Speciation of Np(V) uptake by Opalinus Clay using synchrotron microbeam techniques

Area of Science:

• Geochemistry
• Materials Science
• Nuclear Chemistry

Background:

• Opalinus Clay is a key argillaceous rock formation for nuclear waste disposal research.
• Understanding neptunium (Np) speciation is crucial for predicting its environmental behavior and long-term fate.
• Micro-scale heterogeneity of Opalinus Clay influences contaminant sorption and transport.

Purpose of the Study:

• To determine the chemical speciation of neptunium (Np) sorbed on Opalinus Clay (OPA) at a molecular level.
• To investigate the influence of aerobic and anaerobic conditions on Np speciation.
• To identify the mineral phases responsible for Np reduction and sorption.

Main Methods:

• Synchrotron-based X-ray absorption spectroscopy (XAS) for chemical speciation.
• Micro-X-ray fluorescence (μ-XRF) mapping for elemental distribution (Np, Fe, Ca).
• Micro-X-ray diffraction (μ-XRD) for mineral phase identification.
• Electron microprobe analysis for light element distribution (Si, Al).

Main Results:

• Np(IV) species were detected even in samples exposed to aerobic conditions.
• Significant Np(IV) formation was observed, with some spectra showing almost pure Np(IV).
• Under anaerobic conditions, Np(V) reduction correlated with iron (Fe) distribution, linked to pyrite (FeS2).

Conclusions:

• Neptunium readily reduces to Np(IV) on Opalinus Clay, irrespective of oxygen availability.
• Iron(II)-bearing minerals, particularly pyrite, are key reductants for Np(V) in this geological formation.
• Spatially resolved techniques are essential for understanding complex Np-clay interactions in heterogeneous environments.