Extending the prediction of the thermodynamic properties of clay minerals to the trapping of trace elements

The thermodynamic properties of clay minerals, which control the stability of these minerals in solution, are still a matter of debate in spite of recent advances (Gailhanou et al., submitted). This is especially the case for the minerals that may structurally include trace elements and potential radionuclides such like Ni, Cd, Co, Cr, Mn, Pb, ... The usual methods developed in order to predict thermodynamic properties are parameterised using a given set of minerals. For clay minerals, the latter are mainly composed by Si, Al, Fe and Mg, apart from the alkalis elements (Chermak and Rimstidt, 1989), which means that predictions are limited to minerals whose layers are composed by Si, Al, Fe and Mg. At the vicinity of H&ILW disposal cells, the possible interactions between clay rock or engineered barrier and waste degradation products can result in the appearance of clay minerals that may structurally include radionuclides within an irreversible trapping process. This work aims at proposing a method for predicting the thermodynamic properties of such minerals. Theoretical principle and selection of calibration phases Vieillard (1994) has developed a methodology of estimation based on the difference of electronegativity by considering three scales of values of the parameter HO=(Mz+clay) in the three sites of phyllosilicates. We have considered the work of Vieillard (1994) that originally applies to the estimate of H0f and extended it to the estimate of Cp(T), S0 and V. Some popular estimate methods (Chermak and Rimstidt, 1989) are based on the hypothesis that the thermodynamic property of a mineral can be obtained by combining the properties of its components. An improvement of this principle had consisted in decomposing minerals into their polyhedral components (Chermak and Rimstidt, 1989). Now, we can write the fictive solution equilibrium with a basic polyhedral component MxOy as: and assumming the entropy of this fictive reaction is zero, we can define a SO= parameter as: . The value for the oxide analog of the polyhedral unit is obtained by implementing S0 of the oxide in the S0(MxOy) term. We have also defined, from the same reasoning, similar parameters for heat capacity and volume of the basic polyhedral components: ; . Results and discussion On Figure 1, we have displayed, for entropy, the correlation obtained between calculated values of SO= for the polyhedral unit and for the oxide analog. A straight line and a second-order function are obtained, for the interlayer and octahedral cations, respectively, with a good correlation coefficient. Fig. 1 - Development of predictive capacity for entropy estimates The implementation of the derived semi-empirical, first or second order relations allows to estimate the thermodynamic properties of a clay mineral, MX80 (Na0.409K0.024Ca0.009(Si3.738Al0.262)(Al1.598Mg0.214Fe3+0.173Fe2+0.035)O10(OH)2) in the present case, loaded by 6 radionuclides and to compare the values with the results obtained by Gailhanou et al. (submitted).The results can be expressed in terms of the concentrations for the elements Ni, Cd, Co, Cr, Mn and Pb and in terms of energetic potential with respect to the measurements performed by Gailhanou et al. (submitted)

Bioleaching of silica sand using bioreducing bacteria (Shewanella strains)

Silica sand is generally not pure when mined; often associated with iron hydroxide and oxy-hydroxide impurities which lowers its industrial value and requires purification before use. To join the global community towards environmental consciousness, protection and effective energy utilization, this paper considers microbial purification in a closed non-growth system as an alternative for iron removal. The anaerobic removal of Fe(III)-bearing impurities from the mineral was investigated using small scale microcosm experiments conducted with Fe(III) in silica sand as the sole electron acceptor and lactate as the electron donor in the presence of pure cultures of Shewanella strains (S. putrefaciens CIP8040, S. putrefaciens CN32, S. oneidensis MR-1, S. algae BrY and S. loihica) and anthraquinone 2,6 disulphonate (AQDS) serving as electron shuttling mediator. The reduction of Fe(III) from silica sand was measured as the production of Fe(II) in HCl extracts using modified p-Phenanthroline techniques. Absorbance was measured using UV vis-spectrophotometry at 510nm. Up to 17.6% of the iron bearing impurities (~117mg of bioreducible Fe2O3 per 100g of silica sand) was successfully removed after 15 days. Addition of AQDS as electron transport mediator enhanced the rate and extent of bioreduction by facilitating the exchange of electrons between the iron reducing bacteria and the iron-bearing phase in the mineral. This study revealed that all the selected commercially available Shewanella strains tested were able to reduce and leach iron (III) from Silica sand by coupling oxidation of lactate to anaerobic respiration except Shewanella loihica whereas Shewanella algae BrY was the most efficient.Keywords: Silica sand, iron impurities, iron reducing bacteria, Shewanella strains bioreduction

Hydration thermodynamics of the SWy-1 montmorillonite saturated with alkali and alkaline-earth cations: A predictive model.

International audienceThe aim of the present work was to study the thermodynamic equilibria between water and a homo-ionic montmorillonite SWy-1 saturated by different cations. The choice of this smectite is justified by the large set of experimental data available from the literature for eight different interlayer cations: Na(+), K(+), Rb(+), Cs(+), Mg(2+), Ca(2+), Sr(2+), and Ba(2+). In particular, studies by Cases et al. (1992, 1997) and Berend et al. (1995) are providing heat of adsorption data, pairs of desorption-adsorption isotherms, and information about the partition of adsorption-desorption water molecules between external surfaces and internal spaces. By calculating the effective amount of hydration water as the difference between the so-called gravimetric water and the surface covering water, a thermodynamic model was then developed, based on the concept of Ransom and Helgeson (1994) considering an asymmetric subregular binary solid solution between a fully hydrated and a anhydrous smectite. A set of six thermodynamic parameters (Delta H degrees(hyd), S degrees(hyd) and four Margules parameters) was extracted by a least square method from measurements of enthalpies of adsorption and paired adsorption-desorption isotherms for each interlayer cation. These six initial parameters were then used to determine a complete set of standard thermodynamic hydration parameters (Delta H degrees(hyd), Delta G degrees(hyd), Delta S degrees(hyd), heat capacity, molar volume, and number of interlayer H(2)O) and quantify, for each cation, the number of moles of hydration water molecules as a function of relative humidity and temperature. The validation of the standard state thermodynamic properties of hydration for each end member was carried out using three approaches: (1) a comparison with experimental isotherms obtained on hetero-ionic and homo-ionic SWy-1 smectite at different temperatures; (2) a comparison with the experimental integral enthalpy and entropy of hydration of the SWy-1 smectite; and (3) a comparison with experimental isotherms acquired on various smectites (Upton, MX80, Arizona) with different layer charges. Eventually, the present work demonstrates that, from a limited number of measurements, it is possible to provide the hydration thermodynamic parameters for hydrated smectites with different compositions and under different conditions of temperature and relative humidity, using the newly developed predictive model

A predictive model of thermodynamic entities of hydration for smectites: Application to the formation properties of smectites

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Thermodynamic properties of mixed-layer illite-smectite by calorimetric methods: Acquisition of the enthalpies of mixing of illite and smectite layers

International audienceThe 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, Cp, 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)

In-situ interaction of cement paste and shotcrete with claystones in a deep disposal context

International audienceIn-situ sampling was performed in the Andra Meuse/Haute Marne (France) Underground Research Laboratory (URL) allowing the study of two cement based materials/claystone interfaces that have undergone 4 to 5 years of interaction. The first interface concerned a shotcrete that was sprayed on the wall of an access drift at the laboratory level and the second one, a class G cement paste that was injected in a borehole filled from the surface and was intercepted during the excavation of a new gallery. In the first case, the hydrodynamic conditions were controlled by the ventilation of the drift; while in the second, the cement paste and claystone materials were considered saturated and far from any mechanical perturbation. A multi scale investigation was carried out to identify any evidence of alkaline perturbation in the cement based materials and the claystone. Chemical, mineralogical, and textural measurements were thus performed on the different materials in contact at the nanometer to a centimeter scale. Results showed that all the perturbations resulting from the geochemical contrast between the cement materials and the claystone were limited to a mu m scale on each side of the interfaces. Carbonation was observed in the cement materials leading to an opening or a clogging of the porosity according to the hydrodynamic conditions and the formulation of the cement material. The distribution of the cation exchange population was also modified in the claystone in contact with the cement paste where a potassium saturation of the exchangeable fraction was identified. The originality of the present work is that realistic field controlled samples from the Andra URL were studied, hereby allowing the evaluation of the impact of natural heterogeneities of the in-situ experimental conditions (that is, hydrodynamic conditions, engineered damage zone, mineralogical variations) on the perturbations at the cement paste/claystone interfaces. An important result is that the clogging of porosity was not homogenous along the interfaces

Thermodynamic properties of illite, smectite and beidellite by calorimetric methods: Enthalpies of formation, heat capacities, entropies and Gibbs free energies of formation

International audienceThe thermodynamic properties of three aluminous 2:1 clay minerals were acquired at 1.013 bars and at temperatures between 5 and 500 K using various calorimetric methods. Calorimetric measurements were performed on hydrated and dehydrated <2 mu m clay fractions of smectite MX-80 (Wyoming), illite IMt-2 (Silver Hill) and beidellite SBId-1 (Black Jack Mine). After purification, the mineralogical analyses gave the following structural formulae: Na0.409K0.024Ca0.009(Si3.738Al0.262)(Al1.598Mg0.214 Fe0.1733+Fe0.0352+) O-10(OH)(2,) K0.762Na0.044(Si3.387Al0.613)(Al1.427Mg0.241Fe0.2923+Fe0.0842+)O-10(OH)(2) and Ca0.185K0.104(Si3.574Al0.426)( Al1.812Mg0.09Fe0.1123+)O-10(OH)(2) for smectite MX-80, illite IMt-2 and beidellite SBId-1, respectively. Heat capacities were measured by low temperature adiabatic calorimetry and differential scanning calorimetry, from 5 to 500 K. Standard enthalpies of formation were obtained from solution-reaction calorimetry at 298.15 K. The standard Gibbs free energies of formation of the clay minerals were also calculated, together with the equilibrium constants at 25 degrees C, for anhydrous and hydrated minerals. A comparison between these experimental data and estimated values obtained from prediction models available in the literature, enabled the calculation method that appears to be the most relevant to be selected, at least for aluminous 2:1 clay mineral