Amorphous SiO2 NP-Incorporated Poly(vinylidene fluoride

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Amorphous SiO2 NPs- incorporated Poly(vinylidene fluoride) Electrospun Nanofiber Membrane for High Flux Forward Osmosis Desalination M Obaid, Zafar Ghori, Olfat Fadali, Khalil Khalil, Abdulhakim A. Almajid, and Nasser A. M. Barakat ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09945 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 29, 2015

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ACS Applied Materials & Interfaces

Amorphous SiO2 NPs- incorporated Poly(vinylidene fluoride) Electrospun Nanofiber Membrane for High Flux Forward Osmosis Desalination M. Obaid1,3, Zafar Khan Ghori2, Olfat A. Fadali3, Khalil Abdelrazek Khalil4,5, Abdulhakim A. Almajid4 and Nasser A. M. Barakat2,3,* 1

Bionanosystem Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of South Korea

2

Organic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju 561-756, Republic of South Korea 3

4

Chemical Engineering Department, Faculty of Engineering, Minia University, Minia, Egypt

Mechanical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia 5

Materials Engineering and Design Department, Aswan University, Aswan, Egypt

Corresponding author: Nasser A.M. Barakat, Tel: +82632702363 Fax: +82632702348, E-mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract Novel amorphous silica nanoparticles-incorporated poly(vinylidine fluoride) electrospun nanofiber mats are introduced as effective membranes for forward osmosis desalination technology. The influence of the inorganic nanoparticles content on the water flux and salt rejection was investigated by preparing electrospun membranes having 0, 0.5, 1, 2 and 5 wt. % SiO2 nanoparticles. A laboratory scale forward osmosis cell was utilized to check the performance of the introduced membranes using fresh water as a feed and different brines as drain solution (0.5, 1, 1.5 and 2 M NaCl). The results indicated that the membrane embedding 0.5 wt.% displays constant salt rejection of 99.7 % and water flux of 83 L/m2.h with 2 M NaCl draw solution. Moreover, this formulation displayed the lowest structural parameter (S= 29.7 µm) which represents about 69% reduction compared to the pristine membrane. Moreover, this study emphasizes the capability of the electrospinning process in synthesizing effective membranes as the observed water flux and average salt rejection of the pristine poly(vinylidine fluoride) membrane was 32 L/m2.h (at 2 M NaCl drain solution) and 99 %, respectively. On the other hand, increasing the inorganic nanoparticles to 5 wt.% showed negative influence on the salt rejection as the observed salt flux was 1651 mol/m2.h. Besides the aforementioned distinct performance, study the mechanical properties, porosity and wettability concluded that the introduced membranes are effective for the forward osmosis desalination technology.

Keywords: Electrospinning; Forward Osmosis; PVDF Membrane; Amorphous Silica NPs; Desalination; Nanofibers.


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1. Introduction Seawater is the major water source in the world, meanwhile seawater is widely available so the desalting of the sea water was focused by many researches to overcome the problem of fresh water scarcity in most of the countries. On the other hand, only 2.5% from the whole world waters is fresh so discovery of alternative methods to provide fresh water is the preoccupation of the world

1-2

. Different technologies was used to convert seawater into fresh water such as

electrodialysis 3, thermal (e.g. multi-effect distillation (MED)) and multi-stage flash (MSF)4-5 distillations, and membrane-based techniques. Membrane technologies such as microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and even reverse osmosis (RO) were considered the efficient processes in wastewater treatments and water desalination. Moreover, these technologies overspread in the world and this is attributed to their excellent separation efficiency in removing suspended particles, colloids, and solutes 6. Practically, RO is the most widely used technique for seawater desalination, however its high energy requirements is considered a main problem facing wide applications. Forward osmosis (FO) has attractive for a wide range of applications in brackish and seawater desalination, wastewater treatment, power regenerations and liquid food processing 7-8. FO is an osmotic process that, like reverse osmosis, uses a semi-permeable membrane for effective separation of water from dissolved solutes. The driving force for this separation is an osmotic pressure gradient which is used to induce a net flow of water through the membrane into the draw solution, thus effectively separating the feed water from its solutes. In contrast, the reverse osmosis process uses hydraulic pressure as the driving force for separation which requires high energy requirement. In other words, in contrast to RO, the driving force in FO is the osmotic pressure which is naturally generated due to utilizing higher concentrated solution (draw solution)



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compared to the feed solution. Therefore, in FO, no external energy is required to force the water to pass through the membrane. Accordingly, from the energy consumption point of view, compared to RO, FO is highly preferable. This FO’s strategy strongly increases the role of the membrane; practically the membrane is the backbone of the FO technology. Recently, compared to RO, there is a significant progress in the publications of the FO technology due to the aforementioned low energy requirement advantage and also low fouling tendency 9-11. Many researchers introduced different strategies to develop good performance FO membranes. Among the reported methodologies, loading of the polymer matrix by inorganic nanoparticles showed efficient enhancement of the membrane hydrophilicity, decreased the membrane fouling and increased the mechanical properties12-14. Various review articles were introduced to discuss the advantages of inorganic nanoparticles/polymer composite membranes. For instance, A.C. Lopes15 summarized the main developments in using of Aluminosilicates as polymer fillers for different applications. Furthermore, the influence of inorganic nano-additives on the performance of NPs/polymer composite membranes was reviewed by Mohammadali Baghbanzadeh16. Of course, polymer is the main constituent of the membrane. Accordingly, several polymers have been investigated to fabricate FO membranes such as polyethylene (PE), polysulfone (PSF), cellulose acetate (CA) and poly(vinylidine fluoride) (PVDF) 17. Among the exploited polymers, PVDF has relatively good attention because of its excellent properties such as; thermal stability, good mechanical property and excellent chemical resistance18-20. Consequently, enhancement of PVDF membrane performance was done by many researchers using numerous techniques such as surface modification, blending and chemical grafting

21

. The blending of the PVDF with

inorganic materials was investigated where this technique can change the porous structure of the membrane and enhances its mechanical and wettability properties. In this regard, different


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materials were loaded on the PVDF membrane such as; titanium dioxide nanoparticles (TiO2 NPs) where the influence of particles size was investigated by using 10 and 28 nm TiO2 NPs, the results indicated that the smaller size of TiO2 NPs had better antifouling effect on the membrane22. Moreover, the effect of the TiO2 NPs content (0 to 6 wt.%) on the mechanical and thermal properties, and the

performance of the PVDF membrane was studied, the results

revealed that the optimum concentration of the TiO2 NPs is 0.45 wt.% 23. Further increase in the TiO2 NPs concentration showed negative impact23. Besides TiO2, alumina (Al2O3)24, zirconium dioxide (ZrO2)25 and ferrous chloride (FeCl2)26 were also exploited to enhance the PVDF membrane characteristics. Accordingly, based on these reports, it can be concluded that hydrophilicity, crystallinity and surface structure of the PVDF membrane can be enhanced by loading of inorganic NPs if the content is optimized. It is noteworthy mentioning that all the aforementioned studies did not check the performance of the proposed PVDF-based membranes in FO technology. Moreover, the utilized inorganic nanoparticles were prepared by chemical procedures which adds some environmental constraints upon using in water desalination processes. Besides the membrane composition, membrane fabrication strongly influences the membrane characteristics such as porosity, mechanical properties and wettability. Recently, in literature, most of the researchers are using simple casting technique to prepare thin film membrane. However, it is known that it is difficult to control the membrane characteristics using casting process. Electrospinning is another effective and simple nano-technological approach can be used to synthesize the membrane

27

. The realizable high specific area-to-volume ratio of the

electrospun nanofibers provides the ability to fabricate membranes with high porosity and slight distribution of pore size 19.



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Recently, we introduced extraction of amorphous nanosilica from rice husk 28. In that study, the utilized physicochemical characterizations indicated that the extracted nanoparticles have active surface due to presence of some function groups. In this study, novel electrospun PVDF nanofiber membranes loaded by active amorphous silica nanoparticles extracted from rice husk are introduced to be utilized in the FO process. The influence of the inorganic nanoparticles content on the flux and salt rejection was studied. The utilized silica nanoparticles were extracted from the rice husk using the hydrothermal method at subcritical water conditions. The results indicated that incorporation of the extracted silica nanoparticles strongly enhances the water flux and slat rejection upon utilizing the prepared membrane in FO cell.

2. Experimental 2.1 Materials Unless specified, all the used chemicals and reagents in this study were of analytical grade and used as received without further purification. Poly (vinylidene fluoride) (PVDF) (average Mn ~275,000, Sigma Aldrich) and N, N-dimethylformamide (DMF, >99.5%, Sigma Aldrich) were purchased from Merck Chemicals and used to prepare the electrospun nanofiber mats. M-Phenylenediamine (MPD,>98%, Alfa Aesar) and 1,3,5-Benzenetricarbonyl chloride (TMC, 98%, Alfa Aesar) were utilized in the activation of the electrospun nanofibers. Silica NPs (SiO2), synthesized in the lab by hydrothermal method at subcritical water conditions, were used as additives. Furthermore, sodium chloride (NaCl) and distilled water were used for FO experiments.


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2.2 Preparation 2.2.1 Synthesis amorphous silica NPs from rice husk As aforementioned the utilized SiO2 nanoparticles were extracted from rice husk using a simple hydrothermal instrument

28

. Briefly, the rice husk was cut into small pieces and washed

by water to remove the soluble particles and dust or any other contaminants. Then, the clean husk was dried in an air oven at about 80 °C for 24 h. Then, 2 g of the dried rice husk were mixed with 10 ml of nitric acid and 10 ml of distilled water. The colloid was placed in a Teflon crucible inside a stainless steel reactor. The reactor was placed in the furnace at 160 oC for 2 h. After that, the reactor was immediately cooled down by immersing it into a cold-water bath. The obtained product was filtered off, washed several times by distilled water and dried in vacuum oven at 60 oC for 24 h.

2.2.2 Preparation of the PVDF/Amorphous SiO2 membrane The PVDF/SiO2 composite nanofiber membranes were synthesized using PVDF solution (20 wt. % in DMF), different amounts of amorphous SiO2 NPs were added to prepare final electrospun sol-gels having 0.0, 0.5, 1.0, 2.0 and 5.0 wt.% SiO2. The required amount of SiO2 NPs was added to 2 ml of DMF and sonicated for 20 min before addition to the polymer solution. The mixture was stirred for 24 h at 55±2 oC. The membrane was fabricated by a simple laboratory scale electrospinning apparatus. The aim of the study was introducing an effective membrane with simple synthesis procedure, so the electrospinning process was carried out in an open atmosphere (no humidity control was performed) and simple setup (no syringe pump was exploited; the polymer solution was put in an inclined syringe). But, the other parameters were



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adjusted such as the needle-to-collector distance (15±0.5 cm) and applied voltage (20±0.1 kV) which were very close to the optimum electrospinning conditions for PVDF29 . A rotating stainless steel drum (170 rpm) covered by polyethylene sheet was used as a collector. The electrospinning process was carried at room temperature (23±03 oC) and 47±0.7% humidity. The tip diameter was = 0.45 mm. The produced membranes were dried at 70±0.3 oC in a vacuum oven overnight and then stored until use.

2.2.3 Preparation of polyamide layer Using interfacial polymerization (IP), polyamide thin layer was deposited on the surface of the prepared electrospun nanofiber membranes as follow: the membranes were immersed in a 2 % (w/v) MPD aqueous solution for 2 min to have good penetration of MPD monomer in the pores. Then, TMC solution (0.1 % (w/v) in hexane) was poured onto the membrane surface. After 1 min, the TMC solution was then drained off from the surface. Finally, the membrane surface was rinsed with pure n-hexane and distilled water and dried at 80±0.3

o

C for 5 min.

Later on, all the membranes were stored in distilled water until using.

2.3 Characterization The hydrophilicity of the membrane was measured by DPRO image standard device which is used to measure water contact angle of the membrane. Surface morphology was studied by scanning electron microscope equipped with EDX analysis tool (Hitachi S-7400, Japan). Information about the phase and crystallinity was obtained by using Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu Kα (λ = 1.540 Å) radiation over Bragg angle ranging from 10 to 100o. Moreover, mechanical properties of the different membranes were analyzed using a universal testing machine (UTM, LLOYD Instrument) where three samples of each membrane 8 ACS Paragon Plus Environment

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with 7.62 mm length and 3.18 mm width are installed between two jaws; one fixed and the other was moved by 5 mm/min speed to measure the membrane properties, then the average of the three results was calculated and presented. Moreover, Data Analysis and Descriptive Statistics in Excel 2013 were exploited to estimate the statistical parameters. Furthermore, the water flux of the membrane was determined from the FO system, AL-FO mode, and the salt rejection of the membrane was measured using (LabQuest 2) connected to data logger. FTIR spectroscope (Mode: FTLA 2000 series, ABB) was used to investigate the chemical structure. To determine the porosity (ε) of the electrospun nanofiber membranes, the weights of the wet (m1) and dry (m2) membranes were measured and the porosity of the membranes were calculated by eq.1 using the densities of the ethanol (ρ1=789 kg/m3) and membranes (ρ2=997 kg/m3). In details, three samples of each membrane were immersed in ethanol for 1 h. The excess ethanol was removed from the surfaces using tissue papers, then the membranes were weighed using 4 digital balance to measure the wet mass of the membrane (m1). Later on, the membranes were dried in a vacuum oven at 70±0.3

o

C overnight then the dry mass (m2) of the membranes were determined using

the same balance. The average weight of the three wet and dry membranes were calculated and the porosity was determined accordingly.

/

=

eq.1

2.4 FO system and membrane evaluation All FO experiments were done in the FO cell designed and fabricated locally in South Korea, where the feed solution was faced to the activated surface of the membrane (the surface having



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the polyamide (PA) thin layer, AL-FS mode). Both the feed and draw solutions were maintained at room temperature where the FO system was operated in co-current flow with 7.5 ml/min flow rate for both the draw and feed solution; the system setup is presented in Fig.S1 (Supplemental information). All the membranes were investigated against different concentrations of draw solution (0.5, 1, 1.5 and 2 M NaCl solution) where distilled water was used as the feed solution (FS). The water flux of the membrane was determined by measuring the mass of the FS which recorded continuously using a digital balance for 2 h. On the other hand, the conductivity of FS and DS was monitored using a conductivity meter (LabQuest2) to study the salt rejection of the membrane. The water flux (from the feed to the draw solution) of the membranes (Jv, L m−2h−1) and the reverse salt flux (from the drain solution to the feed solution) (Js, mol.m−2h−1) were calculated using eq.2 and eq.3, respectively

30

. The experiments were carried at room

temperature with an effective membrane area of approximately 5 cm2.

∆

= ∗ ∆ =

eq.2

∆ ∗ ∆

eq.3

Where ΔV (L) is the volume of water passed through the membrane having an effective surface area A (m2) at interval time Δt (h). Ct is the salt concentration (g/L) and Vt is the volume of the feed (L) at the end of the tests. Excel-based algorithm established by Alberto Tiraferri and et al. 31

was used to calculate the water permeability coefficient (A, L/(m2.h.bar)), salt permeability

coefficient (B, L/(m2.h)) and structural parameter (S, µm). Herein, 4 stages of experimental were done using 4 different concentrations of draw solution namely; 0.5, 1, 1.5, 2 M NaCl. The salt rejection (R) of the membranes were calculated at any time by using eq.4 10 ACS Paragon Plus Environment

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= −

∗

eq.4

Where R is the salt rejection (%) at time t, t,

!#

!" is

the NaCl concentration in feed solution at time

is the initial NaCl concentration in the feed solution, and

$%# is

the initial NaCl

concentration in the draw solution 32. The conductivity, water flux and the salt rejection (R, %) were determined to evaluate the membrane performance.

3. Results and Discussion

3.1 SiO2 NPs characterization Fig. 1A displays the XRD pattern of the extracted silica NPs. As shown, a wide and high intensity peak at 22° indicates formation of amorphous nature of the obtained silica

33

. In

addition, the absence of other sharp peaks of possible impurities such as alkaline earth metals confirms the purity of the as-separated silica. Fig. 1B displays SEM image of the extracted inorganic nanoparticles. As shown, the obtained silica was in nanometer scale with a slight agglomeration. The silicon atoms in the rice husks are often naturally uniformly dispersed by molecular units34. As a result, a very fine particle size with a high purity and high surface area powder was obtained. Moreover, the observed agglomerated clusters can be easily dispersed using ultrasonication technique.



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B

A

Fig. 1: XRD pattern (A) and FE-SEM image (B) for the produced silica NPs

3.2 Characterization of PVDF-SiO2 membranes 3.2.1 The morphology of the membrane One of the important trends in membrane characterization is membrane surface morphology. Therefore, the membrane surface was studied by SEM where Fig.2 reveals the SEM images of PVDF and PVDF-SiO2 membranes. Fig. 2 (a) displays SEM image of the pristine PVDF electrospun nanofibers. As shown, smooth and beads-free nanofibers were obtained due to optimizing the polymer solution concentration; 20 wt. %.

The results of the PVDF-SiO2

nanofiber membranes are demonstrated in Fig. 2 (b-e), as exhibited, the nanofibers loading by SiO2 NPs gave also beads-free nanofiber. Actually, ability of spinning of the colloidal solution is a distinct advantage for the electrospinning process27, 35-37. Moreover, as it is clear from FE-SEM images in Fig. S2 in the supplementary information that at the 0.5% SiO2 no inorganic nanoparticles are observed on the surface of the fibers. However, when the silica content was increased to 1%, some silica nanoparticles appeared on the surface. As shown in that figure, more increase in the silica content led to have more discrete nanoparticles on the surface. However, agglomeration of the inorganic nanoparticles was observed at the highest content; 5%.


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Accordingly, at the low silica content (0.5 %), it can be claimed that all the inorganic nanoparticles are embedded inside the polymer nanofibers. Appearance on the surface starts with further increasing in the silica content. a

b

c

d

f

e

Fig. 2: SEM image of pristine PVDF (a), PVDF-0.5%SiO2 (b), PVDF-1%SiO2 (c), PVDF2%SiO2 (d), PVDF-5%SiO2 (e) and the average diameter (f).



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Furthermore, the observed diameters of the pristine PVDF and PVDF-SiO2 nanofibers were analyzed using image analyzer software (Image J) to estimate the average diameter of the nanofibers; the results are shown in Fig. 2(f) in which the average diameter of 400 fibers was measured. As shown, slight incorporation of SiO2 NPs leads to decrease the nanofiber average diameter compared to the pristine PVDF membrane where the average nanofiber diameter reduced by 47% when 0.5% SiO2 was added. On the other hand, more increase of the silica NPs (more than 0.5 %) led to increase the average nanofiber diameter again but still lower than the diameter of the pristine PVDF membrane. The detailed results of the average diameter frequency curves for all the prepared nanofiber mats can be seen in Fig. S3 (Supplemental data). As shown, the results indicated that 12.5% of the nanofiber in the range between 800-900 nm for pristine membrane, on the other hand, 0.5% SiO2 decreased the fiber diameter strongly where 21.5% in the range 200-300 nm and 24% of nanofibers was in the range 300-400 nm. From the other side, increase the silica NPs content results in re-increasing of the nanofiber diameter, the distribution of the nanofiber became uniform. The obtained observation about the influence of silica NPs addition on the average diameter of the produced nanofibers can be explained as follow. There are main two factors strongly affecting the electrospun nanofiber diameters; viscosity and electrical conductivity of the electrospun solution. Therefore, it can be claimed that, because of all the utilized silica NPs were suspended in the same amount of DMF solvent (2 ml), so at small silica content, the viscosity of the solution decreased due to addition of more solvent. However, increasing the silica content led to enhance the viscosity which resulted in increase the average nanofiber diameter as shown in Fig. 2(f) and Fig. S3. Moreover, increasing the amount of the solid nanoparticles can lead to swelling of the produced nanofibers which also results in increase of the average diameter.


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Fig.3 displays SEM images of the introduced PVDF-based nanofiber membranes after activation by deposition of the polyamide (PA) layer and the cross section of these polyamide layers (the insets). As shown, incorporation of silica NPs enhances formation of the PA layer on the membrane surface. From the images, it can be observed that the PA layer cannot be seen in the pristine and 0.5 % SiO2 membrane which indicates that the polyamide is covering the individual nanofibers and this attributed to the small thickness of these layer where the thickness are 90±2 and 152±1.5 nm for PVDF and PVDF-0.5SiO2, respectively. However, increasing the silica led to appear some wings from PA on the nanofibers (Fig. 3c and 3d) and the thickness of the polyamide layer increased to 340±1.6 and 407±1.7 nm for 1% and 2% SiO2, respectively. At the highest silica NPs content (5%), the polyamide forms a thin layer covering the whole membrane with 108±1.7 nm thickness. Panel (f) displays the thickness of the observed PA layers for all formulations.



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a

b

c

d

e

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f

Fig. 3: FE-SEM Image of Pristine PVDF (a), PVDF-0.5%SiO2 (b), PVDF-1%SiO2 (c), PVDF2%SiO2 (d), PVDF-5%SiO2 (e) and the thickness of PA layers (f).

3.2.2 Chemical structure of the membrane (FT-IR analysis and XRD) FT-IR spectra for the pristine and SiO2-loaded PVDF nanofiber membranes after activation are plotted in Fig.4A, B and C.


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B

A

D

C

Fig. 4: FT-IR for the PVDF and PVDF-SiO2 membranes; un-activated membrane (A and B), activated membrane (C) and XRD Pattern for PVDF and PVDF-SiO2 membranes (D)

Fig. 4A displays the full spectra for the PVDF and PVDF-SiO2 membranes without polyamide layer, while Fig. 4B shows high magnification window at the specific wavenumber ranges (1500 – 500 cm-1). On other hand, the full spectra of the membrane with polyamide layer is represented in Fig. 4C. As shown in Fig. 4A and 4B, a series of peaks appeared at around 1401 cm-1 indicating the presence of –CH2– groups. The strong peak at 1174 cm-1 refers to the C-F stretching where it was found over a wide frequency range; 1400 -1000 region 830 – 520 cm-1 refers to the C-F deformation vibrations


38

cm-1. The peak at the

. Moreover, SiO2 NPs as


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shown in Fig. 4A have the strong peak at 1070 cm-1, this peaks refers to the asymmetric Si-O-Si stretching vibration which appeared in range 1100-1000 cm-1. Besides, the broad absorption band from 3659-3392 cm-1 is related to the Si-OH, silanols group, which present in a wide range 3700-3200 cm-1. From the specific wavenumber range, the difference in the peaks intensity is attributed to presence of SiO2 NPs at different concentration. Furthermore, for the PVDF granules (PVDF-Raw) the peaks of α-phase at 610, 762, 797 and 976 cm−1 and β-phase at 840, 1276 and 1431 cm-1 can be observed39-40 which indicates presence of α- and β- phases in the PVDF polymer, this result is in consistent with other researchers39. Moreover, for the pristine PVDF electrospun membrane, the absorption peaks of α-phase at 610 and 976 cm-1 disappeared as well as for the composite PVDF-SiO2 membranes. Furthermore, the peak at 762 cm−1, αphase, decreased for the PVDF membrane as well as for PVDF-SiO2. In addition, the content of the β-phase increased by 16% for the electrospun PVDF compared to the PVDF-Raw as shown in Fig. S4 (Supplemental information), this finding was also approved by Clarisse et. al29 where they studied the effect of the electrospinning on the crystallinity and β-phase content of PVDF. On the other hand, the content of β-phase increased from 86.5% for PVDF to 93.5% for PVDF.0.5SiO2. More increase of silica content led to a slight increase in the β-phase as the observed content was 94.7 % for the membrane having 5 wt. % silica NPs as can be seen in Fig. S4. Besides FT IR, XRD analysis was done for further checking (Fig. 4D). As shown, the XRD patterns confirmed the FT-IR results in which the strong peak at 20.7o which is related to the (200)/(110) reflection of the β-phase41 could be detected for all membranes. However, the (020) reflection peak of the α-phase appears as a shoulder peak at 18.4o 41 for the pristine PVDF as well as with very weak intensity for PVDF-0.5SiO2, but this peak completely disappeared for the other membranes. Also the peak at 38.9o which is related to α-phase42 was only observed for


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the PVDF and disappeared for all the composite membranes. Overall, XRD results are supporting the FT-IR ones which confidently explain the influence of SiO2 addition on the crystal structure of PVDF. The FT-IR for activated membranes, covered with polyamide layer, are plotted in Fig.4C. There is a vibrational band at 1722 cm-1 which can be assigned to the C=O stretching of carboxylic acids due to the polyamide layer, while the peaks at1541and 1609 cm-1 were assigned to Amide II band (N–H in-plane bending and N–C stretching vibration of a –CO–NH– group)

43-44

and

aromatic amide (N–H) deformation vibration 44 or C=C ring stretching vibration 45, respectively. A broad weak peak appeared at 3380-3350 cm-1 which represents N–H stretching of primary and secondary amines. Also, the different strength in the Fig.4C indicate the presence of the silica NPs at different concentration and the peaks at 1724 cm-1 is assigned to C=O stretching vibration which present in 1740-1700 cm-1 due to the polyamide layer formation, as sown in the right side of Fig.4C.

3.2.3 Mechanical properties of the membranes The mechanical properties, such as strength and flexibility, of the membrane are very important for a wide-ranging of application, processing and handling. The mechanical properties of the introduced PVDF/SiO2 NPs membranes in terms of stress strain, Young’s modulus, tensile strength and elongation are shown in Fig.5 in which the average of three samples for each membrane was plotted. The results showed that the tensile strength increased for all membranes that having silica NPs than the pristine PVDF membrane, except 0.5% silica which appeared the lowest tensile strength and modulus as shown in Fig.5A and 5B. The extension at breaks exhibits the same behavior as shown in Fig.5C. These results suit the previous estimations of the



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average nanofiber diameter. In other words, it can be claimed that the mechanical properties are related to the nanofiber diameters so the nanofiber mat having 0.5 silica nanoparticles revealed the worst properties. Moreover, the tensile strength reached to the peak at 1% SiO2 NPs concentration where it increased by 38% higher than PVDF but as the SiO2 increased, over than 1%, the tensile strength decreased. Nevertheless, as the wt. % of SiO2 NPs increased the modulus increases to reach the maximum value for 2%SiO2 which 16% larger than PVDF but the modulus didn’t change for 1%SiO2/PVDF composite compared to pristine PVDF. In addition, 0.5% and 5% SiO2 reduce the modulus less than the pristine membrane where the modulus diminished by 33% and 23%, respectively as presented in Fig.5B. However, the increase in the tensile strength and extension at break for 1% SiO2 and tensile modulus for 2%SiO2 is recognized to the high degree of orientation and the good dispersion of the SiO2 NPs in the nanofiber membranes at low concentration (1%SiO2). The agglomeration takes place at high silica concentration as shown in the FE-SEM images (Fig. S2) which decreases the degree of anisotropy and mobility of the nanofibers where the high strength structures could be resulted from the flexible chain fibers. Overall, the mechanical properties study indicated that

incorporation of the suggested silica nanoparticles does not strongly affect the known good mechanical properties of the PVDF membranes since even the membranes showing relatively weak properties still being in the acceptable range.


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A

6

B

PVDF PVDF-0.5SiO2

5 5

PVDF-1SiO2 PVDF-2SiO2 PVDF-5SiO2

4 4

Stress (MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3

2

1 1

0 0

0

50

100

150

200

250

300

350

400

Percentage Strain

C

Fig. 5: Stress Strain curve (A), tensile strength and Young’s modulus (B) and Extension at break (C) for PVDF membrane loaded with different SiO2 NPs Concentrations.

3.3.4 The membrane porosity The membrane porosity (ε) is defined as the dividing of pores volume to the total volume of the membrane 21.



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A

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B

Fig. 6: (A) Influence of SiO2 in the membrane porosity and (B) Water Contact angle (WCA) for pristine PVDF and PVDF containing different concentration of SiO2 NPs for Support and active layer.

As shown in Fig. 6A, the porosity decreases first with increasing SiO2 content, however at the highest silica loading (5 wt.%), the porosity increased to reach 3% higher than the pristine PVDF membrane. It is noteworthy mentioning that the applied methodology can evaluate the porosity among the nanofibers as well as the porosities present on the surface of the nanofibers. Accordingly, at the low silica content, it can be claimed that the utilized nanosilica starts to block the pores present on the nanofibers surface as the used nanosilica has very small size. However, at high silica content, as shown in Fig. S2, more silica nanoparticles appear on the surface which creates a rough surface as well as increase the annular porosity among the nanofibers which results in the observed enhancement in the porosity as shown in the figure.

3.3.5 Wettability of the membrane The wettability of the surface depends on the roughness, porosity and the surface chemistry. In this study, the wettability was expressed by the water contact angle (WCA); the results are


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presented in Fig. 6B which displays the relationship between SiO2 loading in electrospun nanofiber membranes and water contact angle for the support layer and the active layer. As shown in the figure, the deposition of the polyamide layer strongly enhances the wettability for all formulations and this is supported by many previous reports. On the other hand, the pristine PVDF shows hydrophobic characteristics as shown in Fig. 6B. It is noticed that the loading of SiO2 NPs decreased the WCA by 11% at small concentration; 0.5%. After that the WCA slightly increased with increasing the SiO2 concentration until 2% SiO2 concentration but at more high concentration (5 wt. % SiO2) the WCA decreased again. The singular increase in the membrane hydrophilicity at the lowest silica content can be attributed to the smallest average nanofiber diameter of this electrospun membrane compared to the other formulations (Fig. 2f). Actually, there is a distinct relation between the nanofiber diameters and the water contact angle as it was discussed in other reports

46

. However, due to the porosity decrease at relatively high silica

content (5 wt. %; Fig. 6A), the water contact angle decreased. For the active layer of the membranes, the same trend was observed also as shown in Fig. 6B. As aforementioned, there are many parameters affecting the wettability of electrospun nanofibers such as the nanofiber diameter, porosity, the surface morphology, and surface energy. Where, decreasing fiber diameter and/or the porosity leads to increase the wettability. On the other hand, increasing the nanofiber surface roughness results in enhancement in the wettability 46

. However, among the aforementioned factors affecting the wettability, specifically for the

electrospun nanofiber membranes, porosity and average nanofiber diameter are the main parameters. To properly explain the influence of silica incorporation on the porosity and the average diameter, and the corresponding impacts on the wettability, Fig. S5 was established. Fig. S5 displays, at different silica loadings, the relative increase or decrease in nanofiber average



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diameter, porosity and water contact angle with respect to pristine PVDF electrospun nanofiber membrane. As shown, the water contact angle was affected by the porosity and fiber diameter of the membrane where it increased by 9.58%, 9.54% and 1.39% for 1%, 2%, and 5% SiO2 NPs membranes, respectively compared to the pristine PVDF membrane. For 1% and 2% silica, the WCA strongly increased and this can be attributed to the simultaneous decrease of fiber diameter and porosity for these two samples and simultaneously follows the general influence of the porosity and average diameter on the wettability. On the other hand, in case of the nanofibers containing 5 wt. % silica nanoparticles, the increase of WCA is very small and this refers to the increase in the porosity which is supposed leading to decrease the water contact angle. However, there is an opposite impact of the decrease in the fiber diameter which is trying to increase the WCA, so the resultant is a slight increase in the WCA. It is noteworthy mentioning that, specifically for the nanofiber average diameter, the influence on the WCA depends on the size. In other words, in the large size (as in case of the membranes having 1, 2 and 5 wt.% silica; Fig. 2f) the aforementioned impact of the fiber diameter on WCA is valid. However, for the small size, an opposite influence can be observed 47. Accordingly, the sharp decrease in the nanofiber average diameter in case of the membrane containing 0.5 wt. % silica led to unexpected decrease the WCA compared to pristine PVDF as shown in the figure.

3.3 Evaluation of membrane in FO system. The anticipated structural and transport parameters of the membranes such as the water permeability coefficient (A, L/(m2.h.bar)), salt permeability coefficient (B, L/(m2.h)) and structural parameter (S, µm) variables are displayed in Table 1. As shown, the coefficient of variation of Jw/Js (CV) is less than 10 % which is required for any proposed membrane to be 24 ACS Paragon Plus Environment

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used in the FO process. Moreover, the coefficients of determination for both salt flux and water flux; R(' and R(, , respectively are greater than 95% which is also recommended for the good membrane utilized in the FO technology.

Table 1: the properties and structure parameters of the PVDF and PVDF-SiO2 membranes (±Standard error) S

A B (L/m2.h) 2

τ (µm)

R2 - J w

R2 - J s

CV%

(µm)

(L/m .h.bar) PVDF

0.559±0.024

0.757±0.033

94.9±1.8

1.67±0.03

0.95

0.968

4.04

PVDF-0.5SiO2

1.36±0.01

0.884±0.008

29.7±0.6

0.46±0.009

0.994

1

5.76

PVDF-1SiO2

0.889±0.015

1.67±0.008

41.6±0.7

0.57±0.01

0.994

0.988

9.77

PVDF-2SiO2

1.14±0.02

4.05±0.07

168±1.2

1.1±0.008

0.977

0.981

9.02

As shown in the table, the PVDF-0.5SiO2 membrane has the lowest structure value for S parameter and the lowest tortuosity (τ) which are suitable for FO membrane owing to enhance the performance of the membrane and accomplish high water flux. Moreover, the S value of the pristine PVDF electrospun membrane is considered very small compared to some literatures such as; Miao Tian et.al48 who synthesized two different PVDF nanofiber substrate namely, ES-1# and ES-2#, having S values of 812 and 325 µm, respectively. Comparing these results with the corresponding value of PVDF-0.5SiO2 membrane can conclude that S value was decreased by 28 and 11 times with respect to the ES-1# and ES-2#, respectively. The performance of the membranes, to determine water flux and salt flux, was evaluated using a lab scale FO system (Fig. S1). In which the draw solution (0.5, 1, 1.5 and 2 M NaCl) was circulated facing to the support layer side of membrane (without PA layer) and its



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conductivity was measured using conductivity meter (LABQUEST2). On the same time the feed (distilled water) was circulated facing to the PA layer parallel to the draw solution and the conductivity was also measured. The experiments were carried out for 2 h and the flux of the system was measured constantly by measuring the mass of the feed using digital balance and the volume of water calculated using the density of water (1 g/ml).

Fig. 7: The average water flux for the PVDF and PVDF-SiO2 membranes at different concentrations of draw solution Fig.7 represents the water flux for different membrane at various concentration of draw solution; it can be noticed that the water flux for all composites membrane is higher than for the pristine one and the water flux of the 0.5% SiO2 membrane displayed the highest value with all the utilized draw solution concentrations and this attributed to the lowest S value and hence the smallest τ value of this support layer of the membrane. To explain the influence of S and τ on the water flux, a schematic diagram (Fig. 8) is presented. Basically, in this study and others, it was proved that the polyamide layer has an excellent salt rejection characteristic. Therefore, as it 26 ACS Paragon Plus Environment

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will be explained below, most of the active membranes reveal excellent salt rejection while the PVDF-5SiO2 membrane showed negligible salt rejection (i.e. very high salt flux). Typically, as the draw solution is facing the inactivated side of the membrane, the salt ions can go through easily until reach to the active layer. As shown in Fig. 8A and C, at high S and τ values, the pass is long which means high resistance, moreover, water molecules are diffusing through the PA layer in the opposite direction (from feed side). Consequently, it is expected that the salt concentration (the osmotic pressure of the draw solution) at the inner face of the PA layer (Ci) will be small. Accordingly, the osmotic pressure difference across the PA layer, which is driving water diffusion through the PA layer, will be small as it is graphically explained in Fig. 8 A. Therefore, the final result will be having low water flux. On the other hand, at low S and τ values, the transfer pass of the salt ions will be short (or having low resistance). Therefore, the expected Ci value will be high which enhances the water flow driving force (osmotic pressure) and consequently improves the water flux as shown in Fig. 8 B and D. The aforementioned explanation describes the interfacial concentration polarization (ICP) phenomenon in the FO membranes. Accordingly, this hypothesis can be exploited to explain the highest values of water flux obtained with membrane having 0.5 % SiO2 which reveals the lowest values for S and τ.



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Fig.8. 3D Schematic diagram to explain the mechanism of ICP and effect of tortuosity on the ICP. Cb, Cm and Ci represent bulk, membrane and interfacial salt concentration, respectively. (A) Salt concentration gradient at high tortuosity, (B) Salt concentration gradient at low tortuosity, (C) Salt ions pass in case of high tortuosity, and (D) Salt ions pass in case of low tortuosity. From the above explanation, it can be claimed that mixing is useless to overcome the ICP. Typically, only the change of the support layer properties by decreasing the membrane toroutisty, thickness, and increasing the porous can improve the mass transfer of the salt molecules and consequently overcome the ICP problem. Therefore, it can be concluded that the decrease of the structure parameter by 69% (τ by 72%) for PVDF-0.5SiO2 distinctly enhanced the water flux by about 3 times due to the decreasing of the ICP. On the other hand, the flux increase by increasing the concentration of the draw solution and this refers to the increase of osmotic pressure of the draw solution. Furthermore, 1 wt.% SiO2 and 2 wt.% SiO2 membranes have also high water flux value but less than the flux of 0.5% silica, in descending order of PVDF-0.5SiO2> 28 ACS Paragon Plus Environment

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PVDF-1SiO2> PVDF-2SiO2, and this is related to the increase of tortuosity by increasing the silica contents in the membranes as shown in table 1 which means increasing the ICP. Overall, comparing to the pristine PVDF membrane, the obtained average water fluxes of the silicaloaded membranes still very high. Concerning the membrane having the highest silica nanoparticles content, 5% SiO2, the reverse salt flux was higher than the water flux (i.e. it cannot be used as FO membrane), so it revealed negative water flux values (data are not shown) based on the data in Fig. 7. This hypothesis is approved by the salt flux estimation as shown in Fig. 9A which recorded the highest values for 5%SiO2 about 186, 492, 839, and 1651 mol./m2.h for 0.5, 1, 1.5 and 2 M NaCl draw solutions, respectively and as it was mentioned before these values are higher than the water flux from the feed to draw solutions. This behavior is attributed to the high porosity of this membrane and the thin formed polyamide layer on its surface. Another important finding in Fig. 9A is that the pristine PVDF electrospun membrane has low salt flux which indicates that electrospinning is very successful technology to prepare FO membrane. In other words, the known high porosity of the electrospun mats does not affect the FO membrane performance. Moreover, and most important finding, the membrane revealing the maximum water flux (0.5 SiO2) displays very small salt flux up to 1 M NaCl draw solution and also the corresponding values at the higher draw solution concentration still in the acceptable range. The salt flux increases with increasing the silica content up to 2 wt. % and this is attributed to increasing in the salt permeability coefficient, B, as shown in table 1. On the other hand, the salt flux for the membrane containing 5 wt. % silica nanoparticles is extremely high. In the first moment, it was thought that the membrane has a hole, but in viewing of the Fig.S6 B which displays the salt rejection change with time, it was found that the rejection became constant after



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around 600 and 2000 s for the 1 and 2 M NaCl draw solution, respectively. However, the salt flux was increasing until ending of the experiments in the case of 0.5 NaCl solutions (Fig. S6 A).

A

B

Fig.9: (A) Average reverse salt flux at different concentrations of draw solution for the PVDF-and PVDF-SiO2 membrane and (B) Salt rejection (%) for membrane incorporated by different concentration of SiO2 NPs at different concentrations of draw solution.

Influence of experimental time on slat rejection displayed in Fig. S6. As shown in Fig. S6 A, at the lowest draw solution concentration (0.5 M NaCl), the salt rejection for the pristine and PVDF membranes having 0.5 and 1 wt. % silica nanoparticles did not change with time as it was almost 99.9 % within all the experiment time. However, for the other two formulations, the salt rejection slightly decreased with the time. Moreover, as shown in Fig. S6 B, C and D, the salt rejection was almost constant at its maximum value for the pristine and PVDF membranes having 0.5 and 1 wt. % silica nanoparticles which indicates excellent salt rejection at even high DS concentration. However, for the 5 wt. % membrane, the salt rejection decreased sharply at high DS concentration (1, 1.5 and 2 M NaCl) until certain time it became constant as shown in Fig. S6 B, C and D. As shown, in Fig.9B, the pristine and the membranes having 0.5% and 1% silica 30 ACS Paragon Plus Environment

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nanoparticles achieved salt rejection higher than 99%.

Further increase of silica content

decreases the salt rejection to be about 93 % and 80% at 2M for 2%SiO2 and 5%SiO2, respectively. Overall, based on the obtained results, it can be concluded that the incorporation of PVDF with silica nanoparticles extracted from the rice husk strongly enhances its performance to be suitable for water desalination using FO process as it leads to strong enhancement in the water flux and salt rejection. Moreover, the optimum silica NPs content is 0.5 wt.% at which maximum salt rejection and water flux can be obtained. To properly evaluate the introduced membranes, comparison with other reported membranes was tabulated in Table 2. As shown in the table, the PVDF-0.5SiO2 membrane achieved the highest water flux compared to the other membranes at 1M, 1.5M and 2M draw solutions. Overall, the introduced membrane can open our mind to thinking how to transfer it from lab scale to industrial scale and produce commercial FO membrane having high water flux and very low structure parameter. Table 2: Performance in FO unit of the introduced membranes and some reported ones

The membrane

FS

Jw

Js / Jw

(LMH)

(g/L)

DS

Ref.

DI water

1M NaCl

37.5

0.156

49

TFN0.1 membrane

DI water

1M NaCl

17.5

0.468

50

TFC-1#

DI water

1M NaCl

40 mol % PTA/PEG additive supported TFC (IP-II)

TFC-2#

DI water

1M NaCl

GOT-0.25a

DI water

TFC-0 membrane

DI water

11.6

0.3 48

28

0.46

1M NaCl

~30

~0.2

1M NaCl

15

0.03


51


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TFC-0.75

DI water

1M NaCl

21

0.24

52

TFC membrane

DI water

1M NaCl

22.9

0.3

53

TFC (PK80(35/65)-3)

DI water

1.2M NaCl

40.4

---

54

CNF-0.5

10mM

1M NaCl

15.6

0.06

55

1M NaCl

52±0.05

0.67

This

NaCl Electrospun PVDF-0.5SiO2

DI water

work

HTI-CTA commercial

DI water

1.5M NaCl

8

0.88

56

TFC-0.2 membrane

DI water

1.5M NaCl

10

0.3

56

Electrospun PVDF-0.5SiO2

DI water

1.5M NaCl

70±0.4

0.74

This work

DI water

2M NaCl

41.4

0.24

57

GOT-0.25a

DI water

2M NaCl

~35

~0.22

51

TFN0.1 membrane

DI water

2M NaCl

21.5

----

50

TFN0.6 membrane

DI water

2M NaCl

13.9

1.13

58

TFC membrane

DI water

2M NaCl

33

0.18

58

TFC hollow fiber (PESWater/NMP/PEG support)

DI water

2M NaCl

34.5

0.29

59

TFC-PES/SPSf flat sheet membrane

DI water

2M NaCl

26

0.315

60

TFC32 membrane

DI water

2M NaCl

25.4

2.3

61

Electrospun PVDF-0.5SiO2

DI water

2M NaCl

83±1.3

0.77

This

0.5 wt% CN/rGO in the substrate of TFC

work

“a” refers to the data extracted from figures. 4. Conclusion


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Active amorphous silica nanoparticles can be extracted from rice husk using the hydrothermal treatment process. Electrospinning of sol-gel composed poly(vinylidine fluoride) and the extracted silica nanoparticles leads to produce SiO2 NPs-incorporated electrospun membrane can be effectively utilized in the forward osmosis desalination process. However, the silica nanoparticles content should be optimized as the results concluded that the electrospun poly(vinylidine fluoride) nanofibers membrane having 0.5 wt.% reveals the maximum water flux and the highest salt rejection. Overall, the introduced study opens new avenue for using the electrospinning and the extracted inorganic compounds from agricultural wastes in preparation of effective membranes for the forward osmosis desalination technology.

Supporting Information Forward osmosis cell setup, method for calculating the crystal structure phase content, the morphology of the PVDF-SiO2 composite membranes, nanofiber diameters distribution and the salt rejection of the membranes can be found in the Supporting Information. Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes: The authors declare no competing financial interest

Acknowledgment - 33 ACS Paragon Plus Environment


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The Authors extend their appreciation to the Deanship of Scientific Research at King Saud University for funding the work through the research group project No. IRG14-11.

References 1. Khawaji, A. D.; Kutubkhanah, I. K.; Wie, J.-M., Advances in Sea water Desalination Technologies. Desalination 2008, 221 (1), 47-69. 2. Charcosset, C., A Review of Membrane Processes and Renewable Energies for Desalination. Desalination 2009, 245 (1), 214-231. 3. Sadrzadeh, M.; Mohammadi, T., Sea Water Desalination Using Electrodialysis. Desalination 2008, 221 (1), 440-447. 4. Ophir, A.; Lokiec, F., Advanced MED Process for Most Economical Sea Water Desalination. Desalination 2005, 182 (1–3), 187-198. 5. Gabriel, K. J.; Linke, P.; El-Halwagi, M. M., Optimization of Multi-Effect Distillation Process Using A Linear Enthalpy Model. Desalination 2015, 365 (0), 261-276. 6. Zhao, Z.; Zheng, J.; Wang, M.; Zhang, H.; Han, C. C., High Performance Ultrafiltration Membrane Based on Modified Chitosan Coating and Electrospun Nanofibrous PVDF Scaffolds. J. Membr. Sci. 2012, 394, 209-217. 7. Cath, T. Y.; Childress, A. E.; Elimelech, M., Forward Osmosis: Principles, Applications, and Recent Developments. J. Membr. Sci. 2006, 281 (1), 70-87. 8. Zhang, J.; Loong, W. L. C.; Chou, S.; Tang, C.; Wang, R.; Fane, A. G., Membrane Biofouling and Scaling in Forward Osmosis Membrane Bioreactor. J. Membr. Sci. 2012, 403, 8-14. 9. Zhao, S.; Zou, L.; Tang, C. Y.; Mulcahy, D., Recent Developments in Forward Osmosis: Opportunities and Challenges. J. Membr. Sci. 2012, 396, 1-21. 10. Lee, S.; Boo, C.; Elimelech, M.; Hong, S., Comparison of Fouling Behavior in Forward Osmosis (FO) and Reverse Osmosis (RO). J. Membr. Sci. 2010, 365 (1), 34-39. 11. Yip, N. Y.; Tiraferri, A.; Phillip, W. A.; Schiffman, J. D.; Elimelech, M., High Performance Thin-Film Composite Forward Osmosis Membrane. Environ. Sci. Technol. 2010, 44 (10), 3812-3818. 12. Zhang, Y.; Wang, L.; Xu, Y., Effect of Doping Porous ZrO2 Solid Superacid Shell/Void/TiO2 Core Nanoparticles (ZVT) on Properties of Polyvinylidene Fluoride (PVDF) Membranes. Desalination 2015, 358 (0), 84-93. 13. Liao, C.; Zhao, J.; Yu, P.; Tong, H.; Luo, Y., Synthesis And Characterization of Low Content Of Different SiO2 Materials Composite Poly (Vinylidene Fluoride) Ultrafiltration Membranes. Desalination 2012, 285, 117-122. 14. Emadzadeh, D.; Lau, W. J.; Matsuura, T.; Rahbari-Sisakht, M.; Ismail, A. F., A Novel Thin Film Composite Forward Osmosis Membrane Prepared From PSF–TiO2 Nanocomposite Substrate for Water Desalination. Chem. Eng. J. 2014, 237 (0), 70-80. 15. Lopes, A. C.; Martins, P.; Lanceros-Mendez, S., Aluminosilicate and Aluminosilicate Based Polymer Composites: Present Status, Applications and Future Trends. Prog. Surf. Sci. 2014, 89 (3–4), 239-277. - 34 ACS Paragon Plus Environment

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PVDF/SiO2 Membrane

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PVDF Membrane

Amorphous SiO2 NP-Incorporated Poly(vinylidene fluoride

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