Monovalent Selective Cation-Exchange Membranes for Electrodialysis Pretreatment of Brackish Water Reverse Osmosis

Brackish water reverse osmosis (BWRO) can supplement dwindling freshwater resources because of its inland availability and energy efficient treatment compared to seawater. However, water recovery is limited to 50-85% by mineral scaling from primarily divalent cation-based scalants, such as Ca²⁺ and Mg²⁺, leaving significant quantities of water unusable and in need of costly disposal. Thus, removing scale-forming divalent ions before RO is the key to increasing water recovery, but current approaches, such as solvent extraction or adding scale inhibitors, are thermally or chemically intensive, creating a need for an effective, additive-free scale-controlling strategy. Electrodialysis (ED) could be a thermal and chemical input free brackish water pre-treatment that can remove scale forming ions before the RO step by separating incoming brackish water, using valent selective cation exchange membranes (vsCEMs), into streams that are rich with monovalent or divalent cations. This valent selective ED could be used in a coupled ED-RO hybrid system to achieve high water recovery (&gt;90%) and would be tunable for use with a variety of BW feeds, thus mitigating scaling, improving zero liquid discharge (ZLD) management, reducing the total levelized cost of treatment and process downtime, and extending the overall system lifespan. Here, we focus on monovalent selective CEMs (msCEMs) which enable monovalent cation transport and reject divalent cations. There are a few commercially available msCEMs but they suffer from low monovalent-divalent selectivity. Previous work to improve msCEMs revolves around charge-based exclusion, which is typically achieved by including a thin coating of oppositely charged functional groups (having the same charge as the counter-ions) on the membrane surface, which more strongly repels divalent ions. However, a systematic study of the fabrication and operating conditions and their effect on membrane performance has not yet been performed, preventing them from further improvement. <br/>Here, we use a commercial CEM and two polyelectrolytes of opposite charge, poly(allylamine hydrochloride) (PAH) and poly(styrene sulfonate) (PSS), to deposit a thin selective layer using layer-by-layer assembly, whose facile fabrication and tunability allows for a cohesive study of structure-property-performance relationships within the same framework. Our results using a (PAH/PSS)_2.5 deposition on a commercial support show that we can make consistent, defect-free membranes with average Na⁺/Mg²⁺ selectivities of ~25, showing a noticeable increase over the support layer’s 0.5 Na⁺/Mg²⁺ selectivity. Moreover, we show that the limiting current of these membranes is highly dependent on the ionic strength of the diluate and concentrate solutions, regardless of the ionic species present, with limiting current increasing with solution ionic strength. We show that our membranes are significantly monovalent cation-selective, while systematically revealing the relationships between the separation performance and operating conditions including applied current density, diluate and concentrate solution composition, and thin film composition. This work would advance the fundamental understanding of ion transport in ion exchange membranes and offer critical guidance for applying valent-selective ion exchange membranes in water and wastewater treatment.