Systematic evaluation of stearic acid -based hydrophobic deep eutectic solvents on the extraction of selected light and heavy rare earth elements from water

In this study, the extraction of representative rare earth elements from both light rare earth elements (LREE) and heavy rare earth elements (HREE) groups was evaluated. Praseodymium (Pr) and neodymium (Nd) were selected as representatives of LREE, while europium (Eu), dysprosium (Dy), and erbium (Er) represented HREE. The extraction process was carried out using hydrophobic deep eutectic solvents (HDES) composed of stearic acid as the hydrogen bond donor (HBD) and menthol, thymol, tetraoctylammonium bromide, and trioctylphosphine oxide as the hydrogen bond acceptors (HBAs). The hydrophobicity, solid-liquid equilibria (SLE), σ-surface, σ-profile and chemical bonding of the solvents were studied using Conductor-like Screening Model for Real Solvents (COSMO-RS). The analysis of the σ-surfaces and σ-potentials revealed that the HDES are predominantly composed of nonpolar regions, accompanied by smaller peaks corresponding to hydrogen bond donor and acceptor sites. Prepared HDES exhibit density ranging from 861 to 903 kg/m³ and viscosities ranging from 0.35 to 0.34 Pa.s. Finally, the study investigated the impact of hydrophobicity on the REEs extraction performance and discussed the removal mechanisms in all the different solvents. The effect of the lanthanide contradiction was also investigated. Among the various HDES developed, TOPO:stearic acid (x₁ = 0.59) exhibited the best extraction efficiency with an overall removal of 72.23 ± 1.29% from 0.01 M REEs solution.