Thermodynamics of Dissolution for Bis(triazine)−Bipyridine-Class Ligands in Different Diluents and Its Reflection on Extraction

Abstract

Hydrochemical separation processes are one of the methods used for the treatment of spent nuclear fuel. Solvent extraction is also used in many other non-nuclear applications like the mining industry. In the nuclear case, hydrochemical separation processes are already employed in the world today for the recovery of uranium and plutonium. The method is however also considered for future separation systems for use in combination with the transmutation of the minor actinides. In a hydrochemical separation process the two phases are the pregnant (usually) aqueous feed and the organic phase comprising a diluent together with one or more extractants. One such class of extractants developed for partitioning and transmutation purposes is the bis(triazine)−bipyridine-type (BTBP) molecules. When assessing the feasibility and loading properties of such an extraction system, the solubility of the ligands is of the outmost importance. The understanding of whether the dissolution is enthalpically or entropically driven will also help the understanding of the differences in extraction observed between various diluents and temperatures. In this paper the enthalpy and entropy of dissolution of the BTBP-class ligands have been determined for different diluents. It has also been shown that it is possible to predict the extraction behavior of these molecules in the selected diluent once the solubility is known.

Synthesis and Evaluation of Lipophilic BTBP Ligands for An/Ln Separation in Nuclear Waste Treatment: The Effect of Alkyl Substitution on Extraction Properties and Implications for Ligand Design

“…The approximate solubilities of the ligands 12 and 13 in these diluents were first determined and are shown in Table 2. Quite unexpectedly, both ligands 12 and 13 showed slightly lower solubilities than CyMe 4 -BTBP 1 and BTBP 2 in both 1-octanol and cyclohexanone, [13] despite the presence of the additional alkyl groups. A higher solubility of the ligands was anticipated in these diluents.…”

“…The tert-butyl-substituted derivative 2 [12] (Figure 1) posesses a higher solubility than CyMe 4 -BTBP 1 in suitable diluents such as 1-octanol and cyclohexanone, [13] although its solvent extraction kinetics are slower than those of 1. More recently, a symmetrical BTP ligand derived from camphor has shown both improved solubility and fast extraction kinetics compared to related BTP ligands.…”

“…[10] Recently, it has been shown that the extraction properties of ligands such as 1 can be improved considerably if the ligand is pre-organized for metal binding with a phenanthroline moiety, which locks the ligand into the required cis conformation. [11] The tert-butyl-substituted derivative 2 [12] ( Figure 1) posesses a higher solubility than CyMe 4 -BTBP 1 in suitable diluents such as 1-octanol and cyclohexanone, [13] although its solvent extraction kinetics are slower than those of 1. More recently, a symmetrical BTP ligand derived from camphor has shown both improved solubility and fast extraction kinetics compared to related BTP ligands.…”

Development of the Chalmers Grouped Actinide Extraction Process

“…Cyclohexanone ( = 0.95 g·mL −1 [28]) was a diluent chosen for the Chalmers GANEX process because it well dissolves CyMe 4 -BTBP [29] and ensures relatively fast extraction kinetics with this extractant [30]. It is also a cheap diluent that is mass produced for the production of nylon 6 [31].…”

“…When actinide distribution ratios were investigated in long-chained alcohols (using C5-BTBP and a low-acidic aqueous phase, 0.99 M NaNO 3 and 0.01 M HNO 3 ) it was discovered that the distribution ratios increased with a decrease in chain length, being equal to 5.74 for hexanol and 1.21 for decanol [34]. Hexanol also displays a slightly higher solubility of CyMe 4 -BTBP compared to octanol [29], which is why the main focus was put on 1-hexanol when investigating long--chained alcohols for GANEX purposes. However, the comparatively slow kinetics and low solubility of CyMe 4 -BTBP in these types of diluents [18], led to investigations of other more innovative solvents for the Chalmers GANEX process.…”