where respectively: k26 = (9.2±0.2)×10-4 l2.2mol-2.2min-1 at 60° C; k19 = 254±10 min-1 at 26.0° C; k6 = 25.3±1.9 mol1.1 l-1.1min-1 at 19.5°C. The activation energies were found to be: E19 = 62.6±2.6 kJmol-1 and E26 = 87.7±9.8 kJmol-1. Np(V) was generally found to be stable for long periods in nearly all the kinetic experiments and the reduction of Np(V) to Np(IV) could only be studied at elevated temperatures and reactant concentrations.
Possible reaction mechanisms for the reduction of Np(VI) and Pu(IV) have been suggested; proceeding, in both cases, via the hydrolysis product and an intermediate CH3CHN¤O radical. Simple solvent extraction experiments have shown that Np(VI) and Pu(IV) can be reductively stripped from 30% TBP/n-dodecane in the presence of U(VI).
The rapid reduction of NpO22+ ions to NpO2+ 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 HClO4. The major difference is that in HNO3 the reaction proceeds via two parallel routes. The first is via the hydrolysed UOH3+ ion, as in the reaction in HClO4 and the second is via the non-hydrolysed U4+ ion. These parallel routes lead to the observed order of reaction with respect to H+ ions being reduced from −1 in HClO4 to −0.7 in HNO3. After accounting for the reaction mechanism the rate equation is described by:
where k8 = 404 min−1, k19 = 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 HNO2 and hydrolysis.