Effect of current density, concentration of ternary electrolyte and type of cations on the monovalent ion selectivity of surface-sulfonated graft anion-exchange membranes: modelling and experiment

Separation of Chloride and Sulfate Ions from Desulfurization Wastewater Using Monovalent Anions Selective Electrodialysis

The high concentration of chloride ions in desulphurization wastewater is the primary limiting factor for its reusability. Monovalent anion selective electrodialysis (S-ED) enables the selective removal of chloride ions, thereby facilitating the reuse of desulfurization wastewater. In this study, different concentrations of NaCl and Na2SO4 were used to simulate different softened desulfurization wastewater. The effects of current density and NaCl and Na2SO4 concentration on ion flux, permselectivity (PSO42-Cl-) and specific energy consumption were studied. The results show that Selemion ASA membrane exhibits excellent permselectivity for Cl- and SO42-, with a significantly lower flux observed for SO42- compared to Cl-. Current density exerts a significant influence on ion flux; as the current density increases, the flux of SO42- also increases but at a lower rate than that of Cl-, resulting in an increase in permselectivity. When the current density reaches 25 mA/cm2, the permselectivity reaches a maximum of 50.4. The increase in NaCl concentration leads to a decrease in the SO42- flux; however, the permselectivity is reduced due to the elevated Cl-/SO42- ratio. The SO42- flux increases with the increase in Na2SO4 concentration, while the permselectivity increases with the decrease in Cl-/SO42- ratio.

Composite Anion Exchange Membranes Based on Quaternary Ammonium-Functionalized Polystyrene and Cerium(IV) Phosphate with Improved Monovalent-Ion Selectivity and Antifouling Properties

The possibility of targeted change of the properties of ion exchange membranes by incorporation of various nanoparticles into the membranes is attracting the attention of many research groups. Here we studied for the first time the influence of cerium phosphate nanoparticles on the physicochemical and transport properties of commercial anion exchange membranes based on quaternary ammonium-functionalized polystyrenes, such as heterogeneous Ralex^® AM and pseudo-homogeneous Neosepta^® AMX. The incorporation of cerium phosphate on one side of the membrane was performed by precipitation from absorbed cerium ammonium nitrate (CAN) anionic complex with ammonium dihydrogen phosphate or phosphoric acid. The structures of the obtained hybrid membranes and separately synthesized cerium phosphate were investigated using FTIR, P31 MAS NMR, EDX mapping, and scanning electron microscopy. The modification increased the membrane selectivity to monovalent ions in the ED desalination of an equimolar mixture of NaCl and Na₂SO₄. The highest selectivities of Ralex^® AM and Neosepta^® AMX-based hybrid membranes were 4.9 and 7.7, respectively. In addition, the modification of Neosepta^® membranes also increased the resistance to a typical anionic surfactant, sodium dodecylbenzenesulfonate.

Cation Exchange Membranes and Process Optimizations in Electrodialysis for Selective Metal Separation: A Review

The selective separation of metal species from various sources is highly desirable in applications such as hydrometallurgy, water treatment, and energy production but also challenging. Monovalent cation exchange membranes (CEMs) show a great potential to selectively separate one metal ion over others of the same or different valences from various effluents in electrodialysis. Selectivity among metal cations is influenced by both the inherent properties of membranes and the design and operating conditions of the electrodialysis process. The research progress and recent advances in membrane development and the implication of the electrodialysis systems on counter-ion selectivity are extensively reviewed in this work, focusing on both structure-property relationships of CEM materials and influences of process conditions and mass transport characteristics of target ions. Key membrane properties, such as charge density, water uptake, and polymer morphology, and strategies for enhancing ion selectivity are discussed. The implications of the boundary layer at the membrane surface are elucidated, where differences in the mass transport of ions at interfaces can be exploited to manipulate the transport ratio of competing counter-ions. Based on the progress, possible future R&D directions are also proposed.

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.

Improvement of Selectivity of RALEX-CM Membranes via Modification by Ceria with a Functionalized Surface

Ion exchange membranes are widely used for water treatment and ion separation by electrodialysis. One of the ways to increase the efficiency of industrial membranes is their modification with various dopants. To improve the membrane permselectivity, a simple strategy of the membrane surface modification was proposed. Heterogeneous RALEX-CM membranes were surface-modified by ceria with a phosphate-functionalized surface. Despite a decrease in ionic conductivity of the prepared composite membranes, their cation transport numbers slightly increase. Moreover, the modified membranes show a threefold increase in Ca²⁺/Na⁺ permselectivity (from 2.1 to 6.1) at low current densities.

Ion and Water Transport in Ion-Exchange Membranes for Power Generation Systems: Guidelines for Modeling

Artificial ion-exchange and other charged membranes, such as biomembranes, are self-organizing nanomaterials built from macromolecules. The interactions of fragments of macromolecules results in phase separation and the formation of ion-conducting channels. The properties conditioned by the structure of charged membranes determine their application in separation processes (water treatment, electrolyte concentration, food industry and others), energy (reverse electrodialysis, fuel cells and others), and chlore-alkali production and others. The purpose of this review is to provide guidelines for modeling the transport of ions and water in charged membranes, as well as to describe the latest advances in this field with a focus on power generation systems. We briefly describe the main structural elements of charged membranes which determine their ion and water transport characteristics. The main governing equations and the most commonly used theories and assumptions are presented and analyzed. The known models are classified and then described based on the information about the equations and the assumptions they are based on. Most attention is paid to the models which have the greatest impact and are most frequently used in the literature. Among them, we focus on recent models developed for proton-exchange membranes used in fuel cells and for membranes applied in reverse electrodialysis.

Mathematical Modeling of the Selective Transport of Singly Charged Ions Through Multilayer Composite Ion-Exchange Membrane during Electrodialysis

Abstract

The deposition of several alternating anion- and cation-exchange surface layers (layer-by-layer method) is a promising technique for the modification of ion-exchange membranes, which makes it possible to essentially increase their selectivity to singly charged ions. This paper presents a one-dimensional model, which is based on the Nernst–Planck–Poisson equations and describes the competitive transfer of singly and doubly charged ions through a multilayer composite ion-exchange membrane. It has been revealed for the first time that, as in the earlier studied case of a bilayer membrane, the dependence of the specific permselectivity coefficient (P1/2) of a multilayer membrane on the electrical current density passes through a maximum $$\left( {P_{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}}}^{{\max }}} \right).$$ It has been shown that an increase in the number of nanosized modification bilayers n leads to the growth of $$P_{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}}}^{{\max }},$$ but the flux of a preferably transferred ion decreases in this case. It has been established that $$P_{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}}}^{{\max }}$$ is attained at underlimiting current densities and relatively low potential drop. The simulated dependences $$P_{{{1 \mathord{\left/ {\vphantom {1 2}} \right. \kern-0em} 2}}}^{{\max }}$$(n) qualitatively agree with the known literature experimental and theoretical results.

Mathematical Modeling of Monovalent Permselectivity of a Bilayer Ion-Exchange Membrane as a Function of Current Density

Modification of an ion-exchange membrane with a thin layer, the charge of which is opposite to the charge of the substrate membrane, has proven to be an effective approach to obtaining a composite membrane with permselectivity towards monovalent ions. However, the mechanism of permselectivity is not clear enough. We report a 1D model based on the Nernst–Planck–Poisson equation system. Unlike other similar models, we introduce activity coefficients, which change when passing from one layer of the membrane to another. This makes it possible to accurately take into account the fact that the substrate membranes usually selectively sorb multiply charged counterions. We show that the main cause for the change in the permselectivity coefficient, P_1/2, with increasing current density, j, is the change in the membrane/solution layer, which controls the fluxes of the competing mono- and divalent ions. At low current densities, counterion fluxes are controlled by transfer through the substrate membrane, which causes selective divalent ion transfer. When the current increases, the kinetic control goes first to the modification layer (which leads to the predominant transfer of monovalent ions) and then, at currents close to the limiting current, to the depleted diffusion layer (which results in a complete loss of the permselectivity). Thus, the dependence P_1/2 − j passes through a maximum. An analytical solution is obtained for approximate assessment of the maximum value of P_1/2 and the corresponding fluxes of the competing ions. The maximum P_1/2 values, plotted as a function of the Na⁺ ion current density at which this maximum is reached, gives the theoretical trade-off curve between the membrane permselectivity and permeability of the bilayer monovalent selective ion-exchange membrane under consideration.

Subnanometer Ion Channel Anion Exchange Membranes Having a Rigid Benzimidazole Structure for Selective Anion Separation

Ion-conductive polymers having a well-defined phase-separated structure show the potential application of separating mono- and bivalent ion separation. In this work, three side-chain-type poly(arylene ether sulfone)-based anion exchange membranes (AEMs) have been fabricated to investigate the effect of the stiffness of the polymer backbone within AEMs on the Cl^-/NO₃^- and Cl^-/SO₄^2- separation performance. Our investigations via small-angle X-ray scattering (SAXS), positron annihilation, and differential scanning calorimetry (DSC) demonstrate that the as-prepared AEM with a rigid benzimidazole structure in the backbone bears subnanometer ion channels resulting from the arrangement of the rigid polymer backbone. In particular, SAXS results demonstrate that the rigid benzimidazole-containing AEM in the wet state has an ion cluster size of 0.548 nm, which is smaller than that of an AEM with alkyl segments in the backbone (0.760 nm). Thus, in the electrodialysis (ED) process, the former exhibits a superior capacity of separating Cl^-/SO₄^2- ions relative to latter. Nevertheless, the benzimidazole-containing AEM shows an inability to separate the Cl^-/NO₃^- ions, which is possibly due to the similar ion size of the two. The higher rotational energy barrier (4.3 × 10^-3 Hartree) of benzimidazole units and the smaller polymer matrix free-volume (0.636%) in the AEM significantly contribute to the construction of smaller ion channels. As a result, it is believed that the rigid benzimidazole structure of this kind is a benefit to the construction of stable subnanometer ion channels in the AEM that can selectively separate ions with different sizes.