Task-specific ionic liquid with coordinating anion for heavy metal ion extraction: Cation exchange versus ion-pair extraction

Novel ionic liquids (ILs) were generated by association between a cationic ester derivative of glycine betaine and bis(trifluoromethylsulfonyl)imide (Tf2N-) or dicyanamide (Dca(-)) anions. Extraction of Cu(II) ions from water was performed with these ILs at room temperature. The use of Dca(-) greatly enhances the extraction efficiency for metal ions. The extraction mechanism has been studied by the determination of the concentration of the organic cation released upon extraction and those of counter ions co-extracted. Our results show that the extraction mechanism proceeds via a mixed process involving both cation exchange and ion-pairing. Increasing the concentration of electrolyte in aqueous solution induces an increase in the proportion of metal extracted via ion-pairing without altering the extraction yield. The Cu(II) coordination in IL phase was also investigated by spectroscopic techniques (UV-vis, XAS). The obtained results contribute to understand the mechanisms of the metal ion transfer between the aqueous and ionic liquid phases in order to design "green" extraction processes which present a real environmental benefit. (C) 2013 Elsevier B.V. All rights reserved.