Selective extraction of vanadium from chromium by pure [C8mim][PF6]: An anion exchange process

[Display omitted] •The maximum βV/Cr by pure [C8mim][PF6] was 100.6 in case of pH 3.5.•The extraction mechanism could be the anion exchange between HV10O273− and PF6−.•A novel approach to prepare ionic liquids with metal oxo-anions as counter anion.•Vanadate-loaded IL phase can be quantitatively stripped by KPF6 and NaOH.•Pure [C8mim][PF6] could be used to recover V(V) from Cr(VI)-containing solutions. 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 (βV/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 HV10O273− and PF6−. Furthermore, V(V) in IL phase can be quantitatively stripped by KPF6 solution under the basic condition (COH−=1mol/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.