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.
ABSTRACT
Iron (oxyhydr)oxide nanoparticles are known to sorb metals, including radionuclides, from solution in various environmental and industrial systems. Effluent treatment processes including the Enhanced Actinide Removal Plant (EARP) (Sellafield, UK) use a neutralisation process to induce the precipitation of iron (oxyhydr)oxides to remove radionuclides from solution. There is a paucity of information on mechanism(s) of U(VI) removal under conditions relevant to such industrial processes. Here, we investigated removal of U(VI) from simulated effluents containing 7.16 mM Fe(III) with 4.2 × 10-4-1.05 mM U(VI), during the base induced hydrolysis of Fe(III). The solid product was ferrihydrite under all conditions. Acid dissolutions, Fourier Transform infrared spectroscopy and thermodynamic modelling indicated that U(VI) was removed from solution by adsorption to the ferrihydrite. The sorption mechanism was supported by X-ray Absorption Spectroscopy which showed U(VI) was adsorbed to ferrihydrite via a bidentate edge-sharing inner-sphere species with carbonate forming a ternary surface complex. At concentrations ≤0.42 mM U(VI) was removed entirely via adsorption, however at 1.05 mM U(VI) there was also evidence for precipitation of a discrete U(VI) phase. Overall these results confirm that U(VI) sequestered via adsorption to ferrihydrite over a concentration range from 4.2 × 10-4-0.42 mM confirming a remarkably consistent removal mechanism in this industrially relevant system.
