Experimental and model phase diagrams for Eu(NO 3 ) 3 -NaNO 3 -H 2 O and Eu(NO 3 ) 3 -Mg(NO 3 ) 2 -H 2 O systems at 25 °C, based on solubility experiments, XRD characterization, Schreinemakers' method, controlled relative humidity experiments and Pitzer equations.

Solubility studies in the systems Eu(NO3)3-NaNO3-H2O and Eu(NO3)3-Mg(NO3)2-H2O were conducted at T = (22 ± 2) °C including solid phase identification by X-ray powder diffraction and Schreinemakers' method. Iso-water activity data of both systems at 25 °C were determined by iso-water activity (IWA) and dynamic vapor sorption (DVS) experiments. These experimental results were collectively used to verify and complement existing sets of Pitzer interaction parameters for the ternary systems. The resulting thermodynamic models assume the full dissociation of all involved salts. The geochemical code PhreeSCALE was used together with the parameter estimation software PEST to determine new ternary interaction parameters Ψ Eu3+,Na+,NO3 - , and Ψ Eu3+,Mg2+,NO3 - . In combination with our previous work on Na2SO4 and MgSO4 solutions [F. dos Santos et al., 2024, Dalton Trans., 53, 6289-6299], the updated model allows an accurate description of Eu(iii) solubility in complex nitrate-sulphate systems of relevance for radioactive waste disposal.

The present work improves the understanding on uranium(VI) nitrate systems in acidic, moderate to high ionic strength conditions. Undersaturation solubility studies with the systems UO2(NO3)2-NaNO3-H2O and UO2(NO3)2-Mg(NO3)2-H2O were conducted at T = (22 ± 0.2) °C. Corresponding solid phases UO2(NO3)2·6H2O, NaNO3, and Mg(NO3)2·6H2O were identified via X-ray powder diffraction. Experimental results are compared with literature data available for the ternary systems at T = 25 °C and indicate a strong dependence on temperature. Based on solubility and isopiestic data, an updated Pitzer model assuming the full dissociation of all involved salts is derived, including new Pitzer interaction parameters θUO22+,Na+, θUO22+,Mg2+, ΨUO22+,Na+,NO3-, and ΨUO22+,Mg2+,NO3-. The geochemical code PhreeSCALE was used together with the PhreeSCALE database and the parameter estimation software PEST.

Results of a recent benchmarking exercise for the thermodynamic database ThermoChimie (www.thermochimi e-tdb.com), focused on radionuclides and chemo-toxic elements, are described. This benchmarking is largely based on the comparison of calculated solubility and associated aqueous speciation in solutions representative of clay or cement porewaters. Those exercises help to check if the selected thermodynamic data are accurate and consistent and contribute to identify if all relevant aqueous species and solid phases are included in the database. The comparison has shown some specific cases for which estimations provide a useful tool to cover data gaps, especially those related to temperature effects. Also, it has highlighted that the possibility of improvement of some data sets is dependent on the availability of reliable and accurate experimental information.

ThermoChimie is a thermodynamic database project intended for use across the radioactive waste management community to support repository performance assessment, research and development activities, and potentially inform decisions about waste conditioning, reprocessing, and disposability. Established in 1995 by Andra (France), and now managed by a consortium that also includes NWS (UK) and ONDRAF/NIRAS (Belgium), it is currently moving towards the end of Phase 3 (December 2025) of the project with the recent release of ThermoChimie version 12a and a further version update anticipated in the next 18 months. This paper provides an overview of the ThermoChimie database with emphasis on the activities undertaken during Phase 3, including major updates of the Pd, Fe, Se, Ni, U, Np, Pu, Am, Cm, and Tc systems, inclusion of data for Cu, Be, and Hg, the further development of the XCheck (c) tool that facilitates database management, increased database compatibility with a range of geochemical codes (PHREEQC, TOUGHREACT, CrunchFlow, CHESS, GWB, Spana, PFLOTRAN), and expert consultation on the subjects of cement and zeolite phases and redox reactions to inform future development of the database. This considerable body of work ensures that the ThermoChimie database continues to provide accurate, consistent and complete (as far as practicable) thermodynamic data to the radioactive waste community and beyond.

Thermodynamic description of complex sulfate systems of relevance for nuclear waste disposal. Pitzer and SIT models including aqueous complexes derived for the Eu(iii)–Mg/Na–SO4–H2O system based on solubility experiments and TRLFS measurements.

A thermodynamic description of complex sulfate systems of relevance for nuclear waste disposal is provided. A full dissociation Pitzer model is derived for the system Eu(iii)–Mg/Na–SO4–H2O based on solubility experiments and comprehensive solid phase characterization.

In the context of management of the radioactive waste in deep geological formations, the effect of temperature (20-80 degrees C) on U(VI) adsorption by Callovo-Oxfordian claystone (COx) was studied. A step-by-step approach was followed, starting with the single mineral, illite, followed by an increase in the complexity of the system, through the analysis of the clay fraction and the natural samples of the Callovo-Oxfordian formation. Depending on the study conditions, and the speciation of U(VI) in solution (hydrolysed species, carbonate species and presence of ternary U(VI)-Ca(Mg)-carbonate complexes), the temperature effect was either negligible, or positive (where the increase in temperature favours retention). The most important positive effect was observed for the U(VI)/COx system in the presence of ternary complexes. The data were modelled considering an existing sorption model at 20 degrees C and the thermodynamic data available to describe the evolution of the speciation of U(VI) in solution in function of temperature. The enthalpy values associated with the surface complexes were fitted from the experimental data following a stepwise approach based on the van't Hoff equation.

In the context of the radioactive waste management in deep geological formations, U(VI) retention by intact Callovo-Oxfordian claystone (COx) was studied by percolation-type experiments at 20 and 80 degrees C. The experimental results were confronted with modelling prediction based on a published adsorption model developed from dispersed media in the 20-80 degrees C temperature range. For the experiments at 20 degrees C, the adsorption model allowed to explain the results for the intact system; the retention was weak (R-d similar to 10 L.kg(-1)) and the analysis of the COx phases at the end of the experiment confirmed a retention of U by the clay fraction. The adsorption model in temperature also explained the observed trend of increasing retention with increasing temperature. However, it underestimated the temperature effect on the adsorption of U(VI) by the COx clay fraction, and other phases contributed to the retention. Solid-state analysis of the percolation-doped samples indicated a reactivity in the order pyrite>clay>calcite phases. The transposition of the knowledge at 20 degrees C from the dispersed system to the intact medium was therefore not possible at 80 degrees C for the studied U(VI)/COx system.

For the performance assessment of radioactive waste disposal, it is critical to predict the mobility of radionuclides in the geological barrier that hosts it. A key challenge consists of assessing the transferability of current knowledge on the retention properties deduced from model systems to in natura situations. The case of the redox-sensitive element uranium in the Callovo-Oxfordian clay formation (COx) is presented herein. Extensive experimental work was carried out with respect to parameters affecting uranium speciation (pH, PCO2, [Ca] and redox potential) with illite, COx clay fraction and raw COx claystone. The "bottom-up" approach implemented, with illite and montmorillonite as reactive phases, quantitatively explains the adsorption results of U(VI) and U(IV) on COx. While retention is high for U(IV) (Rd∼104 L kg-1), it remains very low for U(VI) (Rd∼4 L kg-1) due to the formation of soluble ternary Ca(Mg)-U(VI)-carbonate complexes. The applicability of the sorption model was then assessed by comparing predictive analyses with data characterizing the behavior of naturally-occurring U (<3 mg kg-1). The COx clay phase is the largest reservoir of naturally-occurring U (∼65%) but only a small fraction appears to be adsorbed (∼1%). Under representative site conditions (especially with respect to reducing conditions), we have concluded that ternary U(VI) complexes control U speciation in solution while U(IV) surface species dominate U adsorption, with Rd values > 70 L kg-1.

We present two computing tools, ClayTherm and ISTherm, devoted to the estimation of the thermodynamic properties of both anhydrous and hydrated clay minerals (ClayTherm), and of illite/smectite (I/S) mineral series (ISTherm). The first computing tool, ClayTherm, is devoted to thermodynamic property estimates for clay minerals. It combines several previously published estimation models, including hydration aspects. Verification is provided, against a set of solubility data, selected from previous literature. A specific application ISTherm was subsequently developed based on the first tool. It focuses on the smectite-to-illite transformation, and is able to calculate the thermodynamic properties of a series of illite/smectite (I/S) interstratified minerals, starting from the composition of a single I/S sample. The thermodynamic functions have been completed for the mixing energies and the tool was then used in order to investigate the case of a natural I/S hydrothermal series from the Shinzan geothermal field (Japan). Activity diagrams have been calculated including illite/smectite and phase relations are found to be in agreement with previous mineralogical observations and solution chemical analyses. The I/S series from Shinzan is further investigated through reactive transport modelling by using a site-specific, augmented version of the geochemical database. Illitization through the formation of I/S is predicted over realistic reaction times, consistently with available mineralogical observations.

Degradation of organic compounds is a key issue in the performance assessment programs for Low and Intermediate Level radioactive Waste (L-ILW) repositories. Despite significant progress in recent decades, parameterization and implementation of biotic degradation of these compounds in reactive transport models still remain a challenging task, mainly due to uncertainties linked to their complex chemical behaviour. In this work we have explored the application of thermodynamic models in order to stablish a predictive kinetic model for nineteen organic molecules of particular interest in the frame of Low and Intermediate Level Waste disposals. Firstly, from a compilation of experimental data taken from the literature, we observed a decreasing trend in the zero-order kinetic degradation rates as the number of carbon atoms increases. Even though this conclusion is based on thermodynamic and experimental data obtained at circumneutral pH conditions, it can be considered as indicators of most probable processes expected at foreseen alkaline storage conditions. Based on the model published by LaRowe and Van Cappellen (2011) for natural organic matter degradation, a set of organic molecules of interest were analysed and compared. This thermodynamic calculations allowed us to classify the molecules of interest in three different groups regarding their most probable behaviour: i) adipate, succinate, pimelate, benzoate, TPB- and DBP- been identified as persistent molecules as their degradation is thermodynamically inhibited and they represent potential radionuclides complexing agents; ii) oxalate, formate, gluconate, lactate, NTA-, propionate, acetate, ISA- and sorbate will be easily oxidized and their kinetic degradation will proceed while the substrate for bacterial growth remain available. A third group can be mentioned and comprises organic molecules having a partly inhibited degradation from the thermodynamic point of view (DBP- EDTA-, DTPA-). Different biodegradation pathways are mentioned in the literature for these molecules. Given the high complexity of the involved processes, more experimental studies are needed before concluding on their availability for microbial processes.

Clay-rock formations are under investigation in several countries as a potential host of radioactive waste repositories. In order to assess the long-term safety of these concepts, retention distribution data (Rd) are needed to describe the interaction between radionuclides and natural rock. While Rd values obtained from batch-type experiments with dispersed material are essential for the development of retention models, it is also important to generate retention data under more realistic conditions, in particular with compact samples. Specifically, the literature data on strongly retained radionuclides proves to be very scarce. The present study focuses on the retention of europium (Eu) at trace concentrations on clay samples extracted from the Callovo-Oxfordian (COx) formation in France. Rd values obtained with compact COx samples are assessed by a methodology coupling percolation experiments (in pressurized microcells), performed using submillimeter-sized samples in thickness, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry measurements. The Rd found at trace concentrations under non-transient conditions are high (Rd > 10(4) mL g(-1)) and in agreement with those recorded under classical batch-type conditions or those deduced from the distribution of naturally-occurring Eu between the pore water and the clay rock. They can be described by modeling with existing sorption databases, considering an adsorption process only on the clay fraction. This was validated by solid state LA-ICP-MS analyses of one of the Eu-spiked COx samples; the added europium was found exclusively in the COx clay fraction.

A predictive model is developed for estimating the amount of clay water and the hydration thermodynamic properties as a function of relative humidity (R.H.), for any 2:1 clay composition. From a methodology performed by Gailhanou et al. (2017) upon MX80-Na smectite, based on experimental water adsorption isotherms and structural data, a regular solid solution between anhydrous and highly hydrated clay end-members is parameterized, providing a set of 4 thermodynamic parameters Delta H-cem.w , , W-H and W-s characterizing the theoretical mechanisms of adsorption of the clay water versus the relative humidity. To complete literature data, water adsorption isotherms are acquired at 25 degrees C and 45 degrees C for five natural 2:1 clays, with various compositions, interlayer charges and interlayer cations (Na, Ca, K, Mg). The amounts of clay water are extracted from the total adsorbed water amounts by removing the contribution of capillary water and the thermodynamic entities of the hydration reaction are calculated and are closely related to the nature of the clay (characterized by its interlayer and tetrahedral charges and the nature of the interlayer cation), allowing two predictive models, for di-octahedral smectites and for tri-octahedral 2:1 clays. These two models are assessed by using experimental data for other clay minerals provided by literature. For a given 2:1 clay composition, the model allows - to predict quite well the clay water adsorption isotherm, - to provide hydration thermodynamic properties (G, H, S) for the di-octahedral model and for the tri-octahedral model and-to predict the thermodynamic properties of formation for some hydrated clays of fictive compositions.

The stability of illite-smectite interstratified with respect to discrete illite and smectite minerals was investigated by measuring their thermodynamic properties. The standard thermodynamic properties of the illite-smectite ISCz-1 mineral (G, H, S, C-p, and V) were determined between 298.15 K and 375 K by using calorimetric methods. Moreover, the enthalpies of mixing between the illite and smectite layers were measured at 298.15 K by acid solution calorimetry from a complete series of illite-smectite interstratified minerals (Shinzan area, Japan). The measured values were slightly negative, with a minimum value of -3.7 kJ.mol(-1). This contributed to the stability of the mixed-layer with respect to a mechanical mixture of illite and smectite. In addition, the model from Blanc et al. (2015) was implemented to estimate the thermodynamic properties of the interstratified illite-smectite ISCz-1 mineral. The predicted values were consistent with the experimental results. However, the estimates were slightly improved by considering the thermodynamic properties of the mixture of the illite and smectite components and then adding the terms of energies of mixing. This could be confirmed by establishing the stability domains of ISCz-1 and those of the corresponding illite and smectite end-members according to Meunier and Velde's determination of the smectite to illite reaction pathways (Meunier and Velde, 1989). (C) 2019 Elsevier Ltd.

Clay minerals (kaolinite KGa-2, smectite MX-80, illite IMt-2, vermiculite SO and Chlorite Cca-2) were equilibrated in aqueous media during long-term batch experiments up to seven years. Dissolved element concentrations were measured, allowing calculation of the ionic activity product of the minerals. An improved protocol for equilibration has been established to limit evaporation over the seven years of the experiments and to optimize the analytical results accuracy. The protocol could be verified with respect to the solubility of kaolinite. For the other minerals, the equilibrium constants obtained could be compared with the values calculated after calorimetric measurements previously performed on the same samples. The ionic activity products extracted for illite, smectite and, to a lesser extent, vermiculite were found to agree with the values issued from the calorimetric measurements. For chlorite in particular, a discrepancy with calorimetric measurements still remains. This was interpreted in terms of dissolution rates getting slower for this mineral, formed at temperatures higher than 25 degrees C.

The physical and chemical properties of clay-rocks are, at least partly, controlled by the chemical composition of their pore water. In evaluating the concept of disposing of radioactive waste in clay-rock formations, determining pore water composition is an important step in predicting how a clay-rock will behave over time and as a function of external forces, such as chemical and thermal perturbations. This study aimed to assess experimental and modeling methodology to calculate pore water composition in a clay-rock as a function of temperature (up to 80°C). Hydrothermal alteration experiments were carried out on clay-rock samples. We conducted comprehensive chemical and mineralogical characterization of the material before and after reaction, and monitored how the chemical parameters in the liquid and gas phases changed. We compared the experimental results with the a priori predictions made by various models that differed in their hypotheses on the reactivity of the minerals present in the system. Thermodynamic equilibrium could not be assessed unequivocally in these experiments and most of the predicted mineralogy changes were too subtle to be tracked quantitatively. However, from observing the neo-formation of minerals such as goethite we were able to assess the prominent role of Fe-bearing phases in the outcome of the experiments, especially for the measured pH and pCO2 values. After calibrating the amount of reacting Fe-bearing phases with our data, we proposed a thermodynamic model that was capable of predicting the chemical evolution of the systems under investigation as well as the evolution of other systems already published in the literature, with the same clay-rock material but with significant differences in experimental conditions.

We present here the global results of an eight-year project, where we attempted to complete the ThermoChimie database for cement minerals, zeolites and clay minerals. This work reports the methodology adopted to ensure the consistency between the three groups of minerals. Our first step concerns the relations between the three groups of minerals from field observations and experimental results gathered from the literature. A summary of the selection process is then given for clay and cement minerals and a specific focus is proposed for zeolites, with examples of analcime and mordenite. The global consistency of the selection is investigated with respect to the main chemical tendencies found for cement/ clay interactions, especially considering the "alkalinity" of the minerals. Consistency is checked by establishing predominance diagrams in the chemical sub-systems of interest for the three groups of minerals, then comparing the phase relations with respect to experimental results or field observations. The methodology is illustrated using the case of gismondine and zeolite P(Ca). (C) 2014 Elsevier Ltd. All rights reserved.