Mohamed Azaroual

Solubility of platinum in aqueous solutions at 25°C and pHs 4 to 10 under oxidizing conditions

Platinum has been found to be mobile under supergene conditions, including placers and weathering profiles. To elucidate the nature of Pt mobility in supergene environments, the dissolution of platinum metal was investigated under physicochemical conditions similar to those found in such environments. The solubility of platinum metal was measured at 25°C in several systems: Pt-K-HC8H4O4-H2O (pH 4.02), Pt-Na-HCO3-Cl-H2O (pH 6.40), Pt-Na-K-H2PO4-HPO4-H2O (pH 6.90), Pt-Na-HCO3-H2O (pH 8.30), Pt-Na-OH-H2O (pH 8.54), and Pt-Na-HCO3-H2O (pH 9.91). The redox conditions of these experiments were relatively oxidizing, with measured Eh values ranging from +280 to +590 mV. The ionic strength of the aqueous solutions did not exceed 0.30 (molal scale). The interpretation of the solubility results, in terms of the following reaction and its equilibrium constant, Pt(s)+nH2O left right double arrow Pt(OH)2−nn+nH++2e− served to identify the importance of the hydroxylated complex PtOH+ in the pH range (4 to 10) and to determine its stability constant at 25°C. Linear regression of the solubility data using the function log [Pt]−2pe=n pH+log Kn yielded a value of 1.01 ± 0.07 for n, the average ligand number, and −31.76 ± 0.55 for the thermodynamic equilibrium constant of reaction. The resulting stability constant (β1) of PtOH+ (Pt2+ + OH− left right double arrow PtOH+) is 24.91 ± 0.50, assuming the same value of the free energy of formation of Pt2+, ΔGfo(Pt2+) as that given by Glushko et al. (Thermodynamic Constants of Matter, Academy of Science, USSR, 1972). The range of values of ΔGfo(Pt2+) proposed to date is +185.63 to +258.74 kJ/mol. The value of Glushko et al. (1972) (+244.11 kJ/mol) appears to fit better with our measurements at pH 4 to 10 and with those of Wood (Wood S. A., “Experimental determination of the hydrolysis constants of Pt2+ and Pd2+ at 25°C from the solubility of Pt and Pd in aqueous hydroxide solutions,” Geochim. Cosmochim. Acta 55, 1759–1767, 1991) at pH 9 to 15.5. Finally, according to these new measurements of the solubility of platinum, the recommended values for Gibbs free energy (ΔGfo, in kiloJoule per mole) of the different aqueous species of Pt are +244.11 (Pt2+), −55.96 (PtOH+), and −234.48 (Pt(OH)2(aq)). The integration of data from the literature for chloride and sulfate complexes was used to calculate the speciation of platinum in seawater and in solutions with variable chlorinity (0.1, 1, and 3 mol/L NaCl) at 25°C. These calculations showed that in the absence of strong ligands (i.e., S2O32−, CN−), the transport of platinum in supergene environments primarily occurs in the form of PtOH+ (90%) and Pt(OH)2(aq) (9.7%). Chloride complexes (PtCl42− and PtCl3−) account for less than 1% of the dissolved platinum. This study clearly shows that the hydroxylated complexes (PtOH+ and Pt(OH)2(aq)) can play a very important role in controlling platinum transfer mechanisms in surface fluids (soils, placers, weathering profiles, etc.). Because the charged species PtOH+ is largely predominant, the mobility and transfer of platinum can also be affected by adsorption-desorption mechanisms onto oxides and hydroxides.

Thermodynamic properties of solutions in metastable systems under negative or positive pressures

Metastable systems are created when the interface between the atmosphere (in which Patm = 1 bar) and water forms a spherical meniscus either concave toward the air (water filling capillaries, wherein Pwater Patm). Soil water, undergoing negative pressure (“capillary potential”) remains bound to the solid matrix (instead of flowing downward) by the capillary meniscus, concave toward the undersaturated dry atmosphere. The positive counterpart of tensile water in soils is the pressurized water contained in fine droplets suspended in oversaturated humid air, as in clouds. All these systems are anisobaric domains the phases of which have different pressures. Geochemical consequences of such characteristics are assessed here by calculating the consequences of the positive or negative water potential on the equilibrium constants of reactions taking place in stretched or pressurized aqueous solutions. Thermodynamic properties of aqueous species are obtained by using the TH model, used explicitly for positive pressures but extrapolated to negative ones for soil solutions. It appears that soil water dissolves gases, offering an alternative explanation of the observed enrichment of atmospheric noble gases in groundwater and of carbonic gas in the unsaturated zone below the root zone. Water droplets obviously show the opposite behavior, that is, a decreasing dissolutive capability with decreasing droplet size (water pressure increases), inducing some climatic consequences. An application of this approach to the solid-solution equilibria is performed by comparing experimental solubility of amorphous silica in unsaturated media on the one hand, to theoretical calculations taking account of the negative water pressure on the other hand. This comparison outlines the potential complexity of anisobaric situations in nature and the necessity to develop a suitable approach for solid pressure.

A double layer model of the gas bubble/water interface

Zeta potential is a physico-chemical parameter of particular importance to describe sorption of contaminants at the surface of gas bubbles. Nevertheless, the interpretation of electrophoretic mobilities of gas bubbles is complex. This is due to the specific behavior of the gas at interface and to the excess of electrical charge at interface, which is responsible for surface conductivity. We developed a surface complexation model based on the presence of negative surface sites because the balance of accepting and donating hydrogen bonds is broken at interface. By considering protons adsorbed on these sites followed by a diffuse layer, the electrical potential at the head-end of the diffuse layer is computed and considered to be equal to the zeta potential. The predicted zeta potential values are in very good agreement with the experimental data of H(2) bubbles for a broad range of pH and NaCl concentrations. This implies that the shear plane is located at the head-end of the diffuse layer, contradicting the assumption of the presence of a stagnant diffuse layer at the gas/water interface. Our model also successfully predicts the surface tension of air bubbles in a KCl solution.

Influence of surface conductivity on the apparent zeta potential of TiO2 nanoparticles: Application to the modeling of their aggregation kinetics

Titanium dioxide nanoparticles (TiO2 NPs) are extensively used in consumer products. The release of these NPs into aquatic environments raises the question of their possible risks to the environment and human health. The magnitude of the threat may depend on whether TiO2 NPs are aggregated or dispersed. Currently, limited information is available on this subject. A new approach based on DLVO theory is proposed to describe aggregation kinetics of TiO2 NPs in aqueous dispersions. It has the advantage of using zeta potentials directly calculated by an electrostatic surface complexation model whose parameters are calibrated by ab initio calculations, crystallographic studies, potentiometric titration and electrophoretic mobility experiments. Indeed, the conversion of electrophoretic mobility measurements into zeta potentials is very complex for metal oxide nanoparticles. This is due to their very high surface electrical conductivity associated with the electromigration of counter and co-ions in their electrical double layer. Our model has only three adjustable parameters (the minimum separation distance between NPs, the Hamaker constant, and the effective interaction radius of the particle), and predicts very well the stability ratios of TiO2 NPs measured at different pH values and over a broad range of ionic strengths (KCl aqueous solution). We found an effective interaction radius that is significantly smaller than the radius of the aggregate and corresponds to the radius of surface crystallites or small clusters of surface crystallites formed during synthesis of primary particles. Our results confirm that DLVO theory is relevant to predict aggregation kinetics of TiO2 NPs if the double layer interaction energy is estimated accurately.

Long term predictions of CO2 storage by mineral and solubility trapping in the Weyburn Midale reservoir

Publisher Summary The chapter uses geochemical modeling to interpret the potential of long term storage of carbon dioxide in the Weyburn Midale reservoir, and to interpret if geochemically reactive zones exist around the reservoir. Although there are differences between the various flow units within the reservoir, the net long-term reactions are similar in all of them. Up to approximately 10 years, the precipitation of calcite and kaolinite, and the dissolution of anhydrite and various silicate minerals are predicted. In close cooperation with the International Energy Agency, an international multi-disciplinary research initiative to study the short and long-term potential of geological storage of CO 2 in a carbonate reservoir has been established. Water-rock reactions are critical to the short- and long-term storage of injected CO 2 and the quantification of CO 2 storage via such reactions is an important piece of the research puzzle.

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.

In-pore tensile stress by drying-induced capillary bridges inside porous materials

We present here some evidences that capillary liquid bridges are able to deform micrometric cylindrical pores by tensile stress. Brine-soaked filter membranes are submitted to drying conditions leading to NaCl precipitation inside the 5-10 μm pores. A close examination demonstrated that two forms of NaCl crystallites are successively generated. First, primary cubic crystals grow, driven by the permanent evaporation. When this angular primary solid gets near the pore wall, while the evaporation makes the pore volume to be partly invaded by air, capillary liquid can bridge the now-small gap between the halite angles and the pore wall. In a second step, these small capillary bridges are frozen by a secondary precipitation event of concave-shaped NaCl. The proposed interpretation is that the liquid capillary bridges deform the host matrix of the membrane, and the situation is fossilized by the growth of solid capillary bridges. A quantitative interpretation is proposed and the consequences towards the natural media outlined.

Corundum solubility and aluminum speciation in KOH aqueous solutions at 400°C from 0.5 to 2.0 kbar

Abstract Corundum solubility has been measured in KOH aqueous solutions under supercritical conditions (400°C, from 0.5 to 2 kbar). The investigated concentration range (0.001 KOH 2 O) allowed to vary the species balance between diluted solutions dominated by aluminate ion, Al(OH) 4 − , and more concentrated ones in which association of aluminate ion with K + ion is appreciable. The dissociation constant of the complex KAl(OH) 4 0 and the equilibrium constant for hydrolysis of corundum have been simultaneously fitted to the measured solubilities through a speciation calculation. The corundum hydrolysis (1/2 Al 2 O 3 + 5/2 H 2 O ⇔ Al(OH) 4 − + H + ) equilibrium constant is in close agreement with the values calculated from thermodynamical data for corundum (Berman, 1988) and aqueous species (Johnson et al., 1992). Using the Density Model (Anderson et al., 1991) for extrapolating thermodynamical properties of aqueous species results in appreciably higher values for this equilibrium constant, not consistent with experimental data, due to the inadequacy of the Density Model for extrapolating water properties in the supercritical P-T range. For the KA1 (OH) 4 0 dissociation reaction, the following values of log K: −1.94 ± 0.17, −1.73 ± 0.14, and −1.56 ± 0.11, are obtained at 0.5, 1.0, and 2.0 kbar, respectively. These constants correspond to equal contributions of the aluminate ion and of the KAl(OH) 4 0 complex to the total dissolved A1 in 0.1 m KOH solutions. At 600 and 700°C, 2 kbar, the same equilibrium constants calculated from literature experimental corundum solubilities are −2.43 and −3.16, respectively. From the calculated aluminate ion concentration in the system Al 2 O 3 H 2 O and the literature values of corundum solubility in water, the neutral hydroxyde Al(OH) 3 is inferred to be probably the dominant A1 aqueous species in the neutral pH range in this system.

MODELING THE GEOCHEMICAL IMPACT OF AN INJECTION OF CO2 AND ASSOCIATED REACTIVE IMPURITIES INTO A SALINE RESERVOIR

Carbon dioxide storage in deep geological structures is a strategic technology to mitigate climate change and to promote green development. However, despite continuous efforts to develop cost-effective capture processes to clean the CO2\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}