Reduction of Np(VI) to Np(V) with butanal oxime in the presence of excess reductant is presumably described by the equation 4NpO 2 2+ + 2C 3 H 7 CHNOH + H 2 O = 4NpO 2 + + 2C 3 H 7 CHO + N 2 O + 4H + , and the reaction rate, by the equation -d[Np(VI)]/d t = k [Np(VI)][C 3 H 7 CHNOH]/[H + ], with k = 230±15 min -1 at 25°C and the ionic strength of the solution μ = 2. This equation holds for solutions with different values of the ionic strength and HNO 3 concentration. The activation energy is 69.4±12.4 kJ mol -1 .

The rate equations for the reduction of Np(V), Np(VI) and Pu(IV) ions by acetaldoxime in nitric acid have been determined as:-d[Np(V)]/dt = k26[Np(V)][CH3CHNOH][HNO3]1.2- d[Np(VI)/dt = k19[Np(VI)][CH3CHNOH]/[HNO3]- d[Pu(IV)/dt = k6[Pu(IV)]2[CH3CHNOH]1.1/[Pu(III)][HNO3]2.2where respectively: k26= (9.2±0.2)×10-4l2.2mol-2.2min-1at 60° C; k19= 254±10 min-1at 26.0° C; k6= 25.3±1.9 mol1.1l-1.1min-1at 19.5°C. The activation energies were found to be: E19= 62.6±2.6 kJmol-1and 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 kinetics of the reduction of neptunium (VI) and plutonium (IV) ions in nitric acid solution by a new rapid salt free reductant, N,N-ethyl (hydroxyethyl) hydroxylamine, have been studied and rate equations determined. Under equivalent conditions, both Np(VI) and Pu(IV) are reduced faster than by the related reagent, N,N-diethyl hydroxylamine, and it is suggested that this is due to the introduction of the hydroxy group into the reductant molecule. Possible reaction mechanisms have been suggested to account for the observed reaction stoichiometry.

A variety of salt free organic reducing agents (carboxylic acids, aldehydes, ketones, guanidines and substituted hydrazines) have been screened for their reactivity with Np(VI) and Pu(IV) ions. The objective was to find reductants that have fast kinetics for Np(VI) reduction but which also showed a high selectivity for Np(VI) reduction over Pu(IV). Substituted hydrazines, particularly 1,1-dimethyl hydrazine and tert-butyl hydrazine, were shown to be the most promising of the screened reductants. The kinetics of Np(VI) and Pu(TV) reduction reactions with 1,1-dimethyl hydrazine and tert-butyl hydrazine were studied in greater detail and demonstration solvent extraction experiments were performed. The reduction of Pu(TV) was shown to be enhanced in the presence of U(VI) and this made 1,1-dimethyl hydrazine unsuitable as a selective reductant. Tert-butyl hydrazine may however show some promise as a selective reductant for Np(VI) under carefully controlled conditions.

Distribution ratios of Np(IV) and Np(VI) between 30 vol.% TBP/n-dodecane and 1 M nitric acid were measured with varying concentrations of acetohydroxamic acid (AHA). Distribution experiments were undertaken by contacting a 1:1 (v/v) of 30% TBP/n-dodecane with a simulated solution, which contained a variety of elements such as Ce, Nd, Mo, Zr, Fe, Sr, Ni, Pd and Ru. Distribution ratio values of Np(IV) and Np(VI) in a 1.0 M nitric acid concentration were about DNp(IV) ≅ 1.0 and DNp(VI) ≅ 6.4, respectively. But, the addition of AHA brought about a remarkable reduction in the Np extraction with the AHA concentration.