Sonochemical oxidation of Np(V) in aqueous nitric acid medium and sonochemical extraction of Np in two-phase TBP (30 vol.%)-n-dodecane/HNO3/H2O system

The kinetics of Np(V) oxidation was studied in nitric acid solutions in the presence of argon under the effect of power ultrasound (U.S.) with a frequ ency of 20 kHz and intensity of 1 W.cm(-2). Np(V) sonochemical oxidation in 3-4 mol/L HNO3 solutions was only observed for short-time sonication. Long -time sonication reduces the Np(VI) formed back to Np(V). The use of "anti- nitrous" reagent, such as urea, which exerts a buffering effect related to nitrous acid concentration, helps to stabilise Np(VI) formed under sonicati on. Ultrasonic treatment of HNO3 solutions does not lead to complete Np(V) oxidation. It is shown that the Np(VI)/Np(V) equilibrium ratio observed for the sonochemical process can be expressed by the same model as the one use d for Np(V) oxidation in the absence of ultrasound. It was concluded that t he mechanism of Np(V) sonochemical oxidation is related to HNO2 sonochemica l formation due to HNO3 sonolysis, followed by Np(V) oxidation with HNO3, c atalysed by HNO2. The effect of power U.S. on Np(V) oxidation was also studied in the two-pha se solvent extraction system made of tri-n-butylphosphate (30 vol.%)/n-dode cane/HNO3/H2O for the following conditions: U.S. frequency: 20 kHz, I = 1.8 to 3.7 W.cm(-2), T = (32+/-2)degrees C, Ar atmosphere, [NHO3](aq) = 2.8 to 5.2 mol/L. It was shown that sonication of the two-phase mixture causes in tense emulsification and Np(V) oxidation to Np(VI), which is mostly extract ed into the organic phase. Np was found extracted up to 92% in a single ste p. For long-time sonication, neptunium concentration in the organic phase d ecreases due to Np(VI) reduction. The presence of TBP organic phase is suff icient here to buffer the aqueous nitrous acid concentration, so that no "a nti-nitrous" reagent is required. As for single phase experiments, Np(V) so nochemical mechanism of oxidation leading to Np(VI) extraction into the org anic phase is related to Np(V) oxidation in water by nitric acid, the react ion being catalysed by the nitrous acid formed during nitric acid sonolysis . Nitrogen dioxide, formed as a result of nitrous acid sonochemical decompo sition, can also contribute to Np(V) oxidation. For long-time sonication, N p(VI) is reduced to Np(V) in the aqueous phase by the excess nitrous acid f ormed and, consequently, the neptunium is stripped from the organic phase.