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 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.     

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.     

Rate of Generation of Ions H<Superscript>+</Superscript> and OH<Superscript>−</Superscript> at the Ion-Exchange Membrane/Dilute Solution Interface as a Function of the Current Density

Partial currents of water dissociation products through cation- and anion-exchange membranes that form thin desalination channels in electrodialyzers are measured. The investigations are performed in a broad interval of flow rates during desalination of dilute sodium chloride solutions at overlimiting currents. A water dissociation theory, which was developed for bipolar membranes, and a mass transfer theory that allows for the space charge formation at overlimiting currents are used to derive an expression, according to which the rate of generation of the H⁺ and OH^? ions is defined by the ratio of the current density to its critical value at which water starts undergoing discernible dissociation.     

Mass transport in a boundary layer and in an ion exchange membrane: Mechanism of concentration polarization and water dissociation

In an unforced flowing NaCl solution in bulk, gravitational or electro convection supplies ions from bulk toward the membrane surface through a boundary layer. In a boundary layer formed on an anion exchange membrane, the convection converts to migration and diffusion and carries an electric current. In a boundary layer formed on a cation exchange membrane, the convection converts to migration and carry an electric current. In a forced flowing solution in bulk, the boundary layer thickness is reduced and gravitation or electro convection is disappeared. An electric current is carried by diffusion and migration on the anion exchange membrane and by migration on the cation exchange membrane. Ion transport in a boundary layer on the cation exchange membrane immersed in a NaCl solution is more restricted comparing to the phenomenon on the anion exchange membrane. This is due to lower counter-ion mobility in the boundary layer and the restricted water dissociation reaction in the membrane. The water dissociation reaction is generated in an ion exchange membrane and promoted due to the increased forward reaction rate constant. However, the current efficiency for the water dissociation reaction is generally low. The intensity of the water dissociation is more suppressed in the strong acid cation exchange membrane comparing to the phenomenon in the strong base anion exchange membrane due to lower forward reaction rate constant in the cation exchange membrane. In the strong acid cation exchange membrane, the intensity of electric potential is larger than the values in the strong base anion exchange membrane. Accordingly, the stronger repulsive force is developed between ion exchange groups (SO ₃ ^? groups) and co-ions (OH^? ions) in the cation exchange membrane, and the water dissociation reaction is suppressed. In the strong base anion exchange membrane, the repulsive force between ion exchange groups (N⁺(CH₃)₃ groups) and co-ions (H⁺ ions) is relatively low, and the water dissociation reaction is not suppressed. Violent water dissociation is generated in metallic hydroxides precipitated on the desalting surface of the cation exchange membrane. This phenomenon is caused by a catalytic effect of metallic hydroxides. Such violent water dissociation does not occur on the anion exchange membrane.     

Transport properties of separating membranes MF-4SK during alkaline electrolysis of water

The transport properties of separating membranes MF-4SK are studied during electrolysis of H₂O in solutions of KOH. The effective diffusion coefficients of molecules of KOH and H₂O and the transfer coefficients of ions K⁺ and OH^? and molecules of H₂O are measured at KOH concentrations reaching 11 M, currents reaching 0.31 A cm^?2, at ambient temperature and at 80°C. In contact with a KOH solution in the concentration interval 0.1 to 11 M, the membranes that initially swelled in H₂O lose a considerable fraction of water that was present in them and the overall volume of clusters and solution-filled channels in them noticeably decreases. The coefficients of transfer by current of ions K⁺ out of anodic compartment into cathodic and the OH^? ions in the reverse direction, respectively, happen to be equal to about 0.6 and 0.4 at ambient temperature and 0.8 and 0.2 at 80°C. The coefficients of transfer of water molecules out of the anodic volume into the cathodic volume in the process of electrolysis happen to be in the limits 1.6–1.9 at ambient temperature and in the limits 2.2–2.8 at 80°C. The effective diffusion coefficients of molecules of KOH and H₂O at moderate concentrations of KOH (5.6 M) amount to ～2.6 × 10^?7 and 30 × 10^?7 cm²s^?1 at ambient temperature and ～4 × 10^?7 and 61 × 10^?7 cm²s^?1 at 80°C, respectively. At a high concentration of KOH (～10 M) these quantities substantially diminish.     

Electric mass transport through homogeneous and surface-modified heterogeneous ion-exchange membranes at a rotating membrane disk

Polarization characteristics of the homogeneous MF-4SK perfluorinated sulfonated cation-exchange membrane and the heterogeneous MK-40 sulfonic acid membrane with its surface modified by a homogeneous film of Nafion are studied at a rotating membrane disk in 0.1 and 0.001 M sodium chloride solutions. Partial current-voltage curves (CVC) are obtained for sodium and hydrogen ions, and limiting current densities in the electromembrane systems (EMS) under study are calculated as a function of the rotation rate of the membrane disk. Contribution from different mechanisms (electrodiffusion, electroconvection, dissociation of water, and the effect of the limiting-current exaltation) to the total ion flow is estimated experimentally and theoretically under conditions that the diffusion layer in the EMS has stabilized in thickness. It is established that surface modification of the heterogeneous MK-40 membrane with a 7 μm layer of a modifying agent almost completely eliminates the dissociation of water molecules, and the properties of the heterogeneous MK-40 membrane approximate those of the homogeneous Nafion membrane. From IR spectra and potentiometric titration curves of the MK-40 and MF-4SK membranes, it is shown that the acidity of the sulfonate groups in these membranes is nearly identical, but a difference in the dissociation rate of water at these membranes is determined by a different character of charge-density distribution and potential near the membrane-solution interphase boundary. By means of the theory of the overlimiting state in EMS, the internal parameters of the systems under investigation are calculated: distribution of space-charge density and electric-field potential in the diffusion layer and in the membrane. Partial CVC are calculated for H⁺ ions for the space-charge region in the phase of the MF-4SK and MK-40/Nafion ion-exchange membranes. Partial CVC with similar characteristics are compared for the heterogeneous monopolar MK-40 and the bipolar MB-2 membranes, which contain sulfonate groups. It is concluded that the membrane surface layer, where the space charge is localized, plays a dominant role in speeding up the dissociation of water in EMS.     

Discrimination between the permeation of H+ and OH− ions in precipitation membranes

The unique property of precipitation membranes, which allows determination of permeation rates of single ion species by measurement of current—voltage relationships, has been used to measure the transport rates of H⁺ and Oil ions. The permeation of OH^? was found to be approximately 4-5 times higher than that of H⁺ in both BaSO₄ and calcium oxalate precipitation 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.     

Active and selective transport of halogen ions through poly (1-vinylimidazole-co-styrene) membranes

A membrane made of poly(1-vinylimidazole-co-styrene) is able to actively and selectively transport halogen ions. Halogen ions were transported not only by an antiport mechanism with OH^? transfer, but also by a symport mechanism with H⁺ transfer. This variation in transport mechanism can be explained by a polarity change of the membrane. The transport of halogen ions is represented by flux equations, and the selectivity predicted theoretically is compared to the experimental value.     

Morphology and microtopology of cation-exchange polymers and the origin of the overlimiting current

In electrodialysis desalination processes, the operating current density is limited by concentration polarization. In contrast to other membrane processes such as ultrafiltration, in electrodialysis, current transport above the limiting current is possible. In this work, the origin of the overlimiting current at cation-exchange polymers is investigated. We show that, under certain experimental conditions, electroconvection is the origin of the overlimiting conductance. The theory concerning electroconvection predicts a shortening of the plateau length of membranes with increased conductive or geometrical heterogeneity. We investigate the influence of these two parameters and show that the creation of line undulations on the membrane surface normal to the flow direction, having distances in the range of approximately 50-200% of the boundary-layer thickness, lead to an earlier onset of the overlimiting current. The plateau length of the undulated membranes is reduced by up to 60% compared to that of a flat membrane. These results verify the existence of electroconvection as a mechanism destabilizing the laminar boundary layer at the liquid-membrane interface and causing ionic transport above the limiting current density.     

Precipitation membranes — A review

Precipitation membranes are generated exclusively by diffusion-controlled precipitation. They form an asymmetrical barrier with an electrical polarity. Their electrical charges, which are charges of adsorbed ions, are not fixed and can easily and reversibly be desorbed. In the conditioned state a relatively large potential difference develops, the membranes rectify electrical current and display specific ion permeabilities. A unique property of these membranes allows determination of the permeability of single ionic species. The permeability of OH^? was thus found to be markedly greater than that of H⁺, Precipitation membrane effects may be involved in rectification phenomena in biological membranes such as the frog skin.     

Influence of the nature of membrane ionogenic groups on water dissociation and electrolyte ion transport: A rotating membrane disk study

Polarization properties of electromembrane systems (EMS) consisting of a heterogeneous membrane, either the MK-41 phosphonic acid membrane or the MK-40 sulfonic acid membrane, and dilute sodium chloride solutions are investigated with the rotating membrane disk method. For the MK-41/0.01 M NaCl and MK-41/0.001 M NaCl EMS, effective ion transport numbers and partial current-voltage curves (CVC) are measured for sodium and hydrogen ions, and limiting-current densities and the diffusion-layer thickness are calculated as functions of the rotation rate of the membrane disk. With the theory of the overlimiting state of EMS, internal parameters of the systems under investigation—the diffusion-layer thickness, the space-charge distribution, and electric-field strengths in the diffusion layer and in the membrane—are calculated from experimentally obtained CVC and the dependence of effective transport numbers on current density. The catalytic influence of ionogenic groups on the dissociation rate of water is analyzed quantitatively. Partial CVC for H⁺ ions are calculated for the space-charge region in MK-40 and MK-41 membranes. Analogous CVC for bipolar membranes containing sulfonic acid and phosphonic acid groups are compared. The dissociation mechanism of water is the same in all EMS and is independent of the membrane type and the nature of the functional groups.     

Temperature dependence of DC electrical conductivity of kaolin

The samples from kaolin Sedlec were investigated by the help of DTA, TG, and temperature dependences of DC conductivity using Pt wire electrodes and linear heating up to 1,050 °C. After drying, the samples contained ~1.5 mass% of the physically bound water. DTA and TG reflected generally known facts about a release of the physically bound water, dehydroxylation, and metakaolinite → Si–Al spinel transformation. The results of electrical measurements showed the electric current passed over the maximum at 60 °C. The self-ionization of water results in the process H₂O → H⁺ + OH^? in the water layers on the crystal surfaces; consequently, OH^? and H⁺ are the main charge carriers in the low-temperature region. The water molecules simultaneously evaporate from the sample which decreases the number of the charge carriers. When the physically bound water evaporates, the current is carried mostly by K⁺ and Na⁺ ions. During dehydroxylation, the hydroxyls OH^? split into H⁺ and O^2?. The ions H⁺ jump to the neighboring OH^? groups creating the water molecules. The ions O^2?remain bounded to the newly created metakaolinite lattice. Therefore, mobile protons contribute to the electric current. At the same time, this contribution gradually decreases because of the escape of H₂O from the sample. The sharp current peak and DTA peak at 970 °C imply relatively fast metakaolinite → Si–Al spinel transformation. This DC current peak results from the shift of Al³⁺ and O^2? ions into new positions.     

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.     

Bactericidal effect of an electrodialysis system on E. coli cells

Using a new electrodialysis system with both cation- and anion-exchange membranes, the bactericidal effect on Escherichia coli has been investigated in detail from the standpoint of electrochemistry. Various electrolyte solutions containing E. coli (10⁸ cells/cm³) were passed through a desalting chamber at a flow rate of 3 cm³/min under varying current densities, and the viability of the cell (%) and the pH changes in the effluents were measured. When a 0.1 M NaCl aqueous suspension was used, a disinfection effect emerged in the vicinity of the limiting current density (LCD 0.81 A/dm²) and increased with an increase in the current density. The pH value of the suspensions decreased owing to the dissociation of water to H⁺ and OH⁻ ions by the well-known “neutrality disturbance phenomenon” in the region beyond the LCD. These tendencies were also observed when other electrolyte suspensions were used. Concerning the effect of the various species on the disinfection of E. coli cells, ionic systems in which a LCD was easily attained were found to have a strong effect.The germicidal effect may be due to a synergistic effect of acidic H⁺ and basic OH⁻ ions which are produced on the anion-exchange membrane and cation-exchange membrane, respectively, of the desalting chamber.     

Experimental and theoretical investigations of steady and transient states in systems of ion exchange bipolar membranes

The electrochemical characteristics of one commercial bipolar ion exchange membrane and of two home-made bipolar membranes are investigated over a range of current densities up to 2 kA m⁻². Studies are performed using galvano-potentiometry (i/V) and impedance spectrometry methods. The temperature dependence of i/V curves enables the determination of the activation energies related to the overall electrochemical process of H⁺ and OH⁻ production by water dissociation at the membrane junction. The physical analysis of the experimental data is made on the basis of a neutral layer model for the membrane junction. The theoretical treatment leads first to establish a thermodynamic framework insuring the validity of the criteria used in the interpretation of the results in terms of the model. Application of current electrochemical kinetic concepts at steady state involves the idea that, in the presence of an efficient catalyst, a quasi-reversible state of the water dissociation reaction may be achieved at the junction. A theoretical approach is developed for treating the data obtained with transient measurements in absence of co-ion transport. This study reveals the intrinsic roles played in the overall process of respectively: (a) the H⁺ and OH⁻ ion transport; (b) the electrical double layers at the membrane junction boundaries; and (c) the chemical mechanism of water dissociation.     

Kinetic studies on H+- and OH−-catalysed aquation of bis-aspartatochromium(III)

A potential synthetic biochromium source, bis-aspartatochromium(III) ion (where Asp is a tridentate N,O,O′-ligand, bonded via amine nitrogen and carboxylate oxygen atoms) has been obtained and characterized in aqueous solution. Kinetics of partial dechelation of the complex catalysed by H⁺ and OH^? ions has been studied spectrophotometrically within 0.1–1.0 M HClO₄ and 0.1–1.0 M NaOH ranges under first-order conditions. A linear dependence of the k _obs,H on [H⁺] and independence of the k _obs,OH on [OH^?] were established. The derived rate expression and identification of components of the reaction mixture provide evidence for a reaction mechanism, where the key role in the overall process is the formation of an intermediate species with bidentate N,O-bonded Asp via both spontaneous and H⁺(OH^?)-catalysed reaction paths. The intermediate is meta-stable and at pH 5–7 restores the substrate.     

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.     

Vibrational fine structure in the collisionally induced charge reversal and fragmentation of OH− ions

OH⁺ or OD⁺ ions were generated by collisionally induced charge reversal of the corresponding anions and then submitted to collisionally induced dissociation to H⁺ and D⁺. The resulting ion kinetic energy spectra showed multiple vibrational fine structure which were assigned to predissociations from the a ¹ Δ state. Unimolecular dissociation of OH⁺ and OD⁺ to yield O⁺ gave a single-valued kinetic energy release for predissociation of the ¹£ state.