Redox chemistry, solubility and hydrolysis of uranium in dilute to concentrated salt systems

Uranium is the main element present in spent nuclear fuel and accordingly contributes with the largest mass inventory to the nuclear waste. In spite of uranium being a relatively minor contributor to the overall radiological dose of the waste, it is certainly required to have an accurate knowledge on the solution chemistry and solubility phenomena of this key element. Uranium is also a redox-sensitive actinide, and accordingly its chemical behavior is strongly dependent on the redox boundary conditions of the system. Disposal of spent fuel in deep geological formations such as crystalline/granite, clay and rock salt is the option favored by international consensus. Water intrusion is a possible scenario that needs to be accounted for in the context of the long-term Safety Assessment of these repositories. The composition of the pore water contacting the waste will largely vary depending upon host-rock, backfill and other technical barriers, as well as the waste itself. Although a vast number of studies have previously investigated the solution chemistry of uranium, a number of key uncertainties remain. These affect to redox behavior, solid phases controlling solubility and hydrolysis, especially in the alkaline to hyperalkaline pH conditions of relevance in the context of nuclear waste disposal. U(IV) and U(VI) are the most stable oxidation states of uranium controlling its solution chemistry and solubility within the stability field of water and in the absence of strong complexing ligands. The study of this redox couple in the alkaline reducing conditions relevant in certain concepts for waste disposal (e.g. cementitious) is challenged by the further stabilization of U(VI) in the hyperalkaline pH-region, and the high sensitivity of U(IV) towards oxidation in the presence of traces of oxygen. Accordingly, an adequate knowledge of uranium redox chemistry in the aqueous and solid phases under geochemical boundary conditions (pH, pe, ionic strength, etc.) relevant in the context of nuclear waste disposal is important for a correct assessment of the long-term safety. For this reason, the redox chemistry of uranium in the presence of various reducing chemical systems in dilute to concentrated NaCl solutions is investigated in acidic to hyperalkaline pH conditions, and the results summarized in Chapter 3 of this PhD thesis. The kinetics of the reduction of U(VI) to U(IV), as well as the effect of the type and concentration of the reducing system are investigated by systematic measurements of the pHm (with pHm = –log [H+] in molal units), pe and U concentrations until attaining equilibrium conditions. Complete reduction of U(VI) to U(IV) is observed in most of the cases within the boundary conditions (pe + pHm) ≤ 4, although reduction kinetics are strongly impacted by [U(VI)]0, pHm, type and concentration of the reducing system and NaCl concentration. In (oversaturated) alkaline NaCl systems, solubility data and XANES indicate that the reduction proceeds via fast precipitation of Na2U2O7xH2O(cr), which slowly transforms into a UO2(am, hyd) solid phase. In less favourable conditions, the completion of this process required ≈ 635 days. These results also preclude the predominance of the U(IV) anionic hydrolysis species U(OH)5– and U(OH)62– below pHm ≈ 14.5, previously reported in the literature. Experimental data obtained within this PhD thesis indicate that previous observations reported in the literature can be possibly explained by insufficient equilibration time. Furthermore, this study confirms the key role of U(IV) in controlling the solubility and solution chemistry of uranium in reducing, alkaline systems. Very reducing conditions are expected to develop after the closure of underground repositories for nuclear waste disposal due to the anoxic corrosion of steel and iron components. As demonstrated in Chapter 3, U(IV) is expected to control the solubility and aqueous speciation of uranium under these very reducing conditions over a broad range of pH and background electrolyte concentrations. In spite of this, key uncertainties still affect the solution chemistry of U(IV), in particular with regard to the properties of the oxo-hydroxide/s solid phases forming, the aqueous speciation in alkaline to hyperalkaline pH conditions, as well as the formation and stability of U(IV) “intrinsic colloids”. In this context, Chapter 4 of this PhD thesis focuses on the investigation of the solubility and hydrolysis of U(IV) in reducing, dilute to concentrated NaCl, MgCl2 and CaCl2 solutions. A very thorough solid phase characterization including XRD, SEM-EDS, quantitative chemical analysis, EXAFS and TG-DTA confirms that a (nano-)crystalline phase, UO2xH2O(ncr), is responsible for the control of the solubility of U(IV) in the investigated conditions. The systematic investigation of the solubility of this solid phase in dilute to concentrated, acidic to hyperalkaline pHm conditions allows deriving comprehensive chemical, thermodynamic and SIT activity models for the system U4+–Na+–Mg2+–Ca2+–H+–Cl––OH––H2O(l). The investigation of supernatant solutions in solubility experiments without the use of phase separation methods gives also insight on the colloidal fraction in “equilibrium” with UO2xH2O(cr). Although a systematically increased uranium concentration (ca. 2–3 log10-units) is observed with respect to 10 kD ultrafiltered samples, a clear trend to decreasing [U]aq with longer equilibration times is also indicated in solubility experiments within t ≤ 200 days. Hence, the contribution of U(IV) “intrinsic colloids” to the solubility is evident in all salt systems investigated in this work, but the long-term stability of such species remains unclear. Uranium is mostly found as U(VI) under mildly reducing to oxidizing conditions. In the close vicinity of spent nuclear fuel surfaces, radiolysis effects can also promote the formation of U(VI) even in the presence of H2(g). Under alkaline pH conditions and in the absence of complexing ligands (e.g. carbonate, phosphate, silicate), the solubility of U(VI) is expectedly controlled by M–U(VI)–OH uranate solid phases (with M = Na, K, Ca, among others). In contrast to Ca- and Na-uranates, very little is known on the solubility of K–U(VI)–OH phases in spite of the abundance of K+ in many types of groundwaters and, in particular, the key role of this alkali ion in cementitious systems. In this context, Chapter 5 of this PhD is dedicated to the study of U(VI) solubility in alkaline, dilute to concentrated KCl solutions. Comprehensive solubility experiments with systematic variation of pHm and ionic strength, in combination with an extensive solid phase characterization (XRD, SEM–EDS, quantitative chemical analysis, TG-DTA) resulted in thermodynamic and activity models for the system UO22+–K+–Na+–H+–Cl––OH––H2O(l). Sensitivity analysis conducted using this updated thermodynamic model confirms that K- and Na-uranates (K2U2O7x1.5H2O(cr) and Na2U2O7xH2O(cr), respectively) are responsible of controlling the solubility of U(VI) under boundary conditions defined by cementitious systems. The absence of these solid phases in the corresponding thermodynamic databases leads to a very large overestimation (2–6 log10-units), depending upon pHm and alkali concentration) of U concentration in the underlined conditions. This work provides improved fundamental understanding of uranium solution chemistry, including redox processes, solubility phenomena and hydrolysis of both +IV and +VI redox states. Thermodynamic constants derived in the standard state and (SIT) ion interaction coefficients obtained can be implemented in thermodynamic databases and used in geochemical calculations under a variety of boundary conditions. This covers dilute to concentrated salt systems, thus allowing thermodynamic calculations under conditions representative of the different host-rocks foreseen for repositories for nuclear waste disposal, from crystalline and clay to salt-rock

Interlink between solubility, structure, surface and thermodynamics in the ThO2_2(s, hyd)–H2_2O(l) system

The impact of temperature on a freshly precipitated ThO$_2$(am, hyd) solid phase was investigated using a combination of undersaturation solubility experiments and a multi-method approach for the characterization of the solid phase. XRD and EXAFS confirm that ageing of ThO$_2$(am, hyd) at T = 80°C promotes a significant increase of the particle size and crystallinity. TG-DTA and XPS support that the ageing process is accompanied by an important decrease in the number of hydration waters/hydroxide groups in the original amorphous Th(IV) hydrous oxide. However, while clear differences between the structure of freshly precipitated ThO$_2$(am, hyd) and aged samples were observed, the characterization methods used in this work are unable to resolve clear differences between solid phases aged for different time periods or at different pH values. Solubility experiments conducted at T = 22°C with fresh and aged Th(IV) solid phases show a systematic decrease in the solubility of the solid phases aged at T = 80°C. In contrast to the observations gained by solid phase characterization, the ageing time and ageing pH significantly affect the solubility measured at T = 22°C. These observations can be consistently explained considering a solubility control by the outermost surface of the ThO$_2$(s, hyd) solid, which cannot be properly probed by any of the techniques considered in this work. Solubility data are used to derive the thermodynamic properties (log $*K°_{s,0},\Delta_fG°_m$) of the investigated solid phases, and discussed in terms of particle size using the Schindler equation. These results provide new insights on the interlink between solubility, structure, surface and thermodynamics in the ThO$_2$(s, hyd)–H$_2$O(l) system, with special emphasis on the transformation of the amorphous hydrous/hydroxide solid phases into the thermodynamically stable crystalline oxides