Fractal nature of bentonite - water - NaCl gel systems evidenced by viscoelactic properties and model of gels

International audienceThe effect of ionic strength on the viscoelastic behaviour of bentonite dispersion has been investigated and quantified by the elastic modulus (G′) to insure the fractal nature of the bentonite-water-NaCl gel systems. Whatever the ionic strength of the medium, this rheological parameter followed a scaling law as a function of the bentonite volume fraction. Shih's fractal model (strong-link regime) was used to better fit the experimental data in coagulated and non-coagulated gels. A good agreement between the experimental results at different ionic strength and calculated data was obtained. The fractal dimension (D) varied with ionic strength and had a minimum value at maximum coagulation which is in agreement with the fractal nature of the bentonite gels

Influence of clay addition on the properties of olive oil in water emulsions

International audienceThe behaviour of olive oil-in-water emulsions (O/W) was studied in the presence of smectite particles. The distribution of these particles in the emulsions and the effect of their interaction with the surfactant on the stability of the emulsions were investigated. Whereas the variation of surfactant and/or clay content did not seem to affect the nature of the emulsion, it had a significant influence on emulsion stability. This observation led to two main assumptions on the distribution of clay particles within the emulsion, either exclusively in the continuous phase, or also at the oil-water interface. In the absence of clay, the variation of surfactant concentration (from 0 to 17% (w/w)) allowed to distinguish 3 domains. In the first concentration domain (0-4.2%) emulsions stability increased with the content of surfactant. Within the second domain (4.2-8.5% (w/w)), a slight decrease in stability was observed due to flocculation by depletion. For surfactant concentrations equal or higher than 8.5% (3rd domain), emulsion stability increased sharply, probably due to the development of interactions between surfactant molecules as they came closer to their gelling concentration. The effect of clay addition to the aqueous phase (up to 10% (w/w)) on the physicochemical properties of the studied emulsions was assessed from stability, drop size, interfacial tension, rheological and acido-basic analyses. A model based on the location of clay particles either in the bulk or at the interface could be proposed, in which clay particles would interact with surfactant molecules in the bulk phase, and form a mechanical barrier around the oil droplets, thus increasing emulsion stability

Extraction of cadmium (II) from phosphoric acid media using the di(2-ethylhexyl)dithiophosphoric acid (D2EHDTPA): feasibility of a continuous extraction-stripping process

International audienceThe extraction of cadmium from phosphoric media has been studied. The D2EHDTPA was used as extractant and dodecane as diluent. No third phase was observed in the investigated conditions. A continuous micro-pilot scale mixer-settler was successfully tested for both extraction and stripping. More than 99% extraction rate was obtained in steady-state conditions with a flow rate ratio Aqueous/Organic equal to 1.1. Continuous stripping was performed using HCl 4 M. More than 96% of the cadmium was stripped in one continuous mixer-settler stage for flow rate ratio equal to 0.7. Results were in good agreement with the predicted values based on the McCabe-Thiele method. Experimental mixer-settler stages behave as ideal ones (Murphree efficiency > 98%). An optimal flow sheet is proposed to purify the Wet Phosphoric Acid (WPA) and to recover a relatively concentrated cadmium solution (1 g L− 1). Two ideal stages operating at phase ratio A/S equal to 5/1 are required for the extraction step leading to a very depleted raffinate (< 0.2 µg L− 1). For the stripping step, six stages are required (S/A = 5/1). The recovered organic phase contains less than 2 µg L− 1 and could be recycled in the extraction step

Extraction of cadmium from phosphoric acid media by Di(2-Ethylhexyl) dithiophosphoric acid

International audienceDi(2‐ethylhexyl) dithiophosphoric acid (D2EHDTPA) was examined for cadmium removal from phosphoric acid solutions. High extraction performance was achieved. By means of slope and saturation methods, the stoichiometry of the complex was postulated. Both methods indicated the presence of anhydrous CdR2 as being the metallic extracted complex, HR denoting the D2EHDTPA molecule. The stripping of cadmium from the organic phase was ensured by aqueous solutions of HCl; quantitative stripping was achieved with either HCl (4 M) or an HCl‐NaCl mixture

Physico-Chemical Characterizations of Tunisian Organophilic Bentonites

International audienceOrganoclays were prepared from two tunisian purified Na-bentonites using benzyltetradecyldimethylammonium chloride (C14) and benzyldodecyldimethylammonium chloride (C12). The added quantities of organic salts, expressed as a function of the clay's cationic exchange capacity (CEC), have been varied from 0.3 to 4 CEC. Two different methods were investigated in order to determine the influence of the preparation method on the adsorption properties:  on one hand, dry powder clay (method I) and, on the other hand, aqueous clay suspensions (method II) were mixed with the organic salt solutions. The adsorption of organic cations on clays was studied by adsorption isotherms, FTIR spectroscopy and X-ray diffraction. It appears that more salts are adsorbed in the clay interlayer space when using method II. The arrangement of salts within clay is rather complicated. It depends on clay composition, nature of tensioactive molecules, CEC of the clay, and preparation method. According to these parameters, the inserted surfactants can be arranged in monolayer, paraffin, or admicelles structures