Overlimiting current electrodialysis possibilities • Understanding of the origin of electroconvec... more Overlimiting current electrodialysis possibilities • Understanding of the origin of electroconvection • How current-induced convection enhances mass transfer • Water splitting and its effects on electroconvection • A way to enhance water desalination process a b s t r a c t For about 60 years, it is known that "limiting current" density does not really limit the current density in electrodialysis (ED). The use of intensive currents allows one to decrease the area of expensive ion-exchange membranes, hence to obtain more compact ED plants with lower investment costs. However, in practice overlimiting currents are applied solely in electrodeionization, and not in conventional ED devices. In literature (H. Strathmann [1]), overlimiting current density operation ED is considered as being under ongoing research and development. Where is the research today? What is the actual understanding of overlimiting mass transfer processes in membrane systems? As far as electroconvection is involved as a major mechanism of overlimiting transfer, what could bring specific knowledge and practice acquired in nano-and microfluidics into desalination? Can overlimiting ED be economically advantageous? Is there an alternative for classical ED devices applied? This review is intended to give some responses to these questions.
A method to forecast behavior of electrodialyzers with long demineralization channels is suggeste... more A method to forecast behavior of electrodialyzers with long demineralization channels is suggested. It is based on the data obtained using a laboratory cell or an apparatus with short demineralization channels of similar construction. The results are presented as dimensionless parameters.
A modified static method for investigation of Co,-, HCO; and OH-ions sorption by anion-exchange m... more A modified static method for investigation of Co,-, HCO; and OH-ions sorption by anion-exchange membranes was developed. The ion-exchange equilibrium coefficient ( 1 I?$ in the system of the Russian commercial anionexchange membrane MA-41 in sodium carbonate/hydrocarbonate iolutions within pH range from 8.3 to 10.8 is found. This value is calculated by two methods: taking into account or not the hydrolysis reactions of carbonate in the 2equilibrium solution and the sorption of OH-ions by the membrane. The ion exchange coefficient value KzEi_ is 9. 3 Keyworak Anion-exchange membrane; Carbonate/hydrocarbonate solutions; Ion-exchange equilibrium
Electromembrane methods of amino acids purification and separation are very attractive. They do n... more Electromembrane methods of amino acids purification and separation are very attractive. They do not need any additional chemicals, minimize the desired product loss and are energy efficient and manufacturable. Detailed study of amino acid transfer mechanisms inside membranes and at their surfaces provides a basis to improve these methods. For these purposes, concentration dependencies of cation-exchange (CMX, MK-40) and anion-exchange (AMX, MA-41) membranes electrical conductivity in 0.025-1 M LysHCl and NaCl solutions as well as current-voltage curves (CVC) and chronopotentiograms (ChP) of CMX membrane in 0.01 M solutions are obtained.
Transport of salts of phosphoric and carbonic acids through anion exchange membranes (AEM) is stu... more Transport of salts of phosphoric and carbonic acids through anion exchange membranes (AEM) is studied by means of current-voltage responses. Concentration polarisation of such membrane systems is accompanied by a change in the electrolyte concentration at the membrane-solution interface due to hydrolysis reactions. Current-voltage characteristics (CVC) on HPO 4 2− or PO 4 3− ion-containing systems with modified AEM selectively permeable to mono-charged anions present two waves like in electrode multi-component systems with multi-stage electrochemical reactions. This phenomenon does not occur with nonmodified AEMs. It is also the case with all the AEMs studied for solutions containing HCO 3 − or CO 3 2− salts because of close values of electrical resistances of the membranes in these both ionic forms.
Please cite this article as: Galama et al., On the origin of the membrane potential arising acros... more Please cite this article as: Galama et al., On the origin of the membrane potential arising across densely charged ion exchange membranes: How well does the Teorell-Meyer-Sievers theory work?, HIGHLIGHTS The origin of the membrane potential across ion exchange membranes (IEMs) is discussed. At large concentration differences, Teorell-Meyer-Sievers (TMS) theory deviates from the data. No single assumption in the TMS theory can by itself account for the deviation. For highly charged IEMs the membrane potential can go through a maximum Presence of a maximum in the potential depends on the concentration ratio
Three ion exchange membranes (an AMX homogeneous anion exchange membrane, a MK 40 heterogeneous c... more Three ion exchange membranes (an AMX homogeneous anion exchange membrane, a MK 40 heterogeneous cation exchange membrane, and a Nafion 117 homogeneous cation exchange membrane) have been studied by electrochemical impedance spectroscopy. Processing of the experimental impedance spectra according to the model developed previously has made it possible to find the Nernst diffusion bound ary layer (DBL) thickness δ as a function of current density. The behavior of the AMX membrane has been shown to be close to the "ideal" one described by the model: the impedance spectrum of the membrane is close to the theoretical spectrum and the value of δ is only slightly smaller than the quantity δ Lev calculated by the Leveque equation derived in terms of classical convective diffusion theory. The behavior of the MK 40 and Nafion membranes markedly differs from the "ideal" behavior: the reactive component of the impedance in the region of medium frequencies corresponding to the maximum point in the low frequency range of a Warburg type finite length impedance spectrum is significantly lower than its theoretically predicted value. The value of δ is less than δ Lev even for underlimiting currents, and the deviation increases with the increasing current density. This specific behavior of the membranes correlate well with the voltammetry data. The behavior of the studied membranes is associated with the surface properties: the heterogeneity (case of MK 40) and, especially, high hydrophobicity of the (Nafion 117) surface facilitate the development of elec troconvection. Homogeneity and high hydrophilicity of the surface of the AMX membrane determine its behavior, which is close to the ideal. * The contact angles were measured by K. A. Shevtsova and A. V. Nebavskii on the water swollen in membrane 30 s after applying a drop of distilled water. The measurement procedure is described in .
It is known that the behavior of a membrane with homogeneous and heterogeneous surface under the ... more It is known that the behavior of a membrane with homogeneous and heterogeneous surface under the same current density is different. In order to find out this difference, a twodimensional non-stationary model of mass transfer through electrically inhomogeneous surface of ion-exchange membrane under direct current is proposed. In this paper, the general formulation of the boundary conditions at the conductive sections has the form of the integral function of the local current density. The problem was solved by applying an electric current stream function, which is quite similar to the stream function in hydrodynamics. For the first time, it is proved that the distribution of current lines over the conductive sections of the membrane is not uniform. It is shown experimentally and theoretically that the shape of chronopotentiograms and electrochemical impedance spectra changes with changing conductive portion of the ion-exchange membrane surface.
A relatively simple mathematical model based on the Poisson equation is considered. The model is ... more A relatively simple mathematical model based on the Poisson equation is considered. The model is intended for modeling transport through multilayered ion-exchange membranes operating at overlimiting currents. The boundary-value problem is solved by a numerical method of parallel shooting and by an approximate method based on the assumption that the charge density is distributed quasi-uniformly. Concentration profiles in diffusion layers and membranes, current–voltage curves, and dependences of effective transport numbers on the current density are examined.
ABSTRACT Kinetics of surface water desalination by neutralization dialysis (ND) was studied in a ... more ABSTRACT Kinetics of surface water desalination by neutralization dialysis (ND) was studied in a three-compartment stack with ion-exchange membranes, and the process feasibility was evaluated. The pH and the ionic conductivity of a model saline solution (0.02 M NaCl) were measured during different ND operations in batch mode. The influence of the flow rate, the solution concentration in the alkali and acid compartments was investigated. It was found that, at the beginning of the operation, the ion-exchange kinetics was mainly controlled by the membrane diffusion. The maximal fluxes of exchanging ions (Jmax) (Na+ and H+ across CMX membrane, and Cl− and OH− across AMX membrane) were theoretically evaluated. The calculated and the experimentally measured fluxes at the beginning of batch operation were close. However, after some minutes of solution desalination, the salt concentration decreased, which involved diffusion boundary layer controlled kinetics. Hence, with time, the fluxes decreased and were considerably smaller than Jmax. The increase/decrease in the acid and alkali solution concentration resulted in corresponding increase/decrease in the ion-exchange rate. Recommendations have been provided to reduce the desalination duration.
Three ion exchange membranes (an AMX homogeneous anion exchange membrane, a MK 40 heterogeneous c... more Three ion exchange membranes (an AMX homogeneous anion exchange membrane, a MK 40 heterogeneous cation exchange membrane, and a Nafion 117 homogeneous cation exchange membrane) have been studied by electrochemical impedance spectroscopy. Processing of the experimental impedance spectra according to the model developed previously has made it possible to find the Nernst diffusion bound ary layer (DBL) thickness δ as a function of current density. The behavior of the AMX membrane has been shown to be close to the "ideal" one described by the model: the impedance spectrum of the membrane is close to the theoretical spectrum and the value of δ is only slightly smaller than the quantity δ Lev calculated by the Leveque equation derived in terms of classical convective diffusion theory. The behavior of the MK 40 and Nafion membranes markedly differs from the "ideal" behavior: the reactive component of the impedance in the region of medium frequencies corresponding to the maximum point in the low frequency range of a Warburg type finite length impedance spectrum is significantly lower than its theoretically predicted value. The value of δ is less than δ Lev even for underlimiting currents, and the deviation increases with the increasing current density. This specific behavior of the membranes correlate well with the voltammetry data. The behavior of the studied membranes is associated with the surface properties: the heterogeneity (case of MK 40) and, especially, high hydrophobicity of the (Nafion 117) surface facilitate the development of elec troconvection. Homogeneity and high hydrophilicity of the surface of the AMX membrane determine its behavior, which is close to the ideal. * The contact angles were measured by K. A. Shevtsova and A. V. Nebavskii on the water swollen in membrane 30 s after applying a drop of distilled water. The measurement procedure is described in .
Partial currents of water dissociation products through cation-and anion-exchange membranes that ... more 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 OHions is defined by the ratio of the current density to its critical value at which water starts undergoing discernible dissociation.
Several laws governing ampholyte transport through ion-exchange membranes are established by a co... more Several laws governing ampholyte transport through ion-exchange membranes are established by a comparative analysis of the concentration dependence of electrical conductivity for homogeneous (CMX, AMX) and heterogeneous (MK-40, MA-41) membranes in NaCl, LysHCl, and NaH2PO4 solutions. The increase in the electrical conductivity of membranes in ampholyte solutions as the solutions become more dilute is explained by the increased fraction of
Overlimiting current electrodialysis possibilities • Understanding of the origin of electroconvec... more Overlimiting current electrodialysis possibilities • Understanding of the origin of electroconvection • How current-induced convection enhances mass transfer • Water splitting and its effects on electroconvection • A way to enhance water desalination process a b s t r a c t For about 60 years, it is known that "limiting current" density does not really limit the current density in electrodialysis (ED). The use of intensive currents allows one to decrease the area of expensive ion-exchange membranes, hence to obtain more compact ED plants with lower investment costs. However, in practice overlimiting currents are applied solely in electrodeionization, and not in conventional ED devices. In literature (H. Strathmann [1]), overlimiting current density operation ED is considered as being under ongoing research and development. Where is the research today? What is the actual understanding of overlimiting mass transfer processes in membrane systems? As far as electroconvection is involved as a major mechanism of overlimiting transfer, what could bring specific knowledge and practice acquired in nano-and microfluidics into desalination? Can overlimiting ED be economically advantageous? Is there an alternative for classical ED devices applied? This review is intended to give some responses to these questions.
A method to forecast behavior of electrodialyzers with long demineralization channels is suggeste... more A method to forecast behavior of electrodialyzers with long demineralization channels is suggested. It is based on the data obtained using a laboratory cell or an apparatus with short demineralization channels of similar construction. The results are presented as dimensionless parameters.
A modified static method for investigation of Co,-, HCO; and OH-ions sorption by anion-exchange m... more A modified static method for investigation of Co,-, HCO; and OH-ions sorption by anion-exchange membranes was developed. The ion-exchange equilibrium coefficient ( 1 I?$ in the system of the Russian commercial anionexchange membrane MA-41 in sodium carbonate/hydrocarbonate iolutions within pH range from 8.3 to 10.8 is found. This value is calculated by two methods: taking into account or not the hydrolysis reactions of carbonate in the 2equilibrium solution and the sorption of OH-ions by the membrane. The ion exchange coefficient value KzEi_ is 9. 3 Keyworak Anion-exchange membrane; Carbonate/hydrocarbonate solutions; Ion-exchange equilibrium
Electromembrane methods of amino acids purification and separation are very attractive. They do n... more Electromembrane methods of amino acids purification and separation are very attractive. They do not need any additional chemicals, minimize the desired product loss and are energy efficient and manufacturable. Detailed study of amino acid transfer mechanisms inside membranes and at their surfaces provides a basis to improve these methods. For these purposes, concentration dependencies of cation-exchange (CMX, MK-40) and anion-exchange (AMX, MA-41) membranes electrical conductivity in 0.025-1 M LysHCl and NaCl solutions as well as current-voltage curves (CVC) and chronopotentiograms (ChP) of CMX membrane in 0.01 M solutions are obtained.
Transport of salts of phosphoric and carbonic acids through anion exchange membranes (AEM) is stu... more Transport of salts of phosphoric and carbonic acids through anion exchange membranes (AEM) is studied by means of current-voltage responses. Concentration polarisation of such membrane systems is accompanied by a change in the electrolyte concentration at the membrane-solution interface due to hydrolysis reactions. Current-voltage characteristics (CVC) on HPO 4 2− or PO 4 3− ion-containing systems with modified AEM selectively permeable to mono-charged anions present two waves like in electrode multi-component systems with multi-stage electrochemical reactions. This phenomenon does not occur with nonmodified AEMs. It is also the case with all the AEMs studied for solutions containing HCO 3 − or CO 3 2− salts because of close values of electrical resistances of the membranes in these both ionic forms.
Please cite this article as: Galama et al., On the origin of the membrane potential arising acros... more Please cite this article as: Galama et al., On the origin of the membrane potential arising across densely charged ion exchange membranes: How well does the Teorell-Meyer-Sievers theory work?, HIGHLIGHTS The origin of the membrane potential across ion exchange membranes (IEMs) is discussed. At large concentration differences, Teorell-Meyer-Sievers (TMS) theory deviates from the data. No single assumption in the TMS theory can by itself account for the deviation. For highly charged IEMs the membrane potential can go through a maximum Presence of a maximum in the potential depends on the concentration ratio
Three ion exchange membranes (an AMX homogeneous anion exchange membrane, a MK 40 heterogeneous c... more Three ion exchange membranes (an AMX homogeneous anion exchange membrane, a MK 40 heterogeneous cation exchange membrane, and a Nafion 117 homogeneous cation exchange membrane) have been studied by electrochemical impedance spectroscopy. Processing of the experimental impedance spectra according to the model developed previously has made it possible to find the Nernst diffusion bound ary layer (DBL) thickness δ as a function of current density. The behavior of the AMX membrane has been shown to be close to the "ideal" one described by the model: the impedance spectrum of the membrane is close to the theoretical spectrum and the value of δ is only slightly smaller than the quantity δ Lev calculated by the Leveque equation derived in terms of classical convective diffusion theory. The behavior of the MK 40 and Nafion membranes markedly differs from the "ideal" behavior: the reactive component of the impedance in the region of medium frequencies corresponding to the maximum point in the low frequency range of a Warburg type finite length impedance spectrum is significantly lower than its theoretically predicted value. The value of δ is less than δ Lev even for underlimiting currents, and the deviation increases with the increasing current density. This specific behavior of the membranes correlate well with the voltammetry data. The behavior of the studied membranes is associated with the surface properties: the heterogeneity (case of MK 40) and, especially, high hydrophobicity of the (Nafion 117) surface facilitate the development of elec troconvection. Homogeneity and high hydrophilicity of the surface of the AMX membrane determine its behavior, which is close to the ideal. * The contact angles were measured by K. A. Shevtsova and A. V. Nebavskii on the water swollen in membrane 30 s after applying a drop of distilled water. The measurement procedure is described in .
It is known that the behavior of a membrane with homogeneous and heterogeneous surface under the ... more It is known that the behavior of a membrane with homogeneous and heterogeneous surface under the same current density is different. In order to find out this difference, a twodimensional non-stationary model of mass transfer through electrically inhomogeneous surface of ion-exchange membrane under direct current is proposed. In this paper, the general formulation of the boundary conditions at the conductive sections has the form of the integral function of the local current density. The problem was solved by applying an electric current stream function, which is quite similar to the stream function in hydrodynamics. For the first time, it is proved that the distribution of current lines over the conductive sections of the membrane is not uniform. It is shown experimentally and theoretically that the shape of chronopotentiograms and electrochemical impedance spectra changes with changing conductive portion of the ion-exchange membrane surface.
A relatively simple mathematical model based on the Poisson equation is considered. The model is ... more A relatively simple mathematical model based on the Poisson equation is considered. The model is intended for modeling transport through multilayered ion-exchange membranes operating at overlimiting currents. The boundary-value problem is solved by a numerical method of parallel shooting and by an approximate method based on the assumption that the charge density is distributed quasi-uniformly. Concentration profiles in diffusion layers and membranes, current–voltage curves, and dependences of effective transport numbers on the current density are examined.
ABSTRACT Kinetics of surface water desalination by neutralization dialysis (ND) was studied in a ... more ABSTRACT Kinetics of surface water desalination by neutralization dialysis (ND) was studied in a three-compartment stack with ion-exchange membranes, and the process feasibility was evaluated. The pH and the ionic conductivity of a model saline solution (0.02 M NaCl) were measured during different ND operations in batch mode. The influence of the flow rate, the solution concentration in the alkali and acid compartments was investigated. It was found that, at the beginning of the operation, the ion-exchange kinetics was mainly controlled by the membrane diffusion. The maximal fluxes of exchanging ions (Jmax) (Na+ and H+ across CMX membrane, and Cl− and OH− across AMX membrane) were theoretically evaluated. The calculated and the experimentally measured fluxes at the beginning of batch operation were close. However, after some minutes of solution desalination, the salt concentration decreased, which involved diffusion boundary layer controlled kinetics. Hence, with time, the fluxes decreased and were considerably smaller than Jmax. The increase/decrease in the acid and alkali solution concentration resulted in corresponding increase/decrease in the ion-exchange rate. Recommendations have been provided to reduce the desalination duration.
Three ion exchange membranes (an AMX homogeneous anion exchange membrane, a MK 40 heterogeneous c... more Three ion exchange membranes (an AMX homogeneous anion exchange membrane, a MK 40 heterogeneous cation exchange membrane, and a Nafion 117 homogeneous cation exchange membrane) have been studied by electrochemical impedance spectroscopy. Processing of the experimental impedance spectra according to the model developed previously has made it possible to find the Nernst diffusion bound ary layer (DBL) thickness δ as a function of current density. The behavior of the AMX membrane has been shown to be close to the "ideal" one described by the model: the impedance spectrum of the membrane is close to the theoretical spectrum and the value of δ is only slightly smaller than the quantity δ Lev calculated by the Leveque equation derived in terms of classical convective diffusion theory. The behavior of the MK 40 and Nafion membranes markedly differs from the "ideal" behavior: the reactive component of the impedance in the region of medium frequencies corresponding to the maximum point in the low frequency range of a Warburg type finite length impedance spectrum is significantly lower than its theoretically predicted value. The value of δ is less than δ Lev even for underlimiting currents, and the deviation increases with the increasing current density. This specific behavior of the membranes correlate well with the voltammetry data. The behavior of the studied membranes is associated with the surface properties: the heterogeneity (case of MK 40) and, especially, high hydrophobicity of the (Nafion 117) surface facilitate the development of elec troconvection. Homogeneity and high hydrophilicity of the surface of the AMX membrane determine its behavior, which is close to the ideal. * The contact angles were measured by K. A. Shevtsova and A. V. Nebavskii on the water swollen in membrane 30 s after applying a drop of distilled water. The measurement procedure is described in .
Partial currents of water dissociation products through cation-and anion-exchange membranes that ... more 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 OHions is defined by the ratio of the current density to its critical value at which water starts undergoing discernible dissociation.
Several laws governing ampholyte transport through ion-exchange membranes are established by a co... more Several laws governing ampholyte transport through ion-exchange membranes are established by a comparative analysis of the concentration dependence of electrical conductivity for homogeneous (CMX, AMX) and heterogeneous (MK-40, MA-41) membranes in NaCl, LysHCl, and NaH2PO4 solutions. The increase in the electrical conductivity of membranes in ampholyte solutions as the solutions become more dilute is explained by the increased fraction of
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