Pu~Ⅴ removal from solution by oxidation and adsorption onto manganese dioxide

摘要

To better understand the influence of manganese minerals on contaminant plutonium speciation, solid/solution phase distribution, and overall fate and transport, interactions between Pu and redox-active synthetic birnessite, δ-MnO_2, are being studied. Optical absorbance spectra and radioanalytical counting indicate that in the presence of δ-MnO_2 Pu~Ⅴ(aq) is rapidly oxidized to Pu~Ⅵ(aq) and removed from solution under all conditions studied. X-ray adsorption spectra show that the Pu is adsorbed onto the δ-MnO_2 as Pu~Ⅵ. The model that best matches the EXAFS spectra is based on surface-bound mononuclear dioxo Pu~Ⅵ. Adsorption isotherms show classical cation-mineral behavior, with maximum Pu~Ⅵ adsorption between pH 4 and 7, and the adsorption edge shifting to higher pH with increasing [Pu~Ⅵ]_(init). Nearly all measured and predicted Pu~Ⅴ-mineral interactions invoke Pu~Ⅳ adsorption or precipitation. Our results suggest that common Mn~Ⅳ-containing minerals have the potential to stabilize oxidized forms of plutonium and that transport models should be expanded to include Pu~Ⅵ-mineral surface complexation reactions.