Metal extraction to ionic liquids: the relationship between structure, mechanism and application

Ionic liquids (ILs) have recently emerged as an extremely promising medium for the extraction of metals from aqueous phases. ILs are salts that are liquid at room temperature, and have physical and chemical properties that make them potentially valuable replacements for organic molecular solvents. However, that same ionic character gives rise to extraction mechanisms that have no analogue in molecular liquids. While metals generally exist in hydrophobic neutral complexes in molecular liquids, charged complexes incorporating hydrophobic ligands may be highly soluble in IL phases. Further, the ionic character of ILs permits ion exchange mechanisms involving component ions of the IL, raising the possibility that the extraction process may degrade the IL and contaminate the aqueous phase. The purpose of this review is to provide a broad overview of metal extraction from aqueous to IL phase, using the extraction mechanism as a common basis for analysis of extraction phenomena that might otherwise appear quite different. The goal is to identify both the challenges and opportunities offered by the unique properties of ILs, and to provide a framework for the design of metal extraction processes based on the use of ILs.