Manuel Pelletier

The effects of exchanged cation, compression, heating and hydration on textural properties of bulk bentonite and its corresponding purified montmorillonite

Textural properties of homoionic Na- and Ca-bentonite (MX-80 from Wyoming) were studied by nitrogen adsorption. Bulk samples, which contained 20% of nonclay minerals and purified samples, were taken for the study. Powder samples and compressed pellets, which were prepared by applying a uniaxial stress of 57 MPa, were examined. Presence of nonclay minerals in the bulk samples affects significantly textural properties of the system. In particular, nonclay minerals prevented full compression of the bulk samples. In comparison to Na-saturated purified samples, higher values of surface area and total pore volume were detected in Ca-saturated samples. In addition, amount of the trapped pores was considerably higher in Ca-saturated samples than that in Na-saturated ones. In Ca-saturated samples, values of micropore surface area and micropore volume were very low, or even equal to zero. In Na-saturated samples, on the other hand, considerable percentage of surface area was associated with micropores. Surface area and pore volume of the sample decreased following heating at 200 °C during 24 h, with no difference to the exchanged cation. Following compression, surface areas of Na-saturated samples decreased and those of Ca-saturated samples increased. With no differences to the exchanged cation, considerable reduction in pore volumes was observed for the purified samples following compression. In the case of the compressed nonheated samples, hydration at relative humidity of 100% during 2 days resulted in significant increase of total and micropore surface areas. The increase in surface area was contributed mainly by micropores, i.e., open surface areas of the samples increased only slightly. Behavior of heated Na-saturated sample was similar to that of nonheated one. Hydration capacity of Ca-saturated sample, however, diminished considerably after heating.

Hydration and Dispersion of C60 in Aqueous Systems: The Nature of Water−Fullerene Interactions

The nature of fullerene-water interactions and the role that they play in the fate of C60 in aqueous systems is poorly understood. This work provides spectroscopic evidence for the surface hydroxylation of the initially hydrophobic C60 molecule when immersed in water. This mechanism appears to be the basis for stabilizing the hydrophilic nC60 aggregates in suspension. It is remarkable that such a chemical transformation and dispersion are achieved under mild conditions that are readily produced in an aquatic environment. This acquired affinity for water is likely to play a subsequent role in the reactivity, mobility, and bioavailability of fullerenes in aqueous media.

Fe, Mg and Al distribution in the octahedral sheet of montmorillonites. An infrared study in the OH-bending region

Abstract Ten montmorillonites of different origins with variable Fe contents were analysed using transmission IR spectroscopy. Special attention was devoted to the OH-bending region to obtain information about the distribution of octahedral cations. For low to medium Fe contents (≤ 0.56 per Si8 formula unit), a linear relationship between the position of the δAlFeOH band and Fe content was observed. Such correlation might be explained by changes in the cis-trans occupancy of Fe in the octahedral sheet. Deconvolution of the OH-bending domain allows us to discriminate three components (δAlAlOH, δAlMgOH and δAlFeOH) which are correlated with cation abundances derived from chemical analysis. The relative area of each band can then be compared with theoretical areas calculated assuming a fully random distribution of cations in the octahedral sheet. Using such treatment, eight of the 10 montmorillonites studied presented a nearly randomized octahedral distribution. The two samples from Wyoming were clearly different as they exhibited a strong ordering tendency.

Hydration and swelling of synthetic Na-saponites: Influence of layer charge

Abstract This paper concerns the hydration and swelling behavior of four synthetic sodium saponite samples with general formula Nax(Si4-x,Alx)(Mg3)O10(OH)2, where x = 0.4, 0.5, 0.6, and 0.7. The combination of gravimetric water adsorption measurements and X-ray diffraction experiments under controlled water pressure allows us to analyze the influence of layer charge on swelling mechanisms. When the layer charge increases, swelling occurs for lower values of the chemical potential, i.e., at lower relative pressure. In parallel, as the cohesion between layers increases with increasing layer charge, a greater amount of water is needed to initiate swelling for higher-charged samples. In this last case, due to the proximity of interlayer cations, the transition between the one layer and two-layer hydrate occurs before the sodium cations are surrounded by a complete hydration sphere. This study also shows that the “one-layer” or “two-layer” hydrates cannot be considered as defined states as they appear to be modified with both layer charge and relative pressure.

Morphology and surface heterogeneities in synthetic goethites

In the framework on a study of the acido-basic and sorption properties of iron oxides, a thorough characterization of two types of goethite powders was performed in several laboratories joined in a common project. Chemical analysis by ICPAES; high-resolution SEM, TEM, and AFM observations; XRD with line width analysis; and argon and nitrogen sorption isotherms were used for that purpose. The main crystallographic faces of goethite particles could be identified as {001}, {101}, and {121}, and their abundance correlated with the distribution of low-pressure argon adsorption local isotherms. These results will be very useful for further studies on the relationship between surface reactivity in aqueous solution and orientation of solid surfaces.

Structural–chemical disorder of manganese dioxides

Relationships between lattice parameters of manganese dioxides and their surface properties at the solid-aqueous solution interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered MD samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth) and Tw (rate of microtwinning). Water adsorption isotherms showed that the cross sectional area of water molecules adsorbed in the first monolayer is positively correlated to Pr. Titration of the surface charge of the MD series evidenced a positive linear relationship between the PZC and Pr (Pr=0, Tw=0, PZC=1 for ramsdellite; Pr=1, Tw=0, PZC=7.3 for pyrolusite; gamma-MD with intermediate values of Pr (0.2 to 0.45) have increasing PZC values). The rate of microtwinning appeared as a secondary factor for the increase of the PZC. The above correlations are explained by the chemical defects at the origin of the structural disorder, respectively Mn(3+)/Mn4+ substitution for Pr and Mn vacancies for Tw, which result in proton affinity and thus in increased PZC. The experimental results are compared with data collected in the literature for manganese dioxides as well as for dioxides of transition elements with tetragonal structure.

Affinity of C60 Fullerenes with Water

Abstract In this work, we have studied the affinity of pure C60 fullerenes with water. Although C60 have always been known as hydrophobic particles, we show that this characteristic does not remain when contact between C60 and H2O is forced by mechanical stirring. Light scattering measurements and transmission electron microscopy observations revealed the presence of C60 aggregates remaining stable in water after stirring, with a large nanometric size range. Water adsorption/desorption isotherms, confirmed the hydrophilic character of these nano‐clusters, resulting form hydration of the initial hydrophobic C60.

Influence of layer charge on the hydroxyl stretching of trioctahedral clay minerals: A vibrational study of synthetic Na- and K-saponites

Abstract To understand the behavior of octahedral hydroxyl-stretching modes (νOH) as a function of increasing tetrahedral layer charge, infrared and Raman spectroscopic studies were carried out on a series of synthetic saponites. The nature of the monovalent interlayer cation (Na or K) and the hydration of the samples were shown to strongly influence spectral features. In the case of hydrated Nasamples, the νOH position shifts from 3678 to 3686 cm-1 with increasing layer charge. For hydrated K-saponites, two components are observed around 3720 and 3680 cm-1, assigned to OH perturbed by compensating cation and free OH groups, respectively. In the case of dehydrated samples both K and Na-saponites exhibit two components at 3677 and 3720 cm-1. A linear relationship between the relative area of these two νOH components and the layer charge was then evidenced. To assign the features obtained for hydrated Na-samples in the OH-stretching modes domain, the Raman spectra were decomposed using four νOH components around 3675, 3680, 3685, and 3690 cm-1. The evolution with layer charge of the relative proportions of these four components can be described by taking into account the location of hydrated cations, related to the distribution of tetrahedral substitutions. Such an assumption was confirmed using a simple statistical model that yields four populations of OH groups in the octahedral layer of hydrated Na-saponites.

Berthierine-like mineral formation and stability during the interaction of kaolinite with metallic iron at 90 °C under anoxic and oxic conditions

Abstract The interaction between metallic iron and kaolinite was studied in conditions relevant to those that may be encountered in a high-level radioactive waste disposal facility in geological formation. Experiments were carried out under anoxic atmosphere at 90 °C and in chloride solutions to simulate conditions close to disposal facilities. KGa-2 kaolinite was put in contact with powdered metallic iron in batch experiments for durations of 1, 3, and 9 months. Solutions extracted from the end-products were analyzed (pH, Eh, conductivity, and cation concentrations). End-products were characterized by a set of chemical (oxide analyses, CEC, EDXS) and mineralogical techniques (SEM, TEM, XRD, and FTIR), textural analyses (nitrogen adsorption and low-pressure argon adsorption), XPS, and Mössbauer spectroscopy. In another set of experiments the system was changed from anoxic to oxic conditions to evaluate the stability of the system in the presence of O2. The interaction between metallic iron and kaolinite led to a fast initial reaction as major modifications took place during the first month. The partial oxidation of metallic iron resulted in a pH increase and negative Eh values. Iron was not found in solution but in two new Fe-rich phases: magnetite in very low amounts and a Fe-rich clay phase, belonging to the berthierine family. The Si and Al of the berthierine are derived from the partial alkaline dissolution of kaolinite, mostly along edge faces. TEM-EDXS local analyses showed that the composition of resulting particles consisted in mixtures of berthierine and kaolinite layers. Clay particles became thicker with the epitaxial growth of berthierine layers on the basal surfaces of pristine kaolinite. Neoformed berthierine was not stable in the presence of O2 at 90 °C. Berthierine layers dissolved, iron was mobilized to form iron oxides and oxyhydroxides while kaolinite layers recrystallized from released Al and Si.

Structural–chemical disorder of manganese dioxides: II. Influence on textural properties

Relationships between structural parameters of MnO2 and their surface properties at the solid-gas interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered gamma-MnO2 samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth in the ramsdellite network) and Tw (rate of microtwinning). Analysis of the N2 adsorption isotherm evidenced positive correlations between specific surface area and Tw for gamma-MnO2 only and between the energetic constant C and (1-Pr). No microporosity is evidenced. Water adsorption isotherms evidenced the dependence of the H2O monolayer volume on Tw and showed a positive correlation between the cross-section area of water molecules adsorbed in the first monolayer and Pr, ranging from 13.5 A2 for Pr=1 to 6.3 A2 for Pr=0.2 (12 sites/nm2). Energetic heterogeneity is quantified from Ar and N2 low-pressure adsorption isotherms with the DIS procedure and correlated with H2O adsorption. High-energy adsorption domains are quantified and assigned to the different crystal faces: (110) faces with a common 1 x 1 octahedra layer of pyrolusite and ramsdellite and the (001) face of ramsdellite with 2 x 2 octahedra on which channels and plateaus are differentiated. The specific surface area ratio of ramsdellite high-energy sites to total ramsdellite content is shown to depend on Tw. The dependence on microtwinning of low cross-sectional area of N2 and much lower cross-sectional of residual H2O molecules leads us to assume that their adsorption sites on grain boundaries are represented by the twin planes between the structured nanocrystals generated by oxygen evolution during MD synthesis.