A green extraction process for the selective recovery of Sc(iii) based on hydrophobic betaine derivative ionic liquids

Abstract

The efficient extraction recovery of scandium (Sc(III)) is crucial for its application in high-end technology. Two novel hydrophobic carboxylic acid ionic liquids (ILs), namely, [lauryl betaine][bis(trifluoromethanesulphonyl)imide] ([Laur][Tf₂N]) and [cocamidopropyl betaine][bis(trifluoromethanesulphonyl)imide] ([Coca][Tf₂N]), were synthesized using two inexpensive amphoteric surfactants as cation sources. [Laur][Tf₂N] (257 °C) and [Coca][Tf₂N] (251 °C) exhibited good thermal stability and strong hydrophobicity. The viscosity of [Coca][Tf₂N] (4.29 × 10³ mP s) was higher than that of [Laur][Tf₂N] (2.55 × 10³ mPa s) at 25 °C. The optimal extraction conditions were an extraction equilibrium time of 40 min, an initial Sc(III) concentration of 0.001 mol L⁻¹, a sodium nitrate concentration of 0.5 mol L⁻¹, and a pH of 3. The extraction efficiency of [Laur][Tf₂N] and [Coca][Tf₂N] could even exceed 98.7% and 96.0%, respectively. The cation exchange extraction mechanism was studied by slope analysis, IR spectroscopy and ¹³C NMR spectroscopy. Sc(III) extracted using [Laur][Tf₂N] and [Coca][Tf₂N] could be completely stripped with 0.1 mol L⁻¹ and 0.2 mol L⁻¹ HNO₃ once, respectively. The structure of the ILs was not broken after stripping, and the extraction efficiency of the ILs remained almost unchanged after five cycles. In addition, the extraction differences at different pH levels made it possible to separate Sc(III) from other rare earths using ionic liquids [Laur][Tf₂N] and [Coca][Tf₂N].