Dissolution of rare earth elements: Exploring the ability of deep eutectic solvents and organic acid solutions, the case of lactic acid

Deep eutectic solvents (DES), a new class of solvents discovered twenty years ago, are increasingly proposed as a “green” alternative for extractive applications, such as the recovery of critical metals. However, little is known about the chemistry involved in DES, or what makes them special. This study focuses on reducing this knowledge gap by exploring the ability of DES to dissolve rare earth elements (REEs). It compares one of the most widely used systems in the literature, a choline chloride and lactic acid-based DES, with an aqueous solution of lactic acid (1 mol/L), pure H2O and other variations of the initial DES. Analysis of the dissolution yields and dissolution products in the case of concentrated REE phases (La, Nd, Gd, Dy, Y, Yb) revealed that the speciation and nature of REEs affect the dissolution ability of the solvents, but that both lactic acid-DES and aqueous solution show similar dissolution behaviors. X-ray absorption spectroscopy measurements showed that REEs tend to form lactate complexes in both solvents and that the hydrogen bond acceptor part of DES is not directly involved in the dissolution processes. The proportion and stability of these REE-lactate complexes in solution appear to be a crucial limitation for REE solubilization in high REE-concentrated materials, whereas this was not the case for REE-diluted materials. However, under the conditions of this study, all the DES tested showed a lower dissolution capacity than the lactic acid aqueous solution, raising the question of the value of using lactic/citric acid-based DES as dissolution solvents.