Electrochemical properties and thermodynamic stability of Pu and neighbouring actinides in the (IV) and (V) oxidation states in aqueous alkaline media and radwastes

In alkaline media Pu(V) is reduced to Pu(IV) and Pu(III) at a mercury electrode at -0.72 and -1.1V/S.H.E. respectively, while Am(V) is reduced to hydrolysed Am(IV) at a Pt cathode at +0.2 V. The apparent redox potential Am(V)/Am(IV) is consistent with earlier published data, while that of Pu(V)/Pu(IV) = -0.72 V differs significantly from the published data between +0.4 and +0.9 V. Our data are consistent with the reported high stability of Pu(V) in alkaline media. It is concluded that Pu(V) cannot disproportionate in these conditions. Electrochemical reduction of U(V) and Np(V) is not observed in 1 to 10 M NaOH solutions at significantly negative potentials. Hence the estimations of (V)/(IV) potentials for U and Np in 1 M NaOH should also be corrected. A similar study has been undertaken at a pH closer to neutrality and the stability of pentavalent actinides at this pH is also discussed. (orig.)