Search for optimal conditions for the extraction of thiosulfate complexes of Cu(II) by 4-(5-nonyl)pyridine from aqueous mineral acid solutions

Abstract

Solvent extraction of Cu(II) by 4-(5-nonyl)pyridine (NPy) in benzene from mineral acid solutions containing thiosulfate ions has been studied at room temperature (23±2°C). Mineral acid solutions alone constitute an aqueous phase from which Cu(II) is not extracted. Addition of small amounts of thiosulfate ions augments the extraction to an extent that quantitative recovery is possible. Stoichiometric studies reveal the involvement of ion-pair type complexes (NPy·H)2·Cu(S2O3)2 which are responsible for extraction. Stability constants lg Kex for this complex are 7.2±0.2; 9.1±0.2 and 9.5±0.2 for HCl, HNO3 and H2SO4, respectively. The presence of 0.01 mol/l of some complexing ions like ascorbate, acetate, citrate, oxalate, tartrate or iodide does not affect the extraction, thus allowing the recovery of the metal from diverse matrices. Under optimal conditions (0.1M NPy in benzene-0.1M HNO3 or H2SO4+0.01M S2O3−2 or 0.5M HCl+0.05 M S2O3−2) a clean separation from some elements, e.g. Cs(I). Co(II), Fe(III), Eu(III), Ce(III), Se(IV) and Cr(VI) can be achieved.