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Plutonium Activities in Soil Solutions and the Stability and Formation of Selected Plutonium Minerals¹

ABSTRACT

Selected 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.

Key Words: mineral solubility • chemical equilibria • thermodynamic data • plutonium solution complexes • oxidation reduction • plutonium solution activities

¹ Contribution from Battelle Pacific Northwest Laboratories, Richland, WA 99352. This study was made possible through funds provided by ERDA, Nuclear Fuel Cycle and Production pursuant to disposal media studies.

² Senior Research Scientist and Research Scientist, respectively.

Received for publication June 1, 1976.