To form or not to form: PuO₂ nanoparticles at acidic pH

Area of Science:

• Radiochemistry
• Materials Science
• Environmental Science

Background:

• Understanding plutonium dioxide (PuO₂) nanoparticle behavior is crucial for environmental safety and nuclear waste management.
• Previous studies have focused on PuO₂ NP synthesis in neutral to alkaline conditions.

Purpose of the Study:

• To synthesize PuO₂ nanoparticles (NPs) under low pH conditions (pH 1-4).
• To characterize the atomic-scale properties, including oxidation states and crystal structure, of these NPs.
• To investigate the influence of acidic conditions on PuO₂ NP synthesis and stability.

Main Methods:

• Synthesis of PuO₂ NPs at low pH values.
• Characterization using High-Resolution Transmission Electron Microscopy (HRTEM) for morphology and crystallinity.
• High Energy Resolution Fluorescence Detected (HERFD) X-ray Absorption Spectroscopy at the Pu M₄ edge for oxidation state determination.

Main Results:

• Formation of 2 nm crystalline PuO₂ nanoparticles with a defined crystal structure.
• Pu(iv) was identified as the dominant oxidation state in NPs synthesized at pH 1-4.
• At pH 1, evidence of Pu(iii) and Pu(vi) was observed, indicating redox dissolution of PuO₂ NPs.

Conclusions:

• Acidic conditions (pH 1-4) yield crystalline PuO₂ NPs, with Pu(iv) being the primary oxidation state.
• Redox dissolution occurs at very low pH (pH 1), leading to the formation of mixed oxidation states.
• These findings are significant for understanding plutonium migration in subsurface environments via colloid-facilitated transport.