- Email: [email protected]
nanofiber composite hollow fiber membranes with electrospun support layer for water purification
Journal of Membrane Science xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Conductive CNT/nanofiber composite hollow fiber membranes with electrospun support layer for water purification Lei Du a, Xie Quan a, *, Xinfei Fan b, Gaoliang Wei c, Shuo Chen a a
Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian, 116024, China b College of Environmental Science and Engineering, Dalian Maritime University, Dalian, 116026, China c Key Laboratory of Groundwater Resources and Environment (Ministry of Education, China), College of New Energy and Environment, Jilin University, Changchun, 130026, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrospun nanofiber support layer CNT separation layer Hollow fiber membranes High permeability and mechanical strength Electro-assistance
Hollow fiber membranes (HFMs) have been extensively used to alleviate the crisis of water resources because of their large contact area per unit volume. However, developing high permeability and mechanical strength HFMs remains a great challenge. In this study, we design a novel carbon nanotube (CNT)/nanofiber composite hollow fiber membrane (CNC-HFM) that possesses significant advantages of permeability and mechanical strength. Specifically, the pure water flux and Young’s modulus of CNC-HFMs are 7.3 and 12.7 times higher than the commercial polyacrylonitrile HFMs with comparable pore size, respectively. The outstanding merits derive from the continuous interlaced nanofiber-scaffold structure of electrospun nanofiber support layer and crosslinked CNT separation layer, with low-tortuosity membrane pore, high porosity and mechanical strength. In addition, benefiting from the good electrical conductance of CNT separation layer, CNC-HFMs can exhibit a mitigation of membrane fouling and an improvement of separation performance during surface water treatment under electroassistance. The approach of constructing asymmetrical HFMs with electrospun nanofibers as the support layer and CNTs as separation layer provides unique thoughts for the design and manufacture of efficient separation membranes.
1. Introduction People all over the world have been afflicted with clean water shortage, and this situation will get worse with industrialization and urbanization, unfortunately [1,2]. An effective, stable and environ mentally friendly method to water purification is therefore urgently needed. As an acclaimed phase separation technology, membrane filtration has been widely used to address or alleviate the clean water crisis, which has scored remarkable achievements in water production and reclamation [3–5]. Among the existing membrane morphologies, hollow fiber membranes (HFMs) have gained an increasing interest due to their considerably large contact area per unit volume, simple struc ture, convenience for packaging and system operation [6,7]. Much attention has been paid on the rationalization and optimization of membrane structure [8]. The vast majority of HFMs are asymmetric membrane and made up of separation layer and support layer. Many researchers focus on the improvement of separation layer, but relatively
few researchers care for the support layer [9–11]. The support layer affects the porosity, mechanical strength and permeability of the HFMs, and is consequently as important as the separation layer. It is expected that the well-constructed support layer should be low tortuosity, high porosity and mechanical strength. The support layer of HFMs, prepared by phase inversion method which is the most common method to fabricate HFMs, generally has sponge-like or finger-like pore structures [11]. However, the pore structures are tortuous and susceptible to compaction, which results in the increase of resistance to water trans port and the decline of membranes porosity in the pressure-driven membrane filtration process [12]. Therefore, constructing HFMs with low pore tortuosity, structural stability and strong mechanical strength support layer is highly desirable to improve membrane performance. In the reported literatures, the nanofibers-scaffold nonwovens possess the interconnected pore structure with low tortuosity and high porosity [13–16]. Besides, the nanofibers are endowed with superior mechanical strength due to the high aspect ratio and uniform orientation
* Corresponding author. E-mail address: [email protected] (X. Quan). https://doi.org/10.1016/j.memsci.2019.117613 Received 20 May 2019; Received in revised form 9 October 2019; Accepted 25 October 2019 Available online 31 October 2019 0376-7388/© 2019 Published by Elsevier B.V.
Please cite this article as: Lei Du, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117613
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
[17]. It is therefore reasonable to infer that the nanofibers could be a good choice for preparing the support layer of HFMs. Despite the outstanding advantages, how to construct the HFMs using nanofiber is still a problem. Perhaps the most difficult part will be maintaining the stability of self-supporting hollow fiber structure, and more efforts are worth to investing in fabricating HFMs by nanofibers [18–20]. In this study, carbon nanotube (CNT)/nanofiber composite hollow fiber membranes (CNC-HFMs) were fabricated with continuous inter laced electrospun nanofibers scaffold as support layer and crosslinkedCNTs as separation layer. The electrospun nanofibers were collected on antirust steel wire collector and maintained the hollow fiber structure by the means of heat treatment. The CNTs were vacuumfiltrated on the nanofiber-scaffold support layer and covalently crosslinked to ensure the stability of membrane [21]. The experiment was conducted in different operating conditions to regulate the structure of electrospun nanofibers support layer. The relevant chemical, mechani cal, structural characterizations of the CNC-HFMs were evaluated. The separation performance of the CNC-HFMs was appraised by the purifi cation of surface water under electro-assistance.
entangled on the antirust steel wire collector. After a pre-oxidation treatment in the air (PAN: 250 � C, PVDF: 80 � C, heating rate: 200 � C/ h, hold time: 2 h), the antirust steel wire collector was immersed into 0.1 mol/L CuCl2 solution to divide the fiber-scaffold hollow fiber sub strate from the collector. After being rinsed with HP water, the fiberscaffold hollow fiber support layer was dried at 40 � C. 2.3. Preparation of CNT/nanofiber composite hollow fiber membranes The pristine CNTs were oxidized in H2SO4/HNO3 (3:1, v:v) mixture for 0.5 h at 80 � C before use. Then, 0.5 mg/mL oxidized CNT dispersion was prepared by dispersing the oxidized CNT in the HP water under ultrasonication, and then the CNTs were vacuum-filtrated on the hollow fiber support layer to form the CNC-HFMs. Subsequently, a certain amount of PVA solution (0.2 wt%) was infused in the CNC-HFMs under vacuum-filtrated. The obtained membranes were immersed into 1 wt% SA solution for 1 h. After the reaction, the CNC-HFMs were rinsed with HP water, and immobilized in oven at 80 � C for 4 h. The CNC-HFMs-PAN and CNC-HFMs-PVDF stand for the membranes whose supporting layer is prepared by electrospinning with corresponding material. The PANHFMs and PVDF-HFMs stand for the commercial hollow fiber mem branes preparation by phase inversion with corresponding material.
2. Experimental 2.1. Materials
2.4. Characterization of CNC-HFMs
Commercial grade polyacrylonitrile (PAN, average Mw ¼ 150,000 Da), Polyvinylidene Fluoride (PVDF, average Mw ¼ 275,000 Da) and acetone were purchased from J&K Scientific Ltd. Polyvinyl alcohol (PVA), succinic acid (SA), N, N-Dimethylformamide (DMF, �99%), copper chloride (CuCl2), nitric acid (HNO3, 65%), and sulfuric acid (H2SO4, 98%) were supplied by Sinopharm Chemical Re agent Co., Ltd.(Shanghai, China). The CNTs (multiwalled, diameter: 10–20, 20–40, 40–60, and 60–100 nm) were bought from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). All chemicals and reagents were used as received without further purification. The high-purity water (HP water) was supplied by a high purity water system (re sistivity > 18 MΩ/cm, Laike Instrument Co., Ltd., Shanghai, China). The commercial PAN HFMs were bought from Beihai Spring Trade Co., Ltd. (Beihai, China). The commercial PVDF HFMs were bought from Hang zhou Haotian Membrane Technology Co., Ltd. (Hangzhou, China).
The morphologies of CNC-HFMs were characterized using a scanning electron microscope (SEM, Hitachi S-4800). Digital images were filmed by a digital camera (Sony DSC-WX300, Japan). The pore size distribu tions of the membranes were analyzed by the Capillary Flow Porometer (Porolux 1000, IB-FT GmbH, Germany). The porosity of the CNC-HFMs was obtained by means of dry-wet weighting method [22]. The electro-conductivity of the CNC-HFMs was obtained from a four-point probe meter (RTS-8; 4 Probes Tech, Guangzhou, China). The Zeta po tential of organic matter and colloids in the surface water were measured by a zetasizer nano ZS (Malvern Instruments Ltd., UK). The mechanical strength was measured by an Instron 3345 (Instron Ltd., USA). The tensile stress and Young’s modulus was calculated based on the following equations:
σ¼ 2.2. Preparation of nanofiber-scaffold hollow fiber support layer E¼
The whole fabrication process of CNC-HFMs is illustrated in Fig. 1. A detailed instruction is given in the following. The nanofiber-scaffold hollow fiber substrate was prepared using the high-voltage electro static spinning equipment (Tongli micro nano Instrument Co., Ltd., Shenzhen, China). Typically, the spinning solution (PAN spinning sol vent: DMF; PVDF spinning solvent: DMF/acetone ¼ 9:1, m/m) was prepared and fixed in a micro-injection pump. The injection speed was controlled at 1 mL/h. An antirust steel wire collector (diameter: 1, 2 and 3 mm) was placed at a distance of 15 cm from solution outlet. The operating voltage was set at 9–21 kV. The prepared nanofibers were
4F
π R2
r2
�
σ Δl=l
(1) (2)
Where σ is the tensile stress, R and r are corresponding the outer diameter and inner diameter of the membranes, E is Young’s modulus, Δl is the shape variables when materials in the tensile process, l is the pristine length of materials. The membrane performance tests of CNC-HFMs were performed using a lab-made dead-end filtration system (schematically shown in Fig. S1, Supplementary material). A DC voltage (WYJS-60V/2A, Shanghai Shanjie electrical technology, co., Ltd., China) was applied on
Fig. 1. The fabrication process of CNC-HFMs. 2
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
the filtration system to test the filtration performance under electro chemical assistance (cathode: CNC-HFMs; anode: antirust steel network). The cyclic voltammetry measurement was carried out on an electrochemical work station (CHI660D, Shanghai Chenhua Ltd., China) in a three-electrode system with Pt as the counter electrode, Ag/AgCl electrode as the reference electrode and the surface water sample with 0.05 mol Na2SO4 as the electrolyte. The working electrode was fabri cated by coating the crosslinked CNT on a glassy carbon electrode. The turbidity was analyzed by the turbidimeter (LP2000-11 HanNa, Italy) and TOC was analyzed by TOC analyzer (TOC-VCPH, Shimadzu, Kyoto, Japan). The permeate flux and removal efficiency were calculated based on the following equations: J¼
m STPρ
� R¼ 1
their self-supporting hollow fiber structure. In the preparation of sepa ration layer, the hollow fiber is coated with CNT and turns black. Fig. S2 shows the finished membrane with a length of 15 cm, and the length of membrane can be also controlled according to the length of wire collector. Typical morphology of CNC-HFMs is showed in Fig. 2a–d. It can be seen from the figures that no apparent defect and crack are observed on the surface of membrane. The hollow fiber composed of continuous interlaced nanofibers produced by the electrospinning exhibits an intact circular tube shape. Closer observation of the cross-section shows that the nanofibers tightly wind each other after the preliminary heat treatment in the air (Fig. 2b). This structure can ensure that they will not loose after being stripped from the collector. The CNC-HFMs possess an asymmetric membrane structure, which consists of an electrospun PAN nanofiber support layer (Fig. 2c) and a cross-linked CNTs separation layer (Fig. 2d). The common feature of the two constituent parts is the interconnected pore structure constructed by one-dimension nanomaterials. The thickness of separation layer is as thin as 1.5 μm, which is measured from the magnification of boundary between the support layer and separation layer (Fig. S4, Supplementary material). This interconnected pore structure and thin separation layer can be conducive to enlarge the water permeability in the filtration process [23,24].
(3) Cp Cf
� (4)
� 100%
where J is the permeate flux, m is mass of effluent water under the transmembrane pressure P within the time T, S is the effective mem brane area, ρ is the density of water, R is the removal efficiency, Cp and Cf are the solute concentrations in the permeate and feed solutions, respectively. 3. Results and discussions
3.2. Structural control of CNC-HFMs
3.1. Preparation and characterization of CNC-HFMs
The structure of CNC-HFMs can be controlled to optimize their performance by adjusting the preparation parameters of support layer and separation layer. A series of parameters including electric field in tensities, spinning solution concentrations, electrospinning time, CNT amount and the diameters of CNTs were investigated to evaluate their effects on the membrane performance. The SEM images of hollow fiber substrates prepared at different spinning solution concentration are shown in Fig. S5. When the con centration of spinning solution is 5 wt%, the spinning solution will be stretched to nanospheres rather than nanofiber due to the low viscosity (Fig. S5c). By increasing the concentration, a rise in diameter of the prepared nanofibers can be achieved (diameter of 10 wt% nanofiber:
The preparation process of CNC-HFMs includes two procedures: the electrospinning of hollow fiber support layer and the immobilization of the CNT separation layer on the support layer. Figs. S2–S3 (Supple mentary material) displays the digital photos of CNC-HFMs in the whole preparation process. In the preparation of support layer, the diameter of hollow fibers can be controlled based on the size of collector. It can be observed that the as-prepared PAN nanofiber bundles are loosely twined on the antirust steel wire collector. After a typical pre-oxidation process, the oxidized nanofiber bundles tightly wrap around the collector. If stripped from the collector, the nanofiber bundles can still maintain
Fig. 2. SEM images of (a, b) cross-section, (c) support layer and (d) separation layer (d) of CNC-HFMs. 3
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
200–400 nm; diameter of 15 wt% nanofiber: 500–700 nm). However, excessively high concentration will lead to the problem that spinning solution is too viscous to be extruded and stretched to nanofibers. Therefore, the spinning solution concentration in this study was chosen to be 15 wt%. As shown in Fig. S6, the density of PAN nanofibers in creases with the increasing applied electric field intensity. Taking the efficiency and security into consideration, the electric field intensity in this study was chosen at 1 kV/cm. The thickness of a membrane directly determines its mechanical structure, permeate and water flux. To optimize the thickness of support layer and separation layer, the electrospinning time and CNT amount were controlled and the results were shown in Fig. 3a–b. The spinning time is selected more than or equal to 1 h to ensure the stability of hollow fiber structure. The thickness of support layer changes from 61.5 μm to 295.6 μm with electrospinning time increase and the thick ness of separation layer changes from 1.28 μm to 6.05 μm with CNT amount increase. To determine the effect of thickness change on the pore size and flux of membrane, the relevant experiment was conducted and the results were exhibited in Fig. 3c–d. It can be observed that the average pore sizes of support layer changes from 1.63 μm to 1.01 μm as the electro spinning time increases and the average pore sizes of prepared mem branes changes from 205.6 nm to 64.1 nm as the CNT amount increases. The support layer is composed of ultra-long nanofibers, resulting in the slow decrease of pore size with layer thickness. Compared with nano fibers, CNT is short in length and can gradually fill the interspace of the support layer, which leads to sharp decline of pore size with the CNT amount.
The permeability of support layer and prepared membrane is deeply influenced by the layer thickness. The flux of support layer with different electrospinning time is shown in Fig. 4a and the flux of pre pared membrane with 1 h electrospun support layer and different CNT amount is shown in Fig. 4b. The flux of support layer and prepared membrane falls quickly as the layer thickness increases. Considering the mechanical strength and permeability, the support layer with 1 h elec trospinning time and separation layer with 8.82 g m 2 CNT were selected as optimum conditions for preparing the composite membrane. To control the pore size of membranes, CNTs with different di ameters were used to prepare their separation layer. The result illus trates that the pore size of membranes increases with the diameters of CNTs (Fig. 5a). For the sake of membrane performance analysis, the diameter of applied CNTs in this study was chosen in the range of 20–40 nm. To securely attach to the support layer, CNTs are immobilized on the hollow fiber support layer with PVA and SA crosslinking. Fig. 5b shows the images of the membranes with and without crosslinking after 0.5 h ultrasonic shock. It can be observed that the water which the crosslinked sample is located in is pellucid, but CNTs on the membrane without crosslinking re-dispersed in the water resulting in the dark black solu tion. The result demonstrates that CNTs can be effectively fixed on the support layer by the crosslinking step. 3.3. Mechanical strength evaluation The mechanical strength of HFMs is an important performance parameter. High mechanical strength can avoid the fiber membrane
Fig. 3. The thickness of (a) support layer with different electrospinning time and (b) separation layer with different CNT amount; The pore size of (c) support layer with different electrospinning time and (d) prepared membranes with different CNT amount. 4
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
Fig. 4. The flux of (a) support layer with different electrospinning time and (b) prepared membranes with different CNT amount.
Fig. 5. (a) Average pore diameter of CNC-HFMs with different CNT diameters. (b) Physical photos of CNC-HFMs during ultrasonic concussion test.
break and damage during the processes of capsulation, transportation and application. The tensile stress and Young’s modulus are used to appraise mechanical strength of the membranes. To determine the effect of thickness change on the mechanical strength of the membrane, the relevant experiment was conducted. As shown in Fig. 6a–b, the tensile stress and Young’s modulus of support layer decrease with electro spinning time. When the electrospinning time is 1 h, the tensile stress is 21.18 � 1.71 MPa, and it decreases to 11.67 � 1.14 MPa when the electrospinning time is 6 h. Similarly, when the electrospinning time is 1 h, the Young’s modulus is 253.49 � 19.91 MPa, and it decreases to 158.96 � 27.72 MPa when the electrospinning time is 6 h. The decline can be ascribed to the fact that the tightness of nanofibers decreases as the nanofibers thicken. Besides, the tensile stress and Young’s modulus of prepared membrane was also test, which is 19.46 � 0.55 MPa and 236 � 9.33 MPa, respectively. The difference of tensile stress and Young’s modulus between before and after coating separation layer is tiny. The mechanical strength of the membranes was compared with commercial HFMs, which were prepared by the same material (Fig. 6c–d). The tensile stress of CNC-HFMs-PAN is high up to 19.46 MPa, which is 3.06 times higher than that of commercial PANHFMs, and the tensile stress of CNC-HFMs-PVDF is 1.33 times higher than that of commercial PVDF-HFMs. These results suggest that the membranes whose support layer composed of electrospun nanofibers possess a better tensile stress than the commercial hollow fiber mem branes with the same material. A similar result can be obtained from the Young’s modulus measurement. The Young’s modulus of CNC-HFMsPAN is 236 MPa, which is 12.7 times than the PAN commercial hollow fiber membranes, and the Young’s modulus of CNC-HFMs-PVDF is 2.35
times higher than the PVDF commercial hollow fiber membranes. Even if coating CNT on the PAN-HFMs, the tensile stress and Young’s modulus of PAN-CNT-HFMs is respective 6.59 MPa and 20.04 MPa, which is approximate with the strength of PAN-HFMs. The CNT layer is too thin to impact on the mechanical strength of membranes, which illustrates that the outstanding mechanical strength attribute to the nanofiber support layer. The high mechanical strength of support layer prepared by electro spinning method is mainly ascribed to high degree of crystallite and polymer chain orientation with the help of high electric field induction [17]. In addition, the interlaced nanofibers and interconnected pore structure in the electrospun fabrics produce the effect of load transfer according to the mechanical interlocking mechanism. The ultrahigh length to diameter ratio electrospun nanofibers is also expected to improve the toughness and strength of the hollow fiber substrates [25]. Besides, the pyrolysis and crosslinking of membranes after the pre-oxidation process may also contribute to enhance the mechanical strength of support layer. Owing to the fact that PAN hollow fiber sub strates show better mechanical strength, the membrane support layers prepared in the following experiment is composed of PAN. 3.4. Membrane performance evaluation To compare the membrane permeation, commercially available PAN hollow fiber membrane (average pore size: 1.20 μm) is employed as the support layer fabricate PAN-CNT-HFMs, whose separation layer is fabricated with the same preparation method of CNC-HFMs. We tested the porosity, contact angle, pore size and flux of the both membranes before and after CNT coating. The results are shown in Table 1 and 5
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
Fig. 6. (a) The tensile stress and (b) Young’s modulus of support layers with different electrospinning time; The (c) tensile stress and (d) Young’s modulus of different hollow fiber membranes.
Fig. S7 (Supplementary material). Before CNT coating, the electrospun hollow fiber support layer (average pore size: 1.22 μm) achieved a pure water flux of 8.4 � 104 Lm 2h 1bar 1, which is nearly 14 times as high as that of the commercial available PAN hollow fiber membranes with the similar pore size. After CNT coating, pore size of CNC-HFMs decrease to 221 nm. Their pure water flux is measured to be 5.8 � 103 Lm 2h 1bar 1, which is substantially 7.3 times higher than that of PANCNT-HFMs with a larger pore size of 243 nm. The membranes with electrospun support layer occupy significant advantage of pure water flux over the commercial membranes, which is worth to researching and analyzing [26, 27]. The pure water flux can be calculated according to the Hagen-Poiseuille equation: Jw ¼
r2p ΔP 8μðΔx=Ak Þ
can be affected by the pore size, the applied pressure across the mem brane, solution viscosity, the length of membrane pores and porosity. Noticeably, the pore radius rp, applied pressure ΔP and viscosity μ are almost same for the CNC-HFMs and PAN-CNT-HFMs. It is likely that the porosity Ak and the equivalent length of membrane pores Δx contribute to the gap of flux. As listed in Table 1, the porosity Ak of the hollow fiber substrates is nearly 2 times higher than that of the commercial PAN hollow fiber membranes. The higher porosity can offer more water transport chan nels and reduce the water diffusion resistance through the membrane. Another significant reason for the high flux is the equivalent length of membrane pores Δx, which is proportional positive to the tortuosity of membrane pore and the thickness of membrane. It is reported that the pore tortuosity of the membrane prepared by electrospinning is 1.2-1.3, while that of the membrane prepared by phase inversion is higher than 3.5 [19]. The low pore tortuosity can significantly reduce water diffu sion resistance through membrane. The thickness of CNC-HFMs and CNT-PAN-HFMs is 50.8 μm and 95.4 μm, respectively. The low pore tortuosity and thin thickness shorten the equivalent length of membrane pores, which will contribute to improving the permeability. In addition, the tiered interconnected pore structure in the electrospun hollow fiber substrates decreases the amount of dead pore and is also conducive to the decrease of water diffusion resistance [29]. So the reason for high flux of CNC-HFMs can be summarized that the electrospun support layer, possessing high porosity, low tortuosity, thin thickness and interconnected pore structure, can reduce the mass-transfer resistance and increase the diffusion channel [26,27].
(5)
where Jw is the pure water flux, rp is the radius of membrane pore, ΔP is the applied pressure across the membrane, μ is the viscosity of water, Δx is the equivalent length of ideal identical straight cylindrical membrane pores, and Ak is the porosity of the membrane [28]. The pure water flux Table 1 Textural parameters of PAN-CNT-HFMs and CNC-HFMs. Support layer preparation method Porosity ratio (%) Contact angle (o) Pore size before CNT coating (μm) Pore size after CNT coating (nm) Flux before CNT coating (Lm 2h 1bar 1) Flux after CNT coating (Lm 2h 1bar 1)
PAN-CNT-HFMs
CNC-HFMs
Wet spinning 51% 34.60 1.20 243 6.1 � 103 8 � 102
Electrospinning 94% 39.12 1.22 221 8.4 � 104 5.8 � 103
6
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
3.5. Filtration performance of CNC-HFMs under electro-assistance
the dissolved organic matter and colloids were attracted by the positive membranes, which will aggravate the membrane fouling [30,31]. So the flux loss of membranes with positive voltage is more serious than that without voltage. Conversely, when the membranes were applied with negative voltage, the negative dissolved organic matter and colloids can be excluded by the electronegative membranes. The negative mem branes were protected and reduced exposure to dissolved organic matter and colloids by the repelling interaction, which thus can alleviate the membrane fouling [32]. The result illustrates the electro-assistance has a significant effect on the filtration performance and the negative electro-assistance can improve the membrane fouling situation. It is also found that the removal of turbidity and TOC by the CNCHFMs can be enhanced by the electro-assistance (Fig. 7b and c). For example, the removal efficiencies of turbidity and TOC without electroassistance are 73.3% and 30.5%, respectively. Compared with the result without electro-assistance, the removal efficiencies of turbidity and TOC are respectively improved to 89.4% and 35.7% when the membranes are served as anode with a 2 V voltage. What is more, the removal effi ciencies of turbidity and TOC are further increased to 92.8% and 43.3% when the membranes are served as cathode with a 2 V voltage. The re sults show that both the positive and negative voltages can enhance the removal of turbidity and TOC. Furthermore, the negative voltage has a much more enhancement effect on the separation performance of CNCHFMs. The Zeta potential of organic matter and colloids in the surface water was measured to be 10.9 mV. When the membranes are operated under electro-assistance, the electrostatic interaction between the CNCHFMs and the negatively charged organic matter and colloids is
Due to the crosslinking CNTs in the separation layer, the CNC-HFMs possess a moderate electric conductivity of 39 S/m. It has been reported that the electrical conductivity of membranes can be applied to improve the filtration performance [7,30]. To investigate the anti-fouling and selectivity of CNC-HFMs under electro-assistance, the CNC-HFMs are utilized to purify surface water collected from a suburb reservoir. The parameters of the water sample are determined and listed in Table S2 (Supplementary material). Before filtration, the result of static adsorp tion experiment (Fig. S8) shows that the adsorption behavior of CNC-HFMs makes little difference to the water quality. The result of cyclic voltammetry measurement illustrates that no apparent electrochemical reaction is observed on the membranes when the applied cell voltage is in the range of 2 V (Fig. S9, Supplementary material, the corresponding bias potential versus Ag/AgCl: 1.32 V). Therefore, the filtration performance of the CNC-HFMs under electroassistance is investigated at the cell voltage between 0 and 2 V. The normalized flux during 120 min running time is shown in Fig. 7. The flux decreased with time because of membrane fouling, and the decline rate of it is affected by the applied voltage. For example, a flux loss of 35.3% occurred after 2 h running time when the CNC-HFMs were applied with positive voltage at 2 V. Moreover, a flux loss of 28.2% occurred without electro-assistance. However, the flux loss was improved to 6.2%, when the CNC-HFMs were applied with negative voltage at 2 V. It is known that the dissolved organic matter and colloids are negatively charged in surface water. When the membranes were applied with positive voltage,
Fig. 7. (a) The flux of CNC-HFMs with different applied voltages in the surface water; The removal efficiency of (b) turbidity and (c) TOC under different applied voltages. 7
Journal of Membrane Science xxx (xxxx) xxx
L. Du et al.
responsible for the enhanced removal efficiency [33–35]. It has been reported that the negatively charged contaminants can be driven away from the negatively polarized membranes due to the repulsion in same charges, which leads to the improvement of removal efficiency and the mitigation of membrane fouling. Nevertheless, the negative charged contaminants can be easily adsorbed on the positively polarized mem branes, which can also improve the removal efficiency but results in the deterioration of membrane fouling. So positively electro-assistance is unfavorable for the overall elevation of membrane performance. The results indicate that negative electro-assistance can be preferably used to improve the removal efficiency without sacrificing the permeability of membranes. The energy consumption of electro-assisted filtration has been evaluated and calculated. The energy consumptions caused by electrical assistance is less than 0.02 Wh/m3 at 0~2 V, because the current is far below 0.01 A in the electro-assisted filtration process and negligible electrochemical reaction occurs. The energy consumption of single filtration process is 30 Wh/m3, which is consistent with published literature [36]. Therefore, the electrical assistance can only cause negligible additional energy consumption, which indicates that the electro-assisted filtration process is energy efficient.
[4] F. Xunda, M.E. Tousley, M.G. Cowan, B.R. Wiesenauer, N. Siamak, C. Youngwoo, R. D. Noble, E. Menachem, D.L. Gin, C.O. Osuji, Scalable fabrication of polymer membranes with vertically aligned 1 nm pores by magnetic field directed selfassembly, ACS Nano 8 (2014) 11977–11986. [5] M.T.M. Pendergast, E.M.V. Hoek, A review of water treatment membrane nanotechnologies, Energy Environ. Sci. 4 (2011) 1946–1971. [6] R. Hilke, N. Pradeep, A.R. Behzad, S.P. Nunes, K.V. Peinemann, Block copolymer/ homopolymer dual-layer hollow fiber membranes, J. Membr. Sci. 472 (2014) 39–44. [7] G. Wei, H. Yu, X. Quan, S. Chen, H. Zhao, X. Fan, Constructing all carbon nanotube hollow fiber membranes with improved performance in separation and antifouling for water treatment, Environ. Sci. Technol. 48 (2014) 8062. [8] O. Sae-Khow, S. Mitra, Carbon nanotube immobilized composite hollow fiber membranes for pervaporative removal of volatile organics from water, J. Phys. Chem. C 114 (2015) 16351–16356. [9] S. Hern� andez, S. Lei, W. Rong, L. Ormsbee, D. Bhattacharyya, Functionalization of flat sheet and hollow fiber microfiltration membranes for water applications, ACS Sustain. Chem. Eng. 4 (2016) 907. [10] Q. Zhang, N. Wang, L. Zhao, T. Xu, Y. Cheng, Polyamidoamine dendronized hollow fiber membranes in the recovery of heavy metal ions, ACS Appl. Mater. Interfaces 5 (2013) 1907–1912. [11] P. Sukitpaneenit, T.S. Chung, High performance thin-film composite forward osmosis hollow fiber membranes with macrovoid-free and highly porous structure for sustainable water production, Environ. Sci. Technol. 46 (2012) 7358. [12] O. Yee Kang, C. Tai-Shung, Mitigating the hydraulic compression of nanofiltration hollow fiber membranes through a single-step direct spinning technique, Environ. Sci. Technol. 48 (2014) 13933–13940. [13] W. Xiao, D. Fang, B.S. Hsiao, B. Chu, Nanofiltration membranes based on thin-film nanofibrous composites, J. Membr. Sci. 469 (2014) 188–197. [14] A.K. Gautam, C. Lai, F. Hao, MENKHAUS, J. Todd, Electrospun polyimide nanofiber membranes for high flux and low fouling microfiltration applications, J. Membr. Sci. 466 (2014) 142–150. [15] D.S. Liu, J.N. Ashcraft, M.M. Mannarino, M.N. Silberstein, A.A. Argun, G. C. Rutledge, M.C. Boyce, P.T. Hammond, Spray layer-by-layer electrospun composite proton exchange membranes, Adv. Funct. Mater. 23 (2013) 3087–3095. [16] W.S. Hung, C.L. Lai, Q. An, M.D. Guzman, T.J. Shen, Y.H. Huang, K.C. Chang, C. H. Tsou, C.C. Hu, K.R. Lee, A study on high-performance composite membranes comprising heterogeneous polyamide layers on an electrospun substrate for ethanol dehydration, J. Membr. Sci. 470 (2014) 513–523. [17] A. Greiner Dr, J.H. Wendorff Dr, Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angew. Chem. Int. Ed. 46 (2010) 5670–5703. [18] J. Wang, L.a. Hou, K. Yan, L. Zhang, Q.J. Yu, Polydopamine nanocluster decorated electrospun nanofibrous membrane for separation of oil/water emulsions, J. Membr. Sci. 547 (2018) 156–162. [19] S. Xiaoxiao, L. Zhaoyang, S. Darren Delai, Nano gives the answer: breaking the bottleneck of internal concentration polarization with a nanofiber composite forward osmosis membrane for a high water production rate, Adv. Mater. 23 (2011) 3256–3260. [20] J.J. Li, L.T. Zhu, Z.H. Luo, Electrospun fibrous membrane with enhanced swithchable oil/water wettability for oily water separation, Chem. Eng. J. 287 (2016) 474–481. [21] C.D. Lannoy, D. Jassby, D.D. Davis, M.R. Wiesner, A highly electrically conductive polymer–multiwalled carbon nanotube nanocomposite membrane, J. Membr. Sci. 415–416 (2012) 718–724. [22] X. Fan, Y. Liu, X. Quan, S. Chen, Highly permeable thin-film composite forward osmosis membrane based on carbon nanotube hollow fiber scaffold with electrically enhanced fouling resistance, Environ. Sci. Technol. 52 (2018) 1444. [23] A. Podgorski, A. Balazy, L. Gradon, Application of nanofibers to improve the filtration efficiency of the most penetrating aerosol particles in fibrous filters, Chem. Eng. Sci. 61 (2006) 6804–6815. [24] F. Ludovic, K. Sears, Schütz Jürg, N. Finn, C. Huynh, S. Hawkin, M. Duke, S. Gray, Characterization and evaluation of carbon nanotube bucky-paper membranes for direct contact membrane distillation, J. Membr. Sci. 351 (2010) 36–43. [25] S.F. Fennessey, R.J. Farris, Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns, Polymer 45 (2004) 4217–4225. [26] K. Yoon, K. Kim, X. Wang, D. Fang, B.S. Hsiao, B. Chu, High flux ultrafiltration membranes based on electrospun nanofibrous PAN scaffolds and chitosan coating, Polymer 47 (2006) 2434–2441. [27] X.F. Wang, D.F. Fang, K. Yoon, B.S. Hsiao, B. Chu, High performance ultrafiltration composite membranes based on poly (vinyl alcohol) hydrogel coating on crosslinked nanofibrous poly (vinyl alcohol) scaffold, J. Membr. Sci. 278 (2006) 261–268. [28] A.L. Ahmad, B.S. Ooi, A.W. Mohammad, J.P. Choudhury, Composite nanofiltration polyamide membrane: a study on the diamine ratio and its performance evaluation, Ind. Eng. Chem. Res. 43 (2004) 8074–8082. [29] S.W. Cranford, M.J. Buehler, In silico assembly and nanomechanical characterization of carbon nanotube buckypaper, Nanotechnology 21 (2010) 265706. [30] F. Xinfei, Z. Huimin, L. Yanming, Q. Xie, Y. Hongtao, C. Shuo, Enhanced permeability, selectivity, and antifouling ability of CNTs/Al2O3 membrane under electrochemical assistance, Environ. Sci. Technol. 49 (2015) 2293–2300. [31] R. Md Saifur, C.D. Vecitis, E. Menachem, Electrochemical carbon-nanotube filter performance toward virus removal and inactivation in the presence of natural organic matter, Environ. Sci. Technol. 46 (2012) 1556–1564.
4. Conclusions In this work, the conductive CNC-HFMs have been successfully fabricated by coating and crosslinking CNTs on the hollow fiber support layer made up of electrospun PAN nanofibers. The tensile stress and Young’s modulus of CNC-HFMs are 3.06 and 12.7 times higher than those of commercial HFMs prepared by phase inversion, which confirms that the mechanical strength of CNC-HFMs significantly outperformed that of commercial HFMs. The CNC-HFMs also possess 7.3 times higher permeability than the commercial HFMs with comparable pore size, benefited from the high porosity and low tortuosity of the continuous interlaced nanofiber support layer. Furthermore, compared with the results of positively electro-assisted and single membrane separation, the negative electro-assistance effectively alleviated the fouling on the CNC-HFMs and enhanced the removal efficiency of turbidity and TOC. The high-performance and well-constructed CNC-HFMs are promising for the water purification and can be extended to more application in other fields. Declaration of competing interest The authors declare that we have no conflict of interest in this manuscript. Acknowledgement This work was supported by the National Natural Science Foundation of China (21437001), the Programme of Introducing Talents of Disci pline to Universities (B13012), and LiaoNing Revitalization Talents Program (XLYC1801003). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117613. References [1] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Mari~ nas, A. M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301–310. [2] E.N. Wang, R. Karnik, Water desalination: graphene cleans up water, Nat. Nanotechnol. 7 (2012) 552. [3] A.G. Fane, R. Wang, M.X. Hu, Synthetic membranes for water purification: status and future, Angew. Chem. Int. Ed. 54 (2015) 3368–3386.
8
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
[32] X. Sun, W. Ji, Z. Chen, S. Xin, B.J. Hinds, Fouling characteristics and electrochemical recovery of carbon nanotube membranes, Adv. Funct. Mater. 23 (2013) 1500–1506. [33] R. Avner, D. Wenyan, W. Ian, W. Sharon, J. David, Microbial attachment inhibition through low-voltage electrochemical reactions on electrically conducting membranes, Environ. Sci. Technol. 49 (2015) 12741–12750. [34] C.D. Vecitis, M.H. Schnoor, R. Md Saifur, J.D. Schiffman, E. Menachem, Electrochemical multiwalled carbon nanotube filter for viral and bacterial removal and inactivation, Environ. Sci. Technol. 45 (2011) 3672–3679.
[35] C.D. Vecitis, G. Gao, L. Han, Electrochemical carbon nanotube filter for adsorption, desorption, and oxidation of aqueous dyes and anions, J. Phys. Chem. C 115 (2011) 3621–3629. [36] A.I. Sch€ afer, A.G. Fane, T.D. Waite, Cost factors and chemical pretreatment effects in the membrane filtration of waters containing natural organic matter, Water Res. 35 (2001) 1509–1517.
9
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: http://www.elsevier.com/locate/memsci
Conductive CNT/nanofiber composite hollow fiber membranes with electrospun support layer for water purification Lei Du a, Xie Quan a, *, Xinfei Fan b, Gaoliang Wei c, Shuo Chen a a
Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian, 116024, China b College of Environmental Science and Engineering, Dalian Maritime University, Dalian, 116026, China c Key Laboratory of Groundwater Resources and Environment (Ministry of Education, China), College of New Energy and Environment, Jilin University, Changchun, 130026, China
A R T I C L E I N F O
A B S T R A C T
Keywords: Electrospun nanofiber support layer CNT separation layer Hollow fiber membranes High permeability and mechanical strength Electro-assistance
Hollow fiber membranes (HFMs) have been extensively used to alleviate the crisis of water resources because of their large contact area per unit volume. However, developing high permeability and mechanical strength HFMs remains a great challenge. In this study, we design a novel carbon nanotube (CNT)/nanofiber composite hollow fiber membrane (CNC-HFM) that possesses significant advantages of permeability and mechanical strength. Specifically, the pure water flux and Young’s modulus of CNC-HFMs are 7.3 and 12.7 times higher than the commercial polyacrylonitrile HFMs with comparable pore size, respectively. The outstanding merits derive from the continuous interlaced nanofiber-scaffold structure of electrospun nanofiber support layer and crosslinked CNT separation layer, with low-tortuosity membrane pore, high porosity and mechanical strength. In addition, benefiting from the good electrical conductance of CNT separation layer, CNC-HFMs can exhibit a mitigation of membrane fouling and an improvement of separation performance during surface water treatment under electroassistance. The approach of constructing asymmetrical HFMs with electrospun nanofibers as the support layer and CNTs as separation layer provides unique thoughts for the design and manufacture of efficient separation membranes.
1. Introduction People all over the world have been afflicted with clean water shortage, and this situation will get worse with industrialization and urbanization, unfortunately [1,2]. An effective, stable and environ mentally friendly method to water purification is therefore urgently needed. As an acclaimed phase separation technology, membrane filtration has been widely used to address or alleviate the clean water crisis, which has scored remarkable achievements in water production and reclamation [3–5]. Among the existing membrane morphologies, hollow fiber membranes (HFMs) have gained an increasing interest due to their considerably large contact area per unit volume, simple struc ture, convenience for packaging and system operation [6,7]. Much attention has been paid on the rationalization and optimization of membrane structure [8]. The vast majority of HFMs are asymmetric membrane and made up of separation layer and support layer. Many researchers focus on the improvement of separation layer, but relatively
few researchers care for the support layer [9–11]. The support layer affects the porosity, mechanical strength and permeability of the HFMs, and is consequently as important as the separation layer. It is expected that the well-constructed support layer should be low tortuosity, high porosity and mechanical strength. The support layer of HFMs, prepared by phase inversion method which is the most common method to fabricate HFMs, generally has sponge-like or finger-like pore structures [11]. However, the pore structures are tortuous and susceptible to compaction, which results in the increase of resistance to water trans port and the decline of membranes porosity in the pressure-driven membrane filtration process [12]. Therefore, constructing HFMs with low pore tortuosity, structural stability and strong mechanical strength support layer is highly desirable to improve membrane performance. In the reported literatures, the nanofibers-scaffold nonwovens possess the interconnected pore structure with low tortuosity and high porosity [13–16]. Besides, the nanofibers are endowed with superior mechanical strength due to the high aspect ratio and uniform orientation
* Corresponding author. E-mail address: [email protected] (X. Quan). https://doi.org/10.1016/j.memsci.2019.117613 Received 20 May 2019; Received in revised form 9 October 2019; Accepted 25 October 2019 Available online 31 October 2019 0376-7388/© 2019 Published by Elsevier B.V.
Please cite this article as: Lei Du, Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2019.117613
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
[17]. It is therefore reasonable to infer that the nanofibers could be a good choice for preparing the support layer of HFMs. Despite the outstanding advantages, how to construct the HFMs using nanofiber is still a problem. Perhaps the most difficult part will be maintaining the stability of self-supporting hollow fiber structure, and more efforts are worth to investing in fabricating HFMs by nanofibers [18–20]. In this study, carbon nanotube (CNT)/nanofiber composite hollow fiber membranes (CNC-HFMs) were fabricated with continuous inter laced electrospun nanofibers scaffold as support layer and crosslinkedCNTs as separation layer. The electrospun nanofibers were collected on antirust steel wire collector and maintained the hollow fiber structure by the means of heat treatment. The CNTs were vacuumfiltrated on the nanofiber-scaffold support layer and covalently crosslinked to ensure the stability of membrane [21]. The experiment was conducted in different operating conditions to regulate the structure of electrospun nanofibers support layer. The relevant chemical, mechani cal, structural characterizations of the CNC-HFMs were evaluated. The separation performance of the CNC-HFMs was appraised by the purifi cation of surface water under electro-assistance.
entangled on the antirust steel wire collector. After a pre-oxidation treatment in the air (PAN: 250 � C, PVDF: 80 � C, heating rate: 200 � C/ h, hold time: 2 h), the antirust steel wire collector was immersed into 0.1 mol/L CuCl2 solution to divide the fiber-scaffold hollow fiber sub strate from the collector. After being rinsed with HP water, the fiberscaffold hollow fiber support layer was dried at 40 � C. 2.3. Preparation of CNT/nanofiber composite hollow fiber membranes The pristine CNTs were oxidized in H2SO4/HNO3 (3:1, v:v) mixture for 0.5 h at 80 � C before use. Then, 0.5 mg/mL oxidized CNT dispersion was prepared by dispersing the oxidized CNT in the HP water under ultrasonication, and then the CNTs were vacuum-filtrated on the hollow fiber support layer to form the CNC-HFMs. Subsequently, a certain amount of PVA solution (0.2 wt%) was infused in the CNC-HFMs under vacuum-filtrated. The obtained membranes were immersed into 1 wt% SA solution for 1 h. After the reaction, the CNC-HFMs were rinsed with HP water, and immobilized in oven at 80 � C for 4 h. The CNC-HFMs-PAN and CNC-HFMs-PVDF stand for the membranes whose supporting layer is prepared by electrospinning with corresponding material. The PANHFMs and PVDF-HFMs stand for the commercial hollow fiber mem branes preparation by phase inversion with corresponding material.
2. Experimental 2.1. Materials
2.4. Characterization of CNC-HFMs
Commercial grade polyacrylonitrile (PAN, average Mw ¼ 150,000 Da), Polyvinylidene Fluoride (PVDF, average Mw ¼ 275,000 Da) and acetone were purchased from J&K Scientific Ltd. Polyvinyl alcohol (PVA), succinic acid (SA), N, N-Dimethylformamide (DMF, �99%), copper chloride (CuCl2), nitric acid (HNO3, 65%), and sulfuric acid (H2SO4, 98%) were supplied by Sinopharm Chemical Re agent Co., Ltd.(Shanghai, China). The CNTs (multiwalled, diameter: 10–20, 20–40, 40–60, and 60–100 nm) were bought from Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). All chemicals and reagents were used as received without further purification. The high-purity water (HP water) was supplied by a high purity water system (re sistivity > 18 MΩ/cm, Laike Instrument Co., Ltd., Shanghai, China). The commercial PAN HFMs were bought from Beihai Spring Trade Co., Ltd. (Beihai, China). The commercial PVDF HFMs were bought from Hang zhou Haotian Membrane Technology Co., Ltd. (Hangzhou, China).
The morphologies of CNC-HFMs were characterized using a scanning electron microscope (SEM, Hitachi S-4800). Digital images were filmed by a digital camera (Sony DSC-WX300, Japan). The pore size distribu tions of the membranes were analyzed by the Capillary Flow Porometer (Porolux 1000, IB-FT GmbH, Germany). The porosity of the CNC-HFMs was obtained by means of dry-wet weighting method [22]. The electro-conductivity of the CNC-HFMs was obtained from a four-point probe meter (RTS-8; 4 Probes Tech, Guangzhou, China). The Zeta po tential of organic matter and colloids in the surface water were measured by a zetasizer nano ZS (Malvern Instruments Ltd., UK). The mechanical strength was measured by an Instron 3345 (Instron Ltd., USA). The tensile stress and Young’s modulus was calculated based on the following equations:
σ¼ 2.2. Preparation of nanofiber-scaffold hollow fiber support layer E¼
The whole fabrication process of CNC-HFMs is illustrated in Fig. 1. A detailed instruction is given in the following. The nanofiber-scaffold hollow fiber substrate was prepared using the high-voltage electro static spinning equipment (Tongli micro nano Instrument Co., Ltd., Shenzhen, China). Typically, the spinning solution (PAN spinning sol vent: DMF; PVDF spinning solvent: DMF/acetone ¼ 9:1, m/m) was prepared and fixed in a micro-injection pump. The injection speed was controlled at 1 mL/h. An antirust steel wire collector (diameter: 1, 2 and 3 mm) was placed at a distance of 15 cm from solution outlet. The operating voltage was set at 9–21 kV. The prepared nanofibers were
4F
π R2
r2
�
σ Δl=l
(1) (2)
Where σ is the tensile stress, R and r are corresponding the outer diameter and inner diameter of the membranes, E is Young’s modulus, Δl is the shape variables when materials in the tensile process, l is the pristine length of materials. The membrane performance tests of CNC-HFMs were performed using a lab-made dead-end filtration system (schematically shown in Fig. S1, Supplementary material). A DC voltage (WYJS-60V/2A, Shanghai Shanjie electrical technology, co., Ltd., China) was applied on
Fig. 1. The fabrication process of CNC-HFMs. 2
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
the filtration system to test the filtration performance under electro chemical assistance (cathode: CNC-HFMs; anode: antirust steel network). The cyclic voltammetry measurement was carried out on an electrochemical work station (CHI660D, Shanghai Chenhua Ltd., China) in a three-electrode system with Pt as the counter electrode, Ag/AgCl electrode as the reference electrode and the surface water sample with 0.05 mol Na2SO4 as the electrolyte. The working electrode was fabri cated by coating the crosslinked CNT on a glassy carbon electrode. The turbidity was analyzed by the turbidimeter (LP2000-11 HanNa, Italy) and TOC was analyzed by TOC analyzer (TOC-VCPH, Shimadzu, Kyoto, Japan). The permeate flux and removal efficiency were calculated based on the following equations: J¼
m STPρ
� R¼ 1
their self-supporting hollow fiber structure. In the preparation of sepa ration layer, the hollow fiber is coated with CNT and turns black. Fig. S2 shows the finished membrane with a length of 15 cm, and the length of membrane can be also controlled according to the length of wire collector. Typical morphology of CNC-HFMs is showed in Fig. 2a–d. It can be seen from the figures that no apparent defect and crack are observed on the surface of membrane. The hollow fiber composed of continuous interlaced nanofibers produced by the electrospinning exhibits an intact circular tube shape. Closer observation of the cross-section shows that the nanofibers tightly wind each other after the preliminary heat treatment in the air (Fig. 2b). This structure can ensure that they will not loose after being stripped from the collector. The CNC-HFMs possess an asymmetric membrane structure, which consists of an electrospun PAN nanofiber support layer (Fig. 2c) and a cross-linked CNTs separation layer (Fig. 2d). The common feature of the two constituent parts is the interconnected pore structure constructed by one-dimension nanomaterials. The thickness of separation layer is as thin as 1.5 μm, which is measured from the magnification of boundary between the support layer and separation layer (Fig. S4, Supplementary material). This interconnected pore structure and thin separation layer can be conducive to enlarge the water permeability in the filtration process [23,24].
(3) Cp Cf
� (4)
� 100%
where J is the permeate flux, m is mass of effluent water under the transmembrane pressure P within the time T, S is the effective mem brane area, ρ is the density of water, R is the removal efficiency, Cp and Cf are the solute concentrations in the permeate and feed solutions, respectively. 3. Results and discussions
3.2. Structural control of CNC-HFMs
3.1. Preparation and characterization of CNC-HFMs
The structure of CNC-HFMs can be controlled to optimize their performance by adjusting the preparation parameters of support layer and separation layer. A series of parameters including electric field in tensities, spinning solution concentrations, electrospinning time, CNT amount and the diameters of CNTs were investigated to evaluate their effects on the membrane performance. The SEM images of hollow fiber substrates prepared at different spinning solution concentration are shown in Fig. S5. When the con centration of spinning solution is 5 wt%, the spinning solution will be stretched to nanospheres rather than nanofiber due to the low viscosity (Fig. S5c). By increasing the concentration, a rise in diameter of the prepared nanofibers can be achieved (diameter of 10 wt% nanofiber:
The preparation process of CNC-HFMs includes two procedures: the electrospinning of hollow fiber support layer and the immobilization of the CNT separation layer on the support layer. Figs. S2–S3 (Supple mentary material) displays the digital photos of CNC-HFMs in the whole preparation process. In the preparation of support layer, the diameter of hollow fibers can be controlled based on the size of collector. It can be observed that the as-prepared PAN nanofiber bundles are loosely twined on the antirust steel wire collector. After a typical pre-oxidation process, the oxidized nanofiber bundles tightly wrap around the collector. If stripped from the collector, the nanofiber bundles can still maintain
Fig. 2. SEM images of (a, b) cross-section, (c) support layer and (d) separation layer (d) of CNC-HFMs. 3
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
200–400 nm; diameter of 15 wt% nanofiber: 500–700 nm). However, excessively high concentration will lead to the problem that spinning solution is too viscous to be extruded and stretched to nanofibers. Therefore, the spinning solution concentration in this study was chosen to be 15 wt%. As shown in Fig. S6, the density of PAN nanofibers in creases with the increasing applied electric field intensity. Taking the efficiency and security into consideration, the electric field intensity in this study was chosen at 1 kV/cm. The thickness of a membrane directly determines its mechanical structure, permeate and water flux. To optimize the thickness of support layer and separation layer, the electrospinning time and CNT amount were controlled and the results were shown in Fig. 3a–b. The spinning time is selected more than or equal to 1 h to ensure the stability of hollow fiber structure. The thickness of support layer changes from 61.5 μm to 295.6 μm with electrospinning time increase and the thick ness of separation layer changes from 1.28 μm to 6.05 μm with CNT amount increase. To determine the effect of thickness change on the pore size and flux of membrane, the relevant experiment was conducted and the results were exhibited in Fig. 3c–d. It can be observed that the average pore sizes of support layer changes from 1.63 μm to 1.01 μm as the electro spinning time increases and the average pore sizes of prepared mem branes changes from 205.6 nm to 64.1 nm as the CNT amount increases. The support layer is composed of ultra-long nanofibers, resulting in the slow decrease of pore size with layer thickness. Compared with nano fibers, CNT is short in length and can gradually fill the interspace of the support layer, which leads to sharp decline of pore size with the CNT amount.
The permeability of support layer and prepared membrane is deeply influenced by the layer thickness. The flux of support layer with different electrospinning time is shown in Fig. 4a and the flux of pre pared membrane with 1 h electrospun support layer and different CNT amount is shown in Fig. 4b. The flux of support layer and prepared membrane falls quickly as the layer thickness increases. Considering the mechanical strength and permeability, the support layer with 1 h elec trospinning time and separation layer with 8.82 g m 2 CNT were selected as optimum conditions for preparing the composite membrane. To control the pore size of membranes, CNTs with different di ameters were used to prepare their separation layer. The result illus trates that the pore size of membranes increases with the diameters of CNTs (Fig. 5a). For the sake of membrane performance analysis, the diameter of applied CNTs in this study was chosen in the range of 20–40 nm. To securely attach to the support layer, CNTs are immobilized on the hollow fiber support layer with PVA and SA crosslinking. Fig. 5b shows the images of the membranes with and without crosslinking after 0.5 h ultrasonic shock. It can be observed that the water which the crosslinked sample is located in is pellucid, but CNTs on the membrane without crosslinking re-dispersed in the water resulting in the dark black solu tion. The result demonstrates that CNTs can be effectively fixed on the support layer by the crosslinking step. 3.3. Mechanical strength evaluation The mechanical strength of HFMs is an important performance parameter. High mechanical strength can avoid the fiber membrane
Fig. 3. The thickness of (a) support layer with different electrospinning time and (b) separation layer with different CNT amount; The pore size of (c) support layer with different electrospinning time and (d) prepared membranes with different CNT amount. 4
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
Fig. 4. The flux of (a) support layer with different electrospinning time and (b) prepared membranes with different CNT amount.
Fig. 5. (a) Average pore diameter of CNC-HFMs with different CNT diameters. (b) Physical photos of CNC-HFMs during ultrasonic concussion test.
break and damage during the processes of capsulation, transportation and application. The tensile stress and Young’s modulus are used to appraise mechanical strength of the membranes. To determine the effect of thickness change on the mechanical strength of the membrane, the relevant experiment was conducted. As shown in Fig. 6a–b, the tensile stress and Young’s modulus of support layer decrease with electro spinning time. When the electrospinning time is 1 h, the tensile stress is 21.18 � 1.71 MPa, and it decreases to 11.67 � 1.14 MPa when the electrospinning time is 6 h. Similarly, when the electrospinning time is 1 h, the Young’s modulus is 253.49 � 19.91 MPa, and it decreases to 158.96 � 27.72 MPa when the electrospinning time is 6 h. The decline can be ascribed to the fact that the tightness of nanofibers decreases as the nanofibers thicken. Besides, the tensile stress and Young’s modulus of prepared membrane was also test, which is 19.46 � 0.55 MPa and 236 � 9.33 MPa, respectively. The difference of tensile stress and Young’s modulus between before and after coating separation layer is tiny. The mechanical strength of the membranes was compared with commercial HFMs, which were prepared by the same material (Fig. 6c–d). The tensile stress of CNC-HFMs-PAN is high up to 19.46 MPa, which is 3.06 times higher than that of commercial PANHFMs, and the tensile stress of CNC-HFMs-PVDF is 1.33 times higher than that of commercial PVDF-HFMs. These results suggest that the membranes whose support layer composed of electrospun nanofibers possess a better tensile stress than the commercial hollow fiber mem branes with the same material. A similar result can be obtained from the Young’s modulus measurement. The Young’s modulus of CNC-HFMsPAN is 236 MPa, which is 12.7 times than the PAN commercial hollow fiber membranes, and the Young’s modulus of CNC-HFMs-PVDF is 2.35
times higher than the PVDF commercial hollow fiber membranes. Even if coating CNT on the PAN-HFMs, the tensile stress and Young’s modulus of PAN-CNT-HFMs is respective 6.59 MPa and 20.04 MPa, which is approximate with the strength of PAN-HFMs. The CNT layer is too thin to impact on the mechanical strength of membranes, which illustrates that the outstanding mechanical strength attribute to the nanofiber support layer. The high mechanical strength of support layer prepared by electro spinning method is mainly ascribed to high degree of crystallite and polymer chain orientation with the help of high electric field induction [17]. In addition, the interlaced nanofibers and interconnected pore structure in the electrospun fabrics produce the effect of load transfer according to the mechanical interlocking mechanism. The ultrahigh length to diameter ratio electrospun nanofibers is also expected to improve the toughness and strength of the hollow fiber substrates [25]. Besides, the pyrolysis and crosslinking of membranes after the pre-oxidation process may also contribute to enhance the mechanical strength of support layer. Owing to the fact that PAN hollow fiber sub strates show better mechanical strength, the membrane support layers prepared in the following experiment is composed of PAN. 3.4. Membrane performance evaluation To compare the membrane permeation, commercially available PAN hollow fiber membrane (average pore size: 1.20 μm) is employed as the support layer fabricate PAN-CNT-HFMs, whose separation layer is fabricated with the same preparation method of CNC-HFMs. We tested the porosity, contact angle, pore size and flux of the both membranes before and after CNT coating. The results are shown in Table 1 and 5
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
Fig. 6. (a) The tensile stress and (b) Young’s modulus of support layers with different electrospinning time; The (c) tensile stress and (d) Young’s modulus of different hollow fiber membranes.
Fig. S7 (Supplementary material). Before CNT coating, the electrospun hollow fiber support layer (average pore size: 1.22 μm) achieved a pure water flux of 8.4 � 104 Lm 2h 1bar 1, which is nearly 14 times as high as that of the commercial available PAN hollow fiber membranes with the similar pore size. After CNT coating, pore size of CNC-HFMs decrease to 221 nm. Their pure water flux is measured to be 5.8 � 103 Lm 2h 1bar 1, which is substantially 7.3 times higher than that of PANCNT-HFMs with a larger pore size of 243 nm. The membranes with electrospun support layer occupy significant advantage of pure water flux over the commercial membranes, which is worth to researching and analyzing [26, 27]. The pure water flux can be calculated according to the Hagen-Poiseuille equation: Jw ¼
r2p ΔP 8μðΔx=Ak Þ
can be affected by the pore size, the applied pressure across the mem brane, solution viscosity, the length of membrane pores and porosity. Noticeably, the pore radius rp, applied pressure ΔP and viscosity μ are almost same for the CNC-HFMs and PAN-CNT-HFMs. It is likely that the porosity Ak and the equivalent length of membrane pores Δx contribute to the gap of flux. As listed in Table 1, the porosity Ak of the hollow fiber substrates is nearly 2 times higher than that of the commercial PAN hollow fiber membranes. The higher porosity can offer more water transport chan nels and reduce the water diffusion resistance through the membrane. Another significant reason for the high flux is the equivalent length of membrane pores Δx, which is proportional positive to the tortuosity of membrane pore and the thickness of membrane. It is reported that the pore tortuosity of the membrane prepared by electrospinning is 1.2-1.3, while that of the membrane prepared by phase inversion is higher than 3.5 [19]. The low pore tortuosity can significantly reduce water diffu sion resistance through membrane. The thickness of CNC-HFMs and CNT-PAN-HFMs is 50.8 μm and 95.4 μm, respectively. The low pore tortuosity and thin thickness shorten the equivalent length of membrane pores, which will contribute to improving the permeability. In addition, the tiered interconnected pore structure in the electrospun hollow fiber substrates decreases the amount of dead pore and is also conducive to the decrease of water diffusion resistance [29]. So the reason for high flux of CNC-HFMs can be summarized that the electrospun support layer, possessing high porosity, low tortuosity, thin thickness and interconnected pore structure, can reduce the mass-transfer resistance and increase the diffusion channel [26,27].
(5)
where Jw is the pure water flux, rp is the radius of membrane pore, ΔP is the applied pressure across the membrane, μ is the viscosity of water, Δx is the equivalent length of ideal identical straight cylindrical membrane pores, and Ak is the porosity of the membrane [28]. The pure water flux Table 1 Textural parameters of PAN-CNT-HFMs and CNC-HFMs. Support layer preparation method Porosity ratio (%) Contact angle (o) Pore size before CNT coating (μm) Pore size after CNT coating (nm) Flux before CNT coating (Lm 2h 1bar 1) Flux after CNT coating (Lm 2h 1bar 1)
PAN-CNT-HFMs
CNC-HFMs
Wet spinning 51% 34.60 1.20 243 6.1 � 103 8 � 102
Electrospinning 94% 39.12 1.22 221 8.4 � 104 5.8 � 103
6
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
3.5. Filtration performance of CNC-HFMs under electro-assistance
the dissolved organic matter and colloids were attracted by the positive membranes, which will aggravate the membrane fouling [30,31]. So the flux loss of membranes with positive voltage is more serious than that without voltage. Conversely, when the membranes were applied with negative voltage, the negative dissolved organic matter and colloids can be excluded by the electronegative membranes. The negative mem branes were protected and reduced exposure to dissolved organic matter and colloids by the repelling interaction, which thus can alleviate the membrane fouling [32]. The result illustrates the electro-assistance has a significant effect on the filtration performance and the negative electro-assistance can improve the membrane fouling situation. It is also found that the removal of turbidity and TOC by the CNCHFMs can be enhanced by the electro-assistance (Fig. 7b and c). For example, the removal efficiencies of turbidity and TOC without electroassistance are 73.3% and 30.5%, respectively. Compared with the result without electro-assistance, the removal efficiencies of turbidity and TOC are respectively improved to 89.4% and 35.7% when the membranes are served as anode with a 2 V voltage. What is more, the removal effi ciencies of turbidity and TOC are further increased to 92.8% and 43.3% when the membranes are served as cathode with a 2 V voltage. The re sults show that both the positive and negative voltages can enhance the removal of turbidity and TOC. Furthermore, the negative voltage has a much more enhancement effect on the separation performance of CNCHFMs. The Zeta potential of organic matter and colloids in the surface water was measured to be 10.9 mV. When the membranes are operated under electro-assistance, the electrostatic interaction between the CNCHFMs and the negatively charged organic matter and colloids is
Due to the crosslinking CNTs in the separation layer, the CNC-HFMs possess a moderate electric conductivity of 39 S/m. It has been reported that the electrical conductivity of membranes can be applied to improve the filtration performance [7,30]. To investigate the anti-fouling and selectivity of CNC-HFMs under electro-assistance, the CNC-HFMs are utilized to purify surface water collected from a suburb reservoir. The parameters of the water sample are determined and listed in Table S2 (Supplementary material). Before filtration, the result of static adsorp tion experiment (Fig. S8) shows that the adsorption behavior of CNC-HFMs makes little difference to the water quality. The result of cyclic voltammetry measurement illustrates that no apparent electrochemical reaction is observed on the membranes when the applied cell voltage is in the range of 2 V (Fig. S9, Supplementary material, the corresponding bias potential versus Ag/AgCl: 1.32 V). Therefore, the filtration performance of the CNC-HFMs under electroassistance is investigated at the cell voltage between 0 and 2 V. The normalized flux during 120 min running time is shown in Fig. 7. The flux decreased with time because of membrane fouling, and the decline rate of it is affected by the applied voltage. For example, a flux loss of 35.3% occurred after 2 h running time when the CNC-HFMs were applied with positive voltage at 2 V. Moreover, a flux loss of 28.2% occurred without electro-assistance. However, the flux loss was improved to 6.2%, when the CNC-HFMs were applied with negative voltage at 2 V. It is known that the dissolved organic matter and colloids are negatively charged in surface water. When the membranes were applied with positive voltage,
Fig. 7. (a) The flux of CNC-HFMs with different applied voltages in the surface water; The removal efficiency of (b) turbidity and (c) TOC under different applied voltages. 7
Journal of Membrane Science xxx (xxxx) xxx
L. Du et al.
responsible for the enhanced removal efficiency [33–35]. It has been reported that the negatively charged contaminants can be driven away from the negatively polarized membranes due to the repulsion in same charges, which leads to the improvement of removal efficiency and the mitigation of membrane fouling. Nevertheless, the negative charged contaminants can be easily adsorbed on the positively polarized mem branes, which can also improve the removal efficiency but results in the deterioration of membrane fouling. So positively electro-assistance is unfavorable for the overall elevation of membrane performance. The results indicate that negative electro-assistance can be preferably used to improve the removal efficiency without sacrificing the permeability of membranes. The energy consumption of electro-assisted filtration has been evaluated and calculated. The energy consumptions caused by electrical assistance is less than 0.02 Wh/m3 at 0~2 V, because the current is far below 0.01 A in the electro-assisted filtration process and negligible electrochemical reaction occurs. The energy consumption of single filtration process is 30 Wh/m3, which is consistent with published literature [36]. Therefore, the electrical assistance can only cause negligible additional energy consumption, which indicates that the electro-assisted filtration process is energy efficient.
[4] F. Xunda, M.E. Tousley, M.G. Cowan, B.R. Wiesenauer, N. Siamak, C. Youngwoo, R. D. Noble, E. Menachem, D.L. Gin, C.O. Osuji, Scalable fabrication of polymer membranes with vertically aligned 1 nm pores by magnetic field directed selfassembly, ACS Nano 8 (2014) 11977–11986. [5] M.T.M. Pendergast, E.M.V. Hoek, A review of water treatment membrane nanotechnologies, Energy Environ. Sci. 4 (2011) 1946–1971. [6] R. Hilke, N. Pradeep, A.R. Behzad, S.P. Nunes, K.V. Peinemann, Block copolymer/ homopolymer dual-layer hollow fiber membranes, J. Membr. Sci. 472 (2014) 39–44. [7] G. Wei, H. Yu, X. Quan, S. Chen, H. Zhao, X. Fan, Constructing all carbon nanotube hollow fiber membranes with improved performance in separation and antifouling for water treatment, Environ. Sci. Technol. 48 (2014) 8062. [8] O. Sae-Khow, S. Mitra, Carbon nanotube immobilized composite hollow fiber membranes for pervaporative removal of volatile organics from water, J. Phys. Chem. C 114 (2015) 16351–16356. [9] S. Hern� andez, S. Lei, W. Rong, L. Ormsbee, D. Bhattacharyya, Functionalization of flat sheet and hollow fiber microfiltration membranes for water applications, ACS Sustain. Chem. Eng. 4 (2016) 907. [10] Q. Zhang, N. Wang, L. Zhao, T. Xu, Y. Cheng, Polyamidoamine dendronized hollow fiber membranes in the recovery of heavy metal ions, ACS Appl. Mater. Interfaces 5 (2013) 1907–1912. [11] P. Sukitpaneenit, T.S. Chung, High performance thin-film composite forward osmosis hollow fiber membranes with macrovoid-free and highly porous structure for sustainable water production, Environ. Sci. Technol. 46 (2012) 7358. [12] O. Yee Kang, C. Tai-Shung, Mitigating the hydraulic compression of nanofiltration hollow fiber membranes through a single-step direct spinning technique, Environ. Sci. Technol. 48 (2014) 13933–13940. [13] W. Xiao, D. Fang, B.S. Hsiao, B. Chu, Nanofiltration membranes based on thin-film nanofibrous composites, J. Membr. Sci. 469 (2014) 188–197. [14] A.K. Gautam, C. Lai, F. Hao, MENKHAUS, J. Todd, Electrospun polyimide nanofiber membranes for high flux and low fouling microfiltration applications, J. Membr. Sci. 466 (2014) 142–150. [15] D.S. Liu, J.N. Ashcraft, M.M. Mannarino, M.N. Silberstein, A.A. Argun, G. C. Rutledge, M.C. Boyce, P.T. Hammond, Spray layer-by-layer electrospun composite proton exchange membranes, Adv. Funct. Mater. 23 (2013) 3087–3095. [16] W.S. Hung, C.L. Lai, Q. An, M.D. Guzman, T.J. Shen, Y.H. Huang, K.C. Chang, C. H. Tsou, C.C. Hu, K.R. Lee, A study on high-performance composite membranes comprising heterogeneous polyamide layers on an electrospun substrate for ethanol dehydration, J. Membr. Sci. 470 (2014) 513–523. [17] A. Greiner Dr, J.H. Wendorff Dr, Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angew. Chem. Int. Ed. 46 (2010) 5670–5703. [18] J. Wang, L.a. Hou, K. Yan, L. Zhang, Q.J. Yu, Polydopamine nanocluster decorated electrospun nanofibrous membrane for separation of oil/water emulsions, J. Membr. Sci. 547 (2018) 156–162. [19] S. Xiaoxiao, L. Zhaoyang, S. Darren Delai, Nano gives the answer: breaking the bottleneck of internal concentration polarization with a nanofiber composite forward osmosis membrane for a high water production rate, Adv. Mater. 23 (2011) 3256–3260. [20] J.J. Li, L.T. Zhu, Z.H. Luo, Electrospun fibrous membrane with enhanced swithchable oil/water wettability for oily water separation, Chem. Eng. J. 287 (2016) 474–481. [21] C.D. Lannoy, D. Jassby, D.D. Davis, M.R. Wiesner, A highly electrically conductive polymer–multiwalled carbon nanotube nanocomposite membrane, J. Membr. Sci. 415–416 (2012) 718–724. [22] X. Fan, Y. Liu, X. Quan, S. Chen, Highly permeable thin-film composite forward osmosis membrane based on carbon nanotube hollow fiber scaffold with electrically enhanced fouling resistance, Environ. Sci. Technol. 52 (2018) 1444. [23] A. Podgorski, A. Balazy, L. Gradon, Application of nanofibers to improve the filtration efficiency of the most penetrating aerosol particles in fibrous filters, Chem. Eng. Sci. 61 (2006) 6804–6815. [24] F. Ludovic, K. Sears, Schütz Jürg, N. Finn, C. Huynh, S. Hawkin, M. Duke, S. Gray, Characterization and evaluation of carbon nanotube bucky-paper membranes for direct contact membrane distillation, J. Membr. Sci. 351 (2010) 36–43. [25] S.F. Fennessey, R.J. Farris, Fabrication of aligned and molecularly oriented electrospun polyacrylonitrile nanofibers and the mechanical behavior of their twisted yarns, Polymer 45 (2004) 4217–4225. [26] K. Yoon, K. Kim, X. Wang, D. Fang, B.S. Hsiao, B. Chu, High flux ultrafiltration membranes based on electrospun nanofibrous PAN scaffolds and chitosan coating, Polymer 47 (2006) 2434–2441. [27] X.F. Wang, D.F. Fang, K. Yoon, B.S. Hsiao, B. Chu, High performance ultrafiltration composite membranes based on poly (vinyl alcohol) hydrogel coating on crosslinked nanofibrous poly (vinyl alcohol) scaffold, J. Membr. Sci. 278 (2006) 261–268. [28] A.L. Ahmad, B.S. Ooi, A.W. Mohammad, J.P. Choudhury, Composite nanofiltration polyamide membrane: a study on the diamine ratio and its performance evaluation, Ind. Eng. Chem. Res. 43 (2004) 8074–8082. [29] S.W. Cranford, M.J. Buehler, In silico assembly and nanomechanical characterization of carbon nanotube buckypaper, Nanotechnology 21 (2010) 265706. [30] F. Xinfei, Z. Huimin, L. Yanming, Q. Xie, Y. Hongtao, C. Shuo, Enhanced permeability, selectivity, and antifouling ability of CNTs/Al2O3 membrane under electrochemical assistance, Environ. Sci. Technol. 49 (2015) 2293–2300. [31] R. Md Saifur, C.D. Vecitis, E. Menachem, Electrochemical carbon-nanotube filter performance toward virus removal and inactivation in the presence of natural organic matter, Environ. Sci. Technol. 46 (2012) 1556–1564.
4. Conclusions In this work, the conductive CNC-HFMs have been successfully fabricated by coating and crosslinking CNTs on the hollow fiber support layer made up of electrospun PAN nanofibers. The tensile stress and Young’s modulus of CNC-HFMs are 3.06 and 12.7 times higher than those of commercial HFMs prepared by phase inversion, which confirms that the mechanical strength of CNC-HFMs significantly outperformed that of commercial HFMs. The CNC-HFMs also possess 7.3 times higher permeability than the commercial HFMs with comparable pore size, benefited from the high porosity and low tortuosity of the continuous interlaced nanofiber support layer. Furthermore, compared with the results of positively electro-assisted and single membrane separation, the negative electro-assistance effectively alleviated the fouling on the CNC-HFMs and enhanced the removal efficiency of turbidity and TOC. The high-performance and well-constructed CNC-HFMs are promising for the water purification and can be extended to more application in other fields. Declaration of competing interest The authors declare that we have no conflict of interest in this manuscript. Acknowledgement This work was supported by the National Natural Science Foundation of China (21437001), the Programme of Introducing Talents of Disci pline to Universities (B13012), and LiaoNing Revitalization Talents Program (XLYC1801003). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.memsci.2019.117613. References [1] M.A. Shannon, P.W. Bohn, M. Elimelech, J.G. Georgiadis, B.J. Mari~ nas, A. M. Mayes, Science and technology for water purification in the coming decades, Nature 452 (2008) 301–310. [2] E.N. Wang, R. Karnik, Water desalination: graphene cleans up water, Nat. Nanotechnol. 7 (2012) 552. [3] A.G. Fane, R. Wang, M.X. Hu, Synthetic membranes for water purification: status and future, Angew. Chem. Int. Ed. 54 (2015) 3368–3386.
8
L. Du et al.
Journal of Membrane Science xxx (xxxx) xxx
[32] X. Sun, W. Ji, Z. Chen, S. Xin, B.J. Hinds, Fouling characteristics and electrochemical recovery of carbon nanotube membranes, Adv. Funct. Mater. 23 (2013) 1500–1506. [33] R. Avner, D. Wenyan, W. Ian, W. Sharon, J. David, Microbial attachment inhibition through low-voltage electrochemical reactions on electrically conducting membranes, Environ. Sci. Technol. 49 (2015) 12741–12750. [34] C.D. Vecitis, M.H. Schnoor, R. Md Saifur, J.D. Schiffman, E. Menachem, Electrochemical multiwalled carbon nanotube filter for viral and bacterial removal and inactivation, Environ. Sci. Technol. 45 (2011) 3672–3679.
[35] C.D. Vecitis, G. Gao, L. Han, Electrochemical carbon nanotube filter for adsorption, desorption, and oxidation of aqueous dyes and anions, J. Phys. Chem. C 115 (2011) 3621–3629. [36] A.I. Sch€ afer, A.G. Fane, T.D. Waite, Cost factors and chemical pretreatment effects in the membrane filtration of waters containing natural organic matter, Water Res. 35 (2001) 1509–1517.
9