Luminescence of uranium(VI) after liquid-liquid extraction from HCl by Aliquat^® 336 in n-dodecane:1-decanol by time-resolved laser-induced luminescence spectroscopy

Abstract

To investigate the extraction of uranium(VI) in HCl media by Aliquat® 336 in 1:99 (v:v) 1-decanol:n-dodecane mixture, our objective is to identify the complexe(s) in the organic phase by time-resolved laser-induced luminescence spectroscopy (TRLS). The extraction mechanism is supposed to involve the formation of $[U{O_2}Cl_4^{2 - } \cdot {({R_4}{N^ + })_2}]$ in the organic phase. The occurrence of such a species leads to the presence of the ${\rm{U}}{{\rm{O}}_2}{\rm{Cl}}_4^{2 - }$ species in the organic solution, which luminescence shows particular features. The luminescence spectra and decay time evolutions are obtained in the organic phase as a function of HCl concentration in the aqueous phase (0.5–6 M). The extraction of ${\rm{U}}{{\rm{O}}_2}{\rm{Cl}}_4^{2 - }$ is confirmed by the particular spectrum of uranium(VI) in the organic phase, and the typical splitting of the luminescence bands, due to the crystal field effect, is clearly evidenced. The stoichiometry is verified using luminescence intensity variation as a function of the activity of Cl−, and extraction constants are calculated both using the specific interaction theory and Pitzer model. A decomposition of the spectrum of the extracted complex in the organic phase is also proposed. The decay time variation as a function of temperature allows estimating the activation energy of the luminescence process of the extracted complex.

Investigation of the Luminescence of [UO2X4]2– (X = Cl, Br) Complexes in the Organic Phase Using Time-Resolved Laser-Induced Fluorescence Spectroscopy and Quantum Chemical Simulations

“…A monoexponential decay with a fluorescence lifetime of 0.3 μs was measured for the uranium­(VI) sample in the presence of chloride ions, Aliquate 336, and n -dodecane. This confirms the formation of a unique complex, which was assumed to be [A336] 2 [UO 2 Cl 4 ] with four chloride ions coordinated to uranyl in its equatorial plane, in line with the extracted complex stoichiometry …”

“…The [UO 2 Cl 4 ] 2– dianion in interaction with the organic ligand has been prepared under the conditions described in the Experimental Section. The final compound is predicted to be the uranyl tetrachloride associated with two extractant cations of methyltrioctylammonium [A336] 2 [UO 2 Cl 4 ] as a result of an ion-exchange mechanism . Interestingly, Görller-Warland et al stabilized the [UO 2 Cl 4 ] 2– dianion in solution with tetrabutylammonium chloride (Bu 4 NCl), a ligand belonging to the same group of tetraalkylammonium salts as Aliquate 336.…”

“…The decomposition of the [A336] 2 [UO 2 Cl 4 ] luminescence spectra has been performed by Reiller et al [10]. The resolution of the presently used apparatus did not allow to resolve well some of the transitions, and revealed only the wide components which cover several vibronic progressions in one peak.…”

“…Uranyl tetrachloride complexes have been thoroughly studied as a reference cluster because direct comparison with luminescence data from crystals is possible [6]. Moreover, this complex can also be stabilized in non-aqueous solvents [7][8][9], and is of interest in some solvent extraction protocols [10,11]. The structure of the uranyl tetrachloride is well-established with four chloride ions in the equatorial plane of the UO 2 2+ moiety, resulting in a D 4h symmetry.…”

“…Uranyl tetrachloride complexes have been thoroughly studied as a reference system because direct comparisons with luminescence data from crystals are possible . Moreover, this complex can also be stabilized in nonaqueous solvents − and is of interest in some solvent extraction protocols. , The structure of the uranyl tetrachloride is well-established, with four chloride ions in the equatorial plane of the UO 2 2+ moiety, resulting in a D 4 h symmetry. The effects of second-sphere countercations, solvent molecules, or solvating agents also have to be considered, in order to quantify their influence on the structures, the electronic state energies, and the vibronic progressions.…”