Highly selective extraction of scandium(III) from rare earth elements using quaternary ammonium based ionic liquids: Experimental and DFT studies

Global demand for rare earth elements (REEs) is steadily increasing, emphasizing the necessity of effective extraction and recovery methods. Although REEs recovery from electronic waste using ionic liquid (IL) extractants offers an alternative solution to avoid potential adverse environmental impacts during traditional mining and processing, highly efficient and selective extraction is still essential. Especially, due to similarities with other REEs, recovery of scandium (Sc(III)) is confronting grand challenges. Herein, quaternary ammonium based ILs (QA-ILs), i.e., [N333 MeOAc][Tf₂N] and [N444 MeOAc][Tf₂N], have been successfully synthesized for selective Sc(III) extraction from other REEs under various conditions, e.g., initial acidity and IL concentration. Both ILs demonstrate superior selectivity for Sc(III) extraction from other REEs. Specifically, at pH 4.5 and 5, the extraction efficiency of Sc(III) using [N333 MeOAc][Tf₂N] was 73.6 % and 83.4 %, respectively, while it was high up to 90.5 % and 95.9 %, respectively, using [N444 MeOAc][Tf₂N]. Because of stronger hydrophobicity, [N444 MeOAc][Tf₂N] exhibited a higher extraction efficiency. Furthermore, [N444 MeOAc][Tf₂N] demonstrated a higher selectivity for Sc(III) as compared to other REEs, with high separation factor of 17 for Sc(III)/Lu(III) and Sc(III)/Yb(III). Additionally, the ILs can be recycled with a high stripping efficiency up to 86 % using 4 mol·L⁻¹ H₂SO₄ solution. Density functional theory calculations further validated neutral exchange of Sc(III) in the neutral IL extraction process. This study presents a feasible solvent extraction system using novel QA-ILs for highly effective and selective Sc(III) extraction from waste streams.