Highly efficient separation of Nb from Ta using a unique solvent system containing tri-n-octyl phosphine oxide in an ionic liquid.

• The maximum separation factor of ∼ 4418 was achieved between Nb and Ta. • 'Cation exchange' predominates in ionic liquid; while 'solvation' in n -dodecane. • Spontaneous and exothermic liquid–liquid mass transfer. • Formation of outer-sphere complex. • High degree of radiolytic stability and good recyclability. The present investigation demonstrated the mutual separation Nb and Ta from their mixture in hydrochloric acid medium using TOPO in an ionic liquid (C 6 mim•NTf 2). The maximum separation factor achieved was ∼ 4400. In C 6 mim•NTf 2 , the extraction proceeded via 'cation exchange' mechanism with M O C l 2. T O P O I L + as the species; while in n-dodecane the extraction proceeded via neutral 'solvation' mechanism with MO Cl 3. 2 T O P O as the extracted species (where, M = Nb and Ta). The sluggish extraction in C 6 mim•NTf 2 was attributed to the its high viscosity coefficient compared to n-dodecane. The extraction was found to be spontaneous and exothermic in nature with a reduction in overall entropy indicating the formation of outer-sphere complexes. Dilute acid was found to be effective for the back extraction of Nb and Ta from the loaded n-dodecane phase, while EDTA - guanidine carbonate or DTPA - Guanidine carbonate were effective for back extraction from the ionic liquid phase. It was found that 2–3 contacts were sufficient for the quantitative back extraction of the metal ions. The C 6 mim•NTf 2 based solvent systems exhibited better radiolytic stability compared to that of the n-dodecane based solvent system. [ABSTRACT FROM AUTHOR]