Impact of formate, citrate and gluconate on the retention behavior of Pu(III/IV), Cm(III) and Eu(III) by cement phases

Radioactive waste is generated by different anthropogenic activities, e.g. nuclear power plants, military applications, medicine, research, industry, among others. Underground repositories are one of the most favored options for disposal of radioactive waste. The safety and the long-term performance of such repositories rely on a multi-barrier concept, which includes technical and geological barriers. Cementitious materials are widely used for construction and conditioning purposes, in particular in the context of low and intermediate level wastes (L/ILW). The possible infiltration of groundwater can initiate a series of dissolution and precipitation processes that can affect the composition of cement (cement degradation). Cementitious materials impose alkaline and hyperalkaline conditions (10 ≤ pH ≤ 13) over a very long period of time, whereas strongly reducing conditions are expected due to the anaerobic corrosion process of the steel-based materials present in the repository. The main solid phases in hydrated cements: calcium silicates hydrates (C-S-H), ettringite, monosulfate, monocarbonate, and hemicarbonate (AFm phases), can retain radionuclides present in the waste, thus preventing or slowing down their release from the disposal site. Organic ligands can be present in the waste as decontamination agents (e.g. EDTA, citrate), generated in situ through the decomposition of complex organic materials (e.g. cellulose, spent filter, ion exchange resins, etc.), but also as cement additives widely used in modern cementitious material to improve the workability and the final performance of the material (e.g. formate, citrate, gluconate). These ligands can form stable and soluble complexes with radionuclides (L) (e.g. An(III/IV)-OH-L, Ca-An(III/IV)- OH-L), eventually enhancing their mobility. Although expected to be weak, the interaction of organic ligands with cement hydrates can also alter the surface properties of those hydrates, affecting their retention properties with respect to radionuclides. This study aims at a quantitative description and mechanistic understanding of sorption processes. This includes the uptake of formate, citrate and gluconate uptake on AFm phases, ettringite and C-S-H phases, as well as the impact of these organic ligands on the uptake of Eu(III), Cm(III) and Pu(III/IV) by the cement phases. These radionuclides have been chosen as representative of the actinides in the oxidation states +III and +IV, as expected in the reducing conditions predicted to develop after the repository closure. This study is divided into two main blocks: 1. The first part of this study investigates the cement-ligand (cement-L) interactions (binary system). Sorption experiment and thermodynamic calculations (by using the thermodynamic software GEMS) were carried out to identify the main effect of the organic ligands on the structure and composition of AFm phases (Ms, Mc, Hc), ettringite (AFt) and C-S-H phases (with a variable Ca/Si ratio, C/S =0.8 -1.4) under hyperalkaline conditions (pH = 13.3). 2. The second block focuses on the impact of formate, citrate and gluconate on the retention of Pu (under hyperalkaline and reducing conditions), Eu(III) and, Cm(III) by AFm phases (Ms, Mc, Hc), ettringite (AFt) and C-S-H phases (ternary system) based on sorption experiments. In addition, the Pu-L systems (in absence of Ca) were investigated by systematic solubility experiments. To gain insight on the aqueous speciation and retention mechanism, the uptake of Cm(III) on C-S-H phases in presence and in absence of gluconate was further investigated by TRLFS spectroscopic studies.