Mixed matrix heterogeneous cation exchange membrane filled with clay nanoparticles: membranes’ fabrication and characterization in desalination process

Hosseini, S. M.; Seidypoor, A.; Nemati, M.; Madaeni, S. S.; Parvizian, F.; Salehi, E.

doi:10.2166/wrd.2015.064

In this research mixed matrix PVC based-co-clay nanoparticles heterogeneous cation exchange membranes were prepared by solution casting technique. The effect of clay nanoparticles concentration in the casting solution on membrane electrochemical properties was studied. Scanning optical microscopy (SOM) images showed uniform particles distribution and relatively uniform surfaces for the prepared membranes. The membrane water content was improved initially by an increase of additive content ratio up to 1%wt in casting solution and then it began to decrease by more additive concentration. Moreover, swelling was measured less than 5% in thickness and negligible in length and width for the prepared membranes. Membrane potential, transport number and permselectivity were improved by increase of nanoparticles loading ratio. Utilizing Cloisite nanoparticles up to 1%wt in the casting solution also led to an increase in permeability and flux for prepared membranes. The ionic permeability and flux were decreased again by a further increase in additive concentration from 1 to 4%wt. Also, membranes exhibited lower permeability and flux for bivalent ions in comparison with monovalent type. The membrane E-conductivity and mechanical strength was enhanced by an increase of nanoparticles concentration in membrane matrix. The modified membrane containing 1%wt clay nanoparticles showed more suitable electrochemical properties compared to others.

clay nanoparticles, desalination/water treatment, fabrication/electrochemical characterization, ion exchange membrane, mixed matrix

INTRODUCTION

Nowadays, membrane technology has obtained much attention in diverse industries and human life. Among this, ion exchange membranes (IEMs) have been utilized widely as active separators in electrically driven processes, such as electrodialysis for desalting brackish waters, reconcentrating brine from seawater and production of table salt. Ion exchange membranes are also efficient tools in resource recovery, food and pharmacy processing and environmental protection such as treating industrial and biological effluents as well as manufacturing of basic chemical products (Xu 2005; Kariduraganavar et al. 2006; Nagarale et al. 2006; Hosseini et al. 2014; Zarrinkhameh et al. 2014). In IEMs, charged groups attached to polymer backbone are freely permeable to opposite sign ions under an electrical field influence (Baker 2004). In such processes, ion interactions with membrane, water, and with each other occur in complex fashions. Knowledge of the electro kinetic properties of ion exchange membranes is a major contributing factor behind decisions about their applicability in specific separation processes and energy storage devices (Gohil et al. 2006; Dlugolecki et al. 2010; Hosseini et al. 2014).

Preparing ion exchange membranes with special physico/chemical characteristics may be a vital step in future applications (Kariduraganavar et al. 2006; Hosseini et al. 2014). Considerable research has already been carried out to improve the IEM physico-chemical properties. Variation of functional groups type, selection of different polymeric matrices, polymers blending, using of inorganic additives/filler, alteration of cross-link density and surface modification are important techniques to obtain superior IEMs (Fathizadeh et al. 2011; Hosseini et al. 2013, 2014; Zendehnam et al. 2013; Zarrinkhameh et al. 2014).

Utilizing inorganic particles or fillers, especially nanomaterials, into polymeric matrixes has been examined in many applications to enhance the physico/chemical characteristics and separation properties based on the synergism between the organic-inorganic components (Xu 2005; Hosseini et al. 2014). Clay nanoparticle (NP) is well known as an inorganic material with very interesting features and capacity, such as high adsorption capacity, ion exchange property, low cost, stable property and safety toward the environment, which has been utilized in membrane fabrication (Lin et al. 2009; Ghaemi et al. 2011; Daraei et al. 2013; Rajabi et al. 2014).

Preparing heterogeneous cation exchange membranes with specially adapted physico/chemical properties for application in electrodialysis processes related to water recovery and water treatment was the primary target of the current research. For the purpose, mixed matrix polyvinylchloride-co-clay nanoparticles heterogeneous cation exchange membranes were prepared by solution casting techniques using cation exchange resin powder as the functional group's agent and tetrahydrofuran as the solvent. PVC is a flexible and durable polymer with suitable biological and chemical resistance (Harper 1975; Mark 1999). Clay nanoparticles were also employed as inorganic filler additive in membrane fabrication.

Currently no reports have considered incorporating clay nanoparticles into ion exchange membranes and the literature is silent on characteristics and functionality of electrodialysis IEMs.

The effect of used additives’ concentration in the casting solution on membrane electrochemical properties was studied. During the experiments, sodium and barium chloride were employed as ionic solutions for membrane characterization. The results are valuable for electro-membrane processes, especially the electrodialysis process for water recovery and water treatment.

MATERIALS AND METHODS

Materials

Polyvinylchloride (PVC, grade 7054, Density: 490 g/L) supplied by BIPC Company, Iran, was used as binder. Tetrahydrofuran (THF, Merck Inc., Germany) was employed as solvent. Clay nanoparticle (Cloisite^®15A, organically modified montmorillonite clay, quaternary ammonium compounds, bis (hydrogenated Tallow Alkyl) dimethyl, salt with bentonite, specific gravity 1.4–1.8, Southern Clay Products Inc., USA) was used as inorganic filler additive. Cation exchange resin (ion exchanger Amberlyst^® 15, strongly acidic cation exchanger, H⁺ form more than 1.7 milli equivalent/gr dry, density: 0.6 gr/cm³, particle size (0.355–1.18 mm) ≥90%) by Merck Inc., Germany, was also utilized in membrane fabrication. All other chemicals were supplied by Merck. Throughout the experiment, distilled water was used.

Preparation of cation exchange membranes

In order to undertake preparation of the membranes, resin particles were dried in an oven at 30 °C for 48 h and then pulverized into fine particles in a ball mill and sieved to the desired mesh size. The ion exchange resin with desired particles size (–325 +400 mesh) was used in membrane fabrication. The preparation proceeded by dissolving the polymer binder into THF solvent (polymer binder (PVC): solvent (THF)) (w/v), (1: 20)) in a glass reactor equipped with a mechanical stirrer for more than 5 h. This was followed by dispersing a specific quantity of ground resin particle ((resin particle: polymer binder) (w/w), (1:1)) as functional groups agents and clay nanoparticles (S₁: 0.0, S₂: 0.5, S₃: 1.0, S₄: 2.0 and S₅: 4.0%wt) as additive in polymeric solution, respectively. The mixture was mixed vigorously at room temperature to obtain uniform particle distribution in polymeric solution. In addition, for better dispersion of particles and breaking up their aggregates, the polymeric solution was sonicated for 1 h using ultrasonic instrument. Excessive homogeneity and uniform distribution of particles (resin, additive) in the membrane matrix provide superior conducting regions for the membranes and generate easy flow channels for counter-ions transportation. The presence of more conducting region on the membrane surface can also strengthen the intensity of the uniform electrical field around the membrane and decreases the polarization phenomenon (Kang et al. 2003). Furthermore, uniform distribution of particles in polymeric solution increases the viscosity of solution and reduces the evaporation rate of solvent. This improves the polymer chain's conformation with particle surfaces and improves the membrane selectivity (Powell & Qiao 2006). Then, the mixing process was repeated for another 30 min by the mechanical stirrer. The mixture was then cast onto a clean and dry glass plate at 25 °C. The membranes were dried at ambient temperature and immersed in distilled water. In the final stage, membranes were pretreated by immersing in NaCl solution. The membrane thickness was measured by a digital caliper device around 60–70 μm. A summary of the membrane preparation procedure is given in Table 1.

Table 1

Flow sheet of membrane preparation procedure

The procedure for IEM preparation
Step 1	Resin particles drying (at 30 °C for 48 h)
Step 2	Resin particles pulverizing (–325 +400 mesh)
Step 3	Polymer dissolving into solvent (for 5 h)
Step 4	Resin particles and additive dispersing in polymeric solution
Step 5	Sonication of polymeric solution (for 1 h)
Step 6	Mixing of polymeric solution (for another 30 min)
Step 7	Casting (at 25 °C)
Step 8	Film drying (at 25 °C for 30 min) and immersing in water
Step 9	Membranes pretreatment by HCl and NaCl solutions

The procedure for IEM preparation
Step 1	Resin particles drying (at 30 °C for 48 h)
Step 2	Resin particles pulverizing (–325 +400 mesh)
Step 3	Polymer dissolving into solvent (for 5 h)
Step 4	Resin particles and additive dispersing in polymeric solution
Step 5	Sonication of polymeric solution (for 1 h)
Step 6	Mixing of polymeric solution (for another 30 min)
Step 7	Casting (at 25 °C)
Step 8	Film drying (at 25 °C for 30 min) and immersing in water
Step 9	Membranes pretreatment by HCl and NaCl solutions

Test cell

The membranes’ electrochemical properties measurements were carried out using the test cell (Figure 1). The cell consists of two cylindrical compartments made of Pyrex glass which are separated by a membrane. One side of each vessel was closed by Pt electrode supported with Teflon and the other side was equipped with membrane. The membrane area was 19.63 cm².

Figure 1

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Schematic diagram of test cell: (1) Pt electrode, (2) magnetic bar, (3) stirrer, (4) orifice, (5) rubber ring, (6) membrane.

Membrane characterization

Morphological studies

The behavior of prepared membranes is closely related to their structure, especially the spatial distribution of the ionic site (Li et al. 2005). The structures of prepared membranes were examined by scanning optical microscopy (SOM Olympus, model IX 70) in transmission mode with light going through the membrane for scanning purposes.

Water content

The water content was measured as the weight difference between the dried and swollen membranes. The wet membrane was weighed (OHAUS, Pioneer^TM, Readability: 10⁻⁴ gr, OHAUS Corp.) and then dried in an oven until constant weight was obtained. The following equation (Sata 2004; Li et al. 2005; Tanaka 2007) can be used in water content calculations:

1

Ion exchange capacity (IEC)

The IEC determination was performed using the titration method. For the IEC measurements, the membranes in acid form (H⁺) were converted to Na⁺ form by immersing in 1 M NaCl solution to liberate the H⁺ ions. The H⁺ ions in solution were then titrated with 0.01 M NaOH and phenolphthalein indicator. The IEC can be calculated from the following equation (Sata 2004; Li et al. 2005; Kariduraganavar et al. 2006; Tanaka 2007):

2

where a is the milli-equivalent of ion exchange group in membrane and W_dry is the weight of dry membrane (g).

Membrane potential, transport number and permselectivity

The membrane potential is the algebraic sum of Donnan and diffusion potentials 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 (Nagarale et al. 2004a, 2004b). This parameter was evaluated for the equilibrated membrane with unequal concentrations (C₁ = 0.1 M/C₂ = 0.01 M) of electrolyte solution at ambient temperature on either side of the membrane. During the experiment, both sections were stirred vigorously to minimize the effect of boundary layers. The developed potential across the membrane was measured by connecting both compartments and using saturated calomel electrodes (through KCl bridges) and a digital auto multi-meter (DEC, Model: DEC 330FC, Digital Multimeter, China). The measurement was repeated until a constant value was obtained. Membrane potential (E_Measure) is expressed using the Nernst equation (Nagarale et al. 2004a; Gohil et al. 2006; Zarrinkhameh et al. 2014) as follows:

3

where t_i^m is the transport number of counter-ions in membrane phase, R is gas constant, T is the temperature, n is the electrovalence of counter-ion and a₁, a₂ are solutions electrolyte activities in contact membrane surfaces.

The ionic permselectivity of membranes is also quantitatively expressed based on the migration of counter-ions through the IEMs (Shahi et al. 2003; Nagarale et al. 2004b, 2005; Gohil et al. 2006):

4

where t₀ is the transport number of counter-ions in solution (Lide 2006).

Ionic permeability and flux

The ionic permeability and flux were measured using the test cell. A 0.1 M solution (NaCl/ BaCl₂) was placed on one side of the cell and a 0.01 M solution on its other side. A DC electrical potential with an optimal constant voltage (10 V) was applied across the cell with stable platinum electrodes. During the experiment, both sections were recirculated 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 the cathodic section and increase the pH of this region:

According to Fick's law, ionic flux through the membrane can be expressed as follows (Kerres et al. 1998; Li et al. 2005; Zarrinkhameh et al. 2014):

5

where P is ionic permeability (coefficient diffusion) of ions, d is membrane thickness, N is ionic flux, A is the membrane surface area, V₀ is the volume of each compartment in the used test cell and C is the cations concentration in the compartments.

The boundary conditions were as follows:

6

Therefore:

7

The ionic permeability (P) in membrane phase is calculated from Equation (7) considering pH changes in cathodic section (Digital pH-meter, Jenway, Model: 3510).

Electrical resistance

The electrical resistance of equilibrated membrane was measured in NaCl solution with a concentration of 0.5 M (at 25 °C). Measurement was carried out by an alternating current bridge with 1,500 Hz frequency. The membrane resistance is calculated using the different resistance between the cell (R₁) and electrolyte solution (R₂) (R_m = R₁ − R₂) (Sata 2004; Tanaka 2007). The areal resistance was expressed as follows:

8

where r is areal resistance and A is the surface area of membrane.

Mechanical property

The tear resistance as a mechanical property of prepared membranes was tested according to ASTM1922-03. Before undertaking the tests, all samples were cut into standard shapes in ambient conditions. Three samples were used in each test and the average values were reported.