Christophe Tournassat

Nanomorphology of montmorillonite particles: Estimation of the clay edge sorption site density by low-pressure gas adsorption and AFM observations

Abstract Dry and in situ (fluid-cell) Atomic Force Microscopy (AFM) and Low-Pressure Gas Adsorption experiments were used to investigate the surfaces of pure Na-smectite particles. These two techniques permit the identification of different surfaces of the platelets (lateral, basal, and interlayer surfaces) and to quantify their surface area. Calculation of the surface area was done for AFM, by measuring directly the dimensions of the clay particles on AFM images, and for gas adsorption experiments, by applying the Derivative Isotherm Summation (DIS) procedure designed by Villiéras et al. (Villiéras et al. 1992, 1997a, 1997b). In the present study, we find a discrepancy between measurements of the basal and interlayer surface area. This difference is due to the stacking of platelets in dry conditions compared to their dispersion in aqueous suspension. A particle is estimated to be formed of nearly 20 stacked layers in the dehydrated state used in the gas adsorption experiment, whereas it is estimated to be composed of only 1 or 2 layers in aqueous suspension, on the basis of AFM measurements. However, the two techniques give similar results for the lateral surface area of the platelets (i.e., about 8 m2/g) and the perimeter to area ratio value of the particles because the stacking of platelets does not alter these values. This correlation confirms the effectiveness of the interpretation of the gas adsorption experiments lowest pressure domains as the adsorption on lateral surfaces. The lateral surface area has important implications in the calculation of specific sorption site density on clay material. The relevance of the lateral surface area value (8 m2/g) was tested subsequently with sorption data found in the literature. Based on those results, we show that one essential parameter for the calculation of particle edge-site density is the mean perimeter to area ratio value. This parameter can be obtained by microscopic techniques but the measurement is tedious. The good correlation between the AFM results and the DIS-method results confirms that the latter procedure offers a quick and reliable alternative method for the measurement of the lateral surface area. AFM experiments can be further conducted to constrain the dispersion around the DIS value and the anisotropy of suspended particles.

Comparison of molecular dynamics simulations with triple layer and modified Gouy–Chapman models in a 0.1 M NaCl–montmorillonite system

Molecular dynamics (MD) simulations of a montmorillonite/water interface at the pore scale were carried out at 0.1molL(-1) NaCl concentration in order to constrain cation, anion, and water distribution and mobility influenced by the mineral surface. MD results enabled anion exclusion and cation condensation at the surface to be quantified. MD-derived values could then be compared with macroscopic model results obtained from the Modified Gouy-Chapman (MGC) theory. While the Na concentration profile is well reproduced in the diffuse layer, anion exclusion is overestimated by the MGC theory under our experimental conditions. We also showed that MD simulations can be used to constrain Basic Stern model parameters or, in combination with zeta potential measurements, can be used to constrain triple layer model (TLM) parameters by providing suitable values for the capacitance values. Na sorption intrinsic equilibrium constant values for clay basal surfaces are given accordingly.

Influence of surface conductivity on the apparent zeta potential of TiO2 nanoparticles

Zeta potential is a physico-chemical parameter of particular importance in describing ion adsorption and electrostatic interactions between charged particles. Nevertheless, this fundamental parameter is ill-constrained, because its experimental interpretation is complex, particularly for very small and charged TiO(2) nanoparticles. The excess of electrical charge at the interface is responsible for surface conductance, which can significantly lower the electrophoretic measurements, and hence the apparent zeta potential. Consequently, the intrinsic zeta potential can have a larger amplitude, even in the case of simple 1:1 electrolytes like NaCl and KCl. Surface conductance of TiO(2) nanoparticles immersed in a NaCl solution is estimated using a surface complexation model, and this parameter and particle size are incorporated into Henrys model in order to determine a constrained value of the zeta potential from electrophoresis. Interior conductivity of the agglomerates is calculated using a differential self-consistent model. The amplitude of estimated zeta potential is greater than that derived from the von Smoluchowski equation and corresponds to the electric potential at the outer Helmholtz plane calculated by our surface complexation model. Consequently, the shear plane may be located close to the OHP, contradicting the assumption of the presence of a stagnant diffuse layer at the TiO(2)/water interface.

Modeling specific pH dependent sorption of divalent metals on montmorillonite surfaces. A review of pitfalls, recent achievements and current challenges

Within the context of the clay barrier concept for underground nuclear waste disposal, montmorillonite and bentonite have been widely used as reference materials for sorption. In some cases, accompanying modeling work aims at understanding and predicting sorption in complex natural systems where clays are assumed to be representative of the most reactive phases. This bottom-up approach relies heavily on good confidence in the mechanistic understanding of sorption phenomena. The present study aims at reviewing experimental and modeling work on montmorillonite with a focus on divalent metals experiencing pH dependent specific sorption. Current knowledge points out distinct sorption mechanisms on three types of sites: cation exchange on basal planes and surface complexation on edge surfaces with two types of sites: high energy (or strong) sites (HES) with high affinity for metals but low site density and low energy (or weak) sites (LES) with lower affinity for metals but high site density. Based on this current knowledge, criteria are given to select data relevant for surface complexation model calibration (especially ionic strength, pH, clay preparation and characterization, metal to clay ratio and solubility limits), with an emphasis on data uncertainties and reproducibility. Problematic experimental features are highlighted, especially those related to the reversibility of sorption and to the effect of the solid to liquid ratio (RSL) on sorption distribution coefficients. Guidelines for data acquisition and selection are proposed. Surface complexation models available in the literature are then tested in terms of efficiency (data fit) and mechanistic likelihood. None of the currently available models is able to satisfy both aspects. Models directly adapted from oxide surface complexation models fail in both aspects. The most efficient model (in terms of simplicity and accuracy) is a non-electrostatic model. It is the only one that reproduces pH dependent specific sorption data at a low metal clay ratio (<0.001 mol/kgclay; HES) in all selected experimental conditions, as well as data obtained at medium metal to clay ratio (∼0.01-0.05 mol/kgclay; low energy sites). To account for physical mechanisms, an electrostatic surface complexation model has been developed. It takes into account the spill-over effect of negatively charged basal surfaces over edge surfaces, a typical feature of montmorillonite, and is able to reproduce sorption data for LES but not for HES. The reasons for this failure are explained through the mathematical derivations of model equations. This approach shows that it is impossible to reconcile HES properties with an oxide-like surface complexation electrostatic model. Amongst other alternatives, a successful electrostatic surface substitution model, which is compatible with current knowledge on HES structural properties, is proposed.

Influence of montmorillonite tactoid size on Na–Ca cation exchange reactions

The spatial organisation of swelling clay platelets in a suspension depends on the chemical composition of the equilibration solution. Individual clay platelets can be well dispersed, with surfaces entirely in contact with the external solution, or be stacked in tactoids, where part of the surfaces forms parallel alignments embedding interlayer water and cations. External and interlayer surfaces do not exhibit similar affinities for cations having different hydration and charge properties and the clay platelet stacking arrangement influences the clay affinity for these cations. This paper aims to establish the link between exchange properties and clay tactoid size and organisation for Na-Ca exchange on montmorillonite. Different montmorillonite samples behave differently with regards to Na-Ca exchange, from ideal to non-ideal exchange behaviour. A simple model coupling the tactoid stacking size to different Na/Ca relative affinities of the external and interlayer clay surfaces enables these differences to be reproduced.

Benchmark reactive transport simulations of a column experiment in compacted bentonite with multispecies diffusion and explicit treatment of electrostatic effects

Bentonite clay is considered as a potential buffer and backfill material in subsurface repositories for high-level nuclear waste. As a result of its low permeability, transport of water and solutes in compacted bentonite is driven primarily by diffusion. Developing models for species transport in bentonite is complicated, because of the interaction of charged species and the negative surface charge of clay mineral surfaces. The effective diffusion coefficient of an ion in bentonite depends on the ion’s polarity and valence, on the ionic strength of the solution, and on the bulk dry density of the bentonite. These dependencies need to be understood and incorporated into models if one wants to predict the effectiveness of bentonite as a barrier to radionuclides in a nuclear repository. In this work, we present a benchmark problem for reactive transport simulators based on a flow-through experiment carried out on a saturated bentonite core. The measured effluent composition shows the complex interplay of species transport in a charged medium in combination with sorption and mineral precipitation/dissolution reactions. The codes compared in this study are PHREEQC, CrunchFlow, FLOTRAN, and MIN3P. The benchmark problem is divided into four component problems of increasing complexity, leading up to the main problem which addresses the effects of advective and diffusive transport of ions through bentonite with explicit treatment of electrostatic effects. All codes show excellent agreement between results provided that the activity model, Debye-Hückel parameters, and thermodynamic data used in the simulations are consistent. A comparison of results using species-specific diffusion and uniform species diffusion reveals that simulated species concentrations in the effluent differ by less than 8 %, and that these differences vanish as the system approaches steady state.

The electrophoretic mobility of montmorillonite. Zeta potential and surface conductivity effects.

Clay minerals have remarkable adsorption properties because of their high specific surface area and surface charge density, which give rise to high electrochemical properties. These electrochemical properties cannot be directly measured, and models must be developed to estimate the electrostatic potential at the vicinity of clay mineral surfaces. In this context, an important model prediction is the zeta potential, which is thought to be representative of the electrostatic potential at the plane of shear. The zeta potential is usually deduced from electrophoretic measurements but for clay minerals, high surface conductivity decreases their mobility, thereby impeding straightforward interpretation of these measurements. By combining a surface complexation, conductivity and electrophoretic mobility model, we were able to reconcile zeta potential predictions with electrophoretic measurements on montmorillonite immersed in NaCl aqueous solutions. The electrochemical properties of the Stern and diffuse layers of the basal surfaces were computed by a triple-layer model. Computed zeta potentials have considerably higher amplitudes than measured zeta potentials calculated with the Smoluchowski equation. Our model successfully reproduced measured electrophoretic mobilities. This confirmed our assumptions that surface conductivity may be responsible for montmorillonites low electrophoretic mobility and that the zeta potential may be located at the beginning of the diffuse layer.

Metal speciation in landfill leachates with a focus on the influence of organic matter

This study characterises the heavy-metal content in leachates collected from eight landfills in France. In order to identify heavy metal occurrence in the different size fractions of leachates, a cascade filtration protocol was applied directly in the field, under a nitrogen gas atmosphere to avoid metal oxidation. The results of analyses performed on the leachates suggest that most of the metals are concentrated in the <30 kDa fraction, while lead, copper and cadmium show an association with larger particles. Initial speciation calculations, without considering metal association with organic matter, suggest that leachate concentrations in lead, copper, nickel and zinc are super-saturated with respect to sulphur phases. Speciation calculations that account for metal complexation with organic matter, considered as fulvic acids based on C1(s) NEXAFS spectroscopy, show that this mechanism is not sufficient to explain such deviation from equilibrium conditions. It is therefore hypothesized that the deviation results also from the influence of biological activity on the kinetics of mineral phase precipitation and dissolution, thus providing a dynamic system. The results of chemical analyses of sampled fluids are compared with speciation calculations and some implications for the assessment of metal mobility and natural attenuation in a context of landfill risk assessment are discussed.

Pb(II) and Zn(II) adsorption onto Na- and Ca-montmorillonites in acetic acid/acetate medium: Experimental approach and geochemical modeling

Smectites are usually used as a clay barrier at the bottom of subsurface waste landfills due to their low permeability and their capacity to retain pollutants. The Na- and Ca-saturated SWy2 montmorillonites were interacted with initial Zn(NO(3))(2) or Pb(NO(3))(2) concentrations ranging from 10(-6) to 10(-2)M with a solid/liquid ratio of 10 g L(-1) and using acetic acid/acetate as buffer at pH 5 in order to reproduce a biodegradable leachate of a young landfill. These experiments revealed that Zn and Pb sorption onto Na-SWy2 is higher than that onto Ca-SWy2 in the whole range of concentrations. Metal retention into both montmorillonites increases with the decrease in acetic acid/acetate concentration. The two-site protolysis model with no electrostatic term (2SPNE model) was used to model these experiments. As the experimental data of Zn sorption were well fitted, this model was validated and has been improved by taking into account the metal-acetate complexation in solution. In order to validate the model for Pb sorption, new selectivity coefficients have been determined, namely logK(c)(PbNa)=0.5 for Na-montmorillonite and logK(c)(PbCa)=0.3 for Ca-montmorillonite.

MOLECULAR DYNAMICS SIMULATIONS OF ANION EXCLUSION IN CLAY INTERLAYER NANOPORES

The aqueous chemistry of water films confined between clay mineral surfaces remains an important unknown in predictions of radioelement migration from radioactive waste repositories. This issue is particularly important in the case of long-lived anionic radioisotopes (129I-, 99TcO4-, 36Cl-) which interact with clay minerals primarily by anion exclusion. For example, models of ion migration in clayey media do not agree as to whether anions are completely or partially excluded from clay interlayer nanopores. In the present study, this key issue was addressed for Cl- using MD simulations for a range of nanopore widths (6 to 15 Å) overlapping the range of average pore widths that exists in engineered clay barriers. The MD simulation results were compared with the predictions of a thermodynamic model (Donnan Equilibrium model) and two pore-scale models based on the Poisson-Boltzmann equation under the assumption that interlayer water behaves as bulk liquid water. The simulations confirmed that anion exclusion from clay interlayers is greater than predicted by the pore-scale models, particularly at the smallest pore size examined. This greater anion exclusion stems from Cl- being more weakly solvated in nano-confined water than it is in bulk liquid water. Anion exclusion predictions based on the Poisson-Boltzmann equation were consistent with the MD simulation results, however, if the predictions included an ion closest approach distance to the clay mineral surface on the order of 2.0 ± 0.8 Å. These findings suggest that clay interlayers approach a state of complete anion exclusion (hence, ideal semi-permeable membrane properties) at a pore width of 4.2 ± 1.5 Å.