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

Julien Couturier, Clément Levard, Blanche Collin, Perrine Chaurand, Vladimir Vidal, Olivier Mathon, Adrien Duvivier, Bernard Angeletti, Daniel Borschneck, Jérôme Rose, et al.

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Julien Couturier, Clément Levard, Blanche Collin, Perrine Chaurand, Vladimir Vidal, et al. "Dissolution of rare earth elements: Exploring the ability of deep eutectic solvents and organic acid solutions, the case of lactic acid." Separation and Purification Technology, 2025, 366, pp.132740. 10.1016/j.seppur.2025.132740. hal-05023870

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

Julien Couturier<sup>a,b,*</sup>, Clément Levard<sup>a</sup>, Blanche Collin<sup>a</sup>, Perrine Chaurand<sup>a</sup>, Vladimir Vidal<sup>a</sup>, Olivier Mathon<sup>b</sup>, Adrien Duvivier<sup>a</sup>, Bernard Angeletti<sup>a</sup>, Daniel Borschneck<sup>a</sup>, Jérôme Rose<sup>a</sup>, Stéphane Pellet-Rostaing<sup>c</sup>, Guilhem Arrachart<sup>c,*</sup>

## ARTICLE INFO

Editor: M Freire

**Keywords:**

Deep eutectic solvent
Lactic acid
Rare earth elements
Dissolution
Speciation
Dissolution processes

## ABSTRACT

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.

## 1. Introduction

Deep eutectic solvents (DES) have been broadly defined by Martins et al. [1] as a mixture of two or more pure compounds for which the eutectic point temperature is lower than that of an ideal mixture. This non-ideal behavior of the melting temperature is usually attributed to interactions between the components in the form of hydrogen bonds, although these interactions are actually quite complex (neutral, ionic, doubly ionic) and only partially understood [2]. Generally speaking, a DES is therefore a mixture formed by a hydrogen bond donor (HBD) compound and a hydrogen bond acceptor (HBA) compound, whose melting temperature depression is such that it becomes liquid at a given operating temperature within a certain composition range.

This broad definition gives rise to almost infinite possibilities, and

the use of these solvents has been considered for many applications [3]. Because of their modular properties (intrinsic to the notion of a mixture) and their categorization as "green" solvents (mainly due to their non-volatility) this emerging type of solvent has been the subject of particular interest, especially in the context of green chemistry. Among the different types of DES described over the last twenty years [4–6], those based on a quaternary ammonium salt as a HBA, and an organic molecule as a HBD, (so-called type III DES) are by far the most widely used according to the literature [2].

The acidic character of some DES, whose HBD role is held by a carboxylic acid, has been explored for metallurgical applications (leaching, liquid-liquid extraction, electrodeposition) [7]. Indeed, the ability of the DES to solubilize metal oxides was one of the first properties investigated [8,9]. Since then, this ability has been confirmed, and

<footnote><sup>a</sup> Aix-Marseille Univ, CNRS, IRD, INRAE, CEREGE, 13545 Aix-en-Provence, France</footnote>

<footnote><sup>b</sup> European Synchrotron Radiation Facility (ESRF), Grenoble, France</footnote>

<footnote><sup>c</sup> ICSM, Univ Montpellier, CEA, CNRS, ENSCM, Marcoule, France</footnote>

<footnote>* Corresponding authors.
E-mail addresses: julien.couturier@esrf.fr (J. Couturier), guilhem.arrachart@umontpellier.fr (G. Arrachart).</footnote>
some recent publications have focused on the interactions between rare earth elements and DESS.

Rare earth elements (REEs), are a group of metals with similar physicochemical behavior that include the lanthanide family as well as yttrium (Y, Z = 39). REEs are commonly divided into two subgroups: light rare earth elements (LREEs), which comprise the first half of the lanthanides (La, Ce, Pr, Nd, Pm, Sm, Z = 57 – 63), and heavy rare earth elements (HREEs), which comprise Y and the second half of the lanthanides (Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Z = 64 – 71). However, this classification remains purely arbitrary, since there is no abrupt break in the properties between these two subgroups, but rather a gradual change in properties (such as ionic radius) [10], and Scandium (Sc, Z = 39), although regularly associated with REEs, is not investigated here because it does not belong to any of the subgroups due to divergent properties resulting from a smaller ionic radius.

Due to their particular properties in terms of magnetism, luminescence or catalysis, these metals have become essential in many current key technologies [11,12]. As today, they are of considerable economic importance, the question of a secure and sustainable supply of REEs has become crucial for many countries [13–16], and more and more studies are focusing on the dissolution and separation of REEs using green chemistry, including DES [17,18].

The dissolution behavior of REEs in carboxylic acid-based DES has been studied in several ways.

One type of study concerns the dissolution of synthetic REE phases, such as oxides [19–26], carbonates [27,28], or chloride salts [24,29] in DES. Although numerous combinations of REEs in DES systems have been studied, it is very difficult to draw a general conclusion. Indeed, the dissolution behavior of REEs in DES can vary according to many parameters. First and foremost, the nature of the DES (the nature of the components, the HBD:HBA ratio, the proportion of H₂O added), but also the experimental conditions (temperature, time, liquid to solid L:S ratio) and finally on the nature of the REE phase (element present and specification of the REEs). However, these parameters usually differ from one study to another. In general, such studies attempt to establish proofs of concept of REE phase solubilization or of the difference in solubilization between the phases (selectivity) in DES, and usually constitute a screening step to identify the best DES system for a given application.

Another type of study concerns the leaching into carboxylic acid-based DES of more complex materials containing REEs, such as spent permanent magnets [20,23,30–32], spent fluorescent lamps [22], some industrial or mining wastes [33–36] and even ores [27,37,38]. These studies propose more or less complete processes for recovering REEs from primary or secondary sources, with the aim of producing REEs in an alternative and sustainable way in order to reduce their criticality.

Whereas the majority of studies on the dissolution of REEs in DES focus on applications (efficiency, selectivity), a few try to understand the nature of the interactions between REEs and DES [20,29], or even propose reaction pathways [23]. In addition, a few studies compare the efficiency of carboxylic acid-based DESs with corresponding carboxylic acid solutions for REE dissolution applications, and for those that do, there is no consensus concerning the advantages of using carboxylic acid-based DES compared to using organic acid alone [20,22,33,34].

Due to the difficulties involved in using these emerging solvents for industrial applications [39], certain subjects need to be tackled as a priority before considering REE dissolution applications in real cases with DES. These subjects are: (i) the influence of the nature and specification of REE on dissolution results in DES; (ii) identification of the dissolution processes involved in DES, and in particular, the role of each component; and (iii) the advantages of using a carboxylic acid-based DES over using an equivalent aqueous carboxylic acid solution.

To this end, the present work investigated using a DES as dissolution solvent, i.e. a mixture of choline chloride (ChCl) as HBA, lactic acid (LA) as HBD and H₂O, with an HBA:HBD ratio based on the study of Crespo et al. [40]. Dissolution experiments were performed using six REEs in four types of speciation (oxide, carbonate, phosphate, chloride). The

dissolution capacity and the behavior of the DES were compared with those of lactic acid solution, pure H₂O, and other variations of the original DES after changing the individual components. To better understand these systems, the results were compared with those obtained using simplified speciation calculations and the dissolution products were analyzed using pH, X-ray absorption spectroscopy and X-ray diffraction.

A better understanding of the interaction between REEs and one of the most widely used systems in the literature (lactic acid-based DES) [20,25,26,29,31,34,37,38] will help to produce the knowledge required to guide the use of carboxylic acid-based DES for the dissolution of complex materials. Finally, as a "real case experiment", dissolving REEs from bauxite residues of two different origins, a potential interesting secondary source for their extraction, was tested using both the lactic acid-based DES and the lactic acid solution.

## 2. Materials & methods

### 2.1. Preparation and characterization of the solvents

An aqueous solution of lactic acid at 1 mol/L (Sol LA 1M) was prepared using DL-lactic acid (aqueous solution, 90 wt%) and pure H₂O.

A lactic acid-based deep eutectic solvent (DES LA) was formulated by mixing choline chloride ((CH₃)₃NCH₂·CH₂OH⁺Cl⁻ or ChCl, 99 wt%) as a hydrogen bond acceptor (HBA), DL-lactic acid (aqueous solution, 90 wt %) as a hydrogen bond donor (HBD) and pure H₂O. The final water content of 12 % by weight was chosen to reduce the viscosity of the DES and make it easier to handle during experiments and analysis. Variations in this DES were also formulated using L-(+)-lactic acid (anhydrous, 98 wt%) or citric acid (≥99.5 wt%) as HBD, and using guanidine hydrochloride (CH₅N₃·HCl or GuCl, ≥99 wt%) as HBA (see **Figure S1** for chemical structures).

HBAs were purified by drying at 90 °C (≥2h) and were placed in a desiccator before use. The appropriate quantities of HBA, HBD and H₂O were added (in the presence of a stirring bar) and the mixture was subject to a formulation step by microwave heating under atmospheric conditions until a colorless liquid was obtained (10 min, 70 °C, 600 rpm, 2.45 GHz, variable power) using a microwave reactor (Discover 2.0, CEM). Microwave heating was used to promote agitation, ensure uniform heating, and reduce formulation time, while minimizing energy consumption.

The DESs were used within two weeks of formulation to minimize degradation effects due to ageing [41].

The properties of the DESs were then characterized by measuring the density at (25 °C, DSA 5000, Anton Paar), viscosity at (25 °C, 200 s⁻¹ shear rate, 5 replicates, microVISC, RheoSense) and H₂O content (25 °C, 3 replicates, Karl Fischer titration, 809 Titrando, Metrohm). Fourier transform infrared (615 – 4000 cm⁻¹, ATR cristal, spectrum 100, PerkinElmer) spectroscopy analyses were also performed to confirm the formation of hydrogen bonds and the absence of degradation products in some DESs. (Figure S2).

Details of the formulation and properties of all the types of DESs are listed in **Table S1**.

### 2.2. Materials

REE (La, Nd, Gd, Dy, Y, Yb) synthetic phases of oxides ((REE)₂O₃·xH₂O), carbonates ((REE)₂(CO₃)₃·xH₂O), phosphates ((REE) PO₄·xH₂O), and chlorides ((REE)Cl₃·xH₂O) were used for the dissolution experiments (Table 1). The six elements were selected from the LREE and HREE subgroups in order to obtain results that are representative of all the REEs, and the phases were chosen from the most common speciation found in nature.

All synthetic REE materials were purchased (≥ 99.9 wt% purity) except for phosphates of Nd, Gd, Dy and Yb that were synthesized by dissolving 0.08 mol of REE nitrate ((REE)₂(NO₃)₃·xH₂O) in 80 mL of a
<table><caption>**Table 1**
Nature of rare earth elements studied and each associated ionic radius (using data from reference [42]). Types of synthetic phases used for dissolution experiments and associated solubility equilibria in presence of H₂O.</caption><thead><tr><th>REEs</th><th>La</th><th>Nd</th><th>Gd</th><th>Dy</th><th>Y</th><th>Yb</th></tr></thead><tbody><tr><td>Ionic radius(charge: +3, CN: 6) (pm)</td><td>103.2</td><td>98.3</td><td>93.8</td><td>91.2</td><td>90.0</td><td>86.8</td></tr><tr><td>Types of REE synthetic phase used</td><td colspan="6">Solubility equilibria in H₂O</td></tr><tr><td>Oxide</td><td colspan="6">(REE)₂O₃(s) + 6H⁺ ⇌ 2(REE)₃⁺ + 3H₂O</td></tr><tr><td>Carbonate</td><td colspan="6">(REE)₂(CO₃)₃(s) + 3H⁺ ⇌ 2(REE)₃⁺ + 3HCO₃⁻</td></tr><tr><td>Phosphate</td><td colspan="6">(REE)₃PO₄(s) + 2H⁺ ⇌ (REE)₃⁺ + H₂PO₄⁻</td></tr><tr><td>Chloride</td><td colspan="6">(REE)Cl₃(s) ⇌ (REE)₃⁺ + 3Cl⁻</td></tr></tbody></table>

0.2 mol/L solution of sodium phosphate (H₂NaPO₄·H₂O). After 30 min of stirring at ambient temperature, the resulting solids were washed several times in pure H₂O, frozen at -20 °C for 2 h, and subjected to a freeze-drying cycle (-40 °C, 40 mTorr, 2 days) before being heated to 900 °C for 2 h in an oven. Some of these materials are hygroscopic and/or their degree of hydration is not known, whereas this information is needed to determine the quantity of REEs added during the dissolution experiment. The actual degree of hydration of the materials was then determined by thermogravimetric analyses (from 25 °C to 950 °C, 5 °C/min, under nitrogen flow, Mettler Toledo instrument) under conditions similar to those used in the dissolution experiments (Figure S3). In some cases, the structure of the materials was also checked by X-ray diffraction (0.026° step size, 15 rpm rotation, 40 kV, 40 mA, λ(Co) = 1.79 Å, X'Pert Pro, Malvern Panalytical) (Figure S4-S6).

Two bauxite residues of different geological origin (lateritic from Guinea for L-BR-Gui-E and karstic from France for K-BR-Mar-A), extensively characterized in a previous study [43], were also used for leaching experiments on complex materials containing REEs.

## 2.3. Dissolution experiments

REE synthetic phases were dried in a desiccator for 24 h prior to each experiment. Then, regardless of the nature of the material, 0.2 mmol of REE was added in 2 mL of solvent (i.e. at a constant ratio of 0.1 mol of REE per L of solvent). The mixture was homogenized using a vortex mixer and subjected to a microwave heating step under atmospheric conditions (15 min, 70° C, 1200 rpm, 2.45 GHz, variable power, Discover 2.0, CEM), directly followed by a filtration step (50–70 °C) using a 0.2 µm syringe filter to obtain a leachate.

For the leaching experiment, 60 mg of ground and dried complex materials were added to 3 ml of solvent (L:S = 50), after which the same protocol was used for the experiment as that described above.

Each experiment was carried out in sequence in triplicate, except for the dissolution in pure H₂O, for which only a single replicate was performed.

## 2.4. Aqueous geochemical calculations

Speciation diagrams of REE ions were simulated at 25 °C using Spana software (v2020). The activity coefficients were calculated using the simplified Helgeson-Kirkham-Flowers model [44] (based on Debye-Hückel model) and the ionic strength was calculated iteratively assuming electroneutrality for the system (Na or Cl ions were added to keep each calculation point electrically neutral). The equilibrium constants for complexation between lactic acid and REEs (see Fig. S7) were manually incorporated into the Spana database from the work of Martel and Smith [45].

Thermodynamic modeling of the dissolution of REE phases in H₂O were carried out using Phreeqc software (v3) [46]. The compounds selected were as close as possible to the experimental phases, depending whether they were available in the LLNL (Lawrence Livermore National

Laboratory) database (llnl.dat is based on Debye-Hückel model).

## 2.5. Chemical analysis and characterization of the dissolution products

The leachates resulting from the dissolution of the REE phases were diluted 5000 times with HNO₃ (2 vol%) and dissolved REEs were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES, Agilent 5800 VDV). Calibration curves (linear, forced to 0) from 0.05 mg/L to 10 mg/L were obtained using a multi-element standard solution containing all the REEs (Elemental Scientific Inc.). Measurements were performed at three different wavelengths for each element, avoiding interfering emission peaks. The leachates resulting from the leaching of the complex matrix were diluted 500 times with HNO₃ (2 vol%) and the dissolved elements were analyzed by inductively coupled plasma mass spectroscopy (ICP-MS, Perkin Elmer 300X quadrupole). The same standard solution was used for ICP-MS measurements over a concentration range from 2 to 100 µg/L. A blank was performed for each solvent using the same protocol without addition of the material. The results of the experiments are presented as a % dissolution yield for each element considered (X):

$$ \%_{\text{dissolution}}(X) = 100 \times \frac{[X]_{\text{leachate}}}{[X]_{\text{introduced}}} $$

where [X]leachate is the concentration of the element in the leachate in g/L and [X]introduced is the original concentration of elements added to the mixture in g/L.

The pH of the initial solvents and leachates after the experiments was measured after calibration of the pH meter (FiveEasy 20, Mettler Toledo) by direct immersion of the electrode in the solvent or leachate. Such measurements are only indicative in the case of DES media, due to the low H₂O content and the potential non-equivalence between proton (H⁺) concentration and activity.

Some dried precipitates from leachates were ground and deposited on low background silicon plates for analysis by X-ray diffraction (0.026° step size, 15 rpm rotation, 40 kV, 40 mA, λ(Co) = 1.79 Å, X'Pert Pro, Malvern Panalytical). Phase identification and Rietveld refinement (sample displacement, lattice parameters, peak shape and preferred orientation) were performed using the X'pert Highscore plus Malvern Panalytical software.

## 2.6. X-ray absorption spectroscopy (XAS) measurements

XAS spectra at La K-edge (38.925 keV) were recorded at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) on beamline BM23 (Dataset: [47]). As the EXAFS region of the XAS spectrum is very sensitive to interactions between the absorbing atom and its closest neighbors, it was therefore preferred to the XANES region (see Fig. S8) for identifying the nature of the species present in the La solvation sphere. The La K-edge was studied rather than L₃ edges, in order to avoid EXAFS limitations due to the proximity between the L₃ and L₂ edges. Moreover, without the high-energy resolution fluorescence detection technique (HERFD), the EXAFS study of La is not possible at the L₃ edges (blocked by L₂).

Data were collected in the transmission or fluorescence mode, depending on the La concentration in the samples or model compounds, in an He cryostat at 20 K to prevent beam damage and with an X-ray beam size ranging from 1 x 1 to 1 x 4.5 mm².

All the samples were liquid samples consisting of leachates of La synthetic phases dissolved in various solvents. Model compounds consisted of ad hoc aqueous solutions of La compound (pH = 5): La³⁺ free ions (La³⁺ sol.), tri-lactate complex of La (La-lactate sol.) and citrate complex of La (La-citrate sol.); and also solid pellets (~30 mg) of commercial La compound (purity ≥ 99.9 wt%) diluted with polyvinylpyrrolidone (PVP): lanthanum oxide (La₂O₃, Sigma-Aldrich), lanthanum carbonate hydrate (La₂(CO₃)₃·xH₂O, Sigma-Aldrich),
lanthanum phosphate hydrate (LaPO₄·xH₂O, Sigma-Aldrich), lanthanum chloride heptahydrate (LaCl₃·7H₂O, Alfa Aesar), anhydrous lanthanum chloride (LaCl₃ anhydrous, Sigma-Aldrich).

For liquid samples and aqueous model compounds, the thickness of the liquid cells (x = 0.1–1 cm) was chosen as a compromise between an optimal Δμ<sub>theoretical</sub> of 1 and a maximum of μx < 4 after the edge

calculated using Hephaestus software (v0.9.26) [48]. The organization within liquid samples, including DES, is assumed to be preserved at low temperature due to the rapid cooling of the cells in the cryostat [49]. For solid pellets, the dilution ratio with PVP was calculated to obtain an optimal Δμ<sub>theoretical</sub> of 1 using ABSORBIX software (v3.02) [50].

Each XAS scan was recorded from 38.60 to 39.90 keV (k = 16 Å⁻¹)

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Dissolution yields of REE (La, Nd, Gd, Dy, Y, Yb) oxides, carbonates, phosphates and chlorides after microwave heating (15 min, 70 °C, [REE] = 0.1 mol/L) using (A) pure H₂O, (B) Sol LA 1M and (C) DES LA. pH measurements by direct immersion of the pH meter in the initial solvent and in the resulting leachates using (D) pure H₂O, (E) Sol LA 1M and (F) DES LA. Dissolution values > 100 % come from uncertainties in weighing and determining the degree of hydration of the REE phases. Due to the specific properties of DES, pH measurement cannot be accurate here, meaning these data are only indicative.</figcaption></figure>
with an energy step of 5 eV between 38.60 and 38.89 keV, of 0.5 eV to 38.94 keV, and a k step of 0.035 from 38.94 to 39.90 keV (1 s counting time). A pellet of La₂O₃ model compound was used as a reference to control and calibrate the incident X-ray beam energy during the experiment. The EXAFS spectra presented in this article are the average of 2 to 10 scans depending on the signal-to-noise ratio. Data were analyzed using Fastosh software (v1.0.4) [51]. Linear combination fitting (LCF) of the EXAFS region was performed from k = 2.5 to k = 10, with a maximum of two components (adding a third component did not significantly improve the fits).

# 3. Results & Discussion

## 3.1. Dissolution experiments

Table 1 shows the six REEs classified according to their decreasing ionic radius, the different REE synthetic phases used for the dissolution experiments and their solubility equilibria in the presence of H₂O. To obtain an in-depth understanding of the dissolution behavior of these phases in DES LA, it is useful to examine and explain the results of dissolution experiments carried out in pure H₂O and in the aqueous solution of lactic acid (1 mol/L), Sol LA 1M. The results of all the dissolution experiments using microwave heating, together with resulting pH of the leachates, are shown in Fig. 1.

### 3.1.1. Results of dissolution in pure H₂O

As shown in Fig. 1A, the synthetic REE phases are not soluble in pure H₂O after the dissolution process, except for REE chloride salts which are completely solubilized regardless of the nature of the REEs. Here, the only variations in pH before and after the materials come into contact with H₂O are certainly due to the presence of impurities associated with the processes used for the synthesis of the materials (Fig. 1D). For example, in the case of YPO₄, traces of phosphoric acid can lower the pH of the solvent. However, no dissolution was associated with the variations. On the contrary, the absence of variation in pH in the case of total dissolution of REE chloride salts shows that H⁺ protons are not involved in the reaction, as suggested by the expected solubility equilibrium (Table 1). The solutes REE³⁺ and Cl⁻ are simply stabilized by the action of H₂O (ion-dipole interaction). In the case of oxide, carbonate or phosphate REE phases, the action of H₂O (hydrolysis), the moderate pH of the solvent (4–6) and the temperature (70 °C) are not sufficient to cause dissolution.

### 3.1.2. Results of dissolution in Sol LA 1M

In the case of Sol LA 1M, REE chloride salts are completely solubilized, while the REE phosphates are not soluble, like in pure H₂O. On the other hand, there is some solubilization of REE oxides and carbonates, depending to a great extent on the nature of the REE (Fig. 1B). LREE oxides and carbonates were very well solubilized (> 85 % for La and Nd), but the presence of REE in the solution decreased sharply as a function of ionic radius to values below 10 % for Y or Yb.

#### 3.1.2.1. Effect of pH (acid hydrolysis).
The pH of Sol LA 1M played an important role in these results. In the case of oxides, carbonates and phosphates, the greater the solubilization of REEs by the solvent, the higher the pH of the leachate compared to the initial pH of Sol LA 1M (1.70 ± 0.06) (Fig. 1E). This shows that the consumption of proton H⁺ was consistent with the solubility equilibria described in Table 1. However, the pH of the initial solution seems to be insufficiently acidic to weaken the REE phosphate bonds. These experimental behaviors were confirmed by modeling the effect of pH on the dissolution of different phases of La in H₂O at 70 °C using Phreeqc software (Figure S9).

However, other calculations showed that the marked variations observed experimentally in REE solubilization for the same type of phase

(oxide and carbonate) cannot be reproduced or explained by the effect of pH alone, or by variations in the thermodynamic constants. On the contrary, according to Spahiu and Bruno [52], the evolution of the solubility products (pKₐₚ) between Yb (33.3) and La (35.3) carbonates suggests that, for a given pH, the dissolution yield in the case of Yb should be more pronounced than in the case of La at equilibrium. While the Phreeqc model involves small variations in this direction, it is the reverse of what we observed experimentally, with very little Yb solubilized compared to La in the case of carbonate phases.

Furthermore, the decrease in pH (1.08 ± 0.14) observed in the case of REE chloride leachates, while dissolution did not affect pH in pure H₂O (Fig. 1D), points to the presence of other chemical reactions in this system that lead to the release of H⁺.

#### 3.1.2.2. Effect of ligand-promoted dissolution.
The lactic acid present in Sol LA 1M can associate with REE³⁺ ions to form organometallic complexes characterized by metal-ligand bonds. Lactic acid is a Lewis base capable of delocalizing part of its electron density to stabilize the charge of the REE cation, thereby forming a Lewis acid (presence of an electron hole).

The ability to act as a ligand means lactic acid may be involved in the dissolution process. The ligand-promoted dissolution mechanism is mainly defined as the complexation of free REE³⁺ ions in solution after partial dissolution of the REE material, although other routes have also been proposed [53]. Thus, the formation of these metal-ligand complexes consumes the REE³⁺ ions in solution, causing a shift in the equilibrium of the reactions (Table 1), thereby favoring the dissolution of more solids. Many different parameters may be involved in these reactions, including pH, which affects the initial dissolution of the phase as well as the protonation of the ligand.

The presence of lactic acid (lacH) or its conjugate base (lac⁻) in solution is governed by a pH-dependent acid base equilibrium:

$$lacH \rightleftharpoons lac^{-} + H^{+}$$

As lactic acid is a weak acid (pKa = 3.9), its protonated form (lacH) predominates at pH < 3, which is the case here. The complexation reactions with the REE³⁺ ions in solution can consequently be written as:

$$(REE)^{3+} + n lacH \rightleftharpoons [(REE) lacn]^{(3-n)+} + n H^{+}$$

where n (here 1, 2 or 3) is the order of complexation. The equilibrium is then described by the respective complexation constants βₙ.

When favorable, this complexation reaction leads to the release of H⁺ protons, and therefore to a decrease in pH. The presence of complexation reactions with lactic acid was confirmed by the decrease in pH of the leachates resulting from the complete dissolution of the chloride phases (dissolution independent of the pH) (Fig. 1B, 1E). In the case of the other phases, this effect was less visible because the consumption of H⁺ was balanced between the dissolution reaction and the release of H⁺ due to REE complexation.

However, for a given n, the changes in the values of the complexation constants βₙ (25 °C) of lactic acid with REEs indicate rather higher stability for HREEs complexes than for LREEs (Figure S7). This should result in increased dissolution yields of HREEs compared to those of LREEs, in contrast to what we observed for oxides and carbonates. It therefore seems necessary to consider the proportion of each possible species present in solution.

#### 3.1.2.3. Role of the REE tri-lactate complex.
Speciation of REE³⁺ ions in the presence of the 1 mol/L of lactate ligand were calculated in order to simulate dissolution experiments with Sol LA 1M in a simplified system (at 25 °C and without considering counter ions) (Fig. 2A, 2B). At the pH values considered in the leachates (between 1 and 3), REEs can coexist in different forms in solution: free ions, mono-, bi- or tri-lactate complexes. The proportions of each species vary with the pH of the solution
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Aqueous speciation of La³⁺, as a LREE ion, and Yb³⁺, as a HREE ion, in the presence of the lactate ligand (lac) under the conditions used for the dissolution experiments with Sol LA 1M (1 mol/L) (respectively (A) and (B)), and with DES LA (5.6 mol/L) (respectively (C) and (D)). The proportion of REE tri-lactate complex (REE(lac)₃) in solution is shown in each case at pH = 2 (in red). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

and with the nature of the REE. Indeed, free ions dominate at low pH and the proportion of tri-lactate REE complex increases with an increase in pH. Furthermore, for a given pH above 1–1.5, the proportion of tri-lactate complex is higher for HREEs compared to LREEs. For example, at pH 2, the expected proportion of Yb(lac)₃ (≈ 21 %) is significantly higher than that of La(lac)₃ (≈ 2 %). The fact that, in the presence of LREEs, a lower proportion of n-lactate complexes (with a high complexation order) were present in solution compared to HREEs explains why the pH stabilized at a higher value in the leachate of LREE synthetic oxides and carbonates (fewer H⁺ protons released by complexation) (Fig. 1E).

Moreover, among the possible REE species in solution, the tri-lactate complex (REE(lac)₃) is not necessarily water soluble due to its neutrality (charged complexes (n = 1 or 2) will be easily stabilized in solution by the dipolar nature of the H₂O molecule). According to the literature, precipitation or crystallization phenomena of REE lactates have already been observed after dissolution of REE chlorides, nitrates, oxides or carbonates in the presence of ammonium lactate or lactic acid [54–58]. In addition, it has been shown that the solubility of REE tri-lactate complexes in H₂O at 60 °C decreases sharply with a decrease in the

ionic radius of the REEs, from 76 g/L for La, to 7.5 g/L for Gd and finally 1.5 g/L for Yb [56,57] (Table S2).

Finally, only the combined effect of a higher proportion and lower solubility of the REE tri-lactate complex in solution with the decreasing ionic radius of the REE, tends to explain why the presence of solubilized REE decreases from La to Yb in REE oxide and carbonate leachates. However, it is important to note that this effect was not observed in the dissolution of REE chloride phases, where the very acidic pH of the leachates favored the presence of soluble charged complexes in all leachates.

### 3.1.3. Results of dissolution in DES LA

In the case of DES LA, the REE phosphates were not soluble and the solubility of REEs with other phases depended to a great extent on the nature of the REEs. In the case of oxides and carbonates, La and Nd were completely or partially solubilized, whereas REEs with an ionic radius < 98 pm were insoluble (< 1 % dissolution yield). In contrast to the Sol LA 1M solvent, REE solubilization with chloride salts also decreased with the ionic radius of the REEs (from a radius < 93 pm) in the DES LA (Fig. 1C). These experiments show that, under these conditions, DES LA
appears to be less effective for REE dissolution than a simple 1 mol/L lactic acid solution. However, these results are consistent with the literature, as the lack of solubility of Eu, Gd, Tb or Dy oxides in ChCl:LA (1:2) mixtures (24 h at 50 °C or 80 °C) has already been observed [25,26]. Amphlett et al. [29] also observed a decrease in the solubility of REEs as a function of ionic radius when chloride salts were dissolved in an anhydrous ChCl:LA (1:1) mixture ([REE³⁺] = 0.1 mol/L, ambient temperature), with high solubility for Nd and Eu and little or no solubility for Dy, Er and Yb.

The pH values measured with the pH meter in DES LA leachates showed similar trends to those observed with Sol LA 1M. An increase in pH, relative to the initial solvent, was observed when REE from oxides and carbonates were well solubilized in DES LA, and a decrease in pH was observed in the case of REE from chloride salt leachates (Fig. 1F).

Although similar pH values have been observed in similar lactic acid-based DES using pH electrode probes [59–61], this methodology is not suitable to correctly assess acidity in such media. Indeed, the concentrated species present in DES can influence the activity of H⁺ as a proton donor or acceptor, and the low H₂O content can alter the function of the glass electrode, which at best provides a relative pH measurement [62]. Several alternative methods have been explored to provide information on the acidity of a DES medium [63], including methods based on dilution of DES in water [24,64,65], infrared or NMR probes [66,67], colorimetric indicators, or acid-base titration [61]. In particular, the latter technique could be a good alternative to complete our study, since it has the advantage of expressing the strength of acidity as a number, providing quantified information about the acidity of the system, and has been shown to continue working when the value indicated by the pH electrode is saturated [61]. The implementation of this type of analysis, which can be restrictive, was not implemented since the objective was to provide an indicative value.

In DES LA, the chemical processes can be difficult to identify. Indeed, several parameters differ from the Sol LA 1M solvent: a high concentration of LA (≈ 5.6 mol/L), low H₂O content (≈ 12 wt%), the high proportion of choline chloride (HBA) (≈ 46 wt%), the high viscosity (97 mPa.s) or density (1.16) of the mixture. These parameters can influence the activity of the protons, the effect of ligands, the ionic strength, the mass transfer, the reaction kinetics, among others, and some models are beginning to emerge to explore these interactions in more depth, such as for the viscosity of DES [68].

Nevertheless, at first glance, the results suggest that the dissolution of REE phases in DES LA involves similar behavior to those in Sol LA 1M, but in an enhanced manner. Several hypotheses can explain this trend, based on the chemical processes described above.

### 3.1.3.1. Effect of higher concentrations of lactic acid.
Fig. 2C, 2D show speciation calculations for REE³⁺ ions in the presence of the 5.6 mol/L of lactate ligand to simulate the concentration in DES LA (at 25 °C, without considering counter ions or the effect of ChCl). Increasing the concentration of lactic acid from 1 mol/L to 5.6 mol/L shifted all the predominance curves for the species present toward lower pH values (of about 0.8 pH unit). Thus, for a given pH of 2, La and Yb are more likely to be in the form of a tri-lactate complex (44 % and 74 % respectively) under the conditions of lactic acid concentration in DES LA than in Sol LA 1M (respectively, 2 % and 21 %).

The possible increase in the proportion of REE tri-lactate complexes in DES LA compared to Sol LA 1M may imply a decrease in soluble complexes, especially for HREEs for which the precipitation of tri-lactate complexes is favored. This hypothesis would then explain the lower level of soluble HREEs in DES LA leachates, compared to that in Sol LA 1M leachates. However, this effect has to be balanced by the fact that the high lactic acid concentration in DES LA probably leads to a decrease in pH compared to Sol LA 1M, although the pH could not be measured quantitatively for the DES systems.

### 3.1.3.2. Effect of lower H₂O content.
Assuming the solubility of REE tri-lactate complexes is only described in relation to the aqueous fraction of the solvent, the smaller the aqueous fraction in the solvent, the lower its capacity to load these complexes. The reduced loading capacity of the DES LA compared to the Sol LA 1M leads to amplification of the difference in precipitation/solubilization between tri-lactate complexes of LREEs and of HREEs and to an overall reduction in solubilized REEs in the solvent.

### 3.1.3.3. Effect of a high concentration of ChCl.
The presence of a high concentration of salt (ChCl) in DES LA could result in a kind of “salting-out” effect, where ChCl competes with REE-lactate complexes for solubilization in the aqueous fraction. The availability of H₂O molecules for stabilization of the REE-complexes is then reduced, leading to amplified precipitation of the least soluble species, further affecting those that contain HREEs.

These hypotheses, and more generally the analyses described in this section, call into question the involvement of the HBA component (in this case ChCl) in the dissolution efficiency and processes of the DES, but also the effectiveness of lactic acid-based DES compared to lactic acid-based solution for the dissolution of REEs. To improve our understanding of the chemistry involved in DES LA, we investigated the speciation of REEs in the various leachates using X-ray absorption spectroscopy (XAS) as reported below.

## 3.2. Characterization of dissolution products

### 3.2.1. X-ray absorption spectroscopy (XAS) analyses of dissolution leachates

La K-edge (38925 eV) XAS was used to analyze leachates after dissolution of the La₂O₃, La₂(CO₃)₃ and LaCl₃ phases in DES LA, as well as two aqueous leachates with Sol LA 1M (Fig. 3, Table S3).

Qualitatively, all the leachates showed very similar EXAFS signals close to that of the La-lactate sol. model compound. This suggests that La was dissolved by ligand-promoted dissolution rather than remaining as small nanoparticles that may have been dispersed in the supernatant and not separated during the solid/liquid separation step comprising filtration at 0.2 µm. The results of linear combination fitting (LCF) confirmed and quantified this trend. Indeed, the La-lactate sol. model compound was the main compound present (61–75 %) in all the best fits, and reproduced the experimental signals very well ($\chi^2 < 0.5$). Thus, no difference in speciation in the leachates was found as a function of the nature of the synthetic La phases introduced, or between the two types of solvent studied.

The model compound La-lactate sol. was synthesized at pH = 5, with an excess of lactic acid, in order to favor the formation of tri-lactate complexes of La. However, in the absence of model compounds for mono- or bi-lactate complexes, and given the weak specificity of the signal, it is not certain that EXAFS analysis makes it possible to distinguish between mono-, bi- and tri-lactate complexes of La. However, we are able to affirm that lactic acid (HBD) plays a central role in the dissolution of La in DES LA, and that there was no indication that ChCl (HBA) was involved.

Further analyses of Y speciation (K-edge EXAFS analysis) also confirmed the predominance of Y lactate complexes in the leachates after YCl₃ dissolution in DES LA, while the 2 % dissolution yield observed in the case of YPO₄ (Fig. 1C) appeared to be undissolved YPO₄ nanoparticles that passed through the 0.2 µm filter (Figure S10).

Therefore, the XAS analyses suggest that the REEs in the DES LA leachates are present in the form of soluble lactate complexes.

### 3.2.2. Mineralogical analysis of precipitates formed in leachates during ageing

The strength of REE complexation with the lactate ligand increases with decreasing temperature [69]. As a result, the solubility of REE tri-
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** La K-edge EXAFS spectra of model compounds and some leachates resulting from the microwave dissolution experiments, with the number of average scans (in parentheses). The best linear combination fit for each leachate is superimposed in red and details of fitting parameters and results are shown on the right. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

lactate complexes in water decreased with temperature, from 76 g/L to 48 g/L for La and from 1.5 g/L to 0.7 g/L for Yb with a drop in temperature from 60 °C to 20 °C [56,57] (Table S2). These differences in solubility were observed during cooling (from 50-70 °C after filtration to room temperature) and ageing of the dissolution leachates. Indeed, after a few days or weeks of storage, precipitates (crystalline or less structured) were observed in most of the leachates based on Sol LA 1M and DES LA. Some were analyzed by X-ray diffraction (Fig. 4).

The analysis of the X-ray diffractograms of the solid precipitate from La₂O₃ dissolution leachates in Sol LA 1M and DES LA show very good agreement with one of the La tri-lactate structures proposed by Gashigullin et al. [54], namely: La(H₂O)₂(lac)₃. In the case of the precipitate from the YCl₃ dissolution leachate by DES LA, the majority of the diffraction peaks also correspond to the Y(H₂O)₂(lac)₃ structure proposed by the same authors, but two peaks remain unidentified, possibly indicating the presence of another species that co-precipitated with the

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** (A) Picture of a single crystal growing in Sol LA 1M leachate after La₂O₃ dissolution and ageing. (B) Picture of a crystalline precipitate in DES LA leachate after La₂O₃ dissolution. (C) X-ray diffraction analysis of precipitates from different leachates and comparison with theoretical diffractograms of REE(H₂O)₂(lac)₃ structures described by Gashigullin et al. [54].</figcaption></figure>
complex, or a slightly different structure.

All the analyses performed on the leachates and precipitates suggest that the REEs are present in solution in the form of lactate complexes after dissolution with Sol LA 1M and DES LA. In the case of the formation of tri-lactate complexes of REEs, an equilibrium was observed between the soluble form and the insoluble form that precipitates, which was even more pronounced after ageing at room temperature.

### 3.2.3. Comparison with the literature

In the literature, the question of metal speciation in DES is sometimes addressed. Hartley et al. [70] showed that the speciation of metal salts ($M = Cr, Mn, Fe, Co, Co, Zn, Pt, Au, Pd, Ag, Sn$) dissolved in diol- or urea-based DES ($HBA = ChCl$) was dominated by $[MCl_2]⁺$, $[MCl_3]²⁻$ or $[MCl_4]²⁻$ complexes in solution by XAS measurements, thereby highlighting the fundamental role of the $Cl⁻$ counter ion of HBA ($ChCl$) in the dissolution processes. Pateli et al. [71] showed that in the case of organic acid-based DES ($HBA = ChCl$), the complexation properties of HBD can enhance the solubility of metal oxides ($M = Mn, Fe, Co, Ni, Cu, Zn, Pb$) through surface complexation reactions, but that this is generally followed by ligand exchange with $Cl⁻$ in the bulk solvent.

Nevertheless, in both cases, the authors mention the possibility that the HBD plays a direct role in metal speciation by forming complexes. In the case of the dissolution of $Ni(II)$ salts in diol-based DES ($HBA = ChCl$), soluble $Ni$-HBD complexes were identified in solution [70]. In the case of organic acid-based DES ($HBA = ChCl$), precipitates were observed during the dissolution of the metal oxides depending on the affinity of these metals with the ligands concerned [71]. Precipitates were observed with $ZnO$ and $PbO$ in acetic acid-based DES, with $MnO$, $ZnO$ and $PbO$ in lactic acid-based DES, and with $MnO_2$, $MnO$, $CoO$, $CuO$, $Cu_2O$, $ZnO$ and $PbO$ in oxalic acid-based DES. A possible explanation for the formation of a higher proportion of organometallic complexes with REEs than with transition metals is that the electronic structure of rare earth ions (4f electrons not involved in metal-ligand bonding) and their large ionic radius allow the formation of very high coordination complexes (6 to 12) with no preferential direction [72].

Amphlett et al. [29] studied the speciation of the $NdCl_3$ and $EuCl_3$ phases in a $ChCl:LA$ DES (1:1, anhydrous environment) using XAS analysis, a very similar case to our study. In their case, the authors concluded that the speciation was probably $[(REE)Cl_5]⁻$, i.e. governed by the $Cl⁻$ counter ion of the HBA compound, as is preferentially the case for d- and p-block metals, and contrary to the results reported here. Nevertheless, a previous study by Amphlett & Choi [73] also showed that the addition of $H_2O$ had a significant effect on metal speciation in DESs, even in the presence of small amounts of $H_2O$ (< 5 %). This could explain the differences in REE speciation observed in the case of lactic acid-based DESs.

## 3.3. Variations in $H_2O$ content and in the nature of HBA and HBD

In order to approximate the DES mixture used by Amphlett et al. [29] and to investigate if the nature of the HBA or HBD components of the DES influences the chemical environment of the REEs in solution, three other DESs were formulated.

The first DES mixture was similar to DES LA but contained much less $H_2O$ (1.4 wt% instead of 12 wt%). The second mixture involved a modification of the HBA, in which the $ChCl$ was replaced by guanidine hydrochloride ($GuCl$) to form a $GuCl:LA$ (0.33:0.67) DES, which has already been characterized and used by Liu et al. and Yan et al. [20,26]. Its $H_2O$ content was then adjusted to 12 wt% to match the $H_2O$ content in DES LA. In the third DES formulation, the HBD fraction was modified, lactic acid was replaced by citric acid (CA) and the $ChCl:CA$ (0.69:0.31) mixture was formulated according to the molar ratio defined by Crespo et al. [40]. Subsequently, 10 wt% $H_2O$ was added to the mixture so it could be used with the dissolution protocol and to approximate the amount of $H_2O$ present in DES LA (Table S1).

Chemical analyses of the leachates showed that the composition of

the DES does have a major influence on its ability to dissolve $La_2O_3$ (Fig. 5A). For the same HBA/HBD mixture, complete dissolution of La was observed when the lactic acid-based DES contained 12 wt% $H_2O$, whereas it decreased to 3 wt% when the water content was reduced to 1.4 wt%. While the nature of the HBA does not seem to have much influence on the dissolution capacity in the present study (100 wt% and 85 wt% La dissolution for $ChCl:LA$ and $GuCl:LA$, respectively, in 12 wt% $H_2O$), changing the HBD (i.e. by replacing lactic acid with citric acid in $ChCl$ DES) significantly reduced the La dissolution yield (15 wt%).

However, La K-edge XAS measurements and LCF analysis showed that $H_2O$ content and changing HBA had no effect on REE speciation in the leachates (Fig. 5B, 5C). In both cases, the EXAFS spectra were very similar to the DES LA spectrum in the k range studied, although the signals were noisy when the concentrations of La in the leachates were low. The best LCFs ($χ^2 < 1.15$) showed predominance of the La-lactate sol. model compound (72–73 %), in the same proportions as for DES LA and the other mixtures shown in Fig. 3. In the case of the change in HBD, minor differences were observed in the EXAFS spectrum around k = 7, corresponding to the dominant presence (82 %) of a citrate complex of La in solution according to the LCF results. In any case, the results of the EXAFS refinement of the first coordination sphere showed that the calculated effective distances remained the same ($R_{eff} = 2.57–2.58 Å$), corresponding to the typical La-O bond distances in 9-coordinate structures (consistent with the $La(lac)_3·3H_2O$ complex) reported in the literature [74] (Figure S11, Table S4).

These results show, on the one hand, that La K edge XAS analysis can distinguish different types of REE organometallic complex speciation. On the other hand, it shows that, under the conditions of this study, the dissolution chemistry do not appear to vary with $H_2O$ content or with the nature of the HBA and HBD components in the DES. As mentioned previously, the ability of HBD to form organometallic REE-HBD complexes, which are more or less soluble in the aqueous fraction of DES, determines the amount of REE in solution, while HBA is not involved in the dissolution processes.

This may explain the low amount of solubilized La in DES containing only 1.4 wt% $H_2O$, which was limited by the tiny fraction of $H_2O$ available for the solubilization of La tri-lactate complexes. The low proportion of La in citrate-based DES could in this case be explained by the fact that La citrate complexes are easier to form ($β_1 = 7.1$) than La tri-lactate complexes ($β_3 = 5.7$) and are also particularly insoluble in water [45,75,76].

However, it is possible that the modifications of the physico-chemical properties of DESs associated with these changes (nature of the HBD and $H_2O$ content), such as acidity (proton activity) [24,71], viscosity (mass transfer) [40,77], degradation (by esterification) [41] or even molecular structure of DESs also affect the dissolution capacity of La.

For example, the effect of water content on the physico-chemical properties and molecular structure of $ChCl:LA$ DES mixtures has been studied by experimental means or molecular dynamic simulations [49,78–80]. It appears that a significant addition of water to the DES system (from a few wt% to a few tens of wt%) does not disrupt the hydrogen bonds between the HBA and the HBD; the mixture then consists in DES clusters more or less diluted in the water, leading to smooth changes in properties as a function of the dilution in water [49,78,80]. However, at very low concentrations (< 2000 ppm), the water remains confined within the DES structure, and interacts strongly with the hydrogen bonding network [79]. In our study, it was not possible to obtain such a low water content due to the high hygroscopicity of lactic acid-based DES [78], and the atmospheric conditions of the experiments, but it would be reasonable to assume that a strictly anhydrous environment, such as that used by Amphlett et al., [29] resulted in a very low water content (although this is not known).

This point, as well as the effect of the lower HBA/HBD molar ratio of the DES (1:1 compared to 1:1.5 in our case), seem to be the two hypotheses to be explored in order to better understand the differences in
<figure><img src="image_7.png" /><figcaption>Fig. 5. (A) Results of La₂O₃ dissolution after microwave heating (15 min, 70 °C, [REE] = 0.1 mol/L) using DES LA and the three variants formulated (decrease in H₂O content, change in HBA and HBD). (B) La K-edge EXAFS spectra of the leachates obtained, compared with the model compounds La-lactate sol. and La-citrate sol., with the number of raw spectra (in parentheses). The best linear combination for each leachate is superimposed in red. (C) Details of fitting parameters and results for each leachate. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

REE speciation in lactic acid-based DES, between the results obtained by
Amphlett et al. [29] ([[(REE)Cl₅·₆], Rₐff = 2.62–2.70 Å) and those pre-
sented here (La(lac)₃·3H₂O, Rₐff = 2.57–2.58 Å).

**3.4. Application to the dissolution of bauxite residues**

The aim of this final section is to compare the REE dissolution capacity of solvents in a case that is both real and highly complex. The two

materials studied here, L-BR-Gui-E and K-BR-Mar-A, are industrial
wastes resulting from alumina production that contain, respectively,
565 and 2261 mg/kg of REEs, in a matrix mainly composed of Fe (31.2
and 29.4 wt%, respectively), but also many other major (Al, Ti, Si, Na,
Ca) and minor elements [43]. In addition to the low concentration but
large number of different elements present, the complexity is also due to
the fact that different forms of speciation of REEs can occur in bauxite
residues [81].

<figure><img src="image_8.png" /><figcaption>Fig. 6. Comparative dissolution yields of REE and Fe (major element) with DES LA, Sol LA 1M and pure H2O solvents after leaching of (A) a lateritic bauxite residue (L-BR-Gui-E) and (B) a karstic bauxite residue (K-BR-Mar-A) (under microwave 15 min, 70 °C, L:S = 50, L = 3 mL).</figcaption></figure>
Fig. 6 shows the results of leaching these materials with DES LA, Sol LA 1M and pure H₂O, under experimental conditions similar to those described above. However, we were unable to measure the XAS leachate spectra as the concentration of La was too low.

For each material considered separately, the two lactic acid-based solvents showed similar dissolution patterns depending on the nature of the REEs, confirming the hypothesis of similar dissolution behavior in DES LA and Sol LA 1M. Furthermore, like with the synthetic REE phases, the dissolution capacity of Sol LA 1M was higher than (or at least equivalent to) that of DES LA for all REEs, with a possible increase of more than 20 %. Nevertheless, only very slight dissolution of Fe (the main element) occurred in both solvents and with both materials, making it possible to envisage selective dissolution applications, especially with aqueous solutions of organic acids. As expected from previous results, pure H₂O did not allow solubilization of either REE or Fe in the two materials, underlining the fact that the effect of acidity and of the ligand is paramount in this type of application.

In contrast to what has been reported concerning the dissolution of synthetic phases, a decrease in REE solubilization as a function of decreasing ionic radius was not systematically observed here. In fact, since the concentration of REEs in the leachates was very low (< 1 mg/L on average) compared to that used in previous experiments (up to more than 15 g/L), it can be assumed that the solubility limit of the REE tri-lactate complexes has not been reached and was consequently not the limiting factor for the dissolution yield of REE in this application.

Consequently, the lower REE dissolution yield with DES LA in the case of both bauxite residues could probably be due to differences in kinetics or mass transfer between the two systems due to viscosity of the solvents. Further experiments are now needed to confirm this hypothesis.

Furthermore, the difference in the REE dissolution pattern in the two materials produced by the two solvents seems to be due to the differences in REE speciation in these two materials, as already reported in our previous paper on the dissolution of REEs from lateritic and karstic bauxite residues using aqueous citric acid/citrate solutions at mild pH [82]. For example, the particularly low solubilization yield of HREEs in L-BR-Gui-E are consistent with the fact that, in contrast to K-BR-Mar-A, HREEs were in that case, mainly present in the form of phosphates, which, as previously observed, are relatively insoluble [43].

## 4. Conclusion

In this study, the dissolution behavior of various REE-containing materials was investigated in lactic acid-based DES and compared to their behavior in aqueous solutions.

The speciation and nature of the REEs appeared to play a crucial role in dissolution yield in each case studied. Before conducting dissolution experiments of complex materials in organic acid-based solvents, it is therefore indispensable to conduct in-depth investigations of speciation and to consider each individual REE separately, rather than as all belonging to the same family.

Despite the apparent complexity of the interactions in lactic acid-based DES, their solubility appears to be subject to the same chemical processes as aqueous lactic acid solutions and no beneficial influence of the HBA component was observed. Depending on the speciation of the REEs, the acidic pH of the solvents allows some dissolution of the solid phases, assisted by the lactate ligand that forms REE complexes (mono-, bi- or tri-lactate) in solution. In high concentration REE dissolution experiments (with synthetic phases), the final solubilization capacity of REEs in solvents seems to be mainly limited by the proportion and stability of the REE lactate complexes in solution. The proportion of REE tri-lactate complexes in solution increases with pH and with an increase in the concentration of the ligand in the medium, as well as with decreasing REE ionic radius. The stability of the REE tri-lactate complex increases with a decrease in the REE ionic radius and in the temperature of the system. However, the more stable the neutral complex, the more

insoluble it becomes in the aqueous fraction of the solvent, inducing partial precipitation and hence a loss of apparent solubilization. The addition of HBA to the lactic acid/H₂O mixture has, at best, no apparent effect (spectator ions), or involves a “salting-out” effect, thus reducing the solubilization capacity of the DES. In low concentration REE dissolution experiments, there is no longer a limit to solubility, so the final solubilization capacity of REEs in solvents seems to only rely on their macroscopic properties (pH, viscosity, etc.). These differences in behavior in high and low concentration REE materials should be taken into consideration in future studies. They underline the importance of choosing simplified cases as close as possible to the real case, so that the results can be extrapolated.

In all the cases we studied, the lactic acid-based DES was less effective than, or at best equivalent to, a simple lactic acid aqueous solution (1 M) in solubilizing REEs, and, in the case of chloride salts, less effective even than pure H₂O. These results cannot be extrapolated to all experimental conditions (water content, HBA:HBD ratio, etc.) and all carboxylic acid-based DES, which may have different hydrogen bond networks [78,79,83] or a more pronounced non-ideal mixture behavior [40], but they do highlight the importance of comparison with an existing alternative. Moreover, the use of a DES instead of a simpler aqueous solvent also involves a set of additional handling, analytical and cost constraints. Consequently, it would have to be motivated by drastic changes in dissolution properties (yield, selectivity) or unique advantages for the rest of the REE recovery process (fewer steps). For example, it may be more appropriate to focus on what makes a specific DES unique, such as the possibility of electrochemically assisted dissolution of REEs [84,85], liquid-liquid extraction of REEs with hydrophobic DES [86,87], or even REE recovery by electrodeposition [88,89].

## CRediT authorship contribution statement

**Julien Couturier:** Writing – original draft, Conceptualization, Methodology, Investigation, Formal analysis, Validation.
**Clément Levard:** Writing – review & editing, Resources, Conceptualization, Supervision, Funding acquisition, Project administration.
**Blanche Collin:** Investigation.
**Perrine Chaurand:** Writing – review & editing, Investigation.
**Vladimir Vidal:** Investigation.
**Olivier Mathon:** Investigation, Resources.
**Adrien Duvivier:** Investigation.
**Bernard Angeletti:** Investigation, Resources.
**Daniel Borschneck:** Writing – review & editing, Resources.
**Jérôme Rose:** Writing – review & editing.
**Stéphane Pellet-Rostaing:** Writing – review & editing.
**Guilhem Arrachart:** Writing – review & editing, Investigation, Conceptualization, Supervision, Funding acquisition, Project administration.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The authors acknowledge financial support from the CNRS through the MITI interdisciplinary programs (PRIME 2020:ExtraMet project), the French Agence Nationale de la Recherche through the RECALL project (ANR-20-CE04-0007). The authors thank Cyrielle Rey for technical assistance with TGA analysis and the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities on beamline BM23 (proposal number: CH-6762).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.seppur.2025.132740.
# Data availability

Data will be made available on request.

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