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Thin film composite membranes: Preparation, characterization, and application towards copper ion removal
Accepted Manuscript Title: Thin Film Composite Membranes: Preparation, Characterization, and Application towards Copper Ion Removal Authors: Norin Zamiah Kassim Shaari, Nurul Aida Sulaiman, Norazah Abd Rahman PII: DOI: 102845 Reference:
S2213-3437(18)30768-1 https://doi.org/10.1016/j.jece.2018.102845 JECE 102845
To appear in: Received date: Revised date: Accepted date:
16 October 2018 2 December 2018 16 December 2018
Please cite this article as: Kassim Shaari NZ, Sulaiman NA, Rahman NA, Thin Film Composite Membranes: Preparation, Characterization, and Application towards Copper Ion Removal, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.102845 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.
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Thin Film Composite Membranes: Preparation, Characterization, and Application towards Copper Ion Removal Norin Zamiah Kassim Shaari*, Nurul Aida Sulaiman, Norazah Abd Rahman Department of Chemical and Process Engineering,
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Faculty of Chemical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor,
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*Corresponding email: [email protected]
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Highlight
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Abstract
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The TFC with 3 wt.% TEOS exhibited the best properties and performance The TFC capable to remove more than 90% copper (II) ions from a pure solution Feed at pH 7 is found as optimum for separation process through the TFC The treated wastewater through the TFC has permissible concentration of copper The hydrophilic character ensures anti fouling behaviour of the TFC
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Malaysia
A novel thin film composite (TFC) membrane was fabricated from a thin layer of hybrid
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membrane formulated from a blend of polyvinyl alcohol (PVA)/chitosan and cross linked with tetraethylorthosilicate (TEOS), which was layered on the polysulfone (PSF) membrane. Besides producing membrane from polymer blend of PVA/chitosan only, concentrations of TEOS as cross linker was varied at 1wt%, 3wt% and 5wt% with fixed concentration of PVA solution (10wt.%) and chitosan (2wt.%) respectively during the formulation of hybrid membranes. The thermal properties, mechanical strength, water contact angle, swelling
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measurement and anti-fouling behaviour of these membranes were investigated. The performance of the thin film composite was evaluated through filtration of the pure copper ion solution and the industrial wastewater containing copper respectively. The fabricated TFC with 3 wt.% TEOS was found to possess better tensile strength and strain, and thermal stability as compared with other concentration of TEOS. The membrane also exhibit good
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rejection of copper ion (> 90%) from pure copper solution especially at pH 7 of feed solution. A further treatment conducted on the wastewater reveals that TFC with 3 wt.% TEOS could
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remove the copper ion, where the filtered water could be directly discharged to the river. Furthermore, the TFC portrayed good anti fouling behaviour, where it have 3% of relative
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flux decay and 76% of relative flux recovery even with the presence of humic acid as NOM
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(natural organic matter).
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Keywords: Hybrid membrane; thin film composite membrane; polymer blend; copper ion
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removal; wastewater
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1. Introduction
The presence of heavy metal ions in the water resources can affect human health and
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environment due to its toxicity characteristics and non-biodegradability [1-2]. Copper (Cu) is
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a heavy metal that is highly toxic because it can be very easily absorbed by living organism. In the human body, it can accumulate in lungs, kidneys, liver and also other vital organs [36]. A serious toxic dose of exposure is between 4 mg to 400 mg of copper (Cu) per kg of
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body weight where it can cause gastrointestinal bleeding, intravascular hemolysis, hematuria and acute renal failure. While at lower dose, symptoms like headache, nausea, vomiting and diarrhea appears generally after 15 - 60 minutes of exposure [3-4, 5]. Therefore, various chemical and physical treatments such as chemical precipitation, ion-exchange, flotation and adsorption have been applied to remove copper from wastewater,
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[7]. However, these conventional heavy metal treatments possess some limitations such as high cost, ineffective removal at low concentration of heavy metal ions and generate high amount of sludge [8]. Membrane separation on the other hand has been proven on the capability to remove suspended solid, organic compound, and inorganic contaminant in the wastewater [3, 9]. Other than that, the main benefit of membrane separation as compared to
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other processes is related to its unique separation principle, which is the transport selectivity of the membrane. Additionally, membrane separations do not need any additives as compared
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to the conventional method, thus the operating costs are cheaper [10].
Recently, polymer enhanced ultrafiltration (PEUF) method as shown in Fig. 1, is
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being widely used for heavy metal ions removal where it uses organic polymer as complexing
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agent that can bind heavy metal ion chemically [11-13]. Llorens et al. [11] had reported on
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the use of chitosan as complexing agent to remove Cadmium ion. It was reported that pH 7-
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7.5 is the suitable range for the removal process, where more than 90% Cadmium ions was removed from the feed solution. Chou et al. [13] studied PEUF process to remove copper
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ions by comparing three polyelectrolyte complexing agents, that are poly(styrenesulfonate) (PSS), poly(allylamine)(PAA) and polyethylenimine (PEI), and it was found that PEI
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exhibited the best performance by removing 94% of copper ion at pH 3. Besides that, there
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are other types of complexing agents that are being used to remove heavy metal ions such as chitosan, carboxylmethyl cellulose, poly(acrylic acid), polyvinyl alcohol and polyvinyl
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ethylenimine [14].
Therefore the aim of the current study is to fabricate a thin film composite (TFC) as
an improved method of polymer enhanced ultrafiltration (PEUF) by combining the complexation of heavy metal and separation process only in one equipment, as described in Fig. 1. The thin film composite consists of a hybrid membrane as the thin layer, which is formed from a polymer blend of poly(vinyl alcohol) (PVA) with chitosan (Cs) and then
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cross-linked with tetraethylorthosilicate (TEOS). These organics-inorganic hybrid polymer combinations have benefits such as good thermal stability and strong inorganic substrate, causing strong binding affinities toward selected metal ions as the metal ions are chemically bonded by the organic-inorganic polymer hybrids. Furthermore,
with the presence of
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chitosan as the adsorbent, it will ensure a relatively high metal ion adsorption capacities [15]. In the preparation of organic-inorganic composite materials, sol-gel technology offers simple and convenient method. This method involves a sequence process of hydrolysis and
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condensations which led to the membranes matrix modification. TEOS as the silica precursor has been found to yield membrane with a compact structure to provide more area for ion
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adsorption and also the resultant membrane has good mechanical strength [16]. As an organic
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polymer, PVA has many advantages like high anti fouling potential and good thermal
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resistances. However, PVA has low stability and high swelling in water [17]. Therefore, the
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cross linking reaction on the polymer blend of PVA/chitosan is needed to increase the rigidity structure of the membrane and inhibits the swelling of the resultant membrane [18].
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Hybridisation between organic polymer such as PVA with inorganic particles has received significant interest as it does not only control the swelling of PVA but also provides the
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inherent advantages of the organic and inorganic compounds [19-20]. Besides having good
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biological properties, chitosan is used in this study due to its good complexation material where it can separate and adsorb a wide range of pollutants including heavy metals [21-22].
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In this study, the fabricated TFC membranes were subjected to membrane
characterization such as cross sectional morphology, thermal analysis, tensile strength, hydrophilicity and swelling analysis. Objectives of this study was to identify the best concentration of TEOS that yield TFC with good mechanical, thermal and hydrophilic behaviour, as well as able to remove high percentage of copper ion from pure copper
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solution. TFC with optimum concentration of TEOS will be subjected to filtration on wastewater containing copper as well as evaluation on the anti-fouling behaviour. 2. Materials and method 2.1. Materials
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Polysulfone (PSF) pellet resins with molecular weight (44,000 – 53,000 Da), PVA pellets with a hydrolysis degree of 87-89% and molecular weight (85,000 – 124,000 Da), chitosan
(cs)
powder
(deacetylation
degree
84.8
±
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commercial
1.2
m
n%), Tetraethylorthosilicate (TEOS) with 99% purity, hydrochloric acid with 37% purity as
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catalyst, acetic acid and N-methyl-2-pyrrolidone (NMP) with purity of 99% was obtained
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from Merck, Malaysia. Deionized water was also used as coagulation and dilution medium.
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All materials were employed without further purification.
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2.2. Methods
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2.2.1. Preparation of polysulfone (PSF) porous support membrane In the preparation of PSF support membrane, 13 g of polysulfone bead was dissolved in 87 g
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of NMP to produce polysulfone polymer solution with 13 wt% of polymer concentration. The
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solution was stirred continuously for approximately 4 to 6 hours at 60ºC. The solution was left for several hours at room temperature to remove air bubbles. A casting machine was used to cast the polysulfone solution onto a glass plate with adjusted thickness to 100-120 µm. The
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film was immersed in water as a coagulation medium for 1 hour and subsequently, it was left in a large amount of water for another 24 hours. Then, the film was dried in an open air for one day [18,23]. 2.2.2. Preparation of PVA/Chitosan/TEOS hybrid membrane
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In the preparation of hybrid membrane, PVA solution and Cs solution were prepared separately. 2 wt.% Cs solution was prepared by dissolving 3g of chitosan in 2wt% of acetic acid at room temperature with vigorous stirring [18,22]. Next, 10 gram of PVA pellets was dissolved in 90 mL of distilled water with (9:1) ratio under continuous heating at 90ºC for 6 hours and mixing to produce 10 wt% PVA solution [18, 22]. The Cs solution was mixed
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with PVA solution to produce a blend mixture. The mixture was heated at 60ºC for 4 hours to produce a homogeneous solution. Then, the mixture was left to cool at room temperature.
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Next, 3wt% of TEOS as a silica nano precursor was added to the mixture and 1 ml of hydrochloric acid as a catalyst was added to the mixture solution [16,18]. The solution was
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then heated at 30ºC under continuous mixing for 10 hours [18]. Table 1 is a summary of
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hybrid membranes composition used in this study.
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2.2.3. Preparation of Thin Film Composites (TFC) Membrane
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The thin film composite (TFC) membrane was prepared by coating the hybrid membrane
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onto a PSF support using a glass rod. The composite membrane produced was left to dry for 24 hours at room temperature and it was subsequently heat cured in an oven for 1 hour at
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45ºC [16,18].
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2.3. Membrane Characterization The cross section morphology of thin film composite membrane was examined using Scanning electronic microscope (SEM) (TM3000 Tabletop HITACHI, United State of
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America) with an acceleration voltage of 5kV [23]. Perkin – Elmer Spectrum 2000 (FT-IR) instrument was used to examine the functional group and chemical structure of the hybrid membrane. The wavelength used was in the range of 400 cm-1 to 4000 cm-1 [24].
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The thermal stability of membranes was evaluated by using Thermogravimetry Analyzer, Mettler Toledo model Stare SW. The analysis was performed under a nitrogen atmosphere at a heating rate of 10 °C/min. The sample weight ranged from 5 to 10 mg and it was heated from 30oC to 900°C.
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The mechanical strength analysis was performed by using Universal testing machine (Autograph AG-X, Shidmazu, Japan) model. The sample was cut to 100 mm length and 40 mm width. The thickness of the sample was measured using vernier caliper while the speed
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of the machine is set at 5mm/min based on standard state method for plastic strength (ASTM
D2800) [22,25]. The tensile strength, elongation, and tensile modulus of the samples were
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measured.
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The swelling properties of hybrid membranes were measured according to the method
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reported by Xie et. al [19]. Firstly, the dried TFC membrane was immersed in deionised
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water at room temperature for 48 hours. Then, the surface of wet TFC membrane was wiped
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using tissue paper. Immediately, it was weighed to get the mass, Ws before drying in a vacuum oven at 50°C overnight. Then it was weighed again to obtain the mass of dried
(1) [19].
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membrane (Wd). The swelling degree (S) of membrane was then calculated according to Eq.
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W Wd S % s 100 Wd
(1)
The contact angle of membrane samples were measured by using VCA 3000s. The
membrane sample was placed on the VCA platform and the contact angle was measured by using sessile drop method. A minimum of three drops of water were used and the resulted contact angle captured by VCA image were measured.
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2.4 Copper ion removal from pure copper solution The performance of thin film composite (TFC) membrane was tested by using a dead end filtration mode of membrane testing rig. The membrane with an effective surface area of 17.35 cm2 was placed in a sample compartment. The copper concentration in the permeate solution was measured by using Atomic Absorption Spectroscopy (AAS). A calibration curve
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for the copper ion was firstly generated through series of dilution from 1000 ppm copper ion. From the calibration curve, the concentration of copper ions in the solution was identified.
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Three concentrations of copper ion solution were prepared, 50 ppm, 150 ppm and 250 ppm
from dilution of 1000 ppm of that solution. Prior to the filtration process, pH of feed solution
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was adjusted to three conditions, acidic (pH 3), neutral (pH 7) and basic (pH 10) by the
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addition of 1M HCI or 0.01 M NaOH. During the experiment, the feed solution was placed in
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a 300 mL stainless steel filtration cell. The operation was conducted at room temperature
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with 200 mL of feed solution per batch of operation. The pressure was applied at ± 8 bar by using nitrogen gas. The permeate sample was collected in a beaker for volume measurement.
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The flux rate and the percentage removal of copper (Cu) ion from the feed solution were calculated by using Eq. (2) and Eq. (3) respectively.
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V A.t
(2)
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J
where ∆V (mL) is the volume of the permeate sample, A is an effective membrane area in cm2 and ∆t is filtration time (minutes)
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R(%)
C f Cp Cf
x 100
(3)
Where C f and C p is concentration of copper in feed solution and permeate solution respectively 2.5 Copper ion removal from industrial wastewater
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Based on results from characterization of the thin film composite membranes and performance testing on copper ion solution, membrane M3 was selected to be compared with membrane M1 on their effectiveness to remove copper (Cu) ion from wastewater. The result from this experiment will reveal the significance of cross linking process on a polymer blend. Before subjecting the sample for filtration using the membrane, the wastewater had to go
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through some pre treatment. Firstly, the wastewater was filtered for 3 times by using filter paper brand (Whatman No.1) to remove suspended solid. Then, the initial pH of the sample
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was adjusted to pH 7. The filtration process was conducted in the same procedures as stated in Section 2.4.
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2.6 Anti fouling analysis
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Anti-fouling analysis was examined according to the method reported by Zhu et al. [26].
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Based on the method, the antifouling parameters such as Relative Flux Decay (RFD) and
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Relative Flux Recovery (RFR) were determined analytically by using the equations Eq. (3)
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and Eq. (4) [26]. Besides evaluating on the copper ion solution alone, the antifouling performances of the thin film composites also was being evaluated with the incorporation of
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1000 ppm humic acid (HA) solution, which acts as the foulant model into the copper feed solution. Humic acid was chosen due to it contains major portion of NOM (Natural organic
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matter) in a surface water or ground water, where it is a major contributor to membrane organic fouling in the treatment of wastewater [26]. Membrane M1 and M3 were also
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selected for the antifouling analysis.
J J P RFD O x 100 JO
(3)
J RFR 1 X 100 JO
(4)
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where JO is the pure water flux in the first 30 minutes filtration time, JP is the permeate flux of the feed solution containing copper during 2 hours filtration time, which is subsequently after JO was obtained, J1 is the final pure water flux in 30 minutes filtration time after the
2. RESULT AND DISCUSSION
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3.1. Cross section morphology of thin film composite membrane
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membrane has been backwashed.
The cross-sectional image of TFC membrane which consists of the hybrid
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membrane’s layer on the PSF support membrane is exhibited in Fig. 2(a). In comparison with
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the image of PSF support layer as shown in Fig. 2(b), the top layer of the TFC membrane
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shows a relatively dense cross–section with the changes from finger-like structure to sponge-
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like structure. This situation could be described as reported by Huang & Yang [27], after the hybrid membrane was coated onto the PSF membrane, the silica particles in the hybrid
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membrane dispersed homogeneously into the PSF membrane. Besides that, the diffusion of
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PVA/Cs/TEOS hybrid membrane also could increase the compact resistance and mechanical strength of the resultant membrane [28].
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3.2. Thermal analysis
The thermal stability of hybrid membrane with different concentrations of TEOS was
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identified through the TGA curves as depicted in Fig. 3. It can be seen that all membranes have similar weight loss profile and the weight changes pattern. Three main steps of weight loss were observed. The first weight loss occurred at a temperature below 150°C was related to the removal of the residual moisture from the sample [29]. The second weight loss is around 200ºC to 400ºC corresponds to the removal of hydroxyl group on PVA and chitosan
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[30]. The final weight loss occurred after 500°C was related to the decomposition of the polymer backbones in the hybrid membrane [30]. It was observed that the weight residue at temperature more than 400°C for all membranes increases as the TEOS content increased. This condition indicates that the incorporation of TEOS into the polymer blending of PVA/chitosan significantly stabilized the thermal degradation of the membranes as a result of
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increase in cross linking density of the membrane [30-31].
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3.3. Mechanical properties
As shown in Fig. 4, the tensile strength increases with TEOS concentration. As mentioned earlier, the increase in TEOS content will accelerate the cross linking density to
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encourage the formation of dense structure, that finally improves the mechanical properties of
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hybrid membrane due to it needs more strength to fracture [32]. This situation can be
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described through good compatibility between PVA/Cs and TEOS through formation of
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hydroxyl group when the polymer matrix was trapped between silica precipitates [33]. It was
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observed from Fig. 4 that the tensile strain increases with TEOS content from 1wt% until 3wt% but not for 5 wt% (M4) TEOS, where the strain started to decrease. This finding is in
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agreement with the one reported by Yu et al. [34], where the excess amount of TEOS content can cause a decrease in hybrid homogeneity. The hybrid membrane that has too much TEOS
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caused the film to become more rigid due to the restriction in the movement of polymer molecules, and consequently the membrane exhibited a decrease in flexibility [22].
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3.4. Swelling Studies and Contact Angle Analysis The swelling of any polymer film in a solvent depends upon the diffusion coefficient of the solvent. PVA was chosen as membrane material because of its hydrophilicity and film forming characteristics. However, the excessive solubility on PVA has resulted to membrane swelling, which lead to an open structure of membrane [35]. As a consequence, the
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membrane rejection will be low [36]. The PVA used in the study has a degree of hydrolysis 87-89% , where the blend with chitosan causes high degree of swelling in water due to interaction between water molecules and hydroxyl groups in both polymers [19], where it was proven in Fig. 5 for membrane M1. Cross linking is one of the methods to reduce polymer swelling by balancing the hydrophoblic–lipophilic of the membranes [19, 35-37] and
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as shown in Fig. 5, the swelling degree of the hybrid membranes decreases with the increase of cross linker (TEOS). This is due to the formation of new chemical bonds between the
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blended polymer and nano silica, where the bonds is formed with the organic polymer chains
through the polar hydroxyl group of PVA and amine group of chitosan in the polymer [19].
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Similar findings were also reported by Pandey et. al [38], which demonstrated that there was
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a considerable decrease in the swelling degree of hybrid films from PVA/silica. Xie et al.
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(2011) [19] also found that polymer-inorganic nano composite membranes from PVA that
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was cross linked with maleic acid (MA) and silica respectively has low degree of swelling. Generally, membrane hydrophilicity is greater when its contact angle is smaller [32]. Fig. 5
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shows the comparison of water contact angles between PSF support membrane (M0) and the thin film composite membranes (M1, M2, M3 and M4). Although the hydroxyl group are
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consumed during the cross linking process, where the contact angle is expected to increase, it was not reflected in Fig. 5. Instead of increasing the hydrophobicity of the membrane, the
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increase in TEOS concentration has resulted in lowering contact angle of the membrane. This result is similar with the findings obtained by Shi et al. [32], where extra amount of cross
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linker agent like TEOS addition had restrained the formation of crystallization, lowered the membrane porosity, decreasing the roughness of membrane upper surface and reducing the contact angle of the membrane. Other than that, the presence of coated membrane layer from PVA/Chitosan/TEOS hybrid membrane can improve the surface wettability and affinity of porous support membrane which would possibly increase the water permeation rate during
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membrane separation [39]. On the other hand, M1 from polymer blend of PVA/Cs showed the lowest contact angle due to its properties of good film-forming and high hydrophilicity. 3.5 Performance of TFC membrane on copper ion removal from pure copper solution and wastewater
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As shown in Fig. 6(a), at pH 3, there was a rapid improvement on the percentage of copper removal especially for membrane with 3wt% (M3) and 5wt% (M4) TEOS as compared that
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with 0 wt% (M1) and 1 wt% (M2) TEOS for every concentration of copper ion. For 50 ppm copper solution, membrane M3 and M4 able to remove 85% copper ion, as compared to M1 5% and M2 27%. Similar phenomenon also observed for 250 ppm copper solution where M3
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and M4 exhibited the same performance to remove 87% copper ion, whereas 57% by M1 and
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63% by M2. These result show that TEOS is not only good to form membrane that has high
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thermal and mechanical properties but also able to form dense structure of hybrid membrane
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to trap more heavy metal ion [40]. pH is one of the most important factors in the interaction
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of a metal ion with binding polymer [41]. Fig. 6(b) and (c) show that at pH 7 and pH 10, there is a rapid increase of the percentage removal, where it has achieved more than 90%.
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Ngah et al. [41] also found the similar behaviour in their research, where this could be explained by the fact that at higher pH values, less inhibitory effect of H + exists to protonate
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with amine group from chitosan, which could reduce the number of binding sites for copper ion adsorption. Other than that, the lone pair electrons present in the amine (-NH2) groups
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and lone pairs of electrons of hydroxyl (-OH) groups can establish bonds with transition metal ions. Although adsorption increases at pH more than 7, there is a formation of copper ion hydroxide in a form of precipitation which might affect the adsorption. Therefore, based on Fig. 6(a),(b) and (c), membrane M3 consisted of 3wt% TEOS was chosen as the optimum cross linker added to the hybrid membrane due to its similar performance with membrane consisted of 5wt% TEOS (M4).
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Fig. 7 shows the percentage removal of copper ion from industrial wastewater. The membrane used for this experiment is M3 with pH of feed solution was adjusted to pH 7. It can be observed that the percentage removal for copper ion is over 80% and the value was consistent within 3 hours of filtration time. Calculation has been made and the final concentration of copper ion is around 0.2 to 0.5 mg/L, which is within the acceptable limit of
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standard B of effluent discharge as outlined by the Environmental Quality (Industrial Effluent) Regulations 2009.
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3.6 Anti – fouling behaviour
Fig. 8 shows the permeate fluxes, (J) of M1 and M3 during the filtration of copper solution
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with and without humic acid (HA) solution. The presence of HA causes the membranes to
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have lower flux decays but at the same time they have relatively lower flux recovery as
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compared to that in the solution without HA. Based on Table 2, Relative flux recovery of
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M3 at 76% with humic acid and 95% without humic acid indicates that the fabricated thin film composite with hybrid membrane has anti fouling behaviour, which can effectively
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counter back the fouling caused by NOM. In conclusion, the interaction of HA during the
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Conclusions
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filtration with copper ion solution had less effect on the membrane fouling.
The fabricated TFC membranes have been proven to possess good thermal and mechanical stabilities with hydrophilicity character. This new type of PVA/chitosan/TEOS hybrid
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membrane was found able to remove more than 90% of copper (Cu) from pure copper solution and better separation can be achieved at higher pH. However, in order to avoid the hydroxide precipitation at pH 10, pH 7 was found to be the optimum condition for the removal of copper. In terms of industrial wastewater, it was apparent that the composite
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membrane with 3wt.% TEOS (M3) was successful in treating the wastewater and allowing it to be directly discharged to the groundwater.
Declaration of interest: none
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Acknowledgement
The author would like to thank Ministry of Higher Education Malaysia, which has sponsored
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this research through Fundamental Research Grant Scheme with file no. 600-RMI/FRGS 5/3
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(92/2014).
X. Zhang, Y. Wang, Y. Liu, J. Xu, Y. Han, and X. Xu, Applied Surface Science
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[1]
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[21] G. Crini, N. Morin-Crini, N. Fatin-Rouge, S. Déon, P. Fievet, Metal removal from aqueous media by polymer-assisted ultrafiltration with chitosan, Arab. J. Chem., 2014.
[22] S. B. Bahrami, S. S. Kordestani, H. Mirzadeh, P. Mansoori, Poly (vinyl alcohol) Chitosan Blends: Preparation, Mechanical and Physical Properties, Iran. Polym. J.
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1(2002) 139–146.
[23] G. Gong, H. Nagasawa, M. Kanezashi, T. Tsuru, Reverse osmosis performance of
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layered-hybrid membranes consisting of an organosilica separation layer on polymer supports, J. Memb. Sci., 494(2015) 104–112.
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[24] Z. Xie, Hybrid Organic-Inorganic Pervaporation Membranes for Desalination, thesis
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ASTM D882-12, Standard Test Method for Tensile Properties of Thin Plastic
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Sheeting,” ASTM Int., no. West Conshohocken, PA, (2012).
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[26] X. Zhu, H. Loo, R. Bai, A novel membrane showing both hydrophilic and oleophobic surface properties and its non-fouling performances for potential water treatment
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applications, J. Memb. Sci. 436(2013) 47–56.
[27] H. Huang, S. Yang, Filtration characteristics of polysulfone membrane filters,
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37(2006) 1198–1208.
[28] Y. Yang, P. Wang, Preparation and characterizations of a new PS/TiO2 hybrid membranes by sol-gel process, Polymer (Guildf). 47(2006) 2683–2688.
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[29] Y.H. Han, A. Taylor, M. D. Mantle, K. M. Knowles, Sol–gel-derived organic– inorganic hybrid materials, J. Non. Cryst. Solids 353(2007) 313–320.
[30] R. Xing, W. S. W. Ho, Synthesis and characterization of crosslinked polyvinylalcohol/polyethyleneglycol blend membranes for CO2/CH4 separation, J.
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Taiwan Inst. Chem. Eng. 40(2009) 654–662.
[31] J. G. Varghese, R. S. Karuppannan, M. Y. Kariduraganavar, Development of hybrid
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membranes using chitosan and silica precursors for pervaporation separation of water + isopropanol mixtures, J. Chem. Eng. Data, 55(2010) 2084–2092.
F. Shi, Y. Ma, J. Ma, P. Wang, and W. Sun, “Preparation and characterization of
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389(2012) 522–531.
[33] Y. Wu, C. Wu, Y. Li, T. Xu, Y. Fu, PVA-silica anion-exchange hybrid membranes
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prepared through a copolymer crosslinking agent, J. Memb. Sci. 350(2010) 322–332. L.Y. Yu, Z.L. Xu, H.M. Shen, and H. Yang, “Preparation and characterization of
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PVDF-SiO2 composite hollow fiber UF membrane by sol-gel method,” Journal of
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Membrane Science, vol.337, pp. 257-265, 2009. [35] T. Yamaguchi, S.-I. Nakao, S. Kimura, Swelling behavior of the filling-type
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membrane, J. Polym. Sci. Part B Polym. Phys. 35(1997) 469–477.
[36] J. M. Gohil and P. Ray, “Journal of Colloid and Interface Science Polyvinyl alcohol as the barrier layer in thin film composite nanofiltration membranes : Preparation , characterization , and performance evaluation, J. Colloid Interface Sci. 338(2009) 121– 127.
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[37] T. Uragami, K. Okazaki, H. Matsugi, T. Miyata, Structure and permeation characteristics of an aqueous ethanol solution of organic - Inorganic hybrid membranes composed of poly(vinyl alcohol) and tetraethoxysilane, Macromolecules, 35(2002) 9156–9163.
synthesis , characterizations and applications,(2011) 73–94.
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[38] S. Pandey, S. B. Mishra, Sol – gel derived organic – inorganic hybrid materials :
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[39] G. Gong, J. Wang, H. Nagasawa, M. Kanezashi, T. Yoshioka, T. Tsuru, Sol-gel spin coating process to fabricate a new type of uniform and thin organosilica coating on
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polysulfone film, Mater. Lett. 109(2013) 130–133.
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[40] C. Ding, J. Yin, B. Deng, Effects of Polysulfone (PSf) Support Layer on the
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Performance of Thin-Film Composite (TFC) Membranes, J Chem Proc Eng. 1 (2014)
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1-8.
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[41] W. S. W. Ngah, L. C. Teong, M. A. K. M. Hanafiah, Adsorption of dyes and heavy metal ions by chitosan composites : A review, Carbohydr. Polym. 83(2011), 1446–
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1456.
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Figure captions
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A
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LIST OF FIGURES
A
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complexation process.
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Fig. 1 The principles of polymer enhanced ultrafiltration (PEUF) and integrated
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22
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PT
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A
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Fig. 2(a) Cross sectional image for a thin film composite (TFC) membrane
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Fig. 2(b) Cross section image for 13wt% Polysulfone (psf) support membrane
23
100
M1 M2 M3 M4
60
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Weight loss (%)
80
40
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20
0 0
200
400
600
800 o
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Temperature ( C)
1000
Fig. 3 The TGA curves for hybrid membranes incorporated with different TEOS
A
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PT
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M
A
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concentration
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3.4
80
3.2
M4
3.0
60
2.8
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Strain
40
2.4
M3
2.2
Strength (Mpa)
2.6
M2
2.0
20
1.8 1.6
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M1
0
0
1
2
3
4
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TEOS concentration (wt%)
5
A
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PT
ED
M
A
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Fig. 4 Strength vs strain of membranes at various TEOS concentration
25
90
140 120
70 100
60 50
80
40
60
30
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40
20
Swelling Degree (%)
Contact Angle (⁰C)
80
20
10
0 PSF
M1
M2
M3
Membrane
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0 M4
Fig. 5 Swelling degree and contact angle of polysulfone and hybrid membranes with different
A
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PT
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M
A
N
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concentration of cross linker (TEOS).
26
Copper (Cu) ions solution at pH 3
100 80 60 40 20 0 1 3 TEOS Concentration (wt%)
5
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0
(a)
Copper (Cu) ions solution at pH 7
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Percentage removal (%)
120
N
100
A
80
M
60
20 0
A
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PT
0
ED
40
1 3 TEOS Concentration (wt%) (b)
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Percentage removal (%)
120
5
27
Copper (Cu) ions solution at pH 10
100 80 60 40 20 0 1 3 5 TEOS Concentration (wt%) 50 ppm of Cu 150 ppm of Cu 250 ppm of Cu
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0
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Percentage removal, ( %)
120
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(c)
Fig. 6 Percentage removal of copper ion at various TEOS concentration and various
A
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PT
ED
M
A
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concentration of copper feed solution testing at (a) pH 3, (b) pH 7, (c) pH 10
28
120 99.9
99.89
99.93
1
2
3
80 60 40
20
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0
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Percentage removal (%)
100
Time (hours)
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filtration time
M
A
120 Flux Change Percentage (%)
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Fig. 7 The percentage removal of copper ions from industrial wastewater for 3 hours
100
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80
M1 With HA : PVA/Cs/0wt%TEOS
60
M1 without HA : PVA/Cs/0wt%TEOS
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40
M3 with HA : PVA/Cs/3wt%TEOS
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20
M3 without HA : PVA/Cs/3wt%TEOS
0
0
30
60
90 120 150 Time (minutes)
180
210
240
A
Fig. 8 Flux change percentage vs filtration time during anti-fouling analysis for copper feed solution with and without the presence of humic acid solution
29
Table
10 10 10 10
ED PT CC E A
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M1 M2 M3 M4
2 2 2 2
N
-
A
M0
M
Polysulfone membrane 0wt%TEOS 1wt%TEOS 3wt%TEOS 5wt%TEOS
Chitosan Tetraethylorthosilicate (cs) (TEOS) wt.%) -
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Table 1 Formulations of membrane PVA/Cs membrane Membrane Poly(vinyl code alcohol) (PVA)
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LIST OF TABLES
0 1 3 5
30
Table 2 Relative flux decay (RFD) and relative flux recovery (RFR) of the membrane from anti-fouling experiment with and without the presence of Humic acid (HA)
0 wt% TEOS
M3
J0
Jp
J1
(mL/min.m2)
(mL/min.m2)
(mL/min.m2)
0
0.0060
0.0065
1000
0.0061
0 1000
RFD
RFR
0.0054
29.03
90.32
0.0034
0.0042
26.27
69.84
0.0040
0.0019
0.0038
33.93
95.24
0.0048
0.0038
0.0036
3.00
A
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PT
ED
M
A
N
U
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3 wt% TEOS
M1
HA (ppm)
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PVA/Cs Membrane membrane code
76.00
S2213-3437(18)30768-1 https://doi.org/10.1016/j.jece.2018.102845 JECE 102845
To appear in: Received date: Revised date: Accepted date:
16 October 2018 2 December 2018 16 December 2018
Please cite this article as: Kassim Shaari NZ, Sulaiman NA, Rahman NA, Thin Film Composite Membranes: Preparation, Characterization, and Application towards Copper Ion Removal, Journal of Environmental Chemical Engineering (2018), https://doi.org/10.1016/j.jece.2018.102845 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.
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Thin Film Composite Membranes: Preparation, Characterization, and Application towards Copper Ion Removal Norin Zamiah Kassim Shaari*, Nurul Aida Sulaiman, Norazah Abd Rahman Department of Chemical and Process Engineering,
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Faculty of Chemical Engineering, Universiti Teknologi MARA, 40450 Shah Alam, Selangor,
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*Corresponding email: [email protected]
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Highlight
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Abstract
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The TFC with 3 wt.% TEOS exhibited the best properties and performance The TFC capable to remove more than 90% copper (II) ions from a pure solution Feed at pH 7 is found as optimum for separation process through the TFC The treated wastewater through the TFC has permissible concentration of copper The hydrophilic character ensures anti fouling behaviour of the TFC
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Malaysia
A novel thin film composite (TFC) membrane was fabricated from a thin layer of hybrid
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membrane formulated from a blend of polyvinyl alcohol (PVA)/chitosan and cross linked with tetraethylorthosilicate (TEOS), which was layered on the polysulfone (PSF) membrane. Besides producing membrane from polymer blend of PVA/chitosan only, concentrations of TEOS as cross linker was varied at 1wt%, 3wt% and 5wt% with fixed concentration of PVA solution (10wt.%) and chitosan (2wt.%) respectively during the formulation of hybrid membranes. The thermal properties, mechanical strength, water contact angle, swelling
2
measurement and anti-fouling behaviour of these membranes were investigated. The performance of the thin film composite was evaluated through filtration of the pure copper ion solution and the industrial wastewater containing copper respectively. The fabricated TFC with 3 wt.% TEOS was found to possess better tensile strength and strain, and thermal stability as compared with other concentration of TEOS. The membrane also exhibit good
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rejection of copper ion (> 90%) from pure copper solution especially at pH 7 of feed solution. A further treatment conducted on the wastewater reveals that TFC with 3 wt.% TEOS could
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remove the copper ion, where the filtered water could be directly discharged to the river. Furthermore, the TFC portrayed good anti fouling behaviour, where it have 3% of relative
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flux decay and 76% of relative flux recovery even with the presence of humic acid as NOM
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(natural organic matter).
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Keywords: Hybrid membrane; thin film composite membrane; polymer blend; copper ion
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removal; wastewater
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1. Introduction
The presence of heavy metal ions in the water resources can affect human health and
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environment due to its toxicity characteristics and non-biodegradability [1-2]. Copper (Cu) is
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a heavy metal that is highly toxic because it can be very easily absorbed by living organism. In the human body, it can accumulate in lungs, kidneys, liver and also other vital organs [36]. A serious toxic dose of exposure is between 4 mg to 400 mg of copper (Cu) per kg of
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body weight where it can cause gastrointestinal bleeding, intravascular hemolysis, hematuria and acute renal failure. While at lower dose, symptoms like headache, nausea, vomiting and diarrhea appears generally after 15 - 60 minutes of exposure [3-4, 5]. Therefore, various chemical and physical treatments such as chemical precipitation, ion-exchange, flotation and adsorption have been applied to remove copper from wastewater,
3
[7]. However, these conventional heavy metal treatments possess some limitations such as high cost, ineffective removal at low concentration of heavy metal ions and generate high amount of sludge [8]. Membrane separation on the other hand has been proven on the capability to remove suspended solid, organic compound, and inorganic contaminant in the wastewater [3, 9]. Other than that, the main benefit of membrane separation as compared to
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other processes is related to its unique separation principle, which is the transport selectivity of the membrane. Additionally, membrane separations do not need any additives as compared
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to the conventional method, thus the operating costs are cheaper [10].
Recently, polymer enhanced ultrafiltration (PEUF) method as shown in Fig. 1, is
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being widely used for heavy metal ions removal where it uses organic polymer as complexing
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agent that can bind heavy metal ion chemically [11-13]. Llorens et al. [11] had reported on
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the use of chitosan as complexing agent to remove Cadmium ion. It was reported that pH 7-
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7.5 is the suitable range for the removal process, where more than 90% Cadmium ions was removed from the feed solution. Chou et al. [13] studied PEUF process to remove copper
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ions by comparing three polyelectrolyte complexing agents, that are poly(styrenesulfonate) (PSS), poly(allylamine)(PAA) and polyethylenimine (PEI), and it was found that PEI
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exhibited the best performance by removing 94% of copper ion at pH 3. Besides that, there
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are other types of complexing agents that are being used to remove heavy metal ions such as chitosan, carboxylmethyl cellulose, poly(acrylic acid), polyvinyl alcohol and polyvinyl
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ethylenimine [14].
Therefore the aim of the current study is to fabricate a thin film composite (TFC) as
an improved method of polymer enhanced ultrafiltration (PEUF) by combining the complexation of heavy metal and separation process only in one equipment, as described in Fig. 1. The thin film composite consists of a hybrid membrane as the thin layer, which is formed from a polymer blend of poly(vinyl alcohol) (PVA) with chitosan (Cs) and then
4
cross-linked with tetraethylorthosilicate (TEOS). These organics-inorganic hybrid polymer combinations have benefits such as good thermal stability and strong inorganic substrate, causing strong binding affinities toward selected metal ions as the metal ions are chemically bonded by the organic-inorganic polymer hybrids. Furthermore,
with the presence of
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chitosan as the adsorbent, it will ensure a relatively high metal ion adsorption capacities [15]. In the preparation of organic-inorganic composite materials, sol-gel technology offers simple and convenient method. This method involves a sequence process of hydrolysis and
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condensations which led to the membranes matrix modification. TEOS as the silica precursor has been found to yield membrane with a compact structure to provide more area for ion
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adsorption and also the resultant membrane has good mechanical strength [16]. As an organic
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polymer, PVA has many advantages like high anti fouling potential and good thermal
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resistances. However, PVA has low stability and high swelling in water [17]. Therefore, the
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cross linking reaction on the polymer blend of PVA/chitosan is needed to increase the rigidity structure of the membrane and inhibits the swelling of the resultant membrane [18].
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Hybridisation between organic polymer such as PVA with inorganic particles has received significant interest as it does not only control the swelling of PVA but also provides the
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inherent advantages of the organic and inorganic compounds [19-20]. Besides having good
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biological properties, chitosan is used in this study due to its good complexation material where it can separate and adsorb a wide range of pollutants including heavy metals [21-22].
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In this study, the fabricated TFC membranes were subjected to membrane
characterization such as cross sectional morphology, thermal analysis, tensile strength, hydrophilicity and swelling analysis. Objectives of this study was to identify the best concentration of TEOS that yield TFC with good mechanical, thermal and hydrophilic behaviour, as well as able to remove high percentage of copper ion from pure copper
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solution. TFC with optimum concentration of TEOS will be subjected to filtration on wastewater containing copper as well as evaluation on the anti-fouling behaviour. 2. Materials and method 2.1. Materials
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Polysulfone (PSF) pellet resins with molecular weight (44,000 – 53,000 Da), PVA pellets with a hydrolysis degree of 87-89% and molecular weight (85,000 – 124,000 Da), chitosan
(cs)
powder
(deacetylation
degree
84.8
±
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commercial
1.2
m
n%), Tetraethylorthosilicate (TEOS) with 99% purity, hydrochloric acid with 37% purity as
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catalyst, acetic acid and N-methyl-2-pyrrolidone (NMP) with purity of 99% was obtained
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from Merck, Malaysia. Deionized water was also used as coagulation and dilution medium.
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All materials were employed without further purification.
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2.2. Methods
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2.2.1. Preparation of polysulfone (PSF) porous support membrane In the preparation of PSF support membrane, 13 g of polysulfone bead was dissolved in 87 g
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of NMP to produce polysulfone polymer solution with 13 wt% of polymer concentration. The
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solution was stirred continuously for approximately 4 to 6 hours at 60ºC. The solution was left for several hours at room temperature to remove air bubbles. A casting machine was used to cast the polysulfone solution onto a glass plate with adjusted thickness to 100-120 µm. The
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film was immersed in water as a coagulation medium for 1 hour and subsequently, it was left in a large amount of water for another 24 hours. Then, the film was dried in an open air for one day [18,23]. 2.2.2. Preparation of PVA/Chitosan/TEOS hybrid membrane
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In the preparation of hybrid membrane, PVA solution and Cs solution were prepared separately. 2 wt.% Cs solution was prepared by dissolving 3g of chitosan in 2wt% of acetic acid at room temperature with vigorous stirring [18,22]. Next, 10 gram of PVA pellets was dissolved in 90 mL of distilled water with (9:1) ratio under continuous heating at 90ºC for 6 hours and mixing to produce 10 wt% PVA solution [18, 22]. The Cs solution was mixed
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with PVA solution to produce a blend mixture. The mixture was heated at 60ºC for 4 hours to produce a homogeneous solution. Then, the mixture was left to cool at room temperature.
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Next, 3wt% of TEOS as a silica nano precursor was added to the mixture and 1 ml of hydrochloric acid as a catalyst was added to the mixture solution [16,18]. The solution was
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then heated at 30ºC under continuous mixing for 10 hours [18]. Table 1 is a summary of
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hybrid membranes composition used in this study.
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2.2.3. Preparation of Thin Film Composites (TFC) Membrane
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The thin film composite (TFC) membrane was prepared by coating the hybrid membrane
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onto a PSF support using a glass rod. The composite membrane produced was left to dry for 24 hours at room temperature and it was subsequently heat cured in an oven for 1 hour at
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45ºC [16,18].
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2.3. Membrane Characterization The cross section morphology of thin film composite membrane was examined using Scanning electronic microscope (SEM) (TM3000 Tabletop HITACHI, United State of
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America) with an acceleration voltage of 5kV [23]. Perkin – Elmer Spectrum 2000 (FT-IR) instrument was used to examine the functional group and chemical structure of the hybrid membrane. The wavelength used was in the range of 400 cm-1 to 4000 cm-1 [24].
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The thermal stability of membranes was evaluated by using Thermogravimetry Analyzer, Mettler Toledo model Stare SW. The analysis was performed under a nitrogen atmosphere at a heating rate of 10 °C/min. The sample weight ranged from 5 to 10 mg and it was heated from 30oC to 900°C.
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The mechanical strength analysis was performed by using Universal testing machine (Autograph AG-X, Shidmazu, Japan) model. The sample was cut to 100 mm length and 40 mm width. The thickness of the sample was measured using vernier caliper while the speed
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of the machine is set at 5mm/min based on standard state method for plastic strength (ASTM
D2800) [22,25]. The tensile strength, elongation, and tensile modulus of the samples were
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measured.
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The swelling properties of hybrid membranes were measured according to the method
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reported by Xie et. al [19]. Firstly, the dried TFC membrane was immersed in deionised
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water at room temperature for 48 hours. Then, the surface of wet TFC membrane was wiped
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using tissue paper. Immediately, it was weighed to get the mass, Ws before drying in a vacuum oven at 50°C overnight. Then it was weighed again to obtain the mass of dried
(1) [19].
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membrane (Wd). The swelling degree (S) of membrane was then calculated according to Eq.
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W Wd S % s 100 Wd
(1)
The contact angle of membrane samples were measured by using VCA 3000s. The
membrane sample was placed on the VCA platform and the contact angle was measured by using sessile drop method. A minimum of three drops of water were used and the resulted contact angle captured by VCA image were measured.
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2.4 Copper ion removal from pure copper solution The performance of thin film composite (TFC) membrane was tested by using a dead end filtration mode of membrane testing rig. The membrane with an effective surface area of 17.35 cm2 was placed in a sample compartment. The copper concentration in the permeate solution was measured by using Atomic Absorption Spectroscopy (AAS). A calibration curve
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for the copper ion was firstly generated through series of dilution from 1000 ppm copper ion. From the calibration curve, the concentration of copper ions in the solution was identified.
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Three concentrations of copper ion solution were prepared, 50 ppm, 150 ppm and 250 ppm
from dilution of 1000 ppm of that solution. Prior to the filtration process, pH of feed solution
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was adjusted to three conditions, acidic (pH 3), neutral (pH 7) and basic (pH 10) by the
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addition of 1M HCI or 0.01 M NaOH. During the experiment, the feed solution was placed in
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a 300 mL stainless steel filtration cell. The operation was conducted at room temperature
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with 200 mL of feed solution per batch of operation. The pressure was applied at ± 8 bar by using nitrogen gas. The permeate sample was collected in a beaker for volume measurement.
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The flux rate and the percentage removal of copper (Cu) ion from the feed solution were calculated by using Eq. (2) and Eq. (3) respectively.
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V A.t
(2)
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J
where ∆V (mL) is the volume of the permeate sample, A is an effective membrane area in cm2 and ∆t is filtration time (minutes)
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R(%)
C f Cp Cf
x 100
(3)
Where C f and C p is concentration of copper in feed solution and permeate solution respectively 2.5 Copper ion removal from industrial wastewater
9
Based on results from characterization of the thin film composite membranes and performance testing on copper ion solution, membrane M3 was selected to be compared with membrane M1 on their effectiveness to remove copper (Cu) ion from wastewater. The result from this experiment will reveal the significance of cross linking process on a polymer blend. Before subjecting the sample for filtration using the membrane, the wastewater had to go
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through some pre treatment. Firstly, the wastewater was filtered for 3 times by using filter paper brand (Whatman No.1) to remove suspended solid. Then, the initial pH of the sample
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was adjusted to pH 7. The filtration process was conducted in the same procedures as stated in Section 2.4.
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2.6 Anti fouling analysis
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Anti-fouling analysis was examined according to the method reported by Zhu et al. [26].
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Based on the method, the antifouling parameters such as Relative Flux Decay (RFD) and
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Relative Flux Recovery (RFR) were determined analytically by using the equations Eq. (3)
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and Eq. (4) [26]. Besides evaluating on the copper ion solution alone, the antifouling performances of the thin film composites also was being evaluated with the incorporation of
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1000 ppm humic acid (HA) solution, which acts as the foulant model into the copper feed solution. Humic acid was chosen due to it contains major portion of NOM (Natural organic
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matter) in a surface water or ground water, where it is a major contributor to membrane organic fouling in the treatment of wastewater [26]. Membrane M1 and M3 were also
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selected for the antifouling analysis.
J J P RFD O x 100 JO
(3)
J RFR 1 X 100 JO
(4)
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where JO is the pure water flux in the first 30 minutes filtration time, JP is the permeate flux of the feed solution containing copper during 2 hours filtration time, which is subsequently after JO was obtained, J1 is the final pure water flux in 30 minutes filtration time after the
2. RESULT AND DISCUSSION
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3.1. Cross section morphology of thin film composite membrane
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membrane has been backwashed.
The cross-sectional image of TFC membrane which consists of the hybrid
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membrane’s layer on the PSF support membrane is exhibited in Fig. 2(a). In comparison with
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the image of PSF support layer as shown in Fig. 2(b), the top layer of the TFC membrane
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shows a relatively dense cross–section with the changes from finger-like structure to sponge-
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like structure. This situation could be described as reported by Huang & Yang [27], after the hybrid membrane was coated onto the PSF membrane, the silica particles in the hybrid
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membrane dispersed homogeneously into the PSF membrane. Besides that, the diffusion of
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PVA/Cs/TEOS hybrid membrane also could increase the compact resistance and mechanical strength of the resultant membrane [28].
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3.2. Thermal analysis
The thermal stability of hybrid membrane with different concentrations of TEOS was
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identified through the TGA curves as depicted in Fig. 3. It can be seen that all membranes have similar weight loss profile and the weight changes pattern. Three main steps of weight loss were observed. The first weight loss occurred at a temperature below 150°C was related to the removal of the residual moisture from the sample [29]. The second weight loss is around 200ºC to 400ºC corresponds to the removal of hydroxyl group on PVA and chitosan
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[30]. The final weight loss occurred after 500°C was related to the decomposition of the polymer backbones in the hybrid membrane [30]. It was observed that the weight residue at temperature more than 400°C for all membranes increases as the TEOS content increased. This condition indicates that the incorporation of TEOS into the polymer blending of PVA/chitosan significantly stabilized the thermal degradation of the membranes as a result of
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increase in cross linking density of the membrane [30-31].
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3.3. Mechanical properties
As shown in Fig. 4, the tensile strength increases with TEOS concentration. As mentioned earlier, the increase in TEOS content will accelerate the cross linking density to
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encourage the formation of dense structure, that finally improves the mechanical properties of
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hybrid membrane due to it needs more strength to fracture [32]. This situation can be
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described through good compatibility between PVA/Cs and TEOS through formation of
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hydroxyl group when the polymer matrix was trapped between silica precipitates [33]. It was
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observed from Fig. 4 that the tensile strain increases with TEOS content from 1wt% until 3wt% but not for 5 wt% (M4) TEOS, where the strain started to decrease. This finding is in
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agreement with the one reported by Yu et al. [34], where the excess amount of TEOS content can cause a decrease in hybrid homogeneity. The hybrid membrane that has too much TEOS
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caused the film to become more rigid due to the restriction in the movement of polymer molecules, and consequently the membrane exhibited a decrease in flexibility [22].
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3.4. Swelling Studies and Contact Angle Analysis The swelling of any polymer film in a solvent depends upon the diffusion coefficient of the solvent. PVA was chosen as membrane material because of its hydrophilicity and film forming characteristics. However, the excessive solubility on PVA has resulted to membrane swelling, which lead to an open structure of membrane [35]. As a consequence, the
12
membrane rejection will be low [36]. The PVA used in the study has a degree of hydrolysis 87-89% , where the blend with chitosan causes high degree of swelling in water due to interaction between water molecules and hydroxyl groups in both polymers [19], where it was proven in Fig. 5 for membrane M1. Cross linking is one of the methods to reduce polymer swelling by balancing the hydrophoblic–lipophilic of the membranes [19, 35-37] and
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as shown in Fig. 5, the swelling degree of the hybrid membranes decreases with the increase of cross linker (TEOS). This is due to the formation of new chemical bonds between the
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blended polymer and nano silica, where the bonds is formed with the organic polymer chains
through the polar hydroxyl group of PVA and amine group of chitosan in the polymer [19].
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Similar findings were also reported by Pandey et. al [38], which demonstrated that there was
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a considerable decrease in the swelling degree of hybrid films from PVA/silica. Xie et al.
A
(2011) [19] also found that polymer-inorganic nano composite membranes from PVA that
M
was cross linked with maleic acid (MA) and silica respectively has low degree of swelling. Generally, membrane hydrophilicity is greater when its contact angle is smaller [32]. Fig. 5
ED
shows the comparison of water contact angles between PSF support membrane (M0) and the thin film composite membranes (M1, M2, M3 and M4). Although the hydroxyl group are
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consumed during the cross linking process, where the contact angle is expected to increase, it was not reflected in Fig. 5. Instead of increasing the hydrophobicity of the membrane, the
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increase in TEOS concentration has resulted in lowering contact angle of the membrane. This result is similar with the findings obtained by Shi et al. [32], where extra amount of cross
A
linker agent like TEOS addition had restrained the formation of crystallization, lowered the membrane porosity, decreasing the roughness of membrane upper surface and reducing the contact angle of the membrane. Other than that, the presence of coated membrane layer from PVA/Chitosan/TEOS hybrid membrane can improve the surface wettability and affinity of porous support membrane which would possibly increase the water permeation rate during
13
membrane separation [39]. On the other hand, M1 from polymer blend of PVA/Cs showed the lowest contact angle due to its properties of good film-forming and high hydrophilicity. 3.5 Performance of TFC membrane on copper ion removal from pure copper solution and wastewater
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As shown in Fig. 6(a), at pH 3, there was a rapid improvement on the percentage of copper removal especially for membrane with 3wt% (M3) and 5wt% (M4) TEOS as compared that
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with 0 wt% (M1) and 1 wt% (M2) TEOS for every concentration of copper ion. For 50 ppm copper solution, membrane M3 and M4 able to remove 85% copper ion, as compared to M1 5% and M2 27%. Similar phenomenon also observed for 250 ppm copper solution where M3
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and M4 exhibited the same performance to remove 87% copper ion, whereas 57% by M1 and
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63% by M2. These result show that TEOS is not only good to form membrane that has high
A
thermal and mechanical properties but also able to form dense structure of hybrid membrane
M
to trap more heavy metal ion [40]. pH is one of the most important factors in the interaction
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of a metal ion with binding polymer [41]. Fig. 6(b) and (c) show that at pH 7 and pH 10, there is a rapid increase of the percentage removal, where it has achieved more than 90%.
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Ngah et al. [41] also found the similar behaviour in their research, where this could be explained by the fact that at higher pH values, less inhibitory effect of H + exists to protonate
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with amine group from chitosan, which could reduce the number of binding sites for copper ion adsorption. Other than that, the lone pair electrons present in the amine (-NH2) groups
A
and lone pairs of electrons of hydroxyl (-OH) groups can establish bonds with transition metal ions. Although adsorption increases at pH more than 7, there is a formation of copper ion hydroxide in a form of precipitation which might affect the adsorption. Therefore, based on Fig. 6(a),(b) and (c), membrane M3 consisted of 3wt% TEOS was chosen as the optimum cross linker added to the hybrid membrane due to its similar performance with membrane consisted of 5wt% TEOS (M4).
14
Fig. 7 shows the percentage removal of copper ion from industrial wastewater. The membrane used for this experiment is M3 with pH of feed solution was adjusted to pH 7. It can be observed that the percentage removal for copper ion is over 80% and the value was consistent within 3 hours of filtration time. Calculation has been made and the final concentration of copper ion is around 0.2 to 0.5 mg/L, which is within the acceptable limit of
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standard B of effluent discharge as outlined by the Environmental Quality (Industrial Effluent) Regulations 2009.
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3.6 Anti – fouling behaviour
Fig. 8 shows the permeate fluxes, (J) of M1 and M3 during the filtration of copper solution
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with and without humic acid (HA) solution. The presence of HA causes the membranes to
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have lower flux decays but at the same time they have relatively lower flux recovery as
A
compared to that in the solution without HA. Based on Table 2, Relative flux recovery of
M
M3 at 76% with humic acid and 95% without humic acid indicates that the fabricated thin film composite with hybrid membrane has anti fouling behaviour, which can effectively
ED
counter back the fouling caused by NOM. In conclusion, the interaction of HA during the
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Conclusions
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filtration with copper ion solution had less effect on the membrane fouling.
The fabricated TFC membranes have been proven to possess good thermal and mechanical stabilities with hydrophilicity character. This new type of PVA/chitosan/TEOS hybrid
A
membrane was found able to remove more than 90% of copper (Cu) from pure copper solution and better separation can be achieved at higher pH. However, in order to avoid the hydroxide precipitation at pH 10, pH 7 was found to be the optimum condition for the removal of copper. In terms of industrial wastewater, it was apparent that the composite
15
membrane with 3wt.% TEOS (M3) was successful in treating the wastewater and allowing it to be directly discharged to the groundwater.
Declaration of interest: none
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Acknowledgement
The author would like to thank Ministry of Higher Education Malaysia, which has sponsored
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this research through Fundamental Research Grant Scheme with file no. 600-RMI/FRGS 5/3
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(92/2014).
X. Zhang, Y. Wang, Y. Liu, J. Xu, Y. Han, and X. Xu, Applied Surface Science
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[1]
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Figure captions
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A
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LIST OF FIGURES
A
CC E
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complexation process.
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Fig. 1 The principles of polymer enhanced ultrafiltration (PEUF) and integrated
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22
CC E
PT
ED
M
A
N
Fig. 2(a) Cross sectional image for a thin film composite (TFC) membrane
A
Fig. 2(b) Cross section image for 13wt% Polysulfone (psf) support membrane
23
100
M1 M2 M3 M4
60
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Weight loss (%)
80
40
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20
0 0
200
400
600
800 o
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Temperature ( C)
1000
Fig. 3 The TGA curves for hybrid membranes incorporated with different TEOS
A
CC E
PT
ED
M
A
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concentration
24
3.4
80
3.2
M4
3.0
60
2.8
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Strain
40
2.4
M3
2.2
Strength (Mpa)
2.6
M2
2.0
20
1.8 1.6
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M1
0
0
1
2
3
4
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TEOS concentration (wt%)
5
A
CC E
PT
ED
M
A
N
Fig. 4 Strength vs strain of membranes at various TEOS concentration
25
90
140 120
70 100
60 50
80
40
60
30
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40
20
Swelling Degree (%)
Contact Angle (⁰C)
80
20
10
0 PSF
M1
M2
M3
Membrane
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0 M4
Fig. 5 Swelling degree and contact angle of polysulfone and hybrid membranes with different
A
CC E
PT
ED
M
A
N
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concentration of cross linker (TEOS).
26
Copper (Cu) ions solution at pH 3
100 80 60 40 20 0 1 3 TEOS Concentration (wt%)
5
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0
(a)
Copper (Cu) ions solution at pH 7
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Percentage removal (%)
120
N
100
A
80
M
60
20 0
A
CC E
PT
0
ED
40
1 3 TEOS Concentration (wt%) (b)
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Percentage removal (%)
120
5
27
Copper (Cu) ions solution at pH 10
100 80 60 40 20 0 1 3 5 TEOS Concentration (wt%) 50 ppm of Cu 150 ppm of Cu 250 ppm of Cu
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0
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Percentage removal, ( %)
120
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(c)
Fig. 6 Percentage removal of copper ion at various TEOS concentration and various
A
CC E
PT
ED
M
A
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concentration of copper feed solution testing at (a) pH 3, (b) pH 7, (c) pH 10
28
120 99.9
99.89
99.93
1
2
3
80 60 40
20
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0
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Percentage removal (%)
100
Time (hours)
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filtration time
M
A
120 Flux Change Percentage (%)
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Fig. 7 The percentage removal of copper ions from industrial wastewater for 3 hours
100
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80
M1 With HA : PVA/Cs/0wt%TEOS
60
M1 without HA : PVA/Cs/0wt%TEOS
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40
M3 with HA : PVA/Cs/3wt%TEOS
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20
M3 without HA : PVA/Cs/3wt%TEOS
0
0
30
60
90 120 150 Time (minutes)
180
210
240
A
Fig. 8 Flux change percentage vs filtration time during anti-fouling analysis for copper feed solution with and without the presence of humic acid solution
29
Table
10 10 10 10
ED PT CC E A
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M1 M2 M3 M4
2 2 2 2
N
-
A
M0
M
Polysulfone membrane 0wt%TEOS 1wt%TEOS 3wt%TEOS 5wt%TEOS
Chitosan Tetraethylorthosilicate (cs) (TEOS) wt.%) -
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Table 1 Formulations of membrane PVA/Cs membrane Membrane Poly(vinyl code alcohol) (PVA)
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LIST OF TABLES
0 1 3 5
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Table 2 Relative flux decay (RFD) and relative flux recovery (RFR) of the membrane from anti-fouling experiment with and without the presence of Humic acid (HA)
0 wt% TEOS
M3
J0
Jp
J1
(mL/min.m2)
(mL/min.m2)
(mL/min.m2)
0
0.0060
0.0065
1000
0.0061
0 1000
RFD
RFR
0.0054
29.03
90.32
0.0034
0.0042
26.27
69.84
0.0040
0.0019
0.0038
33.93
95.24
0.0048
0.0038
0.0036
3.00
A
CC E
PT
ED
M
A
N
U
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3 wt% TEOS
M1
HA (ppm)
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PVA/Cs Membrane membrane code
76.00