Small-angle neutron scattering study of organic-phase aggregation in the TALSPEAK process

The Trivalent Actinide-Lanthanide Separation by Phosphorus reagent Extraction from Aqueous Komplexes (TALSPEAK) process is a solvent extraction based method for separating trivalent lanthanides (Ln(3+)) from trivalent actinide cations in used nuclear fuel reprocessing. In conventional TALSPEAK, the extractant solution is di(2-ethylhexyl)phosphoric acid (HDEHP) in 1,4-diisopropylbenzene (DIPB). The aqueous medium is diethylenetriamine-N,N,N',N″,N″-pentaacetic acid (DTPA) in a concentrated lactic acid (HL) buffer. Lanthanides are extracted by HDEHP/DIPB, while the actinides remain in the aqueous phase as DTPA complexes. Lactic acid is extracted both independently of the lanthanides and as Ln/HL/HDEHP mixed complex(es). Previous results indicate that lanthanides are extracted both as the mixed complex and as a binary Ln(DEHP·HDEHP)(3) species. Small-angle neutron scattering (SANS) has been applied to study the self-organization properties of solute molecules in xylene solutions containing HDEHP, HL, selected lanthanide ions, and water. The scattering results demonstrate that the dominant HDEHP species is the hydrogen bonded dimer, (HDEHP)(2). Absent lanthanides, lactic acid is extracted as the 1:3 complex (HL·(HDEHP)(3)). Scattering in samples containing up to 0.005 M lanthanides (prepared by extracting lanthanides from aqueous media containing 1.0 M buffered lactic acid) indicates that the dominant metal complex is Ln(DEHP·HDEHP)(3). At 0.013 M extracted lanthanide, the scattering results indicate lower Ln:DEHP stoichiometry and larger scattering particles. At higher metal concentrations, the SANS results indicate large aggregates, the largest aggregates achieving a size equivalent to 20 HDEHP monomers as the primary scattering entity. Analysis of particle shapes indicates best fits with a uniform oblate spheroid particle. These results are discussed in connection with the results of a number of complementary observations that have been made on this system.