The oxidation of U(IV) by nitric acid in 30% tri-butyl phosphate solutions

The oxidation of U(IV) by nitric acid in 30% tri-butyl phosphate has been shown to be a combination of an induction period in which oxidation by HNO3 occurs producing HNO2 and an autocatalytic reaction in which U(IV) is oxidised by HNO2. The rate of the autocatalytic reaction is reduced at increased [HNO3] which is perhaps difficult to explain without considering U(IV) hydrolysis. U(IV) hydrolysis increases as acidity decreases and it has been shown that the observed rate constant of the autocatalytic reaction can be reproduced by calculations which account for U(IV) hydrolysis equilibria. The reaction mechanism of the autocatalytic U(IV) oxidation reaction (Eq. (6)) is therefore via the second hydrolysis product (U(OH)(2)(NO3)(2)). The non-catalytic reaction (Eq. (5)) is more difficult to understand. It is possible that the reaction mechanism also involves the second hydrolysis product interacting with nitronium ions (NO2+). Dissolved oxygen does not appear to affect the oxidation reaction although sparging with gas does by the removal of nitrogen oxide (NO) from the solution and the consequent interruption of the catalytic cycle (Eq. (5-7)). A rate equation and activation energies have been derived and for the first time reproducible and predictable data on U(IV) oxidation in TBP have been obtained.