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The third phase formation during liquid/liquid extraction has been studied for decades by several authors$^{[1-3]}$ starting in the 50’s. Those studies were focused on the determination of the maximum organic loading that can be reached for a given metal in order to avoid organic phase splitting during the industrial applications such as the PUREX process. More recent studies have been conducted using a physical approach$^{[4-5]}$ to propose models accounting for phase segregation phenomena. A new interest on third phase studies appeared in the early 2000’s with the investigations related to the Th-cycle$^{[6-8]}$. Most of the models and results rely on SANS/SAXS or FTIR data acquired on metal-free or single-metal systems, and concentrations are often determined through indirect analysis.
Despite the huge amount of data concerning LOC (Limiting Organic Concentration) of actinides (IV) for different phosphate based solvents, a lack of data remains in the literature for U$^{(VI)}$ alone and in the presence of Th$^{(IV)}$ or Pu$^{(IV)}$ for TBP/dodecane solvent. Besides, there are few values concerning the composition of both heavy and light organic phases after splitting, and when available, the data covers only a narrow range of the 3$^{rd}$ phase existence.
We have determined U$^{(VI)}$LOC values for previously described system at different HNO$_{3}$ and TBP concentrations which have not been yet reported in the literature. The behavior of U$^{(VI)}$ and Th$^{(IV)}$ after phase segregation and the composition of each phase was characterized using X ray fluorescence spectroscopy (XFS), allowing metal and TBP direct titration in organic medium for the first time ever according to our knowledge. Nitric acid and water contents were also estimated using acid/base and Karl Fischer titration respectively. Trends were established for U$^{(VI)}$ and Th$^{(IV)}$ alone in the third phase, as well as for bimetallic U$^{(VI)}$/Th$^{(IV)}$ systems.
A parallel has also been drawn by studying transition metals behavior in HCl/TBP/dodecane systems (analyzed by XFS and ICP-OES). We showed that LOC values and metal distribution can vary considerably from a metal to another even when working in the same conditions of extraction. We are currently investigating molecular species, as some other authors$^{[9-10]}$, that may be involved in phase splitting in order to determine contributions of metal and acid to phase segregation phenomena.
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$^{[2]}$ Z. Kolarik, Proc. Int. Solvent Extraction Conf. ISEC’77, CIM Spec. 21, 178-182, 1977
$^{[3]}$ P. R. Vasudeva Rao, Z. Kolarik, Solvent Extr. Ion Exch., 14(6), 955-993, 1996
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$^{[8]}$ K. Benadict Rakesh, A. Suresh & P. R. Vasudeva Rao, Solvent Extr. Ion Exch., 32, 249-266, 2014
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$^{[10]}$ P. Ivanov, J. Mu, L. Leay, S-Y. Chang, C. A. Sharrad, A. J. Masters, S. L. M. Schroeder, Solvent Extr. Ion Exch., 35(4), 251-265, 2017
