Elucidating mechanism for the extraction of cobalt, nickel, and iron in polymer-electrolyte aqueous biphasic systems with both phases rich in electrolyte

Abstract

Polymer-electrolyte aqueous biphasic systems (ABS) have emerged as environmentally and economically sustainable alternatives for the extraction of metals. However, mechanisms of the metal extraction in ABS formed by top and bottom phases enriched in electrolyte need the best elucidation. In this context, extraction of Co(II), Fe(III), and Ni(II) into ABS, which comprised a polymer (PEO1500) or a triblock copolymer (L64 or F68) and a thiocyanate salt, was studied without the use of any auxiliary complexing agent. Metal extraction from the salt-rich phase to the macromolecule-rich phase depended on the hydrophobicity and size of the macromolecule, electrolyte structure, and pH. Extraction around 100% of Fe(III) was achieved in all ABS investigated at pH = 3.0, Co(II) was extracted around 97% in all ABS, while Ni(II) extraction was smaller than 64% for all conditions evaluated. The L64 + NH4SCN + H2O ABS, with a tie-line length of 29% (w/w) afforded a separation factor of 120 for the separation of Co(II) and Ni(II) at pH = 3.0 and 303 for the separation of Ni(II) and Fe(III) at pH = 6.0. It is proposed that the total extraction of Co(II) and Fe(III) ions into the macromolecule-rich phase was mainly determined by the charge transfer between the metal-thiocyanate complexes formed in the ABS and the ethylene oxide units of the macromolecules.