Evaluation of polymer inclusion membrane efficiency in selective separation of lithium ion from aqueous solution

Abstract

In regards to the unique properties of lithium, lithium separation with high purity is crucial in various industries. Due to the close radii of alkali and alkaline earth metals, this is important to choose a technique with appropriate properties for ion separation in aqueous medium. Among the available methods for lithium enrichment, polymer inclusion membrane (PIM) system has features that meet the needs. In this study, a PIM containing green polyol as the base polymer, 12-Crown-4 (12C4) as the carrier and 1-butyl-3-methylimidazolium chloride (BMIMCl) ionic liquid was synthesized to separate lithium ion from an aqueous solution of Li+, Na+, K+, Ca2+, and Mg2+. Also, the zeolitic imidazolate framework type ZIF-8 was employed in the membrane structure to improve Li+ enrichment process. Characterization techniques such as FTIR, AFM and SEM, verified that ZIF-8 was added to the PIM structure. The results showed that ZIF-8 nanoparticles increased the hydrophobicity and the roughness of PIM that resulted in more selectivity. In addition, this type of MOF has regular porosity that is a suitable pathway for ion transport across the membrane. This feature increases the ion flux of synthesized PIMs. In the optimum combination of membranes, lithium ion flux was 45.7 × 10−3 mmol⋅cm−2⋅h−1 and 63 × 10−3 mmol⋅cm−2⋅h−1 without ZIF-8 and with ZIF-8 nanoparticles, respectively. Also, polyvinylidene fluoride (PVDF), polyvinyl chloride (PVC), and cellulose acetate (CA) were employed as base polymer of PIM to evaluate the effect of polymer on the ion flux. The lithium flux of these polymers was 47.2 × 10−3 mmol⋅cm−2⋅h−1, 44.3 × 10−3 mmol⋅cm−2⋅h−1, and 24.98 × 10−3 mmol⋅cm−2⋅h−1 that are respectively related to PVDF, PVC and CA base PIMs.