Electromembrane Systems Research Articles

Towards optimized cation-exchange membranes for overlimiting current electrodialysis: Correlation between size of resin particles in membranes and mechanism of ion transport through them

Experimental heterogeneous membranes with different particle sizes of cation-exchange resin KU-2–8 (the diameter of resin particles varied from 20 to 71 μm) were manufactured under the production conditions of LLC “IE “Shchekinoazot”. It is established that at currents higher than the limiting current the main mechanisms of electrolyte ion transport are classical electrodiffusion and electroconvection. It is shown for the first time that at the same degree of membrane polarization i/ilim the fraction of electroconvection contribution increases, and the fraction of contribution of water splitting reaction products decreases with decreasing size of resin particles in membranes. The theoretical evaluation of the influence of the size of resin particles in the membrane on the internal parameters of electromembrane systems MK-40/0.01M NaCl solution has shown that the crucial role in the development of electroconvection is played not by the field effect, but by the peculiarities of electrical heterogeneity of the membrane surface.

Enhanced Salt Removal of Fresh Water by Recovery-Reduced Ion Concentration Polarization Desalination.

Here, we examine electromembrane systems for low-concentration desalination applicable to ultrapure water production. In addition to electrodialysis and ion concentration polarization (ICP) desalination, we propose a recovery-reduced ICP strategy for reducing the width of the desalted outlet for a higher salt removal ratio (SRR). The correlation between conductivity changes and thickness of the ion depletion zone is identified for electrodialysis, ICPH (1:1), and ICPQ (3:1) with a low-concentration feed solution (10 mM, 1 mM, 0.1 mM NaCl). Based on the experimental results, the scaling law and SRR for the electroconvection zone are summarized, and current efficiency (CE) and energy per ion removal (EPIR) depending on SRR are also discussed. As a result, the SRR of electrodialysis is mostly around 50%, but that of recovery-reduced ICP desalination is observed up to 99% under similar operating conditions. Moreover, at the same SRR, the CE of recovery-reduced ICP is similar to that of electrodialysis, but the EPIR is calculated to be lower than that of electrodialysis. Considering that forming an ion depletion zone up to half the channel width in the electromembrane system typically requires much power consumption, an ICP strategy that can adjust the width of the desalted outlet for high SRR can be preferable.

Theoretical Analysis of the Influence of Spacers on Salt Ion Transport in Electromembrane Systems Considering the Main Coupled Effects.

This article considers a theoretical analysis of the influence of the main coupled effects and spacers on the transfer of salt ions in electromembrane systems (EMS) using a 2D mathematical model of the transfer process in a desalting channel with spacers based on boundary value problems for the coupled system of Nernst-Planck-Poisson and Navier-Stokes equations. The basic patterns of salt ion transport have been established, taking into account diffusion, electromigration, forced convection, electroconvection, dissociation/recombination reactions of water molecules, as well as spacers located inside the desalting channel. It has been shown that spacers and taking into account the dissociation/recombination reaction of water molecules significantly change both the formation and development of electroconvection. This article confirms the fact of the exaltation of the limiting current studied by Harkatz, where it is shown that the current (flux) of salt ions increases when the dissociation reaction begins by a certain value called the exaltation current, which is proportional to the flow of water dissociation products. A significant combined effect of electroconvection and dissociation/recombination reactions as well as the spacer system in the desalting channel on the transport of salt ions are shown. The complex, nonlinear, and non-stationary interaction of all the main effects of concentration polarization and spacers in the desalting channel are also considered in the work.

Time-Dependent Two-Dimensional Model of Overlimiting Mass Transfer in Electromembrane Systems Based on the Nernst–Planck, Displacement Current and Navier–Stokes Equations

Time-Dependent Two-Dimensional Model of Overlimiting Mass Transfer in Electromembrane Systems Based on the Nernst–Planck, Displacement Current and Navier–Stokes Equations

Ion Transport in Electromembrane Systems under the Passage of Direct Current: 1D Modelling Approaches.

For a theoretical analysis of mass transfer processes in electromembrane systems, the Nernst-Planck and Poisson equations (NPP) are generally used. In the case of 1D direct-current-mode modelling, a fixed potential (for example, zero) is set on one of the boundaries of the considered region, and on the other-a condition connecting the spatial derivative of the potential and the given current density. Therefore, in the approach based on the system of NPP equations, the accuracy of the solution is significantly affected by the accuracy of calculating the concentration and potential fields at this boundary. This article proposes a new approach to the description of the direct current mode in electromembrane systems, which does not require boundary conditions on the derivative of the potential. The essence of the approach is to replace the Poisson equation in the NPP system with the equation for the displacement current (NPD). Based on the system of NPD equations, the concentration profiles and the electric field were calculated in the depleted diffusion layer near the ion-exchange membrane, as well as in the cross section of the desalination channel under the direct current passage. The NPD system, as well as NPP, allows one to describe the formation of an extended space charge region near the surface of the ion-exchange membrane, which is important for describing overlimiting current modes. Comparison of the direct-current-mode modelling approaches based on NPP and NPD showed that the calculation time is less for the NPP approach, but the calculation accuracy is higher for the NPD approach.

Mathematical Modeling of the Influence of the Karman Vortex Street on Mass Transfer in Electromembrane Systems

In electromembrane systems, the transfer of ions near ion-exchange membranes causes concentration polarization, which significantly complicates mass transfer. Spacers are used to reduce the effect of concentration polarization and increase mass transfer. In this article, for the first time, a theoretical study is carried out, using a two-dimensional mathematical model, of the effect of spacers on the mass transfer process in the desalination channel formed by anion-exchange and cation-exchange membranes under conditions when they cause a developed Karman vortex street. The main idea is that, when the separation of vortices occurs on both sides in turn from the spacer located in the core of the flow where the concentration is maximum, the developed non-stationary Karman vortex street ensures the flow of the solution from the core of the flow alternately into the depleted diffusion layers near the ion-exchange membranes. This reduces the concentration polarization and, accordingly, increases the transport of salt ions. The mathematical model is a boundary value problem for the coupled system of Nernst-Planck-Poisson and Navier-Stokes equations for the potentiodynamic regime. The comparison of the current-voltage characteristics calculated for the desalination channel with and without a spacer showed a significant increase in the intensity of mass transfer due to the development of the Karman vortex street behind the spacer.

Preconcentration of Fluorescent Dyes in Electromembrane Systems via Electrophoretic Migration

Microfluidic preconcentration enables the collection or extraction of low-abundance analytes at specific locations. It has attracted considerable attention as an essential technology in bioengineering, particularly for detection and diagnosis. Herein, we investigated the key parameters in the preconcentration of fluorescent dyes based on electrophoresis in a microfluidic electromembrane system. Commercial ion-exchange membrane (IEM)-integrated polydimethylsiloxane microfluidic devices were fabricated, and Alexa Fluor 488 and Rhodamine 6G were used as fluorescent dyes for sample preconcentration. Through experimental studies, the effect of the channel concentration ratio (CCR, concentration ratio of the main and buffer channels) on the performance of the sample preconcentration was studied. The results show that the preconcentration of the target sample occurs more effectively for a high CCR or high salt concentration of the main channel when the CCR is constant. We also demonstrate a phenomenon that the salt concentration in the electrolyte solution increases as the preconcentration progresses. Our results provide consolidated conditions for electrophoresis-based sample preconcentration in electromembrane systems.

Theoretical Analysis of Electroconvection in the Electrodialysis Desalination Channel under the Action of Direct Current.

The development of electroconvection in electromembrane systems is a factor that increases the efficiency of the electrolyte solution desalination process. The desalination of the solution, manifested by a change in the distribution of the ion concentration, can affect the mechanisms of development of electroconvection. The purpose of this work is to study the electroconvective flow developing in the desalination channel under various desalination scenarios. The study was carried out on the basis of a mathematical model of the transfer of binary electrolyte ions in the desalination channel formed between the anion and cation exchange membranes under the action of DC current. An analytical estimation of the threshold current density reflecting the conditions of the system transition into a quasi-stationary state has been obtained. The chronopotentiograms of the desalination channel and the thickness of the electroconvective mixing layer are calculated for both pre-threshold and supra-threshold values of the current density.

Current-Voltage Characteristics of Membranes with Different Cation-Exchanger Content in Mineral Salt-Neutral Amino Acid Solutions under Electrodialysis.

The features of the electrochemical behavior of experimental heterogeneous ion-exchange membranes with different mass fractions of sulfonated cation-exchange resin (from 45 to 65 wt%) have been studied by voltammetry during electrodialysis. Electromembrane systems with 0.01 M NaCl solution and with a mixed 0.01 M NaCl + 0.05 M phenylalanine (Phe) solution have been investigated. A significant influence of the ion-exchanger content on the parameters of current-voltage curves (CVCs) was established for the first time. Electrodialysis of the sodium chloride solution revealed a decrease in the length of the limiting current plateau and in the resistances of the second and third sections of the CVCs with an increase in the resin content in the membrane. The fact of the specific shape of the CVCs of all studied cation-exchange membrane samples in mixed solutions of the mineral salt and the amino acid was established. A specific feature of current-voltage curves is the presence of two plateaus of the limiting current and two values of the limiting current, respectively. This phenomenon in electromembrane systems with neutral amino acids has not been found before. The value of the first limiting current is determined by cations of the mineral salt, which are the main current carriers in the system. The presence of the second plateau and the corresponding second limiting current is due to the appearance of additional carriers due to the ability of phenylalanine as an organic ampholyte to participate in protolytic reactions. In the cation-exchange electromembrane system with the phenylalanine containing solution, two mechanisms of H+/OH- ion generation through water splitting and acid dissociation are shown. The possibility of the generation of H+/OH- ions at the enriched solution/cation-exchange membrane interface during electrodialysis of amino acid containing solutions is shown for the first time. The results of this study can be used to improve the process of electromembrane demineralization of neutral amino acid solutions by both targeted selection or the creation of new membranes and the selection of effective current operating modes.

Theoretical Investigation of the Phenomenon of Space Charge Breakdown in Electromembrane Systems

At present, it is customary to consider the overlimit operating modes of electromembrane systems to be effective, and electroconvection as the main mechanism of overlimiting transfer. The breakdown of the space charge is a negative, "destructive" phenomenon, since after the breakdown the size and number of electroconvective vortices are significantly reduced, which leads to a decrease in mass transfer. Therefore, electromembrane desalination processes must be carried out before space charge breakdown occurs. Thus, the actual problem arises of determining at which potential jumps a breakdown of the space charge occurs at a given concentration of the solution. Electromembrane systems are used for desalination at electrolyte solution concentrations ranging from 1 to 100 mol/m3. In a theoretical study of increasing the efficiency of the desalination process, mathematical modeling is used in the form of a boundary value problem for the system of Nernst-Planck and Poisson (NPP) equations, which refers to "hard" problems that are difficult to solve numerically. This is caused by the appearance of a small parameter at the derivative in the Poisson equation in a dimensionless form, and, correspondingly, a boundary layer at ion-exchange membranes, where concentrations and other characteristics of the desalination process change exponentially. It is for this reason that the numerical study of the boundary value problem is currently obtained for initial concentrations of the order of 0.01 mol/m3. The paper proposes a new numerical-analytical method for solving boundary value problems for the system of Nernst-Planck and Poisson equations for real initial concentrations, using which the phenomenon of space charge breakdown (SCB) in the cross section of the desalination channel in potentiostatic and potentiodynamic modes is studied. The main regularities of the appearance and interaction of charge waves, up to their destruction (breakdown), are established. A simple formula is proposed for engineering calculations of the potential jump depending on the concentration of the solution, at which the breakdown of the space charge begins.

INFLUENCE OF SPACE CHARGE BREAKDOWN ON THE PERFORMANCE OF ELECTROMEMBRANE SYSTEMS

It is known that the existing methods of using electromembrane systems (EMS) do not allow the most efficient water treatment and water treatment at overlimiting current modes, which is why there is a need to determine the operating parameters of electrodialysis plants to prevent complex destructive effects, it is essential limiting mass transfer, and reducing the efficiency of EMS. In addition, in practice, a contradiction is found in practice - an increase in the potential jump in the existing EMS does not provide an intensification of the transfer of salt ions under overlimiting current modes. The article is devoted to the study of the influence of one of the destructive phenomena - the breakdown of the space charge on the performance of electromembrane systems in highly diluted salt solutions. The aim of this work is a theoretical assessment of the effect of space charge breakdown on the transport of salt ions in electromembrane systems. A mathematical model based on the Nernst-Planck and Poisson equations has been developed. The process of electrodialysis desalination of a NaCl solution located in a flow chamber between an anion exchange membrane (AEM) and a cation exchange membrane (CEM) is considered. The solution flow is directed from bottom to top. Numerical modeling of the effect of space charge breakdown on EMS performance in highly dilute salt solutions has been carried out. The new mathematical model and software have been developed for numerical simulation of the effect of space charge breakdown on EMS performance in highly dilute salt solutions. In this work, using a 2D mathematical model and the formula for the current-voltage characteristic of the desalination channel, the effect of space charge breakdown on the transfer of salt ions in electromembrane systems at various values of the potential jump was determined for the first time. The ranges of currents are determined, which correspond to a high and slight increase in the performance of electromembrane systems, as well as a range of currents in which there is no increase. The obtained results can be used to determine the optimal value of the potential jump according to the criterion of maximum productivity. The results obtained can be applied to the design of industrial EMC to intensify mass transfer and increase productivity.

Real-time, dynamic monitoring of selectively driven ion-concentration polarization

Selective ion extraction is a vital element of many resource recovery systems. Carrier-based membranes offer the ability to remove specific, targeted ions from brine and waste streams, but these materials have delivered inconsistent outcomes in their active transport applications. To study non-ideal behaviors of target-ion selective membranes, we introduce a direct, real-time system for selectively measuring concentrations within electro-membrane boundary layers. In a carrier-based membrane system, we applied our method to monitor adverse, current-limiting behaviors. During its operation, we detected loss of transport selectivity and counter-ion discharge. It provided sufficient evidence to identify the mechanism underlying these adverse, over-limiting effects—the internal concentration polarization of free-carrier. Our method may raise new prospects for direct experimental characterization of nonlinear transport phenomena in electro-membrane systems.

Experimental Study on Ion Transport in Microfluidic Electrodialysis Using Partially Masked Ion Exchange Membranes.

Electrodialysis using anion-exchange membranes (AEMs) and cation-exchange membranes (CEMs) has been widely used for water desalination and the management of various ionic species. During commercial electrodialysis, the available area of an ion-exchange membrane is reduced by a non-conductive spacer that is in contact with the AEM/CEM. Although multiple reports have described the advantages or disadvantages of spacers, fewer studies have explored the effects of spacers on the mass transport effect of the reduced membrane area excluding the fluid flow change. In this paper, we present our experimental studies concerning mass transport in microfluidic electrodialysis systems with partially masked ion-exchange membranes. Six different types of masking membranes were prepared by the deposition of non-conductive films on parts of the membranes. The experimental results showed that the overlapped types (in which masking was vertically aligned in the AEM/CEM) exhibited a larger electrical conductance and better current/energy efficiency, compared with the non-overlapped types (in which masking was vertically dislocated in the AEM/CEM). We also observed that a reduction in the unit length of the unmasked ion-exchange membrane enhanced overall mass transport. Our results demonstrate the effects of patterned membranes on electrical resistance and desalination performance; they also identify appropriate arrangements for electromembrane systems.

Removal of Excess Alkali from Sodium Naphthenate Solution by Electrodialysis Using Bilayer Membranes for Subsequent Conversion to Naphthenic Acids.

The processing of solutions containing sodium salts of naphthenic acids (sodium naphthenate) is in high demand due to the high value of the latter. Such solutions usually include an excessive amount of alkali and a pH of around 13. Bipolar electrodialysis can convert sodium naphthenates into naphthenic acids; however, until pH 6.5, the naphthenic acids are not released from the solution. The primary process leading to a decrease in pH is the removal of excess alkali that implies that some part of electricity is wasted. In this work, we propose a technique for the surface modification of anion-exchange membranes with sulfonated polyetheretherketone, with the formation of bilayer membranes that are resistant to poisoning by the naphthenate anions. We investigated the electrochemical properties of the obtained membranes and their efficiency in a laboratory electrodialyzer. Modified membranes have better electrical conductivity, a high current efficiency for hydroxyl ions, and a low tendency to poisoning than the commercial membrane MA-41. We propose that the primary current carrier is the hydroxyl ion in both electromembrane systems with the MA-41 and MA-41M membranes. At the same time, for the modified MA-41M membrane, the concentration of hydroxyl ions in the anion-exchanger phase is higher than in the MA-41 membrane, which leads to almost five-fold higher values of the specific permeability coefficient. The MA-41M membranes are resistant to poisoning by naphthenic acids anions during at least six cycles of processing of the sodium naphthenate solution.

Mathematical modeling of the influence of non-catalytic dissociation / recombination of water molecules in the desalination channel on electric convection

The article formulates a two-dimensional mathematical model of non-stationary transport of 1: 1 electrolyte in a potentiodynamic mode, taking into account electroconvection and non-catalytic dissociation / recombination reaction of water molecules in electromembrane systems, which are considered as the desalting channel of an electrodialysis device. The model is described by a system of coupled Navier-Stokes and Nernst-Planck-Poisson equations taking into account the electric force and physically justified boundary conditions. The article establishes the basic laws of mass transport, taking into account the dissociation / recombination of water molecules. It was shown for the first time that a double electric layer of hydrogen and hydroxyl ions arises in the recombination region. It is shown that between the region of recombination and quasi-equilibrium regions of space charge there are regions of electroneutrality and equilibrium with an almost linear distribution of concentrations. It was found that even under prelimiting, but close enough to the limiting current, modes, non-catalytic dissociation of water molecules in the quasi-equilibrium region of space charge occurs so intensely that the concentration of hydrogen and hydroxyl ions becomes comparable to the concentration of potassium and chlorine ions. At overlimiting current densities, due to the appearance of an extended space charge region and intense dissociation of water molecules in this region, as well as an increase in the electric double layer in the recombination region, the space charge and the dissociation / recombination reaction of water molecules significantly affect each other. In turn, this has a decisive effect on electroconvection and, accordingly, on the transport of salt ions.

Artificial intelligence system for the analysis of theoretical and experimental current-voltage characteristics

Digitalization is one of the development priorities in the global scientific community. Digitalization at this point in time means the introduction of artificial intelligence systems in production, science, economics, management, etc. Artificial intelligence systems have shown their effectiveness in various fields of science and technology, including electrochemistry for such tasks as modeling membrane separation, controlling the variable operation of a simple seawater reverse osmosis plant, etc., however, the study and prediction of theoretical and experimental current-voltage characteristics of electromembrane systems by machine learning and artificial intelligence methods is still practically not carried out. The problem of finding connections between the regularities of changes in the current-voltage characteristics (CVC) and the regularities of the process of salt ion transfer in the electrodialysis apparatus (EDA) desalination channel, the classification of theoretical and experimental CVC depending on the values of the input parameters has not yet been set and investigated, and therefore is a new fundamental problem and an actual practical problem.

(Invited) Current Density Distribution In a Pilot-Scale Electrodialysis Unit - 3D Mathematical Modeling and Experimental Investigation

Electrodialysis (ED) represents a matured and well-established desalination and membrane separation technology. In order to satisfy application demands, there is a trend for increasing dimensions of not only the industrial ED, but also of the other electrochemical membrane units, like electrolyzers, fuel cells etc.), capable of high process intensity and efficiency. These activities are driven mainly by cost reduction. This is also the case of reverse electrodialysis as an allied process, currently meeting rapidly growing popularity as a promising green energy source. However, experiences in a development of larger scale membrane electrochemical devices document scale-up from laboratory or pilot system to industrial ones not to be straightforward. The scale-up based on an direct increase of smaller devises dimensions, like e.g. increasing membrane active area or number of membranes without appropriate modifications of entire unit design, often leads to pronounced nonuniformity of local current density, mass and heat distribution inside the system. Consequently, the performance, reliability and lifetime of industrial scale apparatuses is significantly exacerbated. A uniformity of the above-mentioned fluxes distribution strongly depends on numerous aspects, particularly on the geometry of electrodes, manifolds and fluid distributors. Due to these facts, there is an urgent need of fundamental understanding of the relations between the system geometry and uniformity of the fluxes, especially on an industrial scale.The presented work focuses at investigation on local current density distribution in the pilot-scale ED unit consisting of 200 vertically orientated membrane pairs with working area of 0.64 × 0.32 m2 per membrane. Planar-plate electrodes are horizontally segmented into the 6 identical and electrically insulated segments allowing determination of current distribution along main direction of the solutions flow. The current distribution is investigated at different operating conditions (salt concentration, applied current, solution flow rate, electrode compartment solutions properties etc.). In parallel, a macrohomogeneous (volume-averaged) 3D mathematical model is validated and employed to calculate and visualize the local fields of electric potential, current density and salt concentrations. A nonuniformity in the current distribution mainly due to the by-pass current through the manifold is observed. The qualitative relationships between the nonuniformity of current density distribution and process performance and reliability observed for pilot- or industrial-scale can be generalized to be valid also for other similar electromembrane systems with plate-and-frame design.

One-dimensional mathematical models of salt ion transport in electromembrane systems

This article presents a 1D classification and analysis of mathematical models of binary salt ion transport in electromembrane systems. The various phenomena of concentration polarization occurring in these systems are studied using mathematical models, both individually and in combination with each other. In accordance with this, we have carried out the classification of these phenomena, problems and mathematical models. The article presents a hierarchy of subordination of 1D mathematical models. A brief review, comparison, and analysis of the results of the boundary value problems from the proposed hierarchy are carried out. A numerical study and comparison of some mathematical models from the proposed hierarchy are carried out. The scope of applicability and limitations for each model are defined.

Reactive separation of inorganic and organic ions in electrodialysis with bilayer membranes

This work shows the fundamental possibility of implementing the “reactive separation” of organic and inorganic ions from their mixtures using a bilayer membrane operating in a “mixed mode”. In this work, the electrodialysis process was studied during the processing of a solution containing a mixture of salts of inorganic and organic electrolytes. Sodium chloride was chosen as the inorganic electrolyte, and sodium acetate was chosen as the organic electrolyte. To study the peculiarities of ion transfer and the effect of chemical reactions occurring in the diffusion layer, electromembrane systems with a conventional anion-exchange membrane Ralex AMH and a BM-ac bilayer membrane (obtained by the authors) were studied. It is shown that the Ralex AMH membrane has a high selectivity to chloride ions and the permselectivity coefficient is PCl-/AcΣ=3.3. In the case of a bilayer membrane, which is an active source of hydrogen ions (appearing as a result of the water splitting reaction at the cation-exchanger/anion-exchanger interface), a reaction layer is formed inside the depleted diffusion layer, in which the recombination reaction of acetate ions and hydrogen ions proceeds. The use of a bilayer membrane makes it possible to increase the permselectivity coefficient up to PCl-/AcΣ=61.4. Simultaneously, in the under-limiting current mode, the bilayer membrane exhibits high selectivity to acetate ions, and the specific permselectivity coefficient is PCl-/AcΣ=0.66, which suggests the appearance of specific selectivity between singly charged ions during electrodialysis.

Theoretical Analysis of the Stationary Transport of 1:1 Salt Ions in a Cross-Section of a Desalination Channel, Taking into Account the Non-Catalytic Dissociation/Recombination Reaction of Water Molecules.

In electromembrane systems, the theoretical study of salt ion transport usually uses mathematical models of salt ion transport in the depleted diffusion layer of ion-exchange membranes. This study uses a one-dimensional mathematical model of salt ion transport in a cross-section of a desalination channel formed by anion-exchange and cation-exchange membranes, taking into account an effect of a dissociation/recombination reaction of water molecules. The reaction on the one hand leads to an overlimiting mass transfer due to the effect of exaltation of the limiting current. On the other hand, an appearance of new electric charge carriers (hydrogen and hydroxyl ions) can reduce the space charge that occurs in membranes and suppress an electroconvective mechanism of overlimiting transport. Thus, there is a problem of studying these phenomena together, taking into account their mutual influence, and this article is devoted to the solution of this problem. Theoretically, using a method of mathematical modeling and numerical research, main regularities are established; in particular, it is shown that the dissociation/recombination reaction of water molecules does not lead to the destruction of the double electric layer at the membranes, but also creates a new double electric layer in the middle of the desalination channel. Thus, the space charge and the dissociation/recombination reaction significantly affect each other and simultaneously the transport of salt ions.