Ca-bentonite Modified into Na-bentonite
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In this study,10% Na2CO3 solution as modified to improve Ca-bentonite into Na-bentonite,and under different dosage and stirring time,the value of cation exchange capacity has been explored.The results show that,the value of cation exchange capacity of each soil sample reachs to maximum,when the 10% Na2CO3 solution dosage is 4% and stirring time is 20 min.The value of cation exchange capacity of Ca-bentonite which has been improved into Na-bentonite,is larger than the value of original Ca-bentonite.Keywords:
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Cation-exchange capacity
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Bentonite
Alkalinity
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Bentonite
Swelling capacity
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ABSTRACT A technique has been developed for the determination of the exchangeable cation population of calcareous sandy material with cation exchange capacities of less than 1 meq/100 g. The technique involves the addition of exchange salt in the dry state to samples of porous media using original pore water as the exchange salt solvent. In applying this technique to samples from below the water table, the amount of pore water available for reaction is reduced by centrifuging in the field to bring the moisture content close to field capacity values. By utilizing the minimum amount of pore water, interferences during the exchange process due to calcite precipitation or dissolution are minimized. The extent of calcite dissolution or precipitation that occurs can be appraised by measuring alkalinity or total carbon on the pore water before and after addition of the exchange salt. Three salts, NH 4 C1, CsCl and LiCl were tested for their suitability for this technique. CsCl was found to be preferable because of its low tendency to dissolve carbonate and the preference of Cs + on exchange sites. Application of the method to a field site in southern Ontario yielded a value of 0.51 ± 0·09 meq/100 g over 15 samples for the cation exchange capacity of a near surface glaciofluvial sand deposit. It is believed that this technique could be applied with reasonable accuracy and reproducibility to materials with exchange capacities of as low as 0·1 meq/100 g.
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A small percolation chamber was constructed that allows the investigation of cation exchange kinetics on clay minerals under temperature controlled conditions. A mathematical model for the kinetics of cation exchange was developed that accounts for either linear or Gapon-type exchange isotherms on external (quick) and interlayer (slow) sorption sites, respectively. With montmorillonite, as indicated by the course of the potassium percolation curve, almost the whole of the cation exchange capacity could be attributed to slow sorption sites. Potassium percolation curves could best be modeled assuming that no quick sorption took place. Only the mass exchange rate coefficient was estimated. A linear exchange isotherm was assumed, and the cation exchange capacity was set to the value obtained from batch exchange isotherms. At longer times, however, the simulation deviated considerably from the experiment. We concluded that the presence of easily expandable interlayers added effects to the cation exchange that are not considered by the model. With vermiculite, potassium percolation curves needed three parameters to be estimated: mass exchange rate coefficient, partitioning factor, and Gapon coefficient for quick sorption sites. Cation exchange capacity was set to the value obtained in batch experiments. A linear exchange isotherm was assumed for interlayer sorption sites. According to the model about 64% of the cation exchange capacity (CEC) could be attributed to slow sorption sites. Modeled and observed results are in good agreement. A comparison of parameter estimation results revealed that a Gapon coefficient for slow sorption sites is redundant, whereas the assumption of a linear exchange isotherm for quick sorption sites always leads to erroneous results. A faster mass transfer from slow sorption sites into solution has been observed with increasing temperature.
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The laboratory experiment was done that 1.0mol/L sodium hydroxide solution was injected to the compacted bentonite whose density is the same as the prospected value in the concept of the intermediate-level disposal in Japan in the circumstance of 70°C temperature. After the injection of the alkali solution for approximately 600 days, the bentonite was taken out of the apparatus and some sorts of analysis were done. The accompanying minerals in the bentonite, calcedony and quartz, were dissolved and disappeared in XRD charts. Then analcime was precipitated as a secondary mineral. Although montmorillonite was dissolved, the mass fraction of it was kept approximately. The hydraulic conductivity of the bentonite calculated using the flow rate at the end of the injection of alkali solution was smaller than the prospected value based on a widely-used empirical model of the hydraulic conductivity of compacted bentonite as a function of the equivalent concentration of pore solution, montmorillonite partial void ratio, and the ratio of sodium ion equivalent to the exchangeable cation equivalent. The reasons for the difference were supposed to be the decrease of pore size brought by mineral dissolution and the large viscosity of pore solution involving high concentration aqueous silicon.
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Bentonite
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Laboratory advection-diffusion tests are performed on two regional soils-Brown Earth and Red Earth-in order to assess their capacity to control contaminant migration with synthetic contaminant solution of sodium sulphate with sodium concentration of 1000 mg/L. The test was designed to study the transport/attenuation behaviour of sodium in the presence of sulphate. Effective diffusion coefficient (De) that takes into consideration of attenuation processes is used. Cation exchange capacity is an important factor for the attenuation of cationic species. Monovalent sodium ion cannot usually replace other cations and the retention of sodium ion is very less. This is particularly true when chloride is anion is solution. However, sulphate is likely to play a role in the attenuation of sodium. Cation exchange capacity and type of exchangeable ions of soils are likely to play an important role. The effect of sulphate ions on the effective diffusion coefficient of sodium, in two different types of soils, of different cation exchange capacity has been studied. The effective diffusion coefficients of sodium ion for both the soils were calculated using Ogata Bank’s equation. It was shown that effective diffusion coefficient of sodium in the presence of sulphate is lower for Brown Earth than for Red Earth due to exchange of sodium with calcium ions from the exchangeable complex of clay. The soil with the higher cation exchange retained more sodium. Consequently, the breakthrough times and the number of pore volumes of sodium ion increase with the cation exchange capacity of soil.
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The apparent diffusion coefficients were measured at room temperature (about 23°C) under atmospheric condition by the one-dimensional non-steady state diffusion method for 3H, 99Tc, 137Cs, 237Np and 241Am in compacted sodium-bentonite saturated with water. Sodium-bentonite, which is commercially available as KunigeIV1 ®, was used in this study. Experiments were carried out in the density range of 0.4–2.0(× 103kg/m3). Bentonite in the cell was prepared to be saturated with distilled water. The measured apparent diffusion coefficient decreases with increasing dry density of bentonite. That the apparent diffusion coefficient of 3H decreased as a function of dry density of bentonite appears to be the effect of the change of porous structure with dry density of bentonite. 99Tc may be retarded by anion-exclusion because dominant diffusion specie of 99Tc is pertechnetate ion under atmospheric condition. Retardation for 137Cs may be caused by ion-exchange on bentonite. The sorption, anion-exclusion and molecular filtration are considered as a retardation mechanism for 237Np and 241Am because those dominant species are negatively charged and of large ionic size.
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A major issue for the oil and gas industry is the producing of high water from many fields due to massive water injection. Reducing the water production while improving oil recovery from these fields is a major challenge. Polymer gel has been widely used to shut off water-producing zones, but it is not suitable for high temperature reservoirs (> 100 °C). The aims of the present study were to investigate bentonite particles swelling properties and influence of specific ion type (monovalent/divalent) present in water in swelling behaviour, and hence their potential for plugging high water production zones. In this study series of free swelling tests were conducted on bentonite with varying salinity, pH and temperature. The study found that cation valence, salinities and pH had significant impacts on swelling performance of bentonite particles. Bentonite changed from highly swelled material to aggregate in high salinity. Test results showed that the free swelling of bentonite decreased with the increase of cation valence and concentration. Monovalent cations concentration greatly influenced the swelling volume of bentonite, whereas the influence of the increasing concentration of divalent cations was marginal. As the radius of the hydrated ionic increased, the free swell of bentonite in same valance cation solutions raised. Moderate swelling changes were obtained when the solution pH increased. Meanwhile, increases in temperature will increase the swelling of bentonite due to growth in frequency of interlayer cations and water molecules. Bentonite particles swelling could be controlled and inhibited by varying solution salinities and/or altering interlayer exchangeable cations. Thus, bentonite particles swelling will be triggered by low salinity, after it's been injected and mixed with lower reservoir water salinity.
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