Adsorption of plutonium oxide nanoparticles.

Area of Science:

• Environmental Science
• Geochemistry
• Materials Science

Background:

• Understanding plutonium (Pu) nanoparticle interactions with mineral surfaces is critical for predicting environmental transport and fate.
• Muscovite, a common phyllosilicate mineral, possesses a negatively charged basal plane susceptible to cation and nanoparticle adsorption.
• Previous studies lacked in situ observations of nanoparticle behavior at mineral-water interfaces.

Purpose of the Study:

• To investigate the in situ adsorption behavior of monodisperse cubic plutonium oxide nanoparticles (Pu-NP) onto the muscovite (001) basal plane.
• To quantify the uptake capacity and surface coverage of Pu-NPs.
• To elucidate the structural arrangements and sorption states of adsorbed Pu-NPs using advanced X-ray techniques.

Main Methods:

• In situ observation of Pu-NP adsorption from aqueous solutions onto muscovite (001) using a 100 mM NaCl background electrolyte at pH 2.6.
• Quantification of Pu surface uptake capacity via alpha-spectrometry.
• Determination of surface area coverage by adsorbed Pu-NPs using X-ray reflectivity (XR).
• Structural investigation of Pu-NP sorption states and distribution using resonant anomalous X-ray reflectivity (RAXR).

Main Results:

• High uptake capacity of 0.92 μg Pu/cm(2) (10.8 Pu per unit cell area) was observed, significantly exceeding surface charge requirements.
• Adsorbed Pu-NPs covered 17% of the muscovite surface area, indicating one Pu-NP per 14 unit cells.
• RAXR revealed two distinct sorption states: direct adsorption within 11 Å and extended "stacking" or aggregation up to >100 Å from the surface.
• Nanoparticle aggregation occurred at the interface, not in solution, driven by sorption processes.

Conclusions:

• Plutonium oxide nanoparticles exhibit strong adsorption to muscovite surfaces, with aggregation occurring at the interface.
• The observed high uptake and aggregation behavior have significant implications for the environmental mobility and long-term fate of plutonium.
• This study provides the first in situ evidence of nanoparticle-mineral interface interactions, offering crucial insights into the transport of nanophase inorganic species.