Extraction Behavior of Divalent Metal Cations in Ionic Liquid Chelate Extraction Systems Using 1-Alkyl-3-methylimidazolium Bis(trifluoromethanesulfonyl)imides and Thenoyltrifluoroacetone

Abstract

The extraction behavior of several divalent metal cations (M²⁺) in ionic liquid chelate extraction systems was investigated using several 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids ([Rmim][Tf₂N]) as extraction solvent and thenoyltrifluoroacetone (Htta) as extractant. The behavior was compared with that using less hydrophobic 1-alkyl-3-methylimidazolium hexafluorophosphates ([Rmim][PF₆]). The extracted species in the [Rmim][Tf₂N] systems were neutral M(tta)₂ for M = Cu and anionic M(tta)₃^- for M = Mn, Co, Ni, Zn and Cd. Conversion of ionic liquid anion from PF₆^- to more hydrophobic Tf₂N^- resulted in changing the species extracted for Ni²⁺ from hydrated neutral complex to hydrophobic anionic one. Furthermore, the extractability for these metals was governed by the hydrophobicity of ionic liquid ions. Thus, in the ionic liquid chelate extraction system, selection of a suitable ionic liquid as extraction phase seems to be an important factor for enhancement of extraction selectivity.