Kinetics and Mechanism of Np(IV) Oxidation with Nitric Acid

Abstract

Neptunium (IV) is oxidized to Np(V) with nitric acid in the presence of U(VI) under conditions of low acidity (<∼0.1 M). The reaction rate is described by the equation d[Np(V)]/dt = k1[Np(IV)]/[H+]2 + k2[Np(IV)]2[U(VI)]/[H+]3, in which k1 = (2.0±0.3) × 10−5 mol2 l−2 min−1 and k2 = (5.50±0.47) × 10−2 mol l−1 min−1 at 50°C and solution ionic strength μ = 0.5. The activation energies of the two pathways are 148±31 and 122±12 kJ mol−1. The reaction along the main pathway (with the rate constant k2) is limited by disproportionation of Np(IV) involving NpOH3+ and Np(OH)2UO 2 4+ complex ions.