ZiF-8 induced carbon electrodes for selective lithium recovery from aqueous feed water by employing capacitive deionization system

Abstract

The demand for lithium (Li) will grow from about 500,000 metric tons of lithium carbonate equivalent in 2021 to 3–4 million metric tons in 2030. To meet the Li demand, the separation of Li-mixed monovalent and divalent cations is critical for Li extraction from an aqueous medium. Capacitive deionization (CDI) and membrane capacitive deionization (MCDI) have recently emerged as viable water treatment technologies, yet ion-specific selective recovery using CDI systems is still under-investigated. In this study, the electrode surface of each system was modified to improve Li+ selectivity. Metal-organic frameworks (MOF), particularly zeolitic imidazolate framework-8 (ZiF-8), have shown substantial promise due to their tunable pore size and pore channel chemistry. Through an aqueous medium-based surface modification, we offer a simple technique of synthesizing ZiF-8 on carbon electrodes and underneath the cation exchange membrane (CEM). The bare CDI and MCDI systems initially showed poor selectivity towards Li+ in the mono and divalent ion incorporated simulated solutions. The relative selectivity (ρMLi; (M = metal ions)) in the CDI system was estimated as 0.73, 0.43, 0.67, and 0.58 for Na+, K+, Mg2+, and Ca2+, respectively, which was 0.93, 0.97, 0.39, and 0.30 in the MCDI system. In the case of bare activated carbon (AC) electrodes, the difference of hydration enthalpy played a critical role in Li+ selectivity towards other monovalent ions. However, despite having high hydration enthalpy, the Mg2+ and Ca2+ showed low Li+ selectivity due to the superior charge density of divalent ions. On the other hand, after the modification of AC electrodes with in-situ growth of ZiF-8 on the surface, the Li+ selectivity for monovalent Na+ and K+ was estimated at 3.08 and 1.12, respectively, which is 4.2 and 2.6 times higher than the bare AC electrode, respectively. Besides, compared to Na+, the trade-off between the low dehydration energy of K+ and the rapid ion transit of dehydrated Li+ made separating challenging. Consequently, for divalent Mg2+ and Ca2+, coulombic attraction dominated both in the bare CDI and MCDI systems. This research sheds light on using the newly developed ZiF-8 coating for selective Li recovery.