Diffusion bentonite

The refining process most commonly used involves treatment with hot aqueous alkaH to convert free fatty acids to soaps, followed by bleaching, usually with hydrogen peroxide, although sodium chlorite, sodium hypochlorite, and ozone have also been used. Other techniques include distillation, steam stripping, neutralization by alkaH, Hquid thermal diffusion, and the use of active adsorbents, eg, charcoal and bentonite, and solvent fractionation... 

According to Eq. (4) (Table 1.2), the time required to halve the concentration of the virus particles in the suspension containing the virus particles only would be almost 200 days. In the presence of bentonite (kb = 3.1 10 10 cm3 sec1 and Nd2 = 7.35 106 cm 3) we find after integrating that the free virus concentration after 1 hour of contact is only 2.6 particles cm 3. This example illustrates that the presence of larger particles may aid significantly in the removal of smaller ones, even when Brownian diffusion is the predominant transport mechanism. 

Initially, Oz diffuses through the bentonite and granitic domains, controlling the redox state of the system. Once 02 is exhausted, granitic groundwater controls the redox state of the system. The results of these calculations performed with the PHREEQC geochemical code (Parkhust Appelo 1999) clearly indicate that there is a substantial variability in pH/pe space along the temporal and spatial evolution of the near field of a repositoiy. This has clear consequences for the subsequent interactions with the Fe canister material and finally with the spent fuel matrix. 

Tachi, Y., Shibutani, T., Sato, H. Yui, M. 2001. Experimental and modeling studies on sorption and diffusion of radium in bentonite. Journal of Contaminant Hydrology, 47, 171 -186. 

The origin of Aeolian dust from Chinese desert to Japanese islands was studied with ESR of dust, mostly fine grains of quartz.132 Suspended particulate matter (SPM) collected by environmental protection agency was measured with ESR.133 Dust soot of automobiles was also measured to check the motor combustion rate. Adsorption of NO, Mn2+ and Gd3 on clay minerals, bentonite and se-piolite134 has been studied using paramagnetic ions as tracers for radioactive elements. And studies have been carried out on how divalent and trivalent cations diffuse and blocked by surrounding clay minerals, in the context of their leak from nuclear waste repository to the environment.135... 

Most studies of contaminant transport in slurry walls have relied on the equilibrium assumption, either for convenience or because of the long residence times associated with diffusion-dominated transport. As an alternative, Rabideau and Khandelwal (1998a) have proposed the simple two-compartment mass transfer model to describe nonequilibrium sorption in soil/bentonite systems ... 

Eqs. 3-4 are amenable to semi-analytical solution techniques because of the linear form. The use of more complex kinetic models (e.g., intraaggregate diffusion) has not been attempted, in part because the above models have proved adequate to describe the available data sets, and in part because of a limited understanding of the geometry of the soil/bentonite matrix (gel formation and the resulting diffusion geometry). 

Khandelwal, A., Rabideau, A. J., and Shen, P. (1998). Analysis of diffusion and sorption of volatile organic contaminants in soil/bentonite barrier materials, Environmental Science Technology, 32(9), 1333-1339. 

With the addition of bentonite to a crushed basalt backfill, aqueous diffusion would be the most effective mass transfer process (31). Aagaard and Helgeson (32) state that at temperatures <200°C, aqueous diffusion rates are orders of magnitude greater than rates of silicate hydrolysis even in acid solutions. Therefore, the dissolution rate of backfill phases and the overall mass transfer process could be controlled by reactions at the mineral-fluid interface. As stated earlier, dissolution of basalt phases in the sampling autoclave experiments may also be controlled by interface reactions. 

The experiments demonstrate the development of a streaming potential in consolidated bentonite clay when flushed by a NaCl-solution of either low or high concentration. The streaming potential measured in our experiments is at least 5 to 10 times larger than values reported for bentonite in the literature. Apparently this is caused by a very low electric conductivity of the bentonite samples studied. This low conductivity might be ascribed to overlapping diffuse double layers on the clay particles, caused by the high compaction and the presence of monovalent ions in the equilibrium solution. The bentonite, thus compacted, will be a very effective medium for active application of electroosmosis. Compared with electrically shorted conditions, chemical osmosis will be reduced when the clay is not short-circuited. 

Fig. 23. Diffusion profiles of water into bentonite (a) diffusion of water from a reservoir (left-hand side) into bentonite clay with initially 20% w/b water (b) diffusion of water between two bentonite clay layers initially prepared D20 and H20 as indicated. (Reproduced from Ref 90 with permission from Elsevier.)...

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. 

Manjanna, J., T. Kozaki, and S. Sato. 2009. Fe(III)-montmorillonite Basic properties and diffusion of tracers relevant to alteration of bentonite in deep geological disposal. Appl. Clay Sci. 43 208-217. 

Since cation-exchange kinetics is usually very fast, occurring in millisecond time scales (Tang and Sparks 1993), the kinetics of cesium-137 sorption on bentonite is determined by diffusion steps diffusion through the adhesion layer... 

Taking into account the bimodal structure of the catalyst of this study, in which microporous crystals of zeolite are agglomerated with a binder (bentonite) and with alumina (inert charge), both of high mesopore proportion, the limitation to internal diffusion of oxygen in both regions (in series) of the porous structure has been quantified. 

Many national programs plan to surround containers of their nuclear waste in a geologic repository, with a backfill of compacted bentonite clay (Fig. 13.33). A chief function of the clay backfill is to adsorb radionuclides and so retard their release from the engineered barrier system. Conca (1992) measured the apparent diffusion coefficient (D ) and apparent distribution coefficient (K [ml/g]) of some radionuclides in bentonite clay as a function of clay moisture content and compaction density. Measurements were made for clay densities from 0.2 to 2.0 g/cm, which correspond to porosities of 93 to 25%, respectively. With decreasing porosity, values declined by roughly 10 to 10 -fold. However, for the same porosity reduction, values were usually lowered by 10-fold and more, indicating less adsorption with compaction (Fig. 13.38). 

Bourg, L,C., Bourg, A. C. M., and Sposito, G. (2003). Modeling diffusion and adsorption in compacted bentonite a critical review. J. Contam, Hydrol, 61, 293-302. 

Neretnieks I (1982) Diffusivities of some dissolved constituents in compacted wet bentonite clay-MX80 and the impact of radionuclide migration in the buffer. SKBF/KBS Teknisk Rapport, Stockholm, p 82-87... 

Processes being studied in the modeling of the Kamaishi Test included groundwater and heat flow in the rock matrix, fractures, buffer, and their interfaces under varying unsaturated conditions. Before emplacement of buffer and heater, the inflow of water into the test pit was affected, not only by the presence of fractures, but also by the unsaturated condition of the rock near the test pit. Strong variation in the areal distribution of inflow was observed on the walls of the test pit. After the heater and bentonite were emplaced, diffusion of water into the bentonite from the rock occurred simultaneously with drying of the bentonite near the... 

This report presents our approach for calculation of the Task. Our numerical code THAMES is the three-dimensional finite element simulator of fully coupled processes. First, we defined the input data for THAMES from the supplied properties of FEBEX bentonite. After calibrations of some parameters such as thermal vapour diffusivity, the analysis that treats fully coupled thermal, hydraulic and mechanical processes was carried out. 

First, we identified the input parameter for THAMES on properties of FEBEX bentonite, because the fundamental properties of FEBEX bentonite had been obtained by various laboratory tests to identify the input data for the numerical code CODE BRIGHT (enresa (1998)). After calibrations of the all required parameters for THAMES, such as thermal vapour flow diffusivity and intrinsic permeability, the coupled THM simulations were carried out. 

Figure 6 presents the ionic strength distribution in the buffer and in the rock at various times up to 100 years. Naturally the ionic strength in the granitic water is much lower than that of the bentonite water. It can be noted that the diffusion process is slow, after 100 years the solutes have only penetrated... 

This example is to test the swelling effects under capillary pressures up to 10 Pa occurring in extremely low-permeable bentonite materials. For this purpose, a simple 1-D case is set up. A one meter long bentonite column is heated on the left hand side. Element discretization length is 0.01m. The initial conditions of the system are atmospheric gas pressure, full liquid saturation and a temperature of 12°C. The heater has a constant temperature of 1(X) C. Flow boundary conditions on the left side are gas pressure of 10 Pa and 15% liquid saturation. On the right side we have atmospheric pressure, full liquid saturation and no diffusive heat flux. As a consequence, a typical desaturation process of bentonite is triggered. The complete set of initial and boundary conditions and the material properties for this example was described in detail by Kolditz De Jonge (2003). 

The considered radial process in the bentonite annulus is a complicated one with coupled, highly nonlinear flows that involve many things. There are liquid flow and vapor flow as well as conductive and convective heat flow depending on gradients in pressure, water vapor density and temperature. The flow coefficients depend on water properties such as saturation water vapor pressure and dynamic viscosity of water. They also depend on the properties of bentonite water retention curve, hydraulic conductivity and water vapor diffusion coefficient, and thermal conductivity, all of which are functions of degree of water saturation. 

The highly non-linear coupled equations may, in the form the equations are formulated below, be linearized with a loss of accuracy of around 10%. The equation for the moisture becomes of a convective-diffusive form. An analytical solution involving Bessel functions may be obtained for the transient drying and saturation of the bentonite. 

WATER FLOW AND DIFFUSION PROBLEM IN BENTONITE MOLECULAR SIMULATION AND HOMOGENIZATION ANALYSIS... 

Abstract We here treat a diffusion problem coupled with water flow in bentonite. The remarkable behavior originates from molecular characteristics of its constituent clay mineral, namely montmorillonite, and we show the behavior based on a unified simulation procedure starting with the molecular dynamic (MD) method and extending the obtained local characteristics to a macroscale behavior by the multiscale homogenization analysis (HA Sanchez-Palencia. 1980). Not only the macroscale effective diffusion property but also the adsorption behavior is well defined based on this method. 

The diffusion in a porous media is affected by the flow field, so we treat here the seepage flow problem in the bentonite starting with the Stokes equation and applying the multiscale HA method. 

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