Np(V) Retention at the Illite du Puy Surface

Area of Science:

• Geochemistry
• Environmental Science
• Nuclear Chemistry

Background:

• Understanding neptunium (Np) migration in argillaceous environments is critical for nuclear waste management.
• Clay minerals, particularly illite, play a significant role in radionuclide retention.
• The influence of structural iron on Np speciation and sorption in clays requires further investigation.

Purpose of the Study:

• To investigate Neptunium(V) retention mechanisms on Illite du Puy (IdP).
• To determine the role of structural Fe(II)/Fe(III) in IdP on Np sorption and speciation.
• To assess the potential for Np(V) reduction to Np(IV) in the presence of IdP.

Main Methods:

• Batch sorption experiments were conducted using IdP, Wyoming smectite, and iron-free montmorillonite.
• Fe K-edge X-ray absorption near-edge structure (XANES) and ⁵⁷Fe Mössbauer spectroscopy confirmed structural iron.
• Solvent extraction, Np M₅-edge high-energy resolution (HR-) XANES, and Np L₃-edge extended X-ray absorption fine structure (EXAFS) were used to analyze Np speciation and coordination.

Main Results:

• IdP exhibited higher Np sorption affinity compared to other clay minerals.
• Np(IV) ratio increased with decreasing pH, reaching (24 ± 2)% at pH 5.
• Np speciation analysis indicated Np(IV/V) mixtures and coordination to Fe─O entities, suggesting Np(V) reduction to Np(IV) via structural Fe(II).

Conclusions:

• Structural Fe(II)/Fe(III) in IdP significantly enhances Np(V) sorption.
• Fe(II/III) clay edge sites act as strong Np(V) surface complexation partners.
• Partial reduction of sorbed Np(V) to Np(IV) by structural Fe(II) is a key retention mechanism.