Frédéric Villiéras

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.

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.

Experimental synthesis of chlorite from smectite at 300°C in the presence of metallic Fe

Abstract The alteration and transformation behaviour of montmorillonite (bentonite from Wyoming, MX-80) in low-salinity solutions (NaCl, CaCl2) in the presence of metallic Fe (powder and 8 × 4 × 1 mm plate) and magnetite powder was studied in batch experiments at 300°C to simulate the mineralogical and chemical reactions of clays in contact with steel in a nuclear waste repository. The evolutions of pH and solution concentrations were measured over a period of 9 months. The mineralogical and chemical evolution of the clays was studied by XRD, SEM, Transmission Mossbauer Spectroscopy and TEM (EDS, HR imaging and EELS). Dissolution of the di-octahedral smectite of the starting bentonite was observed, in favour of newly formed clays (chlorite and saponite), quartz, feldspars and zeolite. The formation of Fe-chlorite was triggered by contact with the metallic Fe plate and Fe-Mg-chlorite at distance from the Fe plate (>2 mm).

MIGRATION OF CATIONS IN COPPER(II)-EXCHANGED MONTMORILLONITE AND LAPONITE UPON HEATING

Two clay minerals, a dioctahedral, Na-montmorillonite from Wyoming and a trioctahedral, synthetic Na-laponite, were exchanged by cupric (Cu(II)) ions and subsequently heated at 100 °C intervals up to 500 °C. The resulting materials were analyzed by chemical analysis, X-ray diffraction (XRD), cation exchange capacity (CEC) measurements, combined thermogravimetric and differential thermal analysis (TGA-DTA), infrared (IR) spectroscopy, electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS). Montmorillonite exhibits a well-known Hoffmann-Klemen effect in that, when heated, cupric (Cu) ions migrate into the lacunae of the octahedral sheet, where they compensate the negative charge deficit of the clay layer. In the case of laponite, CEC measurements and spectroscopic measurements reveal that Cu ions migrate into the octahedral sheet where they replace Li and Mg ions. After heating at 200 °C, approximately half the interlayer Cu ions are exchanged. The exchange appears to be 1 Cu for 1 Li, resulting in a slight decrease of the negative charge of the layer. After heating at 300 °C, the remaining Cu ions are exchanged by either 1 Mg or 2 Li, which does not result in any further charge reduction. At 400 °C, some of the extracted Mg remigrates into the structure and exchanges some Li (1 Mg for 2 Li). The final product at 400 or 500 °C is then a Li-laponite with Cu(II) in the octahedral sheet.

Experimental study of the transformation of smectite at 80 and 300°C in the presence of Fe oxides

Abstract The alteration and transformation behaviour of montmorillonite (Wyoming bentonite) was studied experimentally to simulate the mineralogical and chemical reaction of clays in contact with steel in a nuclear waste repository. Batch experiments were conducted at 80 and 300°C, in low-salinity solutions (NaCl, CaCl2) and in the presence or otherwise of magnetite and hematite, over a period of 9 months. The mineralogical and chemical evolution of the clays was studied by XRD, SEM, transmission Mössbauer spectroscopy and EDS-TEM. Experimental solutions were characterized by ICP-AES and ICP-MS. The main results are that no significant change in the crystal chemistry of the montmorillonite occurred at 80°C, while at 300°C, the presence of Fe oxides leads to a partial replacement of montmorillonite by high-charge trioctahedral Fe2+-rich smectite (saponite-like) together with the formation of feldspars, quartz and zeolites.

Separation of hydrocarbons and lipid from water using treated bark

This paper explores the possibility of using treated bark to remove oily compounds from water. Bark was first biologically or chemically treated and saturated with transition metal ions (TMI) to avoid the release of soluble organic compounds from the bark in the treated effluents. Several experimental parameters affecting the oil removal efficiency (RE) were studied (initial oil concentration, temperature, time, etc.). Saturated bark was characterized using Fourier transform infrared (FTIR) spectroscopy and bark wetting index was determined. Results of the retention of lipids suggested that their removal could exceed 95% of initial oil concentration. The uptake of lipid by treated bark varied from 0.2 to 2.0 g of organic oil/g of dry sorbent. No significant chemical modifications of saturated bark were observed in infrared spectroscopy after the sorption of oleic acid on bark treated with transition metal ions. The structure of adsorbed tridimensional layer of oleic acid molecules seemed to take place through the double bond. The hydrocarbon RE exceeded 95% using oil-water mixture with a hydrocarbon/bark ratio of one. The sorption reaction of hydrocarbons and lipids was quasi-instantaneous and seemed to be influenced by the temperature. This indicated that the retention mechanism was related to the capillary action. Results of FTIR spectroscopy suggested that no chemical bonds between barks and oily compounds were established.

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.

Assessment of surface energetic heterogeneity of synthetic Na-saponites. The role of layer charge

Abstract High-resolution gas adsorption techniques were used to analyse the evolution of the aspect ratio and adsorption energy distribution on synthetic saponite samples with increasing layer charge. Using Ar as a gaseous probe, the aspect ratio of the saponite particles can be determined easily by decomposing the derivative adsorption isotherms and taking into account high-energy sites which can be assigned to talc-like ditrigonal cavities. Changes in the shape of the elementary particles are observed for layer charges above 1.30, i.e. when all the ditrigonal cavities contain at least one Al atom substituting for Si. When N2 is used as a probe, high-energy sites that could be wrongly interpreted as micropores on the basis of classical t-plot treatments are observed whatever the layer charge. Using the information obtained from both Ar and N2, schemes for describing adsorption can be proposed for all layer charges and suggest complex adsorption mechanisms for charged clay minerals.

Structural–chemical disorder of manganese dioxides: 1. Influence on surface properties at the solid–electrolyte interface

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.