Homogeneous liquid-liquid extraction of europium from aqueous solution with ionic liquids

Comparing with the traditionally immiscible two-phase extraction, the homogeneous liquid-liquid extraction technique shows potential in industrial separation engineering due to nearly infinite contact interface. In this work the ionic liquid (IL) compounds such as N-(carboxymethyl)-N,N-dimethylethanaminium bis-trifluoromethane-sulfonimide ([DHbet][Tf2N]) and N-(carboxyethyl)-trimethylammonium bistrifluoromethane-sulfonimide ([THbet][Tf2N]) were synthesized. The homogeneous extraction behaviors of europium with two ILs were studied as functions of solution pH, ionic strength, contact time, and initial europium concentration. The results indicated that both homogeneous extractions were dependent on pH and independent on ionic strength. The extraction capacities for [DHbet][Tf2N] and [THbet][Tf2N] were 3.29mmol/L and 3.16mmol/L, respectively. ILs could be recovered using 1.0M hydrochloric acid. The mole-ratio method indicated the formation of a mononuclear complex between the europium ion and IL. Total europium extraction efficiencies of more than 91% for [DHbet][Tf2N] and more than 90% for [THbet][Tf2N] were obtained by quadruple-stage countercurrent extraction. The result proves the feasibility of the homogeneous liquid-liquid extraction technique as an alternative option for europium separation from aquatic solution.