In this study, speciation in aqueous solutions of nitric acid at 25 °C was assessed in two independent ways. First, Raman experiments were carried out and interpreted in terms of free nitrate ions, ion pairs and neutral HNO(3) molecules. In parallel, a model was developed to account for the formation of these two kinds of pairs. It was based on an extension of the binding mean spherical approximation (BiMSA), or associative MSA (AMSA), in which the size and the charge of the ions in the chemical pair may differ from those of the free ions. A simultaneous fit of the osmotic coefficient and of the proportion of free ions (obtained from Raman spectroscopy experiments) led to an estimation of the speciation in nitric acid solutions. The result obtained using this procedure was compared with the estimation obtained from the Raman experiments.
Seven protactinium(V) fluoride compounds have been synthesized, and their crystal structures and Raman spectra are reported. (NH4)2PaF7, K2PaF7, Rb2PaF7, and Cs2PaF7 were found to crystallize in the monoclinic space group P21/c for the ammonium compound and C2/c for the K(+)-, Rb(+)-, and Cs(+)-containing compounds, with nine-coordinate Pa forming infinite chains through fluorine bridges. Na3PaF8 crystallizes in the tetragonal space group I4/mmm with eight-coordinate Pa in tetragonal geometry, while tetramethylammonium fluoroprotactinate shows two different structures: (Me4N)2(H3O)PaF8, an eight-coordinate molecular compound crystallizing in the monoclinic space group C2/c, and (Me4N)PaF6, an eight-coordinate Pa compound forming infinite chains and crystallizing in the orthorhombic space group Pnnm. A comparison of solid- and solution-state Raman data indicates that the PaF8(-) anion could be the predominant Pa(V) complex in concentrated solutions of aqueous HF.
The role of the 5f and 6d orbitals in the chemistry of the actinide elements has been of considerable interest since their discovery and synthesis. Relativistic effects cause the energetics of the 5f and 6d orbitals to change as the actinide series is traversed left to right imparting a rich and complex chemistry. The 5f and 6d atomic states cross in energy at protactinium (Pa), making it a potential intersection between transition metal and actinide chemistries. Herein, we report the synthesis of a Pa-peroxo cluster, A6(Pa4O(O2)6F12) [A = Rb, Cs, (CH3)4N], formed in pursuit of an actinide polyoxometalate. Quantum chemical calculations at the density functional theory level demonstrate equal 5f and 6d orbital participation in the chemistry of Pa and increasing 5f orbital participation for the heavier actinides. Periodic changes in orbital character to the bonding in the early actinides highlights the influence of the 5f orbitals in their reactivity and chemical structure.
Ruthenium extraction by tri-n-butylphosphate (TBP) from nitric acid was studied and modelled in the conditions of the PUREX process. Experimental distribution ratios obtained for water, nitric acid and ruthenium were described with a physicochemical model based on the application of the mass action law on each extraction equilibrium and by taking into account deviations from thermodynamic ideal behaviour both in aqueous and organic phase using Mikulin and Sergievskii-Dannus equations. The best agreement between experimental and calculated ruthenium extraction isotherms was obtained by considering the formation of two complexes and in organic phase. This model was then used to simulate ruthenium behaviour in a counter-current PUREX hot test performed in mixer-settlers. Highlight Extraction of ruthenium from nitric acid media by TBP/TPH was studied at lab-scale A thermodynamic model based on Mikulin equation and Sergievskii-Dannus model was developped Good agreement between experimental and calculated distribution ratios was obtained Mathematical model was tested successfully during a counter-current PUREX hot test performed in mixer-settlers
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