Rare earth elements (REEs), considered critical minerals, include the fifteen lanthanides, yttrium, and scandium. They are classified into light REEs (lanthanum to europium) and heavy REEs (gadolinium to lutetium and yttrium). Their unique properties drive high demand for advanced technologies like electronic displays, magnets, renewable energy, and catalysis. However, supply challenges arise due to fluctuating demand and limited natural abundance. As a result, research is increasingly focused on extracting REEs from secondary sources like mine water, coal fly ash, and electronic waste. In this study, a hydrophobic deep eutectic solvent (HDES) comprised of trioctylphosphine oxide (TOPO) as a hydrogen bond acceptor and stearic acid as a hydrogen bond donor was investigated for REE extraction; in particular, the extraction of Er(III), Sm(III), Nd(III), Dy(III) and Pr(III). The effect of key experimental parameters such as time (1 minute – 24 hours), temperature (20-60℃), pH (3-6), REE loading (10-1000 ppm), and HDES dosage (0.1-2 mL) were examined, with experimental determination of the metals being conducted utilizing Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Overall, the HDES shows excellent extraction performance for both light and heavy REE, with percent removal reaching 99% for all selected REEs after 30 minutes with an HDES loading of 0.75 mL to 2 mL of aqueous solution. Additionally, increasing pH and temperature showed an increase in extraction efficiency > 99.%. Furthermore, data analysis using non-linear regression and diffusion models was performed.
