Nitrous acid-driven reduction of vanadium as a neptunium analogue

Abstract

Neptunium has been previously shown to present challenges within a used nuclear fuel reprocessing scheme due to its tendency to exist in the (IV), (V), and (VI) oxidation states simultaneously. In order to control this neptunium speciation, and informed by relevant work in the literature, we are currently engaged in a study of nitric/nitrous acid redox chemistry with Np(V) and Np(VI). To minimize radiological exposure risks, we are also exploring the validity of using vanadium as an analogue for the study of the kinetics of the Np(VI)/Np(V) reduction by nitrous acid. The kinetics of the reduction of vanadium(V) by nitrous acid in solutions of nitric acid was investigated spectrophotometrically by the method of initial rates. Orders of reaction with respect to V(V), and HNO2 were found to be 0.90, and 1.24 respectively, in reasonable agreement with the analogous reaction orders for the reduction of Np(VI) by nitrous acid previously reported by Precek and Paulenova – suggesting that, for this particular reduction, V(V) can serve as a good kinetic analogue for Np(VI). The value of the rate constant k for the rate law -d[V(V)]/dt = k[V(V)][HNO2]/[H] was found to be 7.5 × 10 M s at 20 °C, two orders of magnitude smaller than that for the reduction of Np(VI) by HNO2, a difference that is attributable to the loss of one of the oxygens during the VO2 to VO reduction reaction.