Uranium retention in a Callovo-Oxfordian clay rock formation: From laboratory-based models to in natura conditions

Predominance of the alkaline earth(II) triscarbonatoactinyl(VI) complexes in different geochemical contexts: Review of existing data and estimation of potentially unidentified species

From the available thermodynamic data in the literature, a review of the impact of the formation of complexes between triscarbonatoactinyl(VI) and alkaline earth(II) (Ae) is estimated under varying conditions. First, after analyzing the literature data and using the ascertained thermodynamic data available from the commissioned reviews from the Nuclear Energy Agency (Organization for the Economic Cooperation and Development) Thermochemical DataBank Project on actinides (An) U, Np, and Pu, and from recently determined AenUO2(CO3)3(4-2n)- thermodynamic functions, the formation of AenAnO2(CO3)3(4-2n)- complexes for Pu(VI) and Np(VI) are estimated using linear free energy relationships (LFERs). The data are in good agreement with the sole determination of AePuO2(CO3)32- from Jo et al. (Dalton Trans. 49, 11605), which gives a relative confidence in the LFERs, and allows the application to actual situations. From existing uranium data, first, the impact of the origin of the data on the calculated predominance is addressed under 0.1 M NaCl and atmospheric CO2(g); second, the influence of ionic strength and salinity on predominance is estimated; and finally, the influence of temperature up to 50 °C on the solubility of uraninite in a deep geological radioactive waste storage or disposal site is calculated. For neptunium and plutonium, the impact of the potential log10β°(AenAnO2(CO3)3(4-2n)-) on Pourbaix diagrams of Pu and Np in Mg-Ca-CO3 media are estimated from Jo et al. (Dalton Trans. 49, 11605) and LFERs. Finally, the application to the speciation of Pu and Np in seawater is proposed.

The redox reactions of U(VI)/UO2 on Tamusu claystone: Effects of Fe2+/Fe3+ and organic matters

The claystone-based Tamusu area in the Bayingebi Basin, Inner Mongolia, is preselected as a China's high-level radioactive waste (HLRW) repository site. This study investigated the redox reactions of U(VI)/UO2 on Tamusu claystone. Five Tamusu claystone samples collected from boreholes Tzk1 and Tzk2 at different depths were used for batch experiments at pH ∼5.0, ∼7.0, and ∼9.0. These claystones contain considerable amounts of organic matters and Fe2+-containing minerals such as pyrite, fluorannite, and ankerite. Results showed that aqueous U(VI) could be partially reduced to U(IV) and/or U(V)-containing precipitates (U3O8, U4O9, etc.) by these Tamusu claystones, and the reaction is more favorable under acidic condition. We proposed that leaching of the structural Fe2+ followed by surface adsorption and interface reaction, is the primary mechanism responsible for U(VI) reduction. Under alkaline condition, organic matters might dominate the partial reduction of aqueous U(VI). Besides, the phosphorus-containing spots on Tamusu claystone surfaces are the reactive sites for U aggregation, implying the possible formation of U(VI)- and/or U(IV)-phosphate minerals. It is important to note that, due to the presence of minor Fe3+ in Tamusu claystones, the high-purity UO2 could undergo partial oxidation to U4O9 and/or U3O8. Therefore, insoluble UO2+x (0 < x ≤ 0.67) is proposed to be the most thermodynamically stable form in Tamusu claystone. This study enhances our comprehension of the essential geochemical processes of uranium in claystone surroundings, but also offers crucial information for the safety evaluation of China's HLRW repository.

Implications of recently derived thermodynamic data and specific ionic interaction theory parameters for (Mg/Ca)nUO2(CO3)3(4-2n)- complexes on the predominance of the Mg2+-Ca2+-UO22+-OH--CO32- systems, and application to natural and legacy-mine waters

The formation of alkaline earth(II)triscarbonatouranyl(VI) (Ae_nUO₂(CO₃)₃^(4-2n)-) species that have been evidenced both in laboratory and in-field studies, is important from slightly acidic pH up to near degraded cementitious in carbonated waters. They are also showing distinctive luminescence properties with a hypsochromic shift relative to UO₂²⁺. The conditions of pH, activities of alkaline earth(II) free ions (mostly Mg²⁺ and Ca²⁺) and carbonate ions (HCO₃^-) can be predicted from the thermodynamic functions and constants. The predictive validity of the activity of major alkaline ions (mostly Na⁺) is determined from the models used to describe the ionic strength comportment of these species, particularly using coefficients from the specific ion interaction theory (SIT). The stability domains of these species are better defined as a function of the activity of the constituents, and applied to natural waters. In this work, using recently obtained complete thermodynamic data and SIT coefficients, we will draw the stability domains of the Ae_nUO₂(CO₃)₃^(4-2n)- species in combinations of activities of H⁺, HCO₃^-, Mg²⁺, Ca²⁺, and Na⁺ for a wide selection of water compositions from the literature. Water samples were collected near a French mining legacy-site (Site du Bosc, Lodève, France). After determining the major ion compositions, we will verify that the luminescence signal of uranium is in agreement with the predicted speciation in the stability domains.