Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles.

Area of Science:

• Environmental Science
• Geochemistry
• Materials Science

Background:

• Understanding uranium speciation is crucial for nuclear waste management and environmental remediation.
• Magnetite nanoparticles are potential sorbents for uranium, but their long-term stability and uranium transformation require investigation.

Purpose of the Study:

• To investigate the redox states and speciation of uranium coprecipitated with magnetite nanoparticles.
• To characterize the transformation of uranium species over time under varying conditions.

Main Methods:

• X-ray absorption spectroscopy (XAS), specifically U M4 high-energy resolution X-ray absorption near-edge structure (HR-XANES) and U L3 XAS.
• Iterative transformation factor analysis (ITFA) for quantifying redox state contributions.
• Transmission electron microscopy (TEM), powder X-ray diffraction (pXRD), scanning electron microscopy (SEM), and Fe 2p X-ray photoelectron spectroscopy (XPS).
• Density functional theory (DFT) calculations for theoretical support.

Main Results:

• Uranium M4 HR-XANES successfully distinguished and quantified simultaneous U(IV), U(V), and U(VI) in magnetite nanoparticles.
• Initially sorbed U(VI) transformed into nonstoichiometric UO2+x nanoparticles under anoxic conditions within 147 days.
• U(IV) species oxidized upon exposure to air.
• A stable U(V) species was observed and quantified, particularly in samples with 1000 ppm U, protected from oxidation within magnetite octahedral sites.

Conclusions:

• Magnetite nanoparticles can host multiple uranium redox states, including a stable U(V) species.
• Uranium speciation in magnetite is dynamic, with transformations influenced by environmental conditions (anoxic vs. air exposure).
• The findings provide insights into uranium behavior in engineered materials relevant to nuclear waste disposal and environmental remediation.