concentration”

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JTICE-999; No. of Pages 10 Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx

Contents lists available at ScienceDirect

Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice

Electrochemical characterization of mixed matrix nanocomposite ion exchange membrane modified by ZnO nanoparticles at different electrolyte conditions ‘‘pH/concentration’’ F. Parvizian a,*, S.M. Hosseini a, A.R. Hamidi a, S.S. Madaeni b, A.R. Moghadassi a a b

Department of Chemical Engineering, Faculty of Engineering, Arak University, Arak 38156-8-8349, Iran Membrane Research Centre, Department of Chemical Engineering, Faculty of Engineering, Razi University, Kermanshah 67149, Iran

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 May 2014 Received in revised form 29 July 2014 Accepted 16 August 2014 Available online xxx

In this research, novel PVC based nanocomposite ion exchange membrane was prepared by casting technique. ZnO nanoparticles were employed as semiconductor additive in membrane fabrication. The effect of nanoparticles concentration on properties of home-made membranes was studied. A relatively uniform surface for the membranes and uniform particles distribution were observed in SOM images. The membranes were characterized in different electrolyte conditions ‘‘concentrations/pH’’. Results revealed that membrane water content, membrane potential, transport number and selectivity were enhanced initially by increase of additive concentration up to 10 wt% in membrane matrix and then showed decreasing trend by more additive loading. The ionic permeability and flux were also decreased initially by increase in additive content up to 5 wt%. The permeability and flux were increased another time by more increase in additive content. An opposite trend was observed for membrane electrical resistance. Moreover, membranes showed higher transport number and selectivity at pH 7 compared to other pH values. Membrane electrical conductivity was decreased by increase of pH value. Also membrane transport number and selectivity were initially enhanced by increase of electrolyte concentration and then began to decrease slightly at high electrolyte concentration. ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: Nanocomposite Cation exchange membrane ZnO nanoparticles Electrolyte condition pH/concentration

1. Introduction Nowadays, easy scaling-up and low energy consumption of membrane separation in comparison with other caused to expand industrial applications of membranes. Thus, various membranes have developed for the use in reverse osmosis, nanofiltration, ultrafiltration, microfiltration, pervaporation separation and electrodialysis [1]. Ion exchange membranes (IEMs) are used in various electrically driven processes such as desalination of saline water by electro dialysis, reconcentrating brine from seawater, salt production, fuel cells, recovery of valuable metals from effluents industries, etc. [2–16]. The Donnan membrane equilibrium principle is the base of ion exchange membrane separation [1]. Ion-exchange membranes can be divided into two major categories of homogeneous or heterogeneous according to structure and preparation procedure. Also there are two types of

* Corresponding author. Tel.: +98 86 32625434; fax: +98 86 32625435. E-mail address: [email protected] (F. Parvizian).

IEMs based on different functional groups, i.e. anionic (bearing – CH2N+R3 groups) and cationic (bearing –SO3 groups) ones [17]. Anion exchange membranes pass anions and repel cations. While cation exchange membranes allow passing cations but reject anions [18]. There are several approaches to improve selectivity and ionic flux of ion exchange membranes. Many research have reported various methods to modify ion exchange membranes features. Change in functional groups or cross-link density, polymers blending, surface modification and use of various additives, are important ways to gain superior IEMs [5,14,18–25]. The latest initiative is development of composite ion exchange membranes using inorganic materials in polymeric matrix [26]. Composite ion exchange membranes besides improvement of efficiency, show remarkable changes in IEMs properties, such as mechanical strength, thermal stability, electrical, magnetic and etc., compared to pure organic polymeric membranes [4,27–32]. In the past decade, semiconductor materials such as metal oxide nanoparticles have been widely studied in various areas because of their unique properties. Among metal oxide materials, ZnO has remarkable physical and chemical properties such as wide

http://dx.doi.org/10.1016/j.jtice.2014.08.017 1876-1070/ß 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article in press as: Parvizian F, et al. Electrochemical characterization of mixed matrix nanocomposite ion exchange membrane modified by ZnO nanoparticles at different electrolyte conditions ‘‘pH/concentration’’. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.08.017

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band gap, good chemical stability, low dielectric constant and represents antibacterial properties [33–42]. Also, Zinc oxide is considerable because of its lower prices, lightness and ease of accessibility and is nontoxic [43,44]. There are a few studies about development of Zinc Oxide composite membranes and the literature is silent on IEMs comprising ZnO nanoparticle. The primary target of current research was preparing the novel nanocomposite cation exchange membranes with appropriate physico-chemical properties for the application in electro-dialysis processes related to water recovery and treatment. For this purpose, Polyvinyl chloride (PVC) based heterogeneous cation exchange membranes were prepared by solution casting techniques using tetrahydrofuran as solvent, cation exchange resin powder as functional groups agent, and ZnO nanoparticle as additive. PVC was chosen because of its low cost, excellent resistance to erosion, acids, alkaline and its stiffness [45–48]. Besides dependency of IEMs’ behavior on their physicochemical properties, the nature of electrolyte solution also affects on their performance [49]. Few researches have reported the effect of environmental conditions on IEMs properties despite they can have significant influences [24,50–52]. This investigation can be important because of the formation of boundary layers on the membranes surface and the occurrence of concentration polarization phenomena and also change in membranes functional groups dissociations under different conditions. Therefore at the next step of this research, the modified ion exchange membranes were characterized in different electrolytic environment to evaluate their potential performance under various conditions. The variations of membrane resistance, ionic selectivity and transport number were studied in NaCl solution with concentration range of 0.001–0.1 M and pH of 2, 7 and 12. The obtained results could be of particular importance for various electro-membrane processes especially the electrodialysis and further improvement of separation efficiency in wastewater treatment.

grinded resin particle as functional groups agents and various amounts of ZnO nanoparticle as additive were dispersed in polymeric solutions, respectively (Table 1). The polymeric solution was mixed severely for more than 6 h to achieve uniform particles distribution. To avoid particles’ aggregation the solution was sonicated for 1 h using an ultrasonic instrument and then the mixture was stirred again for another 30 min using the mechanical stirrer. The prepared solution was casted onto a clean and dry glass plate with 400 mm casting knife at 25 8C. After 60 s air exposure, the membranes were dried at ambient temperature and immersed in distilled water. The membranes were then soaked in NaCl solution. Table 2 represents a brief description of membrane preparation procedure steps. A digital caliper device was used to measure the membrane thickness and ensure its value was around 70 mm. 2.3. Experimental test cell The electrochemical properties of the prepared membranes were evaluated through a series of experiments in a test cell (Fig. 1) as reported earlier [3,26,29]. The sample membrane disk was fixed between rubber rings and separate two cylindrical compartments (each 180 cm3) of cell which made of Pyrexglass. The two ends of each compartment were equipped with Pt electrodes supported with pieces of Teflon. Two openings were placed at the top of each section for feeding and sampling purposes. To minimize the boundary layers’ effect on the vicinity of membrane during experiments both sections were stirred vigorously by magnetic stirrers. 2.4. Membrane characterization 2.4.1. FTIR spectra FTIR test were carried out to provide information about the chemical structure of membranes using a single beam Fourier transform-infrared spectrometer (Galaxy series 5000). Scans of the samples were taken in the spectral range of 4000–500 cm1.

2. Material and method 2.1. Materials Polyvinyl chloride (PVC, grade S-7054, high porosity, bulk density (DI N-53466): 490 g/l, viscosity number (DIN-53726): 105 cm3/g) supplied by Bandar Imam Petrochemical Company (BIPC), Iran, was used as binders. Tetrahydrofuran (THF) as solvent and cation exchange resin (Ion exchanger Amberlyst1 15, strongly acidic cation exchanger, H+ form-more than 1.7 meq/g dry, spec. density 0.6 g/cm3, particlesize (0.355–1.18 mm) 90%) as functional groups agent by Merck Inc., Germany, were used in membrane fabrication. ZnO nanoparticle (ZnO nano-white-powder, average particle size 20 nm, BET area 50 m2/g) was also used as inorganic additive. Distilled water was used for all experiments. The other chemical components were purchased by Merck. 2.2. Preparation of nanocomposite membranes The heterogeneous cation exchange membranes were prepared by solution casting technique and phase inversion method described in the literature [26]. The resin particles were firstly dried in oven (SANEE. V. S. Co.) at 30 8C for 48 h and then pulverized into fine particles in a ball mill (Pulverisette 5, Fritisch Co.). Then the ion exchange resin sieved to desired size (37– 44 mm) for membrane fabrication. Casting solutions were prepared by dissolving the PVC as base polymer in the THF solvent in a glass reactor equipped with a mechanical stirrer (Mode l: VelpSientifica Multi 6 stirrer). Afterward a specific quantity of

2.4.2. Morphological studies Due to the structure of prepared membrane especially the spatial distribution of ionic site can affect the membrane behavior

Table 1 Composition of casting solution used for membranes’ preparation. Membrane Sample Sample Sample Sample Sample

1 2 3 4 5

Zinc oxide nanoparticle (additive:total solid) (w/w) 0:100 5:100 10:100 15:100 20:100

(Solvent (THF):Polymer (PVC) (v/w): (20:1)); (Resin particles:Polymer (w/w): (1:1)).

Table 2 Flowsheet of membrane preparation procedure. The procedure for IEMs preparation Step Step Step Step Step Step Step Step Step

1 2 3 4 5 6 7 8 9

Resin particles draying (at 30 8C for 48 h( Resin particles pulverizing (300 + 400 mesh) Polymer dissolving into solvent (for 5 h) Resin particles and additive dispersing in polymeric solution Sonication of polymeric solution (for 1 h) Mixing of polymeric solution (for another 30 min) Casting (at 25 8C) Film drying (at 25 8C for 30 min) and immersing in water Membranes pretreatment by HCl and NaCl solutions

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counter-ion migration through the ion-exchange membrane [2–4,7,53]: Ps ¼

tim  t 0 1  t0

(3)

where t0 is the transport number of the counter ions in solution [55]. The ionic permselectivity (Ps) is a real quantitative measure of characteristic difference between counter- and co-ions permeability through the membrane [49]. Fig. 1. Schematic diagram of test cell: (1) Pt electrode, (2) magnetic bar, (3) stirrer, (4) orifice, (5) rubber ring, (6) membrane.

[27]; morphological studies were done by scanning optical microscopy (SOM Olympus, model IX 70). In this technique the scanning were done in transmission mode with light going through the membrane. For this purpose, the small samples of prepared membranes were mounted between two lamellas and observation was made by the optical microscope. 2.4.3. Water content The membrane’s water content, f, defined as the weight difference between the dried and swollen membranes, is determined by [2–4,10]:

f¼



Mwet  Mdry M dry



 100

(1)

2.4.5. Ionic permeability and flux of ions The measurements of ionic permeability and flux were carried out using the test cell (Fig. 1). NaCl solutions with unequal concentrations were placed on two side of the cell. A DC electrical potential (Dazheng, DC power supply, Model: PS-302D) with an optimal constant voltage (10 V) was applied across the cell consisting stable platinum electrodes. If the potential be exceed the optimum value, the side reactions (dissociation of water) will be occurred which causes to decrease the current efficiency of the electrodialysis process. During the experiment, both sections were recirculated with the flowrate of 10 ml/min and stirred vigorously to minimize the effect of boundary layers. The cations pass through the membrane to cathodic section. Also, according to anodic and cathodic reactions the produced hydroxide ions remain in cathodic section and increase the pH of this region. 2H2 O þ 2e ! H2 " þ 2OH ðcathodic reactionÞ

(R-1)

Mwet and Mdry represent the mass of the wet and dry membrane, respectively. To find Mwet, the membrane sample was immersed into the deionized water for several days and was weighed (OHAUS, PioneerTM, readability: 104 g, OHAUS Corp.). Then, same membrane was dried in oven at 60 8C until constant weight was obtained as Mdry.

2Cl ! Cl2 " þ 2e ðanodic reactionÞ

2.4.4. Membrane potential, transport number and permselectivity When both surfaces of ion exchange membrane are in contact with a solution having different concentration, an electrical potential would develop across the membrane. Besides electrical characteristic of membrane this electrical potential depends on nature and concentration of the electrolyte solution. The membrane potential is the algebraic sum of Donnan and diffusion potentials [20,24,53]. This parameter was evaluated for the equilibrated membranes with unequal concentrations of ionic solution at ambient temperature on either side of membrane. During the experiment, both sections were stirred vigorously by magnetic stirrers to minimize the effect of boundary layers on the measurement. The developed potential difference across the membrane was measured after 15 min by connecting both compartments and using saturated calomel electrode (through KCl bridges) and digital auto multimeter (DEC, Model: DEC 330F C, Digital Multimeter, China). The measurement was repeated to gain a constant value. The membrane potential (EMeasure) is expressed using the Nernst equation [2–4,20,53] as follows:    
RT a1 ln (2) EMeasure ¼ 2tim nF a2

where, P is the permeability coefficient of ions, D diffusivity of the electrolyte, dCmem/dx change of concentration in membrane, d the membrane thickness, N the ionic flux and C is the cation concentration in the compartments. k is the distribution coefficient for the sorption equilibrium.

where tim is the transport number of the counter ions in the membrane phase, R is the gas constant, T is the temperature, n is the electrovalence of the counter ion, and a1, a2 are solutions electrolyte activities in contact with both surfaces determined by the Debye-Huckel limiting law [54]. The ionic permselectivity of membranes is also expressed quantitatively based on



(R-2)

Based on the first Fick’s law, the flux of ions through the membrane can be expressed as follows [56,57]: N ¼ D

N¼

dC mem dC dC ¼ P ¼ D  k dx dx dx

V dC 1 C1  C2  ¼P A dt d

C10 ¼ 0:1 M;

C20 ¼ 0:01 M;

(4)

(5) C 1 þ C 2 ¼ C10 þ C20 ¼ 0:11 M

(6)

where, A is membrane surface area. Integrating Eq. (5) was as follows: ln

ðC10 þ C20  2C 2 Þ 2PAt ¼ Vd ðC10  C20 Þ

(7)

The permeability coefficient and flux of cations in membrane phase are calculated from Eq. (7) considering pH changes measurements (Digital pH-meter, Jenway, Model: 3510) in cathodic section. Also, these parameters can be determined by variations in conductivity (Digital conduct-meter, Jenway, Model: 4510) in cathodic compartment as described earlier [2–4]. 2.4.6. Electrical resistance The measurement of membrane resistance is important to evaluate the contribution of various functional groups for ion exchange duty and it plays an important role in the practical application of ion exchange membranes. Membrane resistance was calculated based on our previous works by using a test cell [2–4]. The measurement was carried out using 0.1 and 0.5 M NaCl

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solutions. In brief, a membrane sample was immersed in 0.5 M NaCl solution for 24 h. After washing the membrane with distilled water, it was placed into the cell. NaCl solution with same concentration was supplied in the both cell compartments at 25 8C. The electric resistance (R1) was measured by an alternating current bridge with the frequency of 1500 Hz (Audio signal generator, Electronic Afzar Azma Co. P.J.S). In the next step, the membrane sample was pulled out; the apparatus was reintegrated without a membrane, and the electric resistance (R2) was measured. The membrane resistance can be calculated by subtracting R2 from R1 (Rm = R1  R2). The electrical resistance has practical importance due to its relation with energy consumption in the process. The electrical resistance of the membranes was measured as follows [26,29]: r ¼ ðRm AÞ

(8)

where r is areal resistance and A is surface area of the membrane. To minimize the experimental error, measurements were carried out three times for each sample and then their average was reported. 3. Results and discussion 3.1. FTIR analysis FTIR spectroscopy studies were carried out on the pure PVC, pure ZnO nanoparticles and PVC/ZnO nanocomposites membrane as shown in Fig. 2. The spectrum of pure PVC (Fig. 2(a)) presented the absorption bands at 3440, 2913, 1631, 1251, 964 and 611 cm1 [58]. Table 3 represents the assignments of main representative bands of PVC. For ZnO nanoparticle (Fig. 2(b)) a broadband was observed at 3383 cm1, which may be attributed

Table 3 Characteristic absorption bands for pure PVC [60]. Band position (cm1)

Assignment

3440 2913 1631 1251 1096 966

Stretching OH Stretching C–H C5 5O Deformation C–H Stretching C–C Rocking CH2

to OH stretching. The observed adsorption band at 1645 cm1 is due to the presence of moisture in the sample and the bending in the vicinity of 500–600 cm1 is assigned to Zn–O stretching of ZnO. These data are in agreement with previously reported results [58–60]. In Fig. 2(c) the same spectra in the range of 700– 3100 cm1 was shown for PVC/ZnO nanocomposites membrane, while the observed weak peak at 500–700 cm1 is assigned to the Zn–O bond stretching. Also at 1631 cm1, there was an increase in the absorption band of the bending O–H group, which may be because of the complexation between PVC and the ZnO nanoparticles [60]. 3.2. Morphological studies In this work, the morphology, ionic sites condition (resin particles) and also additive particle (ZnO nanoparticle) spatial distribution in the membrane matrix were evaluated by SOM studies. Fig. 3((a)–(e)) exhibit the membranes’ SOM images. In these images the polymer binder (non-conducting area) and resin particles (conducting ion exchange areas) are clearly observed. The dark spots represent the particles. These dark regions are increased with higher additive (zinc oxide nanoparticle) loading as seen in the images. Also, by increasing additive content the distribution of particles in membranes matrix are more uniform because of increase in particle density. Images show sonication has a significant effect on distribution of resin particles in the prepared membranes and results in formation of relatively uniform phase [4,61]. As can be seen, nanoparticles are uniformly distributed in the prepared membranes too. More homogen distribution of resin particles as functional group on the membrane surface and in the bulk of membrane matrix generates superior conducting regions in the membrane. This can improve the membranes electrochemical properties due to provide easy flow channels for the counter-ion transportation. Also the presence of more conducting region on the membrane surface enhances the uniform electrical field around the membrane, so the concentration polarization phenomenon decreases [62]. Increasing the casting solution viscosity due to sonication of solid particles causes to reduce the evaporation rate of casting solvent [63]. Therefore, the polymer chain has more relaxation which creates suitable conformation with particle surfaces. This adaption reduces the possibility of creating cracks and seams between binder and particles in the membrane matrix and improves the membrane selectivity. 3.3. Effect of ZnO nanoparticles’ concentration

Fig. 2. FTIR spectroscopy for (a) pure PVC, (b) pure ZnO nanopowder, (c) PVC/ZnO.

3.3.1. Water content The water content of PVC based zinc oxide nanocomposite membrane is shown in Fig. 4. Results show increasing zinc oxide nanoparticles loading ratio up to 10 wt% increases the membrane water content. This fact is due to the high affinity of hydrophilic ZnO nanoparticles to water that result in increasing hydrophilicity of membrane and cause to diffuse a larger fraction of water through the membrane structure. Also, with increase in additive

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Fig. 3. The SOM images (10 magnifications) of home-made membranes with different concentration of ZnO nanoparticle: (a) 0.0 wt%; (b)5.0 wt%; (c)10.0 wt%; (d)15.0 wt%; (e)20.0 wt%.

loading ratio the membrane heterogeneity improves which increases the voids and cavities throughout the membrane matrix and so increases water absorption and accommodation. However, more increment of nanoparticle concentration from 10 to 20 wt% leads to decrease in water content. This may be due to pores filling [64] by the zinc oxide nanoparticle at high additive loading ratio. This causes to occupy the channels and spaces in the membrane matrix and so reduces the membrane water content. Appropriate membrane water content aids to have better control on ions pathways and improves membrane permselectivity [65]. While the high water content lead to loosening of the membrane structure. Measurements were carried out three times for each sample to minimize the experimental errors, and then their average value was reported.

3.3.2. Membrane potential, permselectivity and transport number The membrane potential, transport number and permselectivity data obtained for prepared nanocomposite membranes in NaCl solution are shown in Figs. 5 and 6. These figures exhibit the dependency of membranes’ properties to ZnO nanoparticle concentration. Results explain all modified membranes has better potential in comparison with pristine one (Fig. 5). At first, the membrane potential increased with the increase of zinc oxide nanoparticle up to 10 wt% in the prepared membranes. This may be due to nanoparticles provide more conducting regions in the membranes and creates suitable flow channels to facilitate counter ions passage. So the Donnan exclusion as responsible of the membrane potential increment enhanced [4,19,20]. As shown in this figure,

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50

(10 %wt)

Transport Number 1

40

1

(5 %wt) (15 %wt)

20

(20 %wt)

10

0.95

0.95

0.9

0.9

0.85

0.85

Transport Number

(0 %wt)

30

Permselectivity

Water Content (%)

Permselectivity

0 0

1

2

3 4 Samples' Number

5

6

0.8

0.8 1

0

2

Fig. 4. The effect of ZnO nanoparticle loading on membrane water content.

3

4

5

6

Samples' Number Fig. 6. Comparison between the transport number and permselectivity of prepared membranes with various additive loading.

3.3.3. Ionic permeability and flux of ions Since the ions pass through the membrane during the experiments to reach the concentrated section, the permeation of counter ions can be calculated from the pH changes in cathodic region. Results revealed that increase in zinc oxide nanoparticles concentration up to 5 wt%, initially decrease the membrane ionic permeability and flux as shown in Fig. 7. This fact may be related to some ionic pathways blocking by added nanoparticles, so the channels are narrower and the ions transport declines. With increasing nanoparticle concentration from 5 to 10 wt% in casting solution, the ion permeability and flux were increased again. The significant effect of zinc oxide nanoparticle on promoting membrane ionic permeability and flux may be because of the hydrophilicity of nano-ZnO [66]. Since ZnO nanoparticle has wurtzite structure and the structure of PVC is partially crystalline, its addition causes to reduce crystallinity degree and increases the amorphous region [60]. This can affect on the membrane water content improvement which provides appropriate channels in membrane matrix to facilitate the ions passage, so ionic flux enhances [65]. Also the results in Fig. 6 shows an overdose of ZnO nanoparticles more than 10 wt% rendered permeability loss generally. This can be due to the addition of zinc oxide nanoparticle to a sufficient extent could cause changes in pore size and membrane microstructure [66].

Flux

54 (5 %wt)

53

(10 %wt)

Flux (mol/m 2.s)*105

(15 %wt)

Potential (mV)

52 51 50

(20 %wt)

49

Permeability

6

24.03

5.9

24.02

5.8

24.01

5.7

24

5.6

23.99

5.5

23.98

Permeability (m/s)*107

the membrane potential decreased with overdose of additive concentration from 10 to 20 wt%. The reason may be due to adding more nanoparticles leads to their agglomeration which decreases the effective surface area of active sites. So the adsorptive characteristic of membranes reduces [64]. Fig. 6 illustrates the transport number and permselectivity of membranes. As can be seen in this figure, with increasing the nano-ZnO particles loading (up to 10 wt%) in casting solution, the transport number and permselectivity enhanced. It may be due to the adsorption property of ZnO nanoparticle that improved ion passage control. Also, the nanoparticle addition causes to occupy the ionic pathways in the membrane matrix and so narrowed them as space limiting factors. Therefore the domination of ionic site on ion traffic intensifies and the membrane permselectivity improves. Transport number and permselectivity are decreased again with more increment of nanoparticle concentration from 10 to 20 wt%. This can be explained by lower charge density of membrane due to the nanoparticles’ agglomeration which facilitates coion percolation through the membrane and reduces the selectivity. Furthermore, at higher additive loading, discontinuity of polymer chain binder due to enhancement of particle density in the casting solution reduces the membrane selectivity [26].

(0 %wt)

48

5.4

47 0

1

2

3

4

5

6

Samples' Number Fig. 5. The membrane potential of prepared membrane with various ratios of additive loading.

23.97 0

1

2

3

4

5

6

Samples' Number Fig. 7. The ionic flux and permeability of sodium ions for the home-made cation exchange membranes at various blend ratios of ZnO nanoparticles concentration.

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solution in the membrane phase [50] is higher. Also occurrences of concentration polarization phenomenon nearby the membrane surface increase the possibility of co-ions percolation [29,49,68].

a

0%wt ZnO

20%wt ZnO

50

40

30

20

10 0.001-0.01 (M)

0.01-0.1 (M)

0.05-0.5 (M)

Solution Concentration (M)

b

0%wt ZnO

10%wt ZnO

20%wt ZnO

1

3.4. Evaluation of electrochemical properties of membranes in various solution concentration and pH

0.9

Transport Number

As shown in Fig. 9((a)–(c)), membrane potential, transport number, and permselectivity initially enhanced with increasing electrolyte concentration. This may be due to the more counter ions presence in the electrolyte environment [24] which increases the possibility of interaction between counter ions and membrane surface. So Donnan exclusion enhanced that is responsible for the increment of membrane potential, transport number, and permselectivity. The obtained results are significantly in contrast with Donnan equilibrium theory [51]. A slightly decrease in all mentioned parameters takes place for more increase in ionic solution concentration. This is because of at high solution concentration the osmotic pressure difference between the external salt solution and internal

10 %wt ZnO

60

Potential (mV)

3.3.4. Electrical resistance Fig. 8 shows the areal electrical resistance of prepared membranes (in 0.5 M NaCl solution) which was initially enhanced with increasing additive concentration to 5 wt%. This may be due to the occupying of ionic channels by ZnO nanoparticles that result in formation of narrow ionic pathways throughout the membrane matrix. With more increase of additive loading from 5 to 10 wt% the areal electrical resistance was decreased due to the hydrophilicity characteristic of nano-ZnO and increase in membrane water content. It is noticeable the suitable amount of water content and also the superior interactions between ions and membrane surface improve the intensity of uniform electrical field around the membrane. The latter can be assigned to adsorption property of zinc oxide nanoparticle, thus the membrane conductivity increased. Moreover, zinc oxide nanoparticles as a semiconductor substance [67] lead to increase the electrical conductivity of PVC/ZnO nanocomposite membrane relative to pure PVC membrane so the electrical resistance decreased. Usually less selective membranes have lower membrane resistances but this may be not true always because of dependency of membrane electrical resistance on its structure and properties [65]. The areal electrical resistance was increased again with more ZnO nanoparticle concentration from 10 to 20 wt% in casting solution. This is attributed to decrease in membrane water content because of narrow channels formation in membrane matrix which in turn reduces the ions transport and so enhances the electrical resistance [26].

7

0.8

0.7

0.6

0.5 0.001-0.01 (M)

0.01-0.1 (M)

0.05-0.5 (M)

Solution Concentration (M)

c

0%wt ZnO

10 %wt ZnO

20%wt ZnO

18 1

17

0.8

16.5

Permselectivity

Areal Resistance (Ohm.cm 2)

17.5

16 15.5

0.6

0.4

15 0.2

14.5 14

0

1

2

3

4

5

Samples' Number Fig. 8. The areal electrical resistance of prepared membranes with various zinc oxide nanoparticle concentrations.

0.001-0.01 (M)

0.01-0.1 (M)

0.05-0.5 (M)

Solution Concentration (M) Fig. 9. The effect of solution (NaCl) concentration variations on (a) membrane potential, (b) transport number and (c) permselectivity

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Moreover, the membrane swelling at high salt solution concentration may decrease the membrane potential, transport number, and selectivity [69]. In addition, the results indicate in pH 7 the membrane potential, transport number, and selectivity is higher in comparison with other pH 2 and pH 12 (Fig. 10((a)–(c))). This is because of

a

0%wt ZnO

10%wt ZnO

20%wt ZnO

60

Potential (mV)

55

3.5. Investigation of environmental pH and electrolyte concentration’s effect on membrane electrical resistance

50 45 40 35 30 2

7

12

pH

b

0%wt ZnO

10%wt ZnO

20%wt ZnO

1

0.9

Transport Number

dissociation of membrane functional groups in various pH values is different which affect the charge nature of membrane matrix [24] and enforces the ionic sites domination on ions passage. Moreover, this behavior may be explained by considering the surface charge of ZnO nanoparticles (–ZnOH2+, –ZnOH, –ZnO) in different pH values. With increasing pH value the concentrations of –ZnO group increases while –ZnOH2+ and –ZnOH groups decrease [58]. Consequently, at suitable electrolyte pH, the membrane charge density increases and dominance of ion-exchange functional groups on ions transport reinforces. This is due to more dissociation and activation of ion-exchange functional groups in the membrane matrix.

0.8

0.7

0.6

0.5 2

7

12

The effects of electrolyte concentration on the membrane electrical resistance are shown in Fig. 11. In the case of pristine membrane (0 wt% nano-ZnO) the electrical resistance is higher at lower salt concentration (0.1 M). This is attributed to the diffusion boundary layer resistance which is more significant at lower salt concentration [29,50]. Moreover, at high salt solution concentration, the membrane swelling decreases the electrical resistance, membrane potential, transport number, and selectivity [29]. The modified nanocomposite membrane with optimized percentage of zinc oxide nanoparticle (10 wt%) showed less electrical resistance variations at high and low solution concentrations. It indicates zinc oxide sorbent active sites in membrane matrix were saturated and further increase of electrolyte concentration had no significant effect on the membrane electrical resistance [70]. However, membrane resistance was slightly lower at low concentration. This may be due to with increasing solution concentrations the adsorption capacity of nano-ZnO particle increases [70]. Also obtained results showed the membrane electrical resistance increased with increasing pH value (Fig. 12). Variations of membrane conductance values may be explained in terms of dissociation of membrane functional groups in the given electrolyte environment [24]. Moreover, this can be explained based on sorption behavior of ZnO nanoparticles in environments with different pH values. Because of presence of OH– groups at high pH the surface charge of zinc oxide is more negative that leads to formation of hydroxyl complexes. Such hydroxyl compounds are responsible for the uptake of the counter ions from the solution. In contrast, the low degree of sorption and thus electrical resistance

pH

c

0%wt ZnO

10%wt ZnO

20%wt ZnO

1

Permselectivity

0.8

0.6

0.4

0.2

0 2

7 pH

12

Fig. 10. The effect of pH variations on (a) membrane potential, (b) transport number and (c) permselectivity.

Fig. 11. The effect of solution concentration on the electrical resistance of used membranes.

Please cite this article in press as: Parvizian F, et al. Electrochemical characterization of mixed matrix nanocomposite ion exchange membrane modified by ZnO nanoparticles at different electrolyte conditions ‘‘pH/concentration’’. J Taiwan Inst Chem Eng (2014), http://dx.doi.org/10.1016/j.jtice.2014.08.017

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JTICE-999; No. of Pages 10 F. Parvizian et al. / Journal of the Taiwan Institute of Chemical Engineers xxx (2014) xxx–xxx

Fig. 12. The effect of pH on the electrical resistance of used membranes.

decline at low pH may be attributed to the competition of sodium ions (Na+) and protons (H+) for the same sites [70,71]. 4. Conclusion The morphology of prepared membranes was studies using SOM images which showed uniform particle distribution and also relatively uniform surface. Moreover, images indicated with increasing zinc oxide nanoparticles concentration more uniform particle distribution in the membrane matrix achieved. The electrochemical properties of prepared nanocomposite ion exchange membranes containing ZnO nanoparticle were studied in different electrolytic environments. Membrane water content was initially improved up to 10 wt% by increase of zinc oxide nanoparticle concentration in casting solution and then decreased. Results illustrated that increasing zinc oxide nanoparticle concentration increased membrane potential, transport number and permselectivity initially and then declined. The maximum membrane ionic permeability and flux obtained for the additive concentration of 10 wt%, while the lowest membrane electrical resistance achieved at this point. Results revealed that membrane potential, transport number, and permselectivity were enhanced with the increase of solution concentration and then decreased slightly with higher concentrations. The results exhibited the environmental pH value had a significant effect on membrane performances and sorption behavior of ZnO nanoparticles. Areal electrical resistance increased with increase of pH value. The modified nanocomposite membrane (with 10 wt% nano-ZnO) showed less electrical resistance variations at different solution concentrations. Considering the desirable electrochemical characteristics of prepared nanocomposite membrane as well as the semiconductor and antibacterial properties of ZnO nanoparticle, this type of ion exchange membrane can be very useful and attractive in the wastewater treatment using electrodialysis process. Acknowledgment The authors gratefully acknowledge Arak University of Iran for the financial support (no. 92-12105) during this research. References [1] Kariduraganavar MY, Nagarale RK, Kittur AA, Kulkarni SS. Ion-exchange membranes: preparative methods for electrodialysis and fuel cell applications. Desalination 2006;197:225–46.

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