Spectroscopic Study into Lanthanide Speciation in Deep Eutectic Solvents

Deep eutectic solvents are a new class of green solvents that are being explored as an alternative for used nuclear fuel and critical material recycling. However, there is a paucity of knowledge regarding metal behavior in them. This paper explores the underlying chemistry of rare-earth elements in choline chloride-based deep eutectic solvents by using a multi-technique spectroscopic methodology. Results show that speciation is highly dependent on the choice of the hydrogen-bond donor. Collected EXAFS data showed Ln³⁺ coordination with ethylene glycol and urea in their respective solvents and coordination with chloride in the lactic acid system. Generalized coordination environments were determined to be [LnL_4-5], [LnL_7-10], and [LnL_5-6] in the ethylene glycol, urea, and lactic acid systems, respectively. Collected UV/vis spectra for Nd³⁺ and Er³⁺ showed variations with changing solvents, showing that Ln-Cl interactions do not dominate in these systems. Luminescence studies were consistent, showing varying emission spectra with varying solvent systems. The shortest luminescent lifetimes were observed in the choline chloride-ethylene glycol deep eutectic solvent, suggesting coordination through O-H groups. Combining all collected data allowed Eu³⁺ coordination geometries to be assigned.