Plutonium(IV) Sorption during Ferrihydrite Nanoparticle Formation

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 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.

Transformation of Ferrihydrite to Goethite and the Fate of Plutonium

“…These were fitted as 6(1) Pu-Fe scatterers at 3.17 43,[65][66][67][68][69] Lastly the EXAFS data are not inconsistent with the formation of a distinct poorly ordered Pu-Fe solid phase as has been hinted at in forensics literature, 56 although the formation of a distinct Pu-Fe phase was not supported by TEM imaging. Overall the TEM and EXAFS data demonstrate that PuO2 exists at the highest concentration Pu coprecipitation samples (3000 ppm) but not the lower concentrations (1000 and 400 ppm).…”

“…For example, constraining the number of Pu-Fe scatterers to 4 reduced σ 2 to 0.013(2) Å 2 while still maintaining a reasonable fit albeit with an increased correlation factor (R %) (7.9 vs 3.5 %); thus, it cannot be excluded that the EXAFS data may represent a surface complex with a lower number of Pu-Fe scatterers. 43 where ferrihydrite precipitation was induced from a Pu-HNO3 solution at pH 9. PuO2 formation was not observed and the data were modeled with 4 Fe scatterers at 3.38(2) Å and attributed it to the formation of a polynuclear multidentate complex with the ferrihydrite surface.…”

“…38,39 Ferrihydrite typically exhibits a high surface area with a high capacity to adsorb dissolved metal species [40][41][42] , including U, Np, and Pu. 22,[43][44][45][46] These traits of ferrihydrite have been utilized to decontaminate radioactive effluent 47 and in various nuclear abatement technologies. 48 When Pu is present in solutions where iron (oxy)hydroxide minerals may be forming, such as in a contaminated ponds, lakes, or streams, or during corrosion of steel, its fate is unknown.…”

Neptunium and Uranium Interactions with Environmentally and Industrially Relevant Iron Minerals

“…Minerals 2022, 12, 165 2 of 12 237 Np is produced from 238 U in nuclear reactors by neutron capture [7] and the α-decay of 241 Am (t1 /2 = 432.5 years) [1] in radioactive wastes, meaning the concentration of 237 Np will increase in radioactive wastes over time. Additionally, 237 Np is often considered a priority radionuclide for removal from radioactive effluents by treatment facilities, for example, the Enhanced Actinide Removal Plant (EARP, Sellafield, UK) [8][9][10], with research into its behaviour offering insight into AnO 2 + (e.g., PuO 2 + ) behaviour more generally. Consequently, there is a need to obtain a mechanistic understanding of the solid-aqueous partitioning of Np in both natural and engineered environments.…”

“…Here, the pH of an acidic effluent is increased through addition of NaOH, resulting in the formation of ferrihydrite via Keggin clusters (Fe 13 ) [35]. As the pH increases, ferrihydrite forms from the clusters and precipitates out of the solution, with the associated radionuclides, including uranium and plutonium, partitioned to the solid phase [9,10]. Studies have explored the speciation of U and Pu associated with the ferrihydrite end-product of precipitation in model systems, with U(VI) forming a bidentate, edge-sharing surface adsorption complex [10] in line with previous work [36], and Pu(IV) forming an inner-sphere, tetradentate complex [9].…”