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

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⁻¹ in nitrate and chloride media. The extraction efficiency, the distribution coefficients, and the separation factor (E_%, 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_Nd/Pr = 1.48 and SF_Nd/Dy = 2.03. Neodymium-to-extractant stoichiometry was evidenced in chloroform to be 1:1 H₂E: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⁻¹) regardless of the aqueous feed.