In recent times, there has been a significant surge in the demand for lanthanides, also referred to as rare earth elements (REEs), which are indispensable raw materials for various high-tech industries. Their versatility in applications, ranging from electric vehicles to catalytic reactions and batteries, has led to widespread recognition. Given that a substantial portion of global REE production is monopolized by China, there's a pressing need to explore alternative sources to meet escalating demands. Among the proposed recycling methods, solvent extraction emerges as a promising technology for REE extraction. However, conventional solvent extraction processes encounter challenges in discerning the selectivity of lanthanides due to their closely resembling chemical properties. This often results in the need for multiple separation stages to achieve the desired purity. In this study, stearic acid (as a hydrogen bond donor) was combined with four separate components, menthol, thymol, trioctylphosphine oxide, and tetraoctylammonium bromide (as hydrogen bond acceptors) to form hydrophobic deep eutectic solvents for the simultaneous extraction of Erbium (Er), Neodymium (Nd), Samarium (Sm) and Praseodymium (Pr) from water samples. The experimental determination of the metals was conducted utilizing ultraviolet–visible spectroscopy (UV-VIS). Furthermore, the research further explored and discussed the extraction behavior of REEs under varying experimental conditions, encompassing different combinations of HDESs, ratios of HBA to HBD, REE loading, and temperatures.
