Development and implementation of experimental methods for improved modelability of uranium-containing saline solutions (EDUKEM). Final Report Subproject C, KIT-INE

The work performed by KIT-INE within EDUKEM is providing improved fundamental scientific understanding on the solution chemistry of uranium, with focus on four main aspects, namely redox behavior, solubility, hydrolysis and complexation with carbonate. This contributes to an improved scientific description of uranium chemistry in the context of the nuclear waste disposal Safety Case, and other fields of environmental concern where a quantitative description of uranium solubility and speciation is required. The geochemical boundary conditions investigated in this project by KIT-INE systematically cover from very acidic to hyperalkaline pH${}_m$ values, from very reducing to oxidizing, and from dilute solutions to concentrated salt brine systems. Focus is given to conditions and systems for which relevant thermodynamic data gaps have been identified or controversial discussions are ongoing in the interested scientific/technical community. Some of these conditions are representative of different potential repository concepts (e.g. crystalline, clay, salt-rock) or wasteforms (e.g. cementitious systems), and can provide relevant direct input to specific application oriented topics in the context of nuclear waste disposal. The reported studies were performed in the controlled area of KIT-INE, making use of the excellent analytical and spectroscopic tools available for the work on radionuclides at this facility.The redox behaviour of uranium was investigated in reducing, dilute to concentrated NaCl solutions from acidic to hyperalkaline pH${}_m$ conditions. The work provides new insight into relevant uranium redox transformation processes, especially regarding the reduction of U(VI) to U(IV) in aqueous solution, but also provides important information on reliable chemical U(IV) redox stabilisation methods as prerequisite for targeted experimental studies in the highly repository relevant U(IV) system. The excellent agreement between experimental observations obtained in this work and thermodynamic calculations supports the use of (pe + pH${}_m$) measurements as an accurate tool to predict the redox behaviour of uranium in dilute to concentrated saline systems under repository relevant boundary conditions. The systematic investigation of redox active chemicals in the studies of KIT-INE within EDUKEM, e.g. Sn(II), Na${}_2$S${}_2$O${}_4$ or mixed (Sn(II) + TiO${}_2$, Sn(II) + Fe(0) and Sn(II) + Fe${}_3$O${}_4$(cr)) reducing systems, allowed to establish highly reliable chemical methods for the stabilization of tetravalent uranium, the relevant uranium oxidation state expected under the strongly reducing conditions of an operative nuclear waste repository. This has significant impact on designing reliable experimental studies in the U(IV) system and can be used in future studies related to aquatic U(IV) chemistry. The solubility of U(VI) was investigated in dilute to concentrated NaCl, KCl and MgCl${}_2$ systems from acidic to hyperalkaline pHm conditions. Solid phases were thoroughly characterized using XRD, SEMEDS, TG-DTA, quantitative chemical analysis and XAFS. Based on the combination of the solubility measurements, spectroscopic data, solid phase characterization, as well as potentiometric data available in the literature, chemical, thermodynamic and (SIT, Pitzer) activity models for the system UO${}_2$${}^{VI}$–H${}^{+}$–K${}^{+}$–Na${}^{+}$–Mg${}^{2+}$–Cl${}^{\text{â}}\text{€}\text{“}$–OH${}^{\text{â}}\text{€}\text{“}$–H${}_2$O(l) were derived. These models represent an accurate and robust tool for the calculation of U(VI) solubility and aqueous speciation in a variety of geochemical conditions relevant in the context of nuclear waste disposal and will be contributing to database projects like THEREDA or NEA-TDB. The solubility of U(VI) was also investigated in the presence of carbonate and calcium under nearneutral pHm conditions. Although the formation and stability of ternary aqueous complexes Ca–U(VI)– CO${}_3$ have been reported in the literature, solubility studies of the analogous ternary solid phases are very sparse. Solubility studies conducted within this work confirm that liebigite, Ca${}_2$UO${}_2$(CO${}_3$)${}_3$·9H${}_2$O(cr), controls the solubility of uranium for the ternary system Ca–U(VI)–CO${}_3$ in dilute (≤ 0.5 M) NaCl solutions.. U(IV) solubility was investigated in reducing, dilute to concentrated NaCl, MgCl${}_2$ and CaCl${}_2$ solutions under acidic to hyperalkaline pHm conditions. A U(IV) solid phase was precipitated in alkaline, reducing conditions and aged for three months before further use in solubility experiments. A thorough characterization of this solid using XRD, EXAFS, SEM-EDS, TG-DTA and quantitative chemical analysis confirmed the formation of a (nano-)crystalline UO${}_2$∙H${}_2$O(ncr) phase. The combination of solubility data determined in the present work, solid phase characterization and thermodynamic data reported in the NEA–TDB and Neck and Kim (2001) for U(IV) hydrolysis species allowed deriving accurate thermodynamic and activity models for the system U${}^{IV}$–Na${}^{+}$–Mg${}^{2+}$–Ca${}^{2+}$–H${}^{+}$– Cl${}^{\text{â}}\text{€}\text{“}$–OH${}^{\text{â}}\text{€}\text{“}$–H${}_2$O(l). This is the most comprehensive thermodynamic study undertaken so far for U(IV) and especially is involving the most accurate solid phase characterization of the solubility-controlling U(IV)-oxohydroxide phase. The generated thermodynamic data feed into thermodynamic database projects, e.g. THEREDA or NEA-TDB. The solubility of U(IV) is significantly enhanced in alkaline NaCl solutions containing carbonate (up to 3 log10-units), compared to carbonate-free systems. Slope analyses and fit of solubility data at different carbonate concentrations and ionic strengths result in a consistent chemical model, where the enhanced solubility can be properly explained by the definition of the complex U(OH)${}_2$(CO${}_3$)${}_3$${}^{4\text{â}\text{ˆ}\text{’}}$. The chemical and thermodynamic data derived for the U(IV)-OH-CO${}_3$ systems will contribute to national (THEREDA) and international (NEA-TDB) database projects, among others.