Third phase formation revisited: The U(VI), HNO3-TBP, n-dodecane system

In this work, the system U(VI), HNO3-tri-n-butylphosphate (TBP), n-dodecane has been revisited with the objective of gaining information on the coordination chemistry and structural evolution of the species formed in the organic phase before and after third phase formation. Chemical analyses, spectroscopic and EXAFS data indicate that U(VI) is extracted as the UO2(NO3)(2).2TBP adduct, while the third phase species have the average composition UO2(NO3)(2).2TBP.HNO3. Small-angle neutron scattering (SANS) measurements on TBP solutions loaded with only HNO3 or with increasing amounts of U(VI) have revealed the presence, before phase splitting, of ellipsoidal aggregates with the major and minor axes up to about 64 and 15 Angstrom, respectively. The formation of these aggregates, very likely of the reverse micelle-type, is observed in all cases, that is, when only HNO3, only UO2(NO3)(2), or both HNO3 and UO2(NO3)(2) are extracted by the TBP solution. Upon third phase formation, the SANS data reveal the presence of smaller aggregates in the light organic phase, while the heavy organic phase contains pockets of diluent, each with an average of about two molecules of n-dodecane.