Adsorption of Rare Earth Elements by Kaolinite and Their Selective Desorption Using Deep Eutectic Solvents

Rare earth elements (REEs) are essential to modern technologies yet are typically sourced from regolith clays containing cation-exchanging minerals such as kaolinite, and conventional extraction methods often rely on environmentally harmful organic solvents. Deep eutectic solvents (DESs) provide a more sustainable alternative due to their tunable solvation properties, low volatility, and high thermal stability. Kaolinite contributes nanoscale features that enhance its suitability for REE separations, including its layered morphology featuring nanometer-scale layer thickness, heterogeneous surface charge, and structurally defined aluminol and silanol edge sites, all of which create reactive interfaces capable of strong electrostatic and inner-sphere interactions with multivalent cations. In this study, DESs were investigated for recovering ytterbium and neodymium (Yb3+ and Nd3+) from kaolinite, with the extraction time, temperature, and solvent composition systematically optimized. At an initial total REE concentration of 2.5 mg, kaolinite removed 99.4% of the REEs from solution, corresponding to a final loading of 0.49 mg g–1. Selective extraction with a choline chloride/lactic acid DES (1:8 molar ratio) yielded a conditional Nd3+/Yb3+ separation factor of 8.85, with Yb3+ showing stronger retention due to its smaller ionic radius and enhanced interaction with the kaolinite nanoscale framework. These findings underscore the potential of integrating kaolinite’s nanoscale structural attributes with DES chemistry to enable efficient, selective, and environmentally benign REE recovery from regolith-derived resources.