Microscopic Structures of Tri-n-butyl Phosphate/n-Octane Mixtures by X-ray and Neutron Scattering in a Wide q Range

Abstract

Tri-n-butyl phosphate (TBP) is an important extractant for separating hexavalent uranium and tetravalent plutonium from used nuclear fuel by solvent extraction. In such solvent extractions using TBP, the organic phase occasionally separates into two organic phases, namely, light and heavy organic phases. The latter one in particular is called the third phase. The purpose of this work is to elucidate the mechanism whereby the third phase forms in biphasic liquid-liquid solvent extraction of heavy metal ions. Toward this end, small- and wide-angle X-ray and neutron scattering (SWAXS and SWANS) experiments were conducted to examine the microscopic structures of TBP/octane mixtures. These investigations of solute associations in TBP-containing organic phases before extraction of heavy metal ions provide insights into system performance. After the extraction of heavy metal ions, for example, the microscopic structures formed in the organic phase are likely to be correlated with the initial microscopic structures, which are revealed here. SWAXS and SWANS, with accurate estimations of incoherent scattering intensities for all solution samples, revealed the following: (i) TBP self-associates in octane, and the average distance between two TBP molecules in the TBP assemblies is evaluated as 0.9-1.0 nm; (ii) the shape of the TBP assembly is ellipsoidal; and (iii) the attractive interaction among TBP assemblies in octane is miniscule, and thus, they tend to be dispersed homogeneously due to the excluded volume effect.

Determination of the Structures of Uranyl–Tri‐n‐butyl‐Phosphate Aggregates by Coupling Experimental Results with Molecular Dynamic Simulations

“…These results are in good agreement with the experimental results published by Motokawa et al . Previous MD simulations reported in the literature describe either TBP dimers in alkane‐diluted organic phases,,, or TBP trimers or even tetramers in more concentrated solutions .…”

“…For the TBP100 % solution, a peak is calculated to be at 8 nm −1 , which is in agreement with the experimentally obtained I ( q ). This signal was interpreted by Motokawa et al . to be characteristic of intermolecular dipole–dipole interactions involving P=O groups of the phosphate ester of two molecules of TBP.…”

Coordination Chemistry of Actinide Nitrates with Cyclic Amide Derivatives for the Development of the Nuclear Fuel Materials Selective Precipitation (NUMAP) Reprocessing Method

“…Rapid shift of complexation equilibria is expected to drive the formation of the higher nitrato complexes of UO 2 2+ and An 4+ , making the solvent extraction successful. Another important point is that solubility of the extractable species in the selected organic phase must be sufficiently high, a third phase immiscible with both aqueous and organic solvents is otherwise formed Such a situation is seriously problematic, and must be avoided, because it precludes the operation of solvent extraction in a biphasic system.…”

Characterization of the Species Formed during the Extraction of Thorium Employing Tri-n-Butyl Phosphate and N,N-Dihexyl Octanamide as Extractants by Laser Desorption/Ionization Time-of-Flight Mass Spectrometry

“…These techniques have helped in identifying the physical/chemical parameters (for example, aggregation, nature and concentration of acids, diluents) responsible for third-phase formation and also identifying species formed in the heavier organic phases. [3][4][5][6][7]27,[32][33][34][35] SANS measurements on TBP solutions loaded with only HNO 3 or with increasing amounts of U(VI) or Th(IV) revealed the presence of ellipsoidal aggregates (reverse micelle type) before phase splitting. The presence of smaller aggregates in the light organic phase upon third-phase formation was observed, while the heavy organic phase contained pockets of diluent.…”