Development of Industrial Extractants into Functional Ionic Liquids for Environmentally Friendly Rare Earth Separation

Rare earth elements (REEs) are critical materials in many cutting-edge technology products. Di(2-ethylhexyl)phosphate (HDEHP) and 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) are the most commonly used industrial extractants in individual REE separation. However, acidic extractants, such as HDEHP and HEH[EHP], must be saponified for REEs separation, which often releases millions of tons of saponification wastewater into the environment annually. To develop effective and environmentally friendly extraction protocols for the REE separation industry, HDEHP and HEH[EHP] were prepared as acid–base coupling bifunctional ionic liquid extractants, and their extraction behavior and stripping properties for REEs were investigated.