Third Phase Formation Revisited: The U(VI), HNO₃–TBP,<i>n</i>‐Dodecane System

Abstract

Abstract 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 Å, 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. #Work performed under the auspices of the U. S. Department of Energy, Office of Basic Energy Science, Division of Chemical Science (for the part performed at the Chemistry Division of ANL) and Division of Material Science (for the part performed at INPS), under contract No. W‐31‐109‐ENG‐38. §The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory ("Argonne") under Contract No. W‐31‐109‐ENG‐38 with the US Department of Energy. The US Government retains for itself, and others acting on its behalf, a paid‐up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Keywords: Third phase formationTributylphosphateUranyl nitrateSmall angle neutronScatteringEXAFS Acknowledgments This work was funded by the U. S. Department of Energy, Office of Basic Energy Science (for use of the Advanced Photon Source), Division of Chemical Science (for the part performed at the Chemistry Division of ANL) and Division of Material Science (for the part performed at INPS), under contract No. W‐31‐109‐ENG‐38. The authors express their gratitude to Denis Wozniak (INPS) for the help provided in the SANS measurements and would also like to thank Paul Rickert, Alice Essling and Susan Lopykinski of ANL for the purification of the TBP sample used in this work and for uranium and water analyses. The EXAFS experiments were conducted with the infrastructure support of the Argonne Actinide Synchrotron Facility and BESSRC‐CAT. Notes #Work performed under the auspices of the U. S. Department of Energy, Office of Basic Energy Science, Division of Chemical Science (for the part performed at the Chemistry Division of ANL) and Division of Material Science (for the part performed at INPS), under contract No. W‐31‐109‐ENG‐38. §The submitted manuscript has been created by the University of Chicago as Operator of Argonne National Laboratory ("Argonne") under Contract No. W‐31‐109‐ENG‐38 with the US Department of Energy. The US Government retains for itself, and others acting on its behalf, a paid‐up, nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government.