Counter-ion Transport Number Research Articles

Ageing of ion-exchange membranes in electrodialysis: A structural and physicochemical investigation

Understanding the ageing mechanisms of ion-exchange membranes (IEMs) used in electrodialysis (ED) for food industry applications constitutes a major challenge. In this regard, four used membranes (two cationic ones and two anionic ones) were analyzed at the end of their useful life (2 years of ED operation) and compared with their respective fresh new samples to assess the evolution of their structural and physicochemical characteristics, and explore their deterioration. The conductivity, ion-exchange capacity, water content, contact angle and counterion transport number were determined for each new and used membrane. Scanning electron microscopy (SEM), energy dispersive X-ray (EDX), and FTIR analyses were also performed.The used cation-exchange membranes (CEMs) suffered significant degradation; nonetheless, they were generally more robust and resistant than the anion-exchange counterparts, which were more unstable. A significant degradation in the polymer matrix of both membrane types was found. Both used CEMs and anion-exchange membranes (AEMs) lost a part of ion-exchange sites, and their specific electrical conductivity decreased. However, the CEMs became denser with lower water content and higher surface hydrophobicity, while the water content and the thickness of the AEMs increased in about two times. The permselectivity of used CEMs decreased, while that of AEM analogues was not modified. A physicochemical model schematically describing the time evolution of membrane structure and properties was built on the basis of the microheterogeneous model and the experimental data. The main idea was to present colloid aggregates adsorbed by the membrane as nanoparticles occupying a part of membrane pores. This model was used to explain the variation of the totality of equilibrium and transport properties, and in particular, the apparent contradiction between the loss of ion-exchange capacity and the increase in permselectivity of the used AEMs.

Characterization, Physicochemical Properties and Transport Studies of Newly Synthesized Polystyrene Blended Inorganic-Organic Nanocomposite Membrane

A nanostructured polystyrene based inorganic-organic composite membrane has been synthesized by sol-gel method under acidic conditions. Investigation on the electrochemical properties showed that higher membrane potential was observed in case of LiCl than other electrolytes used such as NH4Cl, KCl and NaCl for the nanocomposite membrane. Effective fixed charge densities were evaluated from the measured potential and were found to follow the order Li+ > NH4+ > K+ > Na+. The permselectivity, ionic mobility ratio and counter-ion transport number of the composite membrane have also been calculated which suggest that the inorganic-organic nanocomposite membrane is more cation-selective towards Li+ ions; however the selectivity increases with dilution for all monovalent electrolytes. These membranes were comprehensively characterized for their physicochemical properties, morphology, molecular interactions, crystalline nature and thermal stability by water uptake and porosity studies, scanning electron microscopy (SEM), Fourier transform infrared (FTIR), X-ray studies, thermogravimetric and differential thermal analyses (TG-DTA). The ion-exchange capacity, volume void porosity and water uptake of the membrane were found to be highly dependent on polystyrene content in the membrane phase and these properties decreased with increase in the amount of the organic polymer.

Synthesis, characterization and properties of polystyrene incorporated calcium tungstate membrane and studies of its physicochemical and transport behavior

This article reports the synthesis of an inorganic nanocomposite material blended with an organic binder by sol–gel method which has been applied as a potential membrane system. The prepared hybrid compound has been extensively characterized by TG–DTA, SEM and FTIR. The XRD study reveals that the polycrystalline nanocomposite is composed of small crystals with an average granular size of 15.8nm. Ion-exchange capacity, water content and volume void porosity of the composite were determined in order to study the physicochemical properties of the membrane. The values of potentials measured across the polystyrene blended calcium tungstate membrane, using various uni-univalent electrolytes (aqueous KCl, NaCl, LiCl and NH4Cl solutions), were used to evaluate the fixed charge density using different approaches on the basis of irreversible thermodynamics. The counter-ion transport number, mobility ratio and permselectivity of the tungstate membrane were figured out and were found to follow the order Li+>NH4+>K+>Na+ i.e. the inorganic–organic hybrid membrane is more cation selective towards Li+ ions for the same electrolytic concentration.

Current-induced concentration polarization of interfaces between non-ideally perm-selective ion-exchange media and electrolyte solutions

In non-ideally perm-selective ion-exchange media and/or nano-porous/nano-channel systems, ion transport numbers depend on the salt concentration. Under conditions of current-induced polarization, this concentration changes considerably close to the membrane surface or nano-/micro-interface. It is well known that the value of limiting current is controlled (among other things) by the difference of counter-ion transport numbers between the membrane and solution. Thus, a natural question arises about what value of concentration has to be used in the estimates of the difference of transport numbers and, thus, of limiting current. At first glance, it appears that one should use the value of salt concentration occurring at the polarized interface. If this were correct, there would be a positive feedback in the process of concentration polarization making the latter self-sustaining.This communication shows that, rather surprisingly, the limiting current is controlled by the counter-ion transport number taken at the salt concentration occurring at the “opposite” membrane side where the salt concentration goes up (in the limiting case of dominant diffusion resistance of membrane as compared to the unstirred layer). Accordingly, the positive feedback does not occur. Besides, the limiting current can considerably depend on the stirring conditions close to the “back” membrane surface, which has not been generally recognized previously and can be important for the experimental practice. This result has been obtained via explicit solution of the ion-transport problem in the membrane phase.

Stable ion-exchange membranes for water desalination by electrodialysis

In this manuscript, we are using sulfonated poly (ethersulfone) (SPES) based cation-exchange membrane (CEM) and polyvinyl alcohol-4-vinylpyridine (4-VP) based cross-linked based anion-exchange membrane (AEM) for water desalination by electrodialysis (ED). These membranes were characterized for their electrochemical and physicochemical properties such as membrane conductivity, counter-ion transport number, ion-exchange capacity and water content. Prepared ion-exchange membranes (IEMs) showed good stabilities under severe conditions such as high temperature and strong oxidizing conditions. ED experiments in recirculation mode of operation were performed for water desalination. Energy consumption and current efficiency values confirmed suitability of these IEMs for desalination of water by ED.

The effect of aprotic solvent on the selectivity of ion-exchange membranes

The effect of N,N-dimethylacetamide on the selectivity of heterogeneous (MK-40, MA-40, and MA-41) and homogeneous (MF-4SK) ion-exchange membranes is studied for the first time. Concentration dependences of electrical conductivity and diffusion permeability of the membranes were measured experimentally over wide lithium chloride concentrations; on their basis, electrodiffusion coefficients of the co- and counterions were calculated. The interrelation between the electrodiffusion coefficients and the specific moisture capacity of the heterogeneous and homogeneous membranes (which affect their selectivity) is revealed. The calculated electrodiffusion coefficients were used in the calculations of the electromigration transport numbers of counterions in the initial membranes and those processed in mixed solvent. It is shown that the heterogeneous membrane selectivity either increased under the action of the aprotic solvent to polymer material (MA-40, MA-41) or remained practically unchanged (MK-40); the selectivity of homogeneous perfluorinated membranes (e.g., MF-4SK) decreased, thus approaching that of the studied heterogeneous anion-exchange membranes.

Nano-fibrous sulfonated poly(ether ether ketone) membrane for selective electro-transport of ions

We reported fabrication of electrospun nano-fibrous ion-exchange membrane (IEM) composed of sulfonated poly(ether ether ketone) (SPEEK). Prepared membrane was employed for electro-dialytic separation of Na + from Mg 2+ or Ca 2+, which is an urgent problem for the production potable water from brackish water and chlor-alkali industries. Nano-fibrous membrane was successfully fabricated onto spun-bonded non-woven polypropylene fabric. Electrospun SPEEK (ESPEEK) and SPEEK thin film membrane's suitability, for the electro-dialytic separation of cations, was assessed by their conductivity measurements under different electrolytic environment using electrochemical impedance spectroscopy. Counter-ion transport number and membrane permselectivity were strongly depended on the nature of counter-ions. ESPEEK membrane showed very low selectivity (about negligible) for Mg 2+ and Ca 2+, because formation of nano-fibrous pore or channels, which restricted the migration of bi-valent cations. Additionally, very low flux and electro-transport efficiency for bi-valent cations in compare with Na + across ESPEEK membrane, suggested its possible application for electro-driven separation of mono-valent and bi-valent electrolytes. Electrospun nano-fibrous IEMs may be the promising candidate for electro-dialytic selective separation of ions depending on their charge and hydrated ionic size.

Electric conduction of ceramic membranes modified with nanoparticles of an inorganic ion exchanger

The article has investigated electric conduction of systems including composite ceramic membranes whose macropores are filled with an ion-exchange component—hydrated with zirconium dioxide and the solution of 1,1-charged electrolyte HCl. It was found that experimental values of electric conduction of membranes are the values of the same order as the electric conduction of an equilibrium solution. Based on potentiometric transport numbers of counterions we computed electric conduction of the ion-exchanger-solution system with the account of not only electrolyte, which is in intraporous solution, but also of counterions localized in the phase of the ion-exchanger. We have proposed a model equation linking electrical conduction of the membrane, the ion-exchanger, and the solution. An assumption was made about the presence in the pores of nano-dimensional channels being formed when introducing the ion-exchanger to the matrix.

Incorporating ionic size in the transport equations for charged nanopores

Nanopores with fixed charges show ionic selectivity because of the high surface potential and the small pore radius. In this limit, the size of the ions could no longer be ignored because they occupy a significant fraction of the pore and, in addition, they would reach unrealistic concentrations at the surface if treated as point charges. However, most models of selectivity assume point ions and ignore this fact. Although this approach shows the essential qualitative trends of the problem, it is not strictly valid for high surface potentials and low nanopore radii, which is just the case where a high ionic selectivity should be expected. We consider the effect of ion size on the electrical double layer within a charged cylindrical nanopore using an extended Poisson–Boltzmann equation, paying special attention to (non-equilibrium) transport properties such as the streaming potential, the counter-ion transport number, and the electrical conductance. The first two quantities are related to the nanopore selectivity while the third one characterizes the conductive properties. We discuss the nanopore characteristics in terms of the ratio between the electrolyte and fixed charge concentrations and the ratio between the ionic and nanopore radii showing the experimental range where the point ion model can still be useful. Even for relatively small inorganic ions at intermediate concentrations, ion size effects could be significant for a quantitative estimation of the nanopore selectivity in the case of high surface charge densities.

Effect of SiO2 nanoparticle addition on the characteristics of a new organic–inorganic hybrid membrane

Poly(vinylidene fluoride) composite membranes filled with different weight fractions of SiO2 nanoparticles have been prepared by a blending method. Cation-exchange groups were introduced by the copolymerization of glycidyl methacrylate with divinylbenzene and subsequent sulfonation. These hybrid membranes have been characterized by Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), differential scanning calorimetry (DSC), and water uptake and ion-exchange capacity measurements. Membrane potential and membrane conductivity measurements have been carried out with different counter ions to investigate the relationship between ionic migration and the SiO2 nanoparticle content. The counter-ion transport number and permselectivity of these membranes are found to be highly dependent on the SiO2 content in the membrane phase and the nature of the counter ion. Membrane conductance was analyzed in terms of phenomenological coefficients using non-equilibrium thermodynamic principles. It can be concluded that these hybrid membranes exhibit high thermal stability, improved selectivity, and moderate membrane conductivity, and may be suitable for use in the electro-driven separation processes.

Preparation of ultra-and nanoporous glasses and study of their structural and electrokinetic characteristics in 1: 1 electrolyte solutions

Nano-and ultraporous glass membranes with pore radii of 4.5–150 nm are prepared from sodium borosilicate glasses of various compositions. Structural parameters (structure resistance coefficient, volume porosity, and filtration factor) and electrokinetic characteristics (conductivity, counterion transport numbers, and electrokinetic potential ζα*) of membranes are determined at various KCl and NaCl solution concentrations (10−4−10−1 M) in a neutral pH region. The passage from nano-to ultraporous glasses is accompanied by an increase in |ζα*| values, which is apparently related to a decrease in the thickness of a gel layer due to the removal of ion-permeable secondary silica from pore channels. The comparison of electrokinetic characteristics of glass membranes (counterion transport numbers, efficiency coefficients, and electrokinetic potentials) measured in NaCl and KCl solutions indicates a higher specificity of K+ counterions as compared to Na+ ions.

Electric transport properties of ultra-and nanoporous glasses in electrolyte solutions

Electric transport characteristics (conductivity, specific surface conductance, and transport numbers of counterions) for nano-and ultraporous glass membranes with pore radii of 1.3–160 nm are studied and compared for chloride solutions containing single-, double-, and triple-charged cations.

Porous glass membranes as model disperse systems

The structural, adsorption and electrokinetic characteristics of porous glass (PG) membranes, prepared by leaching of various alkali borosilicate glasses, were studied as a function of pH in the presence of 1:1, 2:1 and 3:1 electrolytes. The obtained experimental results were used to calculate both of surface reaction constants and ion adsorption potentials within the framework of a 2-pK model. The electrochemical parameters of membranes (counterion transport numbers, ion concentrations and mobilities in the pore space, Donnan potentials) were also calculated within the framework of a homogeneous model. The dependencies of membrane characteristics on the electrokinetic radius were analysed.

The Effect of Thermal Treatment on the Structural and Electrokinetic Properties of Porous Glass Membranes

The properties of porous glass membranes prepared by acid leaching of sodium borosilicate glasses 8B and 8V and also 8B glass containing small amounts of fluorine and phosphorus (SFP) are comprehensively studied. The effect of the composition and conditions of thermal treatment of the original and porous glasses on their structural (specific surface area, structure resistance coefficient, average pore radius, volume porosity, and filtration factor) and electrokinetic characteristics (conductivity, counterion transport numbers, and electrokinetic potential) in KCl solutions at neutral pH values is studied. It is shown that an increase in thermal treatment temperature TTT of the porous glasses from 120 to 750°C leads to a decrease in structure resistance coefficient β of 8B membranes. For membranes prepared from SFP glass, β values, efficiency coefficients, and counterion transport numbers are virtually independent of TTT at 120–600°C and increase at TTT = 750°C. Specific surface area and volume porosity decrease with a rise in TTT for all studied membranes. The observed regularities of variations in the membrane characteristics are explained by the increasing fraction of large pores because of sintering of small pores with an increase in TTT and by the different amounts of secondary silica in the pore space of porous glasses.

Structure and Electrosurface Properties of Porous Glasses with Different Compositions in KCl and NaCl Solutions

For porous glasses with and without small amounts of fluorine and phosphorus, structural (specific surface area, structure resistance coefficient, and mean pore radius) and electrosurface characteristics (adsorption of potential-determining ions, conductivity, counterion transport numbers, and electrokinetic potential) in sodium and potassium chloride solutions are compared. Results of measuring the equilibrium and transport characteristics of membranes are used to calculate the constants of surface reactions and adsorption potentials of ions within the framework of the 2-pK model of oxide surface charging. Within the framework of the homogeneous model, electrochemical characteristics of porous glasses, namely, concentrations of co- and counterions in a pore-confined liquid, Donnan potentials, convective component of the conductivity of a pore-confined solution, and mobility of counterions in the membranes, are calculated.

Optimal strategy to model the electrodialytic recovery of a strong electrolyte

In this work, mostly Nernst–Planck derived relationships were used to simulate the electrodialytic recovery of a strong electrolyte, namely sodium chloride. To this end, it was set up a five-step experimental procedure consisting of zero-current leaching, osmosis, and dialysis, electro-osmosis, desalination, current–voltage and validation tests. The contribution of leaching and solute diffusion across the electro-membranes was found to be negligible with respect to the electro-migration. On the contrary, solvent diffusion tended to be important as the solute concentration difference at the membrane sides increased or current density was reduced. The electro-osmosis and desalination tests yielded the water and solute transport numbers. By performing several limiting current tests at different solute concentrations and feed flow rates using anionic or cationic membranes, it was possible to determine simultaneously the limiting current intensity, the ratio of the differences between the counter-ion transport numbers in the anion- and cation-exchange membranes and solution, the overall resistance of the electro-membranes, the effective membrane surface area, and the solute mass transfer coefficient. All these process and design parameters allowed the time course of the solute concentration in the concentrating (C) and diluting (D) compartments, as well as the voltage applied to the electrodes, to be reconstructed quite accurately without any further correction factors. The capability of the above parameters to simulate the performance of the electrodialysis (ED) unit was checked by resorting to a few validation tests, that were performed in quite different operating conditions from those used in the training tests, that is by filling tank C with a low feed volume with a low solute concentration and applying a constant current intensity to magnify the effect of electro-osmosis or by changing the current intensity step-wisely to simulate the continuous-mode operation of a multistage ED unit. Finally, a parameter sensitivity analysis made the different contribution of the process and design parameters to be assessed, thus yielding a straightforward procedure for designing or optimising accurately ED desalination units up to a final salt concentration of about 1.7 kmol m −3.

Preparation and electrochemical characterization of sulfonated polysulfone cation‐exchange membranes: Effects of the solvents on the degree of sulfonation

AbstractSulfonated polysulfone cation‐exchange membranes with various degrees of sulfonation were prepared by a treatment with chlorosulfonic acid in different solvents of various polarities, and the effect of the solvent polarity on the degree of sulfonation was explored. These membranes were characterized by their ion‐exchange capacity, volume fraction of water, and electrochemical properties. The counterion transport numbers, permselectivity, and fixed charge densities of these membranes were estimated from membrane potential data and varied with the degree of sulfonation, concentration, and external salt concentration. The counterion mobility in the membrane phase was also estimated from membrane conductance measurements. These membrane were found to have good electrochemical properties and are suitable for various types of electromembrane processes. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2344–2351, 2005

Preparation of organic–inorganic composite anion-exchange membranes via aqueous dispersion polymerization and their characterization

Organic–inorganic composite membranes based on poly(vinyl alcohol)/SiO 2 were prepared via an aqueous dispersion polymerization route and anion-exchange groups were introduced in the membrane matrix by the chemical grafting of 4-vinylpyridine with the desired content. These membranes were extensively characterized for their surface morphology, thermal stability, water content, and surface-charge properties using SEM, TEM, FTIR, TGA, water uptake, and ion-exchange capacity measurements. Counterion transport numbers across these membranes were estimated from membrane potential data. Membrane conductance measurements were also performed and these data were used for the estimation of values of counterion diffusion coefficients in the membrane phase. Physicochemical and electrochemical properties of these membranes and equivalent pore radius (estimated from electroosmotic flux measurements) were found to be highly dependent on the 4-vinylpyridine (4-VP) content in the membrane phase. It was also observed that for better selectivity and membrane conductivity of anion-exchange membranes complete optimization of the loading of 4-VP in the membrane phase is necessary. Furthermore, among these, membrane with 25% loading with 4-VP exhibited very good selectivity, water content, and ion-exchange capacity along with moderate membrane conductivity, which may be used for their application in electro-driven separation at elevated temperatures or for other electrochemical processes.

Comparative studies on performance of interpolymer and heterogeneous ion-exchange membranes for water desalination by electrodialysis

As the preparation of interpolyme- type anion-exchange membranes involves several steps and the use of hazardouschemicals such as chloromethyl ether and trimethyl amine, it was envisaged to prepare heterogeneous anion-exchange membranes through a simple technique involving casting of a solution containing a binder and a polyelectrolyte. Electrochemical characterization of interpolymer and heterogeneous membranes was carried out by measurement of electrical resistance, counter-ion transport number, ion-exchange capacity and volume fraction of water. Interpolymer ion-exchange membranes exhibited better electrochemical and electrodialytic properties in comparison to a heterogeneous membrane. In order to develop alternate anion-exchange membranes, a heterogeneous type of anion-exchange membrane was prepared, and its electrodialytic performance in combination with an interpolymer cation-exchange membrane was studied. Electrodialysis experiments in recirculation mode of operation were performed under varied feed concentrations and applied potential gradients in two configurations: (1) interpolymer cation- and anion-exchange membranes and (2) interpolymer cation- and heterogeneous anion-exchange membranes, packed together, in order to assess their comparative efficiencies for water desalination by electrodialysis. Under both configurations energy consumption and current efficiency were similar under comparable experimental conditions, but the rate of desalination from configuration (1) was much faster.

Preparation of polyvinyl alcohol–silica hybrid heterogeneous anion-exchange membranes by sol–gel method and their characterization

Polyvinyl alcohol–silica hybrid heterogeneous anion-exchange membranes were prepared by sol–gel method by dispersing the anion-exchange resin in the gel formed by acid/base hydrolysis of tetraethylorthosilicate in polyvinyl alcohol solution. The effect of acid and base-catalyzed hydrolysis on membrane preparation and extent of resin loading was studied. Various membranes were prepared with 60% (w/w) anion-exchange resin (Indoin FFIP) loaded with different extents of silica. Membrane potential and membrane resistance measurements have been carried out with different counter-ion to investigate the relationship between ionic migration and silica content in the membrane forming material. Counter-ion transport number and permselectivity of these membranes were found to very due to the silica content and nature of counter-ion. Data indicates that, with increase in compactness of the membranes, its ability to discriminate between counter-ions also increases. Decrease in relative transport of bulkier ions are useful, especially, desirable for selective removal of these type of ions or membrane based electrochemical sensors.