Selective Cobalt over Nickel separation using neat and confined ionic liquids

Abstract

Task Specific Ionic Liquids (TSILs) generated by association between tetraalkylammonium cations and coordinating anions such as dicyanamide (Dca-) and thiocyanate (SCN-) were used for the selective separation of Ni(II) over Co(II). SCN-based TSIL presents higher extraction efficiency than the Dca-based one towards Co(II) (E = 85.4 % vs 54.6 %) and Ni (II) (E = 22 % vs 0.5%) but lower separation factors (βCo/Ni = 21 vs 239). Interestingly, extraction of Co(II) and Ni(II) in Dca-based TSIL can be enhanced using salts with chaotropic anions such as NaNO3 (E > 90 % for Co(II) and E = 85% for Ni(II)). The use of NaCl allows, moreover, the efficient separation of both ions with βCo/Ni >1000. The metal can be recovered from the IL phase using Na2SO4 aqueous solution to limit the release of the IL and to ensure a good recyclability of this latter. The confinement of TSIL in silica –matrix ionogel constitutes a valuable method to improve the handling of the IL. Indeed, the extraction properties of the ionogel containing [THN][Dca] IL are very close to those obtained using the same neat IL with an efficient separation of Co(II) over Ni(II) in NaCl 4 mol.L-1 (βCo/Ni >1000). Finally, the recyclability of the ionogel was studied and found comparable to that of neat IL.