Solubility of U(VI) in chloride solutions. III. The stable oxides/hydroxides in MgCl2 systems: Pitzer activity model for the system UO22+–Na+–K+–Mg2+–H+–OH––Cl––H2O(l)

Yalçıntaş, Ezgi; Çevirim-Papaioannou, Neşe; Gaona, Xavier; Fellhauer, David; Neck, Volker; Altmaier, Marcus

doi:10.1016/j.jct.2018.10.019

Title: Solubility of U(VI) in chloride solutions. III. The stable oxides/hydroxides in MgCl₂ systems: Pitzer activity model for the system UO₂²⁺–Na⁺–K⁺–Mg²⁺–H⁺–OH^––Cl^––H₂O(l)

Journal Article · Wed Oct 24 00:00:00 EDT 2018 · Journal of Chemical Thermodynamics

DOI:https://doi.org/10.1016/j.jct.2018.10.019· OSTI ID:1482016

Yalçıntaş, Ezgi ^[1]; Çevirim-Papaioannou, Neşe ^[2]; Gaona, Xavier ^[2]; Fellhauer, David ^[2]; Neck, Volker ^[2]; Altmaier, Marcus ^[2]

Karlsruhe Inst. of Technology (KIT) (Germany). Inst. for Nuclear Waste Disposal; Los Alamos National Lab. (LANL), Carlsbad, NM (United States)
Karlsruhe Inst. of Technology (KIT) (Germany). Inst. for Nuclear Waste Disposal

Here we have developed new chemical, thermodynamic and activity models for the system UO₂²⁺–Na⁺–K⁺–Mg²⁺–H⁺–OH^––Cl^––H₂O(l) within the Pitzer approach. The new thermodynamic model is based on previously reported data treated within the SIT approach for NaCl and KCl systems, as well as on new experimental data determined in this work for the MgCl₂ system. The solubility of uranium(VI) was studied in 0.01–5.15 mol·kg_w^–1 MgCl₂ solutions at pH_m = 4.1–9.7 (with pH_m = –log [H⁺]). Experiments were performed under Ar atmosphere at T = (22 ± 2) °C. In all investigated systems, the solubility of U(VI) is controlled by metaschoepite, UO₃·2H₂O(cr). In contrast to previously investigated NaCl and KCl systems, no ternary Mg–U(VI)–OH(s) solid phases formed in alkaline MgCl₂ solutions within the timeframe of this study (≤ 200 days). A very significant increase in the solubility (up to 3 log₁₀-units) is observed in acidic to near-neutral pH_m conditions when increasing MgCl₂ concentration from 0.01 to 5.15 mol·kg_w^–1, which reflects the strong ion interaction processes taking place in concentrated MgCl₂ brines. The solubility of UO₃·2H₂O(cr) in the investigated NaCl, KCl and MgCl₂ solutions is well described with the solubility and hydrolysis constants recommended by Altmaier et al., (2017) and NEA–TDB, and a Pitzer activity model derived in the present work. The latter model considers experimental data reported in the present study and available in the literature for NaCl, KCl and MgCl₂ systems (solubility, potentiometric and spectroscopic data), in combination with well-stablished estimation methods and correlations with SIT coefficients. Finally, chemical, thermodynamic and Pitzer activity models provided in this work for the system UO₂²⁺–Na⁺–K⁺–Mg²⁺–H⁺–OH^––Cl^––H₂O(l) accurately describe all evaluated datasets, and represent an adequate tool for the calculation of U(VI) solubility and aqueous speciation in a variety of geochemical conditions including concentrated brine systems of relevance in salt-based repositories for nuclear waste disposal.