Abstract
Fundamental knowledge on intrinsic plutonium colloids is important for the prediction of plutonium behaviour in the geosphere and in engineered systems. The first synthetic route to obtain salt-free intrinsic plutonium colloids by ultrasonic treatment of PuO2 suspensions in pure water is reported. Kinetics showed that both chemical and mechanical effects of ultrasound contribute to the mechanism of Pu colloid formation. In the first stage, fragmentation of initial PuO2 particles provides larger surface contact between cavitation bubbles and solids. Furthermore, hydrogen formed during sonochemical water splitting enables reduction of Pu(IV) to more soluble Pu(III), which then re-oxidizes yielding Pu(IV) colloid. A comparative study of nanostructured PuO2 and Pu colloids produced by sonochemical and hydrolytic methods, has been conducted using HRTEM, Pu LIII-edge XAS, and O K-edge NEXAFS/STXM. Characterization of Pu colloids revealed a correlation between the number of Pu-O and Pu-Pu contacts and the atomic surface-to-volume ratio of the PuO2 nanoparticles. NEXAFS indicated that oxygen state in hydrolytic Pu colloid is influenced by hydrolysed Pu(IV) species to a greater extent than in sonochemical PuO2 nanoparticles. In general, hydrolytic and sonochemical Pu colloids can be described as core-shell nanoparticles composed of quasi-stoichiometric PuO2 cores and hydrolyzed Pu(IV) moieties at the surface shell.
Highlights
We report for the first time the preparation of stable salt-free Pu intrinsic colloids by the action of ultrasonic waves on PuO2 powder in pure water
Most of the sonochemical experiments in this study were performed with 10%CO/Ar gas mixtures
The kinetic curves display an induction period. This observation can be explained by metal oxide particle fragmentation at the initial stage of ultrasonic treatment that leads to an increase of the specific surface area of solids
Summary
PuO2 suspensions in water sparged with pure Ar or with a 10%CO/Ar gas mixture was found to yield optically clear brown-green colloidal solutions stable for at least 6 months. The abnormally low value of the d111-spacing for the hydrolytic Pu colloid results from the diffuse electron diffraction rings that leads to significant uncertainty in the d-spacing determination In this case, the direct measurement of d111-spacing from HRTEM images provides better correlation with the expected Fm3m symmetry for this colloid. The autoxidation of Pu(III) in weakly acid solutions yields larger colloidal particles[25] which is consistent with the proposed redox mechanism of sonochemical Pu colloid formation. Some precipitate is observed in the sonochemical colloidal solution after approximately one year aging This process is accompanied by slight pH decrease from 3.2 to 2.8 which is probably related to the oxolation of Pu(IV) hydroxo species leading to the drop of the particle charge.
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