Selective Limiting Concentration of Electrolyte Solutions with Singly and Doubly Charged Cations

The effect of the anion-exchange layer of the copolymer N,N-diallyl-N,N-dimethylammonium chloride and ethyl methacrylate on the electrochemical properties of a homogeneous cation-exchange membrane based on perfluorosulfonic polymer has been studied. The application of a modifying layer with a thickness of 5 µm onto a 215 µm-thick membrane leads to a reduction in electrical conductivity by no more than 35%, while the diffusion permeability decreases more than fivefold and becomes independent of concentration. During testing of both cation-exchange and bilayer membranes in the process of limiting concentration of sodium chloride solution, comparable degrees of concentration have been achieved. The effectiveness of the bilayer membrane for selective limiting electrodialysis concentration has been demonstrated. When concentrating a solution containing sodium and calcium chlorides, the specific selective permeability coefficient P(Na⁺/Ca²⁺) for the cation-exchange membrane has ranged from 0.5 to 1.2. The use of the bilayer membrane significantly increase the specific selective permeability coefficient to 1.5–2.7, depending on the current density, allowing for efficient separation of electrolytes containing singly and doubly charged cations.