Polyelectrolyte nanofiltration membranes for base separation and recovery

Nanofiltration (NF) membranes have the potential to significantly advance resource recovery efforts where monovalent/divalent ion separation is critical, but their utilization is limited by inadequate stability under extreme conditions. "Base separation"-i.e., separating hydroxide from other ions-has emerged as an essential approach in resource recovery, enabling the extraction of multivalent anions (e.g., carbonates and phosphates) from hydroxide-rich streams. There is a particularly high demand for membranes capable of separating carbonates from hydroxide-rich CO₂ capture solvents and phosphates from hydroxide-rich adsorbent regeneration solvents. However, conventional polyamide NF membranes degrade during long-term exposure to alkaline conditions, limiting their application in extreme conditions. In this study, alkaline-resistant polyelectrolyte membranes are fabricated by depositing alternating layers of polycation, poly(diallyl dimethylammonium chloride) (PDADMAC), and polyanion, poly(sodium 4-styrenesulfonate) (PSS) to a polyethersulfone substrate. The membranes are tested for hydroxide/carbonate and hydroxide/phosphate separation performance, as well as performance stability during prolonged exposure to highly alkaline conditions. Results indicate that higher feed solution pH improves carbonate and phosphate rejection by promoting ion deprotonation and strengthening electrostatic repulsion from the negatively charged membrane. In contrast, increasing carbonate and phosphate concentrations in the feed solution reduces the rejection due to charge screening. The six-bilayer PDADMAC/PSS membrane removes more than 99 % of carbonates and phosphates while allowing extensive passage of hydroxide at pH 13. Stability tests confirm that PDADMAC/PSS membranes maintain excellent ion selectivity over four weeks of exposure to pH 13 KOH, whereas commercial polyamide NF membranes degrade within one week. These findings highlight the potential for PDADMAC/PSS membranes to advance critical resource recovery efforts, providing a durable and effective solution for applications under extreme conditions.