PEG-functionalized ionic liquids: A class of liquid materials for highly efficient extraction of rare earth metals from aqueous solutions

Recycling of rare earth metals is important for their supply risk in the future. In this work, a greener and highly efficient liquid-liquid extraction strategy is presented for rare earth metals from aqueous solution with PEG-functionalized ionic liquids. The factors influencing the solvent extraction process such as phase volume ratio, equilibrium time, pH value of aqueous phase, alkyl chain length of the ionic liquids and temperature of the system were investigated systematically. It was found that under optimal conditions, the single-step extraction efficiency was > 96.8% for the studied 11 kinds of rare earth metal ions Sc(III), La(III), Ce(III), Y(III), Nd(III), Sm(III), Gd(III), Dy(III), Ho(III), Yb(III) and Lu(III). After the extraction, about 100% of the rare earth metals could be stripped from the ionic liquids phases by using dilute aqueous oxalic acid, and the ionic liquids would be recovered and reused in the next extraction process. In addition, the extraction mechanism was investigated in some details, and an ion pair association mechanism was suggested. Since the extraction procedure was performed without saponification, addition of extra extractant, organic diluents or salt, the approach described here has the potential to be used as a sustainable strategy for the separation and enrichment of rare earths from complex matrixes.