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Simple method for preparing thin film composite polyamide nanofiltration membrane based on hydrophobic polyvinylidene fluoride support membrane
Accepted Manuscript Simple method for preparing thin film composite polyamide nanofiltration membrane based on hydrophobic polyvinylidene fluoride support membrane
Ju Sung Lee, Jin Ah Seo, Hyun Ho Lee, Sung Kuk Jeong, Hyun Sic Park, Byoung Ryul Min PII: DOI: Reference:
S0040-6090(17)30031-7 doi: 10.1016/j.tsf.2017.01.031 TSF 35743
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
23 February 2016 11 January 2017 17 January 2017
Please cite this article as: Ju Sung Lee, Jin Ah Seo, Hyun Ho Lee, Sung Kuk Jeong, Hyun Sic Park, Byoung Ryul Min , Simple method for preparing thin film composite polyamide nanofiltration membrane based on hydrophobic polyvinylidene fluoride support membrane. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tsf(2017), doi: 10.1016/j.tsf.2017.01.031
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ACCEPTED MANUSCRIPT
Simple method for preparing thin film composite polyamide nanofiltration membrane based on hydrophobic polyvinylidene fluoride support membrane
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Ju Sung Lee †,a,b, Jin Ah Seo †,a, Hyun Ho Lee c, Sung Kuk Jeong a, Hyun Sic Park a,
Department of Chemical and Biomolecular Engineering, Yonsei University, 50,
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a
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Byoung Ryul Min a,
b
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Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea Fuel Cell Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro
c
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14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Water Industry Team, Korea Water and Wastewater Works Association, 13, Hwanil-gil,
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Mapo-gu, Seoul, 04199, Republic of Korea
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Abstract
This study attempts to prepare thin film composite (TFC) polyamide (PA) membrane using
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polyvinylidene fluoride (PVDF) as support membrane. PVDF has superior physical characteristics but is difficult to be used as TFC PA support membrane because of its strong hydrophobicity. However, in this study TFC PA membrane was prepared using a simple method of flowing trimesoyl chloride (TMC) organic solution at a specific velocity after putting specific quantity of m-phenylenediamine (MPD) aqueous solution on hydrophobic PVDF support membrane. The presence of polyamide formed by the interfacial polymerization of MPD and TMC were verified with x-ray photoelectron spectroscopy and attenuated total reflectance-fourier transform infra-red spectrophotometer, and field emission scanning †
These authors contributed equally to this work. Corresponding author. Tel.: +82-2-2123-2757; Fax: +82-2-312-6401 E-mail address: [email protected] (B.R. Min)
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electron microscopy and atomic force microscopy were used to observe the TFC PA membrane’s surface and structural characteristics. The prepared TFC PA membrane showed maximum performance at TMC organic solution flow velocity of 1.21 m/s and TMC concentration of 0.3% (w/v). The TFC PA membrane prepared under these optimal conditions showed rejection rates of 68.14, 88.80, and 96.06% against NaCl, CaCl2 Na2SO4 5000 ppm, respectively. This proved that the TFC PA membrane prepared in this study had
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nanofiltration performance.
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Keywords: Polyvinylidene fluoride; Hydrophobic support membrane; Thin films; Composite; polyamide
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membrane; Nanofiltration
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1. Introduction
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The world’s water shortage problem is becoming more severe due to climate change, increase in population, and economic growth, among other reasons. Due to the influence of climate change, an additional 200 million people will suffer water shortage by 2025, while the demand for water will
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increase by 150% in developing countries from 2008 to 2025 due to economic growth [1]. In addition, the
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World Health Organization reported that more than 2.5 billion people will be unable to use sanitation facilities [2]. Among the methods to alleviate this water shortage problem are desalination and recycled
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use of waste and sewer water. [3-6]. In particular, desalination is considered an important field for the production of clean water resources. Major desalination technologies include thermal distillation, forward
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osmosis (FO), membrane distillation, and reverse osmosis (RO) [2, 3]. Of these, RO has higher energy efficiency compared to existing thermal desalination technology, and recent active research has made it possible to prepare produce high-performance membranes with low cost [2, 3]. These advantages have made RO the center of attention in desalination as the latest technology, and also as a technology for the production of clean water [2, 3, 7]. Nanofiltration (NF), which requires relatively low pressure compared with RO, is another technology which can be used for desalination [8]. NF has lower cost and energy usage requirements compared to RO, and also has added advantages such as high rejection rate and permeation flux, leading to applications in organic matter and ion removal, in addition to desalination [9]. In particular, NF removes multivalent ions such as Mg2+ and Ca2+ which exist in hard water, and is 2
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regarded a promising technology in the water softening field for the production of soft water, with numerous related on-going and completed studies in recent years. [10-12] Thin film composite (TFC) polyamide (PA) membrane and cellulose acetate (CA) membrane are primarily used for RO and NF [10, 13-15]. TFC PA membrane is characterized by superior water flux and selectivity compared with CA membrane, and also has wider pH and temperature application ranges [15].
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TFC PA membrane is generally comprised of active layer with support membrane, with the active layer composed of polyamide formed through interfacial polymerization [15, 16]. Traditionally, TFC PA
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membrane has been prepared by contacting support membrane with organic phase after wetting in
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aqueous phase [17]. During this process, the amine in the aqueous phase forms the polyamide through interfacial polymerization with the acid chloride existing in organic phase. Trimesoyl chloride (TMC) is
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widely used as acid chloride, while o-phenylenediamine or m-phenylenediamine (MPD) is used as amine [15]. Relatively hydrophilic porous membrane such as polysulfone (PSf) or polyethersulfone (PES) is used as support membrane for TFC PA membrane [13, 16]. However, in such traditional TFC PA
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membrane preparation methods, hydrophobic materials cannot be used as support membrane. This is because the support membrane has to be wetted in aqueous phase for interfacial polymerization, but
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hydrophobic support membrane cannot be wetted in aqueous phase [17, 18]. Of such hydrophobic
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materials, polyvinylidene fluoride (PVDF) has especially superior mechanical strength, chemical resistance, and thermal stability compared to other commercially widely used polymers [19]. Accordingly, if such hydrophobic PVDF could be used as support membrane for TFC PA membrane, the durability of
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NF and RO membrane could be further improved. For this reason, research groups have conducted studies to use PVDF as support membrane for TFC PA membrane by overcoming its hydrophobic
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properties. Studies that have been reported include: trying to be hydrophilic surface of the PVDF support membrane through the use of plasma or chemical treatment [20, 21], and fabricating TFC PA membrane by forcibly wetting aqueous solution onto hydrophobic PVDF support membrane using the pressure of gas and contacting it with organic solution [18, 22]. Also, some papers have conducted research on TFC membranes based on hydrophobic support membrane of PEI material [23, 24]. Unlike other studies, however, in this study TFC PA membrane was prepared by overcoming the hydrophobic property of PVDF using a simple method without the use of additional processes, materials, or pressure. TMC organic solution was run through PVDF support membrane at a specific velocity with a
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specific amount of MPD aqueous solution on it so that the polyamide would form on the hydrophobic PVDF support membrane. The polyamide formed through interfacial polymerization on PVDF support membrane was characterized with x-ray photoelectron spectroscopy (XPS), attenuated total reflectancefourier transform infra-red spectrophotometer (ATR-FTIR), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM), while the optimal condition for this method was
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confirmed through performance test on CaCl2 5000 ppm. Furthermore, rejection rate and permeation flux tests were conducted against NaCl and Na2SO4 feed solution using TFC PA membrane prepared in
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optimal conditions. The rejection rate performance of the TFC PA membrane prepared in this manner
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demonstrated that the adhesion of the polyamide and PVDF were solid.
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2. Experimental
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2.1. Materials
The PET non-woven used as support for PVDF support membrane preparation was purchased from
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Philos (Korea). Polyvinylidene fluoride (PVDF solef® 6020) was purchased from Solvay Korea (Korea)
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and its average molecular weight was 700,000 (g/mol). The solvent used for the PVDF solution was N,Ndimethylacetamide (DMAc, 99%) purchased from Sigma-Aldrich (USA). m-phenylenediamine (MPD, ≥98.0%) and trimesoyl chloride (TMC, ≥98.0%) purchased from TCI (Japan) were used as monomer for
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interfacial polymerization. Also, sodium hydroxide (NaOH, ≥98.0%) purchased from Samchun Chemical (Korea) was used as catalyst for interfacial polymerization reaction, and hexane (mixture of isomers, 99%)
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purchased from Across Organic (Belgium) was used as solvent of TMC. The sodium chloride (NaCl, 99.5%), calcium chloride (CaCl2, 95.0%), and sodium sulfate (Na2SO4, ≥98.5%) used in the feed solution were purchased from Duksan Pure Chemical (Korea), KANTO chemical (Japan), and Samchun Chemical (Korea), respectively. The N2 gas used for the rejection rate and permeation flux tests and supply of TMC organic solution was purchased from Samheung Gas (Korea). Deionized water (DI) made from equipment from AquaMAXTM (Younglin Instrument, Korea) was used in the production of the feed solutions and MPD aqueous solution.
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2.2. Preparation of support membrane
PVDF support membrane was prepared through non-solvent induced phase separation (NIPS) method. PVDF and DMAc were combined in a 13:87 ratio for the production of PVDF 13% (w/w) solution, and stirred at 100 rpm, room temperature conditions. The completely dissolved PVDF 13% (w/w) solution
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was applied on the PET non-woven on the glass sheet at a thickness of 150 µm using casting bar. The PET non-woven with the PVDF solution applied underwent phase inversion in non-solvent (DI water) in
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coagulation bath without any evaporation time. The PVDF membrane in bath was taken out of the
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coagulation bath after 15 minutes, and rinsed in water bath for 1 day. The PVDF membranes prepared were dried at room temperature for 1 day. The hydrophobicity of PVDF support membrane prepared as film was characterized with a water contact angle instrument (Phoenix 300, SEO, Korea). The water
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2.3. Preparation of TFC PA membrane
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contact angle for PVDF support membrane was very hydrophobic at 103.5˚ (Fig. 1).
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Fig. 2 shows the mechanism by which polyamide forms on the hydrophobic PVDF support membrane.
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As shown in t1 of Fig. 2, MPD aqueous solution initially forms on PVDF support membrane surface in bubble form. Then as the TMC organic solution flows on the MPD bubble, it is gradually pushed and the polyamide formed attaches to the surface of the PVDF support membrane due to the velocity of the TMC
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organic solution. At this time, the hydrophobic PVDF support membrane does not wet in MPD aqueous solution, and so the formed polyamide attaches to the PVDF support membrane surface. Accordingly, it
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may be presumed that the pressure formed as the TMC organic solution flows is the driving force that attaches the polyamide to the support membrane.
To prepare the TFC PA membrane, the apparatus in Fig. 3 was used to flow the TMC organic solution at a specific velocity over the PVDF support membrane with specific amount of MPD aqueous solution on it. The velocity of the TMC organic solution was controlled with gas pressure. A precision timer was used in order to control the time of TMC organic solution flow. Table 1 shows the velocity of the TMC organic solution according to gas pressure. The tests were each repeated six times to obtain precise values.
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In this paper, velocity 0 of TMC solution is defined at the speed where MPD droplet does not move. The polyamide manufactured at velocity 0 took the form of initial droplets. Afterwards, the polyamide was attached to support membrane by dry process.
In order to form the polyamide, MPD 2.0% (w/v) aqueous solution and TMC 0.1, 0.3, 0.6, 0.9% (w/v)
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organic solution were prepared [13, 25]. TMC organic solution was prepared using hexane as solvent [14]. NaOH 0.05% (w/v) was added in the MPD aqueous solution in order to accelerate the interfacial
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polymerization reaction [26].
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MPD aqueous solution was placed on the prepared PVDF support membrane using micro-pipette. The TMC solution was provided at a specific velocity to the PVDF support membrane with a specific quantity
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of MPD aqueous solution on it. (As soon as polyamide was formed through interfacial polymerization reaction between TMC organic solution and MPD aqueous solution, the polyamide attached to the PVDF support membrane surface.) After 20 sec of interfacial polymerization reaction, the remaining MPD
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aqueous solution and TMC organic solution were removed with ethanol and hexane.
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2.4. Characterization of TFC PA membrane
2.4.1. XPS
PVDF support membrane and TFC PA membrane elements were analyzed with XPS (K-alpha,
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Thermo VG, UK) in order to confirm the formation of X-ray popolyamide. The X-ray power was set at 12 kv, 3 mA and the X-ray source was monochromatic Al X-ray sources (Al Kα line: 1486.6 eV). The
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diameter of the measured sample area size was 400 µm, and its pass energy 200 eV with step size of 1 eV.
2.4.2. ATR-FTIR
ATR-FTIR (SPECTRUM 100, Perkin Elmer, USA) was used to confirm polyamide formation on PVDF support membrane. Measurements were made in the wave range of 4000-380 cm-1 through accumulation of 32 scans. PVDF support membrane and TFC PA membrane samples were prepared in phase inverted film form without non-woven for the ATR-FTIR analysis.
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2.4.3. FE-SEM FE-SEM (JSM-7001F, JEOL Ltd., Japan) was used to observe the surface and cross-section of the PVDF support membrane and TFC PA membrane samples. Surface samples were prepared in 50 mm × 50 mm sizes for PVDF support membrane and TFC PA membrane. The TFC PA membrane cross-section sample was prepared by breaking after soaking in liquid nitrogen. As the samples were insulators,
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measurements were made after coating with platinum (Pt) for 110 sec.
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2.4.4. AFM
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Surface characteristics of TFC PA membrane were confirmed with AFM (XE-BIO, Park systems, Korea). Samples of 50 mm × 50 mm area were prepared for equipment measurements, although the actual
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area measured was 5 µm × 5 µm. Average roughness (R a) and root-mean square roughness (Rq), which are surface roughness values, were obtained through AFM analysis [27, 28].
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2.5. Performance test of TFC PA membrane
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Rejection rate and permeation flux tests were conducted on CaCl2 5000 ppm in order to determine the
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optimal TMC organic solution velocity and TMC concentration. Then in order to confirm the performance of the TFC PA membrane according to solute type, rejection rate and permeation flux tests were conducted on NaCl 5000 ppm and Na2SO4 5000 ppm feed solution under optimal TMC organic
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solution velocity and TMC concentration conditions on the TFC PA membrane. The tests were conducted with cross-flow at 30 bar of operating pressure.
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The rejection rate (R) was calculated using the following Eq. (1). Here, CP (ppm) is the concentration of the solute in permeate, while CF (ppm) is the concentration of the solute in feed. Conductivity meter (COM-100, HM digital, Inc., USA) was used to measure the concentration and conductivity of the solute in permeate and feed [29].
(1)
Permeation flux (J) was calculated using the following Eq. (2). Here, Δt (hr) is the test time, while V
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(L) is the permeate amount and A (m2) is the effective area of membrane. (This area was consistent in this study at 0.000314 m2.) [29].
(2)
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3. Results and Discussion
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3.1. Confirmation of polyamide formation
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3.1.1. XPS and ATR-FTIR
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Fig. 4 shows the XPS spectra of the PVDF support membrane and prepared TFC PA membrane. PVDF membrane (Fig. 4(a)) showed C (1S) and F (1S) peaks at 284 eV and 688 eV, respectively. In contrast, TFC PA membrane (Fig. 4(b)) showed C (1S), N (1S), and O (1S) peaks at 284, 399, and 532 eV,
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respectively [30]. Notably, the N (1S) and O (1S) peaks mean that polyamide exists on PVDF support membrane. Results from past studies which confirm the presence of C, N, and O peaks in XPS analysis
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on polyamide support the conclusion that PA has formed well on PVDF support membrane in this study
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[30, 31].
Fig. 5(a) represents the ATR-FTIR spectra of PVDF support membrane. Peaks which indicate the β-
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phase of PVDF appear at 510 cm-1 and 840 cm-1, while those which indicate its α-phase appear at 766 cmand 976 cm-1 [32]. These characteristic peaks of PVDF were confirmed in Fig. 5(a). Fig. 5(b) is the
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ATR-FTIR spectra for the prepared TFC PA membrane. It is known that polyamide formed through interfacial polymerization between MPD and TMC show the following three types of peaks. At 1541 cm-1, a peak which indicates presence of amide II bend appears due to C-N stretch and N-H bend in –CO-NHgroup. Then at 1609 cm-1, a peak which indicates presence of aromatic amide appears due to C=C ring stretch or N-H deformation, and finally, at 1663 cm-1 a peak which indicates presence of amide I bend appears due to C-N stretch and C=O stretch. (The influence of the C=O stretch is known to be especially large.) [30]. In addition to the peaks which represent the α-phase and β-phase of PVDF, the three peaks which indicate presence of polyamide (C-N starch, N-H bend at 1541 cm-1 C=C ring stretch or N-H
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deformation at 1609 cm-1, C=O stretch at 1663 cm-1) were all confirmed with ATR-FTIR spectra (Fig. 5(b)) of TFC PA membrane, through which it could be known that polyamide had been successfully formed on PVDF support membrane. Accordingly, our research group confirmed through XPS and ATRFTIR that chemically active layer of polyamide had been successfully formed.
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3.1.2. FE-SEM and AFM Fig. 6 shows FE-SEM images of PVDF support membrane and TFC PA membrane. Here, pores were
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observed on PVDF support membrane surface (Fig. 6(a)). However, no such pores but rather rough
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surfaces were observed for TFC PA membrane surface (Fig. 6(b)). It is known that the surface of the TFC PA membrane formed through interfacial polymerization reaction between MPD and TMC display rough
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surface and ridge and valley structure [33-35]. Accordingly, the SEM surface image of TFC PA membrane prepared in this study clearly show the characteristics of polyamide formed through interfacial polymerization reaction. Fig. 6(c), (d) are cross-section images of TFC PA membrane. The cross-section
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images of the prepared TFC PA membrane show that the polyamide has successfully attached to the surface of the PVDF support membrane according to the mechanism in Fig. 2. In Fig. 6(d), a polyamide
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layer of dense and non-porous structure is observed. In addition, the thickness of the polyamide was
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confirmed to be 59 nm. It is known that the typical thickness of polyamide formed on TFC PA membrane through interfacial polymerization reaction between TMC and MPD is 100 - 1000 nm [36] and polyamide layer in general has very dense and non-porous structure [37]. It may be presumed that the reason that the
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thickness of the polyamide on the TFC PA membrane prepared in this study is lower than those of other polyamides is the influence of PVDF support membrane, and further research is needed to address this
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difference.
Fig. 7 shows the surface roughness values and surface images of TFC PA membrane measured with AFM. Fig. 7 shows the ridge and valley structure of TFC PA membrane. Here, the Ra and Rq values were 45 nm and 59 nm, respectively, and a study result has been reported showing similar surface roughness values for a case of TFC PA membrane prepared through interfacial polymerization reaction between MPD and TMC [35]. These FE-SEM and AFM results confirm that since the polyamide was formed on PVDF support membrane in this study through interfacial polymerization between TMC and MPD, TFC
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PA membrane prepared displays typical rough structure and surface characteristic of typical TFC PA membranes.
3.2. Performance of TFC PA membrane
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3.2.1. Effect of TMC solution velocity Fig. 8 shows the CaCl2 rejection rate and permeation flux of the prepared TFC PA membrane
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according to TMC organic solution velocity. Here, TFC PA membrane displayed rejection rates of 15.15,
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88.80, 87.64, 82.79% and permeation flux of 20.51, 7.78, 8.03, 8.17 LMH for TMC organic solution velocities of 0, 1.21, 2.68, 3.79 m/s, respectively. When the velocity was 0 m/s, the rejection rate was the
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lowest and the permeation flux the highest. The reason for this phenomenon was that with no TMC organic solution flow, the polyamide could not attach firmly to the PVDF support membrane. In this case, the polyamide is ripped from the prepared TFC PA membrane during the performance test, and feed
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solution passes freely through the pores of the PVDF support membrane. When the velocity was 1.21 m/s, the rejection rate was the highest and the permeation flux the lowest, and it was confirmed that as velocity
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increased from 1.21 m/s, the rejection rate decreased and the permeation flux increased. The presumed
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reason for this tendency is that the flowing TMC organic solution affects the polyamide attached to the PVDF support membrane and causes a defect in the active layer of polyamide and allows feed solution to flow into permeation. When the TMC organic solution velocity was 4.81 m/s, the formed polyamide was
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torn off by the fast-flowing TMC organic solution, and so the TFC PA membrane was not properly prepared. Based on this, it was observed that at TMC organic solution velocity of 1.21 m/s, the polyamide
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formed through the mechanism of this study was firmly attached, while higher speeds caused defects in the polyamide’s attachment and hindered the performance of the TFC PA membrane. Accordingly, it was confirmed that the optimal TMC organic solution velocity for the preparation of TFC PA membrane per the method of this study is 1.21 m/s. Meanwhile, m/s which is an SI unit is a velocity unit of the same dimension as LMH, as 1 m/s is equal to 3.6×106 LMH. That the manufactured TFC membrane did not get torn at velocities under 4.81 m/s (1.7316×10 7 LMH) and displayed NF capabilities can be proof that the adhesion stability between the PVDF support membrane and polyamide layer is very stable.
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3.2.2. Effect of TMC concentration Fig. 9 shows the CaCl2 rejection rate and permeation flux results for the prepared TFC PA membrane according to TMC concentration. Here TFC PA membrane displayed rejection rates of 78.82, 88.80, 72.14, 71.10% and permeation flux of 8.71, 7.78, 6.20, 5.71 LMH for TMC concentrations of 0.1, 0.3, 0.6, 0.9%, respectively. It was confirmed that optimal performance is achieved against CaCl 2 5000 ppm when the
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TMC concentration is 0.3%. It has already been reported that if the concentration of the monomer used in interfacial polymerization is too low, then a weak, thin polyamide layer is formed and the performance of
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the TFC PA membrane is lowered. [38]. Accordingly, the reason that the rejection rate is lower at TMC
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concentration of 0.1% compared to TMC concentration of 0.3% is that the concentration is lower than the minimum needed for strong polyamide formation through interfacial polymerization reaction. Also, at
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fixed MPD concentration levels, as the TMC concentration level rises higher than the optimal TMC concentration, the degree of cross-linking of the polyamide decreases [39]. As a result, the CaCl2 rejection rate decreases as TMC concentration increases from 0.3% to 0.9%. As TMC concentration rises,
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permeation flux has a tendency to decrease, which can be explained through study findings that the thickness of active layer increases with increases in TMC concentration [40]. At this time, the increase in
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membrane thickness leads to transports resistance of membrane, which causes a decrease in the pressure
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difference across the membrane. Finally, water flux and solute flux decrease due to the pressure drop [41]. For this reason, as TMC concentration increased from 0.1% to 0.9%, permeation flux gradually decreased. In summary, it may be presumed that permeation flux is more greatly affected by polyamide thickness
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than by its degree of cross-linking.
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3.2.3. TFC PA membrane performance according to different feed solution Fig. 10 shows the rejection rate and permeation flux of prepared TFC PA membrane against CaCl 2 5000 ppm, NaCl 5000 ppm, and Na2SO4 5000 ppm feed solution at previously determined optimal TMC organic solution velocity and TMC concentration conditions. Here, the rejection rate was in NaCl < CaCl2 < Na2SO4 order with values of 68.14, 88.80, and 96.06%, respectively. Electrolytes composed of largesized ions have difficulty passing through membrane and thus show higher rejection rates [42]. The hydrated ion sizes of Na+, Ca2+, and SO42- are 0.178 nm, 0.260 nm, and 0.300 nm, respectively. As a result, that of Ca2+ and SO42- are much larger than that of Na+. This difference in ion size makes it difficult for
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Ca2+ and SO42- to pass through the polyamide compared to Na + [43]. Furthermore, TFC PA membrane which contains polyamide formed through interfacial polymerization have bipolar characteristic, which makes it additionally difficult for ions to pass through. That is, the outer surface of the polyamide has a negative charge because of the leftover acyl chloride in TMC (The acyl chloride in TMC is converted to carboxylic acid due to hydrolysis reaction [44]. Carboxylic acid also has a large negative charge.), while
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the surface of the polyamide on the support membrane side gets a positive charge because of the leftover amine in MPD, and it has been reported that feed solutions such as Na2SO4 and CaCl2, which contain
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multivalent ions as SO42- and Ca2+ have difficulty passing through TFC PA membrane due to a charge
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repulsion effect (multivalent ions of negative charge are repulsed by the negative charge of the polyamide external surface, while those of positive charge are repulsed by the positive charge of the polyamide
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surface on the support membrane side) [45]. Due to the size of the ion and the charge repulsion effect, the rejection rates for CaCl2 and Na2SO4 become higher than for NaCl. Furthermore, the reason that the rejection rate for Na2SO4 is higher than that for CaCl2 is presumably that the effect of the negative
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multivalent ions being repulsed on the outer surface of the polyamide has a greater effect on selectivity than the effect of the positive multivalent ions being repulsed by the polyamide on the support membrane
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side. CaCl2 displayed low permeability compared to Na2SO4, which can be explained by hydrated ion size.
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In the previous study, the crystal radius and hydrated ion radius of Ca 2+ and SO42- have been reported as 0.100 nm and 0.215 nm, and 0.260 nm and 0.300 nm, respectively [46]. In particular, the increase in radius of due to H2O was approximately two times larger for the Ca 2+ (0.160 nm) compared to that of
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SO42- (0.085 nm). This shows that the Ca2+ has strong hold on water [43]. It may be presumed that the
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eater permeability of CaCl2 was relatively lower than that of Na2SO4 for this reason.
4. Conclusions
In this study, polyamide was successfully formed on PVDF support membrane through the use of MPD aqueous solution and TMC organic solution by overcoming the hydrophobic characteristic shortcoming of PVDF support membrane without additional process. TFC PA membrane prepared at TMC 0.3% (w/v) and MPD 2.0% (w/v) with interfacial polymerization reaction for 20 sec showed C (1S), N (1S), O (1S) peaks in XPS spectra indicating presence of polyamide,
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and the three characteristic peaks of polyamide (at 1541, 1609, and 1663 cm-1) in ATR-FTIR spectra. In addition, FE-SEM and AFM images confirm that the polyamide formed through interfacial polymerization between MPD and TMC show rough surfaces and ridge and valley structure typical characteristics of TFC PA membrane. Furthermore, cross-section images of TFC PA membrane prepared through the newly introduced mechanism in this study show well-formed polyamide.
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Also, it was found that the preparation of TFC PA membrane by applying the newly introduced mechanism in this study is optimized according to TMC organic solution velocity and TMC concentration.
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It was confirmed that the optimal TFC PA membrane preparation condition was TMC organic solution
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velocity of 1.21 m/s and TMC concentration of 0.3% (w/v). Under optimal conditions, rejection rates were the highest against multivalent solutes, and especially against Na2SO4.
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To summarize the results of this study, it was confirmed that the new preparation method used in this study can be adequately used for preparation of TFC PA membrane with hydrophobic PVDF porous membrane as support membrane. Furthermore, the rejection rates against Na2SO4 indicate the possibility
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for the prepared TFC PA membrane to be used in the field of nanofiltration. That is, this TFC PA membrane based on PVDF has a relatively simple preparation method, and may be used in water
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treatment and other various fields with polyamide firmly attached to a durable support membrane. In
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addition, the TFC PA membrane prepared through the method proposed in this study raises hopes for usage in organic solution nanofiltration based on its PVDF support membrane with superior chemical
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List of figure captions Fig. 1. The value and image of water contact angle for prepared PVDF UF membrane. Fig. 2. Mechanism by which polyamide forms on hydrophobic support membrane in this study. Fig. 3. Apparatus for supply of TMC organic solution. Fig. 4. XPS spectra for PVDF support membrane and TFC PA membrane (TFC PA membrane preparation
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conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec).
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Fig. 5. ATR-FTIR spectra of PVDF support membrane and TFC PA membrane (TFC PA membrane
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preparation conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec).
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Fig. 6. FE-SEM images of PVDF support membrane and TFC PA membrane (TFC PA membrane preparation conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec).
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Fig. 7. AFM image of TFC PA membrane (TFC PA membrane preparation conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial
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polymerization time: 20 sec).
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Fig. 8. CaCl2 rejection rate and permeation flux of TFC PA membrane prepared at various TMC organic solution velocity (TFC PA membrane preparation conditions - TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec).
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Fig. 9. CaCl2 rejection rate and permeation flux of TFC PA membrane prepared at various TMC concentration (TFC PA membrane preparation conditions - TMC organic solution velocity: 1.21 m/s,
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MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec). Fig. 10. Rejection rate and permeation flux of TFC PA membrane prepared in optimal conditions against NaCl, CaCl2, Na2SO4 (TFC PA membrane preparation conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec). List of Table caption Table 1 TMC organic solution velocity according to gas pressure.
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Table 1 TMC organic solution velocity according to gas pressure. Experimental mean value (m/s)
Standard deviation
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pressure (bar)
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Highlights • Thin polyamide (PA) layer was coated on hydrophobic surface by simple method. PA composite membrane displayed peak performance at 1.21 m/s (organic phase).
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PA composite membrane showed maximum 96.06% rejection rate against Na2SO4 5000 ppm.
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Ju Sung Lee, Jin Ah Seo, Hyun Ho Lee, Sung Kuk Jeong, Hyun Sic Park, Byoung Ryul Min PII: DOI: Reference:
S0040-6090(17)30031-7 doi: 10.1016/j.tsf.2017.01.031 TSF 35743
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
23 February 2016 11 January 2017 17 January 2017
Please cite this article as: Ju Sung Lee, Jin Ah Seo, Hyun Ho Lee, Sung Kuk Jeong, Hyun Sic Park, Byoung Ryul Min , Simple method for preparing thin film composite polyamide nanofiltration membrane based on hydrophobic polyvinylidene fluoride support membrane. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Tsf(2017), doi: 10.1016/j.tsf.2017.01.031
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Simple method for preparing thin film composite polyamide nanofiltration membrane based on hydrophobic polyvinylidene fluoride support membrane
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Ju Sung Lee †,a,b, Jin Ah Seo †,a, Hyun Ho Lee c, Sung Kuk Jeong a, Hyun Sic Park a,
Department of Chemical and Biomolecular Engineering, Yonsei University, 50,
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a
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Byoung Ryul Min a,
b
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Yonsei-ro, Seodaemun-gu, Seoul, 03722, Republic of Korea Fuel Cell Research Center, Korea Institute of Science and Technology, 5, Hwarang-ro
c
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14-gil, Seongbuk-gu, Seoul 02792, Republic of Korea Water Industry Team, Korea Water and Wastewater Works Association, 13, Hwanil-gil,
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Mapo-gu, Seoul, 04199, Republic of Korea
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Abstract
This study attempts to prepare thin film composite (TFC) polyamide (PA) membrane using
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polyvinylidene fluoride (PVDF) as support membrane. PVDF has superior physical characteristics but is difficult to be used as TFC PA support membrane because of its strong hydrophobicity. However, in this study TFC PA membrane was prepared using a simple method of flowing trimesoyl chloride (TMC) organic solution at a specific velocity after putting specific quantity of m-phenylenediamine (MPD) aqueous solution on hydrophobic PVDF support membrane. The presence of polyamide formed by the interfacial polymerization of MPD and TMC were verified with x-ray photoelectron spectroscopy and attenuated total reflectance-fourier transform infra-red spectrophotometer, and field emission scanning †
These authors contributed equally to this work. Corresponding author. Tel.: +82-2-2123-2757; Fax: +82-2-312-6401 E-mail address: [email protected] (B.R. Min)
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electron microscopy and atomic force microscopy were used to observe the TFC PA membrane’s surface and structural characteristics. The prepared TFC PA membrane showed maximum performance at TMC organic solution flow velocity of 1.21 m/s and TMC concentration of 0.3% (w/v). The TFC PA membrane prepared under these optimal conditions showed rejection rates of 68.14, 88.80, and 96.06% against NaCl, CaCl2 Na2SO4 5000 ppm, respectively. This proved that the TFC PA membrane prepared in this study had
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nanofiltration performance.
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Keywords: Polyvinylidene fluoride; Hydrophobic support membrane; Thin films; Composite; polyamide
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membrane; Nanofiltration
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1. Introduction
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The world’s water shortage problem is becoming more severe due to climate change, increase in population, and economic growth, among other reasons. Due to the influence of climate change, an additional 200 million people will suffer water shortage by 2025, while the demand for water will
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increase by 150% in developing countries from 2008 to 2025 due to economic growth [1]. In addition, the
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World Health Organization reported that more than 2.5 billion people will be unable to use sanitation facilities [2]. Among the methods to alleviate this water shortage problem are desalination and recycled
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use of waste and sewer water. [3-6]. In particular, desalination is considered an important field for the production of clean water resources. Major desalination technologies include thermal distillation, forward
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osmosis (FO), membrane distillation, and reverse osmosis (RO) [2, 3]. Of these, RO has higher energy efficiency compared to existing thermal desalination technology, and recent active research has made it possible to prepare produce high-performance membranes with low cost [2, 3]. These advantages have made RO the center of attention in desalination as the latest technology, and also as a technology for the production of clean water [2, 3, 7]. Nanofiltration (NF), which requires relatively low pressure compared with RO, is another technology which can be used for desalination [8]. NF has lower cost and energy usage requirements compared to RO, and also has added advantages such as high rejection rate and permeation flux, leading to applications in organic matter and ion removal, in addition to desalination [9]. In particular, NF removes multivalent ions such as Mg2+ and Ca2+ which exist in hard water, and is 2
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regarded a promising technology in the water softening field for the production of soft water, with numerous related on-going and completed studies in recent years. [10-12] Thin film composite (TFC) polyamide (PA) membrane and cellulose acetate (CA) membrane are primarily used for RO and NF [10, 13-15]. TFC PA membrane is characterized by superior water flux and selectivity compared with CA membrane, and also has wider pH and temperature application ranges [15].
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TFC PA membrane is generally comprised of active layer with support membrane, with the active layer composed of polyamide formed through interfacial polymerization [15, 16]. Traditionally, TFC PA
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membrane has been prepared by contacting support membrane with organic phase after wetting in
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aqueous phase [17]. During this process, the amine in the aqueous phase forms the polyamide through interfacial polymerization with the acid chloride existing in organic phase. Trimesoyl chloride (TMC) is
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widely used as acid chloride, while o-phenylenediamine or m-phenylenediamine (MPD) is used as amine [15]. Relatively hydrophilic porous membrane such as polysulfone (PSf) or polyethersulfone (PES) is used as support membrane for TFC PA membrane [13, 16]. However, in such traditional TFC PA
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membrane preparation methods, hydrophobic materials cannot be used as support membrane. This is because the support membrane has to be wetted in aqueous phase for interfacial polymerization, but
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hydrophobic support membrane cannot be wetted in aqueous phase [17, 18]. Of such hydrophobic
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materials, polyvinylidene fluoride (PVDF) has especially superior mechanical strength, chemical resistance, and thermal stability compared to other commercially widely used polymers [19]. Accordingly, if such hydrophobic PVDF could be used as support membrane for TFC PA membrane, the durability of
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NF and RO membrane could be further improved. For this reason, research groups have conducted studies to use PVDF as support membrane for TFC PA membrane by overcoming its hydrophobic
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properties. Studies that have been reported include: trying to be hydrophilic surface of the PVDF support membrane through the use of plasma or chemical treatment [20, 21], and fabricating TFC PA membrane by forcibly wetting aqueous solution onto hydrophobic PVDF support membrane using the pressure of gas and contacting it with organic solution [18, 22]. Also, some papers have conducted research on TFC membranes based on hydrophobic support membrane of PEI material [23, 24]. Unlike other studies, however, in this study TFC PA membrane was prepared by overcoming the hydrophobic property of PVDF using a simple method without the use of additional processes, materials, or pressure. TMC organic solution was run through PVDF support membrane at a specific velocity with a
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specific amount of MPD aqueous solution on it so that the polyamide would form on the hydrophobic PVDF support membrane. The polyamide formed through interfacial polymerization on PVDF support membrane was characterized with x-ray photoelectron spectroscopy (XPS), attenuated total reflectancefourier transform infra-red spectrophotometer (ATR-FTIR), field emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM), while the optimal condition for this method was
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confirmed through performance test on CaCl2 5000 ppm. Furthermore, rejection rate and permeation flux tests were conducted against NaCl and Na2SO4 feed solution using TFC PA membrane prepared in
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optimal conditions. The rejection rate performance of the TFC PA membrane prepared in this manner
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demonstrated that the adhesion of the polyamide and PVDF were solid.
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2. Experimental
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2.1. Materials
The PET non-woven used as support for PVDF support membrane preparation was purchased from
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Philos (Korea). Polyvinylidene fluoride (PVDF solef® 6020) was purchased from Solvay Korea (Korea)
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and its average molecular weight was 700,000 (g/mol). The solvent used for the PVDF solution was N,Ndimethylacetamide (DMAc, 99%) purchased from Sigma-Aldrich (USA). m-phenylenediamine (MPD, ≥98.0%) and trimesoyl chloride (TMC, ≥98.0%) purchased from TCI (Japan) were used as monomer for
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interfacial polymerization. Also, sodium hydroxide (NaOH, ≥98.0%) purchased from Samchun Chemical (Korea) was used as catalyst for interfacial polymerization reaction, and hexane (mixture of isomers, 99%)
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purchased from Across Organic (Belgium) was used as solvent of TMC. The sodium chloride (NaCl, 99.5%), calcium chloride (CaCl2, 95.0%), and sodium sulfate (Na2SO4, ≥98.5%) used in the feed solution were purchased from Duksan Pure Chemical (Korea), KANTO chemical (Japan), and Samchun Chemical (Korea), respectively. The N2 gas used for the rejection rate and permeation flux tests and supply of TMC organic solution was purchased from Samheung Gas (Korea). Deionized water (DI) made from equipment from AquaMAXTM (Younglin Instrument, Korea) was used in the production of the feed solutions and MPD aqueous solution.
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2.2. Preparation of support membrane
PVDF support membrane was prepared through non-solvent induced phase separation (NIPS) method. PVDF and DMAc were combined in a 13:87 ratio for the production of PVDF 13% (w/w) solution, and stirred at 100 rpm, room temperature conditions. The completely dissolved PVDF 13% (w/w) solution
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was applied on the PET non-woven on the glass sheet at a thickness of 150 µm using casting bar. The PET non-woven with the PVDF solution applied underwent phase inversion in non-solvent (DI water) in
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coagulation bath without any evaporation time. The PVDF membrane in bath was taken out of the
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coagulation bath after 15 minutes, and rinsed in water bath for 1 day. The PVDF membranes prepared were dried at room temperature for 1 day. The hydrophobicity of PVDF support membrane prepared as film was characterized with a water contact angle instrument (Phoenix 300, SEO, Korea). The water
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2.3. Preparation of TFC PA membrane
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contact angle for PVDF support membrane was very hydrophobic at 103.5˚ (Fig. 1).
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Fig. 2 shows the mechanism by which polyamide forms on the hydrophobic PVDF support membrane.
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As shown in t1 of Fig. 2, MPD aqueous solution initially forms on PVDF support membrane surface in bubble form. Then as the TMC organic solution flows on the MPD bubble, it is gradually pushed and the polyamide formed attaches to the surface of the PVDF support membrane due to the velocity of the TMC
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organic solution. At this time, the hydrophobic PVDF support membrane does not wet in MPD aqueous solution, and so the formed polyamide attaches to the PVDF support membrane surface. Accordingly, it
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may be presumed that the pressure formed as the TMC organic solution flows is the driving force that attaches the polyamide to the support membrane.
To prepare the TFC PA membrane, the apparatus in Fig. 3 was used to flow the TMC organic solution at a specific velocity over the PVDF support membrane with specific amount of MPD aqueous solution on it. The velocity of the TMC organic solution was controlled with gas pressure. A precision timer was used in order to control the time of TMC organic solution flow. Table 1 shows the velocity of the TMC organic solution according to gas pressure. The tests were each repeated six times to obtain precise values.
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In this paper, velocity 0 of TMC solution is defined at the speed where MPD droplet does not move. The polyamide manufactured at velocity 0 took the form of initial droplets. Afterwards, the polyamide was attached to support membrane by dry process.
In order to form the polyamide, MPD 2.0% (w/v) aqueous solution and TMC 0.1, 0.3, 0.6, 0.9% (w/v)
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organic solution were prepared [13, 25]. TMC organic solution was prepared using hexane as solvent [14]. NaOH 0.05% (w/v) was added in the MPD aqueous solution in order to accelerate the interfacial
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polymerization reaction [26].
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MPD aqueous solution was placed on the prepared PVDF support membrane using micro-pipette. The TMC solution was provided at a specific velocity to the PVDF support membrane with a specific quantity
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of MPD aqueous solution on it. (As soon as polyamide was formed through interfacial polymerization reaction between TMC organic solution and MPD aqueous solution, the polyamide attached to the PVDF support membrane surface.) After 20 sec of interfacial polymerization reaction, the remaining MPD
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aqueous solution and TMC organic solution were removed with ethanol and hexane.
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2.4. Characterization of TFC PA membrane
2.4.1. XPS
PVDF support membrane and TFC PA membrane elements were analyzed with XPS (K-alpha,
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Thermo VG, UK) in order to confirm the formation of X-ray popolyamide. The X-ray power was set at 12 kv, 3 mA and the X-ray source was monochromatic Al X-ray sources (Al Kα line: 1486.6 eV). The
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diameter of the measured sample area size was 400 µm, and its pass energy 200 eV with step size of 1 eV.
2.4.2. ATR-FTIR
ATR-FTIR (SPECTRUM 100, Perkin Elmer, USA) was used to confirm polyamide formation on PVDF support membrane. Measurements were made in the wave range of 4000-380 cm-1 through accumulation of 32 scans. PVDF support membrane and TFC PA membrane samples were prepared in phase inverted film form without non-woven for the ATR-FTIR analysis.
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2.4.3. FE-SEM FE-SEM (JSM-7001F, JEOL Ltd., Japan) was used to observe the surface and cross-section of the PVDF support membrane and TFC PA membrane samples. Surface samples were prepared in 50 mm × 50 mm sizes for PVDF support membrane and TFC PA membrane. The TFC PA membrane cross-section sample was prepared by breaking after soaking in liquid nitrogen. As the samples were insulators,
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2.4.4. AFM
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Surface characteristics of TFC PA membrane were confirmed with AFM (XE-BIO, Park systems, Korea). Samples of 50 mm × 50 mm area were prepared for equipment measurements, although the actual
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area measured was 5 µm × 5 µm. Average roughness (R a) and root-mean square roughness (Rq), which are surface roughness values, were obtained through AFM analysis [27, 28].
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2.5. Performance test of TFC PA membrane
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Rejection rate and permeation flux tests were conducted on CaCl2 5000 ppm in order to determine the
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optimal TMC organic solution velocity and TMC concentration. Then in order to confirm the performance of the TFC PA membrane according to solute type, rejection rate and permeation flux tests were conducted on NaCl 5000 ppm and Na2SO4 5000 ppm feed solution under optimal TMC organic
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solution velocity and TMC concentration conditions on the TFC PA membrane. The tests were conducted with cross-flow at 30 bar of operating pressure.
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The rejection rate (R) was calculated using the following Eq. (1). Here, CP (ppm) is the concentration of the solute in permeate, while CF (ppm) is the concentration of the solute in feed. Conductivity meter (COM-100, HM digital, Inc., USA) was used to measure the concentration and conductivity of the solute in permeate and feed [29].
(1)
Permeation flux (J) was calculated using the following Eq. (2). Here, Δt (hr) is the test time, while V
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(L) is the permeate amount and A (m2) is the effective area of membrane. (This area was consistent in this study at 0.000314 m2.) [29].
(2)
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3. Results and Discussion
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3.1. Confirmation of polyamide formation
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3.1.1. XPS and ATR-FTIR
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Fig. 4 shows the XPS spectra of the PVDF support membrane and prepared TFC PA membrane. PVDF membrane (Fig. 4(a)) showed C (1S) and F (1S) peaks at 284 eV and 688 eV, respectively. In contrast, TFC PA membrane (Fig. 4(b)) showed C (1S), N (1S), and O (1S) peaks at 284, 399, and 532 eV,
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respectively [30]. Notably, the N (1S) and O (1S) peaks mean that polyamide exists on PVDF support membrane. Results from past studies which confirm the presence of C, N, and O peaks in XPS analysis
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on polyamide support the conclusion that PA has formed well on PVDF support membrane in this study
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[30, 31].
Fig. 5(a) represents the ATR-FTIR spectra of PVDF support membrane. Peaks which indicate the β-
1
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phase of PVDF appear at 510 cm-1 and 840 cm-1, while those which indicate its α-phase appear at 766 cmand 976 cm-1 [32]. These characteristic peaks of PVDF were confirmed in Fig. 5(a). Fig. 5(b) is the
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ATR-FTIR spectra for the prepared TFC PA membrane. It is known that polyamide formed through interfacial polymerization between MPD and TMC show the following three types of peaks. At 1541 cm-1, a peak which indicates presence of amide II bend appears due to C-N stretch and N-H bend in –CO-NHgroup. Then at 1609 cm-1, a peak which indicates presence of aromatic amide appears due to C=C ring stretch or N-H deformation, and finally, at 1663 cm-1 a peak which indicates presence of amide I bend appears due to C-N stretch and C=O stretch. (The influence of the C=O stretch is known to be especially large.) [30]. In addition to the peaks which represent the α-phase and β-phase of PVDF, the three peaks which indicate presence of polyamide (C-N starch, N-H bend at 1541 cm-1 C=C ring stretch or N-H
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deformation at 1609 cm-1, C=O stretch at 1663 cm-1) were all confirmed with ATR-FTIR spectra (Fig. 5(b)) of TFC PA membrane, through which it could be known that polyamide had been successfully formed on PVDF support membrane. Accordingly, our research group confirmed through XPS and ATRFTIR that chemically active layer of polyamide had been successfully formed.
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3.1.2. FE-SEM and AFM Fig. 6 shows FE-SEM images of PVDF support membrane and TFC PA membrane. Here, pores were
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observed on PVDF support membrane surface (Fig. 6(a)). However, no such pores but rather rough
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surfaces were observed for TFC PA membrane surface (Fig. 6(b)). It is known that the surface of the TFC PA membrane formed through interfacial polymerization reaction between MPD and TMC display rough
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surface and ridge and valley structure [33-35]. Accordingly, the SEM surface image of TFC PA membrane prepared in this study clearly show the characteristics of polyamide formed through interfacial polymerization reaction. Fig. 6(c), (d) are cross-section images of TFC PA membrane. The cross-section
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images of the prepared TFC PA membrane show that the polyamide has successfully attached to the surface of the PVDF support membrane according to the mechanism in Fig. 2. In Fig. 6(d), a polyamide
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layer of dense and non-porous structure is observed. In addition, the thickness of the polyamide was
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confirmed to be 59 nm. It is known that the typical thickness of polyamide formed on TFC PA membrane through interfacial polymerization reaction between TMC and MPD is 100 - 1000 nm [36] and polyamide layer in general has very dense and non-porous structure [37]. It may be presumed that the reason that the
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thickness of the polyamide on the TFC PA membrane prepared in this study is lower than those of other polyamides is the influence of PVDF support membrane, and further research is needed to address this
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difference.
Fig. 7 shows the surface roughness values and surface images of TFC PA membrane measured with AFM. Fig. 7 shows the ridge and valley structure of TFC PA membrane. Here, the Ra and Rq values were 45 nm and 59 nm, respectively, and a study result has been reported showing similar surface roughness values for a case of TFC PA membrane prepared through interfacial polymerization reaction between MPD and TMC [35]. These FE-SEM and AFM results confirm that since the polyamide was formed on PVDF support membrane in this study through interfacial polymerization between TMC and MPD, TFC
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PA membrane prepared displays typical rough structure and surface characteristic of typical TFC PA membranes.
3.2. Performance of TFC PA membrane
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3.2.1. Effect of TMC solution velocity Fig. 8 shows the CaCl2 rejection rate and permeation flux of the prepared TFC PA membrane
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according to TMC organic solution velocity. Here, TFC PA membrane displayed rejection rates of 15.15,
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88.80, 87.64, 82.79% and permeation flux of 20.51, 7.78, 8.03, 8.17 LMH for TMC organic solution velocities of 0, 1.21, 2.68, 3.79 m/s, respectively. When the velocity was 0 m/s, the rejection rate was the
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lowest and the permeation flux the highest. The reason for this phenomenon was that with no TMC organic solution flow, the polyamide could not attach firmly to the PVDF support membrane. In this case, the polyamide is ripped from the prepared TFC PA membrane during the performance test, and feed
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solution passes freely through the pores of the PVDF support membrane. When the velocity was 1.21 m/s, the rejection rate was the highest and the permeation flux the lowest, and it was confirmed that as velocity
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increased from 1.21 m/s, the rejection rate decreased and the permeation flux increased. The presumed
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reason for this tendency is that the flowing TMC organic solution affects the polyamide attached to the PVDF support membrane and causes a defect in the active layer of polyamide and allows feed solution to flow into permeation. When the TMC organic solution velocity was 4.81 m/s, the formed polyamide was
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torn off by the fast-flowing TMC organic solution, and so the TFC PA membrane was not properly prepared. Based on this, it was observed that at TMC organic solution velocity of 1.21 m/s, the polyamide
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formed through the mechanism of this study was firmly attached, while higher speeds caused defects in the polyamide’s attachment and hindered the performance of the TFC PA membrane. Accordingly, it was confirmed that the optimal TMC organic solution velocity for the preparation of TFC PA membrane per the method of this study is 1.21 m/s. Meanwhile, m/s which is an SI unit is a velocity unit of the same dimension as LMH, as 1 m/s is equal to 3.6×106 LMH. That the manufactured TFC membrane did not get torn at velocities under 4.81 m/s (1.7316×10 7 LMH) and displayed NF capabilities can be proof that the adhesion stability between the PVDF support membrane and polyamide layer is very stable.
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3.2.2. Effect of TMC concentration Fig. 9 shows the CaCl2 rejection rate and permeation flux results for the prepared TFC PA membrane according to TMC concentration. Here TFC PA membrane displayed rejection rates of 78.82, 88.80, 72.14, 71.10% and permeation flux of 8.71, 7.78, 6.20, 5.71 LMH for TMC concentrations of 0.1, 0.3, 0.6, 0.9%, respectively. It was confirmed that optimal performance is achieved against CaCl 2 5000 ppm when the
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TMC concentration is 0.3%. It has already been reported that if the concentration of the monomer used in interfacial polymerization is too low, then a weak, thin polyamide layer is formed and the performance of
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the TFC PA membrane is lowered. [38]. Accordingly, the reason that the rejection rate is lower at TMC
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concentration of 0.1% compared to TMC concentration of 0.3% is that the concentration is lower than the minimum needed for strong polyamide formation through interfacial polymerization reaction. Also, at
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fixed MPD concentration levels, as the TMC concentration level rises higher than the optimal TMC concentration, the degree of cross-linking of the polyamide decreases [39]. As a result, the CaCl2 rejection rate decreases as TMC concentration increases from 0.3% to 0.9%. As TMC concentration rises,
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permeation flux has a tendency to decrease, which can be explained through study findings that the thickness of active layer increases with increases in TMC concentration [40]. At this time, the increase in
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membrane thickness leads to transports resistance of membrane, which causes a decrease in the pressure
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difference across the membrane. Finally, water flux and solute flux decrease due to the pressure drop [41]. For this reason, as TMC concentration increased from 0.1% to 0.9%, permeation flux gradually decreased. In summary, it may be presumed that permeation flux is more greatly affected by polyamide thickness
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than by its degree of cross-linking.
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3.2.3. TFC PA membrane performance according to different feed solution Fig. 10 shows the rejection rate and permeation flux of prepared TFC PA membrane against CaCl 2 5000 ppm, NaCl 5000 ppm, and Na2SO4 5000 ppm feed solution at previously determined optimal TMC organic solution velocity and TMC concentration conditions. Here, the rejection rate was in NaCl < CaCl2 < Na2SO4 order with values of 68.14, 88.80, and 96.06%, respectively. Electrolytes composed of largesized ions have difficulty passing through membrane and thus show higher rejection rates [42]. The hydrated ion sizes of Na+, Ca2+, and SO42- are 0.178 nm, 0.260 nm, and 0.300 nm, respectively. As a result, that of Ca2+ and SO42- are much larger than that of Na+. This difference in ion size makes it difficult for
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Ca2+ and SO42- to pass through the polyamide compared to Na + [43]. Furthermore, TFC PA membrane which contains polyamide formed through interfacial polymerization have bipolar characteristic, which makes it additionally difficult for ions to pass through. That is, the outer surface of the polyamide has a negative charge because of the leftover acyl chloride in TMC (The acyl chloride in TMC is converted to carboxylic acid due to hydrolysis reaction [44]. Carboxylic acid also has a large negative charge.), while
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the surface of the polyamide on the support membrane side gets a positive charge because of the leftover amine in MPD, and it has been reported that feed solutions such as Na2SO4 and CaCl2, which contain
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multivalent ions as SO42- and Ca2+ have difficulty passing through TFC PA membrane due to a charge
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repulsion effect (multivalent ions of negative charge are repulsed by the negative charge of the polyamide external surface, while those of positive charge are repulsed by the positive charge of the polyamide
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surface on the support membrane side) [45]. Due to the size of the ion and the charge repulsion effect, the rejection rates for CaCl2 and Na2SO4 become higher than for NaCl. Furthermore, the reason that the rejection rate for Na2SO4 is higher than that for CaCl2 is presumably that the effect of the negative
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multivalent ions being repulsed on the outer surface of the polyamide has a greater effect on selectivity than the effect of the positive multivalent ions being repulsed by the polyamide on the support membrane
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side. CaCl2 displayed low permeability compared to Na2SO4, which can be explained by hydrated ion size.
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In the previous study, the crystal radius and hydrated ion radius of Ca 2+ and SO42- have been reported as 0.100 nm and 0.215 nm, and 0.260 nm and 0.300 nm, respectively [46]. In particular, the increase in radius of due to H2O was approximately two times larger for the Ca 2+ (0.160 nm) compared to that of
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SO42- (0.085 nm). This shows that the Ca2+ has strong hold on water [43]. It may be presumed that the
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eater permeability of CaCl2 was relatively lower than that of Na2SO4 for this reason.
4. Conclusions
In this study, polyamide was successfully formed on PVDF support membrane through the use of MPD aqueous solution and TMC organic solution by overcoming the hydrophobic characteristic shortcoming of PVDF support membrane without additional process. TFC PA membrane prepared at TMC 0.3% (w/v) and MPD 2.0% (w/v) with interfacial polymerization reaction for 20 sec showed C (1S), N (1S), O (1S) peaks in XPS spectra indicating presence of polyamide,
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and the three characteristic peaks of polyamide (at 1541, 1609, and 1663 cm-1) in ATR-FTIR spectra. In addition, FE-SEM and AFM images confirm that the polyamide formed through interfacial polymerization between MPD and TMC show rough surfaces and ridge and valley structure typical characteristics of TFC PA membrane. Furthermore, cross-section images of TFC PA membrane prepared through the newly introduced mechanism in this study show well-formed polyamide.
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Also, it was found that the preparation of TFC PA membrane by applying the newly introduced mechanism in this study is optimized according to TMC organic solution velocity and TMC concentration.
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It was confirmed that the optimal TFC PA membrane preparation condition was TMC organic solution
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velocity of 1.21 m/s and TMC concentration of 0.3% (w/v). Under optimal conditions, rejection rates were the highest against multivalent solutes, and especially against Na2SO4.
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To summarize the results of this study, it was confirmed that the new preparation method used in this study can be adequately used for preparation of TFC PA membrane with hydrophobic PVDF porous membrane as support membrane. Furthermore, the rejection rates against Na2SO4 indicate the possibility
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for the prepared TFC PA membrane to be used in the field of nanofiltration. That is, this TFC PA membrane based on PVDF has a relatively simple preparation method, and may be used in water
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treatment and other various fields with polyamide firmly attached to a durable support membrane. In
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addition, the TFC PA membrane prepared through the method proposed in this study raises hopes for usage in organic solution nanofiltration based on its PVDF support membrane with superior chemical
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List of figure captions Fig. 1. The value and image of water contact angle for prepared PVDF UF membrane. Fig. 2. Mechanism by which polyamide forms on hydrophobic support membrane in this study. Fig. 3. Apparatus for supply of TMC organic solution. Fig. 4. XPS spectra for PVDF support membrane and TFC PA membrane (TFC PA membrane preparation
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conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec).
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Fig. 5. ATR-FTIR spectra of PVDF support membrane and TFC PA membrane (TFC PA membrane
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preparation conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec).
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Fig. 6. FE-SEM images of PVDF support membrane and TFC PA membrane (TFC PA membrane preparation conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec).
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Fig. 7. AFM image of TFC PA membrane (TFC PA membrane preparation conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial
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Fig. 8. CaCl2 rejection rate and permeation flux of TFC PA membrane prepared at various TMC organic solution velocity (TFC PA membrane preparation conditions - TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec).
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Fig. 9. CaCl2 rejection rate and permeation flux of TFC PA membrane prepared at various TMC concentration (TFC PA membrane preparation conditions - TMC organic solution velocity: 1.21 m/s,
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MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec). Fig. 10. Rejection rate and permeation flux of TFC PA membrane prepared in optimal conditions against NaCl, CaCl2, Na2SO4 (TFC PA membrane preparation conditions - TMC organic solution velocity: 1.21 m/s, TMC concentration: 0.3% (w/v), MPD concentration: 2.0% (w/v), interfacial polymerization time: 20 sec). List of Table caption Table 1 TMC organic solution velocity according to gas pressure.
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Table 1 TMC organic solution velocity according to gas pressure. Experimental mean value (m/s)
Standard deviation
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0.06
0.490
2.68
0.09
0.980
3.79
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1.470
4.81
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pressure (bar)
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Highlights • Thin polyamide (PA) layer was coated on hydrophobic surface by simple method. PA composite membrane displayed peak performance at 1.21 m/s (organic phase).
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PA composite membrane showed maximum 96.06% rejection rate against Na2SO4 5000 ppm.
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