Effects of the desalination chamber design on the mass-transfer characteristics of electrodialysis apparatuses at overlimiting current densities

The overlimiting current modes are being increasingly used in electrodialysis (ED) of dilute aqueous solutions. Of great importance is establishing a relationship between the design of ED apparatuses and the character of phenomena observed at overlimiting current densities, primarily, electroconvection and H⁺ and OH^? ion generation during water dissociation at the membrane-solution interface. In this work, we analyze the factors governing the efficiency of dilute solutions using modern theoretical concepts and experimental data obtained in laboratory cells and large-scale electrodialysis apparatuses. We also analyze the relationship between the mechanisms of the overlimiting transfer and the design of the desalinating channel. ED apparatuses of different types are considered, namely, apparatuses with profiled membranes, inert spacers, monolayer of ionite granules, and dipolar fillers of unwoven ionite fibers. The optimum concentration ranges of the desalinated solutions were found, and the operating conditions of membrane stacks, providing maximum overlimiting ion transfer, were determined.     

Evaluation of the Zn2+ transport properties through a cation-exchange membrane by chronopotentiometry

In this work the effect of zinc concentration, pH, and boric acid concentration on the zinc transport properties through an IONICS 67-HMR-412 cation-exchange membrane was evaluated. The limiting current density and the transport numbers were determined by means of chronopotentiometry. A model between the limiting current density and the bulk zinc concentration was established, assuming a potential relationship between the zinc transport number through the membrane and the bulk zinc concentration together with the Levich equation for the DBL thickness. A decrease in the initial pH value of the solutions causes considerable modifications both in the plateau region and in the overlimiting current density region of the current–membrane potential curves. The results show that the presence of boric acid produces the precipitation of zinc metaborate on the anodic layer of the cation-exchange membrane.     

Coupled convection of solution near the surface of ion-exchange membranes in intensive current regimes

Mechanisms responsible for the overlimiting ion transfer in membranes systems are discussed. The overlimiting transfer is shown to be due largely to the action of four effects coupled with the concentration polarization of the system. Two of these are connected with the water dissociation near the membrane/solution interface: the emergence of additional charge carriers (ions H⁺ or OH^?) in the depleted solution layer and the exaltation of transfer of salt counterions. The latter effect is connected with the perturbation of electric field caused by the water dissociation products. The other two effects are two versions of coupled convection, which leads to partial destruction of the depleted diffusion layer. These include gravitational convection and electroconvection. The former is caused by the emergence of the solution’s density gradient. The latter develops via a mechanism of electroosmotic slip. In this work, methods of voltammetry and chronopotentiometry and pH measurements are used to study the transfer of ions through homogeneous membranes Nafion-117 and AMX as a function of the concentration of sodium chloride solutions in the underlimiting and overlimiting current regimes. In a 0.1 M NaCl solution, gravitational convection makes a considerable contribution to the transfer of salt ions near the membrane surface in intensive current regimes. The influence of this effect on the electrochemical behavior of membrane systems weakens with the solution dilution and with increasing relative transfer of the H⁺ and OH^? ions that are generated at the membrane/solution interface. In conditions where gravitational convection is suppressed and the water dissociation near the membrane/solution interface is not great, the major contribution to the overlimiting growth of current is made by electroconvection. Topics for discussion in the paper include the mutual influence of effects on one another, in particular, the effect the rate of generation of the H⁺ and OH^? ions exerts on the gravitational convection and electroconvection and the reasons for the different behavior of cation-and anion-exchange membranes in intensive current regimes.     

Experimental Verification of the Electroosmotic Mechanism of Overlimiting Conductance Through a Cation Exchange Electrodialysis Membrane

The paper is concerned with convection at an ion-exchange electrodialysis membrane induced by nonequilibrium electroosmosis as a source of overlimiting conductance current through the membrane. Derivation of nonequilibrium electroosmotic slip condition is recapitulated along with the results of linear stability analysis of quiescent electrodiffusion through a flat ion-exchange membrane. Results of numerical calculation pertaining to nonlinear convection, developing from the respective instability, are reported along with those of recent experiments with modified membranes. These latter rule in favor of electroosmotic versus bulk electroconvective origin of overlimiting conductance through ion-exchange membranes.     

A mathematical model describing voltammograms and transport numbers under intensive electrodialysis modes

A three-layer mathematical model of overlimiting state is developed. A reactive layer with a thickness depending on the current density is introduced into the model. A decrease in the thickness of diffusion layer, which donates the counterions, with increasing current density as a result of electroconvection is also taken into account. A boundary-value problem is formulated within the Nernst-Planck and Poisson’s model in the three-layer region with the boundary conditions of constant concentrations in the bulk solution. It is shown that an increase in the reactive layer thickness with increasing current density determines the behavior of effective transport numbers in the overlimiting state of ion-exchange electromembrane system. In the current range under consideration (from 1 to 20 limiting currents), the reactive layer thickness is several tens nanometers and reaches 70 nm at a 100-fold excess over the limiting current. To calculate the voltammograms, the dependence of effective thickness δ_N of diffusion layer on the current density is required. This dependence can be obtained by solving an inverse problem, from the laser interferometry experiments, or calculated by the Navier-Stokes hydrodynamic model. The model enables one to calculate the distribution of electric field strength, potential, concentrations in the diffusion layers and membrane.     

Effect of anion-exchange membrane surface properties on mechanisms of overlimiting mass transfer

Four effects providing overlimiting current transfer in ion-exchange membrane systems are examined. Two of them are related to water splitting: the appearance of additional current carriers (H+ and OH- ions) and exaltation effect. Two others are due to coupled convection partially destroying the diffusion boundary layer: gravitational convection and electroconvection. Three anion-exchange membranes, which differ in surface morphology and the nature of ion-exchange sites within a surface layer, are examined. The ion transfer across these membranes in NaCl solutions is studied by voltammetry, chronopotentiometry, and pH-metry. By excluding the effects of water splitting and gravitational convection, it is shown that the main mechanism of overlimiting mass transfer in narrow membrane cells at low salt concentrations is electroconvection. The reasons explaining why water splitting suppresses electroconvection are discussed. The scenario of development of potential oscillations with growing current and time is compared with that described theoretically by Rubinstein and Zaltzman.     

Theoretical Analysis of the Effect of Ion Concentration in Solution Bulk and at Membrane Surface on the Mass Transfer at Overlimiting Currents

Overlimiting current modes are of considerable interest for the practice of electrodialysis (ED). However, the economical expedience of such ED modes is evident only for desalination of dilute solutions. Here, we show the theoretical analysis of the effect of concentration on the behavior of an ED cell with homogeneous ion-exchange membranes. The study is based on numerical solution of the two-dimensional system of coupled equations of Nernst–Planck–Poisson–Navier–Stokes. It is shown that as the electrolyte concentration in solution that enters the ED desalination chamber increases, the intensity of electroconvection decreases, which induces a decrease in the relative mass-transfer rate (the decrease in the ratio of current density to its limiting value). This effect is stronger in the region of high potential differences where the electroconvective instability of Rubinstein–Zaltzman is realized under the conditions of a nonuniform concentration field caused by solution desalination. In contrast, the increase in the counterion concentration at the membrane surface (associated with the increase in the surface charge) intensifies the electroconvection.     

Electroconvection in systems with heterogeneous ion-exchange membranes after thermal modification

It is found that the variations in the structure (morphology and microrelief) and chemical composition of surface of heterogeneous ion-exchange membranes as a result of thermal modification have different effects on the current—voltage characteristics and conditions for the generation of electroconvective instability at the membrane/solution interface under intense current modes. After thermal treatment of strongly acidic sulfocation-exchange membrane, which is characterized by a low catalytic activity in the reaction of water dissociation and a high thermal stability of fixed groups, a fraction of conducting surface area increases and the membrane microrelief develops. As a result, the diffusion limiting current density increases and the length of plateau of the current—voltage curve decreases. Therewith, the thickness of the region of electroconvective instability of solution in the near-membrane region increases and the polarization of electromembrane system, at which the mode of unstable electroconvection is reached, decreases. The thermodestruction of strongly basic anion-exchange membranes, conversely, leads to suppression of electroconvection and an increase in the length of plateau of the current—voltage curve due to the formation of fixed weakly basic amino groups, which are catalytically active in the reaction of water dissociation. A linear correlation is found between the dimensions of the region of electroconvective instability and a fraction of weakly basic functional amino groups in the composition of strongly basic membranes.     

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.     

Effect of the ion-exchange-membrane/solution interfacial characteristics on the mass transfer at severe current regimes

In the electrodialysis of dilute solutions, performed at intense current regimes, the membrane electrical conductance and diffusion permeability are no more crucial. Of more importance become the membrane properties that control increase in the overlimiting mass transfer of salt ions, as well as H⁺ and OH^? ions in membrane systems. In this work different methods of the improving of mass-exchange characteristics of commercial ion-exchange membranes intended for operation at intense current regimes are discussed. They are based on modern concepts of mechanisms of the electroconvection (which proceeds as electroosmosis of 2nd kind), as well as mechanisms of H⁺ and OH^? ions generation at the membrane/solution interface. Influence on the membrane electrochemical and mass-exchange characteristics is possible via (a) control of chemical nature of fixed groups at membrane surfaces, in order to weaken their catalytic activity with respect to water dissociation reaction, (b) increase in the surface hydrophobicity, and (c) design of electrical inhomogeneity of the membrane/solution interface; the purpose is the facilitating of intense development of electrical convection.     

Effects of Electrolytes on the Transport Phenomena in a Cation-Exchange Membrane

The effect of electrolyte solutions on the characteristics of the current-voltage (I-V) curve in a cation-exchange membrane (CMX membrane, Tokuyama Soda, Inc.) was studied based on the concentration polarization and electroconvection theory. The study includes the limiting current density (LCD), plateau length, and the ratio of resistance of region III to region I of the I-V curve (R(3rd)/R(1st)). Different electrolyte solutions, HCl, LiCl, NaCl, KCl, CaCl(2), MgCl(2), and AlCl(3), were used in this study. The LCD values of the electrolytes were correlated with the diffusion coefficient of the cation (D(+)) and valence of the cation and anion (z(+), z(-)). Except for the HCl solution, the LCD values of the electrolytes increased linearly with D(+)(1-z(+)/z(-)), implying that the current in this region was governed by the concentration polarization phenomena. The deviation of the HCl solution from the linearity is due to a particular transport mechanism of the proton called the Grotthuss-type transport. The differences in the plateau length and the resistance ratio, R(3rd)/R(1st), with the electrolytes were explained by the Péclet number (Pe) representing a transport pattern in the electroconvection theory. The Péclet number is proportional to the Stokes radius of an ion. An electrolyte with a large Stokes radius has a shorter plateau length and a lower ratio of R(3rd)/R(1st) than those of an electrolyte with a small Stokes radius. Water-splitting measurements for the different electrolyte solutions in the CMX membrane revealed that the contribution of water splitting to the overlimting current was insignificant regardless of the electrolytes used in this study. However, when metal hydroxides, such as Al(OH)(3), formed on the surface of the membrane, significant water splitting was observed. Copyright 2001 Academic Press.     

Structural change of ion-exchange membrane surfaces under high electric fields and its effects on membrane properties

Structural change of an ion-exchange membrane under a high electric field was investigated by comparing water dissociation and the FTIR spectra between the virgin membrane and that used at an overlimiting current density. From a series of water dissociation experiments at overlimiting current densities, it was observed that water dissociation in an anion-exchange membrane used at an overlimiting current density was higher than that in a virgin membrane at the same current density. The FTIR study revealed that the tertiary amine groups are formed from the quaternary ammonium groups on the anion-exchange membrane surface where ion depletion occurs under the influence of the applied strong electric field. The occurrence of increased water dissociation is considered to be caused by the protonation and deprotonation of the tertiary amine groups in the anion-exchange membrane. On the other hand, there was no structural change for the cation-exchange membrane under the electric field investigated in this study, which is coincident with the results of water dissociation experiments for the CMX membrane. In addition, we found that membrane resistance, permselectivity, and plateau length of the current-voltage curve were affected by the converted tertiary amine groups depending on the solution pH.     

Generalization and prognostication of mass exchange characteristics of electrodialyzers operating in overlimiting current regimes with use made of similarity theory and compartmentation method

A semiempirical approach, which allows one to perform generalization, prognostication, and scaling of the mass exchange characteristics of electrodialyzers intended for the desalination of dilute solutions in the overlimiting current regimes, is proposed. The approach is based on modern theoretical notions concerning the dependence of mass transfer on conjugated effects of concentration polarization, such as electroconvection and the exaltation effect and makes use of the similarity theory principles. As a result, there are obtained sufficiently simple equations that describe to within a satisfactory accuracy characteristics of the process of electrodialysis (mass transfer coefficient, Sherwood number, solution desalination degree, and so on) as functions of four parameters, namely, the input concentration, the solution flow rate, the potential drop across the paired chamber of the membrane stack of an electrodialyzer, and the length of the desalination channel. For scaling the obtained dependences with respect to a channel length a compartmentation method is used. Results of experimental investigations and calculations of electrodialyzers with desalination channels without a filler, or containing an inert separator, a monolayer of an ion-exchange resin, a profiled membrane, are presented.     

Direct measurement of concentration distribution within the boundary layer of an ion-exchange membrane

In this study the concentration distributions within the diffusion boundary layer were obtained by directly measuring the potential drops while the currents (under- and overlimiting) passed through the Neosepta CMX cation-exchange membrane (Tokuyama Corp., Japan). Potential drops according to the distance from the membrane surface on the depleted side were measured using a microelectrode to obtain the concentration profile. From the concentration profiles obtained, it was observed that the diffusion boundary layers existed in the range of 300-350 microm, which reasonably coincide with the theoretical diffusion boundary layer thickness calculated from the limiting current density. Although there were some deviations between the concentrations determined from the Nernst model and those from experiments, it was confirmed that the Nernst model effectively depicts the transport phenomena in the ion-exchange membrane system. In addition it was found that the salt concentration at the membrane surface increased when the currents applied exceeded the limiting current. It is thought that the concentration polarization formed in the diffusion boundary layer at currents near or lower than the limiting current was disturbed by a turbulent convection when the current was greater than the limiting current. As a consequence, the concentration at the membrane surface increased to a sufficient level for generation of the overlimiting current.     

Transfer of electrolyte ions and water dissociation in anion-exchange membranes under intense current conditions

Polarization characteristics of electromembrane systems (EMS) based on the Russian commercial heterogeneous membranes MA-40 and MA-41, the anion-exchange heterogeneous membrane AMH (Mega, Czech Republic), and the modified membrane MA-40M are studied by the method of rotating membrane disk in dilute sodium chloride solutions. The effective transport numbers of ions are found; the partial voltammetric characteristics (VAC) with respect to chloride and hydroxyl ions are measured; the limiting current densities are calculated as a function of the membrane disk rotation rate. In terms of the theory of the overlimiting state of EMS, based on experimental VAC and the dependences of the effective transport numbers on the current density, the following internal parameters of systems under study are calculated: the space charge and electric field strength distribution over the diffusion layer and the membrane. It is shown that water dissociation can be virtually completely eliminated by substituting chemically stable quaternary ammonium groups inert with respect to water dissociation in the surface layer of a heterogeneous anion-exchange membrane MA-40 for the active ternary and secondary functional amino groups. The maximum electric field strength values at the membrane/solution interface, which were found in the framework of the theory of over-limiting state, turned out to be close for all anion-exchange membranes studied, namely, (7?C9) × 10⁶ V/cm. This suggests that it is the nature of ionogenic groups in the surface layer rather than the field effect that plays the decisive role in the membrane ability to accelerate the water dissociation reaction. It is proved experimentally that in highly intense current modes of the electrodialysis process, the thermal hydrolysis of quaternary ammonium bases occurs in strongly basic MA-41 and AMH membranes by the Hofmann reaction to form ternary amino groups catalytically active in water dissociation reaction. Based on the concept on the catalytic mechanism of water dissociation, the fraction of ternary amino groups formed by thermal hydrolysis in the surface layer (the space charge region) of monopolar anion-exchange membranes MA-41 and AMH is assessed quantitatively as 0.7 and 6.5%, respectively.     

An approach to fouling characterization of an ion-exchange membrane using current-voltage relation and electrical impedance spectroscopy

Fouling phenomena of an anion-exchange membrane by bovine serum albumin (BSA) were investigated using current-voltage relation and electrical impedance spectroscopy (EIS) in this study. Electrochemical parameters of the Neosepta CMX cation- and AMX anion-exchange membrane (Tokuyama Corp., Japan) such as limiting current density (LCD), transport number, plateau length, and fraction of the conducting phase were measured. Fraction of the conducting phase of the ion-exchange membranes, calculated from the modified Sand equation, played an important role in determining the electrochemical parameters in the presence of foulants such as BSA. Fraction of the conducting phase of the AMX membrane significantly decreased in the presence of BSA. Two distinguishable slopes were observed in the over-LCD region of the current-voltage (I-V) curve, indicating the change of resistance. To further elucidate the phenomena, the electrical impedance spectroscopic study was carried out using the offset alternating current. It was found that the negatively charged loose fouling layer changed to the dense deposited BSA on the surface of the AMX membrane occurring along with enhanced water dissociation phenomena at the surface of the fouled AMX membrane at a higher current density. This result was confirmed by water dissociation experiments in a six-compartment electrodialysis cell.     

Electrochemical behavior of MF-4SK/polyaniline composites as investigated by membrane voltammetry

Membrane voltammetry was used to investigate composites consisting of perfluorinated MF-4SK membranes and polyaniline (PANI) and synthesized under various conditions. A theoretical analysis of the influence of transport-structure parameters of the ion-exchange membranes on their selectivity and limiting-current value is carried out. For MF-4SK/PANI composites, the increase in the exchange potential at the overlimiting state is found to be more than 2V compared to the original membrane. An analysis of how various factors influence the parameters of a current-voltage curve shows that the presence of anilinium and Fe³⁺ ions in the electromembrane system has no effect on the value of the potential at the onset of the overlimiting state. A phenomenological model is proposed to account for the increased plateau length at the limiting current through a change in the energetic state of water in the perfluorinated-membrane matrix resulting from the template synthesis of polyaniline.     

Diffusion layer thickness in a membrane system as determined from voltammetric and chronopotentiometric data

The thickness of the diffusion boundary layer (DBL) in solution near the surface of an ion-exchange membrane is compared at current densities (δ⁰) much less than the limiting value and at the limiting current density (δ_lim). The initial linear part of the current-voltage curve (CVC) of the membrane and the initial part of its chronopotentiogram are used to calculate δ⁰. Values of δ⁰ are found in a flow-through cell with an active membrane area of 2 × 2 cm² and 0.02 M NaCl solution for three membranes: AMX, Nafion-117, and MK-40. It is shown that δ⁰ is more than 20% less for Nafion-117 than for AMX and MK-40. Values of δ⁰ are close together for the latter two membranes and do not differ greatly from the value calculated from convective diffusion theory (the Leveque equation). In all cases, δ⁰ is significantly greater than δ_lim found from the value of the limiting current density by the method of intersecting tangents, which are drawn to the initial segment of the CVC and to the sloping plateau. The effects that determine the dependence of DBL thickness on not only hydrodynamic conditions, but also the state of the membrane surface, are discussed. The principal phenomenon responsible for the decrease in DBL thickness with increasing current is termed coupled convection, more likely, electroconvection. Among the significant properties of the surface are singled out its electrical heterogeneity and degree of hydrophobicity. The different rate of electroconvection near cation- and anion-exchange membranes is related to the Stokes radius of the counterions. The latter explains the well-known observation in the literature that the limiting current density in dilute NaCl solutions is approximately the same for cation- and anion-exchange membranes in spite of the fact that the mobility of Cl⁻ ions is approximately 1.5 times higher than that of Na⁺ ions.     

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.     

Composite sulfonated cation-exchange membranes modified with polyaniline and applied to salt solution concentration by electrodialysis

A method is developed for obtaining anisotropic composites based on the sulfonated cation-exchange MF-4SK and MK-40 membranes and the electroactive polymer polyaniline (PANI). The kinetics of aniline polymerization by successive diffusion in these membranes is investigated, and differences in the transport characteristics of the resulting MF-4SK/PANI and MK-40/PANI composites are identified. It is established from results of electroosmotic and diffusion experiments that the composite MF-4SK/PANI-1 membrane (after 1 h of aniline polymerization) suppresses electrolyte and water flow the most. Diffusion permeability drops by an order of magnitude, and water transport numbers are reduced by 50–70%. In the process of sodium chloride concentration by electrodialysis, the salt content of the concentrate increases by 50–70% with the composite MF-4SK/PANI-1 membrane compared to the base MF-4SK membrane and by 15–20% compared to the electrodialysis MK-40 membrane. Transport characteristics of the membrane pairs under investigation are calculated from the model of limiting concentration by electrodialysis: current efficiency, water transport numbers, osmotic and diffusion permeability. The dominant influence of the electroosmotic mechanism of water transport on the effect of salt solution concentration is established.