Transport of hydrogen and hydroxyl ions through ion-exchange membranes under overlimiting current densities

A direct method is suggested for measurement of fluxes of hydrogen and hydroxyl ions through ion-exchange membranes under overlimiting modes of electrodialysis. Fluxes of hydrogen and hydroxyl ions are measured for different immobilized membrane ions and counterions. The important role of counterions in generation of hydrogen and hydroxyl ions is shown. Exponential decrease in fluxes of hydroxyl or hydrogen ions is established at an increase in the Gibbs energy of counterion hydration.     

Short communications

The transport of carbonic acid anions through an anion-exchange membrane (AEM) during electrodialysis is studied. At current densities above limiting diffusion values, fluxes of hydrocarbonates and carbonates are lower than those of strong-electrolyte anions. The reason for the decreased fluxes is the recombination of carbonic acid ions with hydrogen ions that form during irreversible dissociation of water molecules at the solution/AEM interface     

Electrodialytic separation of potassium ions from sodium ions in the presence of crown ether using a cation-exchange membrane

The separation of potassium and sodium ions from their mixture was performed by electrodialyzing a mixed solution of potassium chloride and sodium chloride in the presence of 18-crown-6 using a commercial cation-exchange membrane. After 18-crown-6 had been impregnated in the membrane, the mixed solution containing 18-crown-6 was electrodialyzed as the desalting-side solution. The permeation of potassium ions through the membrane decreased remarkably and the electrical resistance of the membrane increased during electrodialysis with increasing concentration of 18-crown-6 in the solution. Because potassium ions form a more stable complex with 18-crown-6 than sodium ions and because the complex permeated through the membrane with difficulty, sodium ions are thought to selectively permeate through the membrane. The current efficiency in electrodialysis was greater than 97.0%. Received: 1 June 1999/Accepted in revised form: 13 August 1999     

Ionic Transport in Electrodialysis of Ammonium Nitrate

During the electrodialysis of ammonium nitrate solution, the fluxes of salt ions pass through the maximum, which is observed near the limiting current, with increasing current density. A decrease in the flux of ammonium ions at the overlimiting current densities is caused by the effect of competitive transport of solution ions and by the formation of weak NH₃ ? H₂O electrolyte due to the alkalization of solution layer adjacent to the cation-exchange membrane in the desalination channel. A decrease in the flux of nitrate ions in the overlimiting current modes is caused by a change in the composition and catalytic activity of the functional groups of anion-exchange membrane towards the dissociation of water molecules due to the effect of ammonium ions.     

Reformulating the permselectivity-conductivity tradeoff relation in ion-exchange membranes

Polymer membranes used in separation applications exhibit a tradeoff between permeability and selectivity. That is, membranes that are highly permeable tend to have low selectivity and vice versa. For ion-exchange membranes used in applications such as electrodialysis and reverse electrodialysis, this tradeoff is expressed in terms of membrane permselectivity (i.e., ability to selectively permeate counter-ions over co-ions) and ionic conductivity (i.e., ability to transport ions in the presence of an electric field). The use of membrane permselectivity and ionic conductivity to illustrate a tradeoff between counter-ion throughput and counter-ion/co-ion selectivity in ion-exchange membranes complicates the analysis since permselectivity depends on the properties of the external solution and ionic conductivity depends on the transport of all mobile ions within a membrane. Furthermore, the use of these parameters restricts the analysis to ion-exchange membranes used in applications in which counter-ion/co-ion selectivity is required. In this study, the permselectivity-conductivity tradeoff relation for ion-exchange membranes is reformulated in terms of ion concentrations and diffusion coefficients in the membrane. The reformulated framework enables a direct comparison between counter-ion throughput and counter-ion/co-ion selectivity and is general. The generalizability of the reformulated tradeoff relation is demonstrated for cation-exchange membranes used in vanadium redox flow batteries.     

Effects of circulation and facilitated electromigration of amino acids in electrodialysis with ion-exchange membranes

Dependences of fluxes of the mass of amino acids and mineral ions on the density of a direct electrical current in an electrodialysis process are obtained experimentally. Mutual influence of components on transport at low strengths of the electric field in membranes is discussed. Effects of circulation and facilitated electromigration of amino acids are considered. At a small excess of the limiting current density, the effects lead to a decrease in fluxes of amino acids through membranes. At a large excess of the limiting current, the effects result in an intensive conjugated transport of bipolar ions with the medium ions through membranes.     

Treatment of ion exchange membranes to decrease divalent ion permeability

In the electrodialysis process for concentrating sea water, the addition of a small amount of the polycationic reagent Nonisold into the sea water feed results in a reduction of divalent cation permeability relative to that of monovalent cations. This is due to the formation of a reagent layer on the surface of the cation-exchange membrane. Divalent cations require greater energy to pass over the potential barrier at the reagent layer than do monovalent cations. The relationship between the height of the potential barrier and the ratio of ionic fluxes or the permselectivity coefficient is deduced on the basis of a kinetic-controlled mechanism of uptake of ions from the solution, and the height of the potential barrier is estimated from the experimental results. The difference between the potential barrier for divalent cations and that for monovalent cations may reach up to about 10 kJ/mol.     

Transport of ions of pyrophosphoric acid through chemically different anion-exchange membranes

Characteristics of mass transport of the anions of pyrophosphoric acid are investigated during electrodialysis through MA-40 and MA-41 anion-exchange membranes. It is established that the rate of mass transport for counterions is greater through the MA-41 membrane than the MA-40 membrane, and thus the former can be used for demineralizing solutions with salts of pyrophosphoric acid. Higher selectivity to pyrophosphate ions is found for the MA-40 membrane compared to the MA-41 membrane in the electrodialysis of mixtures of pyrophosphate and hydropyrophosphate ions, and thus the MA-40 membrane can be regarded as promising for the separation of such mixtures.     

Ionic mass transport through a homogeneous membrane in the presence of a uniform electric field

The time-dependent theoretical solution to ionic mass transport through a uniform membrane under the influence of a uniform electric field is derived. Equivalent lag times and steady-state fluxes are then determined. The enhancement ratio of ionic flow (at a given voltage) to passive flow is shown to be proportional to the voltage for large augmenting voltages, and to decrease exponentially for large retarding voltages. Positive enhancement factors can be as large as 120 for triply charged ions, with one volt imposed across a membrane. The lag times decrease with increasing voltage magnitudes, and can (for triply charged ions, with one volt imposed across a membrane) be as small as 1/20 of the passive lag time.     

离子色谱电渗析技术的研究进展

电渗析器件通常定义为在电场作用下操纵离子从一种溶液穿过离子交换膜迁移到另外一种溶液的一种设备。它可以通过电解水产生氢离子或氢氧根离子,从而用于离子色谱系统的淋洗液在线制备、抑制或检测。相较于人工配制淋洗液或再生液,电渗析技术具有绿色、高效、纯度高、自动化程度高等优势。因此基于电渗析器件的离子色谱系统应用范围越来越大。该文简要评述了近几年该器件的研究进展,具体包括电致淋洗液发生器、电致膜抑制器和电渗析样品前处理器。     

Study of pH correction process of chloride-bicarbonate dilute solutions by electrodialysis with bipolar membranes

The pH of a dilute chloride-hydrocarbonate solution and the concentrations of chloride ions and carbonic acid anions at the outlet of the alkaline and acid chambers of the electrodialysis cell formed by bipolar and anion-exchange membranes were determined. The decrease in the concentration of hydrocarbonate ions in the alkaline chamber with growth of current density was not equal to its increase in the acid chamber. This disbalance was caused by two concurrent processes: the electromigration ion transport through the anion-exchange membrane and the chemical reactions of hydrocarbonate ions with the water dissociation products formed on the bipolar and anion-exchange membranes. A mathematical model was suggested to describe the electrodialysis correction of the pH of a dilute chloride-hydrocarbonate solution. The experimental data on the correction of pH of the chloride-hydrocarbonate solution were well approximated by both the model that takes into account water dissociation on the anion-exchange membrane and the simplified model that neglects water dissociation. The experimental data agreed well with the results of calculations by the model in which the effective anion transport numbers were calculated only from ion concentrations and diffusion coefficients in solution. This reflects the outer diffusion character of the kinetics of ion transport through the anion-exchange membrane, with pH of dilute solutions corrected by electrodialysis.     

Reducing power losses caused by ionic shortcut currents in reverse electrodialysis stacks by a validated model

Both in electrodialysis and in reverse electrodialysis ionic shortcut currents through feed and drain channels cause a considerable loss in efficiency. Model calculations based on an equivalent electric system of a reverse electrodialysis stack reveal that the effect of these salt bridges could be reduced via a proper stack design. The critical parameters which are to be optimized are ρ/r and R/r, where ρ is the lateral resistance along the spacers, R is the resistance of the feed and drain channels between two adjacent cells, and r is the internal resistance of a cell. Because these two parameters are dimensionless, different stacks can be easily compared. The model is validated with two experimental stacks differing in membrane type and spacer thickness, one with large ionic shortcut currents and one where this effect is less. The loss in efficiency decreased from 25 to 5% for a well-designed stack. The loss of efficiency in reverse electrodialysis and in electrodialysis can be reduced with the aid of the design parameters presented in this paper.     

Electrodialytic transport properties of cation exchange membranes in the presence of cyclodextrins

After commercial cation exchange membranes (NEOSEPTA) had been immersed in an aqueous cyclodextrin solutions, electrodialysis of a 1:1 mixed solutions of alkaline earth metal ions and sodium chloride was carried out in the presence of cyclodextrins in the desalting side solution. Adsorption of the cyclodextrins in the membrane matrix was confirmed by IR spectrum, X-ray photoelectron spectroscopy (XPS) and the weight increase of each corresponding component of the membranes. As a result of adsorption of the cyclodextrins in the membranes, transport numbers of alkaline earth metal ions relative to sodium ions decreased compared with those of the membranes without cyclodextrins. Water content in the membrane increased and a ratio of calcium ions (alkaline earth metal ions) to sodium ions in the membrane phase decreased after adsorption of the cyclodextrins. This is due to existence of the compounds having hydrophilic outer surface, cyclodextrins, in the membrane matrix. The decrease in the transport number of calcium ions relative to sodium ions was due to both decrease in a ratio of mobility of calcium ions to that of sodium ions and decrease in an ionic ratio of calcium ions to sodium ions in the membrane phase.     

Competitive anion transport in desalting of mixtures of organic acids by batch electrodialysis

Desalting and separation of binary and quaternary acid mixtures via batch electrodialysis are investigated in this article. A monoselective cation exchange membrane and either a non-selective or a monoselective anion exchange membrane are employed in the electrodialysis stack. The effects of current density and composition of the initial feed of the electrodialysis stack (employing a non-selective anion exchange membrane) on its performance are studied in experiments involving mixtures of acetic and succinic acids. The effect of the type of the anion exchange membrane on the process performance is examined in desalting experiments involving a mixture of acetic, formic, lactic, and succinic acids. The trends observed in the experiments are interpreted in terms of species-specific parameters (such as molar concentration, charge on ionic species, molecular weight, degree of ionization, and ionic equivalent conductivity) and characteristics of anion exchange membrane used.     

Transport properties of electrodialysis membranes in the presence of Zn2+ complexes with Cl−

The aim of this paper is to investigate the properties of anion exchange electrodialysis membranes in contact with aqueous solutions containing zinc chloride complexes. Measurements of electric resistance, water content, quantity of sorbed electrolytes, self-diffusion and electrotransport ionic fluxes were performed as a function of the composition of the external solutions. The results were analysed in relation to the complex formation inside the solutions.     

Electrodialytic properties of cation-exchange membranes in the presence of N-dodecyl pyridyl compounds

Electrodialytic transport properties of cation-exchange membranes were observed in the presence of dodecyl pyridyl compounds. The transport properties measured were the relative transport number of calcium ions to sodium ions (P_Na^Ca), the current efficiency of cations and the electric resistance of the membrane during the electrodialysis. Dodecyl pyridyl compounds used were dodecyt pyridinium bromide, dodecyl dipyridyl bromide and dodecyl tripyridyl bromide. When the electrodialysis was carried out in the presence of dodecyl pyridyl compounds, P_Na^Ca decreased and the electric resistance of the membrane increased extremely during the electrodialysis. However, the current efficiency did not change except with dodecyl pyridinium bromide. These phenomena are based on the accumulation of dodecyl pyridyl ions in the definite surface part on the desalting side of the membrane and their permeation through the membrane. The results showed that the hydrophobic strength of the compounds, rather than their molecular weight, is effective in decreasing P_Na^Ca. However, the increase in the electric resistance of the membrane is mainly dependent on the molecular weight of the compounds.     

Self diffusion and conductivity in Nafion membranes in contact with NaCl+CaCl2 solutions

The ion-exchange isotherm of sodium and calcium ions is drawn for the Nafion^R 117 membrane in contact with [NaCl] + 2[CaCl₂] = 0.1 M. The self-diffusion coefficients of sodium and calcium ions, obtained from steady-state self-diffusion ionic fluxes by a radiotracer method are independent of the ionic composition of the membrane phase. They are compared to those obtained from high frequency electrical resistance by means of the ac impedance technique. The values of electric conductivity of the membrane can be derived from a simple additivity law.     

Secondary potential in electrodialysis membranes and the effect on permselectivity

The most important factor in the electrodialysis (ED) process is the permselectivity of the ion exchange membranes, which permit not only the separation of cations and anions in a solution, but also the separation of ions with the same sign but different valences. In this work, the mechanism of the permselectivity has been studied through the measurement of the potentials at different planes of the membrane. The experimental results have shown that there was a secondary potential inside ion exchange membranes in an electrodialysis process. At the membrane side touched with dilute solution, this secondary potential enhanced the external electrical field, and thus speeded up the passage of the corresponding ions in the dilute solution through the membranes; at the membrane side touched with concentrated solution, the secondary potential was contrary to the external electrical field and thus counteracted it, which could be very helpful by preventing the ions in the concentrated solution from entering the membranes. Obviously, the existence of the secondary potential might play an important role in the permselectivity of ion exchange membranes in ED processes.     

Difference in transfer of magnesium and sodium ions through electrodialytic membranes modified with chitosan

The difference in transfer of magnesium and sodium ions through electrodialytic cation-exchange membranes modified with chitosan was studied. The membranes were modified by electrodeposition of chitosan from solution on the membrane surface in the course of electrodialysis, preliminary sorption of chitosan on the membrane surface from aqueous chitosan hydrochloride, or application of a chitosan film to the dry membrane surface. The Mg²⁺/Na⁺ selectivity of transfer through modified electrodialytic membranes was examined in relation to the current density.     

High-impact polystyrene/polyaniline membranes for acid solution treatment by electrodialysis: preparation, evaluation, and chemical calculation

In this study different membranes were produced, aiming to evaluate their use in electrodialysis. These membranes were produced using conventional polymer (high-impact polystyrene) and polyaniline. The membrane characterization was done by FTIR spectroscopy, scanning electron microscopy (SEM), and thermogravimetry (TGA). The studies of the zinc and proton extraction ionic transport through the membranes were evaluated using a three-compartment cell. The results obtained using the produced membranes were compared to the results obtained with the commercial membrane Nafion 450. It was found that a synthesized membrane can be used to recover zinc in acid media. In addition, a preliminary computational essay about the structures of PAni and CSA is presented.