Enhancing Metal Separations Using Hydrophilic Ionic Liquids and Analogues as Complexing Agents in the More Polar Phase of Liquid–Liquid Extraction Systems

The separation of metals by liquid-liquid extraction largely relies on the affinity of metals to the extractants, which normally reside in the organic (less polar) phase because of their high hydrophobicity. Following a different route, using aminopoly(carboxylic acid)s (e.g., EDTA) as complexing agents in the aqueous (more polar) phase was found to enhance metal separations by selectively complexing metal cations. In this study, we demonstrate that, hydrophilic ionic liquids and analogues in the more polar phase could also selectively complex with metal cations and hence enhance metal separations. As an example, Cyanex 923 (a mixture of trialkyl phosphine oxides) dissolved in p -cymene extracts CoCl 2 more efficiently than SmCl 3 from a chloride ethylene glycol (EG) solution. However, when tetraethylammonium chloride is added into the EG solution, CoCl 2 is selectively held back (only 1.2% extraction at 3.0 M tetraethylammonium chloride), whereas the extraction of SmCl 3 is unaffected (89.9% extraction), leading to reversed metal separation with a separation factor of Sm(III)/Co(II) > 700. The same principle is applicable to a range of hydrophilic ionic liquids, which can be used as complexing agents in the more polar phase to enhance the separations of various metal mixtures by liquid-liquid extraction.