Deep eutectic solvents (DES) are a novel type of green solvent composed of a mixture of hydrogen bond donors (HBDs) and hydrogen bond acceptors (HBAs) at a specific stoichiometric ratio [1]. The most characteristic attribute of DES is its lower melting point compared to the individual components. Their main advantages are low toxicity, low cost, simple synthesis, and environmental friendliness [2]. DES are commonly classified into five types, depending on their composition [3]. Additionally, if their individual components are of natural origin, they are referred to as natural deep eutectic solvents (NADES). It’s possible to synthesize two types of NADES, hydrophobic and hydrophilic. Hydrophobic NADES are practically immiscible in water, making them suitable for liquid-liquid extraction studies. Moreover, their natural origin, low toxicity, and low cost made them excellent candidates to replace traditional toxic organic solvents [4]. Currently, approximately 10% of the world’s electricity is generated by nuclear power, making nuclear waste management a topic of significant interest. After the PUREX process, where uranium (U) and plutonium (Pu) are extracted, the remaining raffinate, known as high-level liquid waste (HLLW), is treated to separate lanthanides (Ln) from actinides (An) [5]. Ln are crucial for numerous applications such as the production of permanent magnets, catalysts, optical devices, and batteries. Thus, the Ln demand is expected to grow exponentially next years. The main inconvenient is the intra-Ln separation, due to the similar physicochemical properties between them [6]. In this work, the extraction and separation of Ln by two different hydrophobic NADES were studied and compared with a molecular solvent. Cineole and menthol, both in a 1:1 molar ratio with decanoic acid, were used to synthesize hydrophobic NADES. The hydrophobic NADES, combined with N,N,N′,N′-tetraoctyldiglycolamide (TODGA) as the extractant molecule were used as the organic phase to perform a liquid-liquid extraction of Ln. Different parameters such as the pH, the extractant concentration, or the agitation time were optimized to maximize the extraction efficiency and the selectivity of heavy towards middle and light Ln. Finally, slope analysis was performed to explain the extraction mechanism.
In summary, greater extraction and separation factor values were obtained for heavy Ln, and an extraction mechanism was proposed. Hydrophobic NADES could be a promising alternative to replace volatile molecular solvents, reducing the environmental impact of the process.
[1] C. Wang, Z. Zhou, X. Zhang, H. Guo, G. Boczkaj, Metal extraction using deep eutectic solvents for metal recovery and environmental remediation – A review, Sep. Purif. Technol. 364 (2025) 132533. https://doi.org/10.1016/j.seppur.2025.132533.
[2] R. Cui, Y. Ran, D. Shu, Q. Huang, Q. Song, H. Wang, J. Zhu, W. Yuan, Recent advances in green deep eutectic solvents for lithium-ion battery recycling: A perspective on bibliometric analysis, J. Environ. Manage. 377 (2025) 124670. https://doi.org/10.1016/j.jenvman.2025.124670.
[3] F.J. Alguacil, J.I. Robla, Recent Work on the Recovery of Rare Earths Using Ionic Liquids and Deep Eutectic Solvents, Minerals 13 (2023) 1288. https://doi.org/10.3390/min13101288.
[4] J.L. Trenzado, C. Benito, M.A. Escobedo-Monge, M. Atilhan, S. Aparicio, Cineole – Decanoic acid hydrophobic natural Deep eutectic solvent for toluene absorption, J. Mol. Liq. 384 (2023) 122036. https://doi.org/10.1016/j.molliq.2023.122036.
[5] B. Li, M. Bao, Y. Kang, L. Wang, Y. Liu, L. Wang, C. Xu, Hydrophilic chelators for coordination and separation of radioactive f-block elements, Natl. Sci. Open (2024) 20240028. https://doi.org/10.1360/nso/20240028.
[6] M. Otaki, T. Suominen, S. Hietala, R.T. Koivula, Intra-lanthanide separation performance of DOTP: Solid-phase extraction and selective precipitation studies, Sep. Purif. Technol. 354 (2025) 129082. https://doi.org/10.1016/j.seppur.2024.129082.