Coprecipitation of Ce(III) oxide with UO₂

Area of Science:

• Geochemistry
• Materials Science
• Nuclear Chemistry

Background:

• Understanding the behavior of actinides and lanthanides in solid solutions is crucial for nuclear waste management and environmental remediation.
• Amorphous solid solutions of cerium (Ce) and uranium (U) can form under reducing conditions, impacting their environmental mobility.

Purpose of the Study:

• To synthesize amorphous cerium-uranium solid solutions (Ce$_{x}$U$_{1-x}$O$_{2±y}$) via coprecipitation.
• To determine the solubility of these solid solutions across a wide pH range (2.2–12.8) under reducing conditions.
• To investigate the thermodynamic behavior and mixing properties of Ce(OH)$_{3}$ within the UO$_{2}$ matrix.

Main Methods:

• Coprecipitation of U(IV) and Ce(III) in a glove box atmosphere with dithionite.
• Solubility measurements in 1 M NaClO$_{4}$ solutions.
• Characterization using chemical analysis, SEM-EDX, XRD, XPS, and XAS.

Main Results:

• Amorphous Ce$_{x}$U$_{1-x}$O$_{2±y}$ solid solutions were successfully synthesized and characterized, confirming Ce(III) and U(IV) oxidation states.
• Equilibration was rapid (∼1 week), with Ce release consistently dominated by uranium solubility across the entire pH range.
• The conditional solubility product of Ce(OH)$_{3}$ in the solid solution was significantly lower than for pure Ce(OH)$_{3}$, indicating highly favorable mixing.

Conclusions:

• Ce$_{x}$U$_{1-x}$O$_{2±y}$ amorphous solids behave thermodynamically as solid solutions.
• The high degree of mixing, evidenced by low activity coefficients for Ce(OH)$_{3}$, suggests enhanced stability.
• Uranium solubility in equilibrium with these coprecipitates aligns with literature values for UO$_{2}$(s), providing a predictable solubility limit.