Mechanistic insights into highly efficient rare earth extraction using promising novel trioctylphosphine oxide-based deep eutectic solvents

• Six novel deep eutectic solvents were developed for rare earth extraction. • TOPO: Vanillin showed the highest efficiency, extracting up to 99.83% of Dy. • Extraction mechanisms depend on both the solvent used and the rare earth extracted. • TOPO: Formic acid showed the best selectivity among the tested solvents. • Solvents remained stable after four regeneration cycles using HCl stripping. In this study, six new trioctylphosphine oxide based deep eutectic solvents were developed: trioctylphosphine oxide: octanoic acid (3:7), trioctylphosphine oxide: formic acid (1:4), trioctylphosphine oxide: lactic acid (3:7), trioctylphosphine oxide: acrylic acid (3:7), trioctylphosphine oxide: borneol (7:13), and trioctylphosphine oxide: vanillin (9:11). Viscosity, density, thermal stability, and hydrophobicity of the new solvents have been measured to ensure their suitability as organic solvent in liquid–liquid extraction processes of 5 rare earth elements present in smartphone: Yttrium, Lanthanum, Neodymium, Praseodymium and Dysprosium from an aqueous medium. Optimal operating conditions such as temperature, organic/aqueous ratio, and salt concentration to optimize the process were determined. Five of the new solvents achieved extraction efficiencies exceeding 85 % for all rare earth elements. This study highlights that the extraction complexes formed depend on the solvent used but also the rare earth element extracted. The best efficiency was found using trioctylphosphine oxide: vanillin (9:11) at a O/A ratio of 1, at 310 K when the sodium nitrate concentration in the aqueous phase is 0.7 mol.L -1 . In this condition, the extraction process allows to extract 98.7 % for Neodymium, 91.5 % for Lanthanum, 98.16 % for Praseodymium, 99.83 % for Dysprosium and 99.78 % for Yttrium. These new solvents show relative selectivity for rare earths: trioctylphosphine oxide: formic acid > trioctylphosphine oxide: vanillin > trioctylphosphine oxide: acrylic acid > trioctylphosphine oxide: borneol > trioctylphosphine oxide: lactic acid. Each deep eutectic solvents was recycled using hydrochloric acid stripping. Solvent performance remains constant after four recycling cycles making them promising candidates for use in industrial applications.