High Selectivities among Monovalent Cations in Dialysis through Cation-Exchange Membranes Coated with Polyelectrolyte...

Cation-exchange membranes allow preferential passage of cations over anions, but they show minimal selectivity among cations, which limits their use in ion separations. Recent studies show that modification of cation-exchange membranes with polyelectrolyte multilayers leads to exceptional monovalent/divalent cation electrodialysis selectivities, but no studies report high selectivity among monovalent ions. This work demonstrates that adsorption of protonated poly(allylamine) (PAH)/poly(4-styrenesulfonate) (PSS) multilayers on Nafion membranes leads to high K⁺/Li⁺ selectivities in Donnan dialysis, where K⁺ and Li⁺ ions in a source phase pass through the membrane and exchange with Na⁺ ions in a receiving phase. Addition of 0.01 M HNO₃ to a source phase containing 0.01 M KNO₃ and 0.01 M LiNO₃ increases the K⁺/Li⁺ selectivity from 8 to ∼60 through (PAH/PSS)₅PAH-coated Nafion membranes, primarily because of a ≥fivefold increase in K⁺ flux. These selectivities are much larger than the ratio of 1.9 for the aqueous diffusion coefficients of K⁺ and Li⁺, and uncoated Nafion membranes give a K⁺/Li⁺ selectivity <3. Bi-ionic transmembrane potential measurements at neutral pH confirm that the membrane is more permeable to K⁺ than Li⁺, but this selectivity is less than in Donnan dialysis with acidic solutions. In situ ellipsometry data indicate that PAH/PSS multilayers (assembled at pH 2.3, 7.5, or 9.3) swell at pH 2.0, and this swelling may open cation-exchange sites that preferentially bind K⁺ to enable highly selective transport. The coated membranes also exhibit modest selectivity for K⁺ over H⁺, suggesting selective transport based on preferential partitioning of K⁺ into the coatings. Selectivity declines when increasing the source-phase KNO₃ concentration to 0.1 M, perhaps because the discriminating transport pathway saturates. Moreover, selectivities are lower in electrodialysis than in Donnan dialysis, presumably because electrodialysis engages other transport mechanisms, such as electroosmosis and strong electromigration.