Deciphering the third phase structure in uranium extraction with aliphatic amines using combined small-angle X-ray and neutron scattering

Guerinoni, E.*; Zemb, T.*; Motokawa, Ryuhei   ; Ueda, Yuki   ; Aoyagi, Noboru   ; Pellet-Rostaing, S.*; Dourdain, S.*

Liquid-liquid extraction of uranium involves an acidic aqueous phase in contact with an oil phase containing aliphatic amines and a linear alkane. Excess electrolyte in the oil phase leads to a third phase, hindering metal ion extraction and stripping. We determine the largest admissible extractant concentration (LOC) before macroscopic phase separation. Using small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS), we analyze the microstructure of the phases. In all aliphatic diluents, the third phase consists of weak water-in-oil (w/o) aggregates with interdigitated hydrocarbon chains. Even after centrifugation, SANS detects residual deuterated solvent molecules. A fraction of these w/o aggregates contains uranium(VI) cations (U-loaded aggregates), which exhibit stronger attractive interactions than unloaded aggregates. This leads to the segregation of U-loaded aggregates into large clusters, detected as a Porod-type decay in SAXSbut absent in SANS at a low q-values.