Plutonium Desorption from Mineral Surfaces at Environmental Concentrations of Hydrogen Peroxide

Knowledge of Pu adsorption and desorption behavior on mineral surfaces is crucial for understanding its environmental mobility. Here we demonstrate that environmental concentrations of H₂O₂ can affect the stability of Pu adsorbed to goethite, montmorillonite, and quartz across a wide range of pH values. In batch experiments where Pu­(IV) was adsorbed to goethite for 21 days at pH 4, 6, and 8, the addition of 5–500 μM H₂O₂ resulted in significant Pu desorption. At pH 6 and 8 this desorption was transient with readsorption of the Pu to goethite within 30 days. At pH 4, no Pu readsorption was observed. Experiments with both quartz and montmorillonite at 5 μM H₂O₂ desorbed far less Pu than in the goethite experiments highlighting the contribution of Fe redox couples in controlling Pu desorption at low H₂O₂ concentrations. Plutonium­(IV) adsorbed to quartz and subsequently spiked with 500 μM H₂O₂ resulted in significant desorption of Pu, demonstrating the complexity of the desorption process. Our results provide the first evidence of H₂O₂-driven desorption of Pu­(IV) from mineral surfaces. We suggest that this reaction pathway coupled with environmental levels of hydrogen peroxide may contribute to Pu mobility in the environment

Plutonium(IV) and (V) Sorption to Goethite at Sub-Femtomolar to Micromolar Concentrations: Redox Transformations and Surface Precipitation

Pu­(IV) and Pu­(V) sorption to goethite was investigated over a concentration range of 10^–15–10^–5 M at pH 8. Experiments with initial Pu concentrations of 10^–15 – 10^–8 M produced linear Pu sorption isotherms, demonstrating that Pu sorption to goethite is not concentration-dependent across this concentration range. Equivalent Pu­(IV) and Pu­(V) sorption <i>K</i>_d values obtained at 1 and 2-week sampling time points indicated that Pu­(V) is rapidly reduced to Pu­(IV) on the goethite surface. Further, it suggested that Pu surface redox transformations are sufficiently rapid to achieve an equilibrium state within 1 week, regardless of the initial Pu oxidation state. At initial concentrations >10^–8 M, both Pu oxidation states exhibited deviations from linear sorption behavior and less Pu was adsorbed than at lower concentrations. NanoSIMS and HRTEM analysis of samples with initial Pu concentrations of 10^–8 – 10^–6 M indicated that Pu surface and/or bulk precipitation was likely responsible for this deviation. In 10^–6 M Pu­(IV) and Pu­(V) samples, HRTEM analysis showed the formation of a body centered cubic (bcc) Pu₄O₇ structure on the goethite surface, confirming that reduction of Pu­(V) had occurred on the mineral surface and that epitaxial distortion previously observed for Pu­(IV) sorption occurs with Pu­(V) as well