Selective extraction of vanadium from chromium by pure [C ₈mim][PF₆]: An anion exchange process

Separations among vanadium(V) and chromium(VI) remain challenging. The extraction behaviors of V(V) along with Cr(VI) by a few of pure imidazolium-based hydrophobic ionic liquids have been investigated. Among these hydrophobic ionic liquids, [C₈mim][PF₆] exhibits a special extraction for V(V) while a weak extraction for Cr(VI). The extraction behavior of V(V) by [C₈mim][PF₆] was particularly studied. The extraction of V(V) is strongly dependent on the acidity of the aqueous phase, and the maximum separation factor between V and Cr (β_V/Cr) was calculated to be 100.6 in case of pH 3.5. The most probable extraction mechanism of V(V) has been proposed to be the anion exchange between the main species HV₁₀O₂₇^3- and PF₆^-. Furthermore, V(V) in IL phase can be quantitatively stripped by KPF₆ solution under the basic condition (C_OH^- = 1 mol/l). Recycle test indicates that the extraction capacity shows no apparent decline until the 9th continuous extraction-stripping cycle. The result also provides a new approach for the preparation of a novel ionic liquid with metal oxo-anions as counter anion.