Ind. Eng. Chem. Res. 2010, 49, 8477–8487
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Comparative Studies on Morphological, Electrochemical, and Mechanical Properties of S-Polyvinyl Chloride Based Heterogeneous Cation-Exchange Membranes with Different Resin Ratio Loading A. R. Khodabakhshi, S. S. Madaeni,* and S. M. Hosseini Membrane Research Centre, Department of Chemical Engineering, Razi UniVersity, Tagh Bostan, 67149 Kermanshah, Iran
Heterogeneous types of ion exchange membranes were prepared from S-polyvinyl chloride (S-PVC) as binder and tetrahydrofuran as solvent. Cation exchange resin powder was used in casting solutions to make ion exchange membranes with cationic functional groups. The relationship between resin ratio loading and morphological, electrochemical, and mechanical properties of prepared membranes were studied and evaluated. Membrane potential measurements were performed in monovalent (NaCl) and bivalent (BaCl2) electrolyte solutions in order to observe the selectivity of these membranes for different types of counterions. The results reveal that an increment of resin ratio loading leads to an increase of ion exchange capacity, fixed ion concentration, ionic permeability, current efficiency, water content, hydrophilicity, transport number, conductivity, and specific surface area for prepared membranes. However, the electrical resistance, energy consumption, membrane oxidative stability, and mechanical properties were decreased. Moreover, prepared membranes show different properties for monovalent (Na+) ions and bivalent (Ba2+) ions. 1. Introduction Membrane technology is a green technology. Since their first appearance two centuries ago, membranes have seen a great development in various industrial and fundamental science domains. They are widely used in the fabrication of chemical and food products, and they are playing an important part in environmental protection, water desalination, wastewater treatment, removal of heavy metal ions, nitrates, phosphates, pesticides, phenols, and many other micropollutants.1,2 For years, much attention has been paid to electrochemically driven membrane processes for the desalination of brackish water, concentration of salts, and recovery of organic acids. Such processes have been extensively studied to improve efficiency. Recently, electrochemically driven membrane processes have been applied even to the recovery of heavy metal from industrial wastewater, thus showing a high potential in the environmental area.3 In this field, ion exchange membranes have found many applications ever since they were invented in the last century. Among these, the separation of specific salts from aqueous electrolyte solutions through electrodialysis is one of the most important applications. Using ion exchange membranes in electrodialysis, cations or anions of a wide range of concentrations can be successfully separated, and therefore, many clean separations or productions can be realized not only in aqueous systems but also in nonaqueous systems.4-11 Most commercial ion-exchange membranes can be divided, according to their structure and preparation procedure, into two major categories, either homogeneous or heterogeneous. According to Molau,12 depending on the degree of heterogeneity of the ion-exchange membranes, they can be divided into the following types: (a) homogeneous ion-exchange membranes, (b) interpolymer membranes, (c) microheterogeneous graft- and block-polymer membranes, (d) snake-inthe-cage ion-exchange membranes, and (e) heterogeneous ion-exchange membranes.12,13 * To whom correspondence should be addressed. Phone: +98 831 4274530. Fax: +98 831 4274542. E-mail:
[email protected].
Membranes may differ according to their homogeneity degree: homogeneous and heterogeneous membranes.14 Both homogeneous and heterogeneous, being unique in their nature, supersede each other in one way or another. Heterogeneous membranes can be prepared by (i) calendering ion-exchange particles into an inert plastic film, (ii) dry molding of inert film forming polymers and ion-exchange particles and then milling the mold stock, or (iii) dispersing resin particles in a solution containing a film forming binder and then evaporating the solvent to give an ion-exchange membrane.15 To prepare homogeneous ion exchange membranes, various approaches are available to introduce ionic groups. These approaches can be classified into three categories based on the starting materials:16 (a) Starting with a monomer containing a moiety that either is or can be made into anionic or cationic exchange groups, which can be copolymerized with nonfunctionalized monomer to eventually form an ion exchange membrane; (b) starting with polymer film, which can be modified by introducing ionic characters either directly by grafting of a functional monomer or indirectly by grafting a nonfunctional monomer followed by a functionalization reaction; (c) starting with polymer or polymer blends by introducing anionic or cationic moieties, followed by the dissolving of polymer and casting it into a film. The properties of ion-exchange membranes are determined by two parameters, namely, the basic polymer matrix and the type and concentration of fixed ionic moiety. The type and the concentration of the fixed ionic charges determine the permselectivity and the electrical resistance of the membrane.2 Energy saving, resource recovery, and pollution control are the main reasons for development and application of ion exchange membranes.17 However, preparation of inexpensive membranes with special selective characteristics such as high ion conductivity, high ion permselectivity, moderate swelling, and suitable resistance is a vital step in future applications.18,19 Variation of functional groups, selection of different polymeric matrixes, alteration of cross-link density, nature of surface layer,
10.1021/ie9014205 2010 American Chemical Society Published on Web 08/02/2010
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Table 1. Compositions of Casting Solution for the Preparation of Ion Exchange Membrane
membrane
polymer binder
resin particle (resin/polymer) (w/w)
sample 1 sample 2 sample 3
S-polyvinyl chloride S-polyvinyl chloride S-polyvinyl chloride
40:60 50:50 60:40
solvent (THF/ (polymer + resin) (w/w) 8:1 8:1 8:1
and heterogeneity of the membranes are important tools to achieve superior membranes.6 Preparation of heterogeneous cation exchange membranes with appropriate properties for water recovery and wastewater treatment in industrial electrodialysis processes was the primary target for the current research. For this proposal, heterogeneous cation exchange membranes were prepared by casting-solution techniques using S-polyvinyl chloride as binder and cation exchange resin powder in tetrahydrofuran as solvent. S-PVC is a flexible, durable, and cheap polymer with suitable dimensional stability and appropriate biological and chemical resistance.20,10 Also, the effect of resin ratio loading on morphological, electrochemical, and mechanical properties of the prepared membranes were studied and evaluated. 2. Materials and Methods 2.1. Material. S-Polyvinyl chloride (S-PVC), supplied by BIPC, Iran, grade S-7054, tetrahydrofuran (THF) as solvent, and cation-exchange resin (Ion exchanger Amberlyst 15, strongly acidic cation exchanger, H+ form, more than 1.7 milliequiv/g) by Merck Inc., were used to prepare the membranes. All other chemicals were supplied by Merck Inc. Distilled water was used throughout the study. 2.2. Preparation of Membranes. For the preparation of membranes, particle resin was dried in an oven (BEHDAD Co., Model: oven 5) at 30 °C for 48 h, then powdered in a ball mill (Pulverisette 5, FRITSCH CO.), and sieved to the desired mesh size. Ion exchange resin with desired particles size (-300 + 400 mesh) was used for the preparation of membranes. The membranes were prepared by dissolving the polymer binder in THF solvent in a glass reactor equipped with a mechanical stirrer (Model: Velp Sientifica Multi 6 stirrer) for more than 4 h. This was followed by dispersing a specific quantity of resin particles and casting on a glass plate. The membranes were dried at ambient temperature (25 °C) and immersed in twice distilled water. As the final stage, membranes were pretreated by immersing in HCl and NaCl solutions subsequently before using. The membranes thicknesses were measured by a digital caliper device (Electronic outside Micrometer, IP54 model, OLR) to maintain the membrane thickness around 80-120 micrometers. The composition of casting solution is depicted in Table 1. 2.3. Test Cell. The test cell that was used for measuring transport number, specific electrical resistance, ion diffusion, and other electrochemical properties is shown in Figure 1. As shown in this apparatus, this cell consists of two cylindrical compartments (vessel) from Plexiglas that were separated by the membrane, which was fixed between rubber rings. Each vessel has a volume of 150 cm3. One side of each vessel was closed by a Pt electrode that was supported with a piece of Teflon (polytetrafluoroethylene), and the other side equipped with a piece of porous medium to support the membrane. The top of each compartment contains two orifices for feeding and sampling. In order to minimization the effect of boundary layers on the membranes during the experiments that make concentration polarization on the membrane surface, both sections were
Figure 1. Schematic diagram of test cell: (1) Pt electrode, (2) magnetic bar, (3) stirrer, (4) sampling orifice, (5) rubber ring, and (6) membrane.
stirred vigorously by magnetic stirrers. All experiments were carried out using a membrane area of 13.85 cm2. 2.4. Characterization of Prepared Membranes. 2.4.1. Morphological Studies. The structures of prepared membranes were examined by scanning electron microscopy (Philips, Model XL30, Netherlands) and scanning optical microscopy (Olympus model IX 70). In order to obtain good membrane SEM images, the samples were frozen in liquid nitrogen and fractured. After sputtering with gold, they were viewed with the microscope. Also, in these experiments, optical microscopy was used in transmission mode with light going through the membrane for scanning. 2.4.2. Water Content. Measurement of water content was determined from the difference in weight between the drying and the swollen membranes. The membranes were immersed in distilled water for 24 h and then taken out, and its surface was wiped with filter paper and weighed (Mettler Toledo Group, Model: AL204). The wet membranes were dried at fixed temperature (50 °C) for 4 h until a constant weight as a drymembrane was achieved. The water content can be calculated from the following equation:18,21,22 water content % ) [(Wwet - Wdry)/Wdry] × 100
(1)
2.4.3. Contact Angle Measurements. To study the surface wetting characteristics of prepared membranes, the contact angle between water and membrane surface were measured using a contact angle measuring instrument [G10, KRUSS, Germany]. This test was performed for evaluation of membrane surface hydrophilicity. Deionized water was used as the probe liquid in all measurements. To minimize the experimental error, the contact angle was measured at five random locations for each sample, and then, the average was reported. All experiments have been done in the ambient conditions (23 °C). 2.4.4. Ion Exchange Capacity. Ion exchange capacity (IEC) is defined as milliequivalent of ion exchange group included in a 1 g dry membrane.23 The IEC was determined through titration. In order to measure the ion exchange capacity of the membranes, a sample membrane is left at first in a 1 M HCl solution for 24 h, for converting the exchange group to an H type, and then washed with water sufficiently and kept in it for 24 h until the washed water does not exhibit acidity recognized by the reaction with methyl red. Next, the membrane is immersed in a 1 M NaCl solution for 24 h and washed sufficiently with water. The immersed solution and washed water are collected, and finally, H+ ions dissolved into the collected solution are analyzed with a 0.01 M NaOH and a phenolphthalein indicator (A mequiv). At final, the membrane is wiped with filter paper and dried in oven at 50 °C for 4 h and then weighed (W g). The IEC can be calculated from the following equation:16,21,23 IEC )
( WA )
(2)
Ind. Eng. Chem. Res., Vol. 49, No. 18, 2010
Also, the fixed ion concentration (FIC) can be calculated by FIC )
( waterIECcontent )
(3)
2.4.5. Membrane Potential, Permselectivity, and Transport Number. When both surfaces of an ion exchange membrane are in contact with a solution of different concentration, an electrical potential developed across the membrane due to the tendency of the oppositely charged ions to move with various velocities and mobility. The magnitude of this parameter depends on the electrical characteristic of the membrane along with the nature and concentration of the electrolyte solution used.2,24,25 Membrane potential is an algebraic sum of two Donnan potentials and diffusion potential, determined by the partition of ions into the pores as well as the mobilities of ions within the membrane phase compared with the external phase.21 The membrane potential is measured using a two-cell glassy apparatus as show in Figure 1 in which an equilibrated circular membrane with 13.85 cm2 effective area is integrated between the cells. At ambient temperature, an electrolyte solution with concentration of C1 ) 0.1 M and a solution with C2 ) 0.01 M are poured into the cell compartments on either side of the membrane. Both sections were stirred vigorously by magnetic stirrers, to minimize the effect of boundary layers on the membrane’s surface. After 10 min, a developed potential difference across the membrane is measured by connecting both compartments and the saturated calomel electrode through KCl bridges, with the help of a digital auto multimeter (DEC, Model: DEC 330FC, Digital Multimeter, China). The measurement was repeated for 5 min, in which, meanwhile, a potential was measured every 1 min until the value was constant (Emeasure). The membrane potential generated between the solutions contacting both membrane surfaces is expressed by the Nernst equation18,19,21,23,24,26 that was used for calculation of transport number of ions as follows: Emeasure )
(2tm i
()
a1 RT - 1) ln nF a2
( )
Ps )
- t0 1 - t0
which was fixed between rubber rings. A 0.1 M electrolyte solution was placed on one side of cell (Anodic section), and a 0.01 M solution was placed on the other side diffusion cell. A DC electrical potential (DAZHENG, DC power supply, Model: PS-302D) with constant voltage (10 V) was applied across the cell with platinum electrodes, placed at the end of compartments, and the conductivity changes verses time were measured by the help of a digital conductmeter (JENWAY, Model: 4510). During the experiment, ions permeate through the membrane into the permeation section. Therefore, the conductivity of this section increases with time. The solutions of both compartments were stirred vigorously between two successive measurements to ensure the return of equilibrium condition in two solution-membrane interfacial zones and minimize the effect of boundary layers on the membrane’s surface that makes concentration polarization. The permeation of ions through the membrane phase can be calculated from the variation of conductivity measurement. According to Fick’s law and electrical potential, flux (N) through the membrane can be expressed as follows:18,19 N)P
C1 - C2 d
(6)
where P is the coefficient permeation of ions, d is the membrane thickness, and C1 and C2 are the ion’s concentration in the compartments. The boundary conditions are C10 ) 0.1M,
C20 ) 0.01M,
C1 + C2 ) C10 + C20 ) 0.11M
(7)
Therefore: N)-
C 1 - C2 V dC1 × )P A dt d
(8)
Where A is the membrane surface area,
(4) -
where tm i is the transport number of ions (cationic or anionic) in the membrane phase, R is the gas constant, T is the temperature, n is the electrovalence of counterion, and a1 and a2 are the electrolyte activity in the solutions in contact with both membrane surfaces.27,28 Ion selectivity of membranes is quantitatively expressed in terms of permselectivity based on migration of counterion through the ion-exchange membrane as follows:24,26 tm i
8479
(5)
where t0 is the transport number of these ions in solution phase.29 In fact, permselectivity is a measure of the characteristic difference in the membrane permeability of counterions and coions that confirmed the Law of Donnan equilibrium. On the basis of the Donnan law, cations can ion exchange into a cation exchange membrane as counterions and anions basically cannot enter into the cation exchange membrane. Similarly, cations are also rejected by the cationic charge (anion exchange groups) of the anion exchange membrane. This selective uptake of counterions and rejection of co-ions by the charged membrane is explained as Donnan membrane equilibrium.21 2.4.6. Conductivity and Permeability. The conductivity and cation permeability was carried out via the test cell. The apparatus consists of two compartments that were separated by the membrane,
0 0 (C10 + C20 - C2 - C2) V d(C1 + C2 - C2) × )P A dt d
(9)
∫
c2 0
c2
-
d(C10 + C20 - C2) (C1 + C2 - C2 - C2) 0
ln
[
ln 1 -
0
(C10 + C20 - 2C2) (C10 - C20)
2C2 - 2C20 C10 - C20
-
]
)-
2PAt Vd
(11)
2C2 - 2C20 C10 - C20
