Nitrate ions effects on solvent extraction of rare earth elements from aqueous solutions by D2EHPA: Experimental studies and molecular simulations

Abstract

The number and positioning manner of the ligands present in the first shell of every cation of rare earth elements are key parameters in determining the stoichiometry of the reactions of solvent extraction from nitrate systems, especially when acidic extractants are used. In addition to determining the hydration and coordination of nitrate ligands around lanthanum and yttrium cations in an aqueous electrolyte, this paper investigated stoichiometry by combining the results of studies on molecular dynamics simulation and solvent extraction experiments. The analysis of the results of the Radial Distribution Function clearly showed the presence of one and two nitrate ligands, respectively, in the first hydration shells of lanthanum and yttrium. Considering the effect of nitrate anion, the identified ionic complexes of the rare earth elements in the electrolyte were [LaNO3·(H2O)7]2+ and [Y(NO3)2·(H2O)4]+. The overlap between the results of slope analysis and molecular dynamics elucidated that two and one ligands from the Di-(2-Ethylhexyl) phosphoric acid extractant were required for the complete extraction of lanthanum and yttrium ionic complexes, respectively. The DFT-employing geometry studies on ionic complexes in the aqueous phase revealed that the direction of the nitrate ligand in the first coordination shell of these elements was in a bidentate form. The results of this study, besides assisting with the better interpretation of the difference between the extraction of light and itinerant rare elements by D2EHPA, are greatly effective in designing new extractants in order to extract these elements more selectively.