Task-specific ionic liquids for the separation and recovery of rare earth elements

The diverse and vital applications of rare earth elements (REEs), especially in green technologies, require their effective recovery from natural ores and secondary resources. Although solvent extraction has been developed as the dominant separation technology to obtain high-purity products of REEs with numerous advantages compared to other techniques like chemical precipitation and ion exchange, it suffers from various issues, particularly relating to the human health and environment impacts because of the use of highly volatile, toxic, and flammable organic solvents (extractants and diluents). Interestingly, ionic liquids have been recognized as the promising “greener alternative” for conventional molecular solvents in vogue for REE separation with solvent extraction, which are not only environmentally friendly but also aptly enhance extraction efficiency. This chapter focuses on the use of a new generation of ionic liquids with modification of the chemical structure for the separation of rare earth elements, defined as task-specific ionic liquids or functional ionic liquids. The research exploring the ionic liquids used to extract and separate REEs from various feedstocks/solutions is reviewed, including the generation of task-specific ionic liquids, demonstration of the extraction mechanism, improvements in the extraction efficiency and selectivity by a synergistic effect, potential applications in other separation methods involving membrane technique and electrodeposition, and prospects of possible exploitation. Investigation of the ionic liquid-based extraction systems in detail is urgently needed with the aim of testing and applying the same on an industrial scale, in order to ensure sustainable recovery of REEs from production to the application level in advancing technologies.