Experimental and theoretical simulations investigations of La³⁺/Ce³⁺ extraction and separation with D2EHPA

The main challenge in extracting and separating rare earth elements from industrial waste is their chemical similarity. Thus, this study focuses on the solvent extraction and separation of lanthanum and cerium using diethyl-2-hexylphosphoric acid (D2EHPA). Molecular Dynamics (MD) simulations and Density Functional Theory (DFT) were employed to determine the stoichiometry and the stability of the complexes formed. The results show that the extraction percentage efficiency and the separation factor D_ce/D_La increased in the pH range of 1-5. FTIR studies of loaded D2EHPA organic solutions indicate cation-exchange behavior through the formation of P-O-La and P-O-Ce bonds. The analysis of logD vs. pHeq and logD-2pH vs. log [D₂EHPA] determined that the extracted complex stoichiometries were [La(NO₃) (D₂EHPA)₂] and [Ce(NO)3 (D₂EHPA)₂]. The radial distribution function (RDF) and coordination number of La-H₂O and La-NO₃^-/Ce-H₂O and Ce-NO₃^- gave 8 and 1, respectively, for the number of water molecules and nitrate ions around La³⁺ and Ce³⁺. Optimizing the geometry through the density functional theory (DFT) approach on the optimized structures of the [La(NO₃)(H₂O)₈]²⁺ and [Ce(NO₃)(H₂O)₈]²⁺ ionic complexes illustrated the monodentate form of nitrate ions around the central atom in the first coordination sphere. However, bidentate structures were found for D2EHPA in the extracted complexes. The Born Haber thermodynamic cycle and the Conductor-like Polarizable Continuum Model (CPCM) were applied to calculate the free energies ΔG_ext of lanthanum (-179.02 kcal/mol) and cerium (-358.61 kcal/mol). The energy gap of HOMO-LUMO between [La(NO₃)(D2EPA)₂]  = 0.17 eV) and [Ce(NO₃)(D2EPA)₂]  = 0.03 eV), the Mayer Population Analysis (MPA) and Wiberg Bound Index (WBI) corroborated the stability of the D2EHPA complex with cerium over that with lanthanum. These results could help us to understand how organic extractants interact with rare earth elements and radionuclides. They could also have applications in nuclear technology and the sustainable remediation of polluted environments.