Extraction of copper(II) ions from aqueous solutions with a methimazole-based ionic liquid

The recently synthesized ionic liquid (IL) 2-butylthiolonium bis(trifluoromethanesulfonyl)amide, [mimSBu][NTf₂], has been used for the extraction of copper(II) from aqueous solution. The pH of the aqueous phase decreases upon addition of [mimSBu]⁺, which is attributed to partial release of the hydrogen attached to the N(3) nitrogen atom of the imidazolium ring. The presence of sparingly soluble water in [mimSBu][NTf₂] also is required in solvent extraction studies to promote the incorporation of Cu(II) into the [mimSBu][NTf₂] ionic liquid phase. The labile copper(II) system formed by interacting with both the water and the IL cation component has been characterized by cyclic voltammetry as well as UV-vis, Raman, and ¹H, ¹³C, and ¹⁵N NMR spectroscopies. The extraction process does not require the addition of a complexing agent or pH control of the aqueous phase. [mimSBu][NTf₂] can be recovered from the labile copper-water-IL interacting system by washing with a strong acid. High selectivity of copper(II) extraction is achieved relative to that of other divalent cobalt(II), iron(II), and nickel(II) transition-metal cations. The course of microextraction of Cu²⁺ from aqueous media into the [mimSBu][NTf₂] IL phase was monitored in situ by cyclic voltammetry using a well-defined process in which specific interaction with copper is believed to switch from the ionic liquid cation component, [mimSBu], to the [NTf₂] anion during the course of electrochemical reduction from Cu(II) to Cu(I). The microextraction-voltammetry technique provides a fast and convenient method to determine whether an IL is able to extract electroactive metal ions from an aqueous solution.