Solubility, speciation and thermodynamics of PuCO3OH(cr) in carbonate containing NaCl solutions

In several countries deep geological repositories are the favored option for the final disposal of radioactive waste. Radiological risks may arise due to a potential release of radionuclides from the repository following water intrusion. (Geo)chemical processes such as solubility equilibria, complexation, sorption on and incorporation into mineral phases etc. control the retention of radionuclides, highlighting the relevance of studies on the aquatic chemistry and thermodynamics of key radionuclides. Plutonium is the main transuranic element in spent nuclear fuel. Under the reducing geochemical conditions of a deep geological repository, +III and +IV are the dominating oxidation states for plutonium. In the absence of complexing ligands, the solubility of plutonium is controlled by hydrolysis reactions. Carbonate is a strong ligand for actinides, ubiquitous in natural systems, and to be expected in a deep geological repository, e.g., due to the presence of carbonate minerals like calcite. A significant impact on the solubility behavior of plutonium and other actinides is to be expected. For the trivalent plutonium analogs (Am(III), Cm(III), Ln(III)), the solubility behavior and speciation in carbonate containing solutions are rather well studied. ... mehrFor Pu(III), on the other hand, systematic solubility studies in carbonate containing solutions are missing. Several carbonate-containing solids and complexes were reported with AnCO3OH(cr) being of considerable stability under near neutral pH conditions. The existence of PuCO3OH(cr) was confirmed by Hagan [1], but information on the solubility and stability with respect to the Pu(IV) state were not reported.
In the present work, we focus on the synthesis, characterization, and solubility behavior of PuCO3OH(cr). Blueish, fine crystalline PuCO3OH(cr) was obtained by long-term equilibration of Pu(OH)3(am) in Na-Cl-HCO3-CO3 solution at T = 23°C inside a Ar glove box. The results from powder XRD, SEM-EDX, digestion Vis/NIR, and EXAFS confirmed that the formed crystalline solid is PuCO3OH(cr). The solubility of PuCO3OH(cr) was studied in 0.10-5.61 m Na-Cl-OH-HCO3-CO3-solutions at pHm 5.0-8.0 at 23°C under inert argon atmosphere with 1 % CO2 in a glove box. The experimental solubility data show a systematic dependence on pHm and ionic strength, and can be explained by chemical and thermodynamic models reported for the Pu(III) analogs. Of particular importance is the fact, that the solubility of PuCO3OH(cr) under near neutral pH conditions is up to 4 orders of magnitude lower than that of the Pu(OH)3(am) solid phase (which controls the Pu(III) solubility in absence of complexing ligands). The present work demonstrates that small amounts of CO32- can strongly impact the solubility of Pu(III) and, hence, its mobility and migration behavior.

Lit: [1] Hagan, P.G., Navratil, J.D., Cichorz, R.S. (1981). Characterization of Pu(III) carbonate. J. Inorg. Nucl. Chem. Vol 43, Great Britain