Understanding the synergistic effect on lanthanides(III) solvent extraction by systems combining a malonamide and a dialkyl phosphoric acid

Abstract

To separate minor actinides from lanthanides for used nuclear fuel reprocessing, solvent extraction processes combining neutral and acidic extractants are considered. Mixed systems present singular extraction properties where synergistic effects can be observed but information on chemical equilibria is lacking to properly model such behavior. Speciation of Eu(III) and Nd(III) complexes formed in an organic phase combining N,N′-dimethyl-N,N′-dioctylhexylethoxy malonamide (DMDOHEMA) and di(2-ethylhexyl)phosphoric acid (HDEHP) was investigated through electrospray ionization mass spectrometry (ESI-MS), time-resolved laser-induced fluorescence spectroscopy (TRLIFS), UV–Visible, nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and density functional theory (DFT) calculations. A method consists of adding one of the ligands to a solution containing Ln(III) complexes formed with the other ligand while following changes by each technique was used. In both cases, significant modifications in the metal coordination sphere were observed due to formation of mixed complexes containing both ligands: addition of DMDOHEMA to a Ln-HDEHP solution led to the formation of Ln(DEHP)3(HDEHP)2(DMDOHEMA) complexes while addition of HDEHP to a Ln-DMDOHEMA solution gave Ln(NO3)3-n(DEHP)n(HDEHP)m(DMDOHEMA)x′ (0≤n≤3; n+m≥1 and 1≤x′≤3) complexes.