Preparation, stability, and structural characterization of plutonium(VII) in alkaline aqueous solution

Area of Science:

• Solution chemistry of actinides
• Radiochemistry
• X-ray spectroscopy

Background:

• Plutonium speciation in alkaline solutions is crucial for understanding its behavior.
• Previous studies suggested different coordination environments for Pu(VI) and Pu(VII) but lacked direct in situ characterization.
• Ozonolysis is a common method for oxidizing actinides to higher oxidation states.

Purpose of the Study:

• To determine the in situ coordination environment of Pu(VI) and Pu(VII) in alkaline solution.
• To elucidate the speciation and stability of Pu(VII) under ozonolysis conditions.
• To compare the coordination chemistry of Pu(VII) with that of Np(VII).

Main Methods:

• Preparation of Pu(VI) solution in 2 M NaOH.
• In situ oxidation of Pu(VI) to Pu(VII) via ozonolysis.
• Simultaneous collection of X-ray absorption spectra (XANES and EXAFS).

Main Results:

• Pu(VI) exhibits a dioxo coordination environment (PuO2(2+)), evolving to a tetraoxo environment (PuO4(-)) for Pu(VII).
• Average Pu-O bond distances were determined for both oxidation states.
• Pu(VII) has a half-life of 3.7 hours, attributed to spontaneous reduction to Pu(VI).

Conclusions:

• The study confirms the tetraoxo coordination for Pu(VII) in solution, analogous to Np(VII).
• The observed instability of Pu(VII) is due to reduction, not disproportionation.
• No evidence for Pu(VIII) was found, and the oxidation stoichiometry of Cr(III) supports pure Pu(VII) preparation.