A selection of thermodynamic properties for zeolites: application to the cement/clay interactions

Deep disposal concepts are usually based on a multibarrier concept that may involve a physical contact between clayey and cementitious materials. In such context, zeolites are a group of important phases, which group most of the transition phases between cement and the clayey barrier since zeolites have been shown to form readily after the weathering of clays in a hyperalkaline environment [1]. Thermodynamic properties can be found in the literature for some of the zeolites of interest in deep disposal contexts. However, there is still a lack of consistency among the available thermodynamics datasets. A first task realized in the present work consists in a critical selection of the thermodynamic datasets published so far. The selection could be achieved with some confidence for a rather large list of minerals. Some questions and uncertainties still remain for phases like phillipsite, chabazite or gismondine. Cases from the previous critical selection indicate the role of kinetics in the precipitation of zeolites, which can help in moderating the phase relations indicated by thermodynamics and can be related to field observations or experimental results. In addition, the concept of rock acidity can applied with success in order to investigate the phase relations between cements, zeolites and clayey materials

Role of carbonate minerals in the distribution of trace elements in marine clay formations

International audienceAnthropogenic radionuclides (RN) are generated by a wide range of industrial and medical activities. In the contexts of waste storage, the quantification of RN migration is of paramount importance. RN migration is partly ruled by the interaction of RN with the solid surfaces. Usually experiments are conducted at various scales from laboratory to the field in order to measure retention and retardation parameters of radiotracers. Whereas this experimental approach is mandatory to tackle the issue of RN migration, the understanding of the natural speciation of stable isotopes that are analog to RN brings additional useful information. In particular, the RN natural speciation sheds light on RN isotopic exchange and " irreversible " trapping mechanisms. This study aims at overviewing the association of natural trace elements (U, Th, Ni, I, Sr and Zn) with carbonate minerals in the Callovian-Oxfordian sedimentary formation that is under consideration for deep nuclear waste disposal in France. The combined use of sequential extraction techniques, microscopic and spectrometric techniques, as well as laser-ablation coupled to chemical analysis techniques made it possible to establish the distribution of I, Sr, U, Th and Ni in the various mineral and organic phases present in the clay rock. I and Sr and in a less extent U and Th are mainly carried by carbonates while Ni is distributed in a variety of phases including pyrite, sphalerite, chlorite, organic matter and muscovite

Speciation dynamics of oxyanion contaminants (As, Sb, Cr) in argillaceous suspensions during oxic-anoxic cycles

The final publication is available at Elsevier via http://dx.doi.org/10.1016/j.apgeochem.2017.12.012 © 2018. This manuscript version is made available under the CC-BY 4.0 license https://creativecommons.org/licenses/by/4.0/Argillaceous geological formations are considered promising repositories for waste containing inorganic contaminants. However, the sequestration capacity of an argillaceous natural barrier may change as a result of dynamic environmental conditions, in particular changes in redox state. Here, we imposed redox cycles to argillaceous suspensions amended with a mixture of the inorganic contaminants As(V), Sb(V), Cr(VI), by alternating 7-day cycles of sparging with either oxic or anoxic gas mixtures. During the redox cycles, we assessed the relative importance of different contaminant sequestration mechanisms under sterile and non-sterile conditions. The non-sterile experiments were carried out 1) with the argillaceous material as is, that is, containing the native microbial population, 2) with the addition of ethanol as a source of labile organic carbon, and 3) with the addition of both ethanol and a soil microbial inoculum. In order to capture the observed solid-solution partitioning trends, a series of mixed thermodynamic-kinetic models representing the dynamic conditions in the experiments were developed using PHREEQC. Parameter values for processes such as adsorption, which occurred in all experiments, were kept constant across all the simulations. Additional formulations were introduced as needed to account for the microbial detoxification and respiration of the contaminants in the increasing complexity experiments. Abiotic adsorption of As and abiotic reductive precipitation of Cr dominated sequestration of these two contaminants under all experimental conditions, whereas the presence of microorganisms was essential to decrease the aqueous Sb concentration. Once reduced, As(III), Sb(III), and Cr(III) were not re-oxidized upon exposure to dissolved O2 in any of the experiments. Neither the mineralogy nor the native microbiota catalysed contaminant oxidation processes. Thus, kinetically, the reduction of As(V), Sb(V), and Cr(VI) was irreversible over the experimental duration (49 days). The capacity of the argillaceous matrix to reduce and sequester the aqueous contaminants, without subsequent remobilization to the aqueous phase during the oxic periods, supports the use of argillaceous barriers for geologic waste storage.Andra (French National Radioactive Waste Management Agency)Geochemistry group at ISTerre, a part of Labex OSUG@2020 (ANR 10 LABX 56)Canada Excellence Research Chair progra

Effect of trace elements on carbonate thermodynamic constants

International audienceThis study is a preliminary work on the description and the modeling of mechanisms potentially causing discrepancies between experimental data and thermodynamic calculation performed on carbonate equilibrium at high temperature. A better understanding of dolomite dissolution/precipitation processes that occur during its equilibration is important for investigations of high-level radioactive waste disposal or Carbon and Capture Storage (CCS). The first results demonstrated the fast dolomite equilibration in solution (within 12 days) and the good agreement between experimental and thermodynamic calculations at 25 °C for dolomite and calcite-dolomite systems whereas discrepancies are observed at higher temperature (80 °C). As shown from cathodoluminescence photomicrographs, one of the expected mechanisms causing those differences is the incorporation of trace elements such as Mg, Mn or Ba found in natural systems

A quantitative and mechanistic model for the coupling between chemistry and clay hydration

International audienceIt is proposed here to describe smectite water vapor desorption isotherms using an exchange formalism that quantitatively accounts for the different hydration states and thus different water contents. This approach makes it possible to reproduce both desorption isotherms and relative proportions of the different hydration states as determined by X-ray diffraction. The method is numerically robust and easy to implement in most reactive transport codes. The formalism is satisfactory from a phenomenological point of view and accounts for the influence of external parameters such as interlayer cation composition and solution cation composition and salinity on clay hydration. Furthermore, in contrast to most solid solution models, this approach focuses on the clay reactivity according to the charge and type of interlayer cation rather than on its solubility and therefore does not require the overall thermodynamic properties of the clay. In addition, such an explicit distinction of the hydration/cation exchange processes from the thermodynamic stability of smectite 2:1 layer allows the use of kinetics driving slow dissolution/precipitation rates if the number of exchange sites is related to the amount of clay minerals

Redox Interaction between Selenite and Mackinawite in Cement Pore Water

International audienc

Weathering profile in a near-surface aquitard : example of the teguline Clay, East Paris Basin, France

International audienceNear-surface aquitards are targeted for the containment of low-level radioactive waste (LLW) in near-surface environments because they could act as natural low-permeability protective barriers for underlying groundwater resources. Few detailed mineralogical and geochemical studies have been dedicated to surficial clay formations (Hendry and Wassenaar, 2000), compared to the abundance of data made available for reduced clay formations foreseen for deep nuclear repository systems. Their study is however of primary interest for the near-surface waste storage. The objectives of this study were (1) to describe the distribution of major ions in pore waters of a 80 m-thick clay-rich aquitard near the surface, and (2) to identify geochemical processes controlling the distribution of these ions. These objectives were completed by measurements of several parameters which are currently used to model pore water chemistry in deep reduced aquitards: (1) mineralogy and diagenetic sequence, (2) water content and porosity, (3) cation exchange capacity and cation distribution on the clay exchanger as well as the determination of anion concentrations (Cl and SO4) through milliQ leaching, (4) partial pressure of CO2, O2, and N2

Avancées dans le domaine de la thermodynamique des minéraux argileux

La connaissance des propriétés thermodynamiques des minéraux argileux représente un enjeu important pour la mise en oeuvre et la sécurité à long terme des installations de stockage sous-terrain, pour le stockage de déchets nucléaire, de CO2 ou de toute autre substance dangereuse. Ces propriétés, issues pour la plupart d'expériences de mise à l'équilibre plus ou moins bien contrôlées ou de méthodes d'estimation souffrant d'un déficit de données pour leur calibration, étaient considérées comme mal définies. Afin de répondre à ce manque de connaissance, un programme spécifique a été mis en place. Ce programme, baptisé Thermochimie, a été initié et conduit avec le soutien de l'ANDRA. Il vise à compléter les bases de données thermodynamiques nécessaires aux calculs géochimiques, vis-à-vis des propriétés de ces minéraux argileux

A generalized model for predicting the thermodynamic properties of clay minerals

International audienceA set of models for estimating the enthalpy of formation, the entropy, the heat capacity and the volume of dehydrated phyllosilicates is presented. The model for entropy and heat capacity estimation is essentially based on a method of decomposition into polyhedral units, similar to that published by Holland (1989). The model for predicting the enthalpy of formation is based on the electronegativity scale, as previously developed by Vieillard (1994a, 1994b). For the sake of consistency, the models are parameterized using the same critical selection of thermodynamic properties from the literature. This includes a set of direct measurements especially dedicated to clay minerals that had not been taken into account in previous calculation methods. The accuracy of the predictions is tested for each property. The verification tests are also carried out for minerals that include different chemical elements than the phases used to derive the model constants, especially lithium-bearing micas. Verification tests also concern the Gibbs energy function that combines contributions from both models. Finally, the models are used in order to propose a complete thermodynamic database for clay mineral end-members. The consistency of the stability domains calculated on the basis of these thermodynamic properties is investigated by drawing relevant predominance diagrams for some chemical systems of interest. The models proposed represent a significant improvement with respect to previous works as regards the global accuracy of the estimates and because the developments were realized and tested using the same set of minerals, whose properties had been collected through a critical selection of the literature