Sorption behavior of U(VI) on phyllite: experiments and modeling.

Area of Science:

• Geochemistry
• Environmental Science
• Materials Science

Background:

• Uranium (U-VI) sorption onto geological materials is critical for nuclear waste disposal.
• Low-grade metamorphic rock phyllite is a potential host rock, but its sorption behavior requires detailed understanding.
• Secondary mineral phases, like iron oxyhydroxides, can significantly influence contaminant sorption.

Purpose of the Study:

• To model U(VI) sorption onto phyllite using the diffuse double layer model (DDDL).
• To investigate the role of primary phyllite minerals (quartz, chlorite, muscovite, albite) and secondary ferrihydrite in U(VI) sorption.
• To quantify the contribution of ferrihydrite to the overall U(VI) sorption capacity of phyllite.

Main Methods:

• Batch sorption experiments using phyllite and its mineral components.
• Diffuse Double Layer Model (DDDL) for sorption modeling.
• Mössbauer spectroscopy to identify ferrihydrite formation.
• Centrifugation experiments to detect Fe colloids.
• Acid-base titrations and adsorption isotherms to determine surface properties.
• FITEQL code for optimizing model parameters.

Main Results:

• Ferrihydrite formed as a secondary phase during U(VI) sorption experiments on phyllite.
• Mössbauer spectroscopy confirmed a 2.8% increase in Fe(III), indicating ferrihydrite formation.
• Ferrihydrite nanoparticles (6-25 nm) were detected, influencing sorption.
• DDDL modeling accurately predicted U(VI) sorption using determined constants for phyllite minerals and ferrihydrite.
• Sorption modeling revealed that ferrihydrite significantly dominates U(VI) sorption onto phyllite.

Conclusions:

• Secondary ferrihydrite plays a dominant role in U(VI) sorption onto phyllite.
• Accurate modeling of U(VI) sorption requires considering secondary mineralization phases.
• The findings highlight the importance of iron oxyhydroxides in controlling uranium mobility in geological environments.