Ionic liquids as extraction solvents: Where do we stand?

The unique physicochemical properties of ionic liquids (ILs) and the relative ease with which these properties can be fine-tuned by altering the cationic or anionic moieties comprising the IL have led to intense interest in their use as alternatives to conventional organic solvents in a wide range of synthetic, catalytic, and electrochemical applications. Recent work by a number of investigators has been directed at the application of ionic liquids in various separation processes, among them the liquid-liquid extraction of metal ions. Although certain IL-extractant combinations have been shown to yield metal ion extraction efficiencies far greater than those obtained with molecular organic solvents, other work suggests that the utility of ILs may be limited by solubilization losses and difficulty in recovering extracted metal ions. In this report, recent efforts to overcome these limitations are described, and progress both in achieving an improved understanding of the fundamental aspects of metal ion transfer into ILs and in devising viable IL-based systems for metal ion separation is detailed. In addition, areas upon which future research efforts might profitably be focused are identified.