Electrodialytic concentration of ammonium-containing liquid products of biochemical processing of municipal, industrial, and livestock wastewater is a promising method for obtaining cheap liquid fertilizers for agriculture. At the same time, it is known that electrodialysis of NH4Cl solutions fails to achieve the same high brine concentrations as in the case of other chlorides, for example, KCl. We show that the reason is high diffusion permeability of anion-exchange membranes (AEMs) to NH4Cl, which is due to the protonation–deprotonation reactions of ammonium coions during their transfer from an external solution to an internal AEM solution and vice versa. For the first time, a mathematical model of NH4Cl diffusion through AEM was proposed with allowance for these reactions. The experimental values of the diffusion permeability of an anion-exchange membrane AMX and a cation-exchange membrane CMX in NH4Cl and KCl solutions have been compared. between The results of calculating the diffusion permeability of the membrane AMX in NH4Cl solutions are in qualitative agreement with the experiment data.

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 BMac 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- /Ac? = 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- /Ac? = 61.4. Simultaneously, in the underlimiting current mode, the bilayer membrane exhibits high selectivity to acetate ions, and the specific permselectivity coefficient is PCl- /Ac? = 0.66, which suggests the appearance of specific selectivity between singly charged ions during electrodialysis.

Generation of H+ and OH- ions at anion-exchange membranes is a phenomenon, which substantially reduces the performance of a number of electromembrane processes, such as the wastewater treatment, fuel cell energy production and others. We show that the mechanisms of this phenomenon are different, when a Neosepta AMX anion-exchange membrane is bathed in a strong electrolyte solution (NaCl and KCl) or in an ampholyte solution (NaH2PO4, KH2PO4, KHT and KH(2)Cit). The H+ and OH- ions' generation in the case of strong electrolytes requires a voltage threshold (about 0.3 V of the corrected voltage not including ohmic contribution), which relates to a current close to the limiting current. The process occurs mainly through proton transfer reactions involving the membrane functional groups. In the case of ampholytes, this generation takes place without a threshold; this process is separated in space: the generation of H+ occurs at the depleted solution/membrane interface, that of OH- ions, at the membrane boundary with the enriched solution. Electrochemical impedance spectra (EIS) are used for the determination of the effective rate constants for the H+ and OH- ions' generation. Their values are compared with the rate constants calculated from the acid dissociation constants (K-a).

Long-term (over 20 h) operation of the AMX-Sb membrane in the electrodialysis desalination of 0.02 M NaCl solution in overlimiting current regimes can lead to an increase in experimentally determined limiting current by more than 30% compared to the pristine membrane. This growth is caused by electrochemical degradation of the ion-exchange material at the AMX-Sb/solution interface, which leads to (1) a decrease in hydrophilicity of the membrane surface and (2) the formation of membrane cavities with linear dimensions of 2–3 μm. Both of these effects stimulate electroconvection, which develops in underlimiting current regimes via the mechanism of electroosmosis of the first kind. The resulting microvortex structures deliver a more concentrated solution to the AMX-Sb surface, shifting the limiting state and the onset of the intense generation of H+ and OH− ions to higher currents. The study has been carried out using the techniques of voltammetry and impedance spectroscopy, as well as contact angle measurements and optical visualization of the membrane surface.

It is known that some components of the bathing solution can enhance water splitting at depleted solution/ion-exchange membrane interface via protonation/deprotonation reactions with water. In this paper, we show that not only the presence of such components is important, but also a mechanism ensuring their sufficiently high concentration near the membrane surface. A comparative study of the electrochemical behavior of a Neosepta (R) homogeneous cation-exchange CMX and an anion-exchange AMX membranes (Astom, Japan) in 0.02M KCl or 0.02M NH4Cl solutions is carried out. The NH4+/NH3 couple is an effective catalyst of water splitting. The deprotonation reaction rate constant of the NH4+ ions, which limits the rate of water splitting, is about 10 s(-1). It is almost 6 orders of magnitude greater than the rate constant for direct water dissociation in free solution. A comprehensive electrochemical characterization of the membrane systems is made: voltammetry, chronopotentiometry, electrochemical impedancemetry, and pH-metry (including color indication of the pH of the membrane internal solution). It is found that the water splitting rate at the interface of the AMX membrane in the NH4Cl solution is essentially higher than in the KCl solution. The difference in the rates of this reaction in KCl and NH4Cl solutions at the CMX membrane is insignificant. The reason for the weak effect of the NH4+/NH3 couple on the rate of water splitting at the CMX membrane is that the concentration of both NH4+ and NH3 species is very low near its surface if the current density is close to or higher than its limiting value. As for the AMX membrane, we show that the ammonia-containing species can be transported to its depleted surface from the enriched solution by back "facilitated" diffusion through the membrane and this transport plays a crucial role in the occurrence of the rapid water splitting. (C) 2019 Elsevier Ltd. All rights reserved.

Current-voltage characteristics (CVCs) of a Neosepta AMX membrane are studied in NaH2PO4 (pH = 4.7, C = 0.02 mol/L) and KC4H5O6 (pH = 3.6, C = 0.02 mol/L) solutions. It is shown that in the case of NaH2PO4 there are two plateaus in the CVC, which correspond to achievement of the first, ilimI, and second, ilimII, limiting currents. ilimI occurs when the NaH2PO4 salt diffusion to the membrane surface is saturated, ilimII refers to the saturation of the proton flux when the membrane is almost completely converted into the HPO42− form. In the case of potassium hydrotartrate, there is no state corresponding to ilimI. The difference in CVC for two ampholytes is due to the difference in the ratio between the dissociation constants related to the first (Ka1) and second (Ka2) steps of ampholyte dissociation. In the case of NaH2PO4, where pKa1 and pKa2 differ greatly, a solution of this salt contains nearly exclusively the singly charged phosphate form. However, a KC4H5O6 solution contains, together with the C4H5O6− anion, about 15% of the doubly charged tartrates, which take part in charge transfer. Approximate analytical expressions are obtained for ilimI and ilimII in the case of monosodium hydrophospate solution. Their application gives the ilimI and ilimII values, which are in good agreement with the experiment and with numerical calculations using a mathematical model based on the Nernst-Planck equation, the electroneutrality condition, and the local chemical equilibrium condition in the membrane and two adjacent diffusion boundary layers.

The features of gravitational convection in electromembrane systems with electrolytes whose ions exhibit amphoteric properties have been studied using voltammetry techniques in the vertical and horizontal positions of a membrane stack. It has been shown that gravitational convection has approximately the same effect on these characteristics as in the case of solutions that do not exhibit amphoteric properties (sodium chloride). The main difference in the behavior of systems containing amphoteric compounds is the influence of gravitational convection on the values of the second limiting current, which is not observed in systems containing ions that are not capable of protonation/deprotonation reactions.