Electric mass transfer of sodium chloride through cation-exchange membrane MK-40: A rotating membrane disk study

An experimental device consisting of a rotating membrane disk with horizontally positioned cation-exchange membrane MK-40 is described. The device’s design makes it possible to simultaneously obtain current-voltage curves (CVC) and dependences of effective transport numbers for ions of electrolyte and water dissociation products on the current density. Partial CVC are calculated and limiting current densities and diffusion layer thickness are determined at various disk rotation rates. At current densities below the limiting value, the disk’s CVC obey regularities of electrodiffusion kinetics. Upon raising the current density further, the salt ion fluxes increase due to a decrease in the effective diffusion layer thickness, which is caused by the emergence in the near-membrane region of a space charge and electroconvection. At high current densities there occur oscillations of the potential jump that are caused by hydrodynamic instability of the near-membrane solution layer.     

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.     

Mathematical model for the overlimiting state of an ion-exchange membrane system

A three-layered mathematical model is proposed for describing the overlimiting state in an ion-exchange membrane system. The model’s prominent feature is the allowance for the space-charge region; the water dissociation reaction, which is catalyzed by active ionogenic groups; and the coupled gravitational and electroosmotic convection, which leads to the emergence of dependence of the effective diffusion layer thickness on the electric current density. The model is used for calculating, on the basis of known initial current-voltage curves and dependences of effective transport numbers on the current density, such internal characteristics of the system as the diffusion layer thickness, distribution of concentration of ions, space charge, and electric-field strength at various current densities.     

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.     

Limiting current density and water dissociation in bipolar membranes

The behaviour of bipolar membranes in NaCl and Na₂SO₄ solutions is discussed. The membranes are characterized in terms of their limiting current densities. Below the limiting current density the electric current is carried by salt ions migrating from the transition region between the anion and the cation exchange layer of the bipolar membrane. In steady state these ions are replaced by salt ions transported from the bulk solutions into the transition region by diffusion and migration due to the fact that the ion-exchange layers are not strictly permselective. When the limiting current density is exceeded, the salt transport from the transition region can no longer be compensated by the transport into the region and a drastic increase in the membrane resistance and enhanced water dissociation is observed. This water dissociation is described as being a combination of the second Wien effect and the protonation and deprotonation of functional groups in the membrane. The limiting current density is calculated from a mass balance that includes all components involved in the transport. The parameters used in the mathematical treatment are the diffusion coefficients of salt ions and water, the ion mobilities in the membrane, the fixed charge densitiy of the membrane, the pK_b values of the functional groups and the solution bulk concentrations.     

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.     

Electro-osmotic pumping of sodium chloride solutions

Electro-osmotic pumping (EOP) theory and its characteristics (transport numbers, brine concentration, current density, current efficiency, electro-osmotic coefficients, etc.) of Selemion AMV and CMV ion-exchange membranes were studied. The brine concentration increased with increase in current density and feed water concentration. Current efficiency was nearly constant in a wide range of current densities and feed water concentrations. The water flow through the membranes also increased with increasing current density and feed water concentration. The increase in water flow increased the current efficiency significantly. Consequently, water flow through electrodialysis (ED) membranes had a positive effect on ED. Electro-osmotic coefficients decreased with increasing feed water concentration. Osmotic flow in EOP-ED decreased relative to the total flow with increasing current density while the electro-osmotic flow increased relative to the osmotic flow. Osmotic flow significantly contributes to the total water flow in EOP. Selemion AMV and CMV membranes performed well for salt concentration. A simple membrane potential measurement has been demonstrated to function reasonably satisfactorily to predict membrane performance for salt concentration.     

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.     

Electric mass transport of sodium chloride through cation-exchange membrane MK-40 in dilute sodium chloride solutions: A rotating membrane disk study

The polarization properties of an electromembrane system consisting of an MK-40 membrane and a dilute sodium chloride solution are investigated with an experimental apparatus, which includes a rotating membrane disk with a horizontally positioned membrane. For the electrochemical systems of MK-40/0.01 M NaCl and MK-40/0.001 M NaCl, effective ion transport numbers and partial current-voltage curves are determined 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. The space-charge distribution in the diffusion layer and in the membrane is calculated for various current densities and rotation rates of the membrane. It is shown that when electric-current densities are greater than the limiting value, ion fluxes of the salt increase as a result of a decrease in the effective thickness of the diffusion layer. This decrease is caused by the development of space charge, electroconvection, water dissociation, and the exaltation effect in the region near the membrane. It has been established that in dilute solutions the limiting current is not purely electrodiffusive in nature.     

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.     

Salt splitting with radiation grafted PVDF anion-exchange membrane

A radiation grafted poly(vinylidene fluoride) anion-exchange membrane has been formulated and its behaviour is analysed through the splitting of sodium sulphate by electrohydrolysis. Experiments carried out in a two-compartment membrane electrolysis cell, investigated the influence of flow rate, current density and salt concentration on the performance of the membrane. The different flow conditions had a small influence on current efficiencies, while productivity was significantly greater at higher current densities.The new PVDF material gave acceptable selectivity, low electrical resistance and good chemical, thermal and mechanical stability. A comparison with experiments using cation-exchange membranes demonstrated an inferior performance of the anion-exchange PVDF membrane, in terms of current efficiency and transport properties, despite the lower energy requirements.     

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.     

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.     

Electrically modulated membrane permeability

Double tracer flux experiments using the neutral solutes ³H₂O and ¹⁴C-sucrose have demonstrated that an applied electric field can change the permeability of a charged membrane supporting a gradient in pH or salt concentration. The field selectively changed the membrane's permeability to ¹⁴C-sucrose, while permeability to ³H₂O was minimally affected. Membrane ultrastructure and permeability were shown to depend on the membrane's fixed charge density and intramembrane salt content. The applied field, in turn, served to change the pH or salt concentration inside the membrane by means of an electrodiffusion process. This modulated electrostatic repulsion forces between membrane molecules and fibrils, thereby changing the interstitial separation distances which determine the effective pore size of the membrane. The proposed mechanism was tested using a theoretical model for membrane transport along with independent measurements of field-related transport properties. Our results suggest that the electric field acts as a switch to control the swelling state and the resulting pore size of the membrane.     

Barrier effect and coupled transport in the electrodialysis of metal complexonate solutions

The electrodialysis of an aqueous solution of an alkaline earth complex with ethylenediaminetetraacetic acid (EDTA) was studied in a wide range of current densities. The curve of the complexonate flow across an anionite membrane versus current density has three characteristic sections. The first section corresponds to a linear increase in the flow as a function of current density, the second to a decrease in the flow and decomposition of the complex (barrier effect), and the third to an increase in the complexonate flow due to the transport coupled with the flow of hydroxyl ions formed in the dissociation of water molecules at the interface of the solution and the anionite membrane. Conditions for complete separation of the singly and doubly charged cations were found.     

Model verification of limiting concentration by electrodialysis of an electrolyte solution

The concentration of LiCl in brine and brine volume are obtained as functions of current density by the method of limiting concentration by electrodialysis. These relationships are used for model calculations of current efficiency, the diffusion, osmotic, and electroosmotic permeability of an MK-40/MA-40 membrane pair, and also salt hydration numbers. These theoretical values of water transport numbers and LiCl hydration numbers are compared with corresponding experimental and literature data. It is shown that the model adequately describes the phenomena of the mass electrotransport occurring in electrodialyzers with noncirculating concentration compartments, and it can be successfully applied in calculating the technological parameters of the process, finding the transport properties of ion-exchange membranes, and determining salt hydration numbers in aqueous electrolyte solutions.     

Charge-tranfer reactions of oxygen ions and titanium ions at titanium oxide films

The partial current densities for the transfer of titanium(IV) and oxygen ions, and of electrons at the interface between the electrolyte and the titanium(IV) oxide layer on titanium were measured as functions of total current density and pH value. It is shown theoretically, how conclusions regarding the mechanisms of the transfer reactions of both ions can be drawn from various relations between the ionic partial current densities and their dependence on solution composition, even if the electric potential difference at the oxide interface cannot be measured. Mechanisms for the transfer reactions of titanium(IV) and oxygen ions are derived from the experiments.     

Nature of water transport and electro-osmosis in nafion: insights from first-principles molecular dynamics simulations under an electric field

The effects of water content on water transport and electro-osmosis in a representative polymer electrolyte membrane, Nafion, are investigated in detail by means of first-principles molecular dynamics (MD) simulations in the presence of a homogeneous electric field. We have directly evaluated electro-osmotic drag coefficients (the number of water molecules cotransported with proton conduction) from the trajectories of the first-principles MD simulations and also explicitly evaluated factors that contribute to the electro-osmotic drag coefficients. In agreement with previously reported experiments, our calculations show virtually constant values ( approximately 1) of the electro-osmotic drag coefficients for both low and high water content states. Detailed comparisons of each factor contributing to the drag coefficient reveal that an increase in water content increases the occurrence of the Grotthuss-like effective proton transport process, whose contribution results in a decrease in the electro-osmotic drag coefficient. At the same time, an environment that is favorable for the Grotthuss-like effective proton transport process is also favorable for the transport of water arising from water transport occurring beyond the hydration shell around the protons, whose contribution results in an increase in the electro-osmotic drag coefficient. Conversely, an environment that is not favorable for proton conduction is also not favorable for water transport. As a result, the electro-osmotic drag coefficient shows virtually identical values with respect to change in the water content.