Potential Application of Macrocyclic Compounds for Selective Recovery of Rare Earth Scandium Elements from Aqueous Media

Recently, efforts have been made to develop the extraction, recovery, and analysis of rare earth elements, especially the scandium Sc (III). This study describes the use of 15-crown-5, 12-crown-4, cryptand 2.2.2, and DC18-crown-6 as novel extractants using molecular recognition technology to effectively and selectively separate and recover Sc from aqueous media. The results show that 15-crown-5, 12-crown-4, and cryptand 2.2.2 have exhibited high selectivity for Sc, which could be of potential value in the separation and purification of Sc rare earth elements processing industry. Typically, 15-crown-5, 12-crown-4, and cryptand 2.2.2 ligands have achieved the maximum extraction efficiency > 99%, depending on the pH value of the aqueous solution. On the other hand, DC18-crown-6 only reaches 25.81%. Notably, the complex metal ions can be efficiently recovered/stripped out from the complex by HCl and HNO3. The results of the multielement extraction experiments by cryptand 2.2.2 indicate that the pH of the aqueous solution significantly impacts extraction efficiency and selectivity. Where, at pH 2 the extraction efficiency of Sc ~ 97% compared with Y ~ 18.23%, La ~ 10.21% and Ce ~ 9.2%. Moreover, the complexed Sc ions can be efficiently recovered from complexes using a precise concentration of oxalic acid, with a recovery rate of 100% at 0.1 mol/L oxalic acids, compared to 30.7% for Y, 20.3% for La, and 21.12% for Ce.