Size and structure of hexanuclear plutonium oxo-hydroxo clusters in aqueous solution from synchrotron analysis

Area of Science:

• Nuclear Chemistry
• Materials Science
• Analytical Chemistry

Background:

• Plutonium chemistry is complex and understanding its aggregation is crucial for nuclear waste management and remediation.
• Characterizing small plutonium clusters in aqueous solutions presents significant analytical challenges due to radioactivity and potential for aggregation.

Purpose of the Study:

• To develop and validate a combined synchrotron small-angle X-ray scattering (SAXS) and extended X-ray absorption fine structure (EXAFS) method for probing water-soluble hexanuclear plutonium clusters.
• To determine the size and shape of plutonium hexanuclear clusters stabilized by tetraazacyclododecane-tetraacetic acid ligands in aqueous solution.

Main Methods:

• Utilized a specialized setup coupling SAXS and EXAFS techniques on radioactive samples at the MARS beamline endstation, Synchrotron SOLEIL.
• Employed synchrotron radiation for high-flux X-ray scattering and absorption measurements.
• Applied classical fit models (Guinier, sphere form factor) and iterative comparison for SAXS data analysis.

Main Results:

• Successfully optimized the experimental setup, achieving a high signal-to-noise ratio for detecting small plutonium aggregates in an aqueous phase.
• Confirmed the well-defined structure of the hexanuclear plutonium cluster using EXAFS in solution.
• Correlated EXAFS structural data with SAXS processing and modeling, validating the cluster's size and shape.

Conclusions:

• The combined SAXS-EXAFS approach is effective for characterizing plutonium clusters in solution, even under constrained conditions.
• The tetraazacyclododecane-tetraacetic acid ligand effectively stabilizes hexanuclear plutonium cores.
• The developed methodology provides a robust platform for future studies on plutonium speciation under environmentally relevant conditions.