Improved desalination properties of hydrophobic GO-incorporated PVDF electrospun nanofibrous composites for vacuum membrane distillation

Separation and Purification Technology 230 (2020) 115889

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Improved desalination properties of hydrophobic GO-incorporated PVDF electrospun nanofibrous composites for vacuum membrane distillation

T

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Hongbin Lia, , Wenying Shia,b, Xianhua Zenga, Shoufa Huanga, Haixia Zhanga, Xiaohong Qina,c a

School of Textiles Engineering, Henan University of Engineering, Zhengzhou 450007, PR China Henan Collaborative Innovation Center of Textile and garment industry, Zhongyuan University of Technology, Zhengzhou, PR China c School of Textiles Science, Donghua University, Shanghai 201620, PR China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Desalination Graphene oxide Hydrophobic modification Electrospun nanofiber Vacuum membrane distillation

Hydrophobic nanofibers have attracted great attention in the membrane distillation (MD) process because of their extremely high specific surface area. In this study, hydrophobic nanofibrous composites for vacuum membrane distillation (VMD) consisting of a hydrophobic polyvinylidene fluoride (PVDF) nanofibrous layer and a hydrophobic polypropylene (PP) nonwoven fabric (NWF) substrate were presented. PVDF nanofibrous layer was directly fabricated on the surface of PP NWF substrate by electrospinning technique. 1H, 1H, 2H, 2H -perfluorooctyltriethoxysilane (FTES) functionalized GO nanosheets were incorporated in the PVDF nanofiber layer during the electrospinning process to enhance the hydrophobicity and permeation water flux for VMD. Membrane surface morphology and composition were characterized through SEM and FTIR. The effects of FTESGO nanosheets content on the VMD desalination properties were investigated. And the transfer mechanism of water vapors through the FTES-GO incorporated nanofibrous layers was also proposed. Compared with the original PVDF nanofibrous layer, the surface hydrophobicity, liquid entry pressure (LEP) and water permeability of the FTES-GO incorporated nanofibrous layers had all been improved. When FTES-GO content was 4 wt%, the WCA increased from 104.0° for the neat PVDF nanofibrous layer to 140.5° for the modified nanofibrous layer. The permeation water flux reached a maximum value of 36.4 kg m−2 h−1. It was two times the water flux of the original membrane and meanwhile the salt rejection remained above 99.9% (50 °C, 3.5 wt% NaCl aqueous solution and permeation pressure of 31.3 kPa). No obvious wetting phenomenon was observed for FTES-GO incorporated membrane during the continuous VMD experiment for 60 h.

1. Introduction Membrane Distillation (MD) is a thermally-driven membrane separation process using porous hydrophobic membranes as the separation media and vapor pressure difference existing between the porous membrane surfaces as the driving force. As a promising separation technology for desalting highly saline waters, MD has many attractive advantages, such as low energy consumption, low pretreatment requirements and high purity product water with 100% theoretical solute rejection. It has been used in the applications such as sea and brine desalination, food and pharmaceutical processing, and wastewater treatment [1]. Compared with other membrane separation technologies such as reverse osmosis (RO), nanofiltration (NF), MD has so many technical advantages. However, MD is also attended by some drawbacks such as low permeation water flux and membrane pore wetting during the long-term operation. In order to overcome these

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disadvantages, it is urgent to develop a high-performance separation membrane for MD with high hydrophobicity, high porosity, appropriate pore size and narrow pore size distribution As a porous membrane made up of ultrafine fibers, electrospun nanofiber membranes have received great attention in recent years and have been successfully used in many separation processes [2]. The interconnected pore structure formed by the cross stacking of nanofibers endows membrane a rough nano-scale surface with high porosity, high specific surface area and low tortuosity. The hydrophobic and highly porous nanofiber membrane can effectively prevent pore wetting and enhance the water flux. With the development of membrane distillation technology, hydrophobic electrospun nanofiber membranes have been gradually applied in the field of MD and show excellent distillation performance. Some traditional hydrophobic membrane materials have been successfully spun through the electrospinning process to prepare hydrophobic nanofiber membranes, including polyvinylidene fluoride

Corresponding author at: 1 Xianghe Road, Zhengzhou, Henan Province 450007, PR China. E-mail address: [email protected] (H. Li).

https://doi.org/10.1016/j.seppur.2019.115889 Received 26 February 2019; Received in revised form 2 August 2019; Accepted 2 August 2019 Available online 03 August 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.

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reports on the hydrophobic modification of nanofiber membranes by adding hydrophobic GO into nanofiber membranes during the electrospinning process. If the hydrophobic GO nanosheets can be introduced into the nanofiber membrane, it is expected that the hydrophobicity and MD performance of the nanofiber membranes can be greatly improved. In this study, hydrophobic nanofibrous composites for vacuum membrane distillation (VMD) consisting of a hydrophobic polyvinylidene fluoride (PVDF) nanofibrous layer and a hydrophobic polypropylene (PP) nonwoven fabric (NWF) substrate were prepared by the direct electrospinning of PVDF nanofibers on the surface of PP NWF substrate. Superhydrophobic siloxane, 1H, 1H, 2H, 2H -perfluorooctyltriethoxysilane (FTES) was grafted onto the surface of GO nanosheets. And, the FTES-GO nanosheets with different concentrations were introduced into PVDF spinning solution. The modified nanofibrous layers were thoroughly characterized in terms of surface composition and morphology by FTIR and SEM. Surface hydrophobicity was examined through the dynamic water contact angle (WCA) measurements. The effects of FTES-GO nanosheets concentration on the VMD desalination performance were well studied. Finally, a continuous VMD experiment for 60 h was carried out to determine the performance stability of the prepared nanofibrous layer.

(PVDF) [3], polytetrafluoroethylene (PTFE) [4,5] and PVDF copolymers such as polyvinylidene fluoride-co-hexafluoropropylene (PVDFco-HFP) [6], PVDF-co- trifluorochloroethylene (PVDF-CTFE) [7] and PTFE-co-hexafluoropropylene (FEP) [8]. In addition to developing new hydrophobic polymers, another simple and effective attempt to further improve the MD performance is to incorporate hydrophobic nanomaterials into polymeric nanofiber membrane matrix. Some hydrophobic nanomaterials such as nano-SiO2 [9], nano-Al2O3 [10], carbon nanotubes (CNTs) [11], graphene [12], montmorillonite (MMT) [13], fluorosiloxane surface modified macromolecule (SMM) [14] and graphene quantum dots (GODs) [15] have been introduced into the conventional hydrophobic polymeric spinning solution such as PVDF and PVDF-HFP to prepare the modified nanofiber membranes by electrospinning process. The spun nanofiber membranes are successfully used in MD and show improved distillation performance, especially water flux. Among these hydrophobic nanomaterials, graphene based material as an emerging nanometer material has many characteristics including excellent thermal and chemical stability, high mechanical strength and toughness, etc. It has been widely used in various industrial fields including water treatment and other separation processes [1,12].The hydrophobicity of grapheme itself and its highly specific surface area make it a suitable candidate of membrane modification materials for MD. Some researchers have introduced the graphene nanosheets into the membrane matrix to prepare the electrospun nanofiber membranes for MD. Woo [12] incorporated different concentrations of graphene into electrospun polyvinylidene fluoride-cohexafluoropropylene (PH) membrane to enhance membrane hydrophobicity and permeation water flux in air gap membrane distillation (AGMD). In order to further enhance the hydrophobicity of graphene itself, graphene oxide (GO) with higher chemical activity is often used as a matrix material. Its surface has a large number of reactive functional groups such as hydroxyl group, epoxy group, carbonyl group and carboxyl group. Hydrophobic substances can be grafted onto the GO surface to obtain hydrophobic GO nanosheets [16]. Nowadays, some hydrophobic GO nanosheets have been successfully prepared and incorporated in the membrane matrix for MD, such as 3-(aminopropyl) triethoxysilane (APTS)-GO [17], octadecylamine (ODA)-GO [16], reduced GO [18] and n-butylamine (NBA)-GO [19]. These hydrophobically functionalized GO nanosheets were introduced into the hydrophobic membrane matrix prepared by conventional phase separation (such as PVDF flat sheet membrane [16–18] and PVDF hollow fiber membrane [19]). However, to our knowledge there are few

2. Experimental 2.1. Materials Poly(vinylidene fluoride) (PVDF, FR-904) powder was purchased from Shanghai 3F New Materials Co., Ltd., China and dried in a vacuum oven at 60 °C for 12 h before use. Acetone, sodium chloride and N, Ndimethylformamide (DMF) were obtained from Tianjin Kemiou reagent Co., Ltd. (China). Graphene oxide (GO) nanosheets and 1H, 1H, 2H, 2Hperfluorooctyltriethoxysilane (FTES) used in the fluorination process were purchased from Shanghai Aladdin reagent Co., Ltd. (China). All chemicals were used as received without further purification. Polypropylene (PP) nonwoven fabric (NWF) (Shanghai Tianlue Advanced Textile CO., China) with a grammage of 150 g/m−2 was used as the substrate. 2.2. Hydrophobic modification of GO nanosheets and preparation of spinning solution GO nanosheets were hydrophobically modified by FTES. The aim is

F

F

F

F

F

F

F

F F

F F

F

F F

F F

O

F Si O

CH3

O CH3

O HO

Si O

F

F

F F

F F F

Si O

F

F

F Adsorption

F

F

F

F

F

F O

F F

F

F

F F

F Hydroxylation F

F F

F

F

F

F

H3C

F

F

F

F

F

Si

O

O

O

O O O OH

O O

HO

OH

O

FTES

O

OH

OH OH O

OH O

HO

GO Fig. 1. Schematic diagram of hydrophobic modification of GO nanosheets with FTES. 2

OH

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the average fiber diameter, fiber diameter distribution, average pore size and pore size distribution (PSD) were obtained by analyzing at least 150 fibers and 100 pore voids in the SEM images using Image J software (NIH) [10]. In addition, the bead density (μm−2) was defined as the number of the beads per unit membrane area. Similar values have also been reported in previous literature to compare the number of beads in different electrospun nanofiber membranes [10,21]. The data of the bead density was obtained by analyzing the number of the beads in the SEM photographs using Image J software. The thickness of different layer was measured by using a digital micrometer (Mitutoyo, Japan) with the detection accuracy of 0.001 mm. The thickness of each sample was measured at five different locations, and then the average value was taken to reduce the error. The porosity was defined as the volume of pores divided by the total volume of the sample. It was measured via a gravimetric method [22,23]. Membrane samples with equal sizes of (3 cm × 3 cm) were fully wetted by ethanol for replacing the air within the pores. The weight of the samples before and after saturation of ethanol was measured. Both the porosities of NWF substrate (εs) and nanofibrous layer (εn) were calculated by the following equations:

to bond the hydrophobic FTES hydrolysates onto the surface of GO nanosheets. Fig. 1 illustrates the process of hydrophobic modification of GO nanosheets. The detailed modification process was described as follows. 1.6 g GO nanosheets were added in 50 ml toluene and dispersed homogeneously under sonication vibration for 2 h (Solution 1). 0.5 ml FTES and 0.75 ml pure water were added into 50 ml toluene simultaneously under a moderate stirring for 2 h to complete the hydroxylation of FTES (Solution 2, as illustrated in Fig. 1). Then, these two solutions prepared above were fully mixed and stirred for 18 h in a tightly sealed glove box filled with nitrogen. This was the process of bonding FTES molecules onto the surface of GO nanosheets by grafting reaction (as shown in Fig. 1). Afterwards, the modified GO nanosheets were separated from the mixed solution under high speed centrifugation (10 000 rpm). Thereafter, the modified GO nanosheets were thoroughly washed with pure water and further purified by centrifugation. Finally, the modified GO nanosheets (i.e. FTES-GO nanosheets) were freeze-dried for 48 h to constant weight. A certain amount of FTES-GO nanosheets were added into DMF under sonication vibration for 2 h. PVDF powders were fully dissolved at 60 °C in a mixed solvent of DMF and acetone. Then, the PVDF solution was mixed with the FTES-GO nanosheets suspension under moderate stirring at 60 °C for 24 h to obtain the spinning solution. Afterwards, the spinning solution was degassed in vacuum at room temperature for 6 h. The weight ratio of DMF and acetone in spinning solution was kept at 4: 1. PVDF concentration in spinning solution was 14 wt%. The content of FTES-GO nanosheets in spinning solution was 0.5, 1, 4 and 7 wt%, respectively. And the corresponding nanofibrous layers were named as G1, G2, G3 and G4 nanofibrous layer, respectively. Another PVDF (14 wt%) spinning solution without adding any GO nanosheets was also spun and the obtained nanofibrous layer was used as the control subject designated as G0 nanofibrous layer. Because the nanofibrous layers/NWF substrate prepared in this study are composites, G0-G4 nanofibrous layers and G0-G4 nanofibrous composites mentioned in this study referred to the single layer of nanofibers and the dual layer of the nanofibrous layer/NWF substrate, respectively.

εs =

W1 − W3 × 100% AlρL

W − W3 ⎞ εn = ⎜⎛1 − 2 ⎟ × 100% Aln ρP ⎠ ⎝

(1)

(2)

where W1, W2 and W3 are the weight of wet substrate, dry composite sample and dry substrate, respectively. A is the sample area. L and ln are the thickness of the composite sample and its nanofibrous layer, respectively. ρL and ρP are the density of ethanol and PVDF, respectively. Dynamic water contact angle (WCA) of the surface of different nanofibrous layers was examined on a Kruss Instrument (CM3250DS3210, Germany) at ambient temperature. Membrane samples were pasted smoothly on glass coverslip surface using double-sided adhesive. One water droplet (1 μL) was dropped on the sample surface with an automatic piston syringe. The images were continuously captured by a camera. Liquid entry pressure (LEP) can well evaluate the wetting resistance. Different nanofibrous samples were mounted in a filtration cell. The top of the samples was filled with pure water, and a slight vacuum was applied to the permeate side until the first water droplet appeared. The corresponding applied pressure was record as the LEP.

2.3. Electrospinning of FTES-GO/PVDF nanofibrous layers Nanofibrous layers were fabricated via the electrospinning machine. 10 ml volume of the spinning solution with different contents of modified GO nanosheets was electrospun at a rate of 1.23 ml/h by applying a direct current voltage of 24.1 kV. The distance between the tip of the spinneret and the rotating metal drum (collector) was kept at 27.7 cm. In the large-scale production of electrospun nanofibers, the incorporation of multiple nozzles instead of the current single nozzle in the electrospinning device can solve the problem of low yield as studied in previous literature [20]. It should be pointed that PP NWF densely covered the drum surface. As a substrate, PP NWF can enhance the mechanical strength of the nanofibrous layer and ensure its use in the subsequent experiment of vacuum membrane distillation. After 6 h of steadily spinning, the nascent nanofibrous layer/PP NWF composites were carefully separated from the metal drum and dried in an air-circulating oven for 24 h at 50 °C to remove the solvents. The dried membranes were preserved in a desiccator before use.

2.4.2. Vacuum membrane distillation experiments The nanofibrous composites with effective membrane area of 33.2 cm2 were tested in a VMD setup as illustrated in Fig. 2. The feed solution was 35 g/L sodium chloride (NaCl) aqueous solution with the conductivity around 60 mS/cm. The hot feed solution with a temperature of 50 °C was circulated by a diaphragm pump at a velocity of 0.85 L/min. On the permeation side, the water vapors permeated through the inner pores of the nanofibrous composites and were condensed by the stainless steel spring tube under the ice-water bath. The condensed water was sucked into the suction flask. The pressure on the permeate side was accurately regulated at 31.3 kPa through the vacuum gauge. The feed temperature was measured by a thermometer. The conductivity of the feed and permeate liquid were monitored by a conductivity meter (DDS-11A, Shanghai LeiciInstrument Works, China). During the VMD experiments, all exposed tubes were thermally insulated to prevent heat loss. The permeation water flux and salt rejection were calculated as Eqs. (3) and (4).

2.4. Characterization of electrospun nanofibrous layers 2.4.1. Characterization of morphology and structure The surface chemical compositions of different nanofibrous layers were characterized through the Attenuated Total Reflection-Fourier transform infrared spectroscopy (ATR-FTIR) on a Vector 22 FTIR spectrometer (BRUKER Corporation, Germany) in a wave number of 600–4000 cm−1. The surface morphologies of different nanofibrous layers were observed through scanning electron microscopy (SEM, FEI Quanta 250, USA). Membrane samples were freeze-dried and coated with gold before SEM observation. The structural parameters including

Jw =

m Ao × t

(3)

where m, Ao and t are the weight of permeation water (kg), membrane effective area (m2) and filtration time (h), respectively. 3

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Filtration cell

Nanofibrous composite

Thermometer Flowmeter

Spring tube

Diaphragm pump

Vacuum gauge

Ice-water bath

Feed solution

Control valve

Vacuum pump

Heating kettle

Suction flask Fig. 2. Schematic diagram of the experimental setup for vacuum membrane distillation.

Cp ⎞ R = ⎜⎛1 − ⎟ × 100% Cf ⎠ ⎝

It is difficult to distinguish the ν(C-F2) absorption bands of FTES and the C-F and C-F2 stretching bands of PVDF due to the overlaps of their characteristic bands. In addition, the peaks in the range of 800–900 cm−1 in the spectra of G2, G3 and G4 can be attributed to ν(C–F3) absorption bands of FTES and the vibration absorption peak of PVDF crystal phase. It is also difficult to distinguish these characteristic peaks due to their overlaps. These results indicate that the compatibility between FTES-GO and PVDF molecules is good and there is no obvious surface microscopic phase separation.

(4)

where Cp and Cf are the salt concentrations (mg/L) in the permeation and feed solution, respectively. Concentrations of the permeation and permeate were obtained from the conductivity data. 3. Results and discussion 3.1. Chemical compositions of membrane surface

3.2. Morphology of different electrospun nanofibrous layers

FTIR was used to characterize the chemical structure of PVDF nanofibrous layers with different contents of FTES-GO nanosheets. The typical spectra are shown in Fig. 3. The similar curves of G0 and G1 indicate that the addition of a small amount of FTES-GO nanosheets does not change the surface chemical structure of PVDF nanofibrous layer. In the range of wave number 1110–1300 cm−1, three characteristic peaks of FTES at 1273, 1191, 1134, and 1111 cm−1 are assigned to the stretching vibration of C-F2 [24,25]. Similarly, three vibrational bands and reflection peaks of PVDF in this wavenumber range at 1209, 1165 and 1276 cm−1 can be attributed to the stretching frequencies of C-F and C-F2. For the spectra of G2, G3 and G4 nanofibrous layers, four prominent peaks appears in the range of 1110–1300 cm−1.

Fig. 4 shows the surface SEM images of different PVDF electrospun nanofibrous layers. It can be observed from Fig. 4(a) that a large number of beaded nanofibers are formed in the neat PVDF nanofibrous layer. This special structure composed of beaded fibers is expected to be used in the MD process in this study. Membrane surface densely covered with bead-fibers can effectively hold the water droplets on it, thereby improving the hydrophobicity of membrane surface [26,27]. This favors the acceleration of water vapor transfer rate during the membrane distillation process. After the incorporation of FTES-GO nanosheets, some beads change into larger spheres, that is, spherical beads, as shown in Fig. 4(b). With the increasing concentration of FTESGO nanosheets, the number of spherical beads gradually increased, as shown in Fig. 4(c) and (e). Bead density was used to quantitatively characterize the number of beads in fibers. It was defined as the number of beads per unit membrane area and obtained through the SEM images of different nanofibrous layer surface. The bead densities of different nanofibrous layers are listed in Table 1. It can be seen that the bead density firstly increased from 309.8 to 722.9 × 10−3 μm−2 followed by an obvious increase to 1106.7 × 10−3 μm−2. Under the same electrospinning conditions, low surface tension tends to form more beads in the electrospun nanofiber membrane as the jet can be broken down easily into drops [21]. The incorporation of hydrophobic FTES-GO nanosheets in PVDF spinning solution reduces the solution surface tension and thus induces the formation of more beads under the high voltage electrostatic force. Besides, polymer solution with insufficient molecular chain entanglements can also cause an increase of beads number in electrospun mat due to the breakup of the polymer chains [10,28]. Compared with the pure PVDF spinning solution, the good compatibility between FTES containing fluorine siloxane groups and the fluorinated hydrocarbon PVDF molecules as well

Transmittance (a. u.)

G4 G3

4000

G2 G1 G0

3600

3200

2800

2400

2000

1600

1200

800

Wave number (cm-1) Fig. 3. FTIR spectra of different nanofibrous layers. 4

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Fig. 4. SEM images of different PVDF nanofibrous layer surface (left column × 5.0 k) and the magnified photos (right column × 10.0 k). (a/a′) G0; (b/b′) G1; (c/c′) G2; (d/d′) G3; (e/e′) G4.

5

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Table 1 Structural parameters of different nanofibrous layers. Sample

G0 G1 G2 G3 G4

nanofibrous nanofibrous nanofibrous nanofibrous nanofibrous

layer layer layer layer layer

Thickness (μm)

Porosity (%)

Average fiber diameter (nm)

Average pore size (nm)

Maximum pore size (nm)

Bead density (×10−3 μm−2)

LEP (kPa)

21 23 25 26 22

94.6 95.2 96.1 96.8 96.3

23.4 33.7 35.3 51.1 52.2

142.8 153.0 157.2 171.2 186.1

313.9 379.7 417.3 437.6 460.3

309.8 ± 8.7 355.7 ± 7.9 452.9 ± 10.2 722.9 ± 11.3 1106.7 ± 10.5

182.3 215.6 226.8 233.9 232.3

± ± ± ± ±

3 2 1 2 3

± ± ± ± ±

1.4 1.3 1.0 1.2 1.3

± ± ± ± ±

8.2 13.5 10.1 11.0 9.6

as the excellent adsorption capacity of the flaky GO nanosheets in the GO-incorporated PVDF solution weakens the intermolecular interaction and entanglements of PVDF molecules, especially the PVDF molecules arranged irregularly. The higher the content of FTES-GO nanosheets, the stronger the affinity between FTES-GO nanosheets and PVDF molecules, and the stronger the adsorptive force between them. Consequently, with the increasing contents of FTES-GO nanosheet more and more beads are formed under the high voltage electrostatic force due to the broken down of the liquid jet, as shown in Fig. 4(b-d). Therefore, there should be some GO nanosheets embedded in beads. As illustrated in Fig. 8, some FTES-GO nanosheets are embedded in PVDF nanofiber matrix meanwhile some FTES-GO nanosheets protrude on the nanofiber surface under the extreme stretching of high voltage electrostatic force. Similar results that GO nanosheets appeared on the surface and inside of nanofibers have been observed in previous literature [21]. It should be noted that unlike the smooth spindle-like beads as shown in Fig. 4(a), some granular pellets with a size of about 20 nm appears on the surface of the spherical beads of G3 membrane. This special structure of the nanopellets adhering to the spherical bead surface is similar to the double micronano-papillae on the lotus leaf surface. The appearance of micro-nanospheres on G3 membrane surface would help to enhance the hydrophobicity (as confirmed by the WCA measurements).

± ± ± ± ±

10.2 18.6 15.2 16.8 13.5

± ± ± ± ±

15.6 16.5 19.8 22.3 23.7

± ± ± ± ±

4.2 3.7 5.6 7.3 6.6

electrospun nanofibrous layers including average pore size and maximum pore size were also measured and the results are listed in Table 1. It can be found from Fig. 6 and Table 1 that with the increase of GO content the pore size distribution widens gradually and the average pore size and maximum pore size increase. These results are similar with those of fiber diameter distribution. Previous reports confirmed that fiber diameter plays an important effect on pore size and pore size distribution [31]. The increase of nanofiber diameter will lead to an enlargement of pore size of the membrane. Thus, the variation of fiber diameter is consistent with that of pore size. Previous studies showed that membrane pore size for MD should be less than 0.5 μm, so that the pore wetting phenomenon could be well avoided [29]. It can be seen from Fig. 6 that the pore size of all pores in different nanofibrous layers prepared in this study is below 0.5 μm. This will help to maintain the long-term stability of membrane performance during MD process. Membrane thickness is another important parameter affecting the MD performance, especially the mass transfer resistance. As the membrane thickness decreases, the resistance of vapor mass transfer decreases and membrane water flux increases [32]. Meanwhile, the decreased membrane thickness would result in an increased heat loss derived from the hot feed stream to the cold permeation stream. This would induce a decreased interfacial temperature differences (vapor pressure difference) and hence the permeation water flux declines [32,33]. Therefore, membrane thickness should be considered during MD process. Table 1 lists the thickness data of different nanofibrous layers. The thickness of PP NWF substrate was also measured which is 73 ± 2 μm. It is considerably thinner than that of commercially available porous separation membranes like PVDF, PP and PTFE. The thickness of these commercial membranes ranges from 80 to 200 μm [34,35]. Compared with PP NWF substrate, the thickness of different nanofibrous layers is relatively small and the value is between 21 and 26 μm. With the introduction of FTES-GO nanosheets into the nanofibers, the thickness of all FTES-GO modified layers does not change obviously and remains at a low level. The thinner nanofibrous layer can endow the nanofibrous composites with smaller mass transfer resistance and higher water flux (18–36 kg m−2 h−1) as confirmed in Fig. 10. The increase of membrane porosities can expand the steam transfer channels and enhance the thermal resistance of the porous membranes, thus improving the heat transfer efficiency and mass transfer rate. Membrane water flux will also be increased [13]. However, too loose and porous structure of separation membranes is not conducive to the stability of membrane mechanical properties [22]. This is particularly important for nanofibers with weak mechanical properties. Therefore, it is very important to increase the porosity of nanofibers meanwhile maintaining their mechanical strength in order to keep their advantages in membrane distillation applications. The porosity data of different nanofibrous layers are listed in Table 1. The porosity of PP NWF substrate was also measured and the value is 68.8%. As listed in Table 1, the porosities of all nanofibrous layers are above 90% which are much higher than that of the substrate. High void volume fraction and special interconnected open structure endow the nanofibrous layer with very high porosity. With the introduction and increase of FTES-GO nanosheets in PVDF nanofibers, the porosity of nanofibrous layer exhibits a slight increasing trend. This

3.3. Structural parameters of different nanofiber layers The fiber diameter distribution of different PVDF nanofibrous layers was measured and the results are shown in Fig. 5. With the increase of FTES-GO concentration in PVDF spinning dope, the fiber diameter distribution of the nanofibrous layers becomes wider and gradually shifts to a larger value. The average fiber diameter of different nanofibrous layers was also measured and the results are listed in Table 1. It can be seen that with the increase of GO content in nanofibers, the diameter of nanofibers increases from 23.4 to 52.2 nm. The widening of fiber diameter distribution and the enlargement of fiber diameter indicate that with the introduction of GO nanosheets into the spinning solution, more and more fibers with large diameter are formed. The viscosity of spinning dope has an important effect on fiber diameter. The increase of fiber diameter is related to the viscosity increase of spinning dope. Nanofibers with larger diameter can be obtained by increasing the viscosity of spinning solution under the same electrospinning conditions [25]. The viscosity can be increased by adding other substances such as nanoparticles (TiO2 [25], GO [12] and Al2O3 [10]) or macromolecules such as fluorosiloxane [14]. The introduction of the fluorosiloxane nanosheets of FFES-GO can effectively increase the viscosity of PVDF spinning solution which further leads to an increase of nanofiber diameter. The changes of some beads from smaller spindle shape to larger spherical shape also widen the distribution of fiber diameter. Pore size distribution and pore size are important parameters affecting the MD performance [29,30]. Nanofiber membrane with a narrow pore size distribution would be preferable for the long-term application in MD process. Membranes for MD should have an appropriate pore size to prevent wetting, enhance Knudsen diffusion and viscous flow [24]. The pore size distribution of different PVDF nanofibrous layers is shown in Fig. 6. Structural parameters of the 6

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Fig. 5. Fiber diameter distribution of different nanofibrous layers. (a) G0; (b) G1; (c) G2; (d) G3; (e) G4.

result is due to the formation of loose fibrous layer caused by the gradual increase in the number of beads as confirmed by the above SEM analysis. Similar result has been reported on the CNTs-incorporated PVDF-HFP nanofiber membranes in previous literature [32]. Compared with the NWF substrate, the increased porosity of the nanofibrous layer can effectively enhance the permeation water flux of the nanofibrous composites during the MD process. Although nanofibers do not develop well and a large number of beads are embedded in nanofibers, NWF, as a substrate with good mechanical strength, can provide excellent physical support for nanofiber layers to ensure their long-term use in MD

process.

3.4. The hydrophobicity different electrospun nanofibrous layers The surface hydrophobicity of different nanofibrous layers was evaluated using dynamic water contact angle (WCA) measurements within 300 min of contact time. The results of WCA are shown in Fig. 7. The WCA of PP NWF substrate surface was also measured and the value is 96.96°. The water contact angle over 90° means that PP NWF surface is hydrophobic in nature. After coating the PVDF nanofibrous layer, the 7

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Fig. 6. Pore distribution of different nanofibrous layers. (a) G0; (b) G1; (c) G2; (d) G3; (e) G4.

characteristic and a higher WCA. The introduction of hydrophobic fluorosiloxane GO in PVDF nanofibrous layer is bound to enhance the hydrophobicity of PVDF materials itself. In addition, as shown in SEM images, the introduction of FTES-GO nanosheets causes a large number of beads on the surface of nanofibrous layer. Meanwhile, its number gradually increases with the increase of GO content as listed in Table 1. These results greatly increase the surface roughness of the nanofibrous layers, thus improving the hydrophobicity of the membrane surface. With the further introduction of FTES-GO nanosheets, the size of beads in nanofibers in G4 nanofibrous layer decreases as shown in Fig. 4(e). This reduces the surface roughness of the nanofibrous layer to a certain

WCA values of all nanofibrous layers are higher than that of NWF substrate and vary from 100 to 140.5°. Obviously, the GO-incorporated nanofibrous layers show much better hydrophobicity than the PP NWF substrate. With the incorporation and increase of FTES-GO nanosheet content up to 4 wt%, the WCA value increase from 104.0° for G0 nanofibrous layer to 140.5° for G3. Afterwards, the WCA value shows a slow decrease to 132.5° for G4 nanofibrous layer. There are two major factors affecting the WCA value including the membrane surface roughness and the intrinsic hydrophobicity of the material itself [26,36]. Membrane surface with a more hydrophobic material and a rougher surface structure can exhibit a stronger water repellency 8

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140

4

130

WCA reduction percentage (%)

G3 G4

WCA (o)

G2 120

110

G1 G0

100

NWF 90

0

50

100

150

200

250

2

1

0

300

Time (s)

G0

G1

G2

G3

G4

Nanofibrous layers

Fig. 7. Dynamic water contact angle of different nanofibrous layers.

Fig. 9. WCA reduction percentage of different nanofibrous layers after 300 s of waterdrop contact time.

extent and results in a slight decrease in WCA value. Fig. 8 illustrates the contact between water droplets and the surface of different nanofibrous layers. When water droplets are in contact with a rough and porous surface, most droplets tend to be trapped in the air pockets beneath. The WCA (θc) can be predicted and calculated through the Cassie-Baxter equation [37,38].

cosθc = fsl (Rf cosθ0 + 1) - 1

3

means the fraction of solid-liquid interface (fsl) of FTES-GO incorporated nanofibrous layers would be lower and the value of θc would be higher. Especially for G3 with some micro-nanostructured beads on its surface, it would have a lowest solid-liquid interface and a highest WCA value as shown in Fig. 8(b). For the hydrophobic membrane used in membrane distillation process, its hydrophobicity should remain unchanged for a long time to ensure its good wettability. It can be seen from Fig. 7 that the WCA values of all nanofibrous layers decrease within the water contact of 300 s. In order to observe the decrease of WCA value in the contact time more clearly, the WCA reduction percentages of different nanofibrous layers were calculated through the ratio of the reduced value of WCA to the initial value of WCA. The results are shown in Fig. 9. The WCA reduction percentage of the NWF surface was also calculated by the same method and its WCA value decreases by 0.8% during the 300 min

(5)

where fsl corresponds to the fraction of solid-liquid interface per unit area. Rf is the non-dimensional surface factor which is equal to the ratio of the surface area to its flat solid area. θ0 denotes the static water contact angle for a flat surface of the same material and is obtained via the Young equation. Compared with the original PVDF nanofibrous layer (G0) as illustrated in Fig. 8(a), the surface of the FTES-GO incorporated nanofibrous layers with spherical beads, would have more air pockets. This

Fig. 8. Schematic diagram of contact between water droplets and the surface of different nanofibrous layers. (a) original PVDF nanofibrous layer (G0); (b) FTES-GO incorporated nanofibrous layers (G3). 9

Separation and Purification Technology 230 (2020) 115889

Permeation water flux (kg.m-2.h-1)

of water contact time. This suggests that the hydrophobic characteristic of the substrate material can be well maintained. With the incorporation and increase of FTES-GO nanosheets the WCA reduction percentage shows a decrease from 3.85% for G0 nanofibrous layer to 0.38% for G4 nanofibrous layer. This indicates that the hydrophobicity of the FTES-GO incorporated nanofibrous layer surface is stable especially when FTES-GO nanosheet content is high. The stability of surface hydrophobicity would enhance the anti-wetting properties and the performance durability of the composites. 3.5. The liquid entry pressure (LEP) of different electrospun nanofibrous composites

100

LEP =

−2Bγl cos θ rmax

Rejection

100

90

90

80

80

70

70

60

60

50

50

40

40

30

30

20

20

10

10

0

Liquid entry pressure (LEP) is a critical parameter to evaluate the wetting resistance of hydrophobic membrane for MD. It represents the pressure at which liquid water will penetrate into the pores of the hydrophobic membranes. Table 1 lists the LEP data of different electrospun nanofibrous composites. The LEP of PP NWF substrate was also measured and its value is 96.8 kPa. Compared with the LEP value of NWF substrate, the LEP values of all nanofibrous composites show an obvious increase ranging from 182.3 to 232.3 kPa, as listed in Table 1. The LEP values of the nanofibrous composites increase slowly with the increase of FTES-GO nanosheets content and the LEP values basically remain at about 230 kPa. LEP is the minimum entry pressure before the liquid (water) penetrates into the membrane micropores, which mainly depends on the pore structure and membrane hydrophobicity according to the Laplace equation [39].

Flux

G0

G1

G2

G3

G4

NaCl rejection (%)

H. Li, et al.

0

Nanofibrous composites Fig. 10. VMD desalination performance of different nanofibrous composites.

characteristics. By comparing the water flux of different nanofibrous composites in Fig. 10, it can be found that the permeation water flux exhibits an obvious increase from 18.1 kg m−2 h−1 for the neat PVDF nanofibrous composite (G0) to 36.4 kg m−2 h−1 for G3 nanofibrous composite followed by a decrease to 33.2 kg m−2 h−1 for G4 nanofibrous composite. Overall, the water flux of all FTES-GO incorporated nanofibrous composites is significantly higher than that of neat PVDF nanofibrous composites with a maximum increase of 2 times. On the premise of satisfying the basic requirements of membrane distillation for separation membrane pore size, high flux and selective properties during MD process mainly depends on membrane hydrophobicity, which accelerates vapor transport and alleviates the wetting [42,32]. Under the condition that the pore size can meet the basic requirements for MD, the hydrophobicity of nanofibrous composites determines the vapor transfer rate and the permeation water flux [32]. There are three mechanisms that affect the mass and heat transfer processes of water vapor molecules through nanofibrous layer, including hydrophobicity enhancement, the introduction of GO nanosheets with strong adsorption characteristics and the increase of surface roughness. The hydrophobicity enhancement of FTES-GO incorporated nanofibrous layers as shown in Fig. 7 can effectively accelerate the mass transfer. In order to better explain the changes of MD mass transfer process after introducing FTES-GO nanosheets into PVDF nanofibers, the diagram of the mass transfer of FTES-GO incorporated nanofibrous layer during MD process is illustrated in Fig. 11. The improvement hydrophobicity of of nanofibrous layer weakens the adhesion between water vapor molecules and pore walls. The so-called “repelling phenomenon” is strengthened. Water vapor molecules can pass through the pore channels rapidly and reach the permeation side through the specular reflection and refraction as illustrated in Fig. 11. Hence, Knudsen diffusion and molecular diffusion are enhanced. In addition, the hydrophobicity improvement of nanofibrous layer can also enhance the viscosity flow through the reinforcement of repulsion force. The interconnected pores formed by nanofiber stacking constitute the flow channel of vapor molecules. According to the fluid boundary layer theory, the viscosity flow of the fluid can generate a boundary layer near the wall of the flow channel [12]. The fluid molecules in the boundary layer would appear turbulence, collision and friction with each other. This interactions form a strong resistance to the forward flow of the fluid. The thickness of the boundary layer has an important effect on the mass transfer of the fluid. When the boundary layer is thin, the slip flow between the fluid molecules and the walls of the flow channel can be strengthened and the fluid molecules would pass through the flow channel quickly. The reinforcement of the repulsion force between water vapor molecules and micopore walls makes the boundary layer thinner so that more water vapor molecules can pass through the micropores rapidly with slip flow. The vapor transfer process would be accelerated.

(6)

where B, γl, θ, and rmax are the pore geometric factor, liquid (water) surface tension, liquid (water) contact angle and maximum pore size, respectively. Small pore size and narrow pore size distribution are beneficial to the enhancement of liquid entry pressure (LEP). Meanwhile, the membrane with smaller pore size has higher mass transfer resistance, thus affecting the mass transfer efficiency of water vapor in membrane distillation process [40,41]. It can be obtained from Table 1 that with the increase of FTES-GO nanosheet content in PVDF nanofiber matrix, the maximum pore size of nanofiber layer shows an increase and the pore size distribution widens. Although these results are not conducive to the enhancement of LEP, the obvious improvement of surface hydrophobicity would be the main factor leading to the increase of LEP. Obviously, the spun nanofibrous composites can effectively increase the LEP value without reducing the pore size. This will improve the water flux of the nanofibrous composites in the MD process. 3.6. The VMD desalination performance of different electrospun nanofibrous composites Fig. 10 shows the VMD desalination performance of different electrospun nanofibrous composites. The VMD desalination performance of NWF substrate is also measured under the same VMD conditions. The NaCl rejection and permeation water flux of the hydrophobic PP NWF substrate are 79.23% and 42.3 kg m−2 h−1, respectively. After coating of a thin electrospun nanofibrous layer on the surface of NWF substrate, the NaCl rejection improves greatly as shown in Fig. 10. The data of the NaCl rejection of all nanofibrous composites are kept between 99.9% and 100%. These results confirm that the coating of hydrophobic nanofibrous layer can significantly improve the desalination performance of the NWF substrate. The largest pore size and the weakest hydrophobicity of the NWF substrate make it have the highest permeation water flux as shown in Fig. 10. By contrast, the improved salt rejection of the nanofibrous composites is ascribed to the appropriate pore size for membrane distillation and relatively strong hydrophobic 10

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Viscosity flow

Knudsen diffusion

Molecular diffusion

Activated diffusion

Feed side Salt ion

Permeate side Water molecule

Water vapor molecule

FTES-GO nanosheet

Fig. 11. Schematic diagram of the mass transfer of FTES-GO incorporated nanofibrous layers for MD process.

nanofibrous composite (G0) for the long-term VMD testing. The variations of the conductivities of permeation liquid and water flux of different nanofibrous composites are shown in Fig. 12(a) and (b), respectively. As shown in Fig. 12(a), the conductivities of permeation liquid of all nanofibrous composites are basically kept at a low level of 5–15 μS/cm. With the prolongation of operation time, the conductivities of G0 nanofibrous composite show a slight increase from about 10 to 13 μS/cm. This suggests that some salt ions tend to penetrate the micropores and reach the condensation side after a long-term operation. It can be seen from Fig. 12(b) that the water flux of G0 nanofibrous composite initially remains stable and then slowly decreases especially when the operation time exceeds 20 h. By comparison, G3 nanofibrous composite has a higher water flux and the dada of water flux almost level off during the whole testing process. The declined water flux and deteriorated salt rejection of G0 nanofibrous composite suggest that with the prolongation of operation time some micropores in neat PVDF nanofibrous composite are wetted. This greatly increases the transfer resistance of water vapors through micropores and induces the decline of the permeability and water flux. The hydrophobicity improvement of G3 nanofibrous layer after the incorporation of FTES-GO nanosheets can effectively alleviate the pore wetting in VMD process. Besides, the enlarged LEP value of the FTES-GO incorporated nanofibrous composite endows it a superior anti-wetting property and a stable desalination performance.

Previous studies have shown that GO nanosheet possesses excellent adsorption properties due to its extremely high specific surface area [43,44]. When water vapor molecules pass through the pore channels of nanofibers, some of the water vapor molecules can quickly adsorb on the surface of GO nanosheets which are exposed to the nanofiber surface. These vapor molecules are driven by the driving forces on the permeate side such as vacuum condensation. The so-called activated diffusion as shown in Fig. 11 is formed by the subsequent rapid desorption. This effect also promotes the Knudsen diffusion of vapor molecules to a certain extent. The surface roughness of nanofibrous layer also affects the heat transfer efficiency of MD, thereby affecting MD performance. The rough structure plays a positive effect on the Nusselt number and the convection heat transfer especially under low Reynolds number (laminar flow) during the MD process [32,45]. Membranes with rough surface can improve the MD performance through the increase of the convection heat transfer coefficient. This is similar to the function of external spacer assembled to the membrane modules used for MD [46]. Obviously, the increased surface roughness of nanofibrous layers due to the formation of beaded nanofibers as observed in SEM images in Fig. 4 improves the heat transfer efficiency so that the permanent water flux is enhanced. Table 2 lists the MD performance of nanofiber membranes incorporated with different nanoparticles before and after hydrophobic modification. Although the WCA of the nanofibrous layer prepared in this study is not too high compared with other nanoparticle-modified nanofiber membranes, the increase of WCA and the water flux are more obvious.

4. Conclusions 3.7. Long-term VMD performance of different PVDF nanofibrous composites

The FTES-GO incorporated nanofibrous composite was prepared through the coating of FTES functionalized GO-incorporated PVDF nanofibrous layer onto the surface of PP nonwoven fabric during the electrospinning process. FTIR results demonstrated the good compatibility of FTES-GO nanosheets and PVDF polymer matrix. The emergence of large amounts of beaded nanofiber on the surface of modified nanofibrous layers resulted in the hydrophobicity enhancement. The reinforcements of viscosity flow, Knudsen diffusion and molecular diffusion and the generation of activated diffusion in the pore channels of

A continuous VMD experiment for 60 h was carried out for the further investigation of the effect of operation time on VMD performance of different PVDF nanofibrous composites. In order to better compare the changes of the long-term VMD performance of PVDF nanofibrous composites before and after the incorporation of FTES-GO nanosheets, G3 nanofibrous composite with the best comprehensive performance was selected and compared with the neat PVDF 11

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Table 2 MD performance of electrospun nanofiber membranes incorporated with different nanoparticles before and after hydrophobic modification. Sample

Nanoparticles

MD type

WCA (°)

MD operation parameters

Before

After

PVDF-HFP PVDF

SiO2 Al2O3

DCMD AGMD

148 132

155 150

PVDF-HFP PVDF-HFP PVDF PVDF

CNTs GO MMT Stearic acid functionalized nano Al2O3 FTES modified CNTs SMM/PVPa FTES modified TiO2 GODs FTES-GO nanosheets

DCMD AGMD DCMD AGMD

149 142.3 128 124

158.5 162 154.2 150

DCMD DCMD DCMD AGMD VMD

142.9 129.6 143.5 131.9 104.0

150.5 151.4 149 121 140.5

PVDF-HFP PVDF PVDF-HFP PVDF PVDF

(a)

35 g/L NaCl, 80 °C Air gap 8 mm, 60 °C, 1 g/L lead nitrate 35 g/L NaCl, 60 °C 35 g/L NaCl, 60 °C 35 g/L NaCl, 60 °C Air gap 8 mm, 60 °C, 1 g/L lead nitrate 35 g/L NaCl, 60 °C 35 g/L NaCl, 60 °C 70 g/L NaCl, 60 °C Air gap 2 mm, 35 g/L NaCl, 60 °C 35 g/L NaCl, 50 °C

(b)

25 G0 G3

Before

After

Before

After

99.99 73.2

99.99 99.36

37.5 18.5

48.6 19.8

[9] [10]

99.99 99.99 99 –

99.99 100 99.9 99

28 13.2 5.3 –

29.5 22.9 5.7 19.5

[11] [12] [13] [47]

– 99.97 99.99 (30 h) 99.5 99.96

– 99.98 99.99 (50 h) 99.7 99.97

31 3.2 40 14.7 18.1

34.5 10.8 40 17.6 36.4

[34] [14] [24] [15] This study

35

Water flux (kg.m-2.h-1)

Conductivity (μS.cm-1)

Ref.

G0 G3

40

20

Flux (L/m2h)

Rejection (%)

15

10

5

30 25 20 15 10

0 0

4

8

5

12 16 20 24 28 32 36 40 44 48 52 56 60

Time (h)

0

4

8

12 16 20 24 28 32 36 40 44 48 52 56 60

Time (h)

Fig. 12. Long-term VMD performance of different nanofibrous composites.

the FTES-GO incorporated nanofibrous layers contributed to the improvements of the permeation water flux and anti-wetting properties. Based on the above results, the hydrophobic modification through introducing FTES-GO nanosheets in electrospun nanofibrous layers is proven to be an effective way to obtain high-performance hydrophobic separation materials for membrane distillation.

[4]

[5]

[6]

Acknowledgments [7]

The authors gratefully acknowledge the funding for the Project supported by the Training plan for Young Scholar in Colleges and Universities in Henan Province (No. 2018GGJS151), the Central Plains Thousand People Program-Top Young Talents in Central Plains (No. ZYQR201810135), and Key Research Project of Higher Education of Henan Province (No. 18A540001).

[8]

[9]

[10]

Appendix A. Supplementary material [11]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.seppur.2019.115889.

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