In 30% TBP/OK Np(V) is unstable and disproportionates to Np(IV) and Np(VI). Np(V) readily coordinates to Np(IV) in solution to form a “cation–cation” complex by bonding through an axial oxo group on Np(V). The rate of disproportionation in 30% TBP/OK is >500 times that in aqueous solution.
The kinetics of the reduction of NpO22+ to NpO2+ in nitric acid aqueous solution by formohydroxamic acid (FHA) were studied to allow more accurate flow sheet modelling of neptunium separation from uranium and plutonium in an Advanced PUREX process. The rate of the reaction was monitored using stopped-flow spectrophotometry with near-infrared detection. The conditional reduction rate in 2 M nitric acid at 22°C could be described by the following equation:
-d [NpO22+]/dt = k [NpO22+][FHA](M/s)
with k = 1.17×103 M-1 s-1 at [H+] = 2.0 M. Comparison with other data available in the literature, indicates that formohydroxamic acid is an unusually strong reducing agent for NpO22+.
Hydroxamic acids are promising complexant based alternatives to the reductant (U4+ or Fe2+) based selective stripping of Np (and Pu) from a uranium loaded 30% TBP/OK solvent during the reprocessing of irradiated nuclear fuels. Acetohydroxamic acid (AHA) has the benefits of being both a reductant and complexant that efficiently strips Np (and Pu) from solvent phase without adding salt wastes to actinide separation processes. To model these processes, an understanding of Np-hydroxamate chemistry in aqueous and organic solvent is necessary. Three aspects of this system are discussed.
where, k2a=0.405±0.055 M-1 min-1 at 55 °C (β3≈0.08; β4≈0.007) and the activation energy was E=112±17 kJ mol-1. The reaction mechanism appeared to involve interaction with the 1st hydrolysis product of U(IV)–UOH3+. The data is compared with a previous study of the nitric acid oxidation of U(IV) in 30% TBP. This reaction is autocatalytic due to the formation of nitrous acid during the reaction. The kinetics of the decomposition of HNO2 in 30% TBP (in the absence of U(IV)) have also been reported.
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 oxidation of hydroxylamine by nitric acid in the presence of technetium ions at temperatures above ~60°C is an autocatalytic process comprising an induction period and then a catalysed reaction involving HNO2, which has accumulated in the solution. Tc ions have no appreciable effect on the reaction rate, which is governed only by the nitric and nitrous acid oxidation reactions of hydroxylamine, but the presence of Tc ions does extend the initial induction period. The rate of hydroxylamine oxidation by HNO3 in the presence of HNO2, that is, after the induction period, was found to be:
-d[NH3OH+]/dt = k[NH3OH+][HNO2][HNO3]3.5
where k = 120±10 l4.5 mol-4.5 min-1 at T = 80°C, μ = 2 and [H+] ≤ 2 M. Under these conditions, the reaction apparently has a high activation energy of 160-180 kJ mol-1. At low temperatures (20-40°C) hydroxylamine is effectively stable in solutions of HNO3 up to concentrations of ~2 M, whether or not Tc(VII) ions are present. Tc(V) was also observed to form at least one complex on reduction with excess hydroxylamine with an absorption maximum between 467 and 480 nm dependent on the solution acidity.
Simple hydroxamic acids such as formo- and aceto-hydroxamic acids have been proposed as suitable reagents for the separation of either Pu and/or Np from U in modified or single cycle Purex based solvent extraction processes designed to meet the emerging requirements of advanced fuel cycles. The stability of these hydroxamic acids is dominated by their decomposition through acid hydrolysis. Kinetic studies of the acid hydrolysis of formo- and aceto-hydroxamic acids are reported in the absence and the presence of Pu(IV) ions. The slow reduction of these plutonium(IV) hydroxamate complexes to Pu(III) aquo-ions has been characterised by spectrophotometry and cyclic voltammetry. The reductions of Pu(IV) in the presence of FHA and AHA are consistent with a mechanism in which free hydroxamic acid in solution is hydrolysed whilst Pu(IV) ions remain fully complexed to hydroxamate ligands; then at some point close to a 1 : 1 Pu(IV) : XHA ratio, some free Pu4+ is released from the complex and reduction is initiated. Electrochemical and kinetic data suggest that the reductant is the hydroxamic acid rather than the hydroxylamine.
The oxidation of U(IV) ions in the diluted solvent phase, 30% TBP/n-dodecane, has been investigated in the presence of plutonium ions, which can act as catalysts for U(IV) oxidation. The reaction was shown to follow the cycle below, with the first and third stages being rate determining.
U4+ + 2Pu4+ + 2H2O → UO22+ + 2Pu3++4H+
2Pu3+ + HNO3 + 2H+ → 2Pu4+ + HNO2 + H2O
Pu3+ + HNO2 + H+ → Pu4+ + NO + H2O
2NO + HNO3 + H2O ⇔ 3HNO2
The overall reaction stoichiometry is the same as for the oxidation of U(IV) by HNO3 in TBP:
The rate equations of both these rate limiting steps have been determined, with that for the U(IV)-Pu(IV) reaction (5) being given by the equation below, where k1=74.4±6 M-1.2 min-1 at 25.2 °C and the activation energy is 72±11 kJ mol-1 (in 0.5 M HNO3).
The rate of the second slow stage, the Pu(III)-HNO2 reaction, is given by the equation below, where the rate constant is k2=627±28 M-1 min-1 at 25.2 °C and the activation energy is 87.2±1.4 kJmol-1 (in 0.5 M HNO3).
Mechanistically, it was shown that the U(IV)-Pu(IV) reaction may proceed via the interaction of the hydrolysed actinide ions U(OH)22+ and PuOH3+ and the Pu(III)-HNO2 reaction was found to most probably involve oxidation of Pu(III) ions by nitrinium nitrate (NONO3) ions in its rate determining step.
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.
The careful control of actinide distribution between 30% tributylphosphate (TBP) in organic diluent and nitric acid is vital to the successful operation of nuclear fuel reprocessing plants. Uranium (VI) and tetravalent actinides are extracted into the organic phase as the nitrate complexes UO2(NO3)2(TBP)2 and An(NO3)4(TBP)2 respectively. The presence of dibutylphosphate (HDBP), a tributylphosphate decomposition product, affects this distribution due to the formation of strong, organic soluble, complexes between the phosphate and the actinide ions. This paper describes the investigation of U(VI), U(IV), Np(IV) and Pu(IV) complexation with HDBP in 30% TBP/organic diluent solutions. The distribution of U(VI) between nitric acid and 30% TBP/organic diluent solutions containing HDBP and subsequent analysis of the organic phases by 31P nuclear magnetic resonance spectroscopy, absorption spectroscopy and extended X-ray absorption spectroscopy indicates that U(VI) can form a range of complexes with HDBP. At comparatively high HNO3 loading in the organic phase, HDBP can displace TBP groups to form either UO2(NO3)2(HDBP)(TBP) or UO2(NO3)2(HDBP)2. At lower HNO3 loadings HDBP can also deprotonate and act as a chelate ligand displacing nitrates to form either UO2(DBP)2 (HDBP)x or UO2(NO3)(DBP)(HDBP)x (where x = 1 or 2). Distribution data and absorption spectroscopy also indicates that Np(IV) forms at least two complexes in which nitrate groups are displaced by the DBP- anion. In contrast, for U(IV), it is almost certain that TBP groups are displaced by HDBP at increased HDBP loading forming U(NO3)4(HDBP) (TBP). There is no evidence for the displacement of nitrates. Finally, Pu(IV) distribution data suggested that complex reactions were taking place in both phases ensuring that equilibrium extraction was difficult to ascertain. Nevertheless, HDBP does readily complex with Pu(IV) increasing extraction and displacing nitrates from the Pu(IV) species formed in the organic phase.