- Email: [email protected]
Freeway single and multi-vehicle crash safety analysis: Influencing factors and hotspots
Journal of Membrane Science 592 (2019) 117375
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Hydrophilic polymers of intrinsic microporosity as water transport nanochannels of highly permeable thin-film nanocomposite membranes used for antibiotic desalination Zhanghui Wang, Shuang Guo, Bin Zhang, Liping Zhu
T
⁎
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China
ARTICLE INFO
ABSTRACT
Keywords: Polymer of intrinsic microporosity β-Cyclodextrin Thin-film nanocomposite nanofiltration membrane Antibiotic desalination
Nanofiltration (NF) membranes with superior perm-selectivity are highly desirable in separation and purification processes. In this work, a highly permeable thin film nanocomposite (TFN) NF membrane containing hydrophilic polymer nanoparticles of intrinsic microporosity (PIM NPs) in polyamide (PA) separation layer is developed for antibiotic desalination. The hydrophilic PIM NPs were synthesized by one-pot polycondensation with hydrophilic β-cyclodextrin (β-CD) as one of building units and used as an additive of aqueous phase in the preparation TFN membranes by interfacial polymerization. Unlike commonly-used inorganic nanomaterials, the β-CD-PIM NPs are well compatible with PA matrix due to their polymer characteristic. Moreover, the hydroxyl groups in βCD can participate in interfacial polymerization and thus the interfacial defects between β-CD-PIM NPs and PA matrix are effectively eliminated. As a result, the TFN membranes exhibit not only high water permeance (15.3 L m−2 h−1 bar−1), about three times as large as that of the prinstine membrane, but also high divalent salt rejection (RNa2SO4 = 95.1%). It is believed that the interior micropores inside β-CD-PIM NPs play a role of water transport nanochannels and contribute to high water permeance of the membranes. The long-term durability of the membranes are superior due to the chemical bonding between β-CD-PIM NPs and PA matrix. In addition, the TFN membranes were used for antibiotics desalination due to their high rejection towards antibiotics, but low rejection towards sodium chloride. This work offers a new strategy for constructing water transport nanochannels in separation layer to develop high performance TFN NF membranes.
1. Introduction Antibiotics have been widely used in medical treatment of bacterial infection due to their antimicrobial charicteristics. In pharmaceutical industry, the extraction of antibiotics from fermentation broth after removing microorganisms and protein impurities is a key procedure to obtain pure antibiotics. Conventional methods e.g. adsorption, precipitation, solvent extraction and ion exchange for antibiotic extraction are limited by their complicated procedure, long operation time, severe pollution and easy inactivation of biomolecules as well as intensive energy consumption. Membrane separation technology is emerging as a promising alternative resolution to separation of antibiotics due to simple ambient operation, no phase change, high separation efficiency, cost-saving and energy-efficient virtues and has been paid much attention in recent years [1]. Nanofiltration (NF), one of membrane separation processes with
⁎
molecular weight cut-off ranging from 200 to 1000 Da, exhibits great potential in antibiotic desalination [2]. Some researchers have made many efforts on the construction of so-called thin-film nanocomposite (TFN) membranes incorporating nanomaterials inside active separation layers to achieve high permeability without sacrifice of selectivity [3]. Previously reported nanomaterials used for TFN membranes mainly are inorganic materials, including graphene oxide [4,5], carbon nanotube [6–8], zeolite [9] and silica [10]. However, inorganic nanomaterials are easy to leach out from membrane matrix due to their poor compatibility and interfacial interaction with polymer matrix, which is disadvantageous to long-term stability in separation operation [11]. Surface chemical modification is a feasible strategy to improve the compatibility of inorganic nanomaterials with polymer matrix prior to use. But this brings extra steps and thus higher costs to the preparation of TFN membranes. To solve above-mentioned problems, some organic nanomaterials
Corresponding author. E-mail address: [email protected] (L. Zhu).
https://doi.org/10.1016/j.memsci.2019.117375 Received 25 May 2019; Received in revised form 9 August 2019; Accepted 13 August 2019 Available online 14 August 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
are recently developed as alternatives of inorganic materials for preparing high-performance TFN membranes [12–15]. A notable example is polymers of intrinsic microporosity (PIMs), a class of nanoporous polymer materials with abundant interconnected pores and permanent porosity due to their highly rigid and contorted molecular structures [16–18]. Actually, PIMs have shown great potential as NF membrane materials used in organic solvents. In 2012, Fritsch et al. [19] reported a TFC membrane prepared by dip-coating of PIM on polyacrylonitrile (PAN) support membrane, which exhibited high separation performances in solvent-resistant NF. Livingston et al. [20] successfully prepared an ultrathin polyarylate microporous nanofilms with thickness down to 20 nm by interfacial polymerization and the composite membranes showed outstanding separation performance in organic solvents. Although many important breakthroughs have been acheived on PIMs as membrane materials, rare work has been done on the applications of PIMs membranes in aqueous separation systems. In our recent work, a kind of novel crosslinked and nonsoluble PIM nanoparticles (NPs) modified by β-cyclodextrin (β-CD), named β-CDPIM, were successfully synthesized by one-pot polycondensation reaction [21]. This nanoporous material exhibited superior performances in adsorption removal of organic micropollutants from water [22]. The nanosized and polymeric characteristics make β-CD-PIM NPs great potential as nanofillers compatible with polymer matrix in TFN membranes. The interconnected micropores inside the NPs could act as water transport nanochannels in active separation layer and thus strengthen mass transfer in membrane. The residual hydroxyl groups in β-CD-PIM may participate in interfacial polymerization to eliminate the interfacial defects between β-CD-PIM NPs and PA (PA) and obtain defect-free separation layer. The objective of this work is to verify the feasibility of β-CD-PIM NPs as an additive of aqueous phase in interfacial polymerization to fabricate highly permeable and highly selective TFN NF membranes. The application of the developed TFN membranes in antibiotic desalination were investigated in detail. To the best of our knowledge, this should be the first report on TFN membranes with PIM NPs as permeation-enhanced nanofillers.
sufficient hydrochloric acid to neutralize the residual K2CO3 until carbon dioxide evolution stopped. The precipitates were collected by centrifugation and washed thoroughly with water/ethanol repeatedly to remove residual monomers and solvent. Finally, the solid was dried by the lyophilization until the mass was constant. As a control sample, PIM-1 without β-CD, was also synthesized with only TTSBI (10 mmol) and TFTPN (10 mmol) as monomers following a similar procedure. 2.3. Preparation of TFN membranes The TFN membrane was prepared through interfacial polymerization on the PSF-UF support membrane, as schemed in Fig. 1b. In a typical procedure, a PSF-UF membrane was immersed into the aqueous solution (pH = 12.0) containing PIP (0.35%, w/v) and β-CD-PIM NPs (0.01–0.15 w/v%) for 2 min, and the excessive solution was drained off from the membrane surface. Then, the hexane solution containing TMC (0.2 w/v%) was poured on the surface of PSF-UF membrane for 1 min. After removing excessive organic solution, the membrane was cured in an oven at 50 °C for 15 min for further polymerization. Finally, the obtained membrane was rinsed thoroughly with deionized water to remove chemical residues and stored in deionized water for further use. As a control sample, a control TFC membrane without β-CD-PIM NPs in the aqueous phase was also prepared following a similar procedure. 2.4. Characterizations of β-CD-PIM NPs and the membranes The chemical structures of PIM and β-CD-PIM NPs were analysed by Fourier transform infrared spectroscopy (FTIR, Vector-22, Switzerland) in the range of 400–4000 cm−1 and solid-state cross-polarization/ magic angle spinning (CP/MAS) 13C nuclear magnetic resonance spectroscopy (NMR, Bruker Advance-III, Germany). The thermal stability was characterized by thermogravimetric analysis (TGA, TA-Q500, USA) under nitrogen atmosphere by heating samples up to 800 °C at a rate of 10 °C/min in nitrogen atmosphere. Field emission-scanning electron microscopy (FE-SEM, HitachiS-4800, Japan) was used to observe the morphologies of polymer particles after the samples were sputtered with a 10–20 nm gold layer. Specific surface areas of the synthesized polymers were measured on AUTOSORB-IQ2-MP to characterize the porosity. In a typical procedure, a polymer sample was degassed at 80 °C for 48 h and then backfilled with N2. The isotherms were generated by incremental exposure to ultrahigh-purity N2 up to 1 atm in a liquid nitrogen (77 K) bath. To describe the adsorption process in slit pores of materials, non-localized density functional theory (DFT) and Monte-Carlo simulation were used to offer fluid structure in slit pores. The adsorption isotherms in model pores were measured on basis of the intermolecular potential energy between fluid-fluid and fluid-solid. The parameter of DFT equilibrium model was N2 at 77 K on carbon. The particle size, distribution, and zeta potential of the polymers were measured by dynamic light scattering (DLS, MALVERN, Zeta-sizer Nano ZSP, UK). Prior to characterizations for the membranes, all samples were dried at 50 °C under vacuum for 24 h. The surface and cross-section morphologies were observed with field emission-scanning electron microscopy (FE-SEM, HitachiS-4800, Japan) after sputtered with a 10–20 nm gold layer. To observe cross-sectional morphologies, membrane samples were freeze-fractured in liquid nitrogen. Membrane surface morphology was also analysed by atomic force microscopy (AFM, SPI-3800 N, Seiko Instruments Inc., Japan) with silicon tips (NSG10, NT-MDT, ca. 330 kHz) in tapping mode. Membrane surface chemistry was analysed by infrared spectrophotometer equipped with an attenuated total reflection accessory (ATR-FTIR, Nicolet 6700, USA). To characterize membrane charged properties, surface streaming potentials were measured at various pH values using an electrokinetic analyzer (SurPASS, Anton Paar, Austria). To evaluate hydrophilic/hydrophobic character, membrane surface water contact angle was measured by an OCA-20 contact angle meter at 25 °C.
2. Experimental section 2.1. Materials A polysulfone ultrafiltration membrane (PSF-UF) with molecular weight cut-off of 20 kDa was kindly provided by Development Center of Water Treatment Technology, Hangzhou, China, and used as the support substrate of TFC/TFN membrane. 5,5′,6,6′-Tetrahydroxy-3,3,3′,3′-tetramethylspirobisindane (TTSBI), 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN) and erythromycin (ERY) were obtained from Energy Chemical. PIP and TMC were purchased from Aladdin Reagent and Shanghai Kaisai Chemical Co., Ltd. China, respectively. β-CD, anhydrous potassium carbonate (K2CO3), sodium sulfate (Na2SO4), magnesium sulfate (MgSO4), sodium chloride (NaCl), magnesium chloride (MgCl2), sodium hydroxide (NaOH), polyethylene glycol (PEGs, molecular weight 200, 300, 400, 600 and 800 Da, respectively), hexane, toluene and dimethylacetamide (DMAc) were purchased from Sinopharm Group Chemical Reagent Co. All of the chemicals were analytical grade and used as received without further purification. 2.2. Synthesis of β-CD-PIM NPs The synthesis of β-CD-PIM NPs follows a reported procedure [21]. The synthesis method is illustrated in Fig. 1a. In brief, β-CD (3 mmol), TTSBI (7 mmol), TFTPN (10 mmol), K2CO3 (30 mmol), toluene (15 mL) and DMAc (60 mL) were added into a 250 mL of three-neck round bottom flask equipped with a magnetic stirrer, nitrogen inlet, deanstark trap and reflux condenser. Then the reaction mixture was stirred at 110 °C for 12 h under nitrogen atmosphere. Afterwards, the obtained brown suspension was poured into an excess of water/ethanol with 2
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 1. a, The synthesis route and chemical structure of the β-CD-PIM NPs. b, Schematic diagram for the fabrication of the TFN membranes.
2.5. NF performance tests
synthesized using only TTSBI and TFTPN as monomers. The chemical structures of the synthesized PIM-1 and β-CD-PIM NPs were characterized by FTIR spectroscopies and solid-state 13C NMR. The FTIR spectra of β-CD-PIM NPs (Fig. S1) demonstrate the characteristic absorption peaks of β-CD (3400 cm−1 for –OH) [23] and PIM (2240 cm−1 for –C^N) [24], indicating that the synthesized β-CD-PIM NPs contained the β-CD and PIM-1. The NMR spectrum of β-CD-PIM NPs (Fig. 2a) shows an evident resonance at a chemical shift of 96.7 ppm (labelled 1′), which is ascribed to newly formed alkoxy groups. This result suggests that the hydroxyl hydrogen of β-CD was substituted by TFTPN. The results of NMR and FTIR spectroscopies confirm the successful introduction of β-CD into PIM-1. Calculated from the results of TGA (Fig. S2), the weight ratio of β-CD in the synthesized β-CD-PIM NPs is about 40%. SEM observation shows the synthesized β-CD-PIM appears as uniform globular particles with a diameter of 90 ± 10 nm (Fig. 2b). As shown in the inset of Fig. 2b, the average particle size of βCD-PIM NPs determined by DLS is about 110 nm, which is in well agreement with that from SEM observation. The nanosized and polymeric β-CD-PIM particles are expected to serve as nanofillers well compatible with polymer matrix in interfacial polymerization to construct a defect-free separation layer for TFN membranes. To characterize the porosity of the synthesized β-CD-PIM NPs, the N2 adsorption isotherms were determined by Brunauer–Emmett–Teller (BET) tests. The specific surface area (SBET) of β-CD-PIM NPs (360 m2 g−1) is less than that of PIM-1 (591 m2 g−1) (Fig. S3), which is attributed to the decreased proportion of PIM-1 repeat units. The results of pore size distribution (Fig. 2c and d) show that both PIM-1 and β-CDPIM NPs possess a majority of micropores and a small proportion of mesopores. Calculated from the N2 adsorption isotherms by a density functional theory (DFT) -based method, the average size of the micropores is approximately 1.6 nm, which is well in agreement with the results of molecular dynamic simulation (Fig. S4). The mesopore size ranges from 2.5 to 25 nm, as calculated by the Barrett–Joyner–Halenda (BJH)-based method. Just like PIM-1, β-CD-PIM has a highly rigid and contorted molecular structure that cannot pack space efficiently and thus a continuous network of interconnected intermolecular voids is
The NF performances of the investigated membranes were evaluated by a cross-flow flat apparatus with a 1.0 L of feed tank at 25 °C, 6 bar and pH 6.5. Effective area for each membrane sample was 22.4 cm2 and all tests were carried out for at least 3 times. In a typical procedure, a membrane was pre-compacted by pure water at 6 bar for 1 h until the pure permeance reached a steady state. Then, the NF performance tests were carried out with aqueous solutions of inorganic salts (Na2SO4, MgSO4, NaCl and MgCl2, respectively) or ERY/NaCl mixture as the feed. The permeate was collected for a period of time and weighted using an electric balance to determine the water permeance P (L m−2 h−1 bar−1), which was calculated by the following equation:
P=
Am ×
V t× p
(1)
where V (L) is the volume of permeate, Am (m2) is the effective membrane area, t (h) is the testing time, and p (bar) is the transmembrane pressure. The solute rejection R (%) was calculated according to equation (2):
R = (1
CP ) × 100 Cf
(2)
where Cp and Cf represent the concentrations of solute in permeate and the feed, respectively. The concentrations of inorganic salts were measured by a conductivity meter (FE30, Mettler-Toledo, Switzerland) and the concentrations of organics (ERY, PEG) were tested through total organic carbon analysis meter (TOC-L, Shimadzu, Japan). 3. Results and discussion 3.1. Synthesis and characterizations of β-CD-PIM NPs The β-CD-PIM NPs were synthesized via one-pot nucleophilic substitution polycondensation with TTSBI, TFTPN and β-CD as the monomers. The reaction was carried out in a polar aprotic solvent (DMAc) with K2CO3 as the catalyst. As a control sample, PIM-1 was also 3
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 2. a, Solid-state 13C NMR spectra of the PIM-1 and the β-CD-PIM NPs. b, SEM image of the β-CD-PIM NPs. The inset is the particle size distribution of β-CD-PIM NPs in water (pH = 6.5). c, Pore size distributions of the micropores calculated by the DFT method based on N2 adsorption isotherms. d, Pore size distributions of the mesopores calculated by the BJH method based on N2 adsorption isotherms. e, Water contact angle and images of aqueous suspension of PIM-1 (left) and β-CD-PIM NPs (right). f, Zeta potential of β-CD-PIM NPs in water.
achieved [21]. The unique hierarchically micro/mesoporous character renders the β-CD-PIM NPs potential application as selective transport passages of matters. Hydrophilicity is a vital character for microporous materials used in aqueous system. To evaluate the hydrophilicity/hydrophobicity, static contact angle of water droplet on polymer tablet prepared by filtrating and depositing PIM-1 or β-CD-PIM NPs on a polyvinylidene difluoride microfiltration membrane were examined. It was found the water contact angle on the β-CD-PIM surface was only 51 ± 5°, much lower than that of PIM-1 tablet (125 ± 7°) (Fig. 2e). This result indicated that β-CD-PIM was much more hydrophilic than PIM-1 due to the introduction of β-CD. Moreover, it can also be seen that the β-CD-PIM NPs were well dispersed in water while the PIM-1 was segregated to water surface (Fig. 2e). This phenomenon further showed the hydrophilic nature of the synthesized β-CD-PIM NPs, which is advantageous to the wetting and access of inteior pores by water. The zeta potentials of β-
CD-PIM NPs at various pH were measured and the results are shown in Fig. 2f. It can be seen that the β-CD-PIM NPs are negatively-charged (−30 mV at pH 6.5). This phenomenon should be attributed to the presence of strongly polar electrophilic nitrile groups. When used as a nanofiller in the preparation of TFN membranes, negatively-charged βCD-PIM may have an influence on membrane surface potential and separation performanes. 3.2. Characterizations of the TFN membranes The TFN membranes entrapped with β-CD-PIM NPs were prepared by interfacial polymerization of aqueous solution containing PIP and βCD-PIM NPs (aqueous phase) with hexane solution containing TMC (oil phase). The obtained TFN membranes are named as TFN-X, where X represents the weight percentage of β-CD-PIM NPs in aqueous phase. As a control membrane, control TFC membrane was also prepared with 4
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
are 99 ± 12 nm, 101 ± 9 nm and 98 ± 13 nm, respectively. This indicates the introduction of β-CD-PIM NPs had no obvious influence on the thickness of generated PA separation layer. The root mean square (RMS) roughness of the membranes was examined through AFM and the images and RMS values are shown in Fig. 4g–i. Compared with the control TFC membrane (12.8 nm of RMS value), the RMS roughness for the TFN-0.10% and the TFN-0.15% membranes increased to 20.9 nm and 31.4 nm, respectively. The rougher membrane surface often possesses the larger active permeation area and thus is beneficial to the enhancement of membrane permeability. Membrane surface hydrophilicity was evaluated by static water contact angle measurements. As shown in Fig. 5a, the contact angle decreased with the increasing loading of β-CD-PIM NPs in the membranes. This result is attributed to the increase quantity of carboxylic groups generated by hydrolysis of acyl chloride groups on membrane surface as well as the introduction of hydrophilic β-CD-PIM NPs. The enhanced hydrophlicity facilitates the permeation of water molecules through the membranes. The zeta potentials of the membranes were also examined and the results are shown in Fig. 5b. It can be seen that the zeta potential of the control TFC membrane is about −47 mV, demonstrating a typical negatively-charged characteristic of PA membranes. The potentials for the TFN-0.10%, TFN-0.15% membranes are −65 and −74 mV, respectively. The highly negative potentials should be attributed to the increased quantity of carboxylic groups and the introduction of β-CD-PIM NPs that exhibits −30 mV of potential at pH = 6.5 as discussed above in Section 3.1). The negatively charged character is advantageous to improve the rejection towards multivalent anions for the membranes due to Donnan exclusion effect [29]. To characterize the molecular weight cut-off (MWCO) of control TFC membrane and the TFN membranes, the rejection towards neutral PEGs with various molecular weights were tested. The value of MWCO is defined as the critical molecular weight of PEG molecule whose rejection is more than 90% [30]. As presented in Fig. 5c, the MWCOs for the control TFC, the TFN-0.10% and the TFN-0.15% membranes are about 270, 326 and 800 Da, respectively. The pore size distribution and mean pore size are calculated based on the theory established by Michaels and Singh [31] and the steric and hydrodynamic interactions between organic solute and the membrane can be neglected. The calculated results shown in Fig. 5d demonstrate that the mean effective pore radii for the control TFC, the TFN-0.10% and the TFN-0.15% membranes are around 0.29, 0.31 and 0.33 nm, respectively. It can be seen that the pore size change is not so great although it increases to some extent when β-CD-PIM NPs are introduced into interfacial polymerization. This indicates that the possible interfacial defects in the TFN membranes have been effectively controlled due to good compatibility between β-CD-PIM and PA matrix as well as the participation of β-CD-PIM in interfacial polymerization. Therefore, the increase of membrane pore size should be attributed to the reduced crosslinking degree of PA selective layer. The C/N molar ratio on membrane surface detected by X-ray photoelectron spectroscopy (XPS) increases from 5.4 for the control TFC membrane to 6.0 for the TFN-0.10% membrane (Fig. S5). This phenomenon confirms the decrease of PA crosslinking degree when β-CD-PIM NPs are introduced. The low crosslinking degree implies the formation of relatively loose PA selective layer, which is beneficial to the improvement of membrane permeability. The effective pore radii of the TFN membranes are smaller than the hydrated radius of SO42− (0.379 nm) [32], suggesting the membranes are potential in Na2SO4 removal from water.
Fig. 3. ATR-FTIR spectra of the control TFC membrane and the TFN-CD-PIM membranes.
only PIP and TMC as the monomers. The surface chemistry of the control TFC and the TFN membranes was charicterized by ATR-FTIR. In the spectra of TFN membranes as shown in Fig. 3, a new peak appeared at 2240 cm−1 is assigned to stretching vibration of C^N in β-CD-PIM NPs, verifying the successful incorporation of β-CD-PIM NPs into PA matrix. It can also be seen that the intensity of this peak increases as the dosage of β-CD-PIM NPs in aqueous phase increases, indicating the increasing loading amount of β-CD-PIM NPs in PA layer. Another new peak at 1730 cm−1 assigned to the stretching vibration of ester groups further proves the esterification reaction between residual hydroxyl groups of β-CD and acyl chloride groups of TMC [15]. This phenomenon shows that the amidation between PIP and TMC is accompanied by the esterification although the hydroxyl groups in β-CD are less reactive than PIP. This is advantageous to eliminate possible interfacial defects between the nanoparticles and PA matrix and endow the membranes with high durability. The surface morphologies of the TFN membranes were observed by SEM. For the control TFC membrane (Fig. 4a), a nodular structure can be seen on the top layer of PSF membrane, which is typical in PA TFC membranes prepared by interfacial polymerization [25]. In the SEM image of TFN-0.10% membrane (Fig. 4b), it can be seen that β-CD-PIM NPs distribute uniformly due to their good dispersity in aqueous phase and well compatibility with PA matrix. Compared with the control TFC membrane, the modular structures on the TFN-0.10% membrane are more obscure. It is accepted that the incorporation of nanoparticles hinders the diffusion of PIP and leads to slow interfacial polymerization rate. A a result, PA molecular chains arrange to some extent and thus a relatively smooth surface is achieved [26]. It is worth noting that the boundary between the embedded β-CD-PIM NPs and PA matrix is vague due to well compatibility between organic nanoparticle and PA matrix, which was different from the phenomenon observed in already reported TFN membranes containing inorganic nanomaterials or organic-inorganic hybrid nanomaterials (MOF) [9,27,28]. For the TFN-0.15% membrane (Fig. 4c), the nanoparticles tend to aggregate on membrane surface, which is an inevitable phenomenon in the preparation of TFN membrane when excessive nanomaterials are added. As we know, the thickness of separation layer greatly affects the permeability of membrane. Thin separation layer is advantageous to achieve high permeability. Estimated from the cross-sectional SEM images (Fig. 4d–f), the separation layer thicknesses for the control TFC membrane, the TFN-0.10% membrane and the TFN-0.15% membrane
3.3. NF performance of the TFN membranes The NF performances of the membranes were measured with a cross-flow flat membrane apparatus. The effects of β-CD-PIM NPs addition on pure water permeance (PWP) and Na2SO4 rejection were investigated in detail and the results are shown in Fig. 6a. When the addition of β-CD-PIM NPs increases, the PWP gradually increases while 5
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 4. a, b, c, SEM surface morphologies of control TFC, TFN-0.10% and TFN-0.15% membrane. d, e, f, SEM cross-section morphologies of control TFC, TFN-0.10% and TFN-0.15% membrane. g, h, i, AFM surface morphologies of control TFC, TFN-0.10% and TFN-0.15% membrane.
Fig. 5. a, Water contact angles of control TFC, TFN-0.10% and TFN-0.15% membranes. b, Surface zeta potentials of control TFC, TFN-0.10% and TFN-0.15% membranes. c, Rejections towards PEGs with various molecular weights. d, Pore size distribution of control TFC, TFN-0.10% and TFN-0.15% membranes.
Na2SO4 rejection decreases slightly. The TFN-0.10% membrane shows the optimal NF performance with 15.3 L m−2 h−1 bar−1 of water permeance, three times as large as that of the control TFC membrane, and
95.1% of Na2SO4 rejection. The improvement of water permeability can be attributed to several factors. Firstly, the increase of hydrophilicity facilitates the transport and permeation of water molecules inside 6
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 6. a, Effect of β-CD-PIM dosage on the PWP and Na2SO4 rejection for the investigated TFN membranes. b, Rejections towards various salts for the control TFC and the TFN-0.10% membranes. Operation condition: 1.0 g L−1 of aqueous salt solutions as the feed, pH = 6.5, 25 °C and 6 bar c, Schematic illustration of water channels in the TFN membranes.
membrane pore and thus water permeance increases [33]. Secondly, the rough membrane surface provides large effective area for the permeation of water molecules through membrane [34,35]. In addition, relatively loose PA layer resulted from the introduction of β-CD-PIM NPs also contributes to the enhancement of water permeance [36,37]. More importantly, the hydrophilic and interconnected pores throughout the β-CD-PIM NPs offer low-resistant water transport channels and allow water molecules quickly pass through the membranes [20] (Fig. 6c). It is well-known the rejection of salts is correlated with the electrostatic repulsive effect (density of negative electric charges) and steric hindrance effect (pore size of selective layer). Considering the improved negative charge density of TFN membrane, the slight decrease in Na2SO4 rejection should be attributed to relatively loose PA selective layer. Nevertheless, the Na2SO4 rejection keeps at a high level due to the formation of PA layer atop the entrapped βCD-PIM NPs. For comparison, the influnce of PIM-1 as an additive of aqueous phase on the NF performance of TFC membrane was investigated (Fig. S6). It is found that, as the PWP is higher than 9.5 L m−2 h−1 bar−1, the Na2SO4 rejection decreases drastically. This indicates that, when PIM-1 is used as the nanofiller, the defects may be generated in PA layer due to the aggregation of PIM-1 in water phase. In contrast, the defects can be effectively controlled when β-CD-PIM is used as the additive. Therefore, here it seems indispensable to introduce hydrophilic β-CD into PIM-1 to control the defects of selective layer and extend its application in TFN membranes. As shown in Fig. 6b, the rejections towards various salts for the control TFC and the TFN-0.10% membranes are compared. It can be seen that, for both the investigated membranes, the salt rejections follow an order of Na2SO4 > MgSO4 > MgCl2 > NaCl, which is in accordance with that of typical negative NF membranes [38,39]. According to the Donnan exclusion theory [39], there strong electrostatic repulsion exists between multivalent anion ions and negatively-charged membranes. Thus, the rejections towards Na2SO4 and MgSO4 is often higher than the rejection towards MgCl2 and NaCl. At the meanwhile, the electrostatic attraction between the counterions with NF membranes also has a remarkable influence on the rejection towards multivalent salts. Here the electrostatic attraction between counterions Mg2+ with the negatively-charged membranes is stronger than that between Na+ and the membranes. And thus Mg2+ ions are easier to transport through the membranes. As a result, the MgSO4 rejection is
lower than Na2SO4. NaCl has the lowest rejection due to small hydrated radius for Na+ ions. Compared with the control TFC membrane, the TFN-0.10% membrane keeps high rejection towards Na2SO4 (95.1%) but exhibits low NaCl rejection (35.4%) as a consequence of strong negatively-charged character but relatively loose selective layer. The TFN membranes with high selectivity towards di-/mono-valent salts can find their potential applications in antibiotic desalination. 3.4. Stability of the TFN membranes The stability of membrane during long-term operation is a crucial performance in practical applications. The water permeance and Na2SO4 rejection for the control TFC and the TFN-0.10% membranes were real-time monitored with the operation time and the results are shown in Fig. 7a. It can be seen that both of the TFC and the TFN-0.10% membranes exhibit stable Na2SO4 rejection and water permeance within 168 h (a week) of tests. The good durability of the TFN membranes can be attributed to the covalent bonding and thus no interfacial defects between β-CD-PIM NPs and PA matrix as discussed in Section 3.2. In addition, the hydrogen bonding between β-CD-PIM and PA also contributes to stability of the TFN membranes. Therefore, it is generally believed that the combination of covalent interaction (ester bonding) and non-covalent interaction (hydrogen bonding) are responsible for excellent durability of the TFN membranes, as illustrated in Fig. 7b. The effects of operation pressure on the NF performances of the membranes were evaluated. As shown in Fig. 7c, with the increase of the operation pressure from 2 to 6 bar, the water permeance slightly decreases while the Na2SO4 rejection increases to some extent for both the control TFC and the TFN-0.10% membranes. This can be attributed to the compaction effect of membranes under high pressure. The difference in driven force between water and solute also contributes to the increased Na2SO4 rejection. With the increase of operation pressure, the driven force for water increases while that for Na2SO4 remains steady, and thus the Na2SO4 rejection increases [40,41]. Additionally, the effects of operating temperature on the NF performances were also examined. As shown in Fig. 7d, both water permeance and Na2SO4 rejection for the investigated membranes increases linearly with the increase of operating temperature from 25 to 45 °C. It seems that the water permeance increases more rapidly than Na2SO4 rejection, which is in agreement with the results reported in literature [42]. At 45 °C of 7
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 7. a, The changes of water permeance and Na2SO4 rejection with the operation time (168 h of filtration, 1.0 g L−1 of Na2SO4 solution as the feed, 25 °C, pH 6.5 and 6 bar). b, Schematic illustration of the covalent and non-covalent interactions between the β-CD-PIM NPs and PA matrix. c, The changes of water permeance and Na2SO4 rejection with the operation pressure (1.0 g L−1 of Na2SO4 solution as the feed, 25 °C, pH 6.5 and 2–6 bar). d, The changes of water permeance and Na2SO4 rejection with the operating temperature (1.0 g L−1 of Na2SO4 solution as the feed, 25–45 °C, pH 6.5 and 6 bar).
operation temperature, the water permeance and the Na2SO4 rejection of the TFN-0.10% membrane increase up to 22.1 L m−2 h−1 bar−1 and 98.5%. This shows that the TFN membranes can be used stably at high operation temperature.
operation. In order to remove more salts from antibiotics solution, 300 mL of the concentrated ERY solution was re-diluted to 1000 mL with pure water and the filtration process was re-operated. This process was recycled two more times and the results are shown in Fig. 9. It can be seen that, after three filtration-dilution cycles, the ERY concentration in the retentate increases to 310 mg L−1 while the NaCl concentration decreases to only 3.2 g L−1. The salt concentration can be further decreased by more filtration cycles. Therefore, the desalination from ERY can be effectively achieved by multiple filtration-dilution cycles. Compared with the control TFC membrane, the TFN membranes developed in this work are more competent to antibiotic desalination due to their lower rejection towards NaCl.
3.5. Antibiotic desalination performances Desalination from antibiotics has a great demand in the pharmaceutical industry especially for the extraction of the antibiotics from fermentation broth. Erythromycin (ERY), a typical electro-neutral antibiotic (734 Da of molecular weight), was used as a model molecule to conduct desalination experiments (Fig. 8a) [43]. An aqueous solution containing 100 mg L−1 of ERY and 10 g L−1 of NaCl (the average concentrations in Luria-Bertani fermentation broth) was used as the feed liquid. The filtration was performed in a cross-flow flat apparatus with a 1.0 L of feed tank. In the filtration, the retentate was recycled and backflowed into the feed tank. The changes of water permeance with the operation time for the control TFC and the TFN-0.10% membranes are shown in Fig. 8b. It can be seen that the water permeance for the TFN0.10% membrane is much higher than that for the control membrane, which is similar to the change of pure water permeance as shown in Fig. 6a. The changes of ERY and NaCl rejections with the operation time are shown in Fig. 8c. Compared with the control membrane, the TFN0.10% membrane demonstrates nearly identical ERY rejection (about 97%) but lower NaCl rejection within the whole 6 h of filtration process. It should be noted that the NaCl rejection decreases slowly with the operation time, which is attributed to the enhanced electrostatic shielding effect due to the gradually increased NaCl concentration in feed solution in separation process. The changes of ERY and NaCl concentrations in the recycled retentate with the operation time were monitored and the data are shown in Fig. 8d. It can be seen that, after 6 h of filtration operation using the TFN-0.10% membrane, the ERY concentration in the retentate increases from 100 to 320 mg L−1 while the NaCl concentration changes only a little. In the meanwhile, the volume of the retentate decreases from 1000 mL to 300 mL. This is a typical concentration and desalination process for antibiotics by NF
4. Conclusions The TFN membranes impregnated with β-CD-PIM NPs in PA selective layer were successfully fabricated via interfacial polymerization. βCD-PIM NPs exhibited good hydrophilicity due to the introduction of βCD, facilitating their dispersion in aqueous phase. Meanwhile, the synthesized β-CD-PIM NPs are cross-linked and insoluble polymers. The nanosized and polymeric characteristics make the β-CD-PIM NPs well compatible with PA matrix. The presence of β-CD-PIM NPs in PA selective layer was verified. The formation of ester bonding between βCD-PIM NPs and TMC is advantageous to elimination of possible interfaical defects between NPs and PA matrix. The optimal TFN membrane exhibited high water permeance (15.3 L m−2 h−1 bar−1), nearly three times as large as that of control TFC membrane, without great sacrifice of salt rejection (RNa2SO4 = 95.1%). The enhanced water permeability was attributed to the improvement of membrane hydrophilicity, the increase of surface roughness, reduced crosslinking degree of PA layer and the additional water transport channels inside β-CDPIM NPs. In addition, the TFN membrane exhibited superior stability under different operation conditions due to the covalent bonding (ester bonding formed) and non-covalent bonding (hydrogen bonding) between β-CD-PIM and PA matrix. The TFN membranes demonstrated
8
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 8. a, Schematic illustration of the TFN membranes used for antibiotic desalination. b, The change of water permeance with the operation time. c, The changes of ERY and NaCl rejections with the operation time. d, The changes of ERY and NaCl concentrations in the recycled retentate with the operation time. The feed liquid was the mixed aqueous solution (pH 6.5) containing 100 mg L−1 of ERY and10 g L−1 of NaCl. The filtration was operated at 25 °C under 6 bar of the trans-membrane pressure.
Conflicts of interest There are no conflicts of interest to declare. Acknowledgements We are grateful for the financial supports from the National Natural Science Foundation of China, China (Grant No. 51773175, 51573159 and 51828301) and the Fundamental Research Funds for the Central Universities, China (Grant No. 2019QNA4062). We also thank Ms. Li Xu at State Key Laboratory of Chemical Engineering in Zhejiang University for the assistance in performing polymer and membrane tests. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.117375. Fig. 9. The changes of ERY and NaCl concentrations in the back-flowed retentate with the operation time for three filtration-dilution cycles. The original feed liquid was the mixed aqueous solution (pH 6.5) containing 100 mg L−1 of ERY and10 g L−1 of NaCl. The filtration was operated at 25 °C under 6 bar of the trans-membrane pressure.
References [1] E. Yang, K.-J. Chae, M.-J. Choi, Z. He, I.S. Kim, Critical review of bioelectrochemical systems integrated with membrane-based technologies for desalination, energy selfsufficiency, and high-efficiency water and wastewater treatment, Desalination 452 (2019) 40–67, https://doi.org/10.1016/j.desal.2018.11.007. [2] R. Mulyanti, H. Susanto, Wastewater treatment by nanofiltration membranes, IOP Conf. Ser. Earth Environ. Sci. 142 (2018) 012017, , https://doi.org/10.1088/17551315/142/1/012017. [3] Y. Cheng, Y. Ying, S. Japip, S.-D. Jiang, T.-S. Chung, S. Zhang, D. Zhao, Advanced porous materials in mixed matrix membranes, Adv. Mater. 30 (2018), https://doi. org/10.1002/adma.201802401 1802401.
great potential in antibiotics desalination due to their high rejection towards antibiotics, but low rejection towards NaCl. The overall results highlighted the potential of hydrophilic PIMs in the development of high-performance TFN NF membranes. 9
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al. [4] S. Bano, A. Mahmood, S.-J. Kim, K.-H. Lee, Graphene oxide modified polyamide nanofiltration membrane with improved flux and antifouling properties, J. Mater. Chem. 3 (2015) 2065–2071, https://doi.org/10.1039/C4TA03607G. [5] J. Yin, G. Zhu, B. Deng, Graphene oxide (GO) enhanced polyamide (PA) thin-film nanocomposite (TFN) membrane for water purification, Desalination 379 (2016) 93–101, https://doi.org/10.1016/j.desal.2015.11.001. [6] Y.-S. Guo, Y.-F. Mi, F.-Y. Zhao, Y.-L. Ji, Q.-F. An, C.-J. Gao, Zwitterions functionalized multi-walled carbon nanotubes/polyamide hybrid nanofiltration membranes for monovalent/divalent salts separation, Separ. Purif. Technol. 206 (2018) 59–68, https://doi.org/10.1016/j.seppur.2018.05.048. [7] W.-F. Chan, E. Marand, S.M. Martin, Novel zwitterion functionalized carbon nanotube nanocomposite membranes for improved RO performance and surface antibiofouling resistance, J. Membr. Sci. 509 (2016) 125–137, https://doi.org/10. 1016/j.memsci.2016.02.014. [8] T.-Y. Liu, H.-G. Yuan, Q. Li, Y.-H. Tang, Q. Zhang, W. Qian, B. Van der Bruggen, X. Wang, Ion-responsive channels of zwitterion-carbon nanotube membrane for rapid water permeation and ultrahigh mono-/multivalent ion selectivity, ACS Nano 9 (2015) 7488–7496, https://doi.org/10.1021/acsnano.5b02598. [9] 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 (2007) 1–7, https://doi.org/10. 1016/j.memsci.2007.02.025. [10] 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, https://doi.org/10.1016/j.memsci.2012.10.053. [11] W.J. Lau, S. Gray, T. Matsuura, D. Emadzadeh, J. Paul Chen, A.F. Ismail, A review on polyamide thin film nanocomposite (TFN) membranes: history, applications, challenges and approaches, Water Res. 80 (2015) 306–324, https://doi.org/10. 1016/j.watres.2015.04.037. [12] Y.-L. Ji, Q.-F. An, Y.-S. Guo, W.-S. Hung, K.-R. Lee, C.-J. Gao, Bio-inspired fabrication of high perm-selectivity and anti-fouling membranes based on zwitterionic polyelectrolyte nanoparticles, J. Mater. Chem. 4 (2016) 4224–4231, https://doi. org/10.1039/C6TA00005C. [13] C. Wang, Z. Li, J. Chen, Z. Li, Y. Yin, L. Cao, Y. Zhong, H. Wu, Covalent organic framework modified polyamide nanofiltration membrane with enhanced performance for desalination, J. Membr. Sci. 523 (2017) 273–281, https://doi.org/10. 1016/j.memsci.2016.09.055. [14] C. Li, S. Li, L. Tian, J. Zhang, B. Su, M.Z. Hu, Covalent organic frameworks (COFs)incorporated thin film nanocomposite (TFN) membranes for high-flux organic solvent nanofiltration (OSN), J. Membr. Sci. 572 (2019) 520–531, https://doi.org/10. 1016/j.memsci.2018.11.005. [15] Y.-L. Ji, Q.-F. An, X.-D. Weng, W.-S. Hung, K.-R. Lee, C.-J. Gao, Microstructure and performance of zwitterionic polymeric nanoparticle/polyamide thin-film nanocomposite membranes for salts/organics separation, J. Membr. Sci. 548 (2018) 559–571, https://doi.org/10.1016/j.memsci.2017.11.057. [16] C. Ma, J.J. Urban, Polymers of intrinsic microporosity (PIMs) gas separation membranes: a mini review, Proc. Nat. Res. Soc. 2 (2018) 02002. [17] N.B. McKeown, P.M. Budd, Polymers of intrinsic microporosity (PIMs): organic materials for membrane separations, heterogeneous catalysis and hydrogen storage, Chem. Soc. Rev. 35 (2006) 675–683, https://doi.org/10.1039/B600349D. [18] P.M. Budd, N.B. McKeown, Highly permeable polymers for gas separation membranes, Polym. Chem. 1 (2010) 63–68, https://doi.org/10.1039/B9PY00319C. [19] D. Fritsch, P. Merten, K. Heinrich, M. Lazar, M. Priske, High performance organic solvent nanofiltration membranes: development and thorough testing of thin film composite membranes made of polymers of intrinsic microporosity (PIMs), J. Membr. Sci. 401–402 (2012) 222–231, https://doi.org/10.1016/j.memsci.2012.02. 008. [20] M.F. Jimenez-Solomon, Q. Song, K.E. Jelfs, M. Munoz-Ibanez, A.G. Livingston, Polymer nanofilms with enhanced microporosity by interfacial polymerization, Nat. Mater. 15 (2016) 760–767, https://doi.org/10.1038/nmat4638. [21] Z. Wang, F. Cui, Y. Pan, L. Hou, B. Zhang, Y. Li, L. Zhu, Hierarchically micromesoporous β-cyclodextrin polymers used for ultrafast removal of micropollutants from water, Carbohydr. Polym. 213 (2019) 352–360, https://doi.org/10.1016/j. carbpol.2019.03.021. [22] Z. Wang, B. Zhang, C. Fang, Z. Liu, J. Fang, L. Zhu, Macroporous membranes doped with micro-mesoporous β-cyclodextrin polymers for ultrafast removal of organic micropollutants from water, Carbohydr. Polym. 222 (2019), https://doi.org/10. 1016/j.carbpol.2019.114970 114970. [23] A. Alsbaiee, B.J. Smith, L. Xiao, Y. Ling, D.E. Helbling, W.R. Dichtel, Rapid removal of organic micropollutants from water by a porous β-cyclodextrin polymer, Nature 529 (2016) 190–194, https://doi.org/10.1038/nature16185. [24] F.Y. Li, Y. Xiao, T.-S. Chung, S. Kawi, High-performance thermally self-cross-linked
[25]
[26]
[27]
[28] [29] [30]
[31] [32]
[33] [34] [35] [36]
[37]
[38]
[39] [40]
[41]
[42]
[43]
10
polymer of intrinsic microporosity (PIM-1) membranes for energy development, Macromolecules 45 (2012) 1427–1437, https://doi.org/10.1021/ma202667y. X. Kong, Y. Zhang, S.-Y. Zeng, B.-K. Zhu, L.-P. Zhu, L.-F. Fang, H. Matsuyama, Incorporating hyperbranched polyester into cross-linked polyamide layer to enhance both permeability and selectivity of nanofiltration membrane, J. Membr. Sci. 518 (2016) 141–149, https://doi.org/10.1016/j.memsci.2016.07.037. M. Wu, J. Yuan, H. Wu, Y. Su, H. Yang, X. You, R. Zhang, X. He, N.A. Khan, R. Kasher, Z. Jiang, Ultrathin nanofiltration membrane with polydopamine-covalent organic framework interlayer for enhanced permeability and structural stability, J. Membr. Sci. 576 (2019) 131–141, https://doi.org/10.1016/j.memsci.2019. 01.040. J. Zhu, L. Qin, A. Uliana, J. Hou, J. Wang, Y. Zhang, X. Li, S. Yuan, J. Li, M. Tian, J. Lin, B. Van der Bruggen, Elevated performance of thin film nanocomposite membranes enabled by modified hydrophilic MOFs for nanofiltration, ACS Appl. Mater. Interfaces 9 (2017) 1975–1986, https://doi.org/10.1021/acsami.6b14412. Y. He, Y.P. Tang, D. Ma, T.-S. Chung, UiO-66 incorporated thin-film nanocomposite membranes for efficient selenium and arsenic removal, J. Membr. Sci. 541 (2017) 262–270, https://doi.org/10.1016/j.memsci.2017.06.061. R.J. Petersen, Composite reverse osmosis and nanofiltration membranes, J. Membr. Sci. 83 (1993) 81–150, https://doi.org/10.1016/0376-7388(93)80014-O. R. Rohani, M. Hyland, D. Patterson, A refined one-filtration method for aqueous based nanofiltration and ultrafiltration membrane molecular weight cut-off determination using polyethylene glycols, J. Membr. Sci. 382 (2011) 278–290, https://doi.org/10.1016/j.memsci.2011.08.023. S. Singh, K.C. Khulbe, T. Matsuura, P. Ramamurthy, Membrane characterization by solute transport and atomic force microscopy, J. Membr. Sci. 142 (1998) 111–127, https://doi.org/10.1016/S0376-7388(97)00329-3. L.A. Richards, B.S. Richards, A.I. Schäfer, Renewable energy powered membrane technology: salt and inorganic contaminant removal by nanofiltration/reverse osmosis, J. Membr. Sci. 369 (2011) 188–195, https://doi.org/10.1016/j.memsci. 2010.11.069. Z. Yu, Y. Pan, Y. He, G. Zeng, H. Shi, H. Di, Preparation of a novel anti-fouling βcyclodextrin–PVDF membrane, RSC Adv. 5 (2015) 51364–51370, https://doi.org/ 10.1039/C5RA04894J. Z. Wang, Z. Wang, S. Lin, H. Jin, S. Gao, Y. Zhu, J. Jin, Nanoparticle-templated nanofiltration membranes for ultrahigh performance desalination, Nat. Commun. 9 (2018), https://doi.org/10.1038/s41467-018-04467-3. L.F. Villalobos, T. Huang, K.-V. Peinemann, Cyclodextrin films with fast solvent transport and shape-selective permeability, Adv. Mater.. 29 (n.d.) 1606641. doi:10. 1002/adma.201606641. J. Zhu, M. Tian, J. Hou, J. Wang, J. Lin, Y. Zhang, J. Liu, B.V. der Bruggen, Surface zwitterionic functionalized graphene oxide for a novel loose nanofiltration membrane, J. Mater. Chem. 4 (2016) 1980–1990, https://doi.org/10.1039/ C5TA08024J. J. Lin, W. Ye, H. Zeng, H. Yang, J. Shen, S. Darvishmanesh, P. Luis, A. Sotto, B. Van der Bruggen, Fractionation of direct dyes and salts in aqueous solution using loose nanofiltration membranes, J. Membr. Sci. 477 (2015) 183–193, https://doi.org/10. 1016/j.memsci.2014.12.008. Q. Xie, W. Shao, S. Zhang, Z. Hong, Q. Wang, B. Zeng, Enhancing the performance of thin-film nanocomposite nanofiltration membranes using MAH-modified GO nanosheets, RSC Adv. 7 (2017) 54898–54910, https://doi.org/10.1039/ C7RA11550D. M.J.T. Raaijmakers, N.E. Benes, Current trends in interfacial polymerization chemistry, Prog. Polym. Sci. 63 (2016) 86–142, https://doi.org/10.1016/j. progpolymsci.2016.06.004. X.Q. Cheng, Z.X. Wang, Y. Zhang, Y. Zhang, J. Ma, L. Shao, Bio-inspired loose nanofiltration membranes with optimized separation performance for antibiotics removals, J. Membr. Sci. 554 (2018) 385–394, https://doi.org/10.1016/j.memsci. 2018.03.005. K. Rezzadori, F. Marques Penha, M.C. Proner, G. Zin, J.C. Cunha Petrus, P. Prádanos, L. Palacio, A. Hernández, M. Di Luccio, Evaluation of reverse osmosis and nanofiltration membranes performance in the permeation of organic solvents, J. Membr. Sci. 492 (2015) 478–489, https://doi.org/10.1016/j.memsci.2015.06. 005. M.F.A. Goosen, S.S. Sablani, S.S. Al-Maskari, R.H. Al-Belushi, M. Wilf, Effect of feed temperature on permeate flux and mass transfer coefficient in spiral-wound reverse osmosis systems, Desalination 144 (2002) 367–372, https://doi.org/10.1016/ S0011-9164(02)00345-4. Y. He, G. Chen, Z. Ji, S. Li, Combined UF–NF membrane system for filtering erythromycin fermentation broth and concentrating the filtrate to improve the downstream efficiency, Separ. Purif. Technol. 66 (2009) 390–396, https://doi.org/ 10.1016/j.seppur.2008.12.007.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Hydrophilic polymers of intrinsic microporosity as water transport nanochannels of highly permeable thin-film nanocomposite membranes used for antibiotic desalination Zhanghui Wang, Shuang Guo, Bin Zhang, Liping Zhu
T
⁎
MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China
ARTICLE INFO
ABSTRACT
Keywords: Polymer of intrinsic microporosity β-Cyclodextrin Thin-film nanocomposite nanofiltration membrane Antibiotic desalination
Nanofiltration (NF) membranes with superior perm-selectivity are highly desirable in separation and purification processes. In this work, a highly permeable thin film nanocomposite (TFN) NF membrane containing hydrophilic polymer nanoparticles of intrinsic microporosity (PIM NPs) in polyamide (PA) separation layer is developed for antibiotic desalination. The hydrophilic PIM NPs were synthesized by one-pot polycondensation with hydrophilic β-cyclodextrin (β-CD) as one of building units and used as an additive of aqueous phase in the preparation TFN membranes by interfacial polymerization. Unlike commonly-used inorganic nanomaterials, the β-CD-PIM NPs are well compatible with PA matrix due to their polymer characteristic. Moreover, the hydroxyl groups in βCD can participate in interfacial polymerization and thus the interfacial defects between β-CD-PIM NPs and PA matrix are effectively eliminated. As a result, the TFN membranes exhibit not only high water permeance (15.3 L m−2 h−1 bar−1), about three times as large as that of the prinstine membrane, but also high divalent salt rejection (RNa2SO4 = 95.1%). It is believed that the interior micropores inside β-CD-PIM NPs play a role of water transport nanochannels and contribute to high water permeance of the membranes. The long-term durability of the membranes are superior due to the chemical bonding between β-CD-PIM NPs and PA matrix. In addition, the TFN membranes were used for antibiotics desalination due to their high rejection towards antibiotics, but low rejection towards sodium chloride. This work offers a new strategy for constructing water transport nanochannels in separation layer to develop high performance TFN NF membranes.
1. Introduction Antibiotics have been widely used in medical treatment of bacterial infection due to their antimicrobial charicteristics. In pharmaceutical industry, the extraction of antibiotics from fermentation broth after removing microorganisms and protein impurities is a key procedure to obtain pure antibiotics. Conventional methods e.g. adsorption, precipitation, solvent extraction and ion exchange for antibiotic extraction are limited by their complicated procedure, long operation time, severe pollution and easy inactivation of biomolecules as well as intensive energy consumption. Membrane separation technology is emerging as a promising alternative resolution to separation of antibiotics due to simple ambient operation, no phase change, high separation efficiency, cost-saving and energy-efficient virtues and has been paid much attention in recent years [1]. Nanofiltration (NF), one of membrane separation processes with
⁎
molecular weight cut-off ranging from 200 to 1000 Da, exhibits great potential in antibiotic desalination [2]. Some researchers have made many efforts on the construction of so-called thin-film nanocomposite (TFN) membranes incorporating nanomaterials inside active separation layers to achieve high permeability without sacrifice of selectivity [3]. Previously reported nanomaterials used for TFN membranes mainly are inorganic materials, including graphene oxide [4,5], carbon nanotube [6–8], zeolite [9] and silica [10]. However, inorganic nanomaterials are easy to leach out from membrane matrix due to their poor compatibility and interfacial interaction with polymer matrix, which is disadvantageous to long-term stability in separation operation [11]. Surface chemical modification is a feasible strategy to improve the compatibility of inorganic nanomaterials with polymer matrix prior to use. But this brings extra steps and thus higher costs to the preparation of TFN membranes. To solve above-mentioned problems, some organic nanomaterials
Corresponding author. E-mail address: [email protected] (L. Zhu).
https://doi.org/10.1016/j.memsci.2019.117375 Received 25 May 2019; Received in revised form 9 August 2019; Accepted 13 August 2019 Available online 14 August 2019 0376-7388/ © 2019 Elsevier B.V. All rights reserved.
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
are recently developed as alternatives of inorganic materials for preparing high-performance TFN membranes [12–15]. A notable example is polymers of intrinsic microporosity (PIMs), a class of nanoporous polymer materials with abundant interconnected pores and permanent porosity due to their highly rigid and contorted molecular structures [16–18]. Actually, PIMs have shown great potential as NF membrane materials used in organic solvents. In 2012, Fritsch et al. [19] reported a TFC membrane prepared by dip-coating of PIM on polyacrylonitrile (PAN) support membrane, which exhibited high separation performances in solvent-resistant NF. Livingston et al. [20] successfully prepared an ultrathin polyarylate microporous nanofilms with thickness down to 20 nm by interfacial polymerization and the composite membranes showed outstanding separation performance in organic solvents. Although many important breakthroughs have been acheived on PIMs as membrane materials, rare work has been done on the applications of PIMs membranes in aqueous separation systems. In our recent work, a kind of novel crosslinked and nonsoluble PIM nanoparticles (NPs) modified by β-cyclodextrin (β-CD), named β-CDPIM, were successfully synthesized by one-pot polycondensation reaction [21]. This nanoporous material exhibited superior performances in adsorption removal of organic micropollutants from water [22]. The nanosized and polymeric characteristics make β-CD-PIM NPs great potential as nanofillers compatible with polymer matrix in TFN membranes. The interconnected micropores inside the NPs could act as water transport nanochannels in active separation layer and thus strengthen mass transfer in membrane. The residual hydroxyl groups in β-CD-PIM may participate in interfacial polymerization to eliminate the interfacial defects between β-CD-PIM NPs and PA (PA) and obtain defect-free separation layer. The objective of this work is to verify the feasibility of β-CD-PIM NPs as an additive of aqueous phase in interfacial polymerization to fabricate highly permeable and highly selective TFN NF membranes. The application of the developed TFN membranes in antibiotic desalination were investigated in detail. To the best of our knowledge, this should be the first report on TFN membranes with PIM NPs as permeation-enhanced nanofillers.
sufficient hydrochloric acid to neutralize the residual K2CO3 until carbon dioxide evolution stopped. The precipitates were collected by centrifugation and washed thoroughly with water/ethanol repeatedly to remove residual monomers and solvent. Finally, the solid was dried by the lyophilization until the mass was constant. As a control sample, PIM-1 without β-CD, was also synthesized with only TTSBI (10 mmol) and TFTPN (10 mmol) as monomers following a similar procedure. 2.3. Preparation of TFN membranes The TFN membrane was prepared through interfacial polymerization on the PSF-UF support membrane, as schemed in Fig. 1b. In a typical procedure, a PSF-UF membrane was immersed into the aqueous solution (pH = 12.0) containing PIP (0.35%, w/v) and β-CD-PIM NPs (0.01–0.15 w/v%) for 2 min, and the excessive solution was drained off from the membrane surface. Then, the hexane solution containing TMC (0.2 w/v%) was poured on the surface of PSF-UF membrane for 1 min. After removing excessive organic solution, the membrane was cured in an oven at 50 °C for 15 min for further polymerization. Finally, the obtained membrane was rinsed thoroughly with deionized water to remove chemical residues and stored in deionized water for further use. As a control sample, a control TFC membrane without β-CD-PIM NPs in the aqueous phase was also prepared following a similar procedure. 2.4. Characterizations of β-CD-PIM NPs and the membranes The chemical structures of PIM and β-CD-PIM NPs were analysed by Fourier transform infrared spectroscopy (FTIR, Vector-22, Switzerland) in the range of 400–4000 cm−1 and solid-state cross-polarization/ magic angle spinning (CP/MAS) 13C nuclear magnetic resonance spectroscopy (NMR, Bruker Advance-III, Germany). The thermal stability was characterized by thermogravimetric analysis (TGA, TA-Q500, USA) under nitrogen atmosphere by heating samples up to 800 °C at a rate of 10 °C/min in nitrogen atmosphere. Field emission-scanning electron microscopy (FE-SEM, HitachiS-4800, Japan) was used to observe the morphologies of polymer particles after the samples were sputtered with a 10–20 nm gold layer. Specific surface areas of the synthesized polymers were measured on AUTOSORB-IQ2-MP to characterize the porosity. In a typical procedure, a polymer sample was degassed at 80 °C for 48 h and then backfilled with N2. The isotherms were generated by incremental exposure to ultrahigh-purity N2 up to 1 atm in a liquid nitrogen (77 K) bath. To describe the adsorption process in slit pores of materials, non-localized density functional theory (DFT) and Monte-Carlo simulation were used to offer fluid structure in slit pores. The adsorption isotherms in model pores were measured on basis of the intermolecular potential energy between fluid-fluid and fluid-solid. The parameter of DFT equilibrium model was N2 at 77 K on carbon. The particle size, distribution, and zeta potential of the polymers were measured by dynamic light scattering (DLS, MALVERN, Zeta-sizer Nano ZSP, UK). Prior to characterizations for the membranes, all samples were dried at 50 °C under vacuum for 24 h. The surface and cross-section morphologies were observed with field emission-scanning electron microscopy (FE-SEM, HitachiS-4800, Japan) after sputtered with a 10–20 nm gold layer. To observe cross-sectional morphologies, membrane samples were freeze-fractured in liquid nitrogen. Membrane surface morphology was also analysed by atomic force microscopy (AFM, SPI-3800 N, Seiko Instruments Inc., Japan) with silicon tips (NSG10, NT-MDT, ca. 330 kHz) in tapping mode. Membrane surface chemistry was analysed by infrared spectrophotometer equipped with an attenuated total reflection accessory (ATR-FTIR, Nicolet 6700, USA). To characterize membrane charged properties, surface streaming potentials were measured at various pH values using an electrokinetic analyzer (SurPASS, Anton Paar, Austria). To evaluate hydrophilic/hydrophobic character, membrane surface water contact angle was measured by an OCA-20 contact angle meter at 25 °C.
2. Experimental section 2.1. Materials A polysulfone ultrafiltration membrane (PSF-UF) with molecular weight cut-off of 20 kDa was kindly provided by Development Center of Water Treatment Technology, Hangzhou, China, and used as the support substrate of TFC/TFN membrane. 5,5′,6,6′-Tetrahydroxy-3,3,3′,3′-tetramethylspirobisindane (TTSBI), 2,3,5,6-tetrafluoroterephthalonitrile (TFTPN) and erythromycin (ERY) were obtained from Energy Chemical. PIP and TMC were purchased from Aladdin Reagent and Shanghai Kaisai Chemical Co., Ltd. China, respectively. β-CD, anhydrous potassium carbonate (K2CO3), sodium sulfate (Na2SO4), magnesium sulfate (MgSO4), sodium chloride (NaCl), magnesium chloride (MgCl2), sodium hydroxide (NaOH), polyethylene glycol (PEGs, molecular weight 200, 300, 400, 600 and 800 Da, respectively), hexane, toluene and dimethylacetamide (DMAc) were purchased from Sinopharm Group Chemical Reagent Co. All of the chemicals were analytical grade and used as received without further purification. 2.2. Synthesis of β-CD-PIM NPs The synthesis of β-CD-PIM NPs follows a reported procedure [21]. The synthesis method is illustrated in Fig. 1a. In brief, β-CD (3 mmol), TTSBI (7 mmol), TFTPN (10 mmol), K2CO3 (30 mmol), toluene (15 mL) and DMAc (60 mL) were added into a 250 mL of three-neck round bottom flask equipped with a magnetic stirrer, nitrogen inlet, deanstark trap and reflux condenser. Then the reaction mixture was stirred at 110 °C for 12 h under nitrogen atmosphere. Afterwards, the obtained brown suspension was poured into an excess of water/ethanol with 2
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 1. a, The synthesis route and chemical structure of the β-CD-PIM NPs. b, Schematic diagram for the fabrication of the TFN membranes.
2.5. NF performance tests
synthesized using only TTSBI and TFTPN as monomers. The chemical structures of the synthesized PIM-1 and β-CD-PIM NPs were characterized by FTIR spectroscopies and solid-state 13C NMR. The FTIR spectra of β-CD-PIM NPs (Fig. S1) demonstrate the characteristic absorption peaks of β-CD (3400 cm−1 for –OH) [23] and PIM (2240 cm−1 for –C^N) [24], indicating that the synthesized β-CD-PIM NPs contained the β-CD and PIM-1. The NMR spectrum of β-CD-PIM NPs (Fig. 2a) shows an evident resonance at a chemical shift of 96.7 ppm (labelled 1′), which is ascribed to newly formed alkoxy groups. This result suggests that the hydroxyl hydrogen of β-CD was substituted by TFTPN. The results of NMR and FTIR spectroscopies confirm the successful introduction of β-CD into PIM-1. Calculated from the results of TGA (Fig. S2), the weight ratio of β-CD in the synthesized β-CD-PIM NPs is about 40%. SEM observation shows the synthesized β-CD-PIM appears as uniform globular particles with a diameter of 90 ± 10 nm (Fig. 2b). As shown in the inset of Fig. 2b, the average particle size of βCD-PIM NPs determined by DLS is about 110 nm, which is in well agreement with that from SEM observation. The nanosized and polymeric β-CD-PIM particles are expected to serve as nanofillers well compatible with polymer matrix in interfacial polymerization to construct a defect-free separation layer for TFN membranes. To characterize the porosity of the synthesized β-CD-PIM NPs, the N2 adsorption isotherms were determined by Brunauer–Emmett–Teller (BET) tests. The specific surface area (SBET) of β-CD-PIM NPs (360 m2 g−1) is less than that of PIM-1 (591 m2 g−1) (Fig. S3), which is attributed to the decreased proportion of PIM-1 repeat units. The results of pore size distribution (Fig. 2c and d) show that both PIM-1 and β-CDPIM NPs possess a majority of micropores and a small proportion of mesopores. Calculated from the N2 adsorption isotherms by a density functional theory (DFT) -based method, the average size of the micropores is approximately 1.6 nm, which is well in agreement with the results of molecular dynamic simulation (Fig. S4). The mesopore size ranges from 2.5 to 25 nm, as calculated by the Barrett–Joyner–Halenda (BJH)-based method. Just like PIM-1, β-CD-PIM has a highly rigid and contorted molecular structure that cannot pack space efficiently and thus a continuous network of interconnected intermolecular voids is
The NF performances of the investigated membranes were evaluated by a cross-flow flat apparatus with a 1.0 L of feed tank at 25 °C, 6 bar and pH 6.5. Effective area for each membrane sample was 22.4 cm2 and all tests were carried out for at least 3 times. In a typical procedure, a membrane was pre-compacted by pure water at 6 bar for 1 h until the pure permeance reached a steady state. Then, the NF performance tests were carried out with aqueous solutions of inorganic salts (Na2SO4, MgSO4, NaCl and MgCl2, respectively) or ERY/NaCl mixture as the feed. The permeate was collected for a period of time and weighted using an electric balance to determine the water permeance P (L m−2 h−1 bar−1), which was calculated by the following equation:
P=
Am ×
V t× p
(1)
where V (L) is the volume of permeate, Am (m2) is the effective membrane area, t (h) is the testing time, and p (bar) is the transmembrane pressure. The solute rejection R (%) was calculated according to equation (2):
R = (1
CP ) × 100 Cf
(2)
where Cp and Cf represent the concentrations of solute in permeate and the feed, respectively. The concentrations of inorganic salts were measured by a conductivity meter (FE30, Mettler-Toledo, Switzerland) and the concentrations of organics (ERY, PEG) were tested through total organic carbon analysis meter (TOC-L, Shimadzu, Japan). 3. Results and discussion 3.1. Synthesis and characterizations of β-CD-PIM NPs The β-CD-PIM NPs were synthesized via one-pot nucleophilic substitution polycondensation with TTSBI, TFTPN and β-CD as the monomers. The reaction was carried out in a polar aprotic solvent (DMAc) with K2CO3 as the catalyst. As a control sample, PIM-1 was also 3
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 2. a, Solid-state 13C NMR spectra of the PIM-1 and the β-CD-PIM NPs. b, SEM image of the β-CD-PIM NPs. The inset is the particle size distribution of β-CD-PIM NPs in water (pH = 6.5). c, Pore size distributions of the micropores calculated by the DFT method based on N2 adsorption isotherms. d, Pore size distributions of the mesopores calculated by the BJH method based on N2 adsorption isotherms. e, Water contact angle and images of aqueous suspension of PIM-1 (left) and β-CD-PIM NPs (right). f, Zeta potential of β-CD-PIM NPs in water.
achieved [21]. The unique hierarchically micro/mesoporous character renders the β-CD-PIM NPs potential application as selective transport passages of matters. Hydrophilicity is a vital character for microporous materials used in aqueous system. To evaluate the hydrophilicity/hydrophobicity, static contact angle of water droplet on polymer tablet prepared by filtrating and depositing PIM-1 or β-CD-PIM NPs on a polyvinylidene difluoride microfiltration membrane were examined. It was found the water contact angle on the β-CD-PIM surface was only 51 ± 5°, much lower than that of PIM-1 tablet (125 ± 7°) (Fig. 2e). This result indicated that β-CD-PIM was much more hydrophilic than PIM-1 due to the introduction of β-CD. Moreover, it can also be seen that the β-CD-PIM NPs were well dispersed in water while the PIM-1 was segregated to water surface (Fig. 2e). This phenomenon further showed the hydrophilic nature of the synthesized β-CD-PIM NPs, which is advantageous to the wetting and access of inteior pores by water. The zeta potentials of β-
CD-PIM NPs at various pH were measured and the results are shown in Fig. 2f. It can be seen that the β-CD-PIM NPs are negatively-charged (−30 mV at pH 6.5). This phenomenon should be attributed to the presence of strongly polar electrophilic nitrile groups. When used as a nanofiller in the preparation of TFN membranes, negatively-charged βCD-PIM may have an influence on membrane surface potential and separation performanes. 3.2. Characterizations of the TFN membranes The TFN membranes entrapped with β-CD-PIM NPs were prepared by interfacial polymerization of aqueous solution containing PIP and βCD-PIM NPs (aqueous phase) with hexane solution containing TMC (oil phase). The obtained TFN membranes are named as TFN-X, where X represents the weight percentage of β-CD-PIM NPs in aqueous phase. As a control membrane, control TFC membrane was also prepared with 4
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
are 99 ± 12 nm, 101 ± 9 nm and 98 ± 13 nm, respectively. This indicates the introduction of β-CD-PIM NPs had no obvious influence on the thickness of generated PA separation layer. The root mean square (RMS) roughness of the membranes was examined through AFM and the images and RMS values are shown in Fig. 4g–i. Compared with the control TFC membrane (12.8 nm of RMS value), the RMS roughness for the TFN-0.10% and the TFN-0.15% membranes increased to 20.9 nm and 31.4 nm, respectively. The rougher membrane surface often possesses the larger active permeation area and thus is beneficial to the enhancement of membrane permeability. Membrane surface hydrophilicity was evaluated by static water contact angle measurements. As shown in Fig. 5a, the contact angle decreased with the increasing loading of β-CD-PIM NPs in the membranes. This result is attributed to the increase quantity of carboxylic groups generated by hydrolysis of acyl chloride groups on membrane surface as well as the introduction of hydrophilic β-CD-PIM NPs. The enhanced hydrophlicity facilitates the permeation of water molecules through the membranes. The zeta potentials of the membranes were also examined and the results are shown in Fig. 5b. It can be seen that the zeta potential of the control TFC membrane is about −47 mV, demonstrating a typical negatively-charged characteristic of PA membranes. The potentials for the TFN-0.10%, TFN-0.15% membranes are −65 and −74 mV, respectively. The highly negative potentials should be attributed to the increased quantity of carboxylic groups and the introduction of β-CD-PIM NPs that exhibits −30 mV of potential at pH = 6.5 as discussed above in Section 3.1). The negatively charged character is advantageous to improve the rejection towards multivalent anions for the membranes due to Donnan exclusion effect [29]. To characterize the molecular weight cut-off (MWCO) of control TFC membrane and the TFN membranes, the rejection towards neutral PEGs with various molecular weights were tested. The value of MWCO is defined as the critical molecular weight of PEG molecule whose rejection is more than 90% [30]. As presented in Fig. 5c, the MWCOs for the control TFC, the TFN-0.10% and the TFN-0.15% membranes are about 270, 326 and 800 Da, respectively. The pore size distribution and mean pore size are calculated based on the theory established by Michaels and Singh [31] and the steric and hydrodynamic interactions between organic solute and the membrane can be neglected. The calculated results shown in Fig. 5d demonstrate that the mean effective pore radii for the control TFC, the TFN-0.10% and the TFN-0.15% membranes are around 0.29, 0.31 and 0.33 nm, respectively. It can be seen that the pore size change is not so great although it increases to some extent when β-CD-PIM NPs are introduced into interfacial polymerization. This indicates that the possible interfacial defects in the TFN membranes have been effectively controlled due to good compatibility between β-CD-PIM and PA matrix as well as the participation of β-CD-PIM in interfacial polymerization. Therefore, the increase of membrane pore size should be attributed to the reduced crosslinking degree of PA selective layer. The C/N molar ratio on membrane surface detected by X-ray photoelectron spectroscopy (XPS) increases from 5.4 for the control TFC membrane to 6.0 for the TFN-0.10% membrane (Fig. S5). This phenomenon confirms the decrease of PA crosslinking degree when β-CD-PIM NPs are introduced. The low crosslinking degree implies the formation of relatively loose PA selective layer, which is beneficial to the improvement of membrane permeability. The effective pore radii of the TFN membranes are smaller than the hydrated radius of SO42− (0.379 nm) [32], suggesting the membranes are potential in Na2SO4 removal from water.
Fig. 3. ATR-FTIR spectra of the control TFC membrane and the TFN-CD-PIM membranes.
only PIP and TMC as the monomers. The surface chemistry of the control TFC and the TFN membranes was charicterized by ATR-FTIR. In the spectra of TFN membranes as shown in Fig. 3, a new peak appeared at 2240 cm−1 is assigned to stretching vibration of C^N in β-CD-PIM NPs, verifying the successful incorporation of β-CD-PIM NPs into PA matrix. It can also be seen that the intensity of this peak increases as the dosage of β-CD-PIM NPs in aqueous phase increases, indicating the increasing loading amount of β-CD-PIM NPs in PA layer. Another new peak at 1730 cm−1 assigned to the stretching vibration of ester groups further proves the esterification reaction between residual hydroxyl groups of β-CD and acyl chloride groups of TMC [15]. This phenomenon shows that the amidation between PIP and TMC is accompanied by the esterification although the hydroxyl groups in β-CD are less reactive than PIP. This is advantageous to eliminate possible interfacial defects between the nanoparticles and PA matrix and endow the membranes with high durability. The surface morphologies of the TFN membranes were observed by SEM. For the control TFC membrane (Fig. 4a), a nodular structure can be seen on the top layer of PSF membrane, which is typical in PA TFC membranes prepared by interfacial polymerization [25]. In the SEM image of TFN-0.10% membrane (Fig. 4b), it can be seen that β-CD-PIM NPs distribute uniformly due to their good dispersity in aqueous phase and well compatibility with PA matrix. Compared with the control TFC membrane, the modular structures on the TFN-0.10% membrane are more obscure. It is accepted that the incorporation of nanoparticles hinders the diffusion of PIP and leads to slow interfacial polymerization rate. A a result, PA molecular chains arrange to some extent and thus a relatively smooth surface is achieved [26]. It is worth noting that the boundary between the embedded β-CD-PIM NPs and PA matrix is vague due to well compatibility between organic nanoparticle and PA matrix, which was different from the phenomenon observed in already reported TFN membranes containing inorganic nanomaterials or organic-inorganic hybrid nanomaterials (MOF) [9,27,28]. For the TFN-0.15% membrane (Fig. 4c), the nanoparticles tend to aggregate on membrane surface, which is an inevitable phenomenon in the preparation of TFN membrane when excessive nanomaterials are added. As we know, the thickness of separation layer greatly affects the permeability of membrane. Thin separation layer is advantageous to achieve high permeability. Estimated from the cross-sectional SEM images (Fig. 4d–f), the separation layer thicknesses for the control TFC membrane, the TFN-0.10% membrane and the TFN-0.15% membrane
3.3. NF performance of the TFN membranes The NF performances of the membranes were measured with a cross-flow flat membrane apparatus. The effects of β-CD-PIM NPs addition on pure water permeance (PWP) and Na2SO4 rejection were investigated in detail and the results are shown in Fig. 6a. When the addition of β-CD-PIM NPs increases, the PWP gradually increases while 5
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 4. a, b, c, SEM surface morphologies of control TFC, TFN-0.10% and TFN-0.15% membrane. d, e, f, SEM cross-section morphologies of control TFC, TFN-0.10% and TFN-0.15% membrane. g, h, i, AFM surface morphologies of control TFC, TFN-0.10% and TFN-0.15% membrane.
Fig. 5. a, Water contact angles of control TFC, TFN-0.10% and TFN-0.15% membranes. b, Surface zeta potentials of control TFC, TFN-0.10% and TFN-0.15% membranes. c, Rejections towards PEGs with various molecular weights. d, Pore size distribution of control TFC, TFN-0.10% and TFN-0.15% membranes.
Na2SO4 rejection decreases slightly. The TFN-0.10% membrane shows the optimal NF performance with 15.3 L m−2 h−1 bar−1 of water permeance, three times as large as that of the control TFC membrane, and
95.1% of Na2SO4 rejection. The improvement of water permeability can be attributed to several factors. Firstly, the increase of hydrophilicity facilitates the transport and permeation of water molecules inside 6
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 6. a, Effect of β-CD-PIM dosage on the PWP and Na2SO4 rejection for the investigated TFN membranes. b, Rejections towards various salts for the control TFC and the TFN-0.10% membranes. Operation condition: 1.0 g L−1 of aqueous salt solutions as the feed, pH = 6.5, 25 °C and 6 bar c, Schematic illustration of water channels in the TFN membranes.
membrane pore and thus water permeance increases [33]. Secondly, the rough membrane surface provides large effective area for the permeation of water molecules through membrane [34,35]. In addition, relatively loose PA layer resulted from the introduction of β-CD-PIM NPs also contributes to the enhancement of water permeance [36,37]. More importantly, the hydrophilic and interconnected pores throughout the β-CD-PIM NPs offer low-resistant water transport channels and allow water molecules quickly pass through the membranes [20] (Fig. 6c). It is well-known the rejection of salts is correlated with the electrostatic repulsive effect (density of negative electric charges) and steric hindrance effect (pore size of selective layer). Considering the improved negative charge density of TFN membrane, the slight decrease in Na2SO4 rejection should be attributed to relatively loose PA selective layer. Nevertheless, the Na2SO4 rejection keeps at a high level due to the formation of PA layer atop the entrapped βCD-PIM NPs. For comparison, the influnce of PIM-1 as an additive of aqueous phase on the NF performance of TFC membrane was investigated (Fig. S6). It is found that, as the PWP is higher than 9.5 L m−2 h−1 bar−1, the Na2SO4 rejection decreases drastically. This indicates that, when PIM-1 is used as the nanofiller, the defects may be generated in PA layer due to the aggregation of PIM-1 in water phase. In contrast, the defects can be effectively controlled when β-CD-PIM is used as the additive. Therefore, here it seems indispensable to introduce hydrophilic β-CD into PIM-1 to control the defects of selective layer and extend its application in TFN membranes. As shown in Fig. 6b, the rejections towards various salts for the control TFC and the TFN-0.10% membranes are compared. It can be seen that, for both the investigated membranes, the salt rejections follow an order of Na2SO4 > MgSO4 > MgCl2 > NaCl, which is in accordance with that of typical negative NF membranes [38,39]. According to the Donnan exclusion theory [39], there strong electrostatic repulsion exists between multivalent anion ions and negatively-charged membranes. Thus, the rejections towards Na2SO4 and MgSO4 is often higher than the rejection towards MgCl2 and NaCl. At the meanwhile, the electrostatic attraction between the counterions with NF membranes also has a remarkable influence on the rejection towards multivalent salts. Here the electrostatic attraction between counterions Mg2+ with the negatively-charged membranes is stronger than that between Na+ and the membranes. And thus Mg2+ ions are easier to transport through the membranes. As a result, the MgSO4 rejection is
lower than Na2SO4. NaCl has the lowest rejection due to small hydrated radius for Na+ ions. Compared with the control TFC membrane, the TFN-0.10% membrane keeps high rejection towards Na2SO4 (95.1%) but exhibits low NaCl rejection (35.4%) as a consequence of strong negatively-charged character but relatively loose selective layer. The TFN membranes with high selectivity towards di-/mono-valent salts can find their potential applications in antibiotic desalination. 3.4. Stability of the TFN membranes The stability of membrane during long-term operation is a crucial performance in practical applications. The water permeance and Na2SO4 rejection for the control TFC and the TFN-0.10% membranes were real-time monitored with the operation time and the results are shown in Fig. 7a. It can be seen that both of the TFC and the TFN-0.10% membranes exhibit stable Na2SO4 rejection and water permeance within 168 h (a week) of tests. The good durability of the TFN membranes can be attributed to the covalent bonding and thus no interfacial defects between β-CD-PIM NPs and PA matrix as discussed in Section 3.2. In addition, the hydrogen bonding between β-CD-PIM and PA also contributes to stability of the TFN membranes. Therefore, it is generally believed that the combination of covalent interaction (ester bonding) and non-covalent interaction (hydrogen bonding) are responsible for excellent durability of the TFN membranes, as illustrated in Fig. 7b. The effects of operation pressure on the NF performances of the membranes were evaluated. As shown in Fig. 7c, with the increase of the operation pressure from 2 to 6 bar, the water permeance slightly decreases while the Na2SO4 rejection increases to some extent for both the control TFC and the TFN-0.10% membranes. This can be attributed to the compaction effect of membranes under high pressure. The difference in driven force between water and solute also contributes to the increased Na2SO4 rejection. With the increase of operation pressure, the driven force for water increases while that for Na2SO4 remains steady, and thus the Na2SO4 rejection increases [40,41]. Additionally, the effects of operating temperature on the NF performances were also examined. As shown in Fig. 7d, both water permeance and Na2SO4 rejection for the investigated membranes increases linearly with the increase of operating temperature from 25 to 45 °C. It seems that the water permeance increases more rapidly than Na2SO4 rejection, which is in agreement with the results reported in literature [42]. At 45 °C of 7
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 7. a, The changes of water permeance and Na2SO4 rejection with the operation time (168 h of filtration, 1.0 g L−1 of Na2SO4 solution as the feed, 25 °C, pH 6.5 and 6 bar). b, Schematic illustration of the covalent and non-covalent interactions between the β-CD-PIM NPs and PA matrix. c, The changes of water permeance and Na2SO4 rejection with the operation pressure (1.0 g L−1 of Na2SO4 solution as the feed, 25 °C, pH 6.5 and 2–6 bar). d, The changes of water permeance and Na2SO4 rejection with the operating temperature (1.0 g L−1 of Na2SO4 solution as the feed, 25–45 °C, pH 6.5 and 6 bar).
operation temperature, the water permeance and the Na2SO4 rejection of the TFN-0.10% membrane increase up to 22.1 L m−2 h−1 bar−1 and 98.5%. This shows that the TFN membranes can be used stably at high operation temperature.
operation. In order to remove more salts from antibiotics solution, 300 mL of the concentrated ERY solution was re-diluted to 1000 mL with pure water and the filtration process was re-operated. This process was recycled two more times and the results are shown in Fig. 9. It can be seen that, after three filtration-dilution cycles, the ERY concentration in the retentate increases to 310 mg L−1 while the NaCl concentration decreases to only 3.2 g L−1. The salt concentration can be further decreased by more filtration cycles. Therefore, the desalination from ERY can be effectively achieved by multiple filtration-dilution cycles. Compared with the control TFC membrane, the TFN membranes developed in this work are more competent to antibiotic desalination due to their lower rejection towards NaCl.
3.5. Antibiotic desalination performances Desalination from antibiotics has a great demand in the pharmaceutical industry especially for the extraction of the antibiotics from fermentation broth. Erythromycin (ERY), a typical electro-neutral antibiotic (734 Da of molecular weight), was used as a model molecule to conduct desalination experiments (Fig. 8a) [43]. An aqueous solution containing 100 mg L−1 of ERY and 10 g L−1 of NaCl (the average concentrations in Luria-Bertani fermentation broth) was used as the feed liquid. The filtration was performed in a cross-flow flat apparatus with a 1.0 L of feed tank. In the filtration, the retentate was recycled and backflowed into the feed tank. The changes of water permeance with the operation time for the control TFC and the TFN-0.10% membranes are shown in Fig. 8b. It can be seen that the water permeance for the TFN0.10% membrane is much higher than that for the control membrane, which is similar to the change of pure water permeance as shown in Fig. 6a. The changes of ERY and NaCl rejections with the operation time are shown in Fig. 8c. Compared with the control membrane, the TFN0.10% membrane demonstrates nearly identical ERY rejection (about 97%) but lower NaCl rejection within the whole 6 h of filtration process. It should be noted that the NaCl rejection decreases slowly with the operation time, which is attributed to the enhanced electrostatic shielding effect due to the gradually increased NaCl concentration in feed solution in separation process. The changes of ERY and NaCl concentrations in the recycled retentate with the operation time were monitored and the data are shown in Fig. 8d. It can be seen that, after 6 h of filtration operation using the TFN-0.10% membrane, the ERY concentration in the retentate increases from 100 to 320 mg L−1 while the NaCl concentration changes only a little. In the meanwhile, the volume of the retentate decreases from 1000 mL to 300 mL. This is a typical concentration and desalination process for antibiotics by NF
4. Conclusions The TFN membranes impregnated with β-CD-PIM NPs in PA selective layer were successfully fabricated via interfacial polymerization. βCD-PIM NPs exhibited good hydrophilicity due to the introduction of βCD, facilitating their dispersion in aqueous phase. Meanwhile, the synthesized β-CD-PIM NPs are cross-linked and insoluble polymers. The nanosized and polymeric characteristics make the β-CD-PIM NPs well compatible with PA matrix. The presence of β-CD-PIM NPs in PA selective layer was verified. The formation of ester bonding between βCD-PIM NPs and TMC is advantageous to elimination of possible interfaical defects between NPs and PA matrix. The optimal TFN membrane exhibited high water permeance (15.3 L m−2 h−1 bar−1), nearly three times as large as that of control TFC membrane, without great sacrifice of salt rejection (RNa2SO4 = 95.1%). The enhanced water permeability was attributed to the improvement of membrane hydrophilicity, the increase of surface roughness, reduced crosslinking degree of PA layer and the additional water transport channels inside β-CDPIM NPs. In addition, the TFN membrane exhibited superior stability under different operation conditions due to the covalent bonding (ester bonding formed) and non-covalent bonding (hydrogen bonding) between β-CD-PIM and PA matrix. The TFN membranes demonstrated
8
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al.
Fig. 8. a, Schematic illustration of the TFN membranes used for antibiotic desalination. b, The change of water permeance with the operation time. c, The changes of ERY and NaCl rejections with the operation time. d, The changes of ERY and NaCl concentrations in the recycled retentate with the operation time. The feed liquid was the mixed aqueous solution (pH 6.5) containing 100 mg L−1 of ERY and10 g L−1 of NaCl. The filtration was operated at 25 °C under 6 bar of the trans-membrane pressure.
Conflicts of interest There are no conflicts of interest to declare. Acknowledgements We are grateful for the financial supports from the National Natural Science Foundation of China, China (Grant No. 51773175, 51573159 and 51828301) and the Fundamental Research Funds for the Central Universities, China (Grant No. 2019QNA4062). We also thank Ms. Li Xu at State Key Laboratory of Chemical Engineering in Zhejiang University for the assistance in performing polymer and membrane tests. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.memsci.2019.117375. Fig. 9. The changes of ERY and NaCl concentrations in the back-flowed retentate with the operation time for three filtration-dilution cycles. The original feed liquid was the mixed aqueous solution (pH 6.5) containing 100 mg L−1 of ERY and10 g L−1 of NaCl. The filtration was operated at 25 °C under 6 bar of the trans-membrane pressure.
References [1] E. Yang, K.-J. Chae, M.-J. Choi, Z. He, I.S. Kim, Critical review of bioelectrochemical systems integrated with membrane-based technologies for desalination, energy selfsufficiency, and high-efficiency water and wastewater treatment, Desalination 452 (2019) 40–67, https://doi.org/10.1016/j.desal.2018.11.007. [2] R. Mulyanti, H. Susanto, Wastewater treatment by nanofiltration membranes, IOP Conf. Ser. Earth Environ. Sci. 142 (2018) 012017, , https://doi.org/10.1088/17551315/142/1/012017. [3] Y. Cheng, Y. Ying, S. Japip, S.-D. Jiang, T.-S. Chung, S. Zhang, D. Zhao, Advanced porous materials in mixed matrix membranes, Adv. Mater. 30 (2018), https://doi. org/10.1002/adma.201802401 1802401.
great potential in antibiotics desalination due to their high rejection towards antibiotics, but low rejection towards NaCl. The overall results highlighted the potential of hydrophilic PIMs in the development of high-performance TFN NF membranes. 9
Journal of Membrane Science 592 (2019) 117375
Z. Wang, et al. [4] S. Bano, A. Mahmood, S.-J. Kim, K.-H. Lee, Graphene oxide modified polyamide nanofiltration membrane with improved flux and antifouling properties, J. Mater. Chem. 3 (2015) 2065–2071, https://doi.org/10.1039/C4TA03607G. [5] J. Yin, G. Zhu, B. Deng, Graphene oxide (GO) enhanced polyamide (PA) thin-film nanocomposite (TFN) membrane for water purification, Desalination 379 (2016) 93–101, https://doi.org/10.1016/j.desal.2015.11.001. [6] Y.-S. Guo, Y.-F. Mi, F.-Y. Zhao, Y.-L. Ji, Q.-F. An, C.-J. Gao, Zwitterions functionalized multi-walled carbon nanotubes/polyamide hybrid nanofiltration membranes for monovalent/divalent salts separation, Separ. Purif. Technol. 206 (2018) 59–68, https://doi.org/10.1016/j.seppur.2018.05.048. [7] W.-F. Chan, E. Marand, S.M. Martin, Novel zwitterion functionalized carbon nanotube nanocomposite membranes for improved RO performance and surface antibiofouling resistance, J. Membr. Sci. 509 (2016) 125–137, https://doi.org/10. 1016/j.memsci.2016.02.014. [8] T.-Y. Liu, H.-G. Yuan, Q. Li, Y.-H. Tang, Q. Zhang, W. Qian, B. Van der Bruggen, X. Wang, Ion-responsive channels of zwitterion-carbon nanotube membrane for rapid water permeation and ultrahigh mono-/multivalent ion selectivity, ACS Nano 9 (2015) 7488–7496, https://doi.org/10.1021/acsnano.5b02598. [9] 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 (2007) 1–7, https://doi.org/10. 1016/j.memsci.2007.02.025. [10] 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, https://doi.org/10.1016/j.memsci.2012.10.053. [11] W.J. Lau, S. Gray, T. Matsuura, D. Emadzadeh, J. Paul Chen, A.F. Ismail, A review on polyamide thin film nanocomposite (TFN) membranes: history, applications, challenges and approaches, Water Res. 80 (2015) 306–324, https://doi.org/10. 1016/j.watres.2015.04.037. [12] Y.-L. Ji, Q.-F. An, Y.-S. Guo, W.-S. Hung, K.-R. Lee, C.-J. Gao, Bio-inspired fabrication of high perm-selectivity and anti-fouling membranes based on zwitterionic polyelectrolyte nanoparticles, J. Mater. Chem. 4 (2016) 4224–4231, https://doi. org/10.1039/C6TA00005C. [13] C. Wang, Z. Li, J. Chen, Z. Li, Y. Yin, L. Cao, Y. Zhong, H. Wu, Covalent organic framework modified polyamide nanofiltration membrane with enhanced performance for desalination, J. Membr. Sci. 523 (2017) 273–281, https://doi.org/10. 1016/j.memsci.2016.09.055. [14] C. Li, S. Li, L. Tian, J. Zhang, B. Su, M.Z. Hu, Covalent organic frameworks (COFs)incorporated thin film nanocomposite (TFN) membranes for high-flux organic solvent nanofiltration (OSN), J. Membr. Sci. 572 (2019) 520–531, https://doi.org/10. 1016/j.memsci.2018.11.005. [15] Y.-L. Ji, Q.-F. An, X.-D. Weng, W.-S. Hung, K.-R. Lee, C.-J. Gao, Microstructure and performance of zwitterionic polymeric nanoparticle/polyamide thin-film nanocomposite membranes for salts/organics separation, J. Membr. Sci. 548 (2018) 559–571, https://doi.org/10.1016/j.memsci.2017.11.057. [16] C. Ma, J.J. Urban, Polymers of intrinsic microporosity (PIMs) gas separation membranes: a mini review, Proc. Nat. Res. Soc. 2 (2018) 02002. [17] N.B. McKeown, P.M. Budd, Polymers of intrinsic microporosity (PIMs): organic materials for membrane separations, heterogeneous catalysis and hydrogen storage, Chem. Soc. Rev. 35 (2006) 675–683, https://doi.org/10.1039/B600349D. [18] P.M. Budd, N.B. McKeown, Highly permeable polymers for gas separation membranes, Polym. Chem. 1 (2010) 63–68, https://doi.org/10.1039/B9PY00319C. [19] D. Fritsch, P. Merten, K. Heinrich, M. Lazar, M. Priske, High performance organic solvent nanofiltration membranes: development and thorough testing of thin film composite membranes made of polymers of intrinsic microporosity (PIMs), J. Membr. Sci. 401–402 (2012) 222–231, https://doi.org/10.1016/j.memsci.2012.02. 008. [20] M.F. Jimenez-Solomon, Q. Song, K.E. Jelfs, M. Munoz-Ibanez, A.G. Livingston, Polymer nanofilms with enhanced microporosity by interfacial polymerization, Nat. Mater. 15 (2016) 760–767, https://doi.org/10.1038/nmat4638. [21] Z. Wang, F. Cui, Y. Pan, L. Hou, B. Zhang, Y. Li, L. Zhu, Hierarchically micromesoporous β-cyclodextrin polymers used for ultrafast removal of micropollutants from water, Carbohydr. Polym. 213 (2019) 352–360, https://doi.org/10.1016/j. carbpol.2019.03.021. [22] Z. Wang, B. Zhang, C. Fang, Z. Liu, J. Fang, L. Zhu, Macroporous membranes doped with micro-mesoporous β-cyclodextrin polymers for ultrafast removal of organic micropollutants from water, Carbohydr. Polym. 222 (2019), https://doi.org/10. 1016/j.carbpol.2019.114970 114970. [23] A. Alsbaiee, B.J. Smith, L. Xiao, Y. Ling, D.E. Helbling, W.R. Dichtel, Rapid removal of organic micropollutants from water by a porous β-cyclodextrin polymer, Nature 529 (2016) 190–194, https://doi.org/10.1038/nature16185. [24] F.Y. Li, Y. Xiao, T.-S. Chung, S. Kawi, High-performance thermally self-cross-linked
[25]
[26]
[27]
[28] [29] [30]
[31] [32]
[33] [34] [35] [36]
[37]
[38]
[39] [40]
[41]
[42]
[43]
10
polymer of intrinsic microporosity (PIM-1) membranes for energy development, Macromolecules 45 (2012) 1427–1437, https://doi.org/10.1021/ma202667y. X. Kong, Y. Zhang, S.-Y. Zeng, B.-K. Zhu, L.-P. Zhu, L.-F. Fang, H. Matsuyama, Incorporating hyperbranched polyester into cross-linked polyamide layer to enhance both permeability and selectivity of nanofiltration membrane, J. Membr. Sci. 518 (2016) 141–149, https://doi.org/10.1016/j.memsci.2016.07.037. M. Wu, J. Yuan, H. Wu, Y. Su, H. Yang, X. You, R. Zhang, X. He, N.A. Khan, R. Kasher, Z. Jiang, Ultrathin nanofiltration membrane with polydopamine-covalent organic framework interlayer for enhanced permeability and structural stability, J. Membr. Sci. 576 (2019) 131–141, https://doi.org/10.1016/j.memsci.2019. 01.040. J. Zhu, L. Qin, A. Uliana, J. Hou, J. Wang, Y. Zhang, X. Li, S. Yuan, J. Li, M. Tian, J. Lin, B. Van der Bruggen, Elevated performance of thin film nanocomposite membranes enabled by modified hydrophilic MOFs for nanofiltration, ACS Appl. Mater. Interfaces 9 (2017) 1975–1986, https://doi.org/10.1021/acsami.6b14412. Y. He, Y.P. Tang, D. Ma, T.-S. Chung, UiO-66 incorporated thin-film nanocomposite membranes for efficient selenium and arsenic removal, J. Membr. Sci. 541 (2017) 262–270, https://doi.org/10.1016/j.memsci.2017.06.061. R.J. Petersen, Composite reverse osmosis and nanofiltration membranes, J. Membr. Sci. 83 (1993) 81–150, https://doi.org/10.1016/0376-7388(93)80014-O. R. Rohani, M. Hyland, D. Patterson, A refined one-filtration method for aqueous based nanofiltration and ultrafiltration membrane molecular weight cut-off determination using polyethylene glycols, J. Membr. Sci. 382 (2011) 278–290, https://doi.org/10.1016/j.memsci.2011.08.023. S. Singh, K.C. Khulbe, T. Matsuura, P. Ramamurthy, Membrane characterization by solute transport and atomic force microscopy, J. Membr. Sci. 142 (1998) 111–127, https://doi.org/10.1016/S0376-7388(97)00329-3. L.A. Richards, B.S. Richards, A.I. Schäfer, Renewable energy powered membrane technology: salt and inorganic contaminant removal by nanofiltration/reverse osmosis, J. Membr. Sci. 369 (2011) 188–195, https://doi.org/10.1016/j.memsci. 2010.11.069. Z. Yu, Y. Pan, Y. He, G. Zeng, H. Shi, H. Di, Preparation of a novel anti-fouling βcyclodextrin–PVDF membrane, RSC Adv. 5 (2015) 51364–51370, https://doi.org/ 10.1039/C5RA04894J. Z. Wang, Z. Wang, S. Lin, H. Jin, S. Gao, Y. Zhu, J. Jin, Nanoparticle-templated nanofiltration membranes for ultrahigh performance desalination, Nat. Commun. 9 (2018), https://doi.org/10.1038/s41467-018-04467-3. L.F. Villalobos, T. Huang, K.-V. Peinemann, Cyclodextrin films with fast solvent transport and shape-selective permeability, Adv. Mater.. 29 (n.d.) 1606641. doi:10. 1002/adma.201606641. J. Zhu, M. Tian, J. Hou, J. Wang, J. Lin, Y. Zhang, J. Liu, B.V. der Bruggen, Surface zwitterionic functionalized graphene oxide for a novel loose nanofiltration membrane, J. Mater. Chem. 4 (2016) 1980–1990, https://doi.org/10.1039/ C5TA08024J. J. Lin, W. Ye, H. Zeng, H. Yang, J. Shen, S. Darvishmanesh, P. Luis, A. Sotto, B. Van der Bruggen, Fractionation of direct dyes and salts in aqueous solution using loose nanofiltration membranes, J. Membr. Sci. 477 (2015) 183–193, https://doi.org/10. 1016/j.memsci.2014.12.008. Q. Xie, W. Shao, S. Zhang, Z. Hong, Q. Wang, B. Zeng, Enhancing the performance of thin-film nanocomposite nanofiltration membranes using MAH-modified GO nanosheets, RSC Adv. 7 (2017) 54898–54910, https://doi.org/10.1039/ C7RA11550D. M.J.T. Raaijmakers, N.E. Benes, Current trends in interfacial polymerization chemistry, Prog. Polym. Sci. 63 (2016) 86–142, https://doi.org/10.1016/j. progpolymsci.2016.06.004. X.Q. Cheng, Z.X. Wang, Y. Zhang, Y. Zhang, J. Ma, L. Shao, Bio-inspired loose nanofiltration membranes with optimized separation performance for antibiotics removals, J. Membr. Sci. 554 (2018) 385–394, https://doi.org/10.1016/j.memsci. 2018.03.005. K. Rezzadori, F. Marques Penha, M.C. Proner, G. Zin, J.C. Cunha Petrus, P. Prádanos, L. Palacio, A. Hernández, M. Di Luccio, Evaluation of reverse osmosis and nanofiltration membranes performance in the permeation of organic solvents, J. Membr. Sci. 492 (2015) 478–489, https://doi.org/10.1016/j.memsci.2015.06. 005. M.F.A. Goosen, S.S. Sablani, S.S. Al-Maskari, R.H. Al-Belushi, M. Wilf, Effect of feed temperature on permeate flux and mass transfer coefficient in spiral-wound reverse osmosis systems, Desalination 144 (2002) 367–372, https://doi.org/10.1016/ S0011-9164(02)00345-4. Y. He, G. Chen, Z. Ji, S. Li, Combined UF–NF membrane system for filtering erythromycin fermentation broth and concentrating the filtrate to improve the downstream efficiency, Separ. Purif. Technol. 66 (2009) 390–396, https://doi.org/ 10.1016/j.seppur.2008.12.007.