Sustainable K⁺/Na⁺ monovalent-selective membranes with hot-pressed PSS-PVA saloplastics

Monovalent selective cation exchange membranes could play an important role in balancing the K⁺/Na⁺ ratio in agricultural feed streams to prevent the toxic effects of excess Na⁺ in the plant and soil systems, especially in greenhouses and dry areas. A polyelectrolyte complex of polystyrenesulfonate and polyvinylamine in the monomer ratio 1:2.5 is hot-pressed to form a dense saloplastic. The plastic takes up 42% w/w water when equilibrated, while ion-exchange capacity measurements show that it is negatively charged with a net ion-exchange capacity of 1.1 ± 0.4. Resistance measurements show a very promising preferred conductivity for K⁺ over Na⁺. This was confirmed by measuring K⁺ and Na⁺ transport through the membrane under diffusive conditions from an aqueous mixture of KCl and NaCl. Commercial membranes show resistance-based selectivities of 1.32 ± 0.1 to 1.19 ± 0.1, and diffusion based selectivities of 0.99 ± 0.1 to 0.78 ± 0.1. In contrast, the selectivities for the newly developed saloplastic membrane were 1.80 ± 0.33 for the resistance-based selectivity while the diffusion-based selectivity was 1.91 ± 0.1. The procedure is green as toxic solvents and/or halogenating agents, typically used to make cation exchange membranes, are not needed. This work thus highlights how monovalent selective membranes with a relevant K⁺/Na⁺ selectivity can be prepared by a simple and sustainable hot-pressing approach.