15-crown-5 functionalized layered double hydroxide enabling thin-film nanocomposite membranes for enhanced reverse osmosis desalination and boron removal

Abstract

The escalating global water crisis has intensified the need for advanced and robust desalination technologies. In this study, we report the fabrication of a novel thin-film nanocomposite (TFN) reverse osmosis (RO) membrane by incorporating a specifically engineered nanofiller into the polyamide selective layer. The synergistic integration of 15-crown-5 (CE15) and layered double hydroxide (LDH) markedly enhances membrane hydrophilicity and creates efficient water transport pathways. This results in an exceptional water permeance of 5.36 LMH bar⁻¹ (190 % higher compared to conventional TFC membranes), while maintaining a high NaCl rejection of 99.1 % at 20 bar against brackish water. Moreover, the optimal membrane achieves a boron rejection rate of 84.5 %, outperforming most of reported TFC membranes. Mechanistic investigations, supported by both characterizations and theoretical calculations, reveal that the incorporation of CE15 not only increases the content of ether and carboxyl groups but also reduces the crosslinking density of PA, leading to a thinner and more permeable selective layer. Furthermore, the incorporation of Li⁺ and Na⁺ ions significantly enhance both the performance and long-term stability of the membranes. Specifically, CL.2Na membrane achieves an optimal water permeance of 6.08 LMH bar⁻¹, with an exceptional NaCl rejection of 99.3 % and boron removal rate of 85.7 %. In addition, CL.2Na membrane demonstrates an outstanding stability for long-term RO test, maintaining a high water permeance of 5.84 LMH bar⁻¹ after 72 h of continuous operation. These results confirm that the addition of Li⁺ and Na ⁺ could improve membrane separation performance for prolonged use. This work highlights the potential of crown ether–LDHs synergy, particularly in conjunction with alkali metal ions, for the molecular design of next-generation RO membranes with superior desalination and boron removal capabilities.