Nature Of Bentonite Research Articles

Effect of prehydration, permeant, and desiccation on GCL/Geomembrane interface transmissivity

A laboratory investigation was conducted on two different conventional GCLs (one with fine granular and another one with powdered bentonite) to explore the effect of prehydration and permeant fluid; GCL desiccation on the interface transmissivity, θ, between the interfaces of a 1.5 mm-thick high-density polyethylene (HDPE) geomembrane (GMB) and a GCL. The study also aimed to assess the self-healing capacity of desiccated GCLs for three different permeant solutions under a range of applied stresses (10–150 kPa). It was found that at stresses less than 70 kPa, θ was dominated by variability in the initial contact condition between the GMB-GCL interfaces. The effect of other factors was largely masked by the contact variability. At 100–150 kPa, the effects of initial variability were largely eliminated, but there was no notable effect of other factors on θ in the absence of desiccation. GCL desiccation increased θ by up to three orders of magnitude than an intact specimen at 10–100 kPa. Even at 150 kPa, desiccated specimens had a θ ≤ 8.0 × 10−9 m2/s for all specimens tested. The chemical composition of the permeant solutions, crack width, and nature of bentonite could play an important role in healing the cracks of desiccated GCLs.

Influence of the Nature of the Retainer (Carrier) on the Catalytic Activity of the Catalyst in the Gas-Phase Synthesis of Vinyl Acetate from Ethylene

The article examines the effect of γ-Аl2О3, high-silicon zeolite (HSZ), bentonite, expanded clay, the nature of bentonites, their porous structure, methods of preparation and modes on the activity of the catalyst consisting of palladium, copper and potassium acetate for the vapour phase synthesis of ethylene-vinyl acetate. IR spectra of the synthesized samples were recorded in the reflection mode in the range 400–4000 cm–1 using a Nexus Nicolet Fourier transform IR spectrometer (Thermo Scientific). Thermal studies (TG/DTG-DTA) were carried out on an STA-1500H thermal balance. The specific surface area was determined by the method of low-temperature nitrogen adsorption using a Quantachrome Autosorb-6B instrument. X-ray structural analysis (XRD) was investigated in the range 5–80° 2θ on a DRON 3M diffractometer using a CuKα (λ=1,54178 A) radiation source. X-ray photoelectron spectra were recorded on a SERIES 800XPS Kratos photoelectron spectrometer. Transmission electron microscopy (TEM) photomicrographs were obtained using a JEM100CX-II electron microscope.The specific surface area was determined by low-temperature argon adsorption on a Crystallux-4000M gas chromatograph. The size and radius of the pores were determined using mercury porometry directly on the amount of mercury compressed in them in a mercury porometric device that allows the measurement of the size of the pores. Vinyl acetate synthesis in a pilot device on a catalyst containing 0,4%Рd+4%Cu+7%CH3COOK/HSZ for 2000 hours at a temperature of 165 ℃, a pressure of 0.1 MPa, a vapour-gas mixture at a volumetric rate of 2000 h-1 and the amount of oxygen in the mixture with ethylene Performed when 7 vol.%. During 2000 hours of operation, the catalyst activity was 95-97% selectivity at 370-350 g of vinyl acetate/l. cat. hours.

Swelling properties of natural and modified bentonites by rheological description

Bentonite is used for landfill application because of its high sealing ability. Following the swelling phenomenon the water reacts with the bentonite and expands the solid particles. The voids are filled and this reduces permeability of the soil. However, the drawback is the long time needed to assess the hydraulic performance, whereas the engineers require simple and rapid characterization parameters to design the hydraulic barrier. Present paper investigates the possibility of use of rheology testing to describe the swelling properties of bentonite used for geotechnical engineering issues. Rheology testing is proposed to investigate the swelling properties of different nature of bentonite. Different types of bentonite are tested: natural sodium, natural calcium and sodium-activated-calcium bentonites. Rheological measurements are performed in steady and dynamic conditions. Free swell and oedometer swell tests are also performed and compared.A simple model is proposed to unify the output parameters of the liquid-state bentonite from two rheology tests, with the ones obtained from two swelling tests. This approach propose a single parameter: the swelling ratio β that describes the layer of water surrounding the particles based on the aspect ratio of bentonite packed particles. The method highlights the correlation between swelling characteristics and rheological parameters of bentonites. The results of swelling ratio underline distinctly the difference of mineralogy between the groups of bentonites. A linear equation fits the values of β obtained from the two swelling tests and the values of β obtained from the rheology tests, for all bentonites.Low correlations are observed between the oedometer swelling ratio and the rheology ratio. But, the free swelling ratio is in good agreement with rheological ratio and demonstrates a stronger correlation between the steady state rheology ratio than the dynamic state rheology ratio.

Quantification of Volatile Organic Compounds Diffusion for Virgin Geosynthetic Clay Liners and for a GCL after Contact with a Synthetic Leachate

The effects on the nature of bentonite, natural sodium, and calcium activated, and of the cation exchange on the diffusive properties of two geosynthetic clay liners (GCLs) were examined. Diffusion laboratory tests were conducted to estimate the diffusion coefficients of 1, 2-dichloroethane (DCA), dichloromethane (DCM), thrichloroethylene (TCE), toluene, and benzene. Both GCLs were needle-punched and came from the same manufacturer. The nature of the bentonite contained in both GCLs was different because a natural sodium bentonite and a calcium activated sodium bentonite were tested. Furthermore, one of the GCLs experienced cation exchange from contact with a synthetic leachate used to generate an increase in its hydraulic conductivity by a factor of 8.5. The objective of this process was to evaluate if an increase in the hydraulic conductivity by cation exchange in a GCL would result in a significant increase in the diffusion coefficients of volatile organic compounds (VOCs). The results obtained for both virgin GCLs were very close to one another. No effect on the nature of the bentonite was observed. An increase in the diffusion coefficient was noticed for the case in which the GCL experienced cation exchange while in contact with a synthetic leachate for DCM, DCA, and TCE at a given bulk GCL void ratio. The largest increase, a factor of 2.6, was observed for TCE. However, this increase in the diffusion coefficient could have been balanced by the decrease in the bulk GCL void ratio from 3.9 to 3. The increase in the diffusion coefficient of VOCs, thus, does not seem to be of concern for the range of GCL hydraulic conductivities generated by cation exchange in this study.

Characterisation of permeability to gas of geosynthetic clay liners in unsaturated conditions

ABSTRACT: This paper is a contribution to the characterisation of permeability to gas of unsaturated GCLs, with applications in cover liners of municipal solid waste landfills. Conducting such tests at low gas pressure gradient limits the risk of moisture distribution change in the bentonite, which has been observed under high pressure gradients. However, a consequence of a low pressure gradient is a small gas flow rate that is difficult to measure with good accuracy. Therefore a solution may be the use of transient state tests, such as the falling pressure test adapted to GCL permeability characterisation. This paper shows the influence of the main parameters related to the test, and to the material tested (natural or activated calcium bentonite, moisture content, presence or not of small cracks in the bentonite). The results obtained showed good repeatability for gas differential pressures up to 40 kPa for the test conditions employed in this work. The results showed that the gravimetric moisture content of the GCL necessary to attain a certain permeability value depends on the nature of the bentonite, which was not observed in terms of volumetric moisture content. However, other factors were shown to be more important than the nature of bentonite in GCL permeability to gas: the desiccation due to the gas flow can significantly increase the permeability, which compromises the GCL's performance as a gas barrier.

The Effect of Exchangeable Cations on the Permeability of a Bentonite to Be Used in a Stabilization Pond Liner

The organophilic nature of bentonites exchanged with quaternary ammonium cations is used in sanitary engineering for the adsorption of organic pollutants. This paper deals with five different quaternary ammonium cations: tetramethylammonium, trimethylstearylammonium (C18), dimethylbenzyllaurylammonium (C12), trimethylpalmitylammonium (C16) and dimethyldistearylammonium. A Brazilian bentonite was treated with the above cations and the adsorption of vinasse organics was measured through the total organic carbon present in solution. The results show that tetramethylammonium cation is the most effective of those tested to make sodium bentonite more organophilic and the behaviour follows a Freundlich isotherm. If the isotherms are plotted in milliequivalents of the cation over the weight of the sodium bentonite the present experiments did not show an appreciable difference in the quantity adsorbed. Therefore, if cost is a determining factor, low molecular weight cations should be chosen. The modified bentonites were characterized by the X-ray diffraction patterns. For high molecular weight cations the interlamelar spacing is close to 18 Å but for tetramethylammonium it is 13.5 Å. In any case the replacement of sodium by a quaternary ammonium cation increases the capacity of the clay to adsorb organic molecules.