Metal separation and recovery employing choline chloride-based deep eutectic solvents: Diverse functions of water

Abstract

Deep eutectic solvents (DESs) are considered as environmentally friendly solvents for metal separation and extraction. Compared to aqueous solvents, DESs possess the characteristics of designability, recyclability, and stability; however, they also encounter significant challenges, such as high viscosity and cost constraints that limit their widespread application. The multifunctionality of water in regulating the solvent structure and phase behavior of DESs, modulating the form of metal ions in the system, enhancing efficient separation and recovery of metals, and facilitating the recycling of the solvent system is emphasized in this review. It comprehensively summarizes research progress on water’s roles as an additive, ligand modulator, and anti-solvent in choline chloride (ChCl)-based DESs processes for metal separation and extraction. Moreover, it highlights the pivotal role of water while discussing current research limitations by summarizing the challenges faced by water-modified ChCl-based DESs in metal resource recovery. Finally, future research directions are outlined to provide guidance for process improvement and the practical application of ChCl-based DESs.