Investigation of salt penetration mechanism in hydrolyzed polyacrylonitrile asymmetric membranes for pervaporation desalination

Desalination 463 (2019) 32–39

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Investigation of salt penetration mechanism in hydrolyzed polyacrylonitrile asymmetric membranes for pervaporation desalination

T

⁎

Hannah Faye M. Austriaa,b, Rumwald Leo G. Lecarosa,c, Wei-Song Hunga,d, , ⁎⁎⁎ ⁎⁎ Lemmuel L. Tayob, , Chien-Chieh Hua,d, Hui-An Tsaia,c, Kueir-Rarn Leea,c, , Juin-Yih Laia,d,e a

R&D Center for Membrane Technology, Chung Yuan University, Taoyuan, 32023, Taiwan School of Chemical, Biological, Materials Engineering and Sciences, Mapúa University, Intramuros, Manila 1002, Philippines c Department of Chemical Engineering, Chung Yuan University, Taoyuan, 32023, Taiwan d Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan e Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, 10607, Taiwan b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Asymmetric PAN membrane Pervaporation Desalination Free volume Positron annihilation lifetime

The present work is designed to evaluate the feasibility of using polyacrylonitrile (PAN) asymmetric membrane prepared via diffusion induced phase separation (DIPS). The PAN membrane was hydrolyzed using NaOH for different hours to improve its hydrophilicity and tune the microstructure morphology of HPAN1-5h membrane. The PAN and HPAN1-5h membranes were investigated the physicochemical properties by using the ATR-FTIR, water contact angle, Zeta potential and SEM. Besides, the positron annihilation lifetime spectroscopy results reveal that there was a reduction of free volume by increasing hydrolysis time. The breaking of intermolecular hydrogen bonds after desalination prompts a free structure which affects the growth and decline of the dry and wet zones in the membrane. The HPAN1-5h membranes were revealed the hydrolysis time progresses, the surface of the membrane became denser and pore size decreased. It also showed the salt deposition on the surface of the membranes after pervaporation testing. A permeation flux of 48.0 L/m2 h and rejections above 99% was obtained from 3.5 wt% NaCl aqueous feed solution at 60 °C using the HPAN membrane which was hydrolyzed for 1 h (HPAN1h). The HPAN1h membrane gave the highest permeation flux and has stability for up to 80 h of operation.

⁎

Correspondence to: W.-S. Hung, R&D Center for Membrane Technology, Chung Yuan University, Taoyuan 32023, Taiwan. Correspondence to: K.-R. Lee, Department of Chemical Engineering, Chung Yuan University, Taoyuan 32023, Taiwan. ⁎⁎⁎ Corresponding author. E-mail addresses: [email protected] (W.-S. Hung), [email protected] (L.L. Tayo), [email protected] (K.-R. Lee). ⁎⁎

https://doi.org/10.1016/j.desal.2019.04.012 Received 14 January 2019; Received in revised form 27 March 2019; Accepted 9 April 2019 0011-9164/ © 2019 Published by Elsevier B.V.

Desalination 463 (2019) 32–39

H.F.M. Austria, et al.

1. Introduction

2. Experimental section

The fresh water deficiency is a standout among the most widely recognized issue facing in our society. One of the promising solutions to counteract with this water crisis is the desalination of saltwater [1–3]. Currently, reverse osmosis (RO) is the leading desalination technique utilized in many commercial applications to produce fresh water [4,5]. Besides the advantages of its high efficiency, RO also encountering certain drawbacks specifically the requirement of high-pressure environment which add further energy cost, and fouling [6,7]. On the other hand, the strategy of thermally-driven separation process like membrane distillation (MD) which makes use of hydrophobic membranes has also been extensively considered for desalination [8–14]. As compared to RO process, it has many engaging highlights like high ion rejection, lower temperature prerequisite than normal distillation, and lower pressure environment [14,15]. However, the prolonged employment of MD technology results in wetting, higher operation cost, membrane fouling, lower permeation flux and higher salt rejection [16,17]. Among the various separation processes reported for desalination applications, pervaporation (PV) is the one that utilizes hydrophilic membranes for reduced fouling and consistent separation efficiency with low energy cost [18,19]. Pervaporation is a membrane process wherein specific species in the feed blend pervades through a dense membrane, and dissipates at the permeate side [20–22]. Its mass transfer mechanism has been reported to follow the solution-diffusion theory which consists of three fundamental steps: (a) sorption of the segments into the membrane, (b) diffusion of the broke up species through the membrane, and (c) desorption of the species as a vapor at pervading side [23–26]. PV has been recognized for its liquid mixtures separation applications, drying out of many aqueous-organic mixtures, and separation of low boiling point compounds from complex organic solutions for many years till now [18,27,28]. But at present, its feasibility for desalination application is being studied and has caught many researchers' attention because of some of its advantages [29]. One of these assets is the ability to reject most of the monovalent ions at a very high percentage, being generally above 99%, which is not dependent on the feed condition variations. Another consideration factor is the minimal dependency of the energy required to the salt concentration, owing to its capacity to handle highly saline water without requiring much adjustment in its driving force, as the osmotic pressure of the feed can be disregarded not normal for the pressure-driven processes, for example, RO [30–32]. Also, PV process does not require high temperature as it is associated with phase change like the MD process, so waste heat from industry and other possible economical heat sources (e.g., solar or geothermal energy) can be utilized for the operation. Based on previous studies, the membranes used in PV desalination typically consist of a dense top layer, an asymmetric support layer. The types of membranes used in PV desalination includes organic membranes [21,32], inorganic membranes [33,34], and hybrid organic-inorganic membranes [35,36]. While the fabrication of various membranes for pervaporation desalination is as a rule seriously explored, there are no related reports on the utilization of an asymmetric membrane for desalination appraisal. In this study, the performance of asymmetric polyacrylonitrile (PAN) membranes fabricated via diffusion induced phase separation (DIPS) method was investigated. This technique generally produces a highly porous, open substructure upon the formation of the skin, hence the structure of the membranes produced comprises of a thin top layer and a porous sub-layer having macro voids [37–39]. The PAN membranes were hydrolyzed in aqueous sodium hydroxide solution to control the physicochemical properties of the membrane and evaluate it for PV desalination [37]. The transport mechanism of salts which includes the dry and wet zone changes in asymmetric membranes were examined in this investigation.

2.1. Materials Polyacrylonitrile (PAN) polymer was purchased from Tong-Hua Synthesis Fiber Co. Ltd. (Taiwan). N-Methyl-2-pyrrolidone (NMP) was obtained from TEDIA High Purity Solvents (Farfield, OH, USA). SHOWA Chemical Industry Co., Ltd. (Japan) supplied the sodium hydroxide (NaOH) pellets. 2.2. Fabrication of polyacrylonitrile (PAN) asymmetric membrane An asymmetric PAN membrane was fabricated via diffusion induced phase separation (DIPS) method. Typically, 15 wt% polyacrylonitrile (PAN) solution was prepared using N-Methyl-2-pyrrolidone (NMP) as a solvent followed by solution-casting against a nonwoven polyester substrate using casting knife having a thickness of 200 μm. Further, transferred to the water bath for precipitation, which results in the formation of porous polyacrylonitrile (PAN) membrane. Excess solvent was removed by using distilled water. The membrane was put away in deionized water until use. 2.3. Hydrolysis of PAN membranes An aqueous solution of sodium hydroxide (2 M) was prepared and pre-heated to 50 °C. The PAN membranes were soaked into the prepared NaOH solution and maintained at 50 °C temperature for the desired different hydrolysis time (1–5 h). The membranes were then washed until the rinsed water turns neutral (pH 7). The obtained hydrolyzed PAN membranes were air-dried at room temperature before pervaporation testing. 2.4. Pervaporation testing The execution trial of various fabricated PAN membranes (effective membrane area is 3.14 cm2) were finished by utilizing them to desalt different concentrations of aqueous sodium chloride solution at various temperatures by pervaporation. The surface of the membrane is specifically in contact with the readied feed solution of NaCl in deionized water ranging from 3.5 to 15 wt%. To evaluate the impact of operation temperature on the desalination performance of the membranes, the saltwater was preheated and kept at 30–60 °C. A vacuum pump was utilized to clear the permeate side of the membrane. At time intervals, permeate were collected by using two cold traps immersed in liquid nitrogen. Permeation flux was calculated by dividing the weight of collected permeates, by the effective membrane area and by the sampling time. To determine ion rejection, the osmolality of both the feed solution and collected permeate was tested using Micro-Osmometer (Loser Messtechnik, type 15 M). 2.5. Characterization The physicochemical properties of the asymmetric PAN membranes were characterized by different analytical techniques. The functional groups were investigated and confirmed by using attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR; Perkin Elmer Spectrum One). Water contact angle (Automatic Interfacial Tensiometer, Model PD-VP) was measured to determine the hydrophilicity of the membranes. The scanning electron microscope (SEM; HITACHI S-4800) was used to observe the morphology of all the prepared membranes. 2.6. Determination of free volume Two positron annihilation techniques were used to determine the free volume of PAN and HPAN membranes, firstly doppler broadening 33

Desalination 463 (2019) 32–39

H.F.M. Austria, et al.

hydrophilicity and the negative charge increase from the deprotonation –COOH into –COO– groups. The negative charge of membrane surface exhibits the Donnan exclusion effect leading to high rejection for anions. The physical structure changes of PAN and HPAN membranes can be studied through the surface and cross-sectional SEM images as shown in Figs. 3 and 4. It was measured that the pore size on the surface of the membrane was around 20–30 nm. In the cross-section, the formation of the macro void is noticeable, since this is a characteristic observation of wet-phase inversion method [37–39]. It was observed that as the hydrolysis time proceeds, the size of the both surface pore and macro-voids, as well as porosity were reduced. Based on the above observations, it is proposed that the process of hydrolysis affects the physicochemical properties of PAN. The hydrolysis process created more intermolecular hydrogen bonding that could lead to the rearrangement of molecular chains and denser structure. 3.3. Pervaporation desalination performance of PAN and HPAN membrane The pervaporation desalination execution of the membranes with a feed solution of 3.5 wt% NaCl aqueous solution at 30 °C was plotted in Fig. 5a. The permeation flux of the PAN membrane was calculated to be 6.9 LMH with a salt rejection rate of 99.8%. As compared with PAN and HPAN membrane for the different hydrolyzed times, HPAN membrane hydrolyzed for 1 h shows the higher permeation flux of 24.4 LMH with a salt rejection of 99.9%. The decreasing permeation flux and low salt rejection rate (lesser than 99.9%) was observed for the HPAN membrane with the higher hydrolysis times. During the earlier hydrolysis treatment (1 h), the conversion of nitrile to amide followed by carboxylic acid groups (Fig. 1) increased the hydrophilicity of the membrane (Fig. 2), which is the explanation behind the increase in water flux. On contrary, the longer hydrolysis time treatment leads to the reduction of porosity (Figs. 3 and 4) due to pore swelling and increased population of carboxylic groups which resulting to the decline of permeation flux [50]. The correlation between the feed temperature and pervaporation desalination performance of the HPAN1h membrane presented in Fig. 5b. With the increase of feed temperature, the increase in permeation flux was observed. The following possible reasons are considered for the increase in water flux: (i) As the temperature increases, the driving force simultaneously increased with the vapor pressure in the feed side while that of the permeate side stays unaffected [24,56]; (ii) At higher temperatures, the molecular diffusivity was improved, leading to easier water permeation through the membrane [56]. The pervaporation desalination performance of HPAN1h membrane in different salt concentrations at 30 °C was shown in Fig. 5c. Reduction in water flux can be perceived as the feed saltwater becomes more concentrated. The following can provide evidence the decrease in permeation flux: (i) the diminishing of the thermodynamic activity of water in the feed solution results to a decrease in the water solution in the membrane. Consequently, the driving force, while the concentration gradient is additionally diminished. [57]; (ii) Fouling and concentration polarization on the surface of the membrane may increase resulting in a decline in water flux [23,24,31]; (iii) At higher concentration, the fractional free volume of the membrane matrix is reduced [31]. Fig. 5d shows the outcome of further testing of HPAN1h membrane for over 72 h by separating 3.5 wt% feed solution at 30 °C in a via a batch process. The permeation flux was observed to revolve around 13–15 LMH. For the salt rejection, it was still maintained at > 99.9%. An evaluation in the performance of different pervaporation desalination membranes previously studied is listed in Table 1. All of the membranes showed > 99% NaCl rejection regardless of NaCl concentration in feed. The HPAN1h has a higher flux than most of the membranes listed due to the absence of another dense active layer. The deposition of salt on the membrane surface is clearly visible

Fig. 1. ATR-FTIR spectra of (a) PAN, and (b-f) HPAN membranes at the different hydrolysis time (1–5 h).

energy spectroscopy (DBES) and second positron annihilation lifetime spectroscopy (PALS). Platinum was coated on all the membranes prior to measuring the free volume. Variable mono-energy slow positron beam (VMSPB) uses 50 mCi of 22Na as a source for positron and operation was carried out with positron incident energy in the range of 0–30 keV [40–45]. The accelerator energy was changed to control the implant depth of slow positron in samples. The DBES spectra with 1.0 million total number of counts were identified and recorded at a counting rate of 1800 cps using a solid state HP GE detector. The energy resolution of the detector was 1.5 keV at 0.511 MeV which corresponds to a positron 2γ annihilation peak. 3. Results and discussion 3.1. ATR-FTIR analysis of prepared membranes Fig. 1, an ATR-FTIR is utilized to distinguish the changes in the functional groups of PAN membranes upon hydrolysis at a different time. The display of pristine PAN spectrum peak at 2243 cm−1 confirms the nitrile (C^N) position in Fig. 1(a). Similarly, the occurrence of peaks at 3330, 1660, 1557, and 1407 cm−1 confirms the existence of eOH, eC]O, eNH2 and CeO functional groups, respectively in the HPAN membranes. In addition, the HPAN membrane can be observed the peak response decreases with increasing hydrolysis time as shown in Fig. 1(b–f). It can be observed a peak response increase with hydrolysis time, while indicates the transformation of nitrile group into eCONH2 and eCOOH groups [46–48]. 3.2. Physical property characterization of PAN and HPAN membranes The pristine PAN and HPAN (Fig. 2a) from the hydrolysis time interval to prepared membranes were examined by the water contact angle and the results from 45.38° (PAN) to 27.39° (HPAN1-5h). The increase of hydrolysis time from 1 h to 5 h increases the water solubility coefficient value, which makes the membrane more hydrophilicity increase as well as the promoting separation efficiency also [49]. Furthermore, the effect of hydrolysis time on the zeta potential values are shown in Fig. 2b. The membranes surface experiences more negative charge with the increase of hydrolysis time. The PAN and HPAN membrane surface charge results at −34 mV to −47 mV as shown in Fig. 2b. Both The observation of both water contact angle and zeta potential affirmed that the longer hydrolysis time leads to the increase of the carboxylic acid groups on the surfaces were formed the high 34

Desalination 463 (2019) 32–39

H.F.M. Austria, et al.

Fig. 2. Water contact angle measurement for the hydrophilicity (a) and Zeta potential measurement for the surface charge (b) of PAN and HPAN membranes.

Fig. 3. Surface SEM images of (a) PAN; and HPAN membranes at different hydrolysis time: (b-f) 1–5 h. (Inset: Magnified (×100k) of (a) and (f)).

Fig. 4. Cross-sectional SEM images of (a) PAN and HPAN membranes at different hydrolysis time: (b-f) 1–5 h. (Inset: Magnified (×100k) of (a) and (f)).

35

Desalination 463 (2019) 32–39

H.F.M. Austria, et al.

Fig. 5. (a) The pervaporation desalination performance of PAN and HPAN1-5h membranes at 30 °C. (b) Effect of feed temperature on the pervaporation desalination performance of HPAN1h membrane. (c) Effect of salt concentration on the pervaporation desalination performance of HPAN1h membrane at 30 °C. (d) Long-term pervaporation test of HPAN1h membrane at 30 °C. (Feed: 3.5 wt% aqueous NaCl solution).

annihilation spectroscopy (PAS) to characterize the free volume of a membrane. Comparison of the S- and R-values of the membrane before and after pervaporation desalination tests were shown in Fig. 8. The Svalue measures the free volume within the range of 0.1–1 nm and it is based on 2γ annihilation; while in case of R-value, o-Ps undergoes 3γ annihilation which gives us the information about the presence of bigger pores in the range of 1–100 nm. A higher value of S and R indicates the bigger pores and greater free volume. In Fig. 8a, the S-parameter curve shows that there is a shoulder peak area in the low positron implant energy (< 5 keV) followed by a plateau as positron implant energy increases. The shoulder area refers to the surface of the membrane while the plateau represents the micropore region. It was subsequently found that the bulk S-value decreased with increasing hydrolysis time, which was due to the conversion of CN groups to eCONH2 and eCOOH groups forming more intermolecular hydrogen bonding resulting in smaller free volume. Fig. 8b shows that after desalination the bulk S-value increases indicating larger free volume due to the dissociation of ions (Na+ and Cl−) in water disrupting the intermolecular hydrogen bonding. The results in Fig. 8c indicate the lowest R-value (~0.46) at < 5 keV region which represents the dense surface layer of the membrane after which an increase in R-value (~0.51) was observed indicating the entry into the macropore region. Fig. 8d shows that after desalination test, except for HPAN1h, the Rvalue of the macroporous region decreased significantly. This is due to the salt in water that has penetrated into a certain depth of the membrane (wet zone) then crystallizes. This in turn blocks the macropores of the membrane. Based on the above results, a schematic diagram of a pervaporation

Table 1 Pervaporation desalination performance of different membranes. Membrane

HPAN1h PVA/PAN CTAB-SiO2 PVA-MA-SiO2 GO/PAN GO/PDA-Al2O3 PVA/PVDF PVA-SiO2

NaCl in feed (g/L)

Temp. (°C)

Permeation flux (L/m2h)

Rejection (%)

35 35 35 40 2 35 35 100 2

30 60 25 25 22 90 90 80 60

24.4 48 7.24 2.60 6.93 65.1 48.4 16.4 20.6

99.8 99.8 99.5 99.9 99.5 99.8 99.7 99.9 99.9

Ref.

This work [21] [51] [52] [24] [53] [54] [55]

Abbreviation: HPAN: Hydrolyzed polyacrylonitrile; PVA: Polyvinyl alcohol; PAN: Polyacrylonitrile; CTAB: Cetyltrimethylammonium bromide; MA: Maleic acid; GO: Graphene oxide; PDA: Polydopamine; PVDF: Polyvinylidene fluoride.

from the surface (Figs. 6) and cross-sectional SEM images (Figs. 7) of the membranes respectively. However, salt crystals were not observed within the cross-sectional images. The surface may have provided enough barriers for the crystallized salts penetration through the membrane.

3.4. Positron annihilation spectroscopy analysis of PAN and HPAN membranes The salt penetration mechanism in pervaporation desalination was studied using a non-destructive technique called the positron 36

Desalination 463 (2019) 32–39

H.F.M. Austria, et al.

a

b

c

d

e

f

Fig. 6. Surface SEM images of (a) PAN and (b-f) HPAN membranes at different hydrolysis time (1–5 h) after pervaporation testing.

a

b

c

d

e

f

Fig. 7. Cross-sectional SEM images of (a) PAN and (b-f) HPAN membranes at different hydrolysis time (1–5 h) after pervaporation testing.

NaOH at different times. Pervaporation testing with feed NaCl aqueous solution was done, which revealed that after 1 h of hydrolysis of PAN membrane exhibited the highest performance; the permeation flux and the salt rejection attained 24.4 LMH and 99.9%, respectively. The PAN membrane's physical and chemical properties were confirmed to be affected by hydrolysis. Upon hydrolysis, the CN group was changed into CONH2, then further transformations into COOH. Although the water contact angle results exposed that the membrane became more hydrophilic as the hydrolysis time progresses, the decreasing flux in the PV test indicates that the pore shrinkage on both the surface and the macro voids observed in SEM images has more influence on the performance. The positron annihilation spectroscopy results revealed the depth of both dry and wet zones in the membrane in which at shorter hydrolysis time of PAN resulted in the thinner wet zone and salt only crystallizes on the surface. These data support that membranes without top layer can also be used in pervaporation desalination, which can reduce the membrane's cost production. In actualizing new advances to neutralize the lack in freshwater through desalination, these are imperative industrial factors that must be considered.

desalination transport mechanism on an asymmetric hydrolyzed PAN membrane was proposed in Fig. 9. The pervaporation separation includes the permeation of the preferential component through the membrane then evaporates into the vapor in the permeate side. Therefore, dry and wet areas are formed inside the membrane. It was found that long hydrolysis time of the membrane is accompanied by a small free volume. The intermolecular hydrogen bonds dissociated in the water cause severe swelling, forming a thick wet zone and a thinner dry zone. The salt crystallized less on the surface. At the shorter time of hydrolysis, the membrane has a low degree of swelling forming a relatively thin wet zone and a thicker dry zone. The salt crystallizes more on the surface. In addition, the crystallized salt formed by sodium and chlorine ions can't be vaporized to the permeate side. Therefore, as long as a dry zone can be formed and maintained, a high salt rejection can be obtained. 4. Conclusion This investigation demonstrates that PAN asymmetric membrane can be used for desalination applications. HPAN membranes were successfully fabricated through the hydrolysis of PAN membrane in 2 M 37

Desalination 463 (2019) 32–39

H.F.M. Austria, et al.

Fig. 8. DBES vs. positron incident energy for prepared membranes before and after pervaporation operation (a, b) S parameter; (b, c) R parameter.

Fig. 9. The proposed mechanism of seawater desalination through pervaporation for hydrolyzed PAN asymmetric membrane.

Acknowledgements

desalination, Desalination 245 (2009) 214–231. [5] L. Garcı́, Renewable energy applications in desalination: state of the art, Sol. Energy 75 (2003) 381–393. [6] S.-Y. Kwak, S.H. Kim, S.S. Kim, Hybrid organic/inorganic reverse osmosis (RO) membrane for bactericidal anti-fouling. 1. Preparation and characterization of TiO2 nanoparticle self-assembled aromatic polyamide thin-film-composite (TFC) membrane, Environ. Sci. Technol. 35 (2001) 2388–2394. [7] R.L. McGinnis, M. Elimelech, Global Challenges in Energy and Water Supply: The Promise of Engineered Osmosis, ACS Publications, 2008. [8] A. Alkhudhiri, N. Darwish, N. Hilal, Membrane distillation: a comprehensive review, Desalination 287 (2012) 2–18. [9] L.M. Camacho, L. Dumée, J. Zhang, J.-d. Li, M. Duke, J. Gomez, S. Gray, Advances in membrane distillation for water desalination and purification applications, Water 5 (2013) 94–196. [10] E. Drioli, A. Ali, F. Macedonio, Membrane distillation: recent developments and perspectives, Desalination 356 (2015) 56–84. [11] E. Drioli, A. Criscuoli, E. Curcio, Membrane Contactors: Fundamentals, Applications and Potentialities, Elsevier, 2011. [12] M. El-Bourawi, Z. Ding, R. Ma, M. Khayet, A framework for better understanding membrane distillation separation process, J. Membr. Sci. 285 (2006) 4–29.

The authors would like to acknowledge the Ministry of Science and Technology of Taiwan (MOST 105-2221-E-011-168-MY3 and 1062632-E-033-001) for financially supporting this research. References [1] D. Cohen-Tanugi, R.K. McGovern, S.H. Dave, J.H. Lienhard, J.C. Grossman, Quantifying the potential of ultra-permeable membranes for water desalination, Energy Environ. Sci. 7 (2014) 1134–1141. [2] N. Diban, A.T. Aguayo, J. Bilbao, A. Urtiaga, I. Ortiz, Membrane reactors for in situ water removal: a review of applications, Ind. Eng. Chem. Res. 52 (2013) 10342–10354. [3] S. Lin, M.J. Buehler, Mechanics and molecular filtration performance of graphyne nanoweb membranes for selective water purification, Nanoscale 5 (2013) 11801–11807. [4] C. Charcosset, A review of membrane processes and renewable energies for

38

Desalination 463 (2019) 32–39

H.F.M. Austria, et al.

123–130. [37] C.-L. Lai, W.-C. Chao, W.-S. Hung, Q. An, M. De Guzman, C.-C. Hu, K.-R. Lee, Physicochemical effects of hydrolyzed asymmetric polyacrylonitrile membrane microstructure on dehydrating butanol, J. Membr. Sci. 490 (2015) 275–281. [38] A. Reuvers, J. Van den Berg, C. Smolders, Formation of membranes by means of immersion precipitation: part I. a model to describe mass transfer during immersion precipitation, J. Membr. Sci. 34 (1987) 45–65. [39] C. Smolders, A. Reuvers, R. Boom, I. Wienk, Microstructures in phase-inversion membranes. Part 1. Formation of macrovoids, J. Membr. Sci. 73 (1992) 259–275. [40] P.J. Schultz, K.G. Lynn, Interaction of positron beams with surfaces, thin films, and interfaces, Rev. Mod. Phys. 60 (1988) 701. [41] Q.F. An, W.S. Hung, S.C. Lo, Y.H. Li, M. De Guzman, C.C. Hu, K.R. Lee, Y.C. Jean, J.Y. Lai, Comparison between free volume characteristics of composite membranes fabricated through static and dynamic interfacial polymerization processes, Macromolecules 45 (2012) 3428–3435. [42] W.C. Chao, S.H. Huang, Q.F. An, D.J. Liaw, Y.C. Huang, K.R. Lee, J.Y. Lai, Novel interfacially-polymerized polyamide thin-film composite membranes: studies on characterization, pervaporation, and positron annihilation spectroscopy, Polymer 52 (2011) 2414–2421. [43] H.M. Chen, W.S. Hung, C.H. Lo, S.H. Huang, M.L. Cheng, G. Liu, K.R. Lee, J.Y. Lai, Y.M. Sun, C.C. Hu, R. Suzuki, T. Ohdaira, N. Oshima, Y.C. Jean, Free-volume depth profile of polymeric membranes studied by positron annihilation spectroscopy: layer structure from interfacial polymerization, Macromolecules 40 (2007) 7542–7557. [44] W.S. Hung, M. De Guzman, Q.F. An, K.R. Lee, Y.C. Jean, J.Y. Lai, Solving the charging effect in insulating materials probed by a variable monoenergy slow positron beam, Langmuir 27 (2011) 3020–3023. [45] Y.C. Jean, J.D. Van Horn, W.S. Hung, K.R. Lee, Perspective of positron annihilation spectroscopy in polymers, Macromolecules 46 (2013) 7133–7145. [46] X. Qiao, Z. Zhang, Z. Ping, Hydrophilic modification of ultrafiltration membranes and their application in Salvia Miltiorrhiza decoction, Sep. Purif. Technol. 56 (2007) 265–269. [47] J. Wang, Z. Yue, J.S. Ince, J. Economy, Preparation of nanofiltration membranes from polyacrylonitrile ultrafiltration membranes, J. Membr. Sci. 286 (2006) 333–341. [48] Z.-G. Wang, L.-S. Wan, Z.-K. Xu, Surface engineerings of polyacrylonitrile-based asymmetric membranes towards biomedical applications: An overview, J. Membr. Sci. 304 (2007) 8–23. [49] J. Wijmans, R. Baker, The solution-diffusion model: a review, J. Membr. Sci. 107 (1995) 1–21. [50] H. Lohokare, S. Kumbharkar, Y. Bhole, U. Kharul, Surface modification of polyacrylonitrile based ultrafiltration membrane, J. Appl. Polym. Sci. 101 (2006) 4378–4385. [51] P.S. Singh, S.G. Chaudhri, A.M. Kansara, W. Schwieger, T. Selvam, S. Reuss, V.K. Aswal, Cetyltrimethylammonium bromide-silica membrane for seawater desalination through pervaporation, B Mater. Sci. 38 (2015) 565–572. [52] Z.L. Xie, M. Hoang, T. Duong, D. Ng, B. Dao, S. Gray, Sol-gel derived poly(vinyl alcohol)/maleic acid/silica hybrid membrane for desalination by pervaporation, J. Membr. Sci. 383 (2011) 96–103. [53] K. Xu, B. Feng, C. Zhou, A.S. Huang, Synthesis of highly stable graphene oxide membranes on polydopamine functionalized supports for seawater desalination, Chem. Eng. Sci. 146 (2016) 159–165. [54] L. Li, J.W. Hou, Y. Ye, J. Mansouri, V. Chen, Composite PVA/PVDF pervaporation membrane for concentrated brine desalination: salt rejection, membrane fouling and defect control, Desalination 422 (2017) 49–58. [55] S.G. Chaudhri, J.C. Chaudhari, P.S. Singh, Fabrication of efficient pervaporation desalination membrane by reinforcement of poly(vinyl alcohol)-silica film on porous polysulfone hollow fiber, J. Appl. Polym. Sci. 135 (2018). [56] Z. Xie, M. Hoang, T. Duong, D. Ng, B. Dao, S. Gray, Sol–gel derived poly(vinyl alcohol)/maleic acid/silica hybrid membrane for desalination by pervaporation, J Membrane Sci 383 (2011) 96–103. [57] Q. Wang, N. Li, B. Bolto, M. Hoang, Z. Xie, Desalination by pervaporation: a review, Desalination 387 (2016) 46–60.

[13] K.W. Lawson, D.R. Lloyd, Membrane distillation, J. Membr. Sci. 124 (1997) 1–25. [14] M. Baghbanzadeh, D. Rana, T. Matsuura, C.Q. Lan, Effects of hydrophilic CuO nanoparticles on properties and performance of PVDF VMD membranes, Desalination 369 (2015) 75–84. [15] B. Wu, X. Tan, K. Li, W.K. Teo, Removal of 1,1,1-trichloroethane from water using a polyvinylidene fluoride hollow fiber membrane module: vacuum membrane distillation operation, Sep. Purif. Technol. 52 (2006) 301–309. [16] L.D. Tijing, Y.C. Woo, J.-S. Choi, S. Lee, S.-H. Kim, H.K. Shon, Fouling and its control in membrane distillation—a review, J. Membr. Sci. 475 (2015) 215–244. [17] D.M. Warsinger, J. Swaminathan, E. Guillen-Burrieza, H.A. Arafat, Scaling and fouling in membrane distillation for desalination applications: a review, Desalination 356 (2015) 294–313. [18] P.D. Chapman, T. Oliveira, A.G. Livingston, K. Li, Membranes for the dehydration of solvents by pervaporation, J. Membr. Sci. 318 (2008) 5–37. [19] Z. Xie, D. Ng, M. Hoang, T. Duong, S. Gray, Separation of aqueous salt solution by pervaporation through hybrid organic–inorganic membrane: effect of operating conditions, Desalination 273 (2011) 220–225. [20] C. Cheng, L. Shen, X. Yu, Y. Yang, X. Li, X. Wang, Robust construction of a graphene oxide barrier layer on a nanofibrous substrate assisted by the flexible poly (vinylalcohol) for efficient pervaporation desalination, J. Mater. Chem. A 5 (2017) 3558–3568. [21] B. Liang, K. Pan, L. Li, E.P. Giannelis, B. Cao, High performance hydrophilic pervaporation composite membranes for water desalination, Desalination 347 (2014) 199–206. [22] F.U. Nigiz, N.D. Hilmioglu, Pervaporative desalination of seawater by using composite and blended poly (vinyl alcohol) membranes, Desalin. Water Treat. 57 (2016) 4749–4755. [23] M. Elma, C. Yacou, D.K. Wang, S. Smart, J.C. Diniz da Costa, Microporous silica based membranes for desalination, Water 4 (2012) 629–649. [24] B. Liang, W. Zhan, G. Qi, S. Lin, Q. Nan, Y. Liu, B. Cao, K. Pan, High performance graphene oxide/polyacrylonitrile composite pervaporation membranes for desalination applications, J. Mater. Chem. A 3 (2015) 5140–5147. [25] S. Moulik, S. Nazia, B. Vani, S. Sridhar, Pervaporation separation of acetic acid/ water mixtures through sodium alginate/polyaniline polyion complex membrane, Sep. Purif. Technol. 170 (2016) 30–39. [26] T. Zhu, Y. Luo, Y. Lin, Q. Li, P. Yu, M. Zeng, Study of pervaporation for dehydration of caprolactam through blend NaAlg–poly (vinyl pyrrolidone) membranes on PAN supports, Sep. Purif. Technol. 74 (2010) 242–252. [27] X. Feng, R.Y. Huang, Liquid separation by membrane pervaporation: a review, Ind. Eng. Chem. Res. 36 (1997) 1048–1066. [28] P. Shao, R. Huang, Polymeric membrane pervaporation, J. Membr. Sci. 287 (2007) 162–179. [29] Y. Hartanto, M. Zargar, H.H. Wang, B. Jin, S. Dai, Thermoresponsive acidic microgels as functional draw agents for forward osmosis desalination, Environ. Sci. Technol. 50 (2016) 4221–4228. [30] M. Drobek, C. Yacou, J. Motuzas, A. Julbe, L. Ding, J.C.D. da Costa, Long term pervaporation desalination of tubular MFI zeolite membranes, J. Membr. Sci. 415 (2012) 816–823. [31] E. Huth, S. Muthu, L. Ruff, J.A. Brant, Feasibility assessment of pervaporation for desalinating high-salinity brines, J. Water Reuse Desalin. 4 (2014) 109–124. [32] Y.P. Kuznetsov, E. Kruchinina, Y.G. Baklagina, A. Khripunov, O. Tulupova, Deep desalination of water by evaporation through polymeric membranes, Russ. J. Appl. Chem. 80 (2007) 790–798. [33] M.C. Duke, J. O'Brien-Abraham, N. Milne, B. Zhu, J.Y. Lin, J.C.D. da Costa, Seawater desalination performance of MFI type membranes made by secondary growth, Sep. Purif. Technol. 68 (2009) 343–350. [34] C. Zhou, J. Zhou, A. Huang, Seeding-free synthesis of zeolite FAU membrane for seawater desalination by pervaporation, Microporous Mesoporous Mater. 234 (2016) 377–383. [35] Z. Xie, M. Hoang, T. Duong, D. Ng, B. Dao, S. Gray, Sol–gel derived poly (vinyl alcohol)/maleic acid/silica hybrid membrane for desalination by pervaporation, J. Membr. Sci. 383 (2011) 96–103. [36] B. Feng, K. Xu, A. Huang, Covalent synthesis of three-dimensional graphene oxide framework (GOF) membrane for seawater desalination, Desalination 394 (2016)

39