A kientic isotope effect in Pu(V) disproportionation in nitric acid solutions have been observed in a heterogeneous system with phenyloctylphosphonic acid deposited on hydrophobized silicagel. The isotope separation coefficient for242Pu/239Pu isotopes was found to be 1.022 at 25°C. The rate constant of Pu(V) disproportionation in this heterogeneous system was determined to be 5.0±0.6 12mol–2s–1 at 25°C, [HNO3]= 0.87–1.5 mol l–1 and 11 phase ratio (10% extractant on the support).
A kinetic isotope effect in Pu(V) disproportionation has been observed in nitric acid solution under the effect of power ultrasound with intensity 0.9W·cm–2 and frequency 22 kHz. The isotope separation coefficient for242Pu/239Pu isotopes was found to be 1.0081 at 20°C. Without sonication the k.i.e. was not observed. The rate constant of Pu(V) disproportionation was found to be accelerated under sonication. The rate constant determined was (5.7±0.6)·10–3 12·mol–2·s–1 atl=0.9 W·cm–2,v=22 kHz, [HNO3]=0.5 mol·l–1 andT=20°C. It is supposed that the acceleration of Pu(V) disproportionation and the kinetic isotope effect are due to the activation of plutonoyl groups in the interface between the cavitation bubble and the bulk solvent.
A kinetic study of thermal and photoaccelerated U(IV)-U(VI) isotope exchange has been carried out in oxalate solutions at 11–40°C. The rate and quantum yield were determined as a function of U(IV), U(VI) and oxalate concentration, wavelength of incident light, temperature and absorbed dose of -radiation. The kinetic equations for thermal and photoaccelerated exchange have been obtained. It was assumed that the mechanism of exchange involves formation of U(V) as an intermediate, followed by slow exchange between U(V) and U(IV). The isokinetic dependence confirms the identity of limiting stages for thermal and photostimulated exchange. The upper component of photoexcited T1 level of uranyl is supposed to be the most reactive in the process of U(V) generation. It was observed that the small doses of -radiation evoke the acceleration of isotope exchange, however, at D>100 krad the rate of exchange is reduced to the level of thermal exchange.
Authors:H. Abdunnabi, S. Nikitenko, and T. Afonasieva
The kinetics of oxidation of uranium(IV) monofluoride complex by nitrous acid in nitric acid solution have been studied. The experiments were carried out at constant ionic strength of 2M (HNO3 and NaNO3) and temperature in the range of 18–47 °C. The rate of reaction was determined spectrophotometrically at a wavelength of 621 nm, at which the molar extinction coefficients of UF3+ and UF
are the same. It was shown that reaction orders for [HNO2] and [HNO3] are equal to 0.12 and 0.39, respectively. The values of activation parameters H and S are determined to be 83 kJ mol–1 and 75 J (mol·K)–1, respectively. The rate order of the reaction studied has a weak direct dependence on [H+] in contrary to the strong and reverse dependence in the absence of fluoride ions. In conclusion, fluoride ions may strongly stabilize the U(IV) in nitric acid solutions.