Behaviour of UO22+ in high acid solutions (12 M): characterization and complexation with HPBI

The complexation of U(VI) with a synthetic extractant of actinides was studied. A preliminary work consisted in studying the behaviour of UO22+ in highly concentrated nitric acid solutions, which are the industrial media of extraction of the actinides from nuclear waste. The characterization of uranyl ion was performed through spectrophotometry and time-resolved laser induced spectrofluorometry. The intensities of absorption and emission spectra, and lifetimes of the uranyl ion were measured up to highly concentrated solutions of perchloric and nitric acid (12 M). The influence of NO3- complexation is discussed. Secondly, the complexation of UO22+ with the extractant was studied. The extractant is a β-diketone having a heterocyclic ring (isoxazole) as a substituent which increases the effectiveness of the separation of metals and actinides. Therefore the complexation of UO22+ with HPBI (3-phenyl-4-benzoyl-5-isoxazolone) has been investigated. A series of experiments was carried out on solutions of HPBI and UO22+ in perchloric acid in order to avoid any other complexation, and in nitric acid up to 12 M in order to simulate the industrial conditions which may occur in the waste reprocessing. The previously proposed model involving excited species of the uranyl ion (1), already checked up to 6 M, is extended up to 12 M. In addition, the effect of ionic strength is accounted for, using dedicated models of the literature. The determination of the constants of complexation includes ionic strength calculations