Ion transport in organic and inorganic membranes-2021: Conference Proceedings, Sochi, 20–25 September 2021. , 2021, С. 163-165.

Journal of Membrane Science: joyrnal. V. 601, 2020, P. 117920. 10.1016/j.memsci.2020.117920.

Diffusion dialysis (DD) is an environmentally appropriate method for separating components, which are unstable under the external influences (high temperature, high pressure, presence of electric field). The roadblock to the wider application of the DD method for separation and purification of amino acids is relatively low rate and selectivity of diffusion transport of these substances through ion-exchange membranes. In this paper, mechanisms of amino acid selective transport and possibilities to increase its flux across a cation-exchange membrane are considered. In particular, the effect of replacement of a flat membrane with a profiled one, prepared by the method of hot pressing, when using the same material is examined. Experimental and theoretical study is carried out using solutions of phenylalanine amino acid and NaCl when applying a commercial flat MK-40 cation-exchange membrane and its modification MK-40pr with profiled surface. It is found that the relatively high selective transport of phenylalanine is due to its facilitated diffusion, while the NaCl diffusion is reduced by the Donnan effect. The MK-40pr membrane allows a 8-fold increase in phenylalanine flux in comparison with the MK-40 membrane. This increase in partly due to the increased surface available for mass transfer (in 2.3 times), but also to better hydrodynamics reducing the diffusion layer thickness and to a higher fraction of conductive surface and higher pore radius. The latter is caused by water evaporation within the membrane pores in the course of its profiling.

Membranes: journal. V. 9. - № 12, 2019, Номер статьи 171. 10.3390/membranes9120171.

Membranes: journal. № 9 (9)., 2019, № 114. 10.3390/membranes9090114.

Ion transport in organic and inorganic membranes: material International conference, Sochi, 20-25 May 2019 , 2019, С. 183-185.

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 boundary 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 δLev 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 δLev 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 electroconvection. Homogeneity and high hydrophilicity of the surface of the AMX membrane determine its behavior, which is close to the ideal.

A non-steady state theoretical model is developed using the Nernst-Planck equations in order to study ion transport kinetics through Ion-Exchange Membranes (IEMs) during water desalination by Neutralization Dialysis (ND) in batch mode. The ND cell under study involves three compartments (acid, saline, and alkali) separated by two membranes (a cation-exchange and an anion-exchange ones) assumed ideally permselective and homogeneous. The presence of Diffusion Boundary Layers (DBLs) at the membrane-solution interface is taken into account in the saline compartment. The results of numerical simulation are compared with known experimental data. A good agreement is obtained between experimental and theoretical values representing the pH and the conductivity of the saline circulating solution as functions of time. These experimental results are also compared with the calculations made using the quasi-steady state model developed by Denisov et al. [10]. It is shown that the quasi-steady state approach is not applicable at the beginning of the ND process, during a few tens of minutes, where the concentration profiles in the membranes are far from linear. Within this stage, a few pH fluctuations are possible, while only one or two pH fluctuations occurs in the quasi-steady stage. The mechanism of these fluctuations, which are determined by the periodical change of the “leadership” between the cationexchange and the anion-exchange membranes, delivering the H+ and the OH– ions into the saline solution, respectively, is discussed.

Usually, the current flowing through an electrochemical cell is divided into the faradaic current going to an electrochemical interface reaction, and the current charging electric double layer (EDL). This division leads to the Randles–Ershler equivalent circuit with an EDL capacitance in one branch, and the faradaic impedance in the other, specific for each particular system. However, the physics of the separation of the impedance into faradaic and capacitive components for different electrochemical systems is not sufficiently clear. The most of derivations resulting in the formal construction of the Randles–Ershler or similar equivalent circuits are based on the a priori separation of the electroneutral and the double-layer regions. In this paper, we derive an equation for the impedance of a three-layer system consisted of an ion-exchange membrane and two adjoining diffusion boundary layers (DBL) starting from the Poisson equation. The system is polarized by a constant electric current over which a small sinusoidal signal is applied. The equation shows that the impedance of the considered system can be formally interpreted via an equivalent circuit with a frequency dependent capacitance in one branch and a finite-length Warburgtype impedance in the other. To take into account this dependence, the impedance of the system may be presented as a series connection of five circuits. Three of them are consisted of a geometric capacitance connected in parallel with an ohmic resistance, respectively, for both diffusion layers and for the membrane bulk; the two others being the double-layer capacitance in parallel with the finite-length Warburg impedance for the left and the right interfaces, respectively. The comparison of the impedance spectra calculated within our analytical approach with those obtained by the full numerical solution of the Nernst–Planck–Poisson (NPP) equations shows a good agreement. Different possible situations, which might arise in real systems (different stationary current densities, different thicknesses of the left-hand and right-hand DBLs), are analysed when applying the approximate solution.