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

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The 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 (SO42−, 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−> SO42−. 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].