Abstract
Understanding interactions between iron (oxyhydr)oxide nanoparticles and plutonium is essential to underpin technology to treat radioactive effluents, in cleanup of land contaminated with radionuclides, and to ensure the safe disposal of radioactive wastes. These interactions include a range of adsorption, precipitation, and incorporation processes. Here, we explore the mechanisms of plutonium sequestration during ferrihydrite precipitation from an acidic solution. The initial 1 M HNO3 solution with Fe(III)(aq) and 242Pu(IV)(aq) underwent controlled hydrolysis via the addition of NaOH to pH 9. The majority of Fe(III)(aq) and Pu(IV)(aq) was removed from solution between pH 2 and 3 during ferrihydrite formation. Analysis of Pu–ferrihydrite by extended X-ray absorption fine structure (EXAFS) spectroscopy showed that Pu(IV) formed an inner-sphere tetradentate complex on the ferrihydrite surface, with minor amounts of PuO2 present. Best fits to the EXAFS data collected from Pu–ferrihydrite samples aged for 2 and 6 months showed no statistically significant change in the Pu(IV)–Fe oxyhydroxide surface complex despite the ferrihydrite undergoing extensive recrystallization to hematite. This suggests the Pu remains strongly sorbed to the iron (oxyhydr)oxide surface and could be retained over extended time periods.
Highlights
Plutonium is highly radiotoxic, long-lived (e.g., 239Pu t1/2 = 24,100 years) and ubiquitous in spent nuclear fuel and many radioactive wastes
A key control on plutonium environmental mobility is via sorption reactions with common mineral phases including ironoxides, e.g., ferrihydrite
Characterizing the atomic scale interactions of plutonium with mineral nanoparticles as they form is key to assessing its fate within contaminated environmental and radioactive waste geological disposal systems and in the development of effluent treatment technologies
Summary
Long-lived (e.g., 239Pu t1/2 = 24,100 years) and ubiquitous in spent nuclear fuel and many radioactive wastes It is a key risk driving radionuclide in contaminated land,[1] legacy nuclear facilities such as the Hanford Tanks and Sellafield Ponds,[2,3] and the treatment of radioactive effluents.[4] A key control on plutonium environmental mobility is via sorption reactions (i.e., surface adsorption and/or incorporation) with common mineral phases including iron (oxyhydr)oxides, e.g., ferrihydrite. Past work has indicated that Pu can adsorb to iron (oxyhydr)oxide surfaces via a variety of inner-sphere complexes and/or form discrete PuO2 particles.[9,15−23] Despite this, there is currently no detailed information on the mechanism of Pu(IV) partitioning to solids during ferrihydrite formation This information is crucial to determine key mechanisms of Pu uptake and retention during the radioactive effluent treatment process and, more broadly, within contaminated environments where both Pu and iron (oxyhydr)oxide phases are present. This provides important insights into the pathway of Pu uptake by ferrihydrite with implications for effluent treatment and understanding of Pu fate in engineered and natural environments
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