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V. S. Koltunov, G.I. Zhuravleva, V. I. Marchenko, K. N. Dvoeglazov, O. A. Savilova, G. V. Koltunov, Robin J. Taylor, M. Bankhead

Kinetics and mechanism of the plutonium catalysed nitric acid oxidation of U(IV) ions in 30% tributyl phosphate solution

Keywords: U(IV), Pu(IV), Nitric acid, Tributyl phosphate, Redox

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+H₂⇔3HNO₂

The overall reaction stoichiometry is the same as for the oxidation of
U(IV) by HNO₃ in TBP:

U(NO₃)₄·2TBP+HNO₃·TBP+H₂O·TBP+TBP⇔UO₂(NO₃)₂·2TBP+HNO₂·TBP+2HNO₃·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).

(d [U(IV)])/(d t)=k₁ ([U(IV)] [Pu(IV)]
[H₂O]³)/([HNO₃]^2.8)

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).

-(d [Pu(III)])/(dt) =k₂ ([Pu(III)]
[HNO₂] [HNO₃])/([H₂O])

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.

Radiochimica Acta, Oldenbourg Wissenschaftsverlag

Print ISSN: 0033-8230
Volume: 95, 10/2007
Pages: 559 - 567

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