The kinetics and mechanism of the reduction of neptunium (VI) ions by uranium (IV) ions in nitric acid

Abstract

The rapid reduction of NpO 2 2+ ions to NpO 2 + by U(IV) ions in an aqueous nitric acid solution has been studied and, in many ways, is similar in character to the same reaction in HClO 4 . The major difference is that in HNO 3 the reaction proceeds via two parallel routes. The first is via the hydrolysed UOH 3+ ion, as in the reaction in HClO 4 and the second is via the non-hydrolysed U 4+ ion. These parallel routes lead to the observed order of reaction with respect to H + ions being reduced from −1 in HClO 4 to −0.7 in HNO 3 . After accounting for the reaction mechanism the rate equation is described by:where k 8 = 404 min −1 , k 19 = 275 M −1 min −1 and β = 0.009 M at 10 • C and µ = 2. The activation energy was 66.5 ± 4.9 kJ mol −1 . Nitrate ions had no effect on the reaction rate but sulphamic acid increased the observed rate, probably through catalysis by sulphate ions arising from sulphamic acid reaction with HNO 2 and hydrolysis.

A Theoretical Study of the Inner-Sphere Disproportionation Reaction Mechanism of the Pentavalent Actinyl Ions

“…Second, the suggested mechanism for both U and Pu disproportionation reactions leads to formation of An(OH) 2 2+ ions as pseudostable products (these also require a further one or two water molecules from the solvent to complete their inner hydration sphere). These ions are frequently cited as probable intermediates in actinide redox reactions . No definite mechanism has been suggested for the Np(V) disproportionation here, but in agreement with experimental data, quantum mechanically it differs significantly from that of its neighboring actinides.…”

Multiscale Modeling of Plutonium Radiation Chemistry in Nitric Acid Solutions. 1. Cobalt-60 Gamma Irradiation of Pu(IV)

“…Despite these implications, few irradiation studies can be found in the literature, and these are essentially limited to the impact of plutonium oxidation states on the yields of radiolysis products accompanied by mechanistic speculation. − Although these data have been useful for the construction of empirical and stochastic models, they are insufficient for the development of a predictive, mechanistic, multiscale model. Multiscale models for radiation effects are important as they capture the fundamental chemistry of an irradiated system from the point of energy deposition, through the lifetime of the nonhomogeneous radiation track (microseconds in water), and into bulk homogeneous solution (steady state).…”

Chemical behavior of neptunium in the presence of technetium in nitric acid media

“…U(IV) can reduce Pu(IV) to Pu(III) and can also reduce Np(VI) coming from co-decontamination stage to Np(V) or Np(IV). Basically, Np(IV) is extractable with tributylphosphate (TBP) and it goes with U(VI) into organic stream, whereas Np(V) is unextractable with TBP and will go with Pu(III) into aqueous stream [2][3][4].…”