Concentration polarization and coupled effects of concentration polarization in electromembrane system

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Electrodialysis metathesis

Electrodialysis metathesis (EDM) is used to maximize water recovery and produce concentrated salts from brines obtained as reject or secondary product in other membrane processes, e.g. reverse osmosis [43]. EDM is modified from the conventional ED. In the EDM system, four alternating IEMs form a repeating quad of four chambers and a substitution solution is added to provide the exchangeable ions for the metathesis reaction (Fig. 1-10) [44]. In the reaction, the feed solution, represented by MX, exchange cations and anions with the substitution solution, M′X′, to form new product salts, M′X and MX′. For this purpose, two of the chambers contain the feed and a substitution solution, and are called diluate 1 (D1) and dilute 2 (D2) chambers, respectively. The other two chambers contain each the newly formed non-precipitating product salts, and are called concentrate 1 (C1) and concentrate 2 (C2) chambers (Fig. 1-10). Depending on the intended product solution in the two concentrate chambers, an appropriate substitution salt solution is fed to the D2 chamber to initiate the double decomposition reaction with the feed solution from the D1 chamber. Salts formed in the C1 chamber are predominantly rich in anions and poor in cations from the D1 chamber. Salts formed in the C2 chamber are predominantly rich in cations and poor in anions from the D1 chamber. When sodium chloride is added as the substitution solution, the C1 chamber contains mixed sodium salts, and the C2 chamber contains mixed chloride salts [44]. The membrane selective to the transfer of singly charged ions can be used for mitigation of the scaling and fouling in the concentrate chamber when applying electrodialysis metathesis. The design of electrodialysis stack is organized in a way that there are two concentrate chambers. There are singly charged cations (such as Na+) and multicharged anions (SO42-) in one chamber, multicharged cations (such as Ca2+) and singly charged anions (Cl-) in the other one.

Concentration polarization in electromembrane system

Let us consider a concentration profile of an electrolyte in an IEM and the adjoining diffusion layers of an EMS (Fig. 1-13). Within the membrane, we consider the virtual electrolyte solution, which is in local equilibrium with a thin layer of membrane [50]. As there is equilibrium at the membrane boundaries, the real concentration in the solution and the virtual concentration in the membrane there are identical and the concentration profile is continuous (Fig. 1-13). When a direct current passes normal to the IEM, the counter-ions from the solution pass through the IEM by migration and diffusion. Due to the fact that the counter-ion transport numbers in the membrane, ti , are almost twice as large as in the solution, ti , the electrolyte concentration decreases near the membrane in one (desalination) chamber, C1 , and increases in the neighboring (concentration) chamber, C2 (Fig. 1-13). The phenomenon of concentration variation under the effect of electric current is called concentration polarization (CP).
The ion transport in solution or membrane is described by the extended Nernst–Planck equation with a convective term: F ji Di Ci z i Ci CiV (1.4).
Here ji , Di , Ci and zi are the flux density, diffusion coefficient, concentration and charge number of ionic species i respectively; is the electric potential; F is the Faraday constant; R is the gas constant; T is the temperature; V is the fluid velocity vector. The first term in the right-hand side of Eq. (1.4) represents diffusion, the second, migration, and the third, convection.

Methods for studying the concentration polarization in an electromembrane system

Voltammetry [39, 85, 86], chronopotentiometry [87, 88], chronoamperometry [89-91] impedance [92-94], optical methods [95-97], as well as methods for pH measurement [98, 99] and ion transport numbers [100-103] are used for IEM concentration polarization. Hereafter we give a short review of these methods with focusing attention on those methods, which will be used in our research. Optical methods (laser interferometry, schlieren-method, etc.) are especially convenient for studying the CP in membrane systems. Even an insignificant change in concentration causes a sufficiently large change in the refractive index of the solution [104] and leads to a shift in the interference fringes. The most informative of these methods is laser interferometry based on recording an interference pattern between a wave reflected from the object under study and a reference wave. If an optical disturbance arises in the region of the reflected wave from the object, then, where the path difference reaches a value that is a multiple of the wavelength, a dark band appears. The method of laser interferometry allows one to determine the local electrolyte concentrations in desalination and concentration chambers of complex geometry. The minimum distance from the surface of the membrane on which measurements are possible is several micrometers (≈10 μm). The most common methods of voltammetry and chronopotentiometry are the galvanostatic one [104]. The constant current is passed through the experimental cell until the steady values of potential are reached in the galvanostatic method. I-V curve obtained with a linear current sweep are called galvanodynamic curves. These methods are the most common tools for studying the electrochemical characteristics of membrane systems. As the IEMs are conductors of the second kind, they must be located between two polarizing electrodes to ensure the current flows into the EMS. There are quite a few modifications of equivalent electrical circuit for studying the CP in EMS. The most commonly used four-electrode cell (Fig. 1-17). In this setup, the cathode and anode are located parallel to the membranes and limit the membrane package. The current is applied between the working (anode) and the counter (cathode) electrodes. The potential drop (PD) across the membrane under study, , is measured using sense and reference Ag/AgCl electrodes. These electrodes are placed in the Luggin capillaries. The Luggin tips are installed at both sides of the membrane under study in its geometric center at a distance of about 0.5 mm from the surface. The PD between sense and reference Ag/AgCl electrodes in the electrolyte solution (without the membrane) is zero.
To prevent the ingress of electrolysis products from the near-electrode to the premembrane chambers, they are separated by auxiliary CEM and AEM. In this case, the AEM separates the investigated membrane from the anode chamber, the CEM from the cathode membrane.

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Voltammetry and chronopotentiometry

Voltammetry and chronopotentiometry are the privilege methods to study electrochemical behavior of IEMs in the presence of scaling [70, 98]. The formation of scaling on the membrane surface or inside the membrane leads to the changes in shape of I-V curve. For example, the significant changes in the slope of the initial linear region, ilim and the plateau length of scaled membrane in comparison with the pristine membrane [131].
As was mentioned above, the electrical resistance of the scaled membrane reduces in comparison with the pristine one, that is why the slope angle of the quasi-ohmic region became higher in the case of scaled membrane. The extended plateau is related to the formation of precipitates at the membrane surface. The decrease in ilim for scaled membrane is attributed to the presence of area of different conductance on the surface of IEM. It is possible to obtain important information on operations of ED processes to minimize scaling effects. The disadvantages of this method are that it takes a lot of time and usually requires additional knowledge to interpret the results. Also, during the sample preparation and I-V curve registration, the weakly bounded scaling can be modified and/or detached.
Marti-Calatayud et al. [98] studied the process of scale formation on a homogeneous Nafion 117 (Du Pont) and heterogeneous HDX 100 cation-exchange membranes in mixed Na2SO4 and Fe2(SO4)3 solutions by means of chrononopotentiometric measurements. They showed that the increase with time of the quasi-steady state PD was related to the formation of precipitate on the depleted membrane surface (Fig. 1-27).
Moreover, in the case of Nafion 117 they discovered the second increase of PD. They supposed that this increase relates to the formation of a precipitate layer at the anodic side of the membrane, which was visually confirmed at the end of the experiments. In the case of HDX membrane the PD increases immediately after the current is applied. This different response may indicate the presence in the heterogeneous membrane of some pores more prone to the formation of precipitates where this phenomenon starts from the beginning of the current pulse.

Table of contents :

Chapter 1. Literature review
1.1 Ion-exchange membranes
1.1.1 The structure of ion-exchange membranes
1.1.2 Characterization of ion-exchange membranes
1.1.3 Methods of ion-exchange membrane modification Chemical modification Mechanical modification Electrochemical deposition
1.2 Application of ion-exchange membranes
1.2.1 Conventional electrodialysis
1.2.2 Reverse electrodialysis
1.2.3 Electrodialysis metathesis
1.2.4 Bipolar membrane electrodialysis
1.2.5 Fuel cells
1.3 Concentration polarization and coupled effects of concentration polarization in electromembrane system
1.3.1 Concentration polarization in electromembrane system
1.3.2 Coupled effect of concentration polarization in electromembrane system Water splitting Gravitational convection Electroconvection
1.3.3 Methods for studying the concentration polarization in an electromembrane system Voltammetry Chronopotentiometry Experimental cells for voltammetry and chronopotentiometry
1.4 Fouling
1.4.1 Scale forming mechanisms
1.4.2 Methods for studying scaling on ion-exchange membranes Visualization of ion-exchange membranes scaling Membrane characteristics depending on scaling in electrodialysis Membrane thickness, scaling content, membrane electrical conductivity Voltammetry and chronopotentiometry Contact angles X-ray diffraction analysis
1.4.3 Methods for prevention and control of ion-exchange membrane scaling Ion-exchange membrane modification Cleaning agents Pretreatment: Pressure-driven membrane processes Mechanical action Changing regimes of electrodialysis treatment Control of hydrodynamic conditions Electrodialysis reversal Pulsed electric field Overlimiting current regime
1.5 Conclusions
Chapter 2. Effect of electrolyte nature and membrane surface properties on the development of electroconvection
2.1 Introduction
2.2 Experimental
2.2.2 Ion-exchange membranes
2.2.3 Materials
2.3. Results and discussion
2.3.1 Chronopotentiometry
2.3.2 Current–Voltage Characteristics
2.4 Conclusions
2.5 Acknowledgments
Chapter 3. Effect of homogenization and hydrophobization of a cation-exchange membrane surface on its scaling in the presence of calcium and magnesium chlorides during electrodialysis 
3.1 Introduction
3.2 Experimental
3.2.1 Ion-exchange membranes
3.2.2 Protocol
3.3. Results and discussion
3.3.1 Theoretical value of the limiting current density
3.3.2 Chronopotentiometry
3.3.3 pH changes
3.3.4 Scaling formation
3.3.5 Voltammetry
3.4 Conclusions
3.5 Acknowledgments
3.6 Appendix
Chapter 4. Effect of electroconvection, pH adjustment and pulsed electric field on membrane scaling in electrodialysis
4.1 Introduction
4.2 Experimental
4.2.1. Membranes
4.2.2. Electrodialysis cell and experimental setup
4.2.3. Chronopotentiometric and voltammetry measurements
4.2.4. Analysis methods
4.3. Results and discussion
4.3.1 I-V curves
4.3.2 Chronopotentiograms
4.3.3 Scaling formation in constant current mode
4.3.4 Water splitting and effect of pH
4.3.5 Scale formation in PEF modes
4.4 Conclusions
4.5 Acknowledgments
Chapter 5. Effect of surface modification of cation-exchange membrane with polyaniline on polarization characteristics and scale formation
5.1 Introduction
5.2 Experimental
5.2.1 Preparation of anisotropic MF-4SK/Polyaniline composites under the
conditions of an external electric field
5.2.2 Polarization behavior of anisotropic MF-4SK/PANI composites
5.3 Conclusions
5.4 Acknowledgments
Presentation of the article 2
5.5 Experimental
5.5.1 Membranes
5.5.2 Analyses Membrane thickness Membrane electrical conductivity Contact angle measurements
5.5.3 Electrodialysis cell and experimental setup
5.5.4 Protocol
5.6 Results and discussion
5.6.1. Physico-chemical characteristics of ion-exchange membranes
5.6.2 Voltammetry
5.6.3 Chronopotentiometry
5.7 Conclusions


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