Abstract
The two organic ligands 6,6′-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydrobenzo[1,2,4]triazin-3-yl)[2,2′]bipyridine (CyMe_{4}-BTBP) and tri-butyl phosphate (TBP) have previously been investigated in different diluents for use within recycling of used nuclear fuel through solvent extraction. The thermodynamic parameters, K_{mathrm{S}}, Delta C_{p}, Delta H^{0} and Delta S^{0}, of the CyMe_{4}-BTBP solubility in three diluents (cyclohexanone, octanol and phenyl trifluoromethyl sulfone) mixed with TBP have been studied at 288, 298 and 308 K, both as pristine solutions and pre-equilibrated with 4 molcdot L^{-1} nitric acid. In addition, the amount of acid in the organic phase and density change after pre-equilibration have been measured. The solubility of CyMe_{4}-BTBP increases with an increased temperature in all systems, especially after acid pre-equilibration. This increased CyMe_{4}-BTBP solubility after pre-equilibration could be explained by acid dissolution into the solvent. Comparing the Delta H^{0} and Delta S^{0} calculated using Delta C_{p} with the same parameters derived from a linear fit indicates temperature independence of all three thermodynamic systems. The change in enthalpy is positive in all solutions.
Highlights
Nuclear power plants, just like other sources of energy, produce significant amounts of secondary waste [1,2,3]
Experiments show that the overall solubility of CyMe4-BTBP in FS-13, cyclohexanone and 1-octanol-based organic phases containing 30%vol tri-butyl phosphate (TBP) increase drastically in all solvents when the organic phases are pre-equilibrated with 4 mol⋅L−1 HNO3 compared to the pure solvent, see Figs. 3, 4 and 5
It is known that HNO3 is extracted to a larger degree than HCl [51, 52] while having a similar acid dissociation; single samples with 70%vol FS-13 and 30%vol TBP where the 4 mol⋅L−1 nitric acid was exchanged with 4 mol⋅L−1 hydrochloric acid, Milli-Q water and 4 mol⋅ L−1 NaCl dissolved in Milli-Q water, to rule out the effect of chloride, were prepared to obtain an indication of whether the increased solubility is a result of nitric acid extraction as non-dissociated HNO3, protonation of the organic phase or another phenomenon
Summary
Just like other sources of energy, produce significant amounts of secondary waste [1,2,3]. The most important form of secondary waste from nuclear energy is the used nuclear fuel [1]. This used fuel is highly radiotoxic and has to be isolated from mankind and the environment for a long time [4]. Solvent extraction processes using bis-triazine-bipyridine (BTBP)-type ligand have, shown that sufficient separation can be obtained [12,13,14]. BTBP-type ligands have been suggested for actinide/lanthanide separation in many different types of solvent extraction processes [15,16,17], one of these being the grouped actinide extraction (GANEX) process [18], originally developed in France [19, 20]. Thermodynamic data has been retrieved in order to explain the solvent behavior
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