Effect of temperature on the complexation of triscarbonatouranyl(VI) with calcium and magnesium in NaCl aqueous solution

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.

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.

Chemical thermodynamics of ternary M-An(VI)-CO3 system (M = Mg, Ca, Sr, and Ba)

This review provides an overview of the chemical thermodynamics on ternary earth-alkaline metal-actinyl-tricarbonate systems (i.e., M-AnO2-CO3, M = Mg, Ca, Sr, and Ba) and discusses the aqueous complexation and dissolution/precipitation equilibrium for these ternary aqueous systems. The aqueous ternary U(VI) carbonate species are remarkably predominant in the U(VI) speciation under natural environmental conditions at ambient temperature and moderate ionic strength condition, while the omnipresence, according to recent studies, would be hindered by an increase in temperature and ionic strength. With respect to the ternary solid U(VI) carbonate phases, most of the previously reported data have been focused on physical properties and thus a notable lack of available data on chemical thermodynamic properties, i.e., solubility product constant, has been identified. Nevertheless, substantial influences of these ternary M-AnO2-CO3 systems on the aqueous speciation and the solubility limiting phase under the natural environmental condition are taken into account according to the thermodynamic calculation. The authors point out that the completeness of the chemical thermodynamic model for predicting the chemical behavior of actinides in nature can be further improved on the basis of a sufficient understanding of ternary M-AnO2-CO3 systems.