Plutonium Activities in Soil Solutions and the Stability and Formation of Selected Plutonium Minerals

Abstract

AbstractSelected thermodynamic data was used to develop solid phasesoil solution equilibria diagrams which are used to predict and understand plutonium behavior in terrestrial environments.The plutonium mineral transformations in soils are highly dependent upon the pH, Eh, and the soil solution composition. The predictions of weathering and formation sequences of Pu(OH)3, Pu(OH)4, PuO2(OH), PuO2(OH)2, PuO2, βPu2O3, PuF3, PuF4, PuO2CO3, and Pu(HPO4)2 at pH values from 3 to 11 at pO2 from 0.68 to 80, and in an assumed weathering environment are given. The study shows that the PuO2 is the most stable mineral in the pH and pO2 ranges found in the terrestrial environments.The nature and activity of ions and ion‐complexes and, hence, the total plutonium in solution is a function of the stable phase, pH, Eh, and complexing anions: for example, (i) in a reducing environment (pO2 80) at pH 8 and in equilibrium with PuO2(s), the predominant solution species are Pu3+ and its complexes, and (ii) in an oxidizing environment (pO2 16) at pH 8 and in equilibrium with PuO2(s), the predominant solution specie is PuO2CO3OH−.Empirical predictions regarding the plutonium sorption by soils are outlined and verified by experimental results reported in the literature. The deficiencies in the present thermodynamic data and future research needs are outlined.