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.     

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.     

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 chemical nature of the ionogenic groups of ion-exchange membranes on the size of the electroconvective instability region in high-current modes

The size of the electroconvective instability region on the membrane-solution boundary at currents exceeding the limiting diffusion current was measured by laser interferometry. The influence of the chemical nature of the ionogenic groups of ion-exchange membranes on the development of electroconvective instability was studied. The thickness of the electroconvection region decreased as the catalytic activity of the ionogenic groups of commercial and pilot membrane samples with respect to the heterolytic water dissociation increased. The maximum size of the electroconvective instability region and the minimum currents at which it was recorded for the anion-exchange membranes under study were determined for the highly basic modified anion-exchange membrane MA-41M with an almost completely suppressed water dissociation function. A correlation was found between the size of the convective instability region and the characteristic points on the current-voltage curves.     

Transfer of Amino Acids through a Membrane/Solution Interface in the Presence of Heterogeneous Chemical Protonation Reaction

A physicochemical model for the transfer of amino acids through the interface between a cation-exchange membrane and an amino acid aqueous solution is proposed. The model allows for a heterogeneous reaction of protonation of the amino acid zwitterion. The model calculations are compared with experimentally determined migration transport numbers for glycine cations in an electromembrane system with an MK-40 membrane. The comparison shows that, owing to the protonation reaction, the glycine flux can increase by ca. 30%.     

Potentiometric determination of lysine in aqueous solutions using MF-4SK modified perfluorinated membranes

A selective potentiometric sensor was developed on the basis of MF-4SK modified perfluorinated sulfonic cation-exchange membranes for determining lysine monohydrochloride in mixed aqueous solutions of neutral amino acids. It was shown that the treatment of MF-4SK membranes in ethylene glycol increased the sensitivity of the sensor. The use of MF-4SK perfluorinated sulfonic cation-exchange membranes for determining lysine in aqueous solutions is based on a protolytic reaction; as a result of this reaction, single-charged lysine ions from solution are transferred into doubly charged ions in the membrane phase. The Donnan potential at an individual boundary between the studied solution and the membrane is the sensor response. The response time was 10–15 min. The concentration constants of the sensor selectivity to lysine in the presence of glycine, alanine, and leucine did not exceed 0.019. The relative error of determining lysine monohydrochloride in the studied solutions of neutral amino acids was 2–5%.     

Limiting Current Densities in a System with Cation-Exchange Membrane MK-100 Rotating in a Glycine–HCl Solution

Limiting diffusion current densities in an electromembrane system (EMS) comprising cation-exchange membrane MK-100 and a glycine solution containing hydrochloric acid are studied on a rotating membrane disk. The dependence of the limiting current density at a given rotation rate of the disk on the equilibrium volume concentrations of glycine cations and protons in solution is described by a regression model. The diffusion fluxes of these cations in the EMS are shown to be independent.     

Concentration Polarization and Specific Selectivity of Membranes in Pulse Mode

Nonstationary concentration polarization of membranes was analyzed theoretically. Desalination and transport of single- and double-charged ions through MK-40 cation-exchange membrane was studied in the stationary and pulse modes with the fixed voltage. Main generalities in the changes of the pH and conductivity of desalinated solution as well as concentrations of single- and double-charged ions as the functions of solution flow velocity, voltage, frequency and off-duty ratio of pulses were established.     

Effect of Parameters of Pulsed Electric Field on Average Current Density through Nafion 438 Membrane in Electrodialysis Cell

Pulsed electric fields (PEF) have been used to advantage in electroplating for more than half a century. Recently, interest has increased in the application of these electric modes in the membrane processes. In this work, the effect of parameters (potential, frequency of current, and duty cycle) of pulsed current mode on the average current density in the electrodialysis cell is studied. For this purpose, the plots of current vs. time were measured in the mode of pulsed potential drop on a Nafion 438 (Nafion^TM N438) cation-exchange membrane. It is shown that the largest gain in the current density of 33% can be attained by varying parameters of PEF.     

Cation-exchange reaction with accompanying current discharge

When a cation-exchange membrane swollen with propylene carbonate was placed in tight contact with lithium foil, an ion-exchange reaction of lithium ions with the cation-exchange membrane occurred. The assembly exhibited properties of a lithium cell (3.0-3.2 V without load). The ion-exchange capacity of various cation-exchange membranes and their degree of swelling with propylene carbonte were related to the capacity of the formed lithium cell. A discharge current of 0.32 mA was observed through a 3 cm² cell. Analysis of lithium ions in the membrane phase after discharge revealed that the ion-exchange reaction of lithium ions with the cation-exchange groups of the membrane was directly related to the current discharge of the cell. However, the formed lithium cell showed high self-discharge.     

Electroconvection in systems with heterogeneous ion-exchange membranes

The results of studying the surface morphology of heterogeneous cation-(MK-40) and anion-exchange (MA-40) membranes and calculating the structure of electroconvective vortices generated by the electric body force are shown. The body force and its distribution are estimated by taking into account real parameters of the membrane surface morphology. The calculations of vortices were carried out by solving the Navier-Stokes equation with the no-slip boundary condition and the preset body force distribution. It is shown that the body force induced by the flowing current can generate pairs of electroconvective vortices (electroosmosis of the second kind), where the size of induced vortices is comparable with the intermembrane gap in electrodialysis cells.     

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.     

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 Composition of the MK-40 Cation-Exchange Membrane in Amino Acid Solutions: A Conductimetric Study

The ionic composition of a cation-exchange membrane in solutions of amino acids of three types is studied by measuring its conductivity. The concentration constants of equilibrium of the protonation of zwitterions of these acids in the membrane are estimated. The membrane's ionic composition substantially depends on the type of amino acid.     

Electrodialysis for clean-up of strongly alkaline samples in ion chromatography

An electrodialysis method is described for the off-line neutralization of strongly alkaline samples containing trace levels of common inorganic anions. This method uses an electrodialysis cell comprising three compartments separated from each other by cation-exchange membranes. These compartments comprise an anode compartment housing, a platinum wire anode and 10 ml of a suitable hydrogen ion donating medium, a sample compartment which contains 1 ml of the alkaline sample, and a cathode compartment housing a platinum wire cathode and a dilute solution of sodium hydroxide. During electrodialysis at either constant applied current or constant applied power, hydrogen ions from the anode compartment displace sodium ions from the sample, thereby effecting neutralization.

Experimental parameters, such as the magnitude of the applied current or power, the type of cation-exchange membrane used and the design of the cell have been studied and optimum results were obtained using a Neosepta CM-2 membrane, of area 616 mm² supported between two perspex discs, with an applied current of 150 mA or applied power of 3 W. Under these conditions, a 1 ml sample of 1 M sodium hydroxide could be neutralized in 11 min. The most effective hydrogen ion donating medium consisted of a 2:1 (w/v) slurry of BioRad AG 50W-X8 (200–400 mesh, H⁺ form) cation-exchange resin in 1 mM octanesulfonic acid. Recoveries of solute anions (3–10 μg/ml) from the dialysed solution were close to quantitative, except for fluoride and nitrite, which gave recoveries of less than 60%. It is suggested that low recoveries for these ions are due to formation of neutral, protonated species within the membrane with subsequent loss by diffusion.     

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.     

Multistep mineral fouling growth on a cation-exchange membrane ruled by gradual sieving effects of magnesium and carbonate ions and its delay by pulsed modes of electrodialysis

The aim of this study was to reveal the mechanisms ruling a fouling growth on both sides of a CMX-SB cation-exchange membrane (CEM), run after run during three consecutive electrodialysis (ED) treatments. A model solution containing a high magnesium/calcium ratio (2/5) was demineralized under two different pulsed electric field (PEF) on-duty ratios and dc current. The results showed a series of mechanisms ruling a multilayer mineral fouling growth and its delay by PEFs. The nature of the fouling layer, during a first run, depended on the diluate pH-value evolutions and the ion migration rates through the membrane. A subsequent multilayer fouling growth during consecutive treatments was ruled by the already formed mineral layers, where gradual sieving effects inverted the migration rates and led to a multistep crystal growth. Calcium carbonate grew on the diluate side of CEM, starting from its amorphous phase to then crystallize in a coexisting presence of aragonite and calcite. Amorphous magnesium hydroxide appeared on CEM apparently through fouling dehydration ruled by the mineral layers themselves and by overlimiting current regimes. A delayed fouling growth was observed for PEF ratio 0.3. A long pause lapse during pulse modes was demonstrated as an important parameter for fouling mitigation.     

Three-compartment bipolar membrane electrodialysis for splitting of sodium formate into formic acid and sodium hydroxide: Role of diffusion of molecular acid

Bipolar membrane electrodialysis is used in a three-compartment configuration to regenerate formic acid and sodium hydroxide from sodium formate. A previous study showed that the diffusion of molecular formic acid induces the loss of acid current efficiency. The present study shows the following results: the diffusion of molecular formic acid through the bipolar membrane explains quantitatively the presence of sodium formate in the sodium hydroxide solution. The loss of acid current efficiency is only due to diffusion of molecular acid through both anion-exchange and bipolar membranes. The sodium hydroxide current efficiency is determined by acid diffusion through the bipolar membrane and OH⁻ leakage through the cation-exchange membrane. The flux of acid diffusion through the membranes is proportional to acid concentration and depends on sodium hydroxide concentration for bipolar membrane and on sodium formate concentration for anion-exchange membrane. The flux rates vary with temperature.     

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.     

Effect of an ion-exchange filler on the deionized water quality in electrodialysis

Desalination channels, containing an inert separator and a monolayer of ionites AV-17 and KU-2 taken in various volume ratios, are studied while maintaining concentrations of all solution components invariant. It is shown that the composition of the ion-exchange filler of the desalination channel affects the rate of transport of salt ions through relevant membranes, pH, and specific resistance of desalinated solution. The behavior of membrane systems in an overlimiting state is explained by using notions about different mechanisms governing the so-called overlimiting current through anion-exchange and cation-exchange membranes