Compacted bentonite

Figure 13.38 Apparent distribution coefficients of some radionuclides in bentonite as a function of bentonite compaction density. Except for Np, adsorption of the radionuclides decreases with increasing compaction (decreasing porosity). After Conca (1992).

Figure 1. Water retention curves at constant volume of FEBEX bentonite compacted at dry density of 1.65 g/cm. ...

Three tests on bentonite compacted at an initial dry density of 1.7 g/cm with its hygroscopic water content have been performed at different temperatures (20, 40 and 60 °C), following the suction and stress paths indicated in Figure 4. In the first step, under a vertical load of 0.1 MPa, the suction of the sample was equalised to 14 MPa (due to mechanical limitations of the cell, this is the maximum suction that can be controlled). Afterwards, keeping this suction constant, the vertical load was increased up to 5 MPa. Under this vertical load, suction was decreased by steps until... 

Compacted clay Bentonites, dlites. Most commonly used... 

For compacted, low-permeability soil liners, the U.S. EPA draft guidance recommends natural soil materials, such as clays and silts. However, soils amended or blended with different additives (e.g., lime, cement, bentonite clays, and borrow clays) may also meet the current selection criteria of low hydraulic conductivity, or permeability, and sufficient thickness to prevent hazardous constituent migration out of the landfill unit. Therefore, U.S. EPA does not exclude compacted soil liners that contain these amendments. Additional factors affecting the design and construction of CCLs include plasticity index (PI), Atterburg limits, grain sizes, clay mineralogy, and attenuation properties. 

The effect of operating pressure on flux for various annealing treatments is shown in Figure 3. As is usually observed the optimum pressure, beyond which the curves flaten out, increases as the membranes become denser. Maximum flux of the untreated membrane at a pressure of 30 bar is around 10 m /m d (250 gfd) at 20 % brackish water rejection. The optimum operating pressure of annealed bentonite-containing membranes is lowered by about 10 bar, however, the compaction behavior is comparable to that of the reference membrane. 

Effect of Organophilic Bentonites. Membrane compaction reduces the integral product water output. By implication, membrane stabilization is a means to increase the flux. Stabilization, along with some flux improvement, can be achieved by doping the membranes with organophilic bentonites ( ). 

The objective of employing organophilic bentonite is flux stabilization. In terms of the membrane compaction slope the stabilizing effect is exemplified by the following figures (brackish water conditions) reference, -0.10 bentonite-doped, -0.06. In a field test over 1300 hours on well water of 5200 ppm TDS at a pressure of 60 bar, starting with an initial flux of 1780 1/m d and 95 % rejection, a compaction slope of -0.058 was found under the same conditions the reference membrane had a compaction slope of -0.094. 

Bradbury, M. H. Baeyens, B. 2002. Pore-water Chemistry in Compacted Resaturated MX-Bentonite Physicochemical Characterisation and Geochemical Modelling. Paul Scherrer Institut, Villigen, Switzerland, PSI Berichl Nr. 02-10, 42... 

In the Swedish KBS-project a mixture of bentonite and quartz (10 90) or compacted bentonite were proposed at an early stage as possible backfill materials. Bentonite is a rather common mont-morillonitic clay with good mechanical properties and chemical... 

Abstract A permeameter was developed for measurement of coupled flow phenomena in clayey materials. Results are presented on streaming potentials in a Na-bentonite induced by hydraulic flow of electrolyte solutions. Transport coefficients are derived from the experiments, assuming the theory of irreversible thermodynamics to be applicable. Hydraulic and electro-osmotic conductivities are consistent with data reported elsewhere. However the electrical conductivity of the clay is substantially lower. This is ascribed to the high compaction of the clay resulting in overlap of double layers... 

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. 

The contribution of free salt ions to the electric conductivity may therefore be negligible and only the adsorbed countercharge of cations will then contribute to electric conduction. The derived values of ke enable the prediction of the electro-osmotic water flux by active application of an electric potential gradient. Thus, at 0.01 M NaCl in the compacted bentonite, a gradient of 1 V/m will, in the absence of a hydraulic pressure gradient, cause a water flux of the order of 10 10 m/s. 

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. 

Compacted clay Bentonites, illites, kaolinites Most commonly used sealant for landfills layer thickness varies from 0.15-1.25 m layer must be continuous and not be allowed to dry out and crack. 

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. 

Fernandez, A. M., B. Baeyens, M. Bradbury, and P. Rivas. 2004. Analysis of the porewater chemical composition of a Spanish compacted bentonite used in an engineered barrier. Phys. Chem. Earth 29 105-118. 

The in-line dosing paradigm exploits the observation that protein adsorption by bentonite occurs rapidly - within several minutes (Blade and Boulton 1988 Muh-lack et al. 2006). Furthermore, if combined with centrifugation, the bentonite and wine can be separated and the bentonite lees are simultaneously compacted to reduce value losses. In-line dosing methods for bentonite fining are already used... 

Figure 13.33 Schematic diagram of the engineered barrier system (BBS) showing the high-level nuclear waste in its metal container, surrounded by a buffer or backfill (usually of compacted bentonite clay), in contact with the host rock. The BBS and rock affected thermally by the waste are sometimes termed the near field, with more distant surrounding rock termed the far field. After The status of near field modeling. Proc. Technical Workshop, copyright 1995 by OECD. Used by permission.

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. 

Ochs, M., Lothenbach, B., Shibata, M., Sato, H., and Yui, M. (2003). Sensitivity analysis of radionuclide migration in compacted bentonite a mechanistic model approach. 7 Contam. Hydrol. 61, 313—328. 

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... 

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