Copper extraction using protic ionic liquids: Evidence of the Hofmeister effect

Protic ionic liquids (PILs) have been used extensively in extractions of organic compounds but have seen relatively little use in the extraction of metals from aqueous phase. In this work, PILs based on trialkylammonium alkanoates are investigated for their capacity to extract copper from the aqueous phase. The results demonstrate a high degree of copper extraction, and the structures of the PILs make this possible even in the absence of chelating agents. Charge balance indicates that neutral extraction is the dominant mechanism in all cases, avoiding the degradation of the ionic liquid phase associated with ion exchange mechanisms. Further, we observe that the extent of copper extraction depends strongly on the identity of the anion of the copper salt, and that in the presence of multiple anions the PILs display a strong selectivity for particular species. The study presents strong evidence that this is a manifestation of the Hofmeister effect, and a review of literature studies indicates that this phenomenon may be quite common in IL-based extractions. We also demonstrate that the PILs used in this research are highly selective for copper from a mixture of alkali and alkaline earth metals, and that copper may be stripped from the IL phase using standard chelating agents.