Effect of SCN¯ and NO3¯ ions on the extraction of heavy metals from aqueous solutions with the ionic liquid trihexylammonium octanoate

© 2020The protic ionic liquid, trihexylammonium octanoate [T6AC7COO], was used in order to extract heavy metals ions, such as Cu(II), Co(II) and Ni(II), from aqueous solutions at room temperature, using different counter anions (SO4 2¿, Cl¿ and NO3 ¿). This ionic liquid is highly selective towards copper(II) but not to cobalt(II) and nickel(II). The metal ion extraction capacity of [T6AC7COO] also depends on the counter anion of the metal salt: NO3 ¿> Cl¿> SO4 2¿. When nitrate ions (NO3¯) are added to aqueous solutions containing CoSO4, CoCl2, NiSO4, or NiCl2, before the extraction process, the extraction efficiency of [T6AC7COO] is increased from near zero percent to around 20%¿40%, which can be considered as a good rise. However, the addition of thiocyanate ions (SCN¿) to aqueous solutions, instead of nitrate ions, increases substantially the Co(II) and Ni(II) recovery yields. Practically all the metal ions together with SCN¿ ions are transferred from the aqueous phase to the ionic liquid phase. In the extracted phase, SCN¿ anions bind to the metal center through the N atoms. In contrast, the other anions do not coordinate with the metal ion. For all extraction processes, an ion-par extraction is the dominant mechanism. 1H NMR, FT-IR and ion chromatography were used in order to confirm the extraction mechanism, and UV¿visible spectroscopy and ICP-OES were used to estimate the extraction efficiency of [T6AC7COO].