Elsevier

Journal of Membrane Science

Volume 590, 15 November 2019, 117272
Journal of Membrane Science

Transport properties of bilayer and multilayer surface-modified ion-exchange membranes

https://doi.org/10.1016/j.memsci.2019.117272Get rights and content

Highlights

  • Composite bi- and multilayered ion-exchange membranes characteristics are studied.

  • Characteristics of composite membranes are slightly different from the original.

  • Voltammetry is used to show a difference in the polarized behavior of the membranes.

  • Modification may promote water-splitting or electroconvection in the EMS.

Abstract

This work aims to study changes in the electrochemical and transport properties of bilayered and multilayered ion-exchange membranes based on a Ralex AMH heterogeneous ion-exchange membrane. The composites studied are: a bilayer membrane consisting of a thin layer of perfluorocarbon cation-exchanger deposited on the surface of the Ralex membrane (Ralex/MF 4SK) as well as a composite with polyaniline based on it (Ralex/MF 4SK/PANi), and a composite with a polyaniline and Ralex membrane (Ralex/PANi). The transport properties of the composite membranes are studied in electrolyte solutions of different natures by different methods. The concentration dependences of the diffusion permeability and specific conductivity of the initial and composite membranes in NaCl and HCl solutions were investigated, and the water transport numbers in NaCl solutions were measured. Electrodiffusion coefficients were calculated and the concentration dependences of electromigration numbers of counterion transport were obtained. The effect of asymmetry of diffusion permeability when the orientation of the composite membranes with respect to the flux of electrolyte is changed was studied. A study of the current-voltage characteristics of the original membrane and composites based on it shows that the application of the MF-4SK film leads to an increase in the reaction rate of water-splitting reaction at the emerging cation/anion-exchanger interface. In the case of composites with polyaniline, there is an increase in the limiting current density in comparison with the initial sample and a significant potential oscillation in the overlimiting mode, which indicates a significant contribution of electroconvection in the overlimiting masstransfer.

Introduction

The development of industry inevitably leads to ecological problems and destabilization of the environmental situation. One of the key tasks for humanity in the near future is the transition from a linear to a cyclical industry, where the waste or by product of one process is used as an input for another process. The creation of such cyclic technological processes is difficult or even impossible without membrane technologies [[1], [2], [3]]. Recently the role of electro-membrane technologies [[4], [5], [6], [7], [8]] has increased, primarily as technologies capable of improving the relationship between industry and the environment.

Electrodialysis, and especially electrodialysis reversal, is less demanding on the pretreatment of processed solutions, it can run without adding of chemical reagents to the solution. Another important advantage of electrodialysis is the ability to control the pH of a solution during demineralization. This property is very important in such electrodialysis applications as treatment of natural waters, reducing the acidity of fruit juices and processing dairy products [9,10]. For example, the make-up water in the power systems must be free from hardness salts and carbonic acid and have a pH > 8. If the natural water to be desalted contains weak acids (carbon, silicic, etc.), then an increase of pH in the desalination chambers of the electrodialyzer contributes to the dissociation of these acid and, therefore, more complete extraction of weak acids. The problem of removal of weak acids from aqueous solutions is very relevant as an independent task, as well as a stage in the preliminary preparation of solutions for thermal power engineering. The difference in pH between diluate and concentrate channels of the electrodialyzer entails a number of engineering problems. These problems are associated with a decrease in current efficiency due to an increase in the competitive transfer of salt ions and H+, OH-ions, as well as the transformation of carbonates, silicates, and other ampholytes to electrically neutral molecules. Due to the absence of monopolar membranes with a suppressed or, on the contrary, enhanced ability to generate H+, OH ions, now expensive bipolar membranes are used to create the desired pH range.

At the same time, it is possible to control the water-splitting process using composite multilayer membranes [11].

There are several ways to obtain multilayer ion-exchange membranes, each of which can be used to acquire new membranes with unique properties.

  • 1) “Poisoning” of monopolar cationic- or anion-exchange membranes during the electrodialysis of solutions containing surfactants, fats, transition metal ions, etc. [12]. In this case, a thin unstable (except for scaling) layer forms on the membrane surface. Such a layer may contain substances catalytically active towards water-splitting reaction. For example, during the deposition of a layer of cationic surfactants on an anion-exchange membrane, the dissociation of water can occur at a thickness of this layer of the order of only 10 μm [13].

  • 2) Deposition of polymer with fixed charged groups (hereafter referred to as ionpolymers) with the same charge of the fixed groups as the substrate. This method makes it possible to increase the selectivity of one or both layers of the bipolar membrane [14,15]. When used, it is possible to reduce the pollution of the resulting acid and alkali and increase the current efficiency of the electrodialysis process. This assumption was made and confirmed in Ref. [14].

  • 3) Application of a layer of an ionpolymer with fixed groups having a charge opposed to the charge of the substrate in the form of a thin film [16] or a suspension [17]. In recent years a new field has appeared - the creation of a multilayer composition on a membrane surface consisting of alternating positively and negatively charged layers [18]. This technique makes it possible to maintain a high permeability of the substrate membrane (anion- or cation-exchange one) for salt ions, and at the same time this membrane becomes active in the water-splitting reaction due to the formation of a bipolar boundary (or multiple boundaries). The advantage of such membranes is the ability to control the ratio of the functions of salt ion transport and the generation of water dissociation products by selecting the thickness of one of the layers that make up the resulting composite membrane [11].

The properties of a multilayer membrane depend not only on the modifying component but also on the substrate. For example, modification of the surface of a perfluorinated sulfocationic membrane with a layer of polyaniline (PANi) makes it possible to adjust the flow of ions and water through the composite sample [19,20] depending on the method of synthesis. Synthesis of polyaniline in an uncharged perfluorinated matrix leads to the appearance of sensory properties of the resulting film [21].

An important condition for the effective use of composite ion-exchange membranes is that modifying agents should not impair the electrochemical and mass transfer characteristics of the substrate-membrane while giving new features to it. The transport mechanisms in such systems are quite complex since they occur at different size-scales. The system of nanoscale pores and channels determines the equilibrium (sorption, ion-exchange) and conductive properties (electrical conductivity, diffusion, and osmotic permeability), as well as the selectivity of the ion-exchange membrane. The properties and composition of the membrane surface (with a size-scale ranging from nanometers to micrometers) determine the transport parameters in the overlimiting current regimes, where the interaction of the membrane surface with the solution and the processes described by physicochemical hydrodynamics are important. The geometrical parameters of membrane modules (desalination and concentration channels) are of millimeter scale and are also essential for the resulting characteristics of electromembrane systems.

The main feature of multilayer membranes is the anisotropy of their structure and transport characteristics relative to the normal of the membrane surface. Due to the different physicochemical characteristics of each of the layers, the transfer of ions through such membranes should depend on the orientation of the membrane in the concentration and electric field, which causes the asymmetry of the properties of such membranes. For example, asymmetry of diffusion permeability was found in Refs. [22,23], and asymmetry of current-voltage characteristic in Ref. [24]. In this regard, the study of the membrane materials themselves is not yet complete: the methods of their manufacture, modification, and new areas of use are being explored [25]. The problem of membrane characterization has been broadly discussed in several reviews on the preparation and application of ion-exchange membranes [25,26]. Physicochemical properties of ion-exchange membranes were extensively investigated by various researchers [[27], [28], [29], [30]]. It was shown that electrical conductivity, transport number of counter-ions, diffusion, osmotic and electroosmotic permeability, as functions of the equilibrium electrolyte concentration, have to be measured to characterize ion-exchange membranes [31,32]. Using the approach developed in Refs. [33,34], the knowledge of the physicochemical characteristics of individual membranes makes it possible to calculate the characteristics of industrial electro-membrane modules (electrodialyzers) with a high level of accuracy [[35], [36], [37]].

Previously authors studied ion-exchange membranes with a layer of MF-4SK on the surface of the Ralex AMH membrane [[38], [39], [40]]. Various characteristics of homogeneous perfluorinated membranes modified with polyaniline can also be found in Refs. [22,23,[41], [42], [43], [44]]. However, there is only a few works where polyaniline is used as a modification layer for the anion-exchange membrane [41,42]. Deposition of PANi on the surface of anion-exchange membranes slightly increases limiting current and selectivity as well as ion-exchange capacity. Despite the fact that results obtained for homogeneous membranes are promising regarding physico-chemical characteristics of composites modified with polyaniline, there are no papers devoted to the deposition of polyaniline on heterogeneous membranes.

The aim of this work is the study of the physicochemical and electrochemical characteristics of bilayered ion-exchange membranes obtained by applying a MF-4SK cation-exchange film MF-4SK or a layer of polyaniline on the surface of an anion-exchange membrane (Ralex AMH) and membranes in which polyaniline was introduced into the MF-4SK film (a three-layered membrane). Such membranes could find an application in pH adjustment [45], separation of ions [46] and in the processes of electrodialysis concentration of electrolytes [47].

Section snippets

Objects of the study

The Ralex AMH (Mega a.s., Czech Republic) [48] heterogeneous ion-exchange membrane was chosen as the substrate-membrane. This membrane is a strongly basic anion-exchange membrane with quaternized ammonium functional groups. The matrix of the membrane is a polystyrol cross-linked with Lewatit M500 divinylbenzole ion-exchange resin [49]. The membrane was produced by rolling a thermoplastic mixture consisting of a fine powder of the ion-exchange resin and low density polyethylene, at an

Physicochemical characteristics and surface morphology of the studied membranes

The physicochemical characteristics of the Ralex, Ralex/MF-4SK, Ralex/PANi, and Ralex/MF-4SK/PANi samples are presented in Table 1. During the study, the membrane thickness (l), exchange capacity (Q), water content (Ww) and specific water content (nm) were determined.

For all the samples studied the value of the ion-exchange capacity differs from the Ralex AMH membrane, except the Ralex/MF-4SK/PANi sample.

A reduction of the ion-exchange capacity by 7% for the Ralex/MF-4SK membrane is observed

Conclusions

In this work, we have shown the possibility of creating a bilayer and three-layer composite material based on the Ralex AMH membrane. We observe an increase in ion-exchange capacity, electrical conductivity and limiting current density for membranes modified with polyaniline. In a three-layered system (Ralex/MF-4SK/PANi) we achieved a uniform deposition of a layer of polyaniline on the surface of a heterogeneous membrane.

The application of a thin layer of ionpolymer on the heterogeneous

Acknowledgements

This work was supported by the Russian Foundation for Basic Research, grants №№ 19-08-01172_a and 19-08-00925_a.

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