Saturation bentonite

It has been shown that the hydration of the bentonite buffer was essentially independent of the heterogeneous nature of the rock hydraulic conductivity features. This is explained by the fact that the rock matrix permeability is higher than the saturated bentonite permeability. Some 3D analyses performed, where the heterogeneous permeability features of the rock have been included, tend to support also this conclusion. 

Abstract The Canadian Nuclear Safety Commission (CNSC) used the finite element code FRACON to perform blind predictions of the FEBEX heater experiment. The FRACON code numerically solves the extended equations of Biot s poro-elasticity. The rock was assumed to be linearly elastic, however, the poro-elastic coefficients of variably saturated bentonite were expressed as functions of net stress and void ratio using the state surface equation obtained from suction-controlled oedometer tests. In this paper, we will summarize our approach and predictive results for the Thermo-Hydro-Mechanical response of the bentonite. It is shown that the model correctly predicts drying of the bentonite near the heaters and re-saturation near the rock interface. The evolution of temperature and the heater thermal output were reasonably well predicted by the model. The trends in the total stresses developed in the bentonite were also correctly predicted, however the absolute values were underestimated probably due to the neglect of pore pressure build-up in the rock mass. 

The equilibrium equation takes the swelling behavior of the partially saturated bentonite into account. 

Bbrgesson, L. 1984. Water flow and swelling pressure in non-saturated bentonite-based clay barriers. In Clay Barriers for Isolation of Toxic Waste Proc. Intern. Symp., Stockholm, Sweden, 28-30 May 1984. 

Saturated bentonite consists of hydrated smectite minerals, mainly montmorillonite, and quartz particles, called the macro-grain. See a schematic diagram of the microstructure given in Figure 1. 

The ratio of interlayer space of montmorillonite lamella to the volume of external water determines the macroscale diffusion characteristics of saturated bentonite. In our case thickness of one montmorillonite lamella is given as Inm. The hydration number is thought to be n=2. We assume the number of montmorillonite layers of one stack is 10, 8, 6 and 4. Then if the saturated or dry density is given, the average distance of external pore water X given in Figure 6 is decided from the swelling data calculated by MD (see Kawamura Ichikawa, 2001). 

Water reaches the disposal drifts via small fractures, and saturates bentonite in a few decades. Minimum container lifetime due to anaerobic general corrosion is 20,000 years, although in the evaluation it is assumed that a few (up to 10) canisters fail much earlier due to a fabrication defect. After canister failure, and since no credit is given to the cladding as a barrier, there is an instantaneous release of some volatile radionuclides, such as C1 and Cs. When the water reaches the waste, the gradual release of the radionuclides in the UO2 matrix starts. 

Di Malo (1996) also studied the osmotic and mechanical effects of calcium chloride solutions upon bentonite. Water-saturated bentonite when exposed to CaCl2 solutions became more consolidated, had a large decrease in deformability, and an equally large increase in shear strength. These effects could not be changed by water washing, and X-ray diffraction analysis showed that calcium had ion... 

Hydrating bentonite in fresh water before adding it to the mud greatly increases its efficiency when the makeup water is contaminated with salt and/or hardness. Prehydrated bentonite can be protected from dehydration by flgnosulfonate (70) or sulfomethylated tannin when used in saturated salt water. Salt water clays, such as sepioflte and attapulgite, provide no filtration control and are normally used with suitable filtration control agents. 

Seawater muds are composed of bentonite, thinner (lignosulfonate or lignosulfonate and lignite), and an organic filtration control agent. The typical formulation of a seawater mud is 3.5 Ib/bbl of alkali (2 Ib/bbl caustic soda and 1.5 Ib/bbl lime), 8 to 12 Ib/bbl of lignosulfonate, and 2 to 4 Ib/bbl of bentonite to maintain viscosity and filtration. Another approach is to use bentonite/thinner (ligno-sulfonate)/freshwater premix, and mix it with seawater that has been treated for hardness. This technique will be discussed in the saturated saltwater muds section. 

A modified saturated saltwater mud is prepared with bentonite clay by a special technique. First, bentonite is hydrated in freshwater, then treated with lignosulfonate and caustic soda. This premix is then mixed with saltwater (one-part premix to three-part saltwater). The mixture builds up a satisfactory viscosity and develops filtration control. Thinning of the mud is accomplished by saltwater dilutions additional premix is required for viscosity and water loss control. 

For use in high resistivity soils, the most common mixture is 75% gypsum, 20% bentonite and 5% sodium sulphate. This has a resistivity of approximately 50 ohm cm when saturated with moisture. It is important to realise that carbonaceous backfills are relevant to impressed current anode systems and must not be used with sacrificial anodes. A carbonaceous backfill is an electronic conductor and noble to both sacrificial anodes and steel. A galvanic cell would therefore be created causing enhanced dissolution of the anode, and eventually corrosion of the structure. 

The majority of trichloroethylene present on soil surfaces will volatilize to the atmosphere or leach into the subsurface. Once trichloroethylene leaches into the soil, it appears not to become chemically transformed or undergo covalent bonding with soil components. When trichloroethylene was absorbed onto kaolinite and bentonite, the nuclear magnetic resonance (NMR) spectra showed no evidence of chemical reactions (Jurkiewicz and Maciel 1995). Because trichloroethylene is a dense nonaqueous phase liquid, it can move through the imsaturated zone into the saturated zone where it can displace soil pore water (Wershaw et al. 1994). 

Sodium chloride can be used as an accelerator in formulations that are bentonite free. The maximal bottom-hole temperature is 70° C. In concentrations above 5%, the effectiveness is reduced. Saturated sodium chloride solutions act as retarders. 

Figure 1. Integral (aHx) and differential (d(aHx)/dx) enthalpies of Ca - 2K ( ) and 2K + Ca (0) exchange on Wyoming bentonite as a function of the fractional K saturation of the exchange capacity.

Bradbury, M. H. Baeyens, B. 2003. Porewater chemistry in compacted re-saturated MX-80 bentonite. Journal of Contaminant Hydrology, 61, 329-338. 

Ruggiero et al. (1989) investigated the ability of a natural silt loam soil and the clay minerals, montmorillonite (Mte) and kaolinite (Kte), to immobilize laccase. They compared the catalytic abilities of the soil-enzyme and clay-enzyme complexes to degrade 2,4-dichlorophenol. They found that the immobilized laccase remains active in removing the substrate even after 15 repeated cycles of substrate addition (Figure 2.24). However, Claus and Filip (1988) found that the activity of tyrosinase, laccase, and peroxidase is inhibited by immobilization on bentonite. The type of saturating cations on clay surfaces also substantially influences enzymatic activity (Claus and Filip, 1990). 

The fissured basalt cores and bentonite were altered by placing them in an autoclave under simulated groundwater at 320°C for 30 and 60 days to simulate 1000 y and 2000 y aging, respectively The waste-form wafers for both experiments were aged in the same manner by treating them for 17 days in saturated steam at 340°C The details of these procedures and the rationale for their use have been published previously (7). The effects of saturated steam on borosilicate glass were discussed in a recent publication (8) ... 

In these experiments a commercially available bentonite, marketed under the name Colclay A90 (Ankerpoort, Geertruidenberg, The Netherlands) was used. It is a sodium-montmorillonite with a third of the exchange complex occupied by calcium. 5.0 g of the air-dried powdered bentonite was weighed into a stainless steel mould with an ID of 50 mm between two porous stones of the same diameter. Then the clay was subjected to a compaction pressure of 20.3 MPa for 30 minutes. After compaction, the mould was placed in a bowl of NaCl-solution for five days in which the clay became saturated and swollen. Thus samples were obtained with thickness of 3.8 and 2.8 mm respectively and a diameter of 50 mm. 

Solutions. Wear goggles and protective gloves and clothing. Estimate the weight of doxorubicin in the spilled liquid. Pour over the spill 3 M sulfuric acid solution (17 mL of concentrated sulfuric acid slowly added to 83 mL of cold water) containing 4.7 g of potassium permanganate, allowing 100 mL of solution per 50 mg of doxorubicin. Allow to react for at least 2 hours. Decolorize with a saturated solution of sodium bisulfite. Absorb the liquid on a 1 1 1 mixture of soda ash, clay cat litter (bentonite), and sand. Scoop the mixture into a pail of cold water. Decant the liquid into the drain with water and discard the solid as normal refuse.4-6... 

The three-layered clay mineral montmorillonite (bentonite) is characterised by a low-hydraulic conductivity and a capacity to bind water molecules and positively charged ions (cations). As such, water-saturated compacted bentonite powder is used as a hydrological barrier in areas such as waste disposal, for example around land-fill sites where the desire is to prevent leakage of contaminants from the land-... 

Bourg, I. C., G. Sposito, and A. C. M. Bourg. 2008. Modeling the diffusion of Na+ in compacted water-saturated Na-bentonite as a function of pore water ionic strength. Appl. Geochem. 23 3635-3641. 

Trials were conducted on an industrial scale (Fig. 5.5). After fining, the must was pressurized with air in the saturation column (6 bar) and sent (140 liL/h) into the flotation tank (80 hL). An aspiration system collected the flotation foam (60 hL) on the top of the flotation tank. Particles of the foam were removed with a rotary filter and a clear must (35 hL) was obtained. In the following example, two identical tanks, containing exactly the same must, were treated with B20 + Siio + gluten 20 or with B20 + Siio + FG20. (FG, fish glue soluble gelatin B, bentonite Si, silica gel ... 

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