# Lipophilic EDTA-based ligands in different diluents for the extraction of rare earths: Preliminary results with Nd(III), Dy(III) and Pr(III) in chloride and nitrate media

Tamir Sukhbaatar, Raphaëlle Piton, Fabrice Giusti, Guilhem Arrachart, Magali Duvail, Aarti Kumari, Santosh Daware, Shally Gupta, Sriram Goverapet, Sushanta Kumar Sahu, et al.

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Tamir Sukhbaatar, Raphaëlle Piton, Fabrice Giusti, Guilhem Arrachart, Magali Duvail, et al. Lipophilic EDTA-based ligands in different diluents for the extraction of rare earths: Preliminary results with Nd(III), Dy(III) and Pr(III) in chloride and nitrate media. Hydrometallurgy, 2025, 231, pp.106415. 10.1016/j.hydromet.2024.106415. hal-04791847

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# Lipophilic EDTA-based ligands in different diluents for the extraction of rare earths: Preliminary results with Nd(III), Dy(III) and Pr(III) in chloride and nitrate media

Tamir Sukhbaatar<sup>a</sup>, Raphaëlle Piton<sup>a,1</sup>, Fabrice Giusti<sup>a,*</sup>, Guilhem Arrachart<sup>a</sup>, Magali Duvail<sup>a</sup>, Aarti Kumari<sup>b,2</sup>, Santosh Daware<sup>c</sup>, Shally Gupta<sup>c</sup>, Sriram Goverapet<sup>c</sup>, Sushanta Kumar Sahu<sup>b</sup>, Beena Rai<sup>c</sup>, Stéphane Pellet-Rostaing<sup>a</sup>

## ARTICLE INFO

### Keywords:

- Solvent extraction

- Rare earths recycling

- Permanent magnets

- EDTA-bis(N,N'-dialkylamide)

- Aminopolycarboxylate ligands

## ABSTRACT

A separation method for the recovery of neodymium(III) from two different media, chloride and nitrate, by implementing solvent extraction technique with a novel lipophilic diamide derivative of ethylenediamine tetraacetic acid (EDTA), namely 2,2'-N,N'-didecyl-dioxo-N,N',N'',N'''-tetraazatetratriacontane-N'',N'''-diyl diacetic acid (H₂E-4C₁₀), has been investigated. Initial screening results demonstrated acceptable solubility (ca. 15 mM) of the diamide in 1,3-diisopropylbenzene (DiPB) and in a 93:7 v:v *n*-dodecane:*n*-octanol mixture (DOm) as non-polar diluents. Further investigations aiming at determining operational parameters associated with the befitted extractant systems were conducted at pH ranging from 2 to 4 in chloride and nitrate media. A pH-dependent performance of the proposed systems revealed loading capacity peaking at pH = 3 with the respective values of 1.7 and 2.0 g L⁻<fnref n="1" /> in nitrate and chloride media. The extraction efficiency, the distribution coefficients, and the separation factor (*E*₀%, *D* and *SF*, respectively) were determined for an equimolar mixture of neodymium (Nd) and two other potential competitive lanthanides, namely dysprosium (Dy) and praseodymium (Pr). The best selectivity was observed in the chloride medium at pH = 4 yielding SF<sub>Nd/Pr</sub> = 1.48 and SF<sub>Nd/Dy</sub> = 2.03. Neodymium-to-extractant stoichiometry was evidenced in chloroform to be 1:1 H₂E:N:Nd at pH = 2. Thermodynamic constants related to the considered extraction equilibrium have been determined by using the Van't Hoff relation and the slope analysis method. It appeared within the selected pH-range that the mechanism of the Nd³⁺ transfer was strongly influenced by the nature of the diluent. This was illustrated by the enthalpy-driven transfer when Nd³⁺ was extracted with the H₂E-4C₁₀-Dom system (aliphatic diluent) while the transfer became entropy-driven when the extraction was performed with the H₂E-4C₁₀-DiPB system (aromatic diluent). The complexation of Nd³⁺ involved mainly the participation of both the amide and carboxylic functional groups. The transfer of the cation to the organic phase might occur through the formation of the Nd(HE)A₂ complex (where A is either a nitrate or a chloride anion), allowing for the lowest associated standard Gibbs free energy of transfer (−28 < ΔG° < −24 kJ mol⁻<fnref n="1" />) regardless of the aqueous feed.

**Abbreviations:** Alamine 308, triisooctylamine; Alamine 336, trioctyl/decylamine; Aliquat 336, R₃N(CH₃)⁺Cl⁻, R = octyl/decyl; Cyanex 272, bis(2,4,4-trimethylpentyl)phosphinic acid; Cyanex 301, bis(2,4,4,-trimethylpentyl)dithiophosphonic acid; D2EHPA, di(2-ethylhexyl)phosphoric acid; DGA, diglycolamide; DiPB, 1,3-diisopropylbenzene; DOm, 93:7 v:v *n*-dodecane:*n*-octanol mixture; DOS, di-*n*-octyl sulphide; DTPA, diethylenetriamine pentaacetic acid; EDTA, ethylenediamine tetraacetic acid; (H₂E-4C₁₀), 2,2'-N,N'-didecyl-dioxo-N,N',N'',N'''-tetraaza-tetratriacontane-N'',N'''-diyl diacetic acid; (H₂E-4C₁₂), 2,2'-N,N'-didodecyl-dioxo-N,N',N'', N'''-tetraaza-tetratriacontane-N'',N'''-diyl diacetic acid; Lix 63, 5,8-diethyl-7-hydroxyldodecane-6-oxime; Mextral 336At, trioctylmethylammonium salt (nitrate, chloride, etc.); MIBK, Methylisobutylketone; PC88A = HEH(EHP), 2-ethylhexyl hydrogen 2-ethylhexyl phosphonate; TBP, tri-*n*-butyl phosphate; TOPO, tri-*n*-octylphosphine oxide; Versatic-10, highly branched-C10 tertiary carboxylic acid.

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

<footnote><sup>b</sup> Metal Extraction and Recycling Division, CSIR-National Metallurgical Laboratory, Jamshedpur 831007, India</footnote>

<footnote><sup>c</sup> Tata Research Development and Design Centre, 54-B Hadapsar Industrial Estate, Hadapsar, Pune 411013, Maharashtra, India</footnote>

<footnote>* Corresponding author.</footnote>

<footnote n="1">¹ Permanent address: CRIT DUSP/SPR/LMAR, CEA, Marcoule, France.</footnote>

<footnote n="2">² Permanent address: Department of Fuel, Minerals & Metallurgical Engineering, Indian Institute of Technology (Indian School of Mines), Dhanbad India.</footnote>
# 1. Introduction

Rare earth metals are important for the technologies relevant to renewable energy and electronics because of their extensive use in electric vehicles, wind turbines, electronic storage devices, and other various applications (Gupta and Krishnamurthy, 1992). Specifically, rare earth metals are substantially consumed for the production of permanent magnets (neodymium-iron-boron NdFeB). Demand for permanent magnets has been ever-increasing due to rapid digitization and a dire need for high capacity storage devices (Zepf, 2013; Humphries, 2010). For instance, the growing manufacturing of permanent magnets is such that, according to their extensive revolutionary use scenario, a ratio of neodymium (Nd) prognostic demand for 2035 to its supply in 2010 was reported to exceed 700% (Alonso et al., 2012). Furthermore, the marginal addition of praseodymium (Pr) improves the magnetic strength, ability to withstand an external magnetic field without becoming demagnetized (coercivity), oxidation resistance, and mechanical stability of the magnets significantly (Rho et al., 2020). For this purpose, a pure form of Nd and Pr is a prerequisite. Liquid-liquid extraction (LLE) is commonly used for the separation, purification, and recovery of these rare earth elements (REE) due to its ease in implementation, scalability and control over a process (Tunsu et al., 2014; Cheisson and Schelter, 2019). Despite several efforts made by various researchers across the globe, the separation of adjacent rare earth metals in a mixture has still been a challenge due to their chemical similarity.

Particularly, the separation between Nd and Pr (called together didymium) has been rigorously pursued as reported in the literature. In this context, various strategies such as the use of a synergistic mixture or

an extractant in combination with an (aqueous) complexing agent, have been experimented. Table 1 provides a summary of previous findings. Nevertheless, much smaller separation factors have been reported for the separation between Nd and Pr, specifically with commercial extractants. Therefore, a selectivity for Nd over Pr can be improved by designing a suitable extractant, which will preferentially bind to either of them.

The preferred option is the development of new CHON ligands and more specifically cation-exchangers rather than neutral extractants which are efficient only at high acidity (Stamberg et al., 2020; Ellis et al., 2012). In that order, taking into account the affinity of amino-polycarboxylic acids towards lanthanide in water (Seisenbaeva et al., 2021), hydrophobic diethylenetriamine pentaacetic acid (DTPA) and EDTA derivatives were expected to be suitable candidates. Indeed, hydrophobic DTPA and EDTA bitailed-derivatives were formerly designed (Eckelman et al., 1975) and reported (Kabalka et al., 1991a; Kabalka et al., 1991b) as efficient in vivo metal cation carriers once incorporated into liposomes. Moreover, applications of such systems for bioimaging (Kabalka et al., 1991a; Kabalka et al., 1991b) and alignment of phospholipid bilayers in a magnetic field (Prosser et al., 1999), which involved efficient immobilization of lanthanides, suggested that either DTPA or EDTA bitailed-derivatives might feature the targeted properties, such as the ability to dissolve in non-polar solvents (lipophilicity) and to coordinate with lanthanide ions, which allow efficient solvent extraction of REEs. This has been also confirmed by the reported solubility of EDTA diamide derivatives chelates in toluene-butanol mixtures (Takemitsu et al., 1977; Yamashita et al., 1979).

However, to the best of our knowledge, DTPA and EDTA diamide derivatives have been proposed only once in the last decade as

<table><caption>**Table 1**
Previous publications focusing on lanthanide extraction and intra-lanthanide separation.</caption><thead><tr><th>Authors (year)</th><th>Solvent extractant</th><th>Aqueous phase</th><th>Separated REE</th><th>REE</th><th>Separation factor (SF)<sup>a</sup></th><th>Extraction efficiency (E%)<sup>a</sup></th></tr></thead><tbody><tr><td>Goto &amp; Smutz (Goto and Smutz, 1965) (1965)</td><td>D2EHPA/Amsco 125–82</td><td>Perchlorate, chloride, nitrate or chloride-nitrate mixtures</td><td>Pr/Nd</td><td>Nd/Pr</td><td>~1.4</td><td>84%/81%</td></tr><tr><td>Lu et al. (Lu et al., 1989) (1989)</td><td>Aliquat 336/Naphta 100</td><td>DTPA/nitrate</td><td>Pr/Nd</td><td>Nd/Pr</td><td>NA</td><td>95%/NA</td></tr><tr><td>Thakur et al. (Thakur et al., 1993) (1993)</td><td>PC88A/Kerosene</td><td>Chloride</td><td>Pr/Nd</td><td>Nd/Pr</td><td>~1.6</td><td>88%</td></tr><tr><td>Banda et al. (Banda et al., 2012) (2012)</td><td>Cyanex 272/B/Escaid 110 where B was one of the following: TOPO, DOS, Cyanex 301, Versatic-10, Alamine 308, or Alamine 336</td><td>Chloride</td><td>La/Pr/Nd</td><td>Nd/Pr</td><td>~2</td><td>90%/80%</td></tr><tr><td>Banda et al. (Banda et al., 2015) (2015)</td><td>PC 88A/Escaid 110</td><td>H<sub>2</sub>O<sub>2</sub> or citric acid/chloride</td><td>Pr/Nd</td><td>Nd/Pr</td><td>441</td><td>90%/50%</td></tr><tr><td>Batchu et al. (Batchu et al., 2015) (2015)</td><td>Cyanex 301:Lix 63/Kerosene</td><td>Chloride</td><td>La/Ce/Pr/Nd</td><td>Nd/Pr</td><td>~3.5</td><td>53%/33%</td></tr><tr><td>Liu et al. (Liu et al., 2015) (2015)</td><td>Cyanex 272:Alamine 336:TBP/Kerosene</td><td>Chloride</td><td>Pr/Nd</td><td>Nd/Pr</td><td>~1.5</td><td>40%/30%</td></tr><tr><td>Kumari et al. (Kumari et al., 2019) (2019)</td><td>Mextral® 336At/Kerosene</td><td>EDTA/nitrate</td><td>Pr/Nd/Dy</td><td>Nd/Pr</td><td>~2.1</td><td>53%/45%</td></tr><tr><td>Rho et al. (Rho et al., 2020) (2020)</td><td>TOPO/Kerosene</td><td>Nitrate</td><td>B/Fe/Pr/Nd</td><td>Nd/Pr</td><td>~3.7</td><td>53%/10%</td></tr><tr><td>Wang et al. (Wang et al., 2020) (2020)</td><td>Aliquat 336/Column extractor</td><td>DTPA/nitrate</td><td>Pr/Nd</td><td>Nd/Pr</td><td>~1.6</td><td>80.7%/72.4%</td></tr><tr><td>Stamberg et al. (Stamberg et al., 2020) (2020)</td><td>Dissymmetric N,N,N',N''-tetrasubstituted DGAs</td><td>Chloride or nitrate</td><td>Lanthanides</td><td>Nd/Pr</td><td>~2.9</td><td>90%/82%</td></tr><tr><td></td><td></td><td></td><td></td><td>Nd/Pr</td><td>~19.4</td><td>82%/48%</td></tr><tr><td></td><td></td><td></td><td></td><td>Nd/Pr</td><td>~55.6</td><td>90%/48%</td></tr><tr><td></td><td></td><td></td><td></td><td>Nd/Pr</td><td>~174</td><td>80%/28%</td></tr><tr><td></td><td></td><td></td><td></td><td>Nd/Pr</td><td>~177</td><td>99%/28%</td></tr><tr><td></td><td></td><td></td><td></td><td>Nd/Pr</td><td>~22</td><td>90%/50%</td></tr><tr><td></td><td></td><td></td><td></td><td>Nd/Pr</td><td>~3.2</td><td>99%/96%</td></tr><tr><td></td><td></td><td></td><td></td><td>Nd/Pr</td><td>~100</td><td>100%/81%</td></tr></tbody></table>

<footnote>a Only highest yielded values are shown without correspondence between them on a same row.</footnote>
extractants for LLE of metal cations (Chubu Chrest Co., Ltd; Christest Co., Ltd, 2016). The proof of concept consisted of monitoring based on UV-visible absorption during the LLE of Cu(II), mediated by EDTA or DTPA diamide dissolved into a mixture of ternary fatty alcohol, fatty ester and hydrocarbon. No further characterization of this promising system has been undertaken. In another study, extraction performances, more precisely, the An(III)/Ln(III) separation with ethylenediamine diamide derivatives have already been studied (Sharma Joti et al., 2009). Except for this latter work, no effort has been made according to our knowledge to prospect a possibility of extracting Nd³⁺ by using a lipophilic ethylenediamine derivative from scrap leachate.

Following these observations, preliminary studies were focused on the development of some symmetrical N,N,N',N'-tetraalkylated EDTA for lanthanides separation. Tetraalkylated forms were indeed expected to show higher lipophilicity than dialkylated derivatives (modified with octyl, dodecyl or hexadecyl chains) that are known to behave as water-soluble surfactants at neutral pH (Welsh, 2003). Apart from preventing from partial solubility of the ligand in aqueous phase, the aim was also to reach a molar scale solubility in pure aliphatic diluent as this class of diluent has been claimed as the most suitable for cation exchange-based LLE of REEs (Batchu and Binnemans, 2018).

This article describes the synthesis and the extraction properties of two novel purpose-made lipophilic EDTA-diamide derivatives, namely 2,2'-N,N,N'-didecyl- (H₂E-4C₁₀) and 2,2'-N,N'-didodecyl-dioxo-N,N',N'', N'''-tetraaza-tetratriacontane-N'',N'''-diyl diacetic acid (H₂E-4C₁₂), designed to allow efficient and selective LLE of Nd(III). Solubilities of both compounds were investigated in various diluents and extraction of Nd³⁺ from chloride and nitrate media was studied in parallel. The loading capacity (Q) and the separation factor (SF) of the novel solvent were investigated and reported herein. The behaviour of H₂E-4C₁₀ was tested by focussing on its loading capacity and separation factors, SFₙₐ.

Dₚ and SFₙₐ/pₚ. From a fundamental perspective, the stoichiometric co-efficient of the complex formed and thermodynamic constants were determined by using, respectively, the slope analysis method and the Van't Hoff relation.

## 2. Materials and methods

### 2.1. Material

All chemicals were purchased from Sigma-Aldrich (France) and were used without further purification. Anhydrous N,N-dimethylformamide (DMF) (AcroSeal) was purchased from Acros (France). The NMR spectra reported in supplementary information (section S1) were recorded at room temperature on a Bruker AVANCE III 400 MHz apparatus from Bruker (Wissembourg, France). Deionized water was produced by a milli-Q® Direct 8 system from Merck (France). Inductively coupled plasma optical emission spectroscopy (ICP-OES) on an ICAP 7000 series spectrometer (ThermoScientific, France) was used to determine the concentrations of metal ions in aqueous phase.

### 2.2. Synthesis of H₂E-4C₁₀ and H₂E-4C₁₂ and solubility tests in various diluents

The EDTA tetra-tailed derivatives H₂E-4C₁₀ and H₂E-4C₁₂ were synthesized in two steps according to procedure reported elsewhere (Eckelman et al., 1975; Yamashita et al., 1979; Takeshita et al., 1982; Capretta et al., 1995) (Scheme 1 and supplementary information in section S1).

A precise amount of compound H₂E-4C₁₀ or H₂E-4C₁₂ was weighed and put in a small bottle, into which 1.00 mL (precision pipette) of diluent was firstly added and the resulting mixture was stirred for

<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Synthesis of EDTA tetra-tailed derivatives. The dianhydride was yielded after heating EDTA at 70 °C in a 30:40 v:v acetic anhydride-pyridine mixture (Eckelman et al., 1975; Yamashita et al., 1979; Takeshita et al., 1982; Capretta et al., 1995). Ring opening of the dianhydride was achieved in the presence of an excess of N,N-dialkylamine to produce the expected diamide derivative (Yamashita et al., 1979) (experimental details and NMR data in supplementary notes S1).</figcaption></figure>
<table><caption>**Table 2**
Compositions of both aqueous feed and extractant solutions used for detailed investigations.</caption><thead><tr><th rowspan="2">#</th><th rowspan="2">Focus of the study</th><th colspan="4">Diluents and concentrations /(mol L<sup>-1</sup>)</th></tr><tr><th>Diluent</th><th>[H<sub>2</sub>E-4C<sub>10</sub>](org)</th><th>[Nd<sup>3+</sup>](aq)</th><th>[HA]<sup>a</sup>(aq)</th></tr></thead><tbody><tr><td>1</td><td>Loading capacity</td><td>DiPB</td><td>0.015</td><td>0.001-0.1</td><td>0.01</td></tr><tr><td>2</td><td>Selectivity, SF<sub>Nd/Ln</sub></td><td>DiPB</td><td>0.01</td><td>[Nd] = [Pr] = [Dy] = 0.01</td><td>10<sup>-4</sup>-0.8</td></tr><tr><td>3</td><td>Stoichiometry: slope analysis</td><td>CHCl<sub>3</sub><sup>b</sup></td><td>0.001-0.1</td><td>0.01</td><td>0.01</td></tr><tr><td rowspan="2">4</td><td rowspan="2">Thermodynamic quantities: temperature effect</td><td>DiPB</td><td>0.01</td><td>0.01</td><td rowspan="2">0.01</td></tr><tr><td>DOm<sup>c</sup></td><td>0.006</td><td>0.006</td></tr></tbody></table>

a HA represents either HNO₃ or HCl.

b CHCl₃ stands for chloroform.

c DOm stands for 93:7 n-dodecane:n-octanol mixture.

different time intervals at room temperature (RT) depending on the diluent. For example, stirring time was 5 to 10 min for 1,3-diisopropylbenzene (DiPB) or methyl isobutyl ketone (MIBK), compared to 24 h for cyclohexane or octane. In case of total or partial insolubility, the suspension was supplemented with an additional 1.00 mL (precision pipette) of the corresponding diluent and the mixture was then stirred for another 24 h. The final solubility was verified by observation of disappearance or persistence of suspensions.

## 2.3. Solvent extraction experiments

### 2.3.1. General procedure and screening tests

Series of solvent extraction experiments were carried out by contacting an aqueous acid solution containing lanthanide (either Nd³⁺ or an equimolar mixture of Nd³⁺, Dy³⁺ and Pr³⁺) at a given concentration, with a solution of H₂E-4C₁₀ at known concentration in diluent. Contacts were performed at a unit organic/aqueous phase volume ratio (V₀/Vₐ) with V₀ = Vₐ = 1 or 2 mL, in 5 mL test tubes, which were shaken at 400 rpm in a mixer (INFORS HT) for 30 min at room temperature. After the equilibration, the tubes were centrifuged at 15025g (10,000 rpm) for 10 min allowing optimal separation of the aqueous phase from the organic phases. Light organic phases and the “organic-aqueous interface layer” were thereafter removed. For both ionic media, aqueous feed solutions of six different pH values: 0.1, 0.5, 1, 2, 3, and 4 were prepared. All experiments were performed twice and standard deviation was determined. The concentration of lanthanide [Ln³⁺] in a given aqueous raffinate phase was analyzed by ICP-OES. Considering the unit volume ratio of V₀/Vₐ= 1, the concentration of lanthanide extracted into the relevant organic phase at equilibrium, [Ln³⁺], where the overbar is used to denote species present in the organic phase hereafter, was estimated from the mass balance:

$$ \overline{[Ln^{3+}]} = [Ln^{3+}]_i - [Ln^{3+}] \quad (1) $$

with [Ln³⁺]ᵢ denoting the concentration of Ln³⁺ which was initially present in the aqueous phase before the contact.

Hence, the extraction efficiency Eₚ, and the distribution coefficient D<sub>Ln³⁺</sub> can be then deduced as follows:

$$ E_{\%} = \frac{\overline{[Ln^{3+}]}_i}{[Ln^{3+}]_i} \times 100 \quad (2) $$

$$ D_{Ln^{3+}} = \frac{\overline{[Ln^{3+}]}_i}{[Ln^{3+}]_i} \quad (3) $$

For a mixture of two lanthanides Ln₁³⁺ and Ln₂³⁺, the relative affinity of the extractant for one element compared to the other is reflected by the separation factor SF₁/₂ as follows:

$$ SF_{1/2} = \frac{D_1}{D_2} \quad (4) $$

When [Ln³⁺] is expressed in g L⁻¹, the loading capacity Qe at

equilibrium is:

$$ Q_e = \overline{[Ln^{3+}]} \quad (5) $$

### 2.3.2. Determination of the main operating parameters related to the extraction of Nd(III) from nitrate or chloride media using H₂E-4C₁₀ in the diluents

The corresponding compositions of the extractant and feed solutions have been summarized in Table 2. Analyses were achieved as described above.

## 2.4. Theoretical background

### 2.4.1. Expression of the equilibrium constant K<sub>eq</sub>

The following overall equilibrium (Eq. (6)) can be considered for the Nd³⁺ extraction by ligand H₂E-4C₁₀, which can be assimilated to a di (aminoacid), noted as H₂E. Note that considering the pH of a given aqueous phase with which the extractant phase is in contact, the two amine groups of the ethylenediamine moiety are expected to be protonated, although non-included in this notation.

$$ Nd^{3+}_{(aq)} + a A^{-}_{(aq)} + n \overline{H_2E} \rightleftharpoons Nd A_a \overline{(H_xE)_n} + (3 - a) H^{+}_{(aq)} \quad (6) $$

In this equilibrium, A⁻ refers to a chloride or nitrate anion (Cl⁻, NO₃⁻), a (a ≥ 1) and n (n ≥ 1) correspond, respectively, to stoichiometric coefficients of the anion and of the extractant, and finally x denotes the protonation state of the extractant defined as follows:

$$ x = 2 - \frac{(3 - a)}{n} \quad (7) $$

The values of x = 2, 1 and 0 correspond, respectively, to the diacetic acid form H₂E, the half-deprotonated form HE and the fully deprotonated ligand or diacetate form E (ignoring the charge).

At the equilibrium condition, the corresponding equilibrium constant K<sub>eq</sub> is given by:

$$ K_{eq} = \frac{[Nd A_a (H_x E)_n]_{eq} \cdot [H^+]_{eq}^{3-a}}{[Nd^{3+}]_{eq} \cdot [A^-]_{eq}^a \cdot [H_2E]_{eq}^n} \quad \ast Y \quad (8) $$

where Y represents the ratio of the activity coefficients (γᵢ) of each species (i) involved in the equilibrium to the power of its corresponding stoichiometric coefficients vᵢ such as:

$$ Y = \frac{\gamma_{Nd} A_a (H_x E)_n \cdot (\gamma_H^+)^{3-a}}{\gamma_{Nd^{3+}} \cdot (\gamma_A^-)^a \cdot (\gamma_{H_2E})^n} \quad (9) $$

In the case of an infinite dilution condition, the activity coefficient of the related species can be considered unity (γ ~ 1). At a higher concentration, the activity coefficients of aqueous species must be determined or can be extracted from pre-existing tables. It can be assumed to remain constant during the experiment by maintaining a constant ionic strength in the aqueous phase. As the activity coefficients of species present in (or transferred to) the organic phase are still not accessible,
their activities are assimilated to their concentrations.

## 2.4.2. Determination of the metal: Ligand stoichiometry n with the slopes analysis method

By using Eq. (8) and the definition of the distribution ratio, which is determined experimentally, it is possible to obtain a relation between the distribution ratio and concentration of the extractant H₂E at equilibrium. From this relation, by making several assumptions (see below), it is possible to determine the stoichiometric number n.

$$D_{\text{Nd}} = \frac{[\text{Nd}]_{\text{eq}}}{[\text{Nd}^{3+}]_{\text{eq}}} = \frac{[\text{Nd}^{3+}]_{\text{i}} - [\text{Nd}^{3+}]_{\text{eq}}}{[\text{Nd}^{3+}]_{\text{eq}}} \quad (10)$$

By assuming that $\overline{NdA_a(H_xE)_n}$ is the only form of Nd³⁺ ion present in the related organic phase at equilibrium gives:

$$[\text{Nd}]_{\text{eq}} = [\text{Nd A}_a(\text{H}_x\text{E})_n]_{\text{eq}} \quad (11)$$

In contrast, Nd³⁺ cations in the aqueous phase are subject to different interactions leading to a formation of complexes, ion-pair salts with inorganic anions (nitrate or chloride ions) in addition to a hydrolysis. In the current pH-range of 0-4 hydrolysis can be neglected. Therefore, the concentration of free Nd³⁺ cation is accounted by considering the coefficient α (as fraction of free cation) in the total concentration of Nd³⁺ at the equilibrium:

$$\alpha_{\text{Nd}^{3+}} [\text{Nd}^{3+}]_{\text{eq}} = \alpha_{\text{Nd}^{3+}} ([\text{Nd}^{3+}]_{\text{i}} - [\text{Nd}]_{\text{eq}}) \quad (12)$$

So, an insertion of Eq. (11) and Eq. (12) in Eq. (10) gives:

$$D_{\text{Nd}} = \frac{[\text{Nd A}_a(\text{H}_x\text{E})_n]_{\text{eq}}}{[\text{Nd}^{3+}]_{\text{eq}}} \bullet \frac{1}{\alpha_{\text{Nd}^{3+}}} \quad (13)$$

Replacing Eq. (13) in Eq. (8) gives:

$$D_{\text{Nd}} = K_{\text{eq}} \bullet [\text{A}^{-}]_{\text{eq}}^{\text{a}} \bullet [\overline{\text{H}_2\text{E}}]_{\text{eq}}^{\text{n}} \bullet [\text{H}^{+}]_{\text{eq}}^{-(3-a)} \bullet Y^{-1} \bullet (\alpha_{\text{Nd}^{3+}})^{-1} \quad (14)$$

$$\log D_{\text{Nd}} = n \log[\overline{\text{H}_2\text{E}}]_{\text{eq}} + \log K_{\text{eq}} + (3-a) p \text{H}_{\text{eq}} + \log \frac{[\text{A}^{-}]_{\text{eq}}^{\text{a}}}{Y \bullet \alpha_{\text{Nd}^{3+}}} \quad (15)$$

Based on a plot of $\log D_{\text{Nd}} = f(\log[\overline{\text{H}_2\text{E}}]_{\text{eq}})$, the stoichiometric number n can be determined by assuming that $a_{\text{H}_2\text{E}} \approx [\overline{\text{H}_2\text{E}}]$ (valid for $a_{\text{H}_2\text{E,initial}} \gg \alpha_{\text{Nd}^{3+}}$), the remaining terms being not affected by a variation of the total/initial extractant concentration. In other condition, $[\overline{\text{H}_2\text{E}}]_{\text{eq}}$ can be estimated (considering $V_A = V_O$) by combining Eq. (11) with Eq. (12) as follows:

$$[\overline{\text{H}_2\text{E}}]_{\text{eq}} = [\overline{\text{H}_2\text{E}}]_{\text{i}} - n \bullet [\text{Nd A}_a(\text{H}_x\text{E})_n]_{\text{eq}} = [\overline{\text{H}_2\text{E}}]_{\text{i}} - n \bullet [\text{Nd}]_{\text{eq}} \quad (16)$$

$$[\overline{\text{H}_2\text{E}}]_{\text{eq}} = [\overline{\text{H}_2\text{E}}]_{\text{i}} - n \bullet \left( [\text{Nd}^{3+}]_{\text{i}} - \frac{[\text{Nd}^{3+}]_{\text{eq}}}{\alpha_{\text{Nd}^{3+}}} \right) \quad (17)$$

The value of $[\text{Nd}^{3+}]_{\text{eq}}$ is determined by ICP and $[\overline{\text{H}_2\text{E}}]_{\text{eq}}$ can be calculated by iteration (Artese et al., 2020), considering the values n varying from 1 to x with x to be defined.

## 2.4.3. Determination of the thermodynamic quantities

The standard reaction Gibbs free energy $\Delta G^\circ$ associated with the hypothetical eq. 6 is related to the equilibrium constant, $K_{\text{eq}}$, at constant temperature T and pressure P according to:

$$\Delta G^0 = -RT \ln K_{\text{eq}} \quad (18)$$

$$\Delta G^0 = \Delta H^0 - T \Delta S^0 \quad (19)$$

$$\ln K_{\text{eq}} = - \frac{\Delta_r H^0}{R} \frac{1}{T} + \frac{\Delta_r S^0}{R} \quad (20)$$

Temperature dependence of lnK_{eq} can be also shown by using the Gibbs-Helmholtz equation followed by a variable change, assuming that the standard reaction enthalpy as well as the standard reaction entropy are not affected by a variation of temperature within a considered temperature range (Atkins, 1979).

$$\left( \frac{\partial \ln K_{\text{eq}}}{\partial \left( \frac{1}{T} \right)} \right)_P = - \frac{\Delta H^0}{R} \quad (21)$$

$$K_{\text{eq}} = a^* \exp\left(-\frac{\Delta H^0}{R} \frac{1}{T}\right) \quad (22)$$

Further, based on relation in Eq. (14), $K_{\text{eq}}$ can be accessed from the distribution ratio values, which are experimentally determined, according to the following relation:

$$K_{\text{eq}} = \frac{D_{\text{Nd}}}{[\text{A}^{-}]_{\text{eq}}^{\text{a}} \bullet [\overline{\text{H}_2\text{E}}]_{\text{eq}}^{\text{n}}} \bullet [\text{H}^{+}]_{\text{eq}}^{-(3-a)} \bullet Y \bullet (\alpha_{\text{Nd}^{3+}}) \quad (23)$$

In a diluted solution (Y=1), the equation becomes:

$$K_{\text{eq}} = \frac{D_{\text{Nd}}}{[\text{A}^{-}]_{\text{eq}}^{\text{a}} \bullet [\overline{\text{H}_2\text{E}}]_{\text{eq}}^{\text{n}}} \bullet [\text{H}^{+}]_{\text{eq}}^{-(3-a)} \bullet (\alpha_{\text{Nd}^{3+}}) \quad (24)$$

The value of $[\text{H}^{+}]_{\text{eq}}$ can be accessed by pH-metry or can also be estimated as follows (as $V_A = V_O$ in this work):

$$[\text{H}^{+}]_{\text{eq}} = [\text{H}^{+}]_{\text{i}} + (3-a) \bullet [\overline{\text{Nd}}]_{\text{eq}} \quad (25)$$

The value of $[\text{A}^{-}]_{\text{eq}}$ can be estimated considering its transferred concentration to the organic phase:

$$[\text{A}^{-}]_{\text{eq}} = [\text{A}^{-}]_{\text{i}} - a \bullet [\overline{\text{Nd}}]_{\text{eq}} \quad (26)$$

The initial concentration $[\text{A}^{-}]_{\text{i}}$ can be easily estimated knowing the dissociation constants of each dissociation equilibrium leading to NdA₂⁺, NdA²⁺ and Nd³⁺, as discussed later.

Hence the thermodynamic quantities can be determined by using ln

$$K_{\text{eq}} = f\left(\frac{1}{T}\right) \text{ plots.}$$

# 3. Results and discussion

## 3.1. Solubility test of compounds H₂E-4C₁₀ and H₂E-4C₁₂

The solubility results are listed in Table 3. It appeared that compound H₂E-4C₁₀, which is comprised of shorter alkyl chains, was more soluble in most of these diluents, particularly in the polar ones, compared to its counterpart containing dodecyl chains - compound H₂E-4C₁₂. This suggested that the solubility of EDTA derivatives in organic diluent did not depend only on the diamide alkyl chain lengths, but also on more complex parameters.

This was further confirmed by the measured solubility of diamides in pure hydrocarbon diluents such as cyclohexane, octane and dodecane. This underlined that the design of the diamide derivatives structure required further significant improvements before recovering the expected lipophilicity (≥ 2 M in hydrocarbon). Finally, additional attempts were made in order to assess the solubility of both compounds in some n-octanol:hydrocarbon mixtures (not shown) since n-octanol is widely used at large scale in industrial LLE processes to prevent precipitation and third-phase formation (Lu et al., 2020).

The solubility of both H₂E-4C₁₀ and H₂E-4C₁₂ in a mixture of n-dodecane (D) and n-octanol (O), denoted by DOM at a 93:7 volume ratio (v:v), was ultimately checked. In contrast to compound H₂E-4C₁₂, it was shown that H₂E-4C₁₀ could be solubilized in this mixture at up to ~0.05 M. Therefore, H₂E-4C₁₂ was discarded and the subsequent experiments
<table><caption>**Table 3**
Solubility of H₂E-4C₁₀ and H₂E-4C₁₂ in different diluents.</caption><thead><tr><th rowspan="2">Diluent</th><th rowspan="2">Dipole moment (||μ̃<sub>D</sub>|| in debye)</th><th rowspan="2">Dielectric constant (ε)</th><th colspan="2">Solubility of H₂E-4C₁₀</th><th colspan="2">Solubility of H₂E-4C₁₂</th></tr><tr><th>g L<sup>-1f</sup></th><th>mM<sup>g</sup></th><th>g L<sup>-1f</sup></th><th>mM<sup>g</sup></th></tr></thead><tbody><tr><td>Chloroform</td><td>1.15<sup>a</sup></td><td>4.89<sup>a</sup></td><td>≥ 17.2</td><td>19.8</td><td>≥ 19.2</td><td>19.6</td></tr><tr><td>Heptanone</td><td>2.97<sup>a</sup></td><td>11.98<sup>a</sup></td><td>≥ 17.2</td><td></td><td>Insoluble</td><td></td></tr><tr><td>MIBK</td><td>2.77<sup>a</sup></td><td>15.87<sup>a</sup></td><td>≥ 8.6</td><td>9.9</td><td>Insoluble</td><td></td></tr><tr><td>4-methylacetophenone</td><td>NA</td><td>NA</td><td>> 17.2</td><td></td><td>8.76</td><td>9.0</td></tr><tr><td>Undecen-1-ol</td><td>NA<sup>b</sup></td><td>NA<sup>b</sup></td><td>> 17.2</td><td></td><td>> 19.2</td><td></td></tr><tr><td>TBP</td><td>3.07<sup>c</sup></td><td>7.79<sup>c</sup></td><td>≥ 8.6</td><td></td><td>Insoluble</td><td></td></tr><tr><td>1,3-diisopropylbenzene</td><td>NA<sup>d</sup></td><td>NA<sup>d</sup></td><td>≥ 8.6</td><td></td><td>≥ 9.6</td><td>9.8</td></tr><tr><td>Cyclohexane</td><td>0<sup>a</sup></td><td>2.02<sup>a</sup></td><td>&lt; 8.6</td><td></td><td>Insoluble</td><td></td></tr><tr><td>Octane</td><td>0<sup>a</sup></td><td>1.95<sup>a</sup></td><td>&lt; 8.6</td><td></td><td>&lt; 9.6</td><td></td></tr><tr><td>n-Dodecane</td><td>0<sup>a</sup></td><td>2<sup>a</sup></td><td>Insoluble</td><td></td><td>Insoluble</td><td></td></tr><tr><td>93:7 n-Dodecane:n-octanol (v:v) (DOm)</td><td>NA<sup>e</sup></td><td>NA<sup>e</sup></td><td>≥ 17.2</td><td></td><td>Insoluble</td><td></td></tr></tbody></table>

were performed with H₂E-4C₁₀.

## 3.2. Screening of diluents with a view to extract Nd(III) using H₂E-4C₁₀ at various pH

By considering the cation exchanger feature of H₂E-4C₁₀, the dependence of the Nd-extraction percentage on the initial pH of the feed solution was investigated. As the LLE of REEs can be achieved from various types of leachates (mainly nitrate (Rho et al., 2020; Lu et al., 1989; Wang et al., 2020; Ellis et al., 2012; Kumari et al., 2019) or chloride (Lu et al., 1989; Thakur et al., 1993; Banda et al., 2012; Banda et al., 2015; Liu et al., 2015; Wang et al., 2020)), seven different systems of H₂E-4C₁₀-diluent were contacted with chloride or nitrate solutions of Nd³⁺. The study was carried out with each one of the diluents previously listed in Table 3, except pure hydrocarbon diluents, in which solubility of H₂E-4C₁₀ was not high enough to comply with the conditions investigated. Moreover, even at low concentrations of H₂E-4C₁₀ (< 9 mM), the contacts of a cyclohexane-based solvent with aqueous solutions of Nd³⁺ at different initial pH have resulted in a precipitate formation either at the aqueous-organic interface (0.1 ≤ pH ≤ 1) or in the

aqueous phase (pH ≥ 2) regardless of the type of acid used (not shown). The extraction results relevant to chloride and nitrate media are illustrated in Fig. 1.

Overall dependence of the Nd extraction on the initial feed pH is in line with a typical cation exchange system such that the extraction is hindered at low pH (Eq. (6)). More precisely, most of the tested systems underwent precipitation once contacted with an aqueous feed whose pH was lower than 2. Exceptions were found with chloroform or DiPB as diluents when precipitation occurred at pH ≤ 0.5 with HCl and pH ≤ 1 with HNO₃ as shown in Fig. 1.

The origin of the precipitation phenomenon has not been elucidated, therefore the proposed explanations (structural instability or anarachic solvating processes occurring at low pH) are left merely as a speculative assumption.

Effective extraction of Nd³⁺ was found to occur at relatively high pH (pH > 2), as expected from Eq. (6). An exception was found when TBP was used as a diluent. The higher extraction efficiency observed at low pH (78 % at pH 1 versus 57 % at pH 4) could be due to a cooperative participation of TBP in Nd extraction enabled by the compound H₂E-4C₁₀. Being neutral, TBP is well known for extracting REEs by solvation

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Influence of initial pH on Nd(III) extraction efficiency H₂E-4C₁₀ from solutions of (a) HCl and (b) HNO₃ in different diluents ([H₂E-4C₁₀] = [Nd³⁺] = 10⁻² mol L⁻¹, [HCl] or [HNO₃]: from 10⁻¹ to 10⁻⁴ mol L⁻¹, T = 298 K, contact time = 30 min, experimental error ~ ± 5 %). (Note: According to the diluent, some third-phase formations and precipitations could occur at pH < 2. Because it could not be detected for each experiment, E%Nd obtained in this pH range could not be accurate and were only considered as apparent values; also see Supplementary Information, Fig. S2.1, for corresponding plots D<sub>Nd</sub> = f(pH)).</figcaption></figure>

<footnote>a Data reported by Y. Marcus (Marcus, 1998).</footnote>

<footnote>b No data available but the saturated analogous should feature values as ||μ̃<sub>D</sub>|| ~ 1.7 D and 6 < ε < 8 (Marcus, 1998).</footnote>

<footnote>c Data from Klatt (Klatt, 1980) and Richaud (Richaud, 1978).</footnote>

<footnote>d Values for m-xylene are ||μ̃<sub>D</sub>|| = 0.3 D and ε = 2.37 (Marcus, 1998).</footnote>

<footnote>e Values for n-octan-1-ol are ||μ̃<sub>D</sub>|| = 1.76 D and ε = 10.34 (Marcus, 1998).</footnote>

<footnote>f The solubility is expressed in g of ligand (H₂E-4C₁₀ or H₂E-4C₁₂) per liter of diluent and is given only as an approximation of the mass concentration expressed in g per liter of solution.</footnote>

<footnote>g Molar concentrations (mM) are estimated considering the density (d ~ 1 g mL⁻¹) of the ligand determined by pycnometry.</footnote>
of the lanthanide salts as Ln(X₃)(TBP)₃, initially dissolved at very low pH, thus at high activity of the aqueous species, in HNO₃, H₂SO₄ and HCl solutions (Nasab et al., 2011). When the pH increases, the activity of aqueous metal ion species decreases and the extraction of Ln with pure TBP is drastically lowered to become negligible at pH 4. Therefore, the extraction percentage of Nd (57 %) obtained at this pH (Fig. 2) can be directly attributed to the extracting performance of H₂E-4C₁₀ (Eq. (6)).

It is noteworthy that the extraction efficiency reaches a plateau after pH = 3 while the pKₐ values of the two carboxylic groups of the EDTA N, N'-diisopropylamide derivative have been reported to be, respectively, pKₐ₁ ≤ 0 and pKₐ₂ ~ 2.0 (Wang et al., 1998). This underlines the co-ordination of both carboxylic functions occurring only after their full deprotonation (to -COO⁻) near pH = 3. Further, it can be specified the slight but noticeable beneficial impact of amidation on the increase of the second acidity of EDTA to a higher value of pK₂ ~ 2.0 (Wang et al., 1998), compared to pKₐ₂ ~ 1.4 (Friend and Wall, 2019) of the parent molecule (plain EDTA).

It can be also noticed that when contacted with a chloride feed, DOm appears to be the most appropriate solvent for H₂E-4C₁₀ as the corresponding E₉₆ value increases from 88 % to 93 %, while D increases from 7.3 to 13.3, in comparison with other formulations which lead to an extraction percentage not exceeding 75 % (53 % in chloroform) (Fig. 1b). This observation was in good agreement with those reported by Batchu and Binnemans who showed that the efficiency of an extraction mediated by cation exchange was typically higher in aliphatic diluents than that yielded in more polar (aromatic more specifically) diluents (Batchu and Binnemans, 2018). This was explained by the decrease in the complex formation, caused by the increase of the diluent-extractant interactions with diluents of high dipole moment and dielectric constant.

Conversely, no change in the Nd³⁺ extraction performance was observed when undecenol, MIBK, or DiPB was used as a diluent, where the Nd-extraction percentage remained in the range between 60 % and 75 %. However, the H₂E-4C₁₀-chloroform system showed enhanced extracting abilities in nitrate medium, where the Nd-extraction percentage increased from 17 % to 82 % with the increase in initial pH from 1 to 4.

At initial unit molar ratio of Nd in the feed to H₂E-4C₁₀ in diluent, the

observed trend is that the Nd-extraction efficiency in nitrate medium (Fig. 1b) is higher than that in chloride medium (Fig. 1a). This is inconsistent with some previously reported results (Belova et al., 2021; Belova, 2023), whereas H₂E-4C₁₀-TBP system shows comparable results (~60 %) in both media.

More broadly, the remarkable extraction efficiency of the H₂E-4C₁₀-diluent systems used in this study, which prevails over other cation exchangers, such as the D₂EHPA, in the same conditions must be highlighted (Batchu and Binnemans, 2018; Belova et al., 2021; Belova, 2023). Table 4 shows the comparison of extraction efficiency with other solvent systems, along with the details of ligand and metal cation concentrations, temperature, diluent and aqueous feed.

<caption>**Table 4**</caption>

Comparison of extraction efficiency obtained in similar conditions with other solvents.

<table><thead><tr><th>Reference (years)</th><th>Solvent</th><th>Aqueous feed</th><th>E<br>%<sub>6Nd</sub></th></tr></thead><tbody><tr><td>This work (2024)</td><td>[H₂E-4C₁₀] = 0.01 mol L⁻¹ in DOm<br>[H₂E-4C₁₀] = 0.01 mol L⁻¹ in DiPB</td><td>[Nd³⁺] = 0.01 mol L⁻¹, [HNO₃] = 0.01 mol L⁻¹, pH<sub>eq</sub> = 2.04<br>Nd³⁺] = 0.01 mol L⁻¹, [HCl] = 0.01 mol L⁻¹, pH<sub>eq</sub> = 2.1</td><td>90<br>%</td></tr><tr><td>Batchu &amp;<br>Binnemans ( Batchu and<br>Binnemans, 2018)<br>(2018)</td><td>[D₂EHPA] = 0.01 mol L⁻¹ in A150<sup>a</sup></td><td>[Nd³⁺] = 0.007 mol L⁻¹, [NaCl] = 1 mol L⁻¹, pH<sub>eq</sub> = 2.4</td><td>17<br>%</td></tr><tr><td>Belova et al. (Belova<br>et al., 2021)<br>(2021)</td><td>[D₂EHPA] = 0.036 mol L⁻¹ in 1:1 hexane: isopropanol</td><td>[Nd³⁺] = 0.02 mol L⁻¹, [NaNO₃] = 0.5 mol L⁻¹, 0.07 mol L⁻¹ acetate buffer, pH<sub>eq</sub> = 7.5</td><td>60<br>%</td></tr><tr><td>Belova et al. ( Belova, 2023)<br>(2022)</td><td>[D₂EHPA] = 0.044 mol L⁻¹ [caprylic acid] = 0.375 mol L⁻¹ in 1:1 hexane:isopropanol</td><td>[Nd³⁺] = 0.0128 mol L⁻¹, [NaNO₃] = 0.1 mol L⁻¹, [HNO₃] = 0.01 mol L⁻¹, pH<sub>eq</sub> not precised</td><td>69<br>%</td></tr></tbody></table>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Distribution ratio of Nd as a function of the initial pH in chloride (a) or nitrate (b) media after contacting a 10⁻³ mol L⁻¹ Nd³⁺ aqueous solution with pure diluents (T = 298 K, contact time = 30 min., experimental error ~ ± 10 %, other conditions same as in Fig. 1).</figcaption></figure>

<footnote>a aromatic diluent.</footnote>
Potential contribution of diluent molecules in the Nd-partitioning was investigated for systems (in Fig. 1), in which E%Nd increases with the initial pH. Distribution ratio values were determined for pure diluents, namely, MIBK, heptanone, undecenol and DiPB, after contact at a unit phase ratio with 10⁻³ mol L⁻¹ Nd³⁺ HCl and HNO₃ solutions at three different pH (Fig. 2). It can be observed that there is no Nd-partitioning between a HNO₃ solution and a diluent-phase, regardless of the type of diluent (Fig. 2b). Conversely, there is a slight Nd-partitioning between an HCl solution and the various diluent-phases with, for instance, an extraction/partition percentage in MIBK reaching up to 29% (D = 0.4 in Fig. 2a), which is not negligible. This implies a certain participation of both the diluent molecules and the chloride anions in the Nd-extraction for which related mechanisms seem to rely on functional as well as structural properties of the diluent. For example, see the difference observed in Fig. 2 between the two ketones, namely MIBK and heptan-2-one, both coordinating with Nd³⁺ through the carbonyl function.

Considering the overall efficiencies in HCl and HNO₃ in Fig. 1 the extracting solvent made up of ligand **H₂E-4C₁₀** and diluent DiPB appears to be the best compromise. The **H₂E-4C₁₀**-DODm system revealed remarkable performance in the nitrate medium whereas the properties of the **H₂E-4C₁₀**-chloroform solvent were highly affected by the nature of the feed solution (Fig. 1b). Thus, series of complementary experiments were subsequently carried out to better characterize the extraction of Nd³⁺ by these three systems.

### 3.3. Extraction of Nd³⁺ from acidic aqueous feed using H₂E-4C₁₀ in DiPB

#### 3.3.1. Loading capacity of H₂E-4C₁₀ in DiPB

Loading capacity has been accessed for both series (nitrate and chloride) by plotting the dependence/variation of Nd(III) concentration in organic phase on its gradual increase in the range from 0.001 to 0.1 mol L⁻¹ in an aqueous feed solution of 0.01 mol L⁻¹ acid concentration, to simulate a continuous flow or a "cascade" extraction process. Note that here pH = 2 corresponds to the value for which the contribution of DiPB is minimum, as evidenced for both media in Fig. 2.

The resulting plots are illustrated in Fig. 3a based on initial concentration of Nd(III) in aqueous phase. Fig. 3b shows the actual equilibrium isotherm showing the concentration of Nd(III) (org, eq), Nd(III) (aq, eq) pH (eq) (all at equilibrium). Maximum loading capacity was estimated by the plateau value reached at equilibrium or the threshold of the slope

discontinuity. According to the results, the Nd-loading capacity values of the extracting system based on **H₂E-4C₁₀** at 1.5 × 10⁻² mol L⁻¹ in DiPB were 1.7 g L⁻¹ (1.2 × 10⁻² mol L⁻¹) and 2.0 g L⁻¹ (1.4 × 10⁻² mol L⁻¹) for nitrate and chloride media, respectively. This showed that the trend previously observed in Fig. 1 was inverted once the molar ratio of Nd in the feed to **H₂E-4C₁₀** in the diluent became more than 1 (typically for [Nd³⁺] > 3 g L⁻¹). In both cases, the capacity at the maximal Nd-charge occurring at [Nd³⁺]ᵢ ~ [H₂E]ᵢ suggests a formation of a complex with a potential metal-to-extractant ratio of 1:1 in the loaded organic phases. In other words, the Nd-loading capacity of the new system appears to be limited by the extractant concentration. Rather small difference (15 %) found between the two loading capacities confirms the previously mentioned observations concerning the low impact of the anion of the feed solutions on the extraction properties of the **H₂E-4C₁₀**-DiPB solvent system.

#### 3.3.2. Separation between Nd, Pr and Dy using H₂E-4C₁₀ in DiPB

The relative affinity of **H₂E-4C₁₀** for Nd(III), Dy(III) and Pr(III), the three main REEs entering into the NdBFe magnet leachates, was investigated by contacting aqueous solutions containing 10⁻² mol L⁻¹ of each REE at three different pH with a 10⁻² mol L⁻¹ **H₂E-4C₁₀** in DiPB. The selection of DiPB as the diluent was based on the higher values of SFₚ₉₉ and SFₚ₉₉₇ with aromatic diluents, compared to those obtained with hydrocarbons (Batchu and Binnemans, 2018). The results are displayed in Fig. 4 in the form of the extraction parameters (E% and SF) as a function of initial pH of Nd(III), Dy(III) and Pr(III) in nitrate or chloride media.

Clearly, the extraction of each element increases with increasing the pH in most cases. Further, considering the standard deviation (ca. ~ ± 6 %), no marked difference could be established between E% obtained in nitrate and chloride media for any of the elements, confirming the trend observed for Nd alone (Fig. 1). However, the slightly higher extraction efficiency (~40 %) found for Nd in each medium accounted for a noticeable selectivity, over Pr and Dy. Indeed, separation factors SF were calculated according to Eq. (4) (see material & method) and could barely exceed 1.5. For instance, the maximum SFₚ₉₉₇ and SFₚ₉₉₇ were estimated to be 1.23 and 1.43, respectively, for HNO₃ at pH 2. In comparison, the SFₚ₉₉₇ and SFₚ₉₉₇ of 2.04 and 1.38, respectively, for HCl at pH 4, show that **H₂E-4C₁₀** in DiPB is slightly selective towards Nd compared to Pr and Dy, at the high pH range.

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Distribution isotherms of Nd³⁺ related to its extraction performed by the **H₂E-4C₁₀**-DiPB system from nitrate and chloride media at initial pH = 2. The isotherm has been plotted (filled symbols) vs. the initial (a) or the equilibrated (b) concentration of Nd³⁺ in the aqueous feed. Corresponding Nd:H₂E molar ratio and pH at equilibrium (deduced from Eq. (25)) have been also plotted (empty symbols) on right axis in panel a and panel b, respectively. ([**H₂E-4C₁₀**] = 1.5 × 10⁻² mol L⁻¹, T = 298 K, contact time = 30 min., experimental error ~ ± 5 %).</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 4.** Extraction efficiencies ($E_{9\%L_n}$) and separation factors $SF_{Nd/D_y}$ (▶), $SF_{Nd/Pr}$ (♦) obtained at various initial pH for the extraction of a 1:1:1 Nd$^{3+}$ (●):Pr$^{3+}$ (○): Dy$^{3+}$ (■) mixture ([Ln$^{3+}$] = 10$^{-2}$ mol L$^{-1}$) in nitrate (a) or chloride media (b) using H$_2$E-4C$_{10}$ at 10$^{-2}$ mol L$^{-1}$ in DiPB ($T$ = 298 K, contact time = 30 min.). Corresponding plots $D = f(pH)$ have been reported in Fig. S2.2.</figcaption></figure>

### 3.4. Effect of extractant concentration and determination of stoichiometry using slope analysis

The composition of the Nd species formed and transferred into the H$_2$E-4C$_{10}$-diluent systems was determined at the conditions specified in Table 2. The extractant H$_2$E-4C$_{10}$ was used at various concentrations (from 1 to 100 mM) in chloroform rather than in DiPB as higher solubility (unreachable in DiPB) of the ligand was required for this experiment. According to results reported by Lécrivain et al. (Lécrivain and Kimberlin, 2019), the slope value found in chloroform should be of the same order as the one that could be found in DOm or in DiPB.

Taking into consideration the relation in Eq. (15), stoichiometric number *n* could be then deduced from the slope obtained by plotting $log D_{Nd} = f(log[H_2E]_{eq})$, where $[H_2E]_{eq}$ is the concentration of H$_2$E-4C$_{10}$ in the organic phase at equilibrium. The results are depicted in Fig. 5.

When extraction was performed either from chloride or nitrate media, the stoichiometry number *n* ~ 1 was found consistent with the prediction established according to results obtained for the loading capacity (Fig. 3). The deduced Nd species are summarized in Table 5.

The type of complex formed, thus the coordination mode of H$_2$E-4C$_{10}$, depends on the pH of the solution, which impacts the *a* and *x* values (see Eq. (7)). The denticity of H$_2$E-4C$_{10}$ increases with the pH of the initial feed (in the absence of a supplementary salt) as illustrated in Table 5. To a greater extent, this may be accompanied with a decrease in the equilibrium pH (pH$_{eq}$).

Unfortunately, this pH$_{eq}$ could not be measured with satisfying accuracy and was only estimated according to Eq. (25). Nevertheless, when plotted against the pH$_{eq}$, $logD_{Nd}$ followed a linear relationship (see SI, fig. S2.3) giving the slope as the number of exchanged protons (see SI, eq. S1 & Table S2.1). This number depended on the nature of both the aqueous feed and the diluent. Besides, the most relevant values were found close to 1 or 2 whereas they were not always in accordance with the imposed value of *a*, suggesting that further experiments are required before determining this number precisely.

Within the present experimental conditions (initial pH = 2 and ionic strength *I* = 0.07 M) and considering the acidity of both carboxylic functions (see Section 3.3), it is assumed that the complexation occurs mainly through acetate groups (Table 5 with *x* ≤ 1) with additional participation of the didecylamides carbonyl functions (Scheme 1). The

<figure><img src="image_9.png" /><figcaption>**Fig. 5.** Dependence of Nd distribution ratio on the extractant concentration with nitrate (●) or chloride (■) medium ([Nd$^{3+}$] = 10$^{-2}$ mol L$^{-1}$, initial pH = 2, *T* = 298 K, contact time = 30 min.). Linear fits were obtained as $log(D_{Nd}) = n \cdot log([H_2E]_{eq}) + \text{Intercept (Int)}$. In nitrate medium (—), $R^2 = 0.976$, *n* = 1.0 ± 0.1, *Int* = 3.1 ± 0.2. In chloride medium (—), $R^2 = 0.993$, *n* = 1.2 ± 0.1, *Int* = 3.1 ± 0.1.</figcaption></figure>

latter should become the main complexing groups at very low pH (when *x* = 2). The two amine groups should not be involved as the $pK_a$ of the tertiary amine groups on the ethylenediamine-scaffold lies within 3.5–4.5 and 6.6–7.3 for EDTA-diamides according to different communications (Welsh, 2003; Wang et al., 1998; Danil de Namor and Alfredo Pacheco Tanaka, 1998; Krot et al., 2005). Note that the anion interacting with the trialkylammonium moiety is not accounted in the previous assumption.

Taking into account the species deduced from the experimental results as well as the aforementioned potential coordination sites, the coordination numbers (CNs) of 5–8 for Nd$^{3+}$ for each medium detailed in Table 6 were obtained.
<caption>**Table 5**</caption>

Nd(III) species expected in the organic phases depending on the initial pH of the feed based on Eq. (6)ª.

<table><thead><tr><th>x</th><th>a</th><th>Nd A<sub>a</sub>(H<sub>x</sub>E)</th><th>pH of the aqueous feed</th></tr></thead><tbody><tr><td>2</td><td>3</td><td>Nd A<sub>3</sub>(H<sub>2</sub>E)</td><td>pH &lt; &lt;&lt; 0</td></tr><tr><td>1</td><td>2</td><td>Nd A<sub>2</sub>(HE)</td><td>0 ≤ pH &lt; pK<sub>a2</sub></td></tr><tr><td>0.5</td><td>1.5</td><td>1 : 1 N<sub>d</sub> A<sub>2</sub>(HE) : N<sub>d</sub>A<sub>E</sub></td><td>pH ~ pK<sub>a2</sub></td></tr><tr><td>0</td><td>1</td><td>N<sub>d</sub>A<sub>E</sub></td><td>pH &gt; pK<sub>a2</sub></td></tr></tbody></table>

ª Overall equilibrium $Nd^{3+} + aA^{-} + n\overline{H_2E} \rightleftharpoons NdA_a(H_xE)_n + (3-a)H^+$ (6), with a, n and x previously defined.

<caption>**Table 6**</caption>

Coordination number (CN) of a Nd metallic centre in extracted species from nitrate or chloride solutionsª.

<table><thead><tr><th rowspan="2">Extracted species</th><th>Denticity</th><th>CN</th></tr><tr><th>H<sub>x</sub>E</th><th>Nd</th></tr></thead><tbody><tr><td>Nd(H<sub>2</sub>E)A<sub>3</sub></td><td>2</td><td>8</td></tr><tr><td>Nd(H<sub>2</sub>E)A<sub>2</sub></td><td>2</td><td>5</td></tr><tr><td>NdA<sub>E</sub></td><td>3</td><td>7</td></tr><tr><td>NdA<sub>E</sub></td><td>3</td><td>5</td></tr><tr><td>NdA<sub>E</sub></td><td>4</td><td>6</td></tr><tr><td>NdA<sub>E</sub></td><td>4</td><td>5</td></tr></tbody></table>

ª Overall equilibrium $Nd^{3+} + aA^{-} + n\overline{H_2E} \rightleftharpoons NdA_a(H_xE)_n + (3-a)H^+$ (6), with a, n and x previously defined.

ᵇ Considering $A^-$ as a bidentate (NO₃).

ᶜ Considering $A^-$ as a monodentate (Cl⁻ or NO₃).

Because a coordination number of 8 or 9 is favored for Ln(III), one or more water molecule(s) will certainly occupy the remaining site(s) to complete the CN of 8 or 9. The number of water molecules cannot be ascertained because of the relative size of each ion (hydrated Cl⁻ is slightly larger than hydrated NO₃) and the resulting steric hindrance on the inner sphere. Considering the acid properties of H₂E-4C₁₀, N<sub>d</sub>A<sub>E</sub> with two to four water molecules in the inner coordination sphere, is expected to be the predominant Nd-species in the organic phases when the pH of the aqueous feed exceeds 2.

### 3.5. Thermodynamic investigation of the Nd extraction

The Nd(III) extractions were carried out using similar conditions described previously, but at temperature ranging from 293 K to 313 K both in DiPB ([H₂E-4C₁₀] = 10⁻² M) and in DOm ([H₂E-4C₁₀] = 6 × 10⁻³ M). Thermodynamic quantities ΔH°, ΔS° and ΔG° have been determined by plotting ln K<sub>eq</sub> = f(1/T). Prior to this, the values of the equilibrium constant K<sub>eq</sub> have been estimated according to Eq. (23).

Considering the initial pH of the aqueous feed (pH = 2), the value of x in H<sub>x</sub>E was 0 ≤ x ≤ 1. The metal:H₂E-4C₁₀ stoichiometry was fixed to 1:1 as previously determined (Table 5). The corresponding values of a were deduced from Eq. (7) as reported in Table 5. Based on reported logβ₁ values, the fraction of free Nd³⁺, α<sub>Nd³⁺</sub>, was estimated both in chloride (α<sub>Nd³⁺</sub> = 0.95) and in nitrate (α<sub>Nd³⁺</sub> = 0.98) media by considering that these values are nearly constant in the 293–313 K temperature range (Migdisov and Williams-Jones, 2002; Rao and Tian, 2009). According to some results previously reported elsewhere (Dash et al., 2015; Poltoratskii et al., 2005; Ziouane and Leturcq, 2018), the activity coefficients of the species remaining in the aqueous feed at the equilibrium were considered constant in this same temperature range. The concentration of protons and anions were estimated from Eqs. (25) and (26), respectively. The value of Y was calculated from Eq. (9) with activity coefficients reported in the literature for hydrochloric acid (Schneider et al., 2004), nitric acid (Davis and De Bruin, 1964), and neodymium in

nitrate (Guignot et al., 2019) and chloride (Spedding et al., 1952) medium.

Results are presented in Fig. 6 and deduced quantities are summarized in Table 7 for each Nd A<sub>a</sub>(H<sub>x</sub>E) species, diluent, and aqueous feed.

The results showed that all transfers of Nd were spontaneous in the standard condition as the standard reaction Gibbs free energy values were all negative. In the present condition, the values of the experimental distribution ratio (D > 1) suggested that the considered equilibrium likely occurred in the forward direction of Eq. (6). The transfer which took place in DiPB was endothermic and typically entropy-driven whereas the transfer occurring in DOm was exothermic. Similarly, but to a lower extent, extractions performed from a chloride medium were entropy-driven and those performed from a nitrate medium were enthalpy-driven.

However, compared to the influence of the counter-anion/aqueous medium, the nature of the organic medium significantly affected the thermodynamic quantities ΔH° and ΔS°. Besides, ΔG° was not affected by the nature of the aqueous feed (confirming the results presented in Fig. 5) and only poorly (ΔΔG° ~ 3.7 kJ mol⁻¹) by that of the diluent as it seemed to depend mainly on the extent of protonation of the ligand, i.e., the value of x in N<sub>d</sub>A<sub>a</sub>(H<sub>x</sub>E)<sub>n</sub>. More specifically, ΔG° was found to be proportional to x (Fig. 7) as expected when considering the rewriting of the Eq. (24) as follows:

$$K_{\text{eq}} = \frac{D_{\text{Nd}} \cdot \alpha_{\text{Nd}^{3+}} \cdot \gamma_{\text{H}^{+}}^{3} \cdot [\text{H}^{+}]_{\text{eq}}^{3}}{\gamma_{\text{Nd}^{3+}} \cdot [\text{H}_{2}\text{E}]_{\text{eq}}^{n}} \cdot (\gamma_{\text{H}^{+}} \cdot \gamma_{\text{A}^{-}})^{-a} \cdot [\text{H}^{+}]_{\text{eq}}^{-a} \cdot [\text{A}^{-}]_{\text{eq}}^{-a} \quad (27)$$

Considering n = 1, from the Eq. (7) it can be deduced that a = x + 1 and thus:

$$K_{\text{eq}} = \frac{D_{\text{Nd}} \cdot \alpha_{\text{Nd}^{3+}} \cdot \gamma_{\text{H}^{+}}^{2} \cdot [\text{H}^{+}]_{\text{eq}}^{2}}{\gamma_{\text{A}^{-}} \cdot \gamma_{\text{Nd}^{3+}} \cdot [\text{H}_{2}\text{E}]_{\text{eq}} \cdot [\text{A}^{-}]_{\text{eq}}} \cdot (\gamma_{\text{H}^{+}} \cdot \gamma_{\text{A}^{-}})^{-x} \cdot [\text{H}^{+}]_{\text{eq}}^{-x} \cdot [\text{A}^{-}]_{\text{eq}}^{-x} \quad (28)$$

$$\ln K_{\text{eq}} = \ln \frac{D_{\text{Nd}} \cdot \alpha_{\text{Nd}^{3+}} \cdot \gamma_{\text{H}^{+}}^{2} \cdot [\text{H}^{+}]_{\text{eq}}^{2}}{\gamma_{\text{A}^{-}} \cdot \gamma_{\text{Nd}^{3+}} \cdot [\text{H}_{2}\text{E}]_{\text{eq}} \cdot [\text{A}^{-}]_{\text{eq}}} - x \cdot \ln(\gamma_{\text{H}^{+}} \cdot \gamma_{\text{A}^{-}} \cdot [\text{H}^{+}]_{\text{eq}} \cdot [\text{A}^{-}]_{\text{eq}}) \quad (29)$$

Hence, at given conditions, ΔG° is a linear function of x. Further, in our conditions, the transfer would be more favored when only one proton was exchanged (x = 1) and therefore the complex predominantly formed in the organic phase is N<sub>d</sub>(HE)A₂.

Except the low entropy value found with the H₂E-4C₁₀-DOm system for x = 0 in the nitrate medium, the positive entropy values in Table 7 agree well with the fact that the complexation of lanthanides by one polydentate ligand (like EDTA) leads to a release of several water molecules that were initially occupying the coordination sphere (Cernochova et al., 2005). Further detailed analysis of these results would require a systemic analysis based on the ienaic approach (Zemb et al., 2015), which is beyond the scope of the current study.

Moreover, the marked difference between the standard reaction Gibbs free energies obtained after Nd³⁺ transfer in the two diluents (one aromatic and the other aliphatic) contrasted significantly with the results presented in Fig. S2.1. Hence, the overall standard free energy does not appear to result, exclusively from the distribution coefficient (found typically higher in DOm), which warrants further interpretations. The ability of H₂E-4C₁₀ to release the metal ions from the loaded organic phase to an aqueous solution (back-extraction or stripping properties) ought to be characterized. Preliminary results from on-going work indicate that the back-extraction vary between 55 and 78 %, depending on the stripping agent and the experimental condition used.

### 4. Conclusions

Herein we investigated the Nd extraction from two different media, nitrate and chloride, using lipophilic EDTA-diamide derivative (H₂E-4C₁₀) solubilized in seven diluents. Although it has been shown that the
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 6. Variation of the equilibrium constant lnK<sub>eq</sub> as a function of the temperature when Nd is extracted from nitrate (●) or chloride (▲) media at initial pH = 2. Extractions were performed with [H<sub>2</sub>E-4C<sub>10</sub>] = 10<sup>-2</sup> M in DiPB (a) or with [H<sub>2</sub>E-4C<sub>10</sub>] = 6 × 10<sup>-3</sup> M in DOm (b), T = 293, 298, 303, 313 K, contact time = 30 min. The average standard error was found to be ~ ± 6 %. Plots details have been reported in Table S2.1 (see SI).</figcaption></figure>

<caption>**Table 7**</caption>

Thermodynamic constants determined from the plots in Fig. 6 for extraction performed with DiPB and DOm as diluents.

<table><thead><tr><th rowspan="3">Experimental Conditions</th><th colspan="6">[H<sub>2</sub>E-4C<sub>10</sub>] = 10<sup>-2</sup> mol L<sup>-1</sup> in DiPB</th></tr><tr><th rowspan="2">x</th><th colspan="3">Chloride medium (pH = 2)</th><th colspan="3">Nitrate medium (pH = 2)</th></tr><tr><th>0</th><th>0.5</th><th>1</th><th>0</th><th>0.5</th><th>1</th></tr></thead><tbody><tr><td>ΔH° (kJ mol<sup>-1</sup>)</td><td>5 ± 0.5</td><td>6.0 ± 0.60</td><td>7.4 ± 0.7</td><td>1.1 ± 0.1</td><td>1.3 ± 0.2</td><td>1.6 ± 0.2</td></tr><tr><td>ΔS° (J mol<sup>-1</sup> K<sup>-1</sup>)</td><td>30 ± 2</td><td>67 ± 2.0</td><td>106.0 ± 2.0</td><td>18 ± 0.4</td><td>50.0 ± 0.5</td><td>87 ± 0.7</td></tr><tr><td>ΔG° (kJ mol<sup>-1</sup>) (± 10 %)</td><td>-4.4</td><td>-14.0</td><td>-24.2</td><td>-4.4</td><td>-13.6</td><td>-24.3</td></tr></tbody></table>

<table><thead><tr><th rowspan="3">Experimental Conditions</th><th colspan="6">[H<sub>2</sub>E-4C<sub>10</sub>] = 6 × 10<sup>-3</sup> mol L<sup>-1</sup> in DOm</th></tr><tr><th rowspan="2">x</th><th colspan="3">Chloride medium (pH = 2)</th><th colspan="3">Nitrate medium (pH = 2)</th></tr><tr><th>0</th><th>0.5</th><th>1</th><th>0</th><th>0.5</th><th>1</th></tr></thead><tbody><tr><td>ΔH° (kJ mol<sup>-1</sup>)</td><td>-0.92 ± 0.1</td><td>-1.1 ± 0.1</td><td>-1.4 ± 0.1</td><td>-7.0 ± 0.8</td><td>-8.6 ± 1.0</td><td>-10.2 ± 1.2</td></tr><tr><td>ΔS° (J mol<sup>-1</sup> K<sup>-1</sup>)</td><td>21 ± 0.3</td><td>54 ± 1.0</td><td>89 ± 1.0</td><td>0.4 ± 2.7</td><td>29 ± 4.0</td><td>60 ± 4.0</td></tr><tr><td>ΔG° (kJ mol<sup>-1</sup>) (± 10 %)</td><td>-7.3</td><td>-17.4</td><td>-27.9</td><td>-7.2</td><td>-17.3</td><td>-27.9</td></tr></tbody></table>

best solubility of H<sub>2</sub>E-4C<sub>10</sub> (molar range) was found in chloroform, the best extraction performances from nitrate medium were obtained in a non-polar aliphatic mixture, namely the 93:7 v:v n-dodecane:n-octanol mixture (DOM) and, to a lower extent, in an aromatic diluent, namely the 1,3-diisopropylbenzene (DiPB). Good agreement has been found between the results from this study and the reported literature data, showing that the highest extraction efficiency was obtained in DOM. The loading capacity and the affinity towards other REEs have been investigated with the H<sub>2</sub>E-4C<sub>10</sub>-DiPB system. Similar loading capacities at equilibrium (Q<sub>e</sub>) suggesting a 1:1 metal- H<sub>2</sub>E-4C<sub>10</sub> stoichiometry were obtained in the two media (Q<sub>e,NO<sub>3</sub></sub> = 1.7 g L<sup>-1</sup> and Q<sub>e,Cl<sup>-</sup></sub> = 2.0 g L<sup>-1</sup>). Likewise only a slight difference could be observed between the values of the separation factor obtained for the extraction from each medium (SF<sub>Nd/Pr</sub> = 1.23 vs SF<sub>Nd/Pr</sub> = 1.38 in nitrate and SF<sub>Nd/Dy</sub> = 1.43 vs SF<sub>Nd/</sub>

<figure><img src="image_12.png" /><figcaption>Fig. 7. The standard reaction Gibbs free energy related to the Nd transfer obtained in H<sub>2</sub>E-4C<sub>10</sub>-DiPB (●) or in H<sub>2</sub>E-4C<sub>10</sub>-DOM (*) solvent and plotted vs. the extent of H<sub>2</sub>E-4C<sub>10</sub> protonation. Plot details have been reported in Table S2.2. Mean values of ΔG° were the average of those found with nitrate and chloride aqueous feeds (see Table 7).</figcaption></figure>

D<sub>y</sub> = 2.04 in chloride medium). Therefore, the counter-anion did not cause a significant influence on the separation factors and the Nd extraction mechanism related to the novel system. The metal:ligand stoichiometry of N<sub>Nd</sub>:N<sub>HxE</sub> = 1:1 determined by the slope method using the H<sub>2</sub>E-4C<sub>10</sub>-chloroform system, in both nitrate and chloride media, is consistent with the loading capacities. The standard Gibbs free energy of transfer, ΔG°, for the two systems (H<sub>2</sub>E-4C<sub>10</sub>-DOM and H<sub>2</sub>E-4C<sub>10</sub>-DiPB) was not affected by the nature of the aqueous feed and depended strongly on the extent of protonation of the extracted complex species (x in Nd A<sub>a</sub>(H<sub>x</sub>E)<sub>n</sub>). The enthalpy and the entropy were typically impacted by the nature of the diluent, as the transfer in DiPB (aromatic) was endothermic and entropy-driven whereas that in DOM (aliphatic) was
an exothermic enthalpy-driven process. Taken as a whole, the results have shown that lipophilic EDTA diamide derivatives are new promising cationic exchangers that need further developments and improvements for the enhancement of their solubility in non-polar diluents, better intra-lanthanide selectivity, and extensive characterization of extraction properties, before finding application at a larger scale. This work is currently in progress.

## CRediT authorship contribution statement

**Tamir Sukhbaatar:** Writing – original draft, Investigation.
**Raphaëlle Piton:** Investigation. **Fabrice Giusti:** Writing – review & editing, Methodology, Investigation, Formal analysis. **Guilhem Arrachart:** Writing – review & editing, Methodology. **Magali Duvail:** Writing – review & editing, Data curation. **Aarti Kumari:** Writing – original draft, Investigation. **Santosh Darew:** Writing – original draft, Investigation. **Shally Gupta:** Investigation. **Sriram Goverapet:** Writing – review & editing, Supervision. **Sushanta Kumar Sahu:** Writing – review & editing, Supervision. **Beena Rai:** Writing – review & editing, Supervision. **Stéphane Pellet-Rostaing:** Writing – review & editing, Supervision, Project administration, Funding acquisition.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Stephane Pellet-Rostaing reports financial support provided by Indo-French Centre for the Promotion of Advanced Research. Fabrice Giusti, Stephane Pellet-Rostaing, Guilhem Arrachart, Raphaëlle Piton has patent issued to Assignee. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

This work was funded by the CNRS, the CEA and the CEFIPRA program (Grant 2017-0089 60/30-D). The authors are grateful to B. Baus-Lagarde for ICP-OES analysis and M. Massonnet for the pycnometry measurements.

## Appendix A. Supplementary data

SI include detailed synthesis procedures and NMR spectroscopy data of ligands H₂E-4C₁₀ & H₂E-4C₁₂, distribution ratio of Nd, Dy and Pr determined in the various diluents at different pH, and linear regressions details. Supplementary data to this article can be found online at https://doi.org/10.1016/j.hydromet.2024.106415.

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