Characterization of a Tri-n-butyl Phosphate−Nitric Acid Complex:  a CO₂-Soluble Extractant for Dissolution of Uranium Dioxide

Abstract

Tri-n-butyl phosphate (TBP) reacts with nitric acid to form a hydrogen-bonded complex TBP-(HNO 3 ) x (H 2 O) y that is highly soluble in supercritical fluid carbon dioxide (SF-CO 2 ). The value of x can be up to 2.5, whereas the value of y varies between 0.4 and 0.8 determined by acid-base and Karl Fischer titrations. The protons of HNO 3 and H 2 O in the complex undergo rapid exchange and exhibit a singlet resonance peak in NMR spectra. When the complex is dissolved in a low dielectric constant solvent, small droplets of nitric acid are formed that can be detected by NMR. Phase behavior studies indicate that the complex forms a single phase with SF-CO 2 above a certain pressure for a given temperature. This CO 2 -soluble Lewis acid-base complex provides a method of introducing nitric acid in SF-CO 2 for effective dissolution of uranium dioxide, lanthanide oxides, and perhaps other metal oxides.

Stoichiometric Relation for Extraction of Uranium from UO2 Powder using TBP Complex with HNO3 and H2O in Supereritical CO2

“…If UO 2 is dissolved by HNO 3 contained in the water cores in the complex, the acidity of the equilibrated aqueous phase of the complex would affect on the dissolution mechanism. Previous research 11) clarified that the acidity of the equilibrated aqueous phase of this complex was greater than 12 M, i.e. the acidity in the waster cores was grater than 12 M. Even if the HNO 3 concentration in the complex is only 4.9 M, the dissolution may follow Eq.…”

Ionic Liquid and Supercritical Fluid Hyphenated Techniques for Dissolution and Separation of Lanthanides, Actinides, and Fission Products

“…An ionic liquid-supercritical CO 2 hyphenated dissolution, extraction and separation process may provide an environmentally sustainable method for nuclear waste management. 2 and Nd(NO 3 ) 3 (TBP) 3 , uranium could be extracted by sc-CO 2 without co-extraction of neodymium. The fraction of uranium extracted by sc-CO 2 depends on the relative amount of the two metals and the amount of TBDGA added to the IL phase. )…”

“…The dissolution of uranium oxides in sc-CO 2 with a CO 2 -soluble TBP-nitric acid complex of the formula TBP(HNO 3 ) 1.8 (H 2 O) 0.6 is of particular interest to nuclear waste treatment. 2 This reagent, basically a Lewis acid-Lewis base complex, is easily prepared in the lab by shaking TBP with an equal volume of the concentrated nitric acid (15.5 M). In the mixing process, HNO 3 and a small amount of H 2 O molecules will migrate to the organic phase via hydrogen bonding with the P=O group of TBP leaving the majority of the water behind in the aqueous phase.…”

A study on selective precipitation ability of cyclic urea to U(VI) for developing reprocessing system based on precipitation method

“…These matters result in complexity of processes and an increase in products of radioactive wastes. In order to overcome such disadvantages, some innovative reprocessing methods for spent nuclear fuels have been developed, e.g., Fluorex [3,4], NEXT [5], Super-Direx [6], Urexþ [7], and pyrochemical methods [8].…”