What are the main contributions to the total enthalpy of displacement accompanying the adsorption of some multivalent metals at the silica-electrolyte interface?

The integral molar enthalpies of displacement, Δdplh, accompanying adsorption of Mg(II), Ca(II), Sr(II), Ba(II), Cd(II), Co(II), Zn(II), and Eu(III) cations from aqueous solutions of metal nitrate onto Spherosil XO75LS at 298K were determined based on the combination of liquid flow calorimetry and classical adsorption for two degrees of surface coverage: 0.035µmolm(-2) and 0.08µmolm(-2), in the presence of 0.1molL(-1) sodium nitrate in the aqueous phase at pH 5, 6, and 7. The displacement was shown to be endothermic and quite independent of the chemical specificity of the adsorbing metal. Two enthalpy effects were postulated to contribute mostly to the positive Δdplh values, depending on the experimental pH value: (i) cation dehydration upon adsorption and (ii) deprotonation of surface silanols to create negatively charged SiO(-) sites. Changing proportions among the various adsorbed species, including "free" Eu(3+) or Cd(2+) cations and hydrolyzed Eu(OH)(2+), Eu(OH)2(+) or Cd(OH)(+) cations, were accepted to explain the downward trends in Δdplh with increasing extent of adsorption for Eu(III) and Cd(II).

Driving force for the hydration of the swelling clays: case of montmorillonites saturated with alkaline-earth cations.

Important structural modifications occur in swelling clays upon water adsorption. The multi-scale evolution of the swelling clay structure is usually evidenced by various experimental techniques. However, the driving force behind such phenomena is still not thoroughly understood. It appears strongly dependent on the nature of the interlayer cation. In the case of montmorillonites saturated with alkaline cations, it was inferred that the compensating cation or the layer surface could control the hydration process and thus the opening of the interlayer space, depending on the nature of the interlayer cation. In the present study, emphasis is put on the impact of divalent alkaline-earth cations compensating the layer charge in montmorillonites. Since no experimental technique offers the possibility of directly determining the hydration contributions related to interlayer cations and layer surfaces, an approach based on the combination of electrostatic calculations and immersion data is developed here, as already validated in the case of montmorillonites saturated by alkaline cations. This methodology allows to estimate the hydration energy for divalent interlayer cations and therefore to shed a new light on the driving force for hydration process occurring in montmorillonites saturated with alkaline-earth cations. Firstly, the surface energy values obtained from the electrostatic calculations based on the Electronegativity Equalization Method vary from 450 mJ m(-2) for Mg-montmorillonite to 1100 mJ m(-2) for Ba-montmorillonite. Secondly, considering both the hydration energy for cations and layer surfaces, the driving force for the hydration of alkaline-earth saturated montmorillonites can be attributed to the interlayer cation in the case of Mg-, Ca-, Sr-montmorillonites and to the interlayer surface in the case of Ba-montmorillonites. These results explain the differences in behaviour upon water adsorption as a function of the nature of the interlayer cation, thereby allowing the macroscopic swelling trends to be better understood. The knowledge of hydration processes occurring in homoionic montmorillonites saturated with both the alkaline and the alkaline-earth cations may be of great importance to explain the behaviour of natural clay samples where mixtures of the two types of interlayer cation are present and also provides valuable information on the cation exchange occurring in the swelling clays.

Bulk hydrolysis and solid-liquid sorption of heavy metals in multi-component aqueous suspensions containing porous inorganic solids: are these mechanisms competitive or cooperative?

Fundamental aspects of the removal of heavy metals from aqueous streams under conditions of competition among the various species have been studied between pH 3 and 9 on Spherosil XO75LS, ordered mesoporous MCM-41 and MCF silicas, as well as a MCF sample grafted with (3-aminopropyl)methoxydimethylsilane (AMPS-MCF). Cd(II), Co(II), Pb(II), or Sr(II) nitrate solutions were used to determine the percentage of metal uptake by each solid at 298 K as a function of the pH of the equilibrium solution, at an initial metal concentration of 0.0001 mol L(-1) and the ionic strength being fixed with 0.01 mol L(-1) NaNO(3). Almost complete retention of the heavy metals on the four solid samples was observed, with the process beginning at pH values smaller than those marking the onset of "bulk" precipitation of a given metal in "free" solution. The heavy metal-uptake mechanism was regarded as hydrolysis-like phenomenon in metal-containing solid suspensions. Weak adsorption of metal species from slightly acidic and neutral solutions was a kind of nucleation step. Adding cadmium to an equimolar solution containing cobalt, lead, or strontium showed no significant effect on the retention of the main metal component. This indicated the great independence of the retention mechanisms.

Experimental approach of the relation between surface tension and interfacial thickness of simple liquids.

It is possible to calculate the thickness of liquid-vapor interfaces starting from ellipsometry data. Some data are available since a long time but not sufficiently known. Using these data, a basic trend showing that the interface thickness increases when the surface tension decreases is shown. This trend is in agreement with which is generally expected from approach of the critical point considerations. Such a trend extended to the solid-liquid interface could be used to interpret some phenomena at the solid-liquid interface, where the thickness of the interface does not seem to vary, when considering only the liquid part of the interface. It implies that the number of molecular layers of the solid participating to adsorption should be different for various systems.

Hydration sequence of swelling clays: evolutions of specific surface area and hydration energy.

In order to identify the key steps and the driving force for the hydration process of swelling clays, the water adsorption isotherms and enthalpies were measured on monoionic montmorillonite samples saturated with alkali or calcium ions, and on bi-ionic samples saturated with a sodium-calcium mixture. The specific surface area evolution along the hydration process was determined using a recent interpretation of the experimental adsorption isotherms of swelling solids. Results are interpreted in structural terms. Compared with additional data from sample-controlled thermal analysis (SCTA), the results confirm experimentally that the hydration of Li- and Na-montmorillonite is mainly a cation-controlled process, in contrast with the hydration of Cs samples in which the cation contribution to hydration is negligible, as we have already demonstrated using electrostatic calculations or conductivity measurements.

Surface energy of talc and chlorite: Comparison between electronegativity calculation and immersion results.

The surface energies of talc and chlorite is computed using a simple model, which uses the calculation of the electrostatic energy of the crystal. It is necessary to calculate the atomic charges. We have chosen to follow Henry's model of determination of partial charges using scales of electronegativity and hardness. The results are in correct agreement with a determination of the surface energy obtained from an analysis of the heat of immersion data. Both results indicate that the surface energy of talc is lower than the surface energy of chlorite, in agreement with observed behavior of wettability. The influence of Al and Fe on this phenomenon is discussed. Surface energy of this type of solids seems to depend more strongly on the geometry of the crystal than on the type of atoms pointing out of the surface; i.e., the surface energy depends more on the physics of the system than on its chemistry.

Determination of the surface energy of kaolinite and serpentine using PACHA formalism-comparison with immersion experiments.

Clays play an important role in a wide variety of industrial processes. Indeed, they present interesting surface properties. For these reasons, we study the surface energy of models of clays by solid state calculations using electronegativities equalization. In this article, we focus on kaolinite and serpentine, two clays characterized by a simple structure of the TO type. We describe the clays and their structures and we develop a simple model from solid state calculations used to determine the surface energy. The results are in agreement with a recent interpretation of the immersion of kaolinite in water. This article must be related to some others focusing on solid surface energy, especially some treating talc and chlorites, and montmorillonites saturated by alkaline cations, to be published.