A Critical Evaluation of the Redox Properties of Uranium, Neptunium and Plutonium Ions in Acidic Aqueous Solutions

Abstract

Abstract Standard redox potentials, E 0 s, and redox processes of U, Np and Pu ions in acidic aqueous solutions are reviewed and evaluated critically. The E 0 s of reversible redox processes, MO 2 2+ /MO 2 + and M 4+ /M 3+ (M: U, Np or Pu) adopted are those proposed mainly by Riglet et al. on the basis of the precise correction of formal potentials, E 0' s, according to the improved theoretical approach to estimate the activity coefficient. Electrode processes of the U, Np and Pu ions are discussed in terms of current-potential curves, measured so far by polarography, voltammetry or flow coulometry. Special attention is payed to the irreversible MO 2 + /M 4+ reactions. Disproportionation reactions of MO 2 + are also discussed. New substances are introduced as intermediates during reductions of MO 2 + to M 4+ or disproportionations of MO 2 + . CONTENTS Introduction Standard redox potentials for uranium, neptunium and plutonium ions in acidic aqueous solutions 2.1 Evaluation of E 0 from E 0' determined by electrochemical measurements 2.2 Temperature dependence of E 0 Redox reactions of uranium, neptunium and plutonium in acidic aqueous solutions investigated by polarography or voltammetry 3.1 Uranium 3.2 Neptunium 3.3 Plutonium 3.4 Disproportionation of NpO 2 + , PuO 2 + , Np 4+ and Pu 4+ 3.5 Reduction of MO 2 + and reduction intermediates Redox reactions of uranium, neptunium and plutonium in acidic aqueous solutions investigated by flow coulometry 4.1 Electrode processes of the uranium, neptunium and plutonium ions investigated by flow coulometry at the column electrode at the column electrode 4.2 Disproportionation of MO 2 + during the electrolysis by flow coulometry 4.3 Reduction mechanisms of MO 2 + (M = Np or Pu) and reduction intermediates investigated by flow coulometry Conclusions List of abbreviations Appendix References