Kinetics of reduction of hexavalent neptunium by nitrous acid in solutions of nitric acid

Nitrous acid is a key redox controlling factor, affecting the speciation of neptunium in the reprocessing of used nuclear fuel by solvent extraction. The kinetics of the reduction of neptunium(VI) by nitrous acid in solutions of nitric acid was investigated spectrophotometrically by the method of initial rates. The reaction is of first order with respect to Np(VI) while the order with respect to HNO2 is 1.20 ± 0.04. The reaction rate is almost inversely proportional to the hydrogen ion concentration (reaction order −0.92 ± 0.06), indicating that the reaction proceeds primarily through the reaction of neptunium(VI) with the nitrate anion. The experimental value of the rate constant k for the rate law −d[Np(VI)]/dt = k·[Np(VI)]·[HNO2]1.2/[H+] is of (0.159 ± 0.014) M−0.2 s−1 in I = 4 M and at 20 °C. The activation energy is (−57.3 ± 1.6) kJ/mol, which is in agreement with previous data on this reaction in perchloric acid.