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superhydrophobic acetylene-based nanocarbon hybrid membrane for seawater desalination via DCMD
Accepted Manuscript Title: PVDF-co-HFP/superhydrophobic acetylene-based nanocarbon hybrid membrane for seawater desalination via DCMD Authors: Mustafa Mohammed Aljumaily, Mohammed Abdulhakim Alsaadi, N. Awanis Hashim, Qusay F. Alsalhy, Farouq S. Mjalli, Muataz Ali Atieh, Ahmed Al-Harrasi PII: DOI: Reference:
S0263-8762(18)30432-5 https://doi.org/10.1016/j.cherd.2018.08.032 CHERD 3326
To appear in: Received date: Revised date: Accepted date:
11-6-2018 7-8-2018 22-8-2018
Please cite this article as: Aljumaily, Mustafa Mohammed, Alsaadi, Mohammed Abdulhakim, Hashim, N.Awanis, Alsalhy, Qusay F., Mjalli, Farouq S., Atieh, Muataz Ali, Al-Harrasi, Ahmed, PVDF-co-HFP/superhydrophobic acetylene-based nanocarbon hybrid membrane for seawater desalination via DCMD.Chemical Engineering Research and Design https://doi.org/10.1016/j.cherd.2018.08.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
PVDF-co-HFP/superhydrophobic acetylene-based nanocarbon hybrid membrane for seawater desalination via DCMD
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Mustafa Mohammed Aljumailyae, Mohammed Abdulhakim Alsaadiab*, N. Awanis Hashimc, Qusay F. Alsalhyd, Farouq S. Mjallie, Muataz Ali Atiehf Ahmed Al-Harrasig
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Nanotechnology & Catalysis Research Centre (NANOCAT), IPS Building, University of
Malaya, 50603 Kuala Lumpur, Malaysia
National Chair of Materials Science and Metallurgy, University of Nizwa, Sultanate of Oman
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Department of Chemical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia
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Membrane Technology Research Unit, Chemical Engineering Department, University of
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Technology, Alsinaa Street No. 52, B. O. 35010, Baghdad, Iraq Department of Petroleum and Chemical Engineering, Sultan Qaboos University, P.O. Box 33,
Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, Qatar
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123 Muscat, Oman
Foundation, PO Box 5825, Doha, Qatar Chair of Oman’s Medicinal Plants & Marine Natural Products, University of Nizwa, Nizwa
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Sultanate of Oman
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*Corresponding authors Dr. Mohammed A. AlSaadi E-mail: [email protected], Tel: +60163630693, Fax: +60 3 7967 5311; Prof. Dr. Qusay F. Alsalhy; [email protected]
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[email protected]; Mobile: +964-7901730181
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Highlights
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Graphical Abstract
Hydrophobic character of PVDF-co-HFP was highly improve by embedded of
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CNMs/PAC
Membrane fabrication parameters was statistically optimized using DoE
Membranes porosity and roughness were highly improved by embedded of CNMs/PAC
DCMD permeation flux was highly improved up to 102 kg/m2h
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Abstract Surface hydrophobicity is the most desirable characteristic for high DCMD performance. Superhydrophobic carbon nanomaterials/powder activated carbon (CNMs/PAC) has unique
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properties and believed to be the proper candidate to increase the membrane hydrophobicity with maintaining good mechanical properties and high porosity at the same time. In this work, we
introduce a phase inversion process based on central composite design, aimed at minimizing the number of experiments required for membrane fabrication. The hydrophobic membrane
fabrication conditions are modeled as independent parameters, with the flux provided as the
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model response. The analyses performed on the membrane structure and surface, as well as its
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mechanical properties revealed that the superhydrophobic CNMs/PAC significantly enhances the
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hydrophobicity of the composite membrane surface. The accuracy measurements obtained by
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analysis of variance showed that the model developed and all the proposed parameters have
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significant effects on the flux. However, the CNMs/PAC emerged as the most significant
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influential factor and interacted with polymer concentration and casting knife thickness to exert effects on the permeate flux. The optimum preparation parameters were 775.21 mg carbon
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loading, PVDF-HFP concentration of 21.86 g and casting knife thickness of 118.93 µm, as these
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values yield the highest flux of about 102 kg/m2h.
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Keywords: Membrane preparation; membrane distillation; surface characterization; phase inversion; carbon nanomaterials; central composite design; optimization.
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1. Introduction Membrane distillation (MD) is a thermally-driven distillation process as a part of which only vapor molecules are transported through a hydrophobic microporous membrane [1-3]. While
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many MD configurations are in use, owing to its simplicity, direct contact membrane distillation process (DCMD) is the most common. It comprises of a partial evaporation step then vapor transmission through the membrane surface, followed by condensation step that takes place within the membrane[4]. To date, DCMD has been used in seawater and brackish water
desalination [5, 6]. In order to achieve high vapor permeability, high salt rejection and good
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fouling surface resistance, membrane characteristics can be controlled by modifying the
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parameters affecting membrane preparation, thus ensuring the most appropriate pore size,
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porosity, hydrophobicity and fouling resistance [7]. As its name suggests, in the DCMD, the hot
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feed side to be treated must be in direct contact with the membrane, without penetrating into the membrane’s dry pores [8, 9]. One of the basic requirements for DCMD with the feed component
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is membrane hydrophobicity, as membrane pore wetting must be avoided to achieve optimal
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results.
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High flux can be achieved by embedding carbon-based nanomaterials (CNMs) into the polymer solution, as this enhances the hydrophobic character of the hybrid membrane [10-12].
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Due to their unique properties, thermal stability and chemical, conductive properties, CNMs are among the most promising advanced materials. Thus, they have been proposed as a suitable
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candidate for increasing the permeate flux [13, 14]. The hydrophobic properties of any surface strongly depend on its geometric features and chemistry [15, 16]. CNMs with their unique properties can contribute to porosity, pore size and hydrophobicity improvements [11].
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To attain many desirable properties, such as porous membrane structure, mechanical characteristics and high flux, the experimental design (DoE) employed in the membrane preparation process must be carefully tuned for its optimum conditions [17]. The DoE for
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composites membrane (CM) preparation seems to be the best method for optimizing the fabrication conditions and minimizing the number of experimental runs [18]. To understand the effects of different parameters and fabrication variables in the DCMD process, response surface methodology (RSM) is typically employed. This method is also used for developing predictive
models based optimization of DCMD-modified CMs by different techniques [19]. This approach
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can also be used to elucidate the link between the membrane fabrication parameters and the
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performance of the prepared CMs in DCMD [20].
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CNMs are increasingly used as additives to different types of polymer-based membranes. The
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most commonly used CNMs are carbon nanofibers (CNFs), carbon nanotubes (CNTs) and graphene oxide (GO) [21-24]. Carbon nanomaterials/powder activated carbon (CNMs/PAC) has
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recently emerged as a cost-effective method based on acetylene as the carbon source[25]. CNMs
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are grown above the PAC surface resulting in hybrid nanocarbon with excellent properties [26-
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28]. Thus far, in the current work, CNMs/PAC has been used as embedded material with PVDFHFP to improve the membrane properties, especially the hydrophobic character. Available data
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indicate that CNMs/PAC exhibits very high contact angle (CA). Thus, it can be used as an additive to improve membrane hydrophobicity and therefore enhance the permeation flux [29].
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Owing to these beneficial properties, extensive research has been conducted on flux enhancement. On the other hand, optimization of the preparation parameters remains insufficiently explored. Therefore, in the current work, various membrane preparation parameters were studied, namely polymer concentration, CNMs/PAC loading, and casting knife
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thickness, to improve the membrane performance. Moreover, the membrane fabrication process was statistically optimized using DoE, focusing on understanding the key aspects of CNMs/PAC behavior on the membrane surface, rather than solely aiming to achieve the maximum possible
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flux. All samples were prepared as flat sheets using the phase inversion methodology and were implemented in DCMD design. Finally, the experimental results were analyzed to identify the
influence of different parameters and to identify the optimal CM flat-sheet production conditions
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for the DCMD system.
Materials
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2.1.
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2. Experimental Details
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CNMs/PAC were prepared based on the previously established optimum conditions and the method developed earlier [29]. The poly(vinylidene fluoride-co-hexafluoropropylene)
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(PVDF-HFP) polymer was used as a matrix while the solvent was N-methyl-2-pyrrolidinone
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(NMP, anhydrous, 99.5%, of 1.03 g/ mL density). Both were purchased from Sigma-Aldrich.
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Sodium chloride (NaCl 99.5%) salt used in synthesizing aqueous solutions was also purchased from Sigma-Aldrich.
Membrane preparation methodology
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2.2.
All flat-sheet CMs (A1–A11) examined in the present study were prepared by the phase
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inversion method, following the statistical approach provided within the Design Expert (Version 7) software. The central composite design (CCD) statistical method was chosen to optimize the response and study the interactions of three preparation parameters with the aim of minimizing the required number of experimental runs. A regression statistical model was derived for use in
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the optimization study. The CMs preparation procedure commenced by homogeneously dispersing different amounts of CNMs/PAC (following the DoE procedure) in NMP for 1 h with an ultrasonic processor (UP400S, 24 kHz) until optimal distribution is attained. The thus
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obtained granular PVDF-HFP and NMP-CNMs/PAC suspension was dispersed at different concentrations using a magnetic stirrer at room conditions, followed by an ultrasonic process (based on DoE, as given in Table 1). The composite mixture was left for 24 hours under lab
conditions to completely remove all trapped air bubbles before casting it (using a casting knife of 100−300 µm thickness, based on DoE) on a glass support on the casting machine. Next, the CMs
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were immersed in a deionized water bath until the sample could be peeled off from the glass
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surface. After precipitation, the membrane samples were placed in another deionized water bath
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for 24 h to complete their formation and to ensure that all residual solvent traces were removed.
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Finally, the CMs were dried in air for two days in order to determine the optimum model. 2.2.1. Membrane structure analysis
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The prepared flat-sheet CMs (coded A1–A11 in Table 1) were subjected to scanning
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electron microscopy (SEM) to determine their structure. The Jeol JSM-5510 (Jeol Ltd., Japan)
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SEM was used in this study. The CM samples (1 cm2) were prepared for SEM analysis by fracturing in liquid nitrogen before securing them to a support holder for sputter coater chamber,
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and were coated homogeneously with a thin layer of platinum.
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2.2.2. Membrane surface analysis The CM top surfaces were examined by atomic force microscopy (AFM). Different
image perspectives were obtained over different locations on the CM surface of each sample by using a tapping mode Nanoscope IIIa. The topography and phase signals were scanned simultaneously for all CM surfaces with the colloidal probe. The surfaces of the prepared flat-
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sheet CMs were characterized in terms of the average surface roughness (Rave) and pore size distribution. The recorded data were processed by Gwyddion software. An scan area of 10 µm × 10 µm (in X and Y direction) was used while the Z-axis was set to 1.0 µm for determining the
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average pore size based on the AFM top and valley distribution for at least 30 measurements by Gwyddion software. Average roughness (Rave) was obtained by applying the following expression: 𝑅𝑎𝑣𝑒 =
1 𝐿 ∫ |𝑍 (𝑋)| 𝑑𝑥 𝐿 𝐷
(1)
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where Z(x) describes the surface profile analyzed in terms of height distribution (Z) and position
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2.2.3. Surface water wettability measurements
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(X), and L is the length of each CM sample.
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The water contact angle (CA) of the CM surface was measured via the sessile drop method, using a goniometer (DSA20E KRÜSS GmbH apparatus, Germany). First, 4 µl of
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distilled water was deposited on the surface of the tested specimens (2.5 cm × 2.5 cm) and the
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contour of the drop was recorded by using an automatic interfacial tension-meter. The contact angle on the CM surface was determined by performing at least 5 measurements in different
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areas of each sample.
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2.2.4. Porosity measurements The prepared flat-sheet CM porosity ε (%) of each sample was determined using the
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gravimetric method, as defined by the following equation: 𝜀(%) = (𝜔1 − 𝜔2 )/(𝐴 × 𝐼 × 𝑑𝑤 ) × 100
(2)
where 𝜔1 is the wet CM sample weight; 𝜔2 is the dry CM sample weight; A is the CM effective area (m2), dw is the distilled water density (0.998 g/cm3) and I is the CM thickness (m).
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2.3.
Chemical structure analysis of the prepared composite membranes The functional groups that resulted from incorporating CNMs/PAC into the membrane
matrix were studied using Attenuated Total Reflection-Fourier Transform Infrared spectroscopy
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(ATR-FTIR) and the spectra were recorded on a NICOLET 510P spectrometer. The CM structure was recorded at wavelengths ranging from 4000 to 500 cm−1. 2.4.
Mechanical properties
Tensile strength test measurements of the CMs at room temperature were carried out using an
Instron 5940 tensile test machine. All samples were secured and held at both membrane ends and
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were pulled at a constant elongation velocity 30 mm/min for the tension test using a 20 mm
Membrane performance measurements in DCMD
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2.5.
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gauge length. The tensile stress-strain and Young's modulus were evaluated.
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DCMD performance measurements for all CMs (A1−A11) were performed in a lab-scale DCMD unit shown in Figure 1. The CM was placed inside a stainless steel membrane module with an
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effective membrane area of 3.4 cm2. Synthetic salt water was used as a hot feed solution comprising of 35 g/L NaCl and distilled water on the permeate stream side. Two peristaltic
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pumps were used to control both the feed and the permeate flow rate. The two streams were pumped in recirculation cycle mode through heat and cool stream exchangers. In all CM
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samples, salt concentrations were maintained at a similar level. The system was operated with a feed side flow rate of 22 mL/min and a permeate side flow rate of 12 mL/min. The specific
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reason for choosing low feed flow rate was to prevent exceeding the liquid entry pressure (LEP) and in turn overcome the wettability phenomenon of the membrane [30, 31]. The temperature was continuously controlled at 60 °C on the hot feed side and at 22 °C on the permeate side. Electrical conductivity, as well as water level transferred to the permeate side, were monitored
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over time. Each CM sample was tested in three replicates and the average value of the DCMD performance was calculated.
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3. Results and Discussion 3.1. FT-IR composite membrane FT-IR analysis is a particularly useful method extensively used for characterization
studies of the crystalline phases and functional groups attached to the polymer. The chemical structure of the CM was compared to that of a pristine polymer membrane (PVDF-HFP), as
shown in Figure 2. According to the presence of transmittance apices at 528, 614, 760, 795, 840,
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876, and 973 cm−1, the spectra-FTIR indicated crystalline phase predominance that may be
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attributed to the changes in the polymeric materials during the phase inversion in membrane
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fabrication and this phenomenon has been observed by many researchers[32, 33]. However,
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transmittance apices at 974, 1070, 1180, 1280 and 1400 cm−1 suggest existence of polymer– solvent NMP interactions [32]. A peak clearly seen at 1640 cm-1 is due to C꞊O stretching
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vibrations, whereas weak and strong peaks at 2853 and 2925 cm−1, respectively, likely arise
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from–CH and CH2 stretching [34]. In general, CNM is contaminated by amorphous carbon
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during the growth process in CVD, which is typically oxidized by air to produce a few functional groups. Generally, the oxy-functional groups would correspond to more negative values, as seen
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in the CA measurement discussed below.
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3.2.
Scanning electron microscopy The SEM images of all CM flat sheet samples (A1–A11) are shown in Figure 3. The
cross-section micrographs reveal different structures that represent different polymer concentrations, CNMs/PAC loading levels and casting knife thicknesses, as suggested by the DoE. Figure 3 reveals the crucial impact of polymer concentration increase in the membrane
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matrix on the reduction in the number of cavities in the membrane. The cross-section micrographs reveal also that the CNMs/PAC is well-dispersed in the membrane matrix even with high CNM loading. All examined CMs have finger-like void structure layers at the top, and a
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sponge-like void structure layer at the bottom. Figure 3 shows the finger-like cavities became narrower and shorter in the cross-section of membranes A2 and A3. While, in agreement with
other researchers [35-37], the low concentration polymer led to reduction in the width and length of the finger-like as can be seen in the cross-section of membranes A5 and A7. Moreover, the
free macro-voids increased in size due to greater CNMs/PAC matrix loading. In a cross-section
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of membranes A1, A4 and A5, it can be seen that the finger-like structure in the top layer is
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wider than the remaining components and the free micro-voids are increased with the increasing
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the amount of CNMs/PAC in the polymer solution. The finger-like pore structure transformed to
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spherical nano-voids and porosity was increased, as shown in the cross-section of membrane A9 due to high CNMs/PAC loading. This amount of loading has made the finger-like structure layer
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separated from the sponge-like pore structure layer, as shown in the cross-section of membranes
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A4 and A9. Although similar amount of CNMs/PAC was utilized for samples A5 and A7, the
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separation of layers could not be seen as in A4 and A9 because of the difference in the polymer concentration. CNMs/PAC loading not only affected the configuration of finger-like voids but
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increased there number as well. This is obvious when the amount of CNMs/PAC increases up to 1000 mg. By comparing the top layer of pristine PVDF-HFP cross-section with that of the
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prepared CMs, it can be noted that the finger-like structure becomes wider with increased CNM loading into the matrix with more free micro-voids. Thus, it can be concluded that the CNMs/PAC loading results to a top membrane layer with a finger-like structure because of the superhydrophobicity of the carbon of CNMs, and sponge-like structure at the bottom. The
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increment in viscosity with the increase in polymer concentration should be hindered or delay the penetration of the NMP through membrane coagulation [9, 35, 38-40]. One of the obvious effects of the lower casting knife thickness in the membrane is the change in the configuration of
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the cavities where it appears in the cross section longer and narrower. The mechanism of this change can be explained by the fast evaporation behavior of the solvent during the late stage of the coagulation through the phase inversion process as shown clearly in the cross-section of membranes A1 and A7. 3.3.
Atomic force microscopy
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The three-dimensional AFM images reveal the surface morphology of the prepared flat-
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sheet membranes (A1–A11). It can be seen in Figure 4 that the nodules aggregate at the surfaces
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with high CNM/PAC loading. In contrast, the less CNM/PAC loading the less aggregate found
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at the surface. Furthermore, the nodule alignment is more pronounced as the polymer concentration increases. However, this nodule alignment is observed clearly at the surfaces of the
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flat-sheet membranes A1, A4, A6 and A9. The nodule surface size and distribution are lower at
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low CNM/PAC loading, likely due to the mitigation of the superhydrophobic CNMs that takes
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place at the top surface of the membranes during the coagulation of the phase inversion process. Moreover, the delay because of the CNMs/loading through the coagulation process results in
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nodule agglomerates, as shown in membrane A3 and differ from A2. On the other hand, samples A5−A7 contain low CNMs/PAC volume and low polymer concentration (20%) with 300 µm
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thickness, which led to a small nodule size. It can be seen in Figure 5 that polymer concentration and different solvent amounts led to different membrane thicknesses after the phase inversion process. This is related to the nodule size and distribution, as observed in A4 and A9, which have the same amount of CNMs/PAC. Also, obvious nodule alignment was found at the surface of
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membranes A4 and A9, due to the high polymer concentration in the casting solution in addition to the high loading of CNM/PAC. Increasing polymer concentration led to delay solvent/nonsolvent exchange rate during the membrane formation which in turn results to uniform nodule
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size or nodule alignment at the surface. Although nodule size and roughness were higher at the CM surfaces than at those of the pristine PVDF-HFP membranes, the surface roughness between the highest peaks and lowest valleys of the CMs A1–A11 was determined, as shown in Figure 6. The surface roughness is strongly correlated with the Z-value and the membrane cross-section at different CNMs/PAC loadings, which were tested in three different regions (lines A, B and C in
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Figure 7) to describe the variation of height across the membrane and pore diameter. The results
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showed that the height decreased from ̴ 250 to ̴100 µm. The AFM results revealed that high
Porosity and pore size
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3.4.
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CNMs/PAC loading leads to a smoother surface than low loading.
It is well known that polymer concentration has strong impact on the membrane surface
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porosity. A comparison with the pristine membrane revealed that the prepared CMs possessed an
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excellent porosity, which was 80–93% higher than the value measured for the pristine PVDF-
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HFP (66%) [9, 35]. Moreover, as shown in Figure 8, high CNM loading resulted in greater porosity for samples A1, A4, A5 and A9, which can be attributed to the macro-void formation
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observed in same samples, this comply with the observation of SEM images (Figure 3). Porosity is affected by interaction of several parameters, such as the CNMs/PAC loading, PVDF-HFP
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concentration and interactions established with the casting solution during the coagulation process [30]. Membrane pore size is one of the most important factors for increasing the water flux. For the tested membranes, it increased as the CNMs/PAC content was increased up to 1000 mg, as shown in Figure 9. The best choice for MD pore size, according to the pertinent literature,
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is 0.2–0.5 µm [41]. Therefore, the samples A2, A5, and A7 were selected for performance evaluation due to exhibiting high flux, whereas A8 and A2 had the lowest and the highest pore size, respectively. It is well known that the effect of interactive tension between the polymer and
3.5.
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solvents during the polymerization process affects membrane porosity and pore size [35, 42, 43]. Contact angle measurement
The hydrophobic properties of a membrane are strongly dependent on the fabrication process and the geometric features of the resulting surfaces [44]. Hydrophobic surface is generally
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assessed in terms of its contact angle (CA). It can be seen in Figure 10 that the CA of all CM
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samples increased as a result of adding the superhydrophobic CNMs/PAC. The increase in the
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CA indicates that the superhydrophobic CNMs/PAC can improve the hydrophobicity of the
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membrane surface relative to the pristine PVDF-HFP, for which the CA was reported to be 90.1° ± 1.7° [45]. The CA of prepared CMs reached the maximum of 125.6o ± 1.9° (measured for A5,
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which had a high concentration of superhydrophobic CNM/PAC). However, despite having the
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same amount of superhydrophobic CNM/PAC, sample A9 has low CA because of high polymer
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concentration and greater thickness, which led to the agglomeration of CNMs/PAC in the membrane structure. As a consequence, high polymer concentration prevents good CNM/PAC
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dispersion, as confirmed by 3D images of A5 and A9. It is well-known that, in composite materials, roughness occurs due to filler accumulation on the surface. In our case, the filler is
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superhydrophobic CNMs/PAC, which increases the total surface area and consequently the CA. 3.6.
Mechanical properties of CMs CM mechanical properties are a good indicator of ability to resist excessive pressure.
Therefore, the characteristics of the prepared CMs were evaluated in this work. Table 2 shows the mechanical properties of all the samples of CMs suggested by DoE. The tensile speed of 140 14
mm/min was exerted on all membrane dog-bone specimen samples at room conditions. The results reported in Table 2 indicate that CNMs/PAC loading maximizes the advantage of effective superhydrophobic CNMs/PAC reinforcement by adding higher strength for the
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mechanically weak PVDF-HFP. It can be seen that low CNMs/PAC loading leads to high tensile strength, as shown in Table 2 for samples A1, A2, A6 and A7. Low polymer concentration
enhanced the CMs with a decrease in agglomeration of CNMs/PAC in the membrane. Moreover, the SEM images confirmed the mechanical properties of A4, A5 and A8, indicating that the
membrane macro-void structure had a distinct effect on membrane strength, as shown in Table 2
3.7.1. Statistical analysis and modeling
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Statistical analyses of the prepared composite membranes
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3.7.
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for same samples.
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Preparation parameters, such as polymer concentration, CNMs/PAC loading and casting
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knife thickness, of all flat-sheet CMs were optimized using the method of multiple variables to
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study the effects of these parameters on flux, as shown in Table 3. Using DoE, quadratic models describing flux were developed. The regressed model was tested statistically for its significance
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using the analysis of variance (ANOVA) technique. The fabrication conditions of superhydrophobic CMs were used as the independent variables, while the flux was set as the
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model response variable. The regression equation for flux is given below:
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𝐹𝑙𝑢𝑥 = 1/(+0.013 + 0.016 𝐴 + 0.025 𝐵 + 9.227 × 10−3 𝐶 + 9.205 × 10−3 𝐴𝐵 + 4.750 × 10−3 𝐴 𝐶 + 5.351 × 10−3 𝐵𝐶 + 0.014 𝐴2 + 0.021𝐵 2 − 6.663 × 10−3 𝐶 2 ) (3)
where A represents CNMs loading, B denotes PVDF-HFP concentration, and C is the casting knife thickness.
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As the P-values were less than 0.05, the CCD model predictions are statically significant and all individual variable effects are pronounced as shown in Table 4. Among the three fabrication parameters studied, only the second-order interaction effect of CNMs/PAC loading
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with of PVDF-HFP concentration (AB) was significant, and emerged as the major determinant of flux. The second-order effects of CNMs loading (A2), PVDF-HFP concentration (B2) and the interaction effect of casting knife thickness (C2) were all insignificant.
The F-value of 548.14 indicates that the prediction model is significant. The values of
Prob > F greater than 0.1 indicated that the model is not significant. It is clearly observed that the
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coefficient of determination (R2 = 0.9998) of the quadratic model was significant, since its
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probability Prob > F value was 0.0331 and was thus below 0.05. A comparison of the actual
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results with the model-predicted values is depicted in Figure 11, which shows that there is a good
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agreement between the predicted and actual values, as indicated by the coefficient of determination (R2) being close to unity.
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3.7.2. Effects of CNMs/PAC loading, PVDF-HFP concentration and casting knife
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thickness
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The effects of interaction parameters on flux were graphically illustrated by CCD curves, as shown in Figure 12. The interaction of the three CM parameters was examined by DCMD and
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the optimum level of each parameter for maximum response was determined accordingly. It can be seen in Figure 3 that CNMs/PAC loading and PVDF-HFP concentration at constant casting
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knife thickness had a strong impact on the flux, as shown in Figure 12(a). It can be thus concluded that the interaction between polymer concentration and CNMs/PAC loading was significant. It is well-known that the polymer concentration in casting solution plays a crucial role in the membrane morphology, as it controls solution viscosity, which in turn affects
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membrane performance [5]. Figure 12(b) indicates a notable increase in flux with a decrease in membrane thickness and a slight increase in CNMs/PAC loading [46]. Similarly, decreasing PVDF-HFP loading results in a greater flux at low membrane thickness [47]. The highest value
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of flux is obtained for the lowest PVDF-HFP loading and thickness, as shown in Figure 12(c). CM fabrication by phase inversion method was optimized simultaneously using a CCD method. All membrane preparation parameters and response were incorporated as variables in the CCD method to eliminate the number of experimental tests and satisfy all the criteria for optimum flux. The ultimate goal of adding superhydrophobic CNMs/PAC is to maximize the vapor
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permeate flux by enhancing the membrane surface hydrophobicity. Multiple parameter
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combinations were evaluated in order to identify the optimum conditions for CM preparation and
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these were listed according the value of module desirability order. Table 5 lists the constraints
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developed to control the optimization solutions of the CCD software. The optimum preparation conditions for the highest desirability were a carbon loading CNMs/PAC of 775.21 mg, PVDF-
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HFP concentration of 21.86 g, and casting knife thickness of 118.93 µm, which gave the highest
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flux of about 102 kg/m2h. These values were used in subsequent experimental tests and the
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predicted maximum flux was obtained. Specifically, we prepared the membrane A12 according to the optimization results
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mentioned above. The morphological properties of A12 are presented in Figure 13. It can be observed that the SEM cross-section of A12 is characterized by finger-like structures at the top
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layer with a thin sponge-like layer at the bottom, whereas in 2D AFM image, the top surface exhibits high average roughness (55.4) and large pore size (55.1 µm). Moreover, the CA of the A12 membrane surface is 128°±1.4°. These results confirm a remarkable agreement between the predicted flux and that obtained experimentally.
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4.
Conclusion
Composite membrane fabrication based on different morphological parameters was performed for DCMD process. The CCD was conducted in order to elucidate the effect of the main
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fabrication parameters using the phase inversion technique in flat sheet form. Three membrane preparation variables were considered, namely CNMs/PAC loading in the casting solution,
polymer concentration, and casting knife thickness. The effect of these variables on the structural characteristics of the CMs was studied and the effect on the performance was examined through the DCMD. The superhydrophobic CNMs/PAC improved the hydrophobicity of the membrane
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surface compared to the pristine PVDF-HFP. The optimum fabrication parameters were carbon
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loading CNMs/PAC of 775.21 mg, PVDF-HFP concentration of 21.86 g, and casting knife
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thickness of 118.93 µm resulted in a membrane with characteristics of high average roughness
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(55.4), large pore size (55.1 µm) and CA (128°±1.4°). The results also confirm that the optimum prepared CMs possess excellent porosity of 93% higher than the pristine PVDF-HFP. The
Acknowledgment
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maximum flux achieved was 102 kg/m2h and the salt rejection was higher than 99.9%.
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The authors would like to acknowledge the National Chair of Materials Sciences and Metallurgy,
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University of Nizwa, Oman and surface science lab, department of physics, college of science, Sultan Qaboos University and the University of Malaya UMRG RP044D-17AET for funding this
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research.
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References [1] S. Al-Obaidani, E. Curcio, F. Macedonio, G. Di Profio, H. Al-Hinai, E. Drioli, Potential of membrane distillation in seawater desalination: thermal efficiency, sensitivity study and cost
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estimation, Journal of Membrane Science, 323 (2008) 85-98. [2] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, science, 333 (2011) 712-717.
[3] A. Alklaibi, N. Lior, Membrane-distillation desalination: status and potential, Desalination, 171 (2005) 111-131.
U
[4] T.Y. Cath, V.D. Adams, A.E. Childress, Experimental study of desalination using direct
N
contact membrane distillation: a new approach to flux enhancement, Journal of Membrane
A
Science, 228 (2004) 5-16.
M
[5] H. Attia, S. Alexander, C.J. Wright, N. Hilal, Superhydrophobic electrospun membrane for
D
heavy metals removal by air gap membrane distillation (AGMD), Desalination, 420 (2017) 318-
TE
329.
[6] S. Srisurichan, R. Jiraratananon, A. Fane, Mass transfer mechanisms and transport resistances
CC
194.
EP
in direct contact membrane distillation process, Journal of Membrane Science, 277 (2006) 186-
[7] K.Y. Wang, S.W. Foo, T.-S. Chung, Mixed matrix PVDF hollow fiber membranes with
A
nanoscale pores for desalination through direct contact membrane distillation, Industrial & Engineering Chemistry Research, 48 (2009) 4474-4483. [8] K.W. Lawson, D.R. Lloyd, Membrane distillation, Journal of membrane Science, 124 (1997) 1-25.
19
[9] Q.F. Alsalhy, K.T. Rashid, S.S. Ibrahim, A.H. Ghanim, B. Van der Bruggen, P. Luis, M. Zablouk, Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) hollow fiber membranes prepared from PVDFcoHFP/PEG600Mw/DMAC solution for membrane distillation,
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Journal of Applied Polymer Science, 129 (2013) 3304-3313. [10] M. Spasova, N. Manolova, N. Markova, I. Rashkov, Tuning the properties of PVDF or
PVDF-HFP fibrous materials decorated with ZnO nanoparticles by applying electrospinning alone or in conjunction with electrospraying, Fibers and Polymers, 18 (2017) 649-657.
[11] V. Kochkodan, N. Hilal, A comprehensive review on surface modified polymer membranes
U
for biofouling mitigation, Desalination, 356 (2015) 187-207.
N
[12] S.A. Rahman, K.T. Rashid, Q.F. Alsalhy, Improvement of PVDF-co-HFP Hollow Fiber
A
Membranes for Direct Contact Membrane Distillation Applications, Indian Journal of Science
M
and Technology, 10 (2017).
[13] K. Wang, A.A. Abdalla, M.A. Khaleel, N. Hilal, M.K. Khraisheh, Mechanical properties of
D
water desalination and wastewater treatment membranes, Desalination, 401 (2017) 190-205.
TE
[14] P. Goh, A. Ismail, B. Ng, Carbon nanotubes for desalination: Performance evaluation and
EP
current hurdles, Desalination, 308 (2013) 2-14. [15] L. Zhu, P. Shi, J. Xue, Y. Wang, Q. Chen, J. Ding, Q. Wang, Superhydrophobic Stability of
CC
Nanotube Array Surfaces under Impact and Static Forces, ACS Applied Materials & Interfaces, 6 (2014) 8073-8079.
A
[16] M. Ma, R.M. Hill, Superhydrophobic surfaces, Current opinion in colloid & interface science, 11 (2006) 193-202. [17] M.K. AlOmar, M.A. Alsaadi, M.M. Aljumaily, S. Akib, T.M. Jassam, M.A. Hashim, N, NDiethylethanolammonium chloride-based DES-functionalized carbon nanotubes for arsenic
20
removal from aqueous solution, DESALINATION AND WATER TREATMENT, 74 (2017) 163-173. [18] M. Khayet, C. Cojocaru, M.d.C. García-Payo, Experimental design and optimization of
membrane science, 351 (2010) 234-245.
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asymmetric flat-sheet membranes prepared for direct contact membrane distillation, Journal of
[19] D. Cheng, W. Gong, N. Li, Response surface modeling and optimization of direct contact membrane distillation for water desalination, Desalination, 394 (2016) 108-122.
[20] M. Khayet, Membranes and theoretical modeling of membrane distillation: a review,
U
Advances in colloid and interface science, 164 (2011) 56-88.
N
[21] Y. Manawi, V. Kochkodan, M.A. Hussein, M.A. Khaleel, M. Khraisheh, N. Hilal, Can
A
carbon-based nanomaterials revolutionize membrane fabrication for water treatment and
M
desalination?, Desalination, 391 (2016) 69-88.
[22] M. Bhadra, S. Roy, S. Mitra, Desalination across a graphene oxide membrane via direct
D
contact membrane distillation, Desalination, 378 (2016) 37-43.
TE
[23] H. Qiu, Y. Peng, L. Ge, B.V. Hernandez, Z. Zhu, Enhanced anti-fouling property in
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membrane distillation by pore channel surface modification, Applied Surface Science, 443 (2018) 217-226.
CC
[24] L. Eykens, K. De Sitter, C. Dotremont, L. Pinoy, B. Van der Bruggen, Membrane synthesis for membrane distillation: A review, Separation and Purification Technology, 182 (2017) 36-51.
A
[25] H.M. Alayan, M.A. Alsaadi, A. Abo-Hamad, M.K. AlOmar, M.M. Aljumaily, R. Das, M.A. Hashim, Hybridizing carbon nanomaterial with powder activated carbon for an efficient removal of Bisphenol A from water: the optimum growth and adsorption conditions, Desalination and water treatment, 95 (2017) 128-143.
21
[26] Y.M. Ahmed, S. Abdullah-Al-Mamun, R. Ma'an Fahmi, A. Iameel, M.A. AlSaadi, Study of Pb Adsorption by Carbon Nanofibers Grown on Powdered Activated Carbon, Journal of Applied Sciences, 10 (2010) 1983-1986.
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[27] A. Mamun, Y. Ahmed, S. Muyibi, M. Al-Khatib, A. Jameel, M. AlSaadi, Synthesis of carbon nanofibers on impregnated powdered activated carbon as cheap substrate, Arabian Journal of Chemistry, 9 (2016) 532-536.
[28] M.A. AlSaadi, A. Al-Mamun, S.A. Muyibi, M.Z. Alam, I. Sopyan, M.A. Atieh, Y.M.
Ahmed, Synthesis of various carbon nanomaterials (CNMs) on powdered activated carbon,
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African Journal of Biotechnology, 10 (2011) 18892-18905.
N
[29] M.M. Aljumaily, M.A. Alsaadi, R. Das, S.B.A. Hamid, N.A. Hashim, M.K. AlOmar, H.M.
A
Alayan, M. Novikov, Q.F. Alsalhy, M.A. Hashim, Optimization of the Synthesis of
M
Superhydrophobic Carbon Nanomaterials by Chemical Vapor Deposition, Scientific Reports, 8 (2018) 2778.
D
[30] T.L. Silva, S. Morales-Torres, J.L. Figueiredo, A.M. Silva, Multi-walled carbon
TE
nanotube/PVDF blended membranes with sponge-and finger-like pores for direct contact
EP
membrane distillation, Desalination, 357 (2015) 233-245. [31] M. García-Payo, M. Essalhi, M. Khayet, Effects of PVDF-HFP concentration on membrane
CC
distillation performance and structural morphology of hollow fiber membranes, Journal of Membrane Science, 347 (2010) 209-219.
A
[32] X. Tian, X. Jiang, Poly (vinylidene fluoride-co-hexafluoropropene)(PVDF-HFP) membranes for ethyl acetate removal from water, Journal of hazardous materials, 153 (2008) 128-135.
22
[33] S. Ramesh, O.P. Ling, Effect of ethylene carbonate on the ionic conduction in poly (vinylidenefluoride-hexafluoropropylene) based solid polymer electrolytes, Polymer Chemistry, 1 (2010) 702-707.
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[34] F. Avilés, J. Cauich-Rodríguez, L. Moo-Tah, A. May-Pat, R. Vargas-Coronado, Evaluation of mild acid oxidation treatments for MWCNT functionalization, Carbon, 47 (2009) 2970-2975. [35] S. Fadhil, T. Marino, H.F. Makki, Q.F. Alsalhy, S. Blefari, F. Macedonio, E. Di Nicolò, L. Giorno, E. Drioli, A. Figoli, Novel PVDF-HFP flat sheet membranes prepared by triethyl
phosphate (TEP) solvent for direct contact membrane distillation, Chemical Engineering and
U
Processing: Process Intensification, 102 (2016) 16-26.
N
[36] J.T. Jung, J.F. Kim, H.H. Wang, E. di Nicolo, E. Drioli, Y.M. Lee, Understanding the non-
A
solvent induced phase separation (NIPS) effect during the fabrication of microporous PVDF
M
membranes via thermally induced phase separation (TIPS), Journal of Membrane Science, 514 (2016) 250-263.
D
[37] A.K. Hołda, I.F. Vankelecom, Understanding and guiding the phase inversion process for
TE
synthesis of solvent resistant nanofiltration membranes, Journal of Applied Polymer Science,
EP
132 (2015).
[38] Q.F. Alsalhy, S.S. Ibrahim, F.A. Hashim, Experimental and theoretical investigation of air
CC
gap membrane distillation process for water desalination, Chemical Engineering Research and Design, 130 (2018) 95-108.
A
[39] K.T. Rashid, S.B.A. Rahman, Q.F. Alsalhy, Optimum Operating Parameters for Hollow Fiber Membranes in Direct Contact Membrane Distillation, Arabian Journal for Science and Engineering, 41 (2016) 2647-2658.
23
[40] K.T. Rashid, S.A. Rahman, Q. Alsalhy, Hydrophobicity Enhancement of Poly (vinylidene fluoride-co-hexafluoropropylene) for Membrane Distillation, Journal of Polymer Science and Technology, 1 (2015) 1-9.
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[41] M.M.A. Shirazi, A. Kargari, M. Tabatabaei, Evaluation of commercial PTFE membranes in desalination by direct contact membrane distillation, Chemical Engineering and Processing: Process Intensification, 76 (2014) 16-25.
[42] M. Kim, G. Kim, J. Kim, D. Lee, S. Lee, J. Kwon, H. Han, New continuous process
developed for synthesizing sponge-type polyimide membrane and its pore size control method
U
via non-solvent induced phase separation (NIPS), Microporous and Mesoporous Materials, 242
N
(2017) 166-172.
A
[43] Z.-L. Xu, F.A. Qusay, Polyethersulfone (PES) hollow fiber ultrafiltration membranes
M
prepared by PES/non-solvent/NMP solution, Journal of Membrane Science, 233 (2004) 101-111. [44] D.-W. Jeong, U.-H. Shin, J.H. Kim, S.-H. Kim, H.W. Lee, J.-M. Kim, Stable hierarchical
D
superhydrophobic surfaces based on vertically aligned carbon nanotube forests modified with
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conformal silicone coating, Carbon, 79 (2014) 442-449.
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[45] L. Xie, X. Huang, K. Yang, S. Li, P. Jiang, “Grafting to” route to PVDF-HFP-GMA/BaTiO 3 nanocomposites with high dielectric constant and high thermal conductivity for energy storage
CC
and thermal management applications, Journal of Materials Chemistry A, 2 (2014) 5244-5251. [46] H.Y. Wu, R. Wang, R.W. Field, Direct contact membrane distillation: An experimental and
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analytical investigation of the effect of membrane thickness upon transmembrane flux, Journal of Membrane Science, 470 (2014) 257-265.
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[47] L. Eykens, I. Hitsov, K. De Sitter, C. Dotremont, L. Pinoy, I. Nopens, B. Van der Bruggen, Influence of membrane thickness and process conditions on direct contact membrane distillation
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CC
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TE
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at different salinities, Journal of Membrane Science, 498 (2016) 353-364.
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Figure 1: Schematic diagram of a lab-scale DCMD system used in this work
Figure 2: FTIR spectroscopy of the composite membrane and pristine PVDF-HFP 26
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Figure 3: Cross-section SEM images of the composite membrane flat-sheet membranes prepared by DoE indicated.
27
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A
CC
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TE
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Continued Figure 3
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A
CC
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TE
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prepared by the DoE indicated.
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Figure 4: 3D AFM images of the top surface of the composite membrane flat-sheet membranes
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A
CC
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Figure 5: Thickness of membrane after phase inversion process.
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CC
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Figure 6: The roughness of composite membrane fabricated through DoE by phase inversion.
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A
CC
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TE
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prepared by the DoE indicated.
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Figure 7: 2D AFM images of the top surface of the composite membrane flat-sheet membranes
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Figure 8: The porosity of composite membrane fabricated through DoE by phase inversion.
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CC
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.
33
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Figure 9: The pore size diameter of composite membrane fabricated through DoE by phase
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CC
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inversion.
34
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A
CC
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Figure 10: Contact Angle of composite membrane fabricated through DoE by phase inversion.
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CC
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Figure 11: Parity plot of experimental and predicted values of flux.
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Figure 12: Response surface plots for the effects of different process parameters on flux: (a) Effect of CNMs loading and concentration of PVDF-HFP; (b) Effect of CNMs loading and
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thickness of CM; (c) effect of concentration of PVDF-HFP and thickness of CM.
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Figure 13: The flat-sheet membrane A12 prepared using the optimum conditions. (a) SEM for
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top layer of cross-section, (b) for bottom layer, (c) for 3D AFM and (d) 2D AFM.
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Table 1: Central composite experimental design for preparation of flat-sheet membranes by phase inversion method SAMPLES *100 Ratio of Cnms Loading Polymer to Solvent Casting Knife (mg/gm) Ratio (µM) (%) 100.00 24.00 100.00 A1
A4
1000.00
A5
1000.00
A6
100.00
A7
100.00
A8
1000.00
A9
1000.00
A10
550.00
A11
550.00
300.00
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1000.00
20.00
300.00
24.00
100.00
20.00
100.00
24.00
300.00
20.00
100.00
22.00
200.00
24.00
300.00
22.00
200.00
22.00
300.00
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A3
20.00
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100.00
A
A2
2.098
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Table 2: Mechanical properties of composite membrane fabricated through DoE by phase inversion. Samples Tensile strength Elongation at break Modulus (mpa) (%) (mpa) 3.601 186.9 63.55 A1 78.7
34.87
3.493
133.1
32.45
2.61
183.7
67.98
2.673
99.37
47.82
1.89
185.7
92.86
A7
2.012
62.05
79.96
A8
2.148
100.3
65.99
A9
2.885
80.89
60.85
A10
3.659
42.35
63.27
A11
2.591
52.49
53.79
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A2
A4 A5
A
CC
A6
EP
A3
39
100.00
20.00
300.00
95
A3
1000.00
20.00
300.00
32.1
A4
1000.00
24.00
100.00
A5
1000.00
20.00
100.00
A6
100.00
24.00
300.00
A7
100.00
20.00
100.00
A8
1000.00
22.00
200.00
A9
1000.00
24.00
300.00
A10
550.00
22.00
200.00
78
A11
550.00
22.00
65
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A2
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Table 3: Experimental design matrix and the value of responses based on experiment run. Samples CNMs/PAC Polymer to Solvent Casting Knife Flux Loading (%) (µm) (kg/m2h) (mg/gm) 31 100.00 24.00 100.00 A1
14 67
19.65 102
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17
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A
300.00
9
0.010
A-CNMs/PAC
1.95710-3
B-PVDF-HFP
4.96410-3
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P-value prob > F
1.15210-3
548.14
0.0331
1
1.95710-3
931.41
0.0209
1
4.96410-3
2362.03
0.0131
9
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Model
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Table 4: The analysis of variance for response surface quadratic model. Source Sum of Degree of Mean F-value squares Freedom square
6.81210-4
1
6.81210-4
324.12
0.0353
AB
6.77910-4
1
6.77910-4
322.57
0.0354
AC
1.80510-4
1
1.80510-4
85.89
0.0684
BC
2.29110-4
1
2.29110-4
109.01
0.0608
A2
6.02710-5
1
6.02710-5
28.68
0.1175
B2
2.09410-4
1
2.09410-4
99.62
0.0636
C2
2.08910-5
1
2.08910-5
9.94
0.1955
Residual
2.10210-6
1
2.10210-6
A
CC
C-Thickness
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Table 5: Constraints for optimization process based on CCD for DCMD performance. Name Goal Lower Limit Upper Limit Importance In range
100
1000
3
B
In range
20
24
3
C
In range
100
300
3
FLUX
Maximize
9
102
A
CC
EP
TE
D
M
A
N
U
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A
41
5
S0263-8762(18)30432-5 https://doi.org/10.1016/j.cherd.2018.08.032 CHERD 3326
To appear in: Received date: Revised date: Accepted date:
11-6-2018 7-8-2018 22-8-2018
Please cite this article as: Aljumaily, Mustafa Mohammed, Alsaadi, Mohammed Abdulhakim, Hashim, N.Awanis, Alsalhy, Qusay F., Mjalli, Farouq S., Atieh, Muataz Ali, Al-Harrasi, Ahmed, PVDF-co-HFP/superhydrophobic acetylene-based nanocarbon hybrid membrane for seawater desalination via DCMD.Chemical Engineering Research and Design https://doi.org/10.1016/j.cherd.2018.08.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
PVDF-co-HFP/superhydrophobic acetylene-based nanocarbon hybrid membrane for seawater desalination via DCMD
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Mustafa Mohammed Aljumailyae, Mohammed Abdulhakim Alsaadiab*, N. Awanis Hashimc, Qusay F. Alsalhyd, Farouq S. Mjallie, Muataz Ali Atiehf Ahmed Al-Harrasig
a
Nanotechnology & Catalysis Research Centre (NANOCAT), IPS Building, University of
Malaya, 50603 Kuala Lumpur, Malaysia
National Chair of Materials Science and Metallurgy, University of Nizwa, Sultanate of Oman
c
Department of Chemical Engineering, University of Malaya, Kuala Lumpur 50603, Malaysia
d
Membrane Technology Research Unit, Chemical Engineering Department, University of
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b
e
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Technology, Alsinaa Street No. 52, B. O. 35010, Baghdad, Iraq Department of Petroleum and Chemical Engineering, Sultan Qaboos University, P.O. Box 33,
Qatar Environment and Energy Research Institute, Hamad Bin Khalifa University, Qatar
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f
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123 Muscat, Oman
Foundation, PO Box 5825, Doha, Qatar Chair of Oman’s Medicinal Plants & Marine Natural Products, University of Nizwa, Nizwa
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g
Sultanate of Oman
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*Corresponding authors Dr. Mohammed A. AlSaadi E-mail: [email protected], Tel: +60163630693, Fax: +60 3 7967 5311; Prof. Dr. Qusay F. Alsalhy; [email protected]
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[email protected]; Mobile: +964-7901730181
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Highlights
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Graphical Abstract
Hydrophobic character of PVDF-co-HFP was highly improve by embedded of
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CNMs/PAC
Membrane fabrication parameters was statistically optimized using DoE
Membranes porosity and roughness were highly improved by embedded of CNMs/PAC
DCMD permeation flux was highly improved up to 102 kg/m2h
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Abstract Surface hydrophobicity is the most desirable characteristic for high DCMD performance. Superhydrophobic carbon nanomaterials/powder activated carbon (CNMs/PAC) has unique
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properties and believed to be the proper candidate to increase the membrane hydrophobicity with maintaining good mechanical properties and high porosity at the same time. In this work, we
introduce a phase inversion process based on central composite design, aimed at minimizing the number of experiments required for membrane fabrication. The hydrophobic membrane
fabrication conditions are modeled as independent parameters, with the flux provided as the
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model response. The analyses performed on the membrane structure and surface, as well as its
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mechanical properties revealed that the superhydrophobic CNMs/PAC significantly enhances the
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hydrophobicity of the composite membrane surface. The accuracy measurements obtained by
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analysis of variance showed that the model developed and all the proposed parameters have
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significant effects on the flux. However, the CNMs/PAC emerged as the most significant
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influential factor and interacted with polymer concentration and casting knife thickness to exert effects on the permeate flux. The optimum preparation parameters were 775.21 mg carbon
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loading, PVDF-HFP concentration of 21.86 g and casting knife thickness of 118.93 µm, as these
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values yield the highest flux of about 102 kg/m2h.
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Keywords: Membrane preparation; membrane distillation; surface characterization; phase inversion; carbon nanomaterials; central composite design; optimization.
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1. Introduction Membrane distillation (MD) is a thermally-driven distillation process as a part of which only vapor molecules are transported through a hydrophobic microporous membrane [1-3]. While
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many MD configurations are in use, owing to its simplicity, direct contact membrane distillation process (DCMD) is the most common. It comprises of a partial evaporation step then vapor transmission through the membrane surface, followed by condensation step that takes place within the membrane[4]. To date, DCMD has been used in seawater and brackish water
desalination [5, 6]. In order to achieve high vapor permeability, high salt rejection and good
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fouling surface resistance, membrane characteristics can be controlled by modifying the
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parameters affecting membrane preparation, thus ensuring the most appropriate pore size,
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porosity, hydrophobicity and fouling resistance [7]. As its name suggests, in the DCMD, the hot
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feed side to be treated must be in direct contact with the membrane, without penetrating into the membrane’s dry pores [8, 9]. One of the basic requirements for DCMD with the feed component
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is membrane hydrophobicity, as membrane pore wetting must be avoided to achieve optimal
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results.
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High flux can be achieved by embedding carbon-based nanomaterials (CNMs) into the polymer solution, as this enhances the hydrophobic character of the hybrid membrane [10-12].
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Due to their unique properties, thermal stability and chemical, conductive properties, CNMs are among the most promising advanced materials. Thus, they have been proposed as a suitable
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candidate for increasing the permeate flux [13, 14]. The hydrophobic properties of any surface strongly depend on its geometric features and chemistry [15, 16]. CNMs with their unique properties can contribute to porosity, pore size and hydrophobicity improvements [11].
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To attain many desirable properties, such as porous membrane structure, mechanical characteristics and high flux, the experimental design (DoE) employed in the membrane preparation process must be carefully tuned for its optimum conditions [17]. The DoE for
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composites membrane (CM) preparation seems to be the best method for optimizing the fabrication conditions and minimizing the number of experimental runs [18]. To understand the effects of different parameters and fabrication variables in the DCMD process, response surface methodology (RSM) is typically employed. This method is also used for developing predictive
models based optimization of DCMD-modified CMs by different techniques [19]. This approach
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can also be used to elucidate the link between the membrane fabrication parameters and the
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performance of the prepared CMs in DCMD [20].
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CNMs are increasingly used as additives to different types of polymer-based membranes. The
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most commonly used CNMs are carbon nanofibers (CNFs), carbon nanotubes (CNTs) and graphene oxide (GO) [21-24]. Carbon nanomaterials/powder activated carbon (CNMs/PAC) has
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recently emerged as a cost-effective method based on acetylene as the carbon source[25]. CNMs
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are grown above the PAC surface resulting in hybrid nanocarbon with excellent properties [26-
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28]. Thus far, in the current work, CNMs/PAC has been used as embedded material with PVDFHFP to improve the membrane properties, especially the hydrophobic character. Available data
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indicate that CNMs/PAC exhibits very high contact angle (CA). Thus, it can be used as an additive to improve membrane hydrophobicity and therefore enhance the permeation flux [29].
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Owing to these beneficial properties, extensive research has been conducted on flux enhancement. On the other hand, optimization of the preparation parameters remains insufficiently explored. Therefore, in the current work, various membrane preparation parameters were studied, namely polymer concentration, CNMs/PAC loading, and casting knife
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thickness, to improve the membrane performance. Moreover, the membrane fabrication process was statistically optimized using DoE, focusing on understanding the key aspects of CNMs/PAC behavior on the membrane surface, rather than solely aiming to achieve the maximum possible
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flux. All samples were prepared as flat sheets using the phase inversion methodology and were implemented in DCMD design. Finally, the experimental results were analyzed to identify the
influence of different parameters and to identify the optimal CM flat-sheet production conditions
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for the DCMD system.
Materials
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2.1.
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2. Experimental Details
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CNMs/PAC were prepared based on the previously established optimum conditions and the method developed earlier [29]. The poly(vinylidene fluoride-co-hexafluoropropylene)
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(PVDF-HFP) polymer was used as a matrix while the solvent was N-methyl-2-pyrrolidinone
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(NMP, anhydrous, 99.5%, of 1.03 g/ mL density). Both were purchased from Sigma-Aldrich.
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Sodium chloride (NaCl 99.5%) salt used in synthesizing aqueous solutions was also purchased from Sigma-Aldrich.
Membrane preparation methodology
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2.2.
All flat-sheet CMs (A1–A11) examined in the present study were prepared by the phase
A
inversion method, following the statistical approach provided within the Design Expert (Version 7) software. The central composite design (CCD) statistical method was chosen to optimize the response and study the interactions of three preparation parameters with the aim of minimizing the required number of experimental runs. A regression statistical model was derived for use in
6
the optimization study. The CMs preparation procedure commenced by homogeneously dispersing different amounts of CNMs/PAC (following the DoE procedure) in NMP for 1 h with an ultrasonic processor (UP400S, 24 kHz) until optimal distribution is attained. The thus
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obtained granular PVDF-HFP and NMP-CNMs/PAC suspension was dispersed at different concentrations using a magnetic stirrer at room conditions, followed by an ultrasonic process (based on DoE, as given in Table 1). The composite mixture was left for 24 hours under lab
conditions to completely remove all trapped air bubbles before casting it (using a casting knife of 100−300 µm thickness, based on DoE) on a glass support on the casting machine. Next, the CMs
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were immersed in a deionized water bath until the sample could be peeled off from the glass
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surface. After precipitation, the membrane samples were placed in another deionized water bath
A
for 24 h to complete their formation and to ensure that all residual solvent traces were removed.
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Finally, the CMs were dried in air for two days in order to determine the optimum model. 2.2.1. Membrane structure analysis
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The prepared flat-sheet CMs (coded A1–A11 in Table 1) were subjected to scanning
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electron microscopy (SEM) to determine their structure. The Jeol JSM-5510 (Jeol Ltd., Japan)
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SEM was used in this study. The CM samples (1 cm2) were prepared for SEM analysis by fracturing in liquid nitrogen before securing them to a support holder for sputter coater chamber,
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and were coated homogeneously with a thin layer of platinum.
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2.2.2. Membrane surface analysis The CM top surfaces were examined by atomic force microscopy (AFM). Different
image perspectives were obtained over different locations on the CM surface of each sample by using a tapping mode Nanoscope IIIa. The topography and phase signals were scanned simultaneously for all CM surfaces with the colloidal probe. The surfaces of the prepared flat-
7
sheet CMs were characterized in terms of the average surface roughness (Rave) and pore size distribution. The recorded data were processed by Gwyddion software. An scan area of 10 µm × 10 µm (in X and Y direction) was used while the Z-axis was set to 1.0 µm for determining the
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average pore size based on the AFM top and valley distribution for at least 30 measurements by Gwyddion software. Average roughness (Rave) was obtained by applying the following expression: 𝑅𝑎𝑣𝑒 =
1 𝐿 ∫ |𝑍 (𝑋)| 𝑑𝑥 𝐿 𝐷
(1)
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where Z(x) describes the surface profile analyzed in terms of height distribution (Z) and position
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2.2.3. Surface water wettability measurements
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(X), and L is the length of each CM sample.
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The water contact angle (CA) of the CM surface was measured via the sessile drop method, using a goniometer (DSA20E KRÜSS GmbH apparatus, Germany). First, 4 µl of
D
distilled water was deposited on the surface of the tested specimens (2.5 cm × 2.5 cm) and the
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contour of the drop was recorded by using an automatic interfacial tension-meter. The contact angle on the CM surface was determined by performing at least 5 measurements in different
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areas of each sample.
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2.2.4. Porosity measurements The prepared flat-sheet CM porosity ε (%) of each sample was determined using the
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gravimetric method, as defined by the following equation: 𝜀(%) = (𝜔1 − 𝜔2 )/(𝐴 × 𝐼 × 𝑑𝑤 ) × 100
(2)
where 𝜔1 is the wet CM sample weight; 𝜔2 is the dry CM sample weight; A is the CM effective area (m2), dw is the distilled water density (0.998 g/cm3) and I is the CM thickness (m).
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2.3.
Chemical structure analysis of the prepared composite membranes The functional groups that resulted from incorporating CNMs/PAC into the membrane
matrix were studied using Attenuated Total Reflection-Fourier Transform Infrared spectroscopy
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(ATR-FTIR) and the spectra were recorded on a NICOLET 510P spectrometer. The CM structure was recorded at wavelengths ranging from 4000 to 500 cm−1. 2.4.
Mechanical properties
Tensile strength test measurements of the CMs at room temperature were carried out using an
Instron 5940 tensile test machine. All samples were secured and held at both membrane ends and
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were pulled at a constant elongation velocity 30 mm/min for the tension test using a 20 mm
Membrane performance measurements in DCMD
A
2.5.
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gauge length. The tensile stress-strain and Young's modulus were evaluated.
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DCMD performance measurements for all CMs (A1−A11) were performed in a lab-scale DCMD unit shown in Figure 1. The CM was placed inside a stainless steel membrane module with an
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effective membrane area of 3.4 cm2. Synthetic salt water was used as a hot feed solution comprising of 35 g/L NaCl and distilled water on the permeate stream side. Two peristaltic
EP
pumps were used to control both the feed and the permeate flow rate. The two streams were pumped in recirculation cycle mode through heat and cool stream exchangers. In all CM
CC
samples, salt concentrations were maintained at a similar level. The system was operated with a feed side flow rate of 22 mL/min and a permeate side flow rate of 12 mL/min. The specific
A
reason for choosing low feed flow rate was to prevent exceeding the liquid entry pressure (LEP) and in turn overcome the wettability phenomenon of the membrane [30, 31]. The temperature was continuously controlled at 60 °C on the hot feed side and at 22 °C on the permeate side. Electrical conductivity, as well as water level transferred to the permeate side, were monitored
9
over time. Each CM sample was tested in three replicates and the average value of the DCMD performance was calculated.
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3. Results and Discussion 3.1. FT-IR composite membrane FT-IR analysis is a particularly useful method extensively used for characterization
studies of the crystalline phases and functional groups attached to the polymer. The chemical structure of the CM was compared to that of a pristine polymer membrane (PVDF-HFP), as
shown in Figure 2. According to the presence of transmittance apices at 528, 614, 760, 795, 840,
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876, and 973 cm−1, the spectra-FTIR indicated crystalline phase predominance that may be
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attributed to the changes in the polymeric materials during the phase inversion in membrane
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fabrication and this phenomenon has been observed by many researchers[32, 33]. However,
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transmittance apices at 974, 1070, 1180, 1280 and 1400 cm−1 suggest existence of polymer– solvent NMP interactions [32]. A peak clearly seen at 1640 cm-1 is due to C꞊O stretching
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vibrations, whereas weak and strong peaks at 2853 and 2925 cm−1, respectively, likely arise
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from–CH and CH2 stretching [34]. In general, CNM is contaminated by amorphous carbon
EP
during the growth process in CVD, which is typically oxidized by air to produce a few functional groups. Generally, the oxy-functional groups would correspond to more negative values, as seen
CC
in the CA measurement discussed below.
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3.2.
Scanning electron microscopy The SEM images of all CM flat sheet samples (A1–A11) are shown in Figure 3. The
cross-section micrographs reveal different structures that represent different polymer concentrations, CNMs/PAC loading levels and casting knife thicknesses, as suggested by the DoE. Figure 3 reveals the crucial impact of polymer concentration increase in the membrane
10
matrix on the reduction in the number of cavities in the membrane. The cross-section micrographs reveal also that the CNMs/PAC is well-dispersed in the membrane matrix even with high CNM loading. All examined CMs have finger-like void structure layers at the top, and a
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sponge-like void structure layer at the bottom. Figure 3 shows the finger-like cavities became narrower and shorter in the cross-section of membranes A2 and A3. While, in agreement with
other researchers [35-37], the low concentration polymer led to reduction in the width and length of the finger-like as can be seen in the cross-section of membranes A5 and A7. Moreover, the
free macro-voids increased in size due to greater CNMs/PAC matrix loading. In a cross-section
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of membranes A1, A4 and A5, it can be seen that the finger-like structure in the top layer is
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wider than the remaining components and the free micro-voids are increased with the increasing
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the amount of CNMs/PAC in the polymer solution. The finger-like pore structure transformed to
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spherical nano-voids and porosity was increased, as shown in the cross-section of membrane A9 due to high CNMs/PAC loading. This amount of loading has made the finger-like structure layer
D
separated from the sponge-like pore structure layer, as shown in the cross-section of membranes
TE
A4 and A9. Although similar amount of CNMs/PAC was utilized for samples A5 and A7, the
EP
separation of layers could not be seen as in A4 and A9 because of the difference in the polymer concentration. CNMs/PAC loading not only affected the configuration of finger-like voids but
CC
increased there number as well. This is obvious when the amount of CNMs/PAC increases up to 1000 mg. By comparing the top layer of pristine PVDF-HFP cross-section with that of the
A
prepared CMs, it can be noted that the finger-like structure becomes wider with increased CNM loading into the matrix with more free micro-voids. Thus, it can be concluded that the CNMs/PAC loading results to a top membrane layer with a finger-like structure because of the superhydrophobicity of the carbon of CNMs, and sponge-like structure at the bottom. The
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increment in viscosity with the increase in polymer concentration should be hindered or delay the penetration of the NMP through membrane coagulation [9, 35, 38-40]. One of the obvious effects of the lower casting knife thickness in the membrane is the change in the configuration of
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the cavities where it appears in the cross section longer and narrower. The mechanism of this change can be explained by the fast evaporation behavior of the solvent during the late stage of the coagulation through the phase inversion process as shown clearly in the cross-section of membranes A1 and A7. 3.3.
Atomic force microscopy
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The three-dimensional AFM images reveal the surface morphology of the prepared flat-
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sheet membranes (A1–A11). It can be seen in Figure 4 that the nodules aggregate at the surfaces
A
with high CNM/PAC loading. In contrast, the less CNM/PAC loading the less aggregate found
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at the surface. Furthermore, the nodule alignment is more pronounced as the polymer concentration increases. However, this nodule alignment is observed clearly at the surfaces of the
D
flat-sheet membranes A1, A4, A6 and A9. The nodule surface size and distribution are lower at
TE
low CNM/PAC loading, likely due to the mitigation of the superhydrophobic CNMs that takes
EP
place at the top surface of the membranes during the coagulation of the phase inversion process. Moreover, the delay because of the CNMs/loading through the coagulation process results in
CC
nodule agglomerates, as shown in membrane A3 and differ from A2. On the other hand, samples A5−A7 contain low CNMs/PAC volume and low polymer concentration (20%) with 300 µm
A
thickness, which led to a small nodule size. It can be seen in Figure 5 that polymer concentration and different solvent amounts led to different membrane thicknesses after the phase inversion process. This is related to the nodule size and distribution, as observed in A4 and A9, which have the same amount of CNMs/PAC. Also, obvious nodule alignment was found at the surface of
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membranes A4 and A9, due to the high polymer concentration in the casting solution in addition to the high loading of CNM/PAC. Increasing polymer concentration led to delay solvent/nonsolvent exchange rate during the membrane formation which in turn results to uniform nodule
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size or nodule alignment at the surface. Although nodule size and roughness were higher at the CM surfaces than at those of the pristine PVDF-HFP membranes, the surface roughness between the highest peaks and lowest valleys of the CMs A1–A11 was determined, as shown in Figure 6. The surface roughness is strongly correlated with the Z-value and the membrane cross-section at different CNMs/PAC loadings, which were tested in three different regions (lines A, B and C in
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Figure 7) to describe the variation of height across the membrane and pore diameter. The results
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showed that the height decreased from ̴ 250 to ̴100 µm. The AFM results revealed that high
Porosity and pore size
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3.4.
A
CNMs/PAC loading leads to a smoother surface than low loading.
It is well known that polymer concentration has strong impact on the membrane surface
D
porosity. A comparison with the pristine membrane revealed that the prepared CMs possessed an
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excellent porosity, which was 80–93% higher than the value measured for the pristine PVDF-
EP
HFP (66%) [9, 35]. Moreover, as shown in Figure 8, high CNM loading resulted in greater porosity for samples A1, A4, A5 and A9, which can be attributed to the macro-void formation
CC
observed in same samples, this comply with the observation of SEM images (Figure 3). Porosity is affected by interaction of several parameters, such as the CNMs/PAC loading, PVDF-HFP
A
concentration and interactions established with the casting solution during the coagulation process [30]. Membrane pore size is one of the most important factors for increasing the water flux. For the tested membranes, it increased as the CNMs/PAC content was increased up to 1000 mg, as shown in Figure 9. The best choice for MD pore size, according to the pertinent literature,
13
is 0.2–0.5 µm [41]. Therefore, the samples A2, A5, and A7 were selected for performance evaluation due to exhibiting high flux, whereas A8 and A2 had the lowest and the highest pore size, respectively. It is well known that the effect of interactive tension between the polymer and
3.5.
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solvents during the polymerization process affects membrane porosity and pore size [35, 42, 43]. Contact angle measurement
The hydrophobic properties of a membrane are strongly dependent on the fabrication process and the geometric features of the resulting surfaces [44]. Hydrophobic surface is generally
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assessed in terms of its contact angle (CA). It can be seen in Figure 10 that the CA of all CM
N
samples increased as a result of adding the superhydrophobic CNMs/PAC. The increase in the
A
CA indicates that the superhydrophobic CNMs/PAC can improve the hydrophobicity of the
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membrane surface relative to the pristine PVDF-HFP, for which the CA was reported to be 90.1° ± 1.7° [45]. The CA of prepared CMs reached the maximum of 125.6o ± 1.9° (measured for A5,
D
which had a high concentration of superhydrophobic CNM/PAC). However, despite having the
TE
same amount of superhydrophobic CNM/PAC, sample A9 has low CA because of high polymer
EP
concentration and greater thickness, which led to the agglomeration of CNMs/PAC in the membrane structure. As a consequence, high polymer concentration prevents good CNM/PAC
CC
dispersion, as confirmed by 3D images of A5 and A9. It is well-known that, in composite materials, roughness occurs due to filler accumulation on the surface. In our case, the filler is
A
superhydrophobic CNMs/PAC, which increases the total surface area and consequently the CA. 3.6.
Mechanical properties of CMs CM mechanical properties are a good indicator of ability to resist excessive pressure.
Therefore, the characteristics of the prepared CMs were evaluated in this work. Table 2 shows the mechanical properties of all the samples of CMs suggested by DoE. The tensile speed of 140 14
mm/min was exerted on all membrane dog-bone specimen samples at room conditions. The results reported in Table 2 indicate that CNMs/PAC loading maximizes the advantage of effective superhydrophobic CNMs/PAC reinforcement by adding higher strength for the
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mechanically weak PVDF-HFP. It can be seen that low CNMs/PAC loading leads to high tensile strength, as shown in Table 2 for samples A1, A2, A6 and A7. Low polymer concentration
enhanced the CMs with a decrease in agglomeration of CNMs/PAC in the membrane. Moreover, the SEM images confirmed the mechanical properties of A4, A5 and A8, indicating that the
membrane macro-void structure had a distinct effect on membrane strength, as shown in Table 2
3.7.1. Statistical analysis and modeling
N
Statistical analyses of the prepared composite membranes
A
3.7.
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for same samples.
M
Preparation parameters, such as polymer concentration, CNMs/PAC loading and casting
D
knife thickness, of all flat-sheet CMs were optimized using the method of multiple variables to
TE
study the effects of these parameters on flux, as shown in Table 3. Using DoE, quadratic models describing flux were developed. The regressed model was tested statistically for its significance
EP
using the analysis of variance (ANOVA) technique. The fabrication conditions of superhydrophobic CMs were used as the independent variables, while the flux was set as the
CC
model response variable. The regression equation for flux is given below:
A
𝐹𝑙𝑢𝑥 = 1/(+0.013 + 0.016 𝐴 + 0.025 𝐵 + 9.227 × 10−3 𝐶 + 9.205 × 10−3 𝐴𝐵 + 4.750 × 10−3 𝐴 𝐶 + 5.351 × 10−3 𝐵𝐶 + 0.014 𝐴2 + 0.021𝐵 2 − 6.663 × 10−3 𝐶 2 ) (3)
where A represents CNMs loading, B denotes PVDF-HFP concentration, and C is the casting knife thickness.
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As the P-values were less than 0.05, the CCD model predictions are statically significant and all individual variable effects are pronounced as shown in Table 4. Among the three fabrication parameters studied, only the second-order interaction effect of CNMs/PAC loading
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with of PVDF-HFP concentration (AB) was significant, and emerged as the major determinant of flux. The second-order effects of CNMs loading (A2), PVDF-HFP concentration (B2) and the interaction effect of casting knife thickness (C2) were all insignificant.
The F-value of 548.14 indicates that the prediction model is significant. The values of
Prob > F greater than 0.1 indicated that the model is not significant. It is clearly observed that the
U
coefficient of determination (R2 = 0.9998) of the quadratic model was significant, since its
N
probability Prob > F value was 0.0331 and was thus below 0.05. A comparison of the actual
A
results with the model-predicted values is depicted in Figure 11, which shows that there is a good
M
agreement between the predicted and actual values, as indicated by the coefficient of determination (R2) being close to unity.
D
3.7.2. Effects of CNMs/PAC loading, PVDF-HFP concentration and casting knife
TE
thickness
EP
The effects of interaction parameters on flux were graphically illustrated by CCD curves, as shown in Figure 12. The interaction of the three CM parameters was examined by DCMD and
CC
the optimum level of each parameter for maximum response was determined accordingly. It can be seen in Figure 3 that CNMs/PAC loading and PVDF-HFP concentration at constant casting
A
knife thickness had a strong impact on the flux, as shown in Figure 12(a). It can be thus concluded that the interaction between polymer concentration and CNMs/PAC loading was significant. It is well-known that the polymer concentration in casting solution plays a crucial role in the membrane morphology, as it controls solution viscosity, which in turn affects
16
membrane performance [5]. Figure 12(b) indicates a notable increase in flux with a decrease in membrane thickness and a slight increase in CNMs/PAC loading [46]. Similarly, decreasing PVDF-HFP loading results in a greater flux at low membrane thickness [47]. The highest value
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of flux is obtained for the lowest PVDF-HFP loading and thickness, as shown in Figure 12(c). CM fabrication by phase inversion method was optimized simultaneously using a CCD method. All membrane preparation parameters and response were incorporated as variables in the CCD method to eliminate the number of experimental tests and satisfy all the criteria for optimum flux. The ultimate goal of adding superhydrophobic CNMs/PAC is to maximize the vapor
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permeate flux by enhancing the membrane surface hydrophobicity. Multiple parameter
N
combinations were evaluated in order to identify the optimum conditions for CM preparation and
A
these were listed according the value of module desirability order. Table 5 lists the constraints
M
developed to control the optimization solutions of the CCD software. The optimum preparation conditions for the highest desirability were a carbon loading CNMs/PAC of 775.21 mg, PVDF-
D
HFP concentration of 21.86 g, and casting knife thickness of 118.93 µm, which gave the highest
TE
flux of about 102 kg/m2h. These values were used in subsequent experimental tests and the
EP
predicted maximum flux was obtained. Specifically, we prepared the membrane A12 according to the optimization results
CC
mentioned above. The morphological properties of A12 are presented in Figure 13. It can be observed that the SEM cross-section of A12 is characterized by finger-like structures at the top
A
layer with a thin sponge-like layer at the bottom, whereas in 2D AFM image, the top surface exhibits high average roughness (55.4) and large pore size (55.1 µm). Moreover, the CA of the A12 membrane surface is 128°±1.4°. These results confirm a remarkable agreement between the predicted flux and that obtained experimentally.
17
4.
Conclusion
Composite membrane fabrication based on different morphological parameters was performed for DCMD process. The CCD was conducted in order to elucidate the effect of the main
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fabrication parameters using the phase inversion technique in flat sheet form. Three membrane preparation variables were considered, namely CNMs/PAC loading in the casting solution,
polymer concentration, and casting knife thickness. The effect of these variables on the structural characteristics of the CMs was studied and the effect on the performance was examined through the DCMD. The superhydrophobic CNMs/PAC improved the hydrophobicity of the membrane
U
surface compared to the pristine PVDF-HFP. The optimum fabrication parameters were carbon
N
loading CNMs/PAC of 775.21 mg, PVDF-HFP concentration of 21.86 g, and casting knife
A
thickness of 118.93 µm resulted in a membrane with characteristics of high average roughness
M
(55.4), large pore size (55.1 µm) and CA (128°±1.4°). The results also confirm that the optimum prepared CMs possess excellent porosity of 93% higher than the pristine PVDF-HFP. The
Acknowledgment
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maximum flux achieved was 102 kg/m2h and the salt rejection was higher than 99.9%.
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The authors would like to acknowledge the National Chair of Materials Sciences and Metallurgy,
CC
University of Nizwa, Oman and surface science lab, department of physics, college of science, Sultan Qaboos University and the University of Malaya UMRG RP044D-17AET for funding this
A
research.
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References [1] S. Al-Obaidani, E. Curcio, F. Macedonio, G. Di Profio, H. Al-Hinai, E. Drioli, Potential of membrane distillation in seawater desalination: thermal efficiency, sensitivity study and cost
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estimation, Journal of Membrane Science, 323 (2008) 85-98. [2] M. Elimelech, W.A. Phillip, The future of seawater desalination: energy, technology, and the environment, science, 333 (2011) 712-717.
[3] A. Alklaibi, N. Lior, Membrane-distillation desalination: status and potential, Desalination, 171 (2005) 111-131.
U
[4] T.Y. Cath, V.D. Adams, A.E. Childress, Experimental study of desalination using direct
N
contact membrane distillation: a new approach to flux enhancement, Journal of Membrane
A
Science, 228 (2004) 5-16.
M
[5] H. Attia, S. Alexander, C.J. Wright, N. Hilal, Superhydrophobic electrospun membrane for
D
heavy metals removal by air gap membrane distillation (AGMD), Desalination, 420 (2017) 318-
TE
329.
[6] S. Srisurichan, R. Jiraratananon, A. Fane, Mass transfer mechanisms and transport resistances
CC
194.
EP
in direct contact membrane distillation process, Journal of Membrane Science, 277 (2006) 186-
[7] K.Y. Wang, S.W. Foo, T.-S. Chung, Mixed matrix PVDF hollow fiber membranes with
A
nanoscale pores for desalination through direct contact membrane distillation, Industrial & Engineering Chemistry Research, 48 (2009) 4474-4483. [8] K.W. Lawson, D.R. Lloyd, Membrane distillation, Journal of membrane Science, 124 (1997) 1-25.
19
[9] Q.F. Alsalhy, K.T. Rashid, S.S. Ibrahim, A.H. Ghanim, B. Van der Bruggen, P. Luis, M. Zablouk, Poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) hollow fiber membranes prepared from PVDFcoHFP/PEG600Mw/DMAC solution for membrane distillation,
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Journal of Applied Polymer Science, 129 (2013) 3304-3313. [10] M. Spasova, N. Manolova, N. Markova, I. Rashkov, Tuning the properties of PVDF or
PVDF-HFP fibrous materials decorated with ZnO nanoparticles by applying electrospinning alone or in conjunction with electrospraying, Fibers and Polymers, 18 (2017) 649-657.
[11] V. Kochkodan, N. Hilal, A comprehensive review on surface modified polymer membranes
U
for biofouling mitigation, Desalination, 356 (2015) 187-207.
N
[12] S.A. Rahman, K.T. Rashid, Q.F. Alsalhy, Improvement of PVDF-co-HFP Hollow Fiber
A
Membranes for Direct Contact Membrane Distillation Applications, Indian Journal of Science
M
and Technology, 10 (2017).
[13] K. Wang, A.A. Abdalla, M.A. Khaleel, N. Hilal, M.K. Khraisheh, Mechanical properties of
D
water desalination and wastewater treatment membranes, Desalination, 401 (2017) 190-205.
TE
[14] P. Goh, A. Ismail, B. Ng, Carbon nanotubes for desalination: Performance evaluation and
EP
current hurdles, Desalination, 308 (2013) 2-14. [15] L. Zhu, P. Shi, J. Xue, Y. Wang, Q. Chen, J. Ding, Q. Wang, Superhydrophobic Stability of
CC
Nanotube Array Surfaces under Impact and Static Forces, ACS Applied Materials & Interfaces, 6 (2014) 8073-8079.
A
[16] M. Ma, R.M. Hill, Superhydrophobic surfaces, Current opinion in colloid & interface science, 11 (2006) 193-202. [17] M.K. AlOmar, M.A. Alsaadi, M.M. Aljumaily, S. Akib, T.M. Jassam, M.A. Hashim, N, NDiethylethanolammonium chloride-based DES-functionalized carbon nanotubes for arsenic
20
removal from aqueous solution, DESALINATION AND WATER TREATMENT, 74 (2017) 163-173. [18] M. Khayet, C. Cojocaru, M.d.C. García-Payo, Experimental design and optimization of
membrane science, 351 (2010) 234-245.
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asymmetric flat-sheet membranes prepared for direct contact membrane distillation, Journal of
[19] D. Cheng, W. Gong, N. Li, Response surface modeling and optimization of direct contact membrane distillation for water desalination, Desalination, 394 (2016) 108-122.
[20] M. Khayet, Membranes and theoretical modeling of membrane distillation: a review,
U
Advances in colloid and interface science, 164 (2011) 56-88.
N
[21] Y. Manawi, V. Kochkodan, M.A. Hussein, M.A. Khaleel, M. Khraisheh, N. Hilal, Can
A
carbon-based nanomaterials revolutionize membrane fabrication for water treatment and
M
desalination?, Desalination, 391 (2016) 69-88.
[22] M. Bhadra, S. Roy, S. Mitra, Desalination across a graphene oxide membrane via direct
D
contact membrane distillation, Desalination, 378 (2016) 37-43.
TE
[23] H. Qiu, Y. Peng, L. Ge, B.V. Hernandez, Z. Zhu, Enhanced anti-fouling property in
EP
membrane distillation by pore channel surface modification, Applied Surface Science, 443 (2018) 217-226.
CC
[24] L. Eykens, K. De Sitter, C. Dotremont, L. Pinoy, B. Van der Bruggen, Membrane synthesis for membrane distillation: A review, Separation and Purification Technology, 182 (2017) 36-51.
A
[25] H.M. Alayan, M.A. Alsaadi, A. Abo-Hamad, M.K. AlOmar, M.M. Aljumaily, R. Das, M.A. Hashim, Hybridizing carbon nanomaterial with powder activated carbon for an efficient removal of Bisphenol A from water: the optimum growth and adsorption conditions, Desalination and water treatment, 95 (2017) 128-143.
21
[26] Y.M. Ahmed, S. Abdullah-Al-Mamun, R. Ma'an Fahmi, A. Iameel, M.A. AlSaadi, Study of Pb Adsorption by Carbon Nanofibers Grown on Powdered Activated Carbon, Journal of Applied Sciences, 10 (2010) 1983-1986.
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[27] A. Mamun, Y. Ahmed, S. Muyibi, M. Al-Khatib, A. Jameel, M. AlSaadi, Synthesis of carbon nanofibers on impregnated powdered activated carbon as cheap substrate, Arabian Journal of Chemistry, 9 (2016) 532-536.
[28] M.A. AlSaadi, A. Al-Mamun, S.A. Muyibi, M.Z. Alam, I. Sopyan, M.A. Atieh, Y.M.
Ahmed, Synthesis of various carbon nanomaterials (CNMs) on powdered activated carbon,
U
African Journal of Biotechnology, 10 (2011) 18892-18905.
N
[29] M.M. Aljumaily, M.A. Alsaadi, R. Das, S.B.A. Hamid, N.A. Hashim, M.K. AlOmar, H.M.
A
Alayan, M. Novikov, Q.F. Alsalhy, M.A. Hashim, Optimization of the Synthesis of
M
Superhydrophobic Carbon Nanomaterials by Chemical Vapor Deposition, Scientific Reports, 8 (2018) 2778.
D
[30] T.L. Silva, S. Morales-Torres, J.L. Figueiredo, A.M. Silva, Multi-walled carbon
TE
nanotube/PVDF blended membranes with sponge-and finger-like pores for direct contact
EP
membrane distillation, Desalination, 357 (2015) 233-245. [31] M. García-Payo, M. Essalhi, M. Khayet, Effects of PVDF-HFP concentration on membrane
CC
distillation performance and structural morphology of hollow fiber membranes, Journal of Membrane Science, 347 (2010) 209-219.
A
[32] X. Tian, X. Jiang, Poly (vinylidene fluoride-co-hexafluoropropene)(PVDF-HFP) membranes for ethyl acetate removal from water, Journal of hazardous materials, 153 (2008) 128-135.
22
[33] S. Ramesh, O.P. Ling, Effect of ethylene carbonate on the ionic conduction in poly (vinylidenefluoride-hexafluoropropylene) based solid polymer electrolytes, Polymer Chemistry, 1 (2010) 702-707.
SC RI PT
[34] F. Avilés, J. Cauich-Rodríguez, L. Moo-Tah, A. May-Pat, R. Vargas-Coronado, Evaluation of mild acid oxidation treatments for MWCNT functionalization, Carbon, 47 (2009) 2970-2975. [35] S. Fadhil, T. Marino, H.F. Makki, Q.F. Alsalhy, S. Blefari, F. Macedonio, E. Di Nicolò, L. Giorno, E. Drioli, A. Figoli, Novel PVDF-HFP flat sheet membranes prepared by triethyl
phosphate (TEP) solvent for direct contact membrane distillation, Chemical Engineering and
U
Processing: Process Intensification, 102 (2016) 16-26.
N
[36] J.T. Jung, J.F. Kim, H.H. Wang, E. di Nicolo, E. Drioli, Y.M. Lee, Understanding the non-
A
solvent induced phase separation (NIPS) effect during the fabrication of microporous PVDF
M
membranes via thermally induced phase separation (TIPS), Journal of Membrane Science, 514 (2016) 250-263.
D
[37] A.K. Hołda, I.F. Vankelecom, Understanding and guiding the phase inversion process for
TE
synthesis of solvent resistant nanofiltration membranes, Journal of Applied Polymer Science,
EP
132 (2015).
[38] Q.F. Alsalhy, S.S. Ibrahim, F.A. Hashim, Experimental and theoretical investigation of air
CC
gap membrane distillation process for water desalination, Chemical Engineering Research and Design, 130 (2018) 95-108.
A
[39] K.T. Rashid, S.B.A. Rahman, Q.F. Alsalhy, Optimum Operating Parameters for Hollow Fiber Membranes in Direct Contact Membrane Distillation, Arabian Journal for Science and Engineering, 41 (2016) 2647-2658.
23
[40] K.T. Rashid, S.A. Rahman, Q. Alsalhy, Hydrophobicity Enhancement of Poly (vinylidene fluoride-co-hexafluoropropylene) for Membrane Distillation, Journal of Polymer Science and Technology, 1 (2015) 1-9.
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[41] M.M.A. Shirazi, A. Kargari, M. Tabatabaei, Evaluation of commercial PTFE membranes in desalination by direct contact membrane distillation, Chemical Engineering and Processing: Process Intensification, 76 (2014) 16-25.
[42] M. Kim, G. Kim, J. Kim, D. Lee, S. Lee, J. Kwon, H. Han, New continuous process
developed for synthesizing sponge-type polyimide membrane and its pore size control method
U
via non-solvent induced phase separation (NIPS), Microporous and Mesoporous Materials, 242
N
(2017) 166-172.
A
[43] Z.-L. Xu, F.A. Qusay, Polyethersulfone (PES) hollow fiber ultrafiltration membranes
M
prepared by PES/non-solvent/NMP solution, Journal of Membrane Science, 233 (2004) 101-111. [44] D.-W. Jeong, U.-H. Shin, J.H. Kim, S.-H. Kim, H.W. Lee, J.-M. Kim, Stable hierarchical
D
superhydrophobic surfaces based on vertically aligned carbon nanotube forests modified with
TE
conformal silicone coating, Carbon, 79 (2014) 442-449.
EP
[45] L. Xie, X. Huang, K. Yang, S. Li, P. Jiang, “Grafting to” route to PVDF-HFP-GMA/BaTiO 3 nanocomposites with high dielectric constant and high thermal conductivity for energy storage
CC
and thermal management applications, Journal of Materials Chemistry A, 2 (2014) 5244-5251. [46] H.Y. Wu, R. Wang, R.W. Field, Direct contact membrane distillation: An experimental and
A
analytical investigation of the effect of membrane thickness upon transmembrane flux, Journal of Membrane Science, 470 (2014) 257-265.
24
[47] L. Eykens, I. Hitsov, K. De Sitter, C. Dotremont, L. Pinoy, I. Nopens, B. Van der Bruggen, Influence of membrane thickness and process conditions on direct contact membrane distillation
A
CC
EP
TE
D
M
A
N
U
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at different salinities, Journal of Membrane Science, 498 (2016) 353-364.
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A
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D
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A
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Figure 1: Schematic diagram of a lab-scale DCMD system used in this work
Figure 2: FTIR spectroscopy of the composite membrane and pristine PVDF-HFP 26
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Figure 3: Cross-section SEM images of the composite membrane flat-sheet membranes prepared by DoE indicated.
27
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A
CC
EP
TE
D
M
A
Continued Figure 3
28
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A
CC
EP
TE
D
prepared by the DoE indicated.
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Figure 4: 3D AFM images of the top surface of the composite membrane flat-sheet membranes
29
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A
CC
EP
TE
Figure 5: Thickness of membrane after phase inversion process.
30
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A
CC
EP
TE
Figure 6: The roughness of composite membrane fabricated through DoE by phase inversion.
31
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A
CC
EP
TE
D
prepared by the DoE indicated.
M
Figure 7: 2D AFM images of the top surface of the composite membrane flat-sheet membranes
32
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TE
Figure 8: The porosity of composite membrane fabricated through DoE by phase inversion.
A
CC
EP
.
33
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TE
Figure 9: The pore size diameter of composite membrane fabricated through DoE by phase
A
CC
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inversion.
34
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A
CC
EP
Figure 10: Contact Angle of composite membrane fabricated through DoE by phase inversion.
35
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A
CC
EP
TE
D
Figure 11: Parity plot of experimental and predicted values of flux.
36
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Figure 12: Response surface plots for the effects of different process parameters on flux: (a) Effect of CNMs loading and concentration of PVDF-HFP; (b) Effect of CNMs loading and
A
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thickness of CM; (c) effect of concentration of PVDF-HFP and thickness of CM.
37
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Figure 13: The flat-sheet membrane A12 prepared using the optimum conditions. (a) SEM for
A
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TE
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top layer of cross-section, (b) for bottom layer, (c) for 3D AFM and (d) 2D AFM.
38
Table 1: Central composite experimental design for preparation of flat-sheet membranes by phase inversion method SAMPLES *100 Ratio of Cnms Loading Polymer to Solvent Casting Knife (mg/gm) Ratio (µM) (%) 100.00 24.00 100.00 A1
A4
1000.00
A5
1000.00
A6
100.00
A7
100.00
A8
1000.00
A9
1000.00
A10
550.00
A11
550.00
300.00
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1000.00
20.00
300.00
24.00
100.00
20.00
100.00
24.00
300.00
20.00
100.00
22.00
200.00
24.00
300.00
22.00
200.00
22.00
300.00
U
A3
20.00
N
100.00
A
A2
2.098
D
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Table 2: Mechanical properties of composite membrane fabricated through DoE by phase inversion. Samples Tensile strength Elongation at break Modulus (mpa) (%) (mpa) 3.601 186.9 63.55 A1 78.7
34.87
3.493
133.1
32.45
2.61
183.7
67.98
2.673
99.37
47.82
1.89
185.7
92.86
A7
2.012
62.05
79.96
A8
2.148
100.3
65.99
A9
2.885
80.89
60.85
A10
3.659
42.35
63.27
A11
2.591
52.49
53.79
TE
A2
A4 A5
A
CC
A6
EP
A3
39
100.00
20.00
300.00
95
A3
1000.00
20.00
300.00
32.1
A4
1000.00
24.00
100.00
A5
1000.00
20.00
100.00
A6
100.00
24.00
300.00
A7
100.00
20.00
100.00
A8
1000.00
22.00
200.00
A9
1000.00
24.00
300.00
A10
550.00
22.00
200.00
78
A11
550.00
22.00
65
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A2
N
Table 3: Experimental design matrix and the value of responses based on experiment run. Samples CNMs/PAC Polymer to Solvent Casting Knife Flux Loading (%) (µm) (kg/m2h) (mg/gm) 31 100.00 24.00 100.00 A1
14 67
19.65 102
U
17
M
A
300.00
9
0.010
A-CNMs/PAC
1.95710-3
B-PVDF-HFP
4.96410-3
EP
P-value prob > F
1.15210-3
548.14
0.0331
1
1.95710-3
931.41
0.0209
1
4.96410-3
2362.03
0.0131
9
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Model
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Table 4: The analysis of variance for response surface quadratic model. Source Sum of Degree of Mean F-value squares Freedom square
6.81210-4
1
6.81210-4
324.12
0.0353
AB
6.77910-4
1
6.77910-4
322.57
0.0354
AC
1.80510-4
1
1.80510-4
85.89
0.0684
BC
2.29110-4
1
2.29110-4
109.01
0.0608
A2
6.02710-5
1
6.02710-5
28.68
0.1175
B2
2.09410-4
1
2.09410-4
99.62
0.0636
C2
2.08910-5
1
2.08910-5
9.94
0.1955
Residual
2.10210-6
1
2.10210-6
A
CC
C-Thickness
40
Table 5: Constraints for optimization process based on CCD for DCMD performance. Name Goal Lower Limit Upper Limit Importance In range
100
1000
3
B
In range
20
24
3
C
In range
100
300
3
FLUX
Maximize
9
102
A
CC
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TE
D
M
A
N
U
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A
41
5