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Over 60 years of nuclear activities have resulted in a global legacy of radioactive wastes, with uranium considered a key radionuclide in both disposal and contaminated land scenarios. With the understanding that U has been incorporated into a range of iron (oxyhydr)­oxides, these minerals may be considered a secondary barrier to the migration of radionuclides in the environment. However, the long-term stability of U-incorporated iron (oxyhydr)­oxides is largely unknown, with the end-fate of incorporated species potentially impacted by biogeochemical processes. In particular, studies show that significant electron transfer may occur between stable iron (oxyhydr)­oxides such as goethite and adsorbed Fe­(II). These interactions can also induce varying degrees of iron (oxyhydr)­oxide recrystallization (<4% to >90%). Here, the fate of U­(VI)-incorporated goethite during exposure to Fe­(II) was investigated using geochemical analysis and X-ray absorption spectroscopy (XAS). Analysis of XAS spectra revealed that incorporated U­(VI) was reduced to U­(V) as the reaction with Fe­(II) progressed, with minimal recrystallization (approximately 2%) of the goethite phase. These results therefore indicate that U may remain incorporated within goethite as U­(V) even under iron-reducing conditions. This develops the concept of iron (oxyhydr)­oxides acting as a secondary barrier to radionuclide migration in the environment.

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 HNO 3 solution with Fe(III) (aq) and 242 Pu(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 PuO 2 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.

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