Scaling behavior of cation exchange membrane induced by Ca2+ during the electrodialysis for enriching lithium-containing solutions

Membrane fouling, particularly Ca2+-induced scaling on cation exchange membranes (CEM), hinders the application of electrodialysis in lithium extraction. This study investigated the fouling mechanisms of Ca2+ coexisting with Cl- and SO42- on sulfonated CEM, using CaCl2 and CaSO4 solution as the targeted foulants. The conductivity, pH, turbidity, particle size distribution, chemical composition and morphology of the membrane surface were monitored to verify the evolution of the bulk solution and the membrane during the Ca2+-induced fouling. Online detection of conductivity and pH revealed distinct trends in fouling solutions containing Ca2+-Cl- and Ca2+-SO42-. Noticeable changes in turbidity and particle distribution were observed in the concentrated chamber of the highly saturated Ca2+-SO42- solution, whereas the concentrated chamber of the Ca2+-Cl- solution showed turbidity and particle size distribution similar to the initial values. SEM showed that there are different morphologies of scaling crystals in the two types of solution. These fundamental differences are the root cause of the distinct fouling mechanisms between the Ca2+-Cl- solution and Ca2+-SO42- solution. To further investigate the fouling mechanisms, the electrochemical impedance spectroscopy (EIS) was employed to differentiate the impedance of the electric double layer and diffusion layer, providing insights into the characteristics of the CEM-solution interfaces fouled by different lithium-containing solutions. Additionally, adsorption experiments, quartz crystal microbalance with dissipation (QCM-D) and computational simulations (COMSOL, DFT) were conducted to provide detailed insights into the effects of fouling solutions on the properties of the desalted solution, the probability of crystal precipitation, and particularly the interaction between Ca2+ and the functional sites on the CEM. Finally, the Ca2+-induced scaling mechanism of CEM for enriching the lithium-containing solutions with and without SO42– was proposed, elucidating the synergistic effects of Ca2+-induced crystallization in the bulk solution and Ca2+ adsorption on the CEM surface. The research results could offer theoretical guidance for developing pollution control strategies.