O. A. Demina

Comparative Study of Methods Used for the Determination of Electroconductivity of Ion-Exchange Membranes

The difference, differential-difference, and mercury-contact ac methods used for determining electroconductivity of ion-exchange membranes of various types in NaCl solutions are analyzed on the basis of the concentration dependence of their electroresistance. The sources of errors in the membrane electroresistivity measurements are analyzed. For each method, sodium chloride concentrations at which the error does not exceed 5% are determined.

Transport–Structural Parameters of Domestic and Foreign Anion-Exchange Membranes

The concentration dependences of the electroconductivity, diffusion penetrability, and transport numbers for water in anion-exchange membranes of domestic and foreign production are studied in identical conditions in NaCl solutions with the aid of a complex of standardized methods of conditioning the samples and measuring their properties. The dependences of the electroconductivity and the diffusion flux of the salt are used to calculate transport–structural parameters, which characterize conducting properties of membranes in the framework of a heterogeneous model. Electrotransport properties of anion-exchange membranes are compared and factors that affect them are discussed.

Ion and water transport during lithium chloride concentration from aqueous organic solutions by electrodialysis

The concentration of lithium chloride from aqueous organic solutions with different volume concentrations of N,N-dimethylacetamide (DMAA) was studied experimentally. An extended model of the limiting electrodialysis concentration of electrolytes from mixed media was developed. The model takes into account the osmotic and electroosmotic mechanisms of the transfer of water and organic solvent, as well as the electromigration and diffusion mechanisms of the salt transfer. Using the experimental data and the extended model, we evaluated the transport parameters of the MK-40/MA-40 membrane pair in aprotic solutions of variable compositions and studied how they changed with an increase in the volume fraction of DMAA in a mixed solution. The contribution of each of these mechanisms of ion and water transport to the electrodialysis concentration of electrolyte from aqueous and organic aqueous solutions was determined. Electroosmotic transfer was found to be the main mechanism of the solvent transfer that limits the stage of the electrodialysis concentration of the electrolyte from aqueous and organic aqueous solutions.

Method of Model Parameter Calculation of Ion-Exchange Resins

A method for determination of parameters of a three-wire model and Lichtenecker model on the basis of experimental data of conductivity measurement for ion-exchange resins and equilibrium solutions is developed and experimentally tested. This method allows determining the required parameters using experimental points available for measurements on the basis of the minimum root-mean-square deviation. Verification of the method using our own experimental data and data from the literature allowed establishing that the three-wire model in the case of a solid-phase resin is practically devoid of the “solution-only channel“ and can be considered as a two-wire model.

Effect of an aprotic solvent on the properties and structure of ion-exchange membranes

The effect of the aprotic solvent dimethylacetamide on the equilibrium and transport properties of heterogeneous (MK-40, MA-40, and MA-41) and homogeneous (MF-4SK) ion-exchange membranes is investigated. On the basis of concentration dependences of the conductivity and diffusion permeability of membranes, model calculations of transport-structural parameters that reflect the structural and kinetic characteristics of conducting phases of the swollen polymer are performed. The effect of the aprotic solvent on the flow of current through the structural fragments of the ion-exchange material is estimated. The causes of changes that are induced in the properties of the membranes by the aprotic solvent are ascertained.

The three-wire model and Lichtenecker’s equation for calculations of the conductivity of ion-exchange columns

The approaches to the evaluation of the conductivity of ion-exchange columns are analyzed. Specifically, the three-wire model and Lichtenecker’s power equation are examined. The problem of how the coefficient of ion-exchange column filling and the parameter characterizing the state of conducting phases with respect to the current flow are related to the parameters involved in the equations of the three-wire model is treated theoretically. The common points and the coincident portions of the functions obtained in each of the approaches and some discrepancies between the functions are found. The discrepancies are determined by the special features of the models under consideration.

Effect of polyaniline on the current passing through structural fragments of ion-exchange sulfonic-cationite resins and membranes

Concentration dependences of the specific electrical conductivity of ion-exchange resin KU-2 and ion-exchange heterogeneous membrane MK-40 made thereof are obtained and compared prior to and after the membrane modification with polyaniline. It is shown that the introducing of the polyaniline to polymer matrices of ionites results in the change of the specific electrical conductivity of both the resin KU-2 and the membrane MK-40, which correlates with the earlier data obtained for the MF-4SK/polyaniline composites. The quantitative evaluation of the polyaniline effect on the mechanism of current passing through structural fragments of initial and modified ionites is carried out. To this purpose, model parameters were used, which reflect the current paths and the conducting phases’ volume fractions and spatial orientation.

Prediction of the mass exchange characteristics of industrial electrodialyzer concentrators

A comparative study of the mass exchange characteristics of an EDC-Y laboratory cell and EDC-II/125 pilot module used in the processing of ammonium nitrate juice vapor was performed. Based on the data obtained on the laboratory cell it was shown that the use of the compartmentation method allows the prediction and scaling of the mass exchange characteristics of industrial electrodialyzer concentrators with hydraulically closed concentration chambers.

Electroosmotic transport of water and aprotic solvent in heterogeneous membranes

Electrosmotic water transport in MK-40, MA-40, and MA-41 electrodialysis membranes before and after their treatment by aprotic solvent solution and electroosmotic permeability of a MK-40/MA-40 membrane couple in water and N,N-dimethyl acetamide were studied for the first time. It is found that N,N-dimethyl acetamide produces practically no effect on the properties of the studied ion-exchange materials and its transport number through the MK-40/MA-40 membrane couple is independent both of the initial lithium chloride concentration in a desalting cell of the concentrator electrodyalizer and its volume fraction in the solution. The initial concentration of lithium chloride in the desalting cell does not affect the transport numbers of water in the studied membrane couple either. However, these depend on the volume fraction of the organic component in the solution and this dependence features an extremum. A method for estimation of electroosmotic permeability of the membrane couple in aqueous-organic solutions of electrolytes is suggested.

Diffusion of electrolytes of different natures through the cation-exchange membrane

Diffusion of different electrolytes through a negatively charged (cation-exchange) membrane into distilled water has been studied. It has been established theoretically (with no regard to the presence of diffusion layers) that the integral diffusion permeability coefficient of an electrolyte depends on the diffusion coefficients and the ratio between the charge numbers of a cation–anion pair, the ratio between the density of charges fixed in the membrane and electrolyte concentration, and the averaged coefficient of equilibrium distribution of cation−anion ion pairs in the membrane matrix. It has been found that, when co-ions have a higher mobility, the dependence of diffusion permeability on electrolyte concentration passes through a maximum. Derived equations have been compared with experimental dependences of the diffusion permeability of an MC-40 membrane with respect to different solutions of inorganic 1: 1 and 2: 1 electrolytes. The developed method has been shown to be applicable for describing diffusion of any electrolytes (including asymmetric ones) through arbitrary uniformly charged membranes.