Plutonium(IV) Sorption during Ferrihydrite Nanoparticle Formation

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.