Integrated sulfonated poly ether ketone membrane capacitive deionization for lithium recovery from diluted binary solutions

Abstract

To harness the abundant lithium (Li)-containing aqueous resources and meet the growing demand for Li, membrane-based processes for selective Li recovery have garnered extensive attention. The incorporation of sulfonate groups into membranes is a crucial strategy for fine-tuning ion selectivity, which has been widely used in nanofiltration and electrodialysis for Li separation; however, its potential in the membrane capacitive deionization (MCDI) process for Li recovery remains largely unexplored. This study bridges this research gap by integrating activated carbon (AC) electrodes of capacitive deionization (CDI) with sulfonate poly ether ketone (SPEK) membranes, featuring varying degrees of sulfonation (SPEK-20, SPEK-40, and SPEK-60), and evaluating their Li recovery performance using Li/M (M representing competing cations such as Na, K, Mg, Ca) binary solutions. The synthesized membrane electrodes underwent thorough characterization, confirming the successful formation of sulfonate groups in the SPEK membrane, revealing the SPEK layer with a cross-section thickness below 10 µm, and identifying the hydrophilicity of the AC/SPEK electrode surface. The MCDI results unveiled the significant impact of incorporating SPEK membranes on Li recovery performance in monovalent-cation competing feeds compared to bare AC electrodes, boosting the Li removal rate and Li selectivity in Li/K feed by over 210 % and 120 %, respectively. Meanwhile, SPEK membranes demonstrate a preferable affinity for capturing and releasing divalent ions, with the variations in sulfonation degree exerting a greater influence on cases involving divalent cations.