Efficient separation of vanadium from chromium by a novel ionic liquid-based synergistic extraction strategy

Ionic liquid (IL)-based extraction is a promising and environmentally benign separation technology. To develop sustainable extraction technologies, quaternary ammonium-based IL extraction strategy is attractable. In this work, the separation of vanadium(V) from chromium(VI) by pure tricaprylmethylammonium nitrate ([A336][NO₃]) and organic acidified primary amine N1923 ([RNH₃][NO₃]) was systematically investigated. The optimal proportion of [A336][NO₃] and [RNH₃][NO₃] was studied and results showed that the mixed [A336][NO₃] and [RNH₃][NO₃] exhibited an obvious synergistic-effect for V(V). The extraction of V(V) was strongly dependent on the acidity of the aqueous phase and reaches maximum at pH 2.5-3, while the maximum separation coefficient (β_V/Cr) was located at about pH 9.0. Moreover, β_V/Cr could be improved through adjusting the molar concentration ratio of V/Cr. The interference of coexisting anions (nitrates, chlorides, sulfate and phosphate) on the extraction of V(V) was examined and the results showed that PO₄^3-, NO₃^- and Cl^- had negative effects at various degrees except for SO₄^2-. The V(V) extraction behaviors could be properly described by Langmuir and pseudo-second-order rate equations. The maximum extraction capacity for V(V) was estimated as 1.877mmol/g at 303K. Increased temperature had little effect on the extraction capacity, but greatly improved the extraction rate. The typical anion exchange mechanism between NO₃^- and V₄O₁₂^4- (or V₃O₉^3-) was proposed for the current extraction system. The IL phase could be renewed through stripping the loaded vanadium by a 0.5M NaNO₃ solution. This work demonstrated that quaternary ammonium IL containing a commercial organic extractant is an efficient and sustainable IL-based extraction strategy for the separation of vanadium from chromium, and as a result, the development of an IL-based extraction process is straightforwardly envisaged.