Selective separation of lithium using concentration polarization near ion exchange membranes: A proof-of-concept...

The growing demand for lithium worldwide requires efficient and sustainable recovery strategies to reduce reliance on resource-intensive primary sources like conventional mining and brine extraction. Secondary sources, such as wastewater from spent lithium-ion batteries and industrial wastewater have emerged as viable alternatives for lithium extraction. This study investigated the concentration polarization (CP) near ion-exchange membranes (IEMs) for selective separation of Li⁺ over K⁺ using the difference in the diffusion coefficient. The smaller diffusion coefficient of Li⁺ (1.03 × 10^-9 m²/s) than that of K⁺ (1.96 × 10^-9 m²/s) allowed the selective Li⁺ separation in shock electrodialysis (SED). A lab-scale SED reactor was built with two cation-exchange membranes (CEMs) to evaluate the effect of the applied voltage, flow rate, and intermembrane distance on Li⁺ selectivity. A successful Li⁺ selection (up to a 40.8% increase in the Li⁺ concentration) was demonstrated in the effluent of the concentrating boundary layer while the effluent of the diluting boundary layer showed a depletion of Li⁺ (up to a 30.4% decrease in the Li⁺ concentration). Mathematical model simulations of a ternary system (Li⁺, K⁺, Cl^-) confirmed the selective Li⁺ separation in SED due to the diffusivity difference. In the model simulations, the Li⁺ selection was substantially affected by the selectivity of CEM, applied voltage and boundary layer thickness. These findings highlight the potential of SED as a scalable and energy-efficient approach for lithium recovery from secondary sources contributing to resource recycling in the circular economy.