Solubility, Speciation and Thermodynamics of PuCO3OH(cr) in Carbonate Containing Solutions

Emplacement in deep geological repositories is the internationally favored option for the disposal of spent nuclear fuel. With respect to the long-term safety of disposal facilities, the potential release of radionuclide due to the interaction with aqueous solutions is analyzed. Important processes that influence the mobility of radionuclides are solubility equilibria, complexation with inorganic and organic ligands, interaction with mineral phases, etc. The quantification and modeling of the individual processes as a function of various solution parameters such as pHm, ionic strength or redox potentials are based on chemical and thermodynamic models and related databases. Plutonium (Pu) is the main transuranic element in spent nuclear fuel and contributes significantly to its radiotoxicity. Under the reducing conditions of a deep geological repository, the +III and +IV oxidation states of plutonium are predominant. In absence of other complexing ligands than OH-, the solubility behavior of Pu is often controlled by hydrolysis reactions. However, a significant influence of dissolved carbonate on speciation and solubility of Pu is expected due to its known strong interaction with actinides. ... mehrWhile the behavior of Am(III), Cm(III) and trivalent lanthanides, Ln(III), have been well investigated in carbonate containing solutions, only very few studies focus on Pu(III). Numerous carbonate-containing An(III)/Ln(III) complexes and solid phases have been characterized. AmCO3OH(cr) was found to be of particular thermodynamic stability under neutral pH conditions. In case of Pu(III), the existence of PuCO3OH(cr) was demonstrated by Hagan in 1981, but systematic studies on its solubility and stability are missing, making reliable calculations of the Pu(III)-Pu(IV) redox borderline in carbonate containing solutions challenging.
In this work, the synthesis, solubility behavior and thermodynamics of PuCO3OH(cr) was investigated. Equilibration of initial Pu(OH)3(am) in a Na-Cl-HCO3-CO3 containing solution inside an argon glove box at T = 23 °C resulted in the formation of a blue fine crystalline solid phase after few months. Results from powder XRD, SEM/EDX, UV/VIS and EXAFS classified the transformation product as PuCO3OH(cr). The solubility of PuCO3OH(cr) was investigated in Na-Cl-OH-HCO3-CO3 solutions at five different ionic strength conditions (0.10, 0.51, 1.02, 3.20 and 5.61 m) with pHm = 5.0-8.0 inside an Ar box with p(CO2) adjusted to 1%, and at T = 23 °C. The experimental solubility data show a systematic dependence on ionic strength and pHm value, and can be well described with the chemical and thermodynamic models derived for the Pu(III) analogs Am(III)/Cm(III). Especially under near-neutral pHm conditions, the solubility of PuCO3OH(cr) is significantly lower (partly up to four orders of magnitude) than that of Pu(OH)3(am) which is currently considered as solubility limiting Pu(III) solid phase in many geochemical modeling analysis. The present work shows that traces of CO32- strongly impact the solubility of Pu(III) and can lead to a pronounced limitation of its mobility.

Acknowledgement: This work was partly funded by the German federal company for radioactive waste disposal, BGE.