The physicomechanical and transport characteristics of heterogeneous anion exchange membranes Ralex AMH-Pes and MA-41, and homogeneous anion exchange membranes AHT and MA-1 in solutions of sodium chloride and succinic acid are studied. It was found that in succinic acid solutions, the concentration dependence of the specific electrical conductivity of anion-exchange membranes has an atypical course compared with the dependence measured in solutions of strong electrolytes. This is due to a change in the pH and ionic composition inside the membrane compared to the external working solution. The concentration dependence of the integral coefficient of diffusion permeability of anion-exchange membranes in succinic acid solution has a decreasing character. This dependence can be explained by the pH shift inside the membrane to a more alkaline region when diluting the external solution, while the equilibrium of the succinic acid dissociation reaction inside the membrane shifts towards the formation of a two-charge succinate anion. With an increase in the proportion of double-charged ions in the membrane phase, an increase in electrostatic forces capable of attracting ions of the opposite charge sign occurs. At the same time, the concentration of co-ions in the membrane phase increases. This effect leads to an increase in the rate of diffusion transfer of the succinic acid molecule with a decrease in its concentration in the working solution.The study of the mass transfer of succinic acid through anion-exchange membranes and the volt-ampere characteristics (CVC) of the anion-exchange membranes AHT and MA-1 showed that succinic acid is capable of being intensively transferred through anion-exchange membranes in the super-limit current mode, as a result of the development of electroconvection at the ion-exchange membrane/diffusion layer interface.

The phenomenon of concentration polarization (CP) in membrane systems refers to the emergence of concentration gradients in solution near the membrane surface due to the selective transport of some solution components through the membrane under the effect of transmembrane driving forces. CP accompanies all types of membrane processes, changing transport conditions and reducing efficiency of separation processes: in most cases, the total transport rate decreases, the energy consumption increases, and the selectivity of the transport process is lost. This review addresses general regularities and specific features of the CP phenomenon in electrodialysis, reverse osmosis, nanofiltration, ultrafiltration, and pervaporation processes, as well as membrane sensing systems and fuel cells. Fundamentals of the CP phenomenon and experimental methods for its investigation are discussed.

At present, the nature of the limiting state of electromembrane systems in solutions of strong electrolytes (e.g., NaCl) is well known. The value of the limiting current in these systems with a rotating membrane disk (RMD) can be calculated quite accurately using the Levich equation. In cases where weak acids and/or their salts are present in the electromembrane system, this equation ceases to be satisfied and the electrodiffusion ion transport is complicated by proton-transfer chemical reactions between these acids, their anions, and water. In this work, the effect of these reactions on the limiting current density in a system with a rotating disk of a cation-exchange membrane and acetic acid has been experimentally studied. The results of voltammetry and their theoretical interpretation using the known mathematical model are presented. Conditions under which the mass transfer rate is limited by the diffusion delivery of acetic acid molecules to the membrane surface, as well as conditions under which the limiting stage is the reaction of their dissociation at the membrane/depleted solution interface, have been revealed.

The paper presents the results of studying the electrochemical characteristics and long-term stability of MA-41 membranes on the surface of which poly-N,N-diallylmorpholinium bromide was applied. The deposition of a polyelectrolyte on the membrane surface leads to an increase in the limiting current from 0.8 to 1.1 mA/cm2. The comparison of the experimental and theoretically calculated values of the limiting current density allows us to conclude that the modification of the membrane surface by poly-N,N-diallylmorpholinium bromide does not lead to the formation of a continuous polyelectrolyte film on the surface, but its fixation occurs due to the sorption of macromolecules on the surface of the ion-exchanger particles. To quantify the rate of the water dissociation reaction at the membrane/solution interface, the method of electrochemical impedance was used, which makes it possible to compare the rate constants of the water dissociation reaction for different membranes, assuming that the reaction is described by the Gericher impedance. It is shown that modification of the MA-41 membrane surface leads to a decrease in the rate of the water dissociation reaction in the current range i = 1.5–4ilim by a factor of 2–6. The reduction in water dissociation reaction rate is attributed to the substitution of catalytically active secondary and tertiary amino groups in the surface layer of the pristine membrane by stable heterocyclic ammonium bases of poly-N,N-diallylmorpholinium. The study of the long-term stability of the resulting membrane showed that when the membrane is polarized with a current equal to twice the limiting current, the desorption of the modifier occurs within 25 h, and the properties of the membrane become close to those of the unmodified MA-41 membrane. It was shown that the electrochemical impedance method can be used as a very sensitive method for studying the long-term stability of ion-exchange membranes.

Ion-exchange membranes with high specific selectivity to singly charged ions are in demand in various industries. One of the ways to increase the specific selectivity can be the formation on the membrane surface of a thin layer with a charge opposite to the charge of membrane fixed groups. The possibility of forming such a layer due to the specific interaction of calcium ions with the sulfonate groups of the membrane during treatment with a high-intensity electric current in a CaCl2 solution has been investigated. The ability of heterogeneous (MK-40, Ralex CMH) and homogeneous (CMX, CJMC-5) sulfonated cation-exchange membranes to specifically adsorb calcium ions on their surface has been studied. It has been shown that the CMX membrane exhibits this ability to the greatest extent, which is due to a higher density of -SO_3^ - groups on its surface compared to other studied membranes. It has been found that formation of a thin positively charged layer on the surface of the CMX membrane increases the membrane permselectivity coefficient P_Na^ + / . -0emCa^2 + by 69

In this study, the electrical conductivity of cation-exchange and anion-exchange membranes was studied in solutions containing both strong (sodium chloride and acetate) and weak (acetic, succinic and citric acids) electrolytes. The results obtained indicate that the concentration dependence of the electrical conductivity of membranes in weak electrolytes differs significantly from that observed for solutions of strong electrolytes. In the case of an acetic acid solution, the electrical conductivity of the membranes is higher than that of the equilibrium solution over the entire range of concentrations that were studied. It has been shown that existing models of the transport and structural organization of membranes allows describing the structural parameters of ion-exchange membranes in contact with strong electrolytes. In sodium chloride and sodium acetate solutions, the obtained dependences were processed within the framework of microheterogeneous and three-wire models to establish the influence of the nature of the electrolyte on the transport and structural characteristics of the membranes. The dependence of electrical conductivity on the concentration of a weak electrolyte solution does not allow the use of either microheterogeneous or extended three-wire models for the description of the structure-property relationship of ion-exchange materials. It has been shown that in acetic acid and partially succinic acid solutions, the solution provides the main contribution to the resistance of the electromembrane system. Based on the results obtained from measuring electrical conductivity, changes in the design of a laboratory electrodialyzer were proposed and experiments were carried out on desalting a solution of acetic acid. It has been shown that the use of thinner intermembrane separators in the desalting chamber leads to an increase in the integral current efficiency (from 0.32 to 0.44 at 0.6 A/dm2 and the same degree of desalination) and a reduction in specific energy consumption (from 3.0 to 1.9 kWh/mol at 0.6 A/dm2) during desalination of acetic acid. The results obtained can be further used to improve the parameters of the process of obtaining weak acids by bipolar electrodialysis.

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 ( P 1/2 ) of a multilayer membrane on the electrical current density passes through a maximum ( P_1 / . -0em 2^max). It has been shown that an increase in the number of nanosized modification bilayers n leads to the growth of P_1 / . -0em 2^max, but the flux of a preferably transferred ion decreases in this case. It has been established that P_1 / . -0em 2^max is attained at underlimiting current densities and relatively low potential drop. The simulated dependences P_1 / . -0em 2^max ( n ) qualitatively agree with the known literature experimental and theoretical results.

Electrodialysis are increasingly used in hybrid membrane technologies for the recovery and separation of weak polybasic acids from municipal and industrial wastewater and other solutions. It is known that the rate of generation of H+ and OH− ions at ion-exchange membrane boundaries during the treatment of such solutions is greater than that in the case of strong electrolytes, such as NaCl. One of the mechanisms of this generation, water splitting involving catalytic participation of functional groups, is relatively well understood. However, the second possible mechanism, where H+ ions are generated during acid dissociation at the depleted solution/anion-exchange membrane interface, is not sufficiently clear. Current-voltage characteristics (CVCs) of a Neosepta AMX anion-exchange membrane in 0.02 M NaH2PO4, NaH2Cit, KHT, and NaCl solutions are recorded. The effective transport number of H+ ion in the depleted boundary solution is measured to quantify the rate of the H+ ion generation. While the "water splitting" mechanism takes place in any kind of electrolytes, the "acid dissociation" mechanism is characteristic only for weak polybasic acids and other ampholytes, the electric charge of which depends on the pH. With that, the first mechanism occurs only at overlimiting currents, and the second occurs both in the underlimiting and overlimiting current modes. The dissociation of acid species at an AEM interface is caused by Donnan exclusion of protons as co-ions from the membrane. The rate of this reaction increases in the order KHT < NaH2PO4 2.6 ilimLev, there is a competition between both mechanisms of H+ ion generation. The obtained results give new insight into the understanding of the kinetics of H+/OH− ion generation and the impact of this process on the efficiency of electrodialysis of solutions containing polybasic acid species.

Membrane scaling and subsequent cleaning efficiency are major factors limiting commercial applications of electrodialysis. The generation of OH− ions during water splitting at the membrane interface promotes scaling formation. We report a modification of a commercial anion-exchange membrane, which improves its scaling resistance by reducing the water splitting rate. This is achieved by replacing the weakly basic amino groups on the surface of membrane with quaternary ones that do not have catalytic activity with respect to water splitting. We have carried out a comprehensive study of membranes modified by ex situ, and new membranes, for the first time modified in situ, directly inside the electrodialyzer. The in situ modified membranes were found to exhibit better properties in decreasing the water splitting rate, which, in turn, significantly reduces the scale formation on the surface facing the concentration chamber.

Anion-exchange membranes modified with a polyquaternium-22 (PQ-22) polymer were studied for their use in electrodialysis. The use of PQ-22 for modification makes it possible to “replace” weakly basic amino groups on the membrane surface with quaternary amino groups. It was found that the content of quaternary amino groups in PQ-22 is higher than the content of carboxyl groups, which is the reason for the effectiveness of this polymer even when modifying Ralex AHM-PES membranes that initially contain only quaternary amino groups. In the case of membranes containing weakly basic amino groups, the PQ-22 polymer modification efficiency is even higher. The surface charge of the modified MA-41P membrane increased, while the limiting current density on the current-voltage curves increased by more than 1.5 times and the plateau length decreased by 2.5 times. These and other characteristics indicate that the rate of water splitting decreased and the electroconvective mixing at the membrane surface intensified, which was confirmed by direct visualization of vortex structures. Increasing the surface charge of the commercial MA-41P anion-exchange membrane, reducing the rate of water splitting, and enhancing electroconvection leads to mitigated scaling on its surface during electrodialysis.

The results of the study of the oscillations in the chronopotentiograms during scaling on the surface of an MA-41P anion-exchange membrane facing the concentration chamber of an electrodialyzer during the processing of dilute stratal water similar in composition to natural brines incidentally produced in the gas and oil fields of the Angara–Lena basin are presented. If Mg2+ ions are present in the solution, porous Mg(OH)2 is predominantly formed on the surface of an MA-41P membrane despite the excess Ca2+ content and sufficient amount of $${\text{HCO}}_{3}^{ - }$$ in the solution. In the absence of Mg2+ ions in the solution, CaCO3 precipitates on the surface of the membrane. During scaling, an increase in the amplitude and period of the oscillation of the potential drop ∆φ from 5 mV and on average 5–10 s (due to the development of equilibrium electroconvection) up to 50 mV and 430 s, respectively, is recorded in the chronopotentiograms of MA-41P membrane. It is established that the amplitude and period of oscillations of the potential drop as well as the pH values of the concentrate can serve as a signal for taking actions to flush and regenerate ion-exchange membranes during the electrodialysis processing of dilute solutions of stratal water.

The article formulates a mathematical model of non-stationary transfer of 1:1 electrolyte in potentiodynamic mode, taking into account electroconvection and non-catalytic reaction of dissociation/recombination of water molecules in membrane systems, which are considered the channel of desalting of the electrodialysis apparatus. Using this model, the main regularities of the space charge distribution are theoretically established, the influence of the input parameters is determined: initial concentration, potential sweep rate, etc. It is shown that the desalination channel consists of narrow quasi-equilibrium regions of space charge adjacent to ion-exchange membranes; an electric double layer appears in the central part, caused by the recombination reaction. Regions of electroneutrality are located between this double electric layer and the near-membrane space charge regions. This study will further be used to analyze the combined effect of the noncatalytic reaction of dissociation of water molecules and recombination and electroconvection on the current-voltage characteristic of the desalting channel.

The work aims to study the competitive ions transfer through bilayer ion-exchange membranes, both in theory and in an experiment. The transport of ions was described within the framework of the Nernst-Planck model in a four-layer membrane system formed by a bilayer membrane and two diffusion layers. For the two cases, when the current is much smaller than the limiting one and for the limiting current, we obtained analytical solutions of the problem. In these two cases, one can calculate the ions separation coefficients, effective transport numbers and permselectivity coefficients using the thermodynamic and kinetic properties of the individual layers of a bilayer membrane. For an arbitrary current density, a numerical solution of the boundary value problem is obtained. The numerical solution allowed calculating the dependencies of the permselectivity coefficient on the current density for membranes with different thicknesses of the modifying layer. The theory was verified experimentally in an electromembrane system with a bilayer membrane in a ternary solution of CaCl2+NaCl electrolyte. Quantitatively and qualitatively, the causes and the mechanism of permselectivity coefficient reduction are analyzed. For bilayer membranes at currents close to the limiting one, it is determined by the diffusion coefficients of the separated counterions in solution and by the charge numbers of counterions and coions.

The broad possibilities of electrochemical impedance spectroscopy for assessing the capacitance of interphase boundaries; the resistance and thickness of the foulant layer were shown by the example of AMX-Sb membrane contacted with red wine from one side and 0.02 M sodium chloride solution from the other side. This enabled us to determine to what extent foulants affect the electrical resistance of ion-exchange membranes, the ohmic resistance and the thickness of diffusion layers, the intensity of water splitting, and the electroconvection in under- and over-limiting current modes. It was established that short-term (10 h) contact of the AMX-Sb membrane with wine reduces the water-splitting due to the screening of fixed groups on the membrane surface by wine components. On the contrary, biofouling, which develops upon a longer membrane operation, enhances water splitting, due to the formation of a bipolar structure on the AMX-Sb surface. This bipolar structure is composed of a positively charged surface of anion-exchange membrane and negatively charged outer membranes of microorganisms. Using optical microscopy and microbiological analysis, it was found that more intense biofouling is observed on the AMX-Sb surface, that has not been in contacted with wine.

The geochemical regional specialization of the behavior of rare earth elements (REE) in the soils of Sochi, manifested in the predominance of REE of the middle group, has been established. The enrichment by the middle group of REE is inherited from the rocks (mainly clayey mudstones) of the territory. At the same time, the features of the distribution of REE, characteristic of natural soils, are not violated for soils formed under the conditions of technogenesis. This indicates that the pollution of urban soils is determined by a natural and technogenic factor. The sources of pollution are man-made sites and talus, which lack a soil-vegetation layer, including those recently formed during the preparation for the 2014 Winter Olympic Games and composed of the same material as the parent rocks, but significantly crushed and, accordingly, subject to active weathering. In mosses of the urban zone, in the eluvial landscape, the predominance of the middle group of REE is not observed. However, as we move down the slope, the REE distribution curve relative to the North American shale standardization becomes similar to the similar graph for soils and rocks. The relationship between the accumulation of elements in mosses, and in soils and underlying rocks is revealed when considering the features of fractionation of REE in mosses of the super-aquatic landscape.

The paper shows the possibility of using a microheterogeneous model to estimate the transport numbers of counterions through ion-exchange membranes. It is possible to calculate the open-circuit potential and power density of the reverse electrodialyzer using the data obtained. Eight samples of heterogeneous ion-exchange membranes were studied, two samples for each of the following types of membranes: Ralex CM, Ralex AMH, MK-40, and MA-41. Samples in each pair differed in the year of production and storage conditions. In the work, these samples were named "batch 1" and "batch 2". According to the microheterogeneous model, to calculate the transport numbers of counterions, it is necessary to use the concentration dependence of the electrical conductivity and diffusion permeability. The electrolyte used was a sodium chloride solution with a concentration range corresponding to the conditional composition of river water and the salinity of the Black Sea. During the research, it was found that samples of Ralex membranes of different batches have similar characteristics over the entire range of investigated concentrations. The calculated values of the transfer numbers for membranes of different batches differ insignificantly: ±0.01 for Ralex AMH in 1 M NaCl. For MK-40 and MA-41 membranes, a significant scatter of characteristics was found, especially in concentrated solutions. As a result, in 1 M NaCl, the transport numbers differ by ±0.05 for MK-40 and ±0.1 for MA-41. The value of the open circuit potential for the Ralex membrane pair showed that the experimental values of the potential are slightly lower than the theoretical ones. At the same time, the maximum calculated power density is higher than the experimental values. The maximum power density achieved in the experiment on reverse electrodialysis was 0.22 W/m2, which is in good agreement with the known literature data for heterogeneous membranes. The discrepancy between the experimental and theoretical data may be the difference in the characteristics of the membranes used in the reverse electrodialysis process from the tested samples and does not consider the shadow effect of the spacer in the channels of the electrodialyzer.

Modification of commercial ion-exchange membranes is the easiest and cheapest way to improve their properties. It has been shown that the treatment of the surface of the MA-41P membrane with a bifunctional polymer (copolymer based on polydiallyldimethylammonium chloride) not only reduces the generation of H + and OH – ions but also contributes to an increase in the limiting current density and a reduction in the length of the current-voltage curve plateau compared to the initial membrane. These changes are signs of increased electroconvection at the surface of the modified membrane (MA-41PM). It has been found that the modified membrane exhibits stability for at least 100 h of operation in an overlimiting current mode. In an electric field, a partial separation of the modifier from the MA-41PM surface takes place as a result of which the rate of generation of H + /OH – ions gradually increases.

The specific features of the sorption activity of silver nanoparticles (AgNPs) on biodegradable polymers of natural (collagen) and artificial (polyamide 6.6) origin have been established. The ability has been demonstrated for resorbable fibers of natural origin to more actively adsorb AgNPs with sizes from 1 to 10 nm during the first hour and stronger hold them on the surface during the first day of exposure to argogel. During the first hour of incubation of these polymers in a gel composition containing AgNPs obtained by the method of cavitation-diffusion photochemical reduction, their higher sorption activity was revealed in the size range from 1 to 10 nm in relation to the polyfilament synthetic material. After 24 h of exposure, a significant increase in the fraction of small AgNPs already on collagen fibers was observed, which was also accompanied by a significantly lower (19 times) content of AgNPs with a diameter above 40 nm on the catgut.