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

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.

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.

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.

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.

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

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.

The membrane water content was decreased again by a further increase in additive concentration from 1 to 4%wt. This may be attributed to filling of voids and cavities by the clay nanoparticles at high additive concentration which occupy the free spaces in membrane matrix and reduces the amount of water molecules’ accommodation. In fact, free spaces in the membrane matrix are surrounded by the nanoparticles and so decline water accommodation. The suitable amount of membrane water content can have better control on the pathways of ion traffic and improve the membrane selectivity. Additionally, high water content can provide more and wider transfer channels for co- and counter-ion transportation and decrease the selectivity and also lead to a loose structure for the membranes. However, this is not always true and depends on the membrane structure and its properties. The measurements were carried out three times for each sample and then their average value was reported in order to minimize experimental errors.

IEC results (Figure 4) indicated that utilizing clay nanoparticles concentration up to 1%wt in the casting solution initially led to an improvement in ion exchange capacity in prepared membranes. This may be due to the adsorption characteristic of Cloisite nanoparticles which makes superior interaction between the ions and the membrane surface. This facilitates the ions transportation between the solution and membrane phase and improves membrane ion exchange possibilities. The membrane ion exchange capacity was declined again by a further increase in additive concentration. This is because of the decrease in accessibility of resin particles in membrane matrix at a high additive content ratio which isolates the resin particles and reduces the accessibility of ion exchange functional groups by their surroundings.

The ionic permeability and flux were decreased again by a further increase in additive concentration from 1 to 4%wt in the prepared membranes. This may be due to the formation of narrow ionic transfer channels in the membrane matrix by the additive particles and the low amount of membrane water content and IEC for the prepared membrane which makes the ions transportation through the membrane difficult and so declines the ionic permeability and flux. The voids and cavities (free spaces) occupied by the nanoparticles in the membrane matrix also limits the pathways and restricts the transportation.

A comparison between the selectivity and areal electrical resistance of prepared membranes in this study and some commercial membranes is given in Table 2.

During the preparation process, the loss of molecules solvent introduces cavities and voids between the particles and polymer binder region due to evaporation. These micro-voids are sufficient to accommodate the solvent molecules for the solvation. Therefore, solvation did not change membranes dimensions manifestly. The amount of swelling in prepared membranes was less than 5% in thickness. Also, their swelling was negligible in length and width.

SOM images showed a relatively uniform surface for the membranes. Also, particles are uniformly distributed in the prepared membranes. It was found that membrane water content was increased initially by using clay nanoparticles up to 1%wt in the casting solution and then began to decrease by a further increase in additive concentration from 1 to 4%wt which is in contrast with the hydrophilic characteristic of Cloisite. Moreover, the amount of swelling in prepared membranes was less than 5% in thickness and negligible in length and width. It was found that membrane potential, transport number and permselectivity were all improved in sodium chloride ionic solution by using clay nanoparticles in the casting solution. Utilizing clay nanoparticles in the casting solution up to 1%wt also led to a sharp increase in sodium permeability and flux in prepared membranes. Results revealed that sodium 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 ones. The membrane areal electrical resistance was decreased by an increase of clay nanoparticles concentration in the casting solution. The opposite trend was found for the membrane mechanical property. Also, the amount of swelling in prepared membranes was less than 5% in thickness. Their swelling was also negligible with respect to length and width measurements. Among the prepared membranes, modified membrane containing 1%wt clay nanoparticles showed more suitable electrochemical properties compared to others. The obtained results are valuable for electro-membrane processes, especially electrodialysis for water recovery and treatment.

The authors gratefully acknowledge Arak University for financial support during this research.