-2) (2007) 1–7. [7] T. Wang, Y. Yang, J. Zheng, Q. Zhang, S. Zhang., A novel highly permeable positively charged nanofiltration membrane based on a nanoporous hypercrosslinked polyamide barrier layer, J. Membr. Sci. 448 (2013) 180–189. [8] J.L.F. Shiguang Li, Richard D. Noble, R. Krishna, Interpreting unary, binary, and ternary mixture permeation across a SAPO-34 membrane with loadingdependent Maxwell–Stefan diffusivities, J. Phys. Chem. C 111 (2007) 5075–5082. [9] T. Wang, X. Lu, Y. Yan, Synthesis of SAPO-34 from metakaolin: crystallization mechanism of SAPO-34 and transformation processes of metakaolin, Microporous Mesoporous Mater. 168 (2013) 155–163. [10] D.S. Wragg, R.E. Johnsen, P. Norby, H. Fjellvåg., The adsorption of methanol and water on SAPO-34: in situ and ex situ X-ray diffraction studies, Microporous Mesoporous Mater. 134 (1-3) (2010) 210–215. [11] X.L. Wang, T. Tsuru, S.I. Nakao, S. Kimura., The electrostatic and sterichindrance model for the transport of charged solutes through nanofiltration membranes, J. Membr. Sci 135 (1997) 19–32.

[12] T.T. Xiao-Lin Wang, Shin-ichi Nakao, Shoji Kimura, o1-s2.0-S0376738897001257main.pdf4., J. Membr. Sci. 135 (1997) 19–32. [13] Z.E. Hughes, L.A. Carrington, P. Raiteri, J.D. Gale., A computational investigation into the suitability of purely siliceous zeolites as reverse osmosis membranes, J. Phys. Chem. C 115 (10) (2011) 4063–4075. [14] R.M. Barrer, B.E.F Fender, The diffusion and sorption of water in zeolites—II. Intrinsic and self-diffusion, J. Phys. Chem. Solids 21 (1961) 12–24. [15] M.L. Lind, A.K. Ghosh, A. Jawor, X. Huang, W. Hou, Y. Yang, E.M.V. Hoek, Influence of zeolite crystal size on zeolite–polyamide thin film nanocomposite membranes, Langmuir 25 (17) (2009) 10139–10145. [16] T. Feng, D.A. Hoagland, T.P. Russell, Assembly of acid-functionalized singlewalled carbon nanotubes at oil/water interfaces, Langmuir 30 (2014) 1072–1079. [17] T. Kamada, T. Ohara, T. Shintani, T. Tsuru, Controlled surface morphology of polyamide membranes via the addition of co-solvent for improved permeate flux, J. Membr. Sci. 467 (2014) 303–312. [18] C. Kong, T. Shintani, T. Tsuru., “Pre-seeding”-assisted synthesis of a high performance polyamide–zeolite nanocomposite membrane for water purification, New J. Chem. 34 (10) (2010) 2101. [19] Y.-M. Zhang, Z.-X. Yang, Y. Chen, F. Ding, Y. Liu, Molecular binding and assembly behavior of β-cyclodextrin with piperazine and 1,4-dioxane in aqueous solution and solid state, Cryst. Growth Des. 12 (3) (2012) 1370–1377. [20] Y. Cui, Q. Zhang, J. He, Y. Wang, F. Wei, Pore-structure-mediated hierarchical SAPO-34: facile synthesis, tunable nanostructure, and catalysis applications for the conversion of dimethyl ether into olefins, Particuology 11 (4) (2013) 468–474. [21] R.X. Zhang, T.Y. Liu, J. Vanneste, L. Poelmans, A. Sotto, X.L. Wang, B. Van Der Bruggen, From microfiltration (MF) to nanofiltration (NF): a design of composite hollow fiber membranes with tunable, reliable performance and reinforced mechanical strength, J. Appl. Polym. Sci. 132 (2015) 41247. [22] P.D. Peeva, N. Million, M. Ulbricht, Factors affecting the sieving behavior of anti-fouling thin-layer cross-linked hydrogel polyethersulfone composite ultrafiltration membranes, J. Membr. Sci. 390–391 (2012) 99–112. [23] M.N. Sarbolouki., A general diagram for estimating pore size of ultrafiltration and reverse osmosis membranes, Sep. Sci. Technol. 17 (1982) 2. [24] R. Oizerovich-Honig, V. Raim, S. Srebnik, Simulation of thin film membranes formed by interfacial polymerization, Langmuir 26 (1) (2010) 299–306. [25] R.M. Barrer, S.D. James, Electrochemistry of crystal-polymer membranes. 1. Resistance measurements, J. Phys. Chem. B 64 (1960) 417. [26] B.J. Hinds., Aligned multiwalled carbon nanotube membranes, Science 303 (5654) (2004) 62–65. [27] W.F. Chan, H.Y. Chen, A. Surapathi, M.G. Taylor, X.H. Shao, E. Marand, J.K. Johnson, Zwitterion functionalized carbon nanotube/polyamide nanocomposite membranes for water desalination, ACS Nano (2013) 7. [28] C. Kong, T. Shintani, T. Kamada, V. Freger, T. Tsuru, Co-solvent-mediated synthesis of thin polyamide membranes, J. Membr. Sci. 384 (1–2) (2011) 10–16. [29] F. Yuan, Z. Wang, X.W. Yu, Z.H. Wei, S.C. Li, J.X. Wang, S.C. Wang, Visualization of the formation of interfacially polymerized film by an optical contact angle measuring device, J. Phys. Chem. C 116 (2012) 11496–11506. [30] G.L. Jadav, P.S. Singh, Synthesis of novel silica–polyamide nanocomposite membrane with enhanced properties, J. Membr. Sci. 328 (1-2) (2009) 257–267. [31] H.A. Shawky, Performance of aromatic polyamide RO membranes synthesized by interfacial polycondensation process in a water–tetrahydrofuran system, J. Membr. Sci. 339 (1-2) (2009) 209–214. [32] E.S. Kim, B. Deng, Fabrication of polyamide thin-film nano-composite (PA-TFN) membrane with hydrophilized ordered mesoporous carbon (H-OMC) for water purifications, J. Membr. Sci. 375 (1–2) (2011) 46–54. [33] H. Wu, B. Tang, P. Wu, Optimizing polyamide thin film composite membrane covalently bonded with modified mesoporous silica nanoparticles, J. Membr. Sci. 428 (2013) 341–348.