Liquid-Liquid extraction of Cd(II) and Zn(II) using a novel tetraalkylphosphonium-based ionic liquid

The recovery of scarce metals from secondary resources is desirable from an economic and ecological perspective. The limitations of natural resources and the continuous growth of advanced metal materials increase the importance of securing metal resources. The purpose of this study was to utilize a newly synthesized ionic liquid (IL), trioctyldodecylphosphonium bromide (P_8,8,8,12Br), as a novel extractant for Cd(II) and Zn(II) from a chloride medium. An investigation into the extraction parameters, including the acid concentration, extractant type, and equilibration time, for both metals was performed. Two IL molecules were involved in the extraction reaction for both metals. The selective stripping of Cd(II) and Zn(II) from the loaded IL phase was achieved with 1 M oxalic acid and 3 M HNO₃, respectively. The presence of other metals did not affect the extraction efficiencies of Cd(II) and Zn(II). The extraction efficiencies of Cd(II) and Zn(II) were maintained at 94% and 95%, respectively, with an equilibration time of 2 h. Although Fe(III) and Cu(II) were extracted into the IL phase in significant amounts, the removal of Fe(III) and Cu(II) could be achieved by scrubbing using 1 M Na₂SO₃ and 3 M H₂SO₄, respectively. P_8,8,8,12Br exhibited excellent recyclability through consecutive extraction-stripping cycles. A recovery scheme for Cd(II) and Zn(II) from a wasted battery-model solution was developed based on the extraction data. P_8,8,8,12Br could recover Cd(II) and Zn(II) from a mixed metal solution in high purity. This study highlights the future potential of P_8,8,8,12Br in metal recycling.