Kinetics of the oxidation of Pu(IV) by manganese dioxide

The kinetics of the oxidation of plutonium(IV) by manganese dioxide were studied in 1.0 M NaCl over the pH range from 2.5 to 8.2 with variable concentrations of manganese dioxide from 0.01 m(2)/L to 4.97 m(2)/L at a constant temperature of 19+/-2 degreesC. The concentration of plutonium in solution was 6.0(+/-1.0) x 10(-10) M. Fractions of Pu(IV), (V) and (VI) as a function of time were determined by removal of plutonium from the solid phase followed by an ultrafiltration/solvent extraction procedure using TTA and HDEHP as extractants. Appropriate removal conditions were established for Pu(IV), Pu(V) and Pu(VI) using Th(IV), Np(V) and U(VI) as oxidation state analogs. In the pH range from 2.0 to 3.5, the oxidation of Pu(IV) by manganese dioxide was first order with respect to the concentration of manganese dioxide and -0.21 with respect to the hydrogen ion concentration. Consequently assuming a first order dependence with respect to the concentration of Pu(IV), the oxidation reaction can be described by the following rate equation: -d[Pu(IV)]/dt = k (.) [Pu(IV)] (.) [MnO2(v)] (.) [H+](-0.21) with k = 3.72(+/-0.13) x 10(-3) (m(-2) L)(mol(-1) L)(-0.21)(min)(-1). Using the kinetic data determined in this study, the influence of manganese dioxide on the oxidation state distribution of plutonium under various environmental conditions as well as in waste solutions can be estimated.