Ca Montmorillonite Research Articles

New sorbing grouts for radioactive and toxic heavy metals

According to European regulations, toxic industrial wastes (classified as class 1) should be stored in an impervious medium. Technically, the impervious medium could be either a natural clay layer, 5 m thick, with a hydraulic conductivity lower than 10 −9 m s −1, or an artificial watertight barrier around and/or under the disposal site. The purpose of these barriers is to prevent the seepage of polluted water from the waste into the environment. The hydraulic grouts named “Petrisol” (used in the case of radioactive wastes) and “Ecosol” (for toxic industrial wastes) presented here are intended to be used as a physicochemical barrier to reduce the pollution of the groundwater around neglected waste deposits. These grouts are made of standard bentonite and cement with the addition of complementary agents as follows: siliceous materials (silica fumes or fly ash), special clays or zeolites, and chemical additives. Their specific characteristic is their capacity to retain the ions of polluting substances found in solution in groundwater. The aim of our research has been to quantify the efficiency of the grouts and to compare their retention properties with grouts conventionally used in public works: mixes of bentonite—cement in defined proportions. We chose out of the most dangerous and frequently occurring elements: lead (Pb 2+), cadmium (Cd 2+), and mercury (Hg 2+). For the radioactive wastes, we chose two elements well known for their great mobility: caesium (Cs +) and strontium (Sr 2+). In the first instance, we were interested in the retention of the different cations by raw materials (clays=Ca montmorillonite, Na bentonite, vermiculite, palygorskite, illite; and zeolithes=chabazite and an artificial zeolite) in determining the maximum capacity of the fixing of the cation by a material using the method of differences or an adsorption test. The values we have obtained vary with a large number of parameters: the contact time with solid solution, the temperature of fixing, the concentration of cations in the initial solution, the nature of the cations, the nature of the absorbing materials, the pH, and so on. From this experimental study of the fixation of the polluting cations passing through hydraulic grouts, after a curing time of 7 to 90 days, we are able to class the cations in the conditions of the previous experiment in order of decreasing affinity with respect to materials and grouts studied: Cd 2+ >Cd 2+ >Pb 2+ >Sr 2+ >Cs + >Hg 2+. Regarding the materials, we established a similar classification in order of decreasing fixing capacity: artificial zeolite>Na bentonite>montmorillonite>vermiculite>illite>chabazite>palygorskite. In the second instance, we considered the hardened grouts and we compared normal percolation of cationic solutions and distilled water, under perfectly defined experimental conditions (notably pressure). Thus, we are able to calculate the quantity of fixed cations by a grout of

Sorption and Desorption of Acetonitrile on Montmorillonite from Aqueous Solutions

The sorption and desorption of acetonitrile on K, Na, Ca, and Mg montmorillonite from aqueous solutions were determined using a 14C isotope tracer method. The heat of sorption of acetonitrile on K, Na, and Ca montmorillonite at two concentrations was determined using a Calvet microcalorimeter. The infrared spectra of the clay-water-organic systems were recorded on a Fourier-transform infrared spectrometer and the c-axis spacings of K, Ca, and Mg montmorillonite in the acetonitrile solutions were determined by x-ray diffraction. It was found that the sorption and desorption of acetonitrile on the four homoionic montmorillonites are reversible, and that the sorption of acetonitrile is exothermic on K and Ca montmorillonite, but endothermic on Na montmorillonite. Further, it was found that the wavenumbers of the C≡N band and the C-H band for the acetonitrile molecules on the clay surfaces do not differ from those in the solution. It was also observed that the sorption of acetonitrile affected the c-axis spacing of the various homoionic montmorillonites differently. These results led to the conclusion that the sorption of acetonitrile on montmorillonite is not due to any specific bonding between the organic molecules and the clay surfaces, but rather to a distribution or partition between the interfacial phase and the bulk-solution phase. Analysis was provided to show that a partition mechanism does not necessarily yield a linear isotherm, especially when the concentration of the organics is high.

Adsorption and Heat of Adsorption of Organic Compounds on Montmorillonite from Aqueous Solutions

AbstractThe adsorption of 1,4‐dioxane, urea, ethanol, phenol, and the acetate anion by <2µm Na‐ and Ca‐saturated Upton montmorillonite from dilute aqueous solutions was determined at different concentrations by using their 14C‐labeled isotopes. The heat of adsorption of each compound at each concentration was also determined. All the neutral organic molecules were positively adsorbed and the acetate anion was negatively adsorbed. Except for the adsorption of 1,4‐dioxane and phenol on Ca montmorillonite, the adsorption isotherms of the neutral molecules were linear. The linear adsorption isotherms mean that the adsorbed molecules were not held to the surface by specific bonds but were distributed proportionally between interfacial and bulk phases. Preference of the neutral molecules for the interfacial phase was due to their increased interaction with the surrounding water molecules. Preference of the acetate ion for the bulk phase was attributed to its electrostatic repulsion by the clay. The adsorption reactions of the 1,4‐dioxane and urea with the Na montmorillonite were exothermic; the other adsorption reactions were endothermic. Had specific bonds formed between the organic molecules and the surface of the montmorillonite, the adsorption reactions would have been exothermic. Hence, the endothermic adsorption reactions confirm the absence of such bonds for the ethanol, phenol, and acetate anions. The adsorption isotherm of 1,4‐dioxane on Ca montmorillonite was of the Langmuir type, explained in the conventional way. The adsorption isotherm of phenol on the same clay exhibited upward curvature. We postulate the existence of interlayer regions containing phenol as a separate phase that grew disproportionally as the phenol concentration in the bulk phase increased.

Water sorption on Ca-saturated clays: II. Internal and external surfaces of montmorillonite

AbstractWater-sorption isotherms and interlamellar spacings were measured for Ca montmorillonite (from Redhill. UK) over the relative pressure range 1 >P/P0> 0. By taking account of the change in interlamellar volume with relative pressure, it is shown that internal and external surfaces can be calculated by a modifiedt-plot analysis. The results also show that sorption on the external surfaces of the montmorillonite is quantitatively similar to that on reference oxide surfaces. Desorption-sorption hysteresis occurred and was apparently caused by an ageing process in which the interlamellar volume increased at the expense of the external surfaces.

Rock-fluid interactions in a temperature gradient: biotite granodiorite + H 2O

A biotite granodiorite was reacted in a controlled temperature gradient with distilled water for 60 days at 1/3 kbar P Tot = P H 2 o . Polished rock prisms were placed in the gradient at 72, 119, 161, 209, 270, and 310°C. Scanning electron microscope and microprobe analyses show the appearance of several secondary phases: Ca montmorillonite at 72°C and 119°C; zeolite, either stillbite or heulandite, at 161°C; and another zeolite, thomsonite, at higher temperatures. Solution analyses show a constant composition after about 2 weeks of reaction. The phase assemblages observed in this experiment are connected by reaction relationships. The reactions show the transition from clay to zeolite I (stilbite or heulandite) to zeolite II (thomsonite) with increasing temperature and decreasing chemical potential of silica. Additional relations can be used to predict mineral assemblages not directly observed in this experiment but which may occur at different pressures and temperatures as well as at different chemical potentials of water and silica.

Hydrothermal Interactions of Cement or Mortar with Zeolites or Montmorillonites

ABSTRACTConcretes, grouts, clays and/or zeolites are candidate borehole, shaft or tunnel plugging materials for any nuclear waste repository. Interactions between these plugging materials may take place under mild hydrothermal conditions during the life of a repository. Class H cement or mortar (PSU/WES mixture) was reacted with one of two montmorillonites, clinoptilolite or mordenite at 100° and 200°C for different periods under a confining pressure of 30 MPa. The solid reaction products were characterized by x-ray powder diffraction and scanning electron microscopy after the hydrothermal treatments. When zeolites were in contact (not intimate mixture) with class H cement, they did not seem to alter but clinoptilolite altered to analcime, and mordenite became poorly crystalline in the presence of mortar (containing NaCl) at both 100° and 200°C. When cement or mortar was intimately mixed with zeolites or montmorillonites and reacted hydrothermally, the reaction resulted in the formation of Al substituted tobermorite (11Å type) in all cases (this type of reaction is expected at the interface) at both 100° and 200°C. The mechanism of tobermorite formation includes the decomposition of zeolites or montmorillonites in the presence of alkaline (pH ≃ 12) cement or mortar and recrystallization to form Al substituted tobermorite. Cesium sorption measurements in 0.01N CaCl2on the reaction products revealed that selective Cs sōrption increased in most cases, even though little or none of the original zeolites and montmorillonites remained in the products. For example, Cs sorption Kd(mL/g) increased from 80 in the untreated mortar + Ca montmorillonite mixture to 1700 in the interaction product which is Al substituted tobermorite. Thus, we discover here that Al substituted tobermorite has good selectivity for Cs.

THE EFFECT OF PARTICLE MORPHOLOGY ON THE K‐Ca EXCHANGE PROPERTIES OF MONTMORILLONITE

SummaryAdsorption isotherms for K on to Ca montmorillonite (< 2 μm and < 0.1 μm. e.s.d. fractions) at ionic strengths from 3 × 10−2M to 3 × 10−5M have been analysed by the methods of Van Bladel and Laudelout (1967) and show that negatively charged sites in the lamellae of both size fractions are the same but when these lamellae are intermeshed in crystals, potassium is favoured, probably in wedge sites between adjacent lamellae. The effectiveness of these wedge sites is lost at zero ionic strength.

EFFECTS OF METHOD OF CLAY PREPARATION ON SUBSEQUENT ADSORPTION OF THE INSECTICIDE FENSULFOTHION

The manner in which montmorillonite and illite adsorbents were prepared for batch-type adsorption experiments determined the extent of fensulfothion (Dasanit®) adsorption. Curve-fitting techniques produced regression equations for the isotherms generated by four clay systems (Na– and Ca–montmorillonite and illite) and by the four preparation treatments per system (freeze-dried; freeze-dried, resuspended; freeze-dried, 100% RH; suspension). Only the regression equation isotherms of the Na–illite system and one treatment comparison of the Ca–illite system (freeze-dried, 100% RH vs. suspension) were not significantly different (5% level). In general, the freeze-drying process increased fensulfothion adsorption relative to that for the same clay in suspension. Conditioning freeze-dried clay with water vapor (100% RH) prior to adsorption decreased fensulfothion adsorption relative to the freeze-dried clay (dried over P2O5).

Tensile Strength of Montmorillonite as a Function of Saturating Cation and Water Content

AbstractThe tensile strength, tensile axial strain, and tensile strain energy were determined for Na‐, K‐, Ca‐, Al‐, and Fe‐saturated montmorillonite over the water vapor pressure range (P/Po) from 0.02 to 0.92. The procedure used was direct tensile stressing of oriented clay films. Measured tensile strengths ranged from 366 kg cm‐2 for Fe montmorillonite at 0.02 P/Po to 19 kg cm‐2 for Ca montmorillonite at 0.92 P/Po. Strength decreased rapidly with added increments of water during initial interlamellar expansion and continued to decrease at a slower rate with further additions of water. In general, strength decreased in the following order: Fe > K ≥ Na > Al > Ca. The tensile strength of Na montmorillonite was of the same order of magnitude as the force required to separate two Na clay platelets as determined from adsorption/desorption water isotherms.Axial strain was independent of P/Po above 0.4 for all systems except the Na‐saturated clay. For Fe montmorillonite, strain was insensitive to water content above 0.15 P/Po. Monovalent clay specimens required the most energy to rupture and divalent samples required the least energy. Aluminum and Fe clays required an intermediate rupture energy.The greater tensile strength and low axial strain of the Fe montmorillonite suggested that a different mechanism controlled the failure characteristics of this clay. Hydroxy iron material may be the major cementing agent in the Fe system. The data show that saturating cation played a dominant role in the strength‐energy properties of the other homoionic clays.

Stabilisation of montmorillonite sols by chemical treatment. Part. II. Effect of polymetaphosphates, sodium metasilicate, oxalate, citrate and orthophosphate on Na and Ca montmorillonite sols

AbstractThree different causes are concluded to contribute to the stabilisation of montmorillonite sols by polymeric phosphates, sodium metasilicate and sodium oxalate: a charging effect due to adsorption of the anions to the edges of the micelle plates. a decrease of the sodium ion activity in the mixture of stabilizer and flocculating salt. an inactivation of strongly flocculating calcium ions by complex formation or precipitation.

Stabilisation of montmorillonite sols by chemical treatment. Part I. Properties of sodium and calcium montmorillonite sols

AbstractAccording to ultramicroscopical counting and relative viscosity data the micelles of Na and Ca montmorillonite sols are minute flat crystal plates with an average thickness corresponding to that of a packet of about five crystal lattice layers only with an average diameter‐thickness ratio of the order of a few hundred. This is corroborated by electronmicrographs and X‐ray studies.The average thickness of the micelles decreases with decreasing sol concentration, indicating a concentration‐dependent association‐dissociation equilibruim of the crystal lattice layers.Montmorillonite is a peculiar colloid in the sense that the platy micelles may be assumed to bear two basically different double layers, one on the surface of the lattice and a second at the broken lattice edges. The two double layer potentials may even have a different sign.