Study of the Specific Adsorption of Calcium Ions on the Surface of Heterogeneous and Homogeneous Cation-Exchange Membranes to Increase Their Selectivity to Singly Charged Ions

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 \(-{\text{SO}}_{3}^{ - }\) 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 \({{P}_{{{{{\text{N}}{{{\text{a}}}^{{\text{ + }}}}} \mathord{\left/ {\vphantom {{{\text{N}}{{{\text{a}}}^{{\text{ + }}}}} {{\text{C}}{{{\text{a}}}^{{{\text{2 + }}}}}}}} \right. \kern-0em} {{\text{C}}{{{\text{a}}}^{{{\text{2 + }}}}}}}}}}\) by 69%. Moreover, the presence of such a layer does not lead to an increase in undesirable water splitting, which occurs when widely used polyelectrolytes with amino groups are applied as modifiers.