Comprehensive Studies on Third Phase Formation: Application to U(VI)/Th(IV) Mixtures Extracted by TBP in N-dodecane

Phase splitting of tributylphosphate (TBP)/n-dodecane organic phases resulting from the extraction of UO2(NO3)(2), Th(NO3)(4) and mixtures of both actinides from aqueous nitrate solutions has been investigated. Limiting organic concentrations (LOC) and metals distribution beyond third phase formation have been determined, with comparison between the cases of single metal-systems and metals mixtures. Simultaneous quantification of TBP and both metals was achieved through X-ray fluorescence (XRF) analyses. LOC studies reveal that thorium (IV) drives the third phase formation as it is the most destabilizing element in the solvent. After organic phase splitting, studies of the distributions of metals between the heavy organic phase (HOP) and the diluted organic phase (DOP) in the case of U-(VI)/Th-(IV) mixtures revealed that they are similar to those observed when both metals are alone in the solvent: Thorium (IV) has a strong affinity for the HOP, whereas uranium (VI) distributes both in HOP and in DOP. A supersaturation coefficient (N-LOC) is proposed as a new tool to account for the data obtained in the present study. Furthermore, the approach was successfully applied to analyse available data in the literature regarding thorium (IV) distribution studies after phase splitting in various TBP-alkane solvents. Such a study beyond third phase formation paves the way for studying the mechanism involved in third phase formation, as the metal is clearly identified as the key parameter.