A Deep Insight into the Selectivity Difference Between Y(III) and La(III) Ions Toward D2EHPA Ligand: Experimental and DFT Study

Abstract

The twofold extraction behavior of light and heavy rare earth elements transforms into a more selective extraction of heavy rare earth elements when Di-(2-ethylhexyl) phosphoric acid (D2EHPA), one of the commonest cation exchange extractants, is employed. However, why this phenomenon has not been fully investigated from the quantum perspective yet. To confirm and interpret the laboratory-observed selectivity results in the extraction of Y(III) over than La(III), this study utilized the Density Functional theory (DFT) connected with Born Haber thermodynamic besides importing the solvent effect through the Conductor-Like Screening Model (COSMO). The hydration reaction energies of La(III) and Y(III) were estimated at -383.7 kcal/mol and -171.83 kcal/mol according to the cluster solvation model. It was observed that, among other influential factors, hydration energy is a critical one in the rate of the extraction free energy of every rare earth element and its tendency to be transferred to the organic phase in reacting to the extractant ligand. It was shown that the experimental ∆∆Gext results (2.1 kcal/mol) enjoyed a proper consonance with the ∆∆Gext results of DFT calculations (1.3 kcal/mol). In the pursuit of discovering the reasons for this phenomenon, the orbital structure of every aqueous and organic complex was studied, and the significant differences in energy magnitudes were discussed. The current comprehensive design of experimental studies and calculations can give birth to a deeper understanding of the interactions of the D2EHPA extractant with La(III) and Y(III).