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

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+ + 2Pu(4+) +2H(2)O -> UO22+ + 2Pu(3+) + 4H(+) 2Pu(3+) + HNO3 + 2H(+) -> 2Pu(4+) + HNO2 + H2O Pu3+ + HNO2 + H+ -> Pu4+ + NO + H2O 2NO + HNO3 + H2O reversible arrow 3HNO(2) The overall reaction stoichiometry is the same as for the oxidation of U(IV) by HNO3 in TBP: U(NO3)(4) center dot 2TBP + HNO3 center dot TBP + H2O center dot TBP + TBP double left right arrow UO2(NO3)(2) center dot 2TBP + HNO3 center dot TBP + 2HNO(3) center dot 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 k(1) = 74.4+/- 6 M-1.2 min(-1) at 25.2 degrees C and the activation energy is 72 +/- 11kJ mol(-1) (in 0.5 M HNO3) d[U(IV)]/dt = k(1) [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 k(2) = 627 +/- 28 M-1 min(-1) at 25.2 degrees C and the activation energy is 87.2 +/- 1.4 kJ mol-1 (in 0.5 M HNO3). -d[Pu(III)]/dt = k(2) [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)(2)(2+) 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.