Bentonite Mineralogy and General Review
Bentonite Mineralogy Literature Review
Preferential Modification of Bentonite Structure
MSc.
By Luke Molloy
Supervised by Dr Michael McLaughlin
Luke Molloy lmolloy@research.ait.ie
Contents
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1 Background and uses
Bentonite clays are used in a verity of applications including the sealing of leachate from landfill sites to the containment of spent nuclear fuels. Bentonite clays have certain desirable material properties such as the ability to swelling and high sorption capacity including adsorption and ion exchange (Carlson 2004). The properties exhibited by
Bentonite clays are dependent on the mineralogy, geochemistry and chemical composition of the material which are the result of varying geological history and source locations.
The swelling capacity of bentonite, which has many commercial advantages, is dependent on the proportion of smectite within the bulk material. Desired material properties such as swelling capacity, cation exchange capacity and plasticity are dependent not only on the proportion of smectite but the smectite species and the value
of exchangeable cations between the layer spacings, see Figure 2.1.
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2 Mineralogy and molecular structure
Bentonite is a naturally occurring mineral mostly composed of the clay mineral smectite.
These clays are formed by the alteration of volcanic ash which is laid down in marine environments which gets slotted between other types of rocks. Most of the smectite in the clay is made up of montmorillonite, which is a dioctahedral smectite but occasionally other types of smectite may be present. (Carlson 2004)
Montmorillonite is constructed of layers 1nm (nanometre, 1 x 10 -9 m) thick. The structure of montmorillonite is that of an octahedral layer containing aluminium, magnesium, oxygen and hydroxyl ions sandwiched between two tetrahedral layers of silicon, oxygen
and hydroxyl ions as seen in Figure 2.1 below.
Figure 2.1 – Montmorillonite lattice structure (Poerpressure 2013)
In montmorillonite, about one in eight of the octahedral aluminium ions, Al 3+ , is replaced by a magnesium ion, Mg 2+ . This results in a charge imbalance which draws any water present into the interlayer space between the sheets. This causes the clay to swell dramatically (Poerpressure 2013) as the net charge on the clay mineral becomes negative thus attracting the H + ion from the water (sorption). Adsorption of water molecules is more intense near the surface of the clay particle with decreasing intensity a function of distance (Cernica 1995). Two adjacent particles with like negative charges will experience repulsion.
The ratio of 1:3 cation to anion ratio of montmorillonite makes it dioctahedral, only 2 out of every 3 octahedral sites around each hydroxyl needs to be filled to obtain
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electrical neutrality. Other forms exist such as potassium (K), sodium (Na), calcium (Ca) bentonites.
The clay mineral montmorillonite is part of a basic 3 group of minerals also containing kaolinites and illites with the lattice structure of the minerals being the basis of their classification.
Under a scanning electron microscope bentonite particles can be almost indistinguishable from the filler clay or the coating clay. The sodium or potassium salts of bentonite exfoliate into very thin plates. Theoretically these plates can be as tiny as
about 1 nm thick, yielding a vast surface area per unit mass (NCSU 2013). Figure 2.2
shows the ratio of length to thickness of a bentonite platelet.
Figure 2.2 – Approximate length to thickness ratio of bentonite platelet
Bentonite from a location 100 km west-northwest of Prague in the Czech Republic was analysed using various techniques to have the following chemical makeup:
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Figure 2.3 – Bentonite chemical composition, 1) raw olive grey bentonite,2) separated
<1µm size fraction (Konta 1986)
3 Particle size and stability in suspension
3.1
Nano bentonite
4 Swelling capacity
Bentonite can be used as buffer for high level waste (HLW) repository due to its swelling ability on contact with free water. The swelling causes pressures to build within the bentonite layer and thus forming a hydraulic barrier with a self-sealing capacity.
Experimental results by (Lee et al. 2012) using compacted calcium (Ca) bentonite (from
Korean), show that swelling increases with an increase in dry density, and its dependence on dry density increases at densities beyond 1.6 Mg/m 3 . The investigation showed that the swelling behaviour of Ca bentonite subjected to NaCl (sodium chloride) solution was different to that of Na bentonite. The swelling pressure of the Ca bentonite was higher with 0.04M concentration of NaCl but decreased thereafter. This can be explained by an ion exchange of Ca 2+ cations for Na + cations from the NaCl solution.
Once the Na + ions transfer to the bentonite a concentration differential occurs in which
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osmotic forces draw more water molecules to the bentonite (Lee et al. 2012). Osmotic swelling is the second phase of the process as the bulk water has a less concentration of ions than that between the particle layers.
5 The law of mass action
6 Isomorphous substation
This is the process where lower charge cations within the clay particle lattice such as
Mg ++ replace higher charged cations such as Al +++ which ultimately results in a net
negative charge on the clay particle. Figure 6.1 shows the 2-1 crystal lattice structure of a
layered clay particle.
Figure 6.1 – Alumina silicate clay particle structure
Figure 6.2 below shows the substitution of Mg cations for Al cations resulting in a net
negative charge on the particle in the octahedral layer. Montmorillonite smectite is
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always negative due to isomorphous substitution which occurs during mineral crystallization.
Figure 6.2 – Isomorphpous substitution
The structural bonding of the oxygen-oxygen or the oxygen-cation leaves the layers weakly held together which allows the adsorption of cations in the interlayer space. This means the mineral is expandable and has a high cation exchange capcity (CEC). `
When dry the interlayer cations hold the layers together. The clays swell in water due to the absorbtion of water to the interlayer space.
The resulting charge imbalance is equalised by hydrated cations like K, Na, Mg and/or
Ca. More than 80% of these are located in the interlayer region (Uskarci 2006).
Bentonite smectite formed in aqueous environments have hydrated ions which results in the ions being only loosely held by the negatively charged clay layer thus making them susceptible to cation exchange.
7 Cation exchange capacity
Ion exchange is the process where ions in an electrolyte solution exchange with the ions in a solid phase material (Yen 2007). Montmorillonite in this case is the solid phase material, making up the majority of the bentonite, which acts as the exchange mechanism for the ions (cationic exchanger). Ions within the diffuse double layer, the area where there are a combination of negatively charged mineral surfaces and
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positively charged spaces around the mineral, may exchange between the ions on the clay particle and the ions within this layer. The thickness of the absorbed water layer can be affected as a result of ion exchange thus affecting the ability to swell. This could have consequences for the integrity of bentonite in engineered barrier systems (EBS).
8 Van der Waals Attractive Forces
9 Particle associations in clay suspensions
10 Particle size and size distribution
Aggregate-size distribution should affect the rate of swell, and can affect the hydraulic conductivity to non-standard liquids (Shackelford et al. 2000).
11 Permeability,
11.1
Mass transport mechanisms (self written)
Water or other solutes are transportable through soil due to its matrix of voids naturally present. Clay mineral particles are not uniform and do not form uniform layers naturally due to a number of factors; particle orientation, density, geological history etc. Hydraulic conductivity in soils is affected in many ways as the liquid makes its journey through interconnecting voids within the soil structure. Understanding or predicting this property
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and manipulating its extent, in whichever context, is of significant importance to engineers.
11.2
Other
Montmorillonite has a high specific surface area, extremely low hydraulic conductivity
(approx. 1 x 109 cm/s) and high cation exchange capacity. Na bentonite is primarily used in geosynthetic clay liners (GCLs) which mean the exchange complex of montmorillonite is dominated by Na + ions. Resulting problems occur in the gradual replacement of Na + ions which exist on the surface of the montmorillonite particle surface by multivalent
Ca ++ ions which may exist in the surrounding permanent liquid in which the bentonite
GCL is in permanent contact with. This can result in an increase in hydraulic conductivity in the order of a magnitude or more. This process continues very slowly until the exchange of Ca ++ ions for Na + is complete (Ho Young et al. 2006) and the possible increase of hydraulic conductivity due to compatibility problems with contaminant if not prehydrated with a compatible water source (Bouazza 2002). In some cases several years can be required to reach equilibrium (Egloffstein 2001).
The primary differences between GCLs are the mineralogy and form of bentonite such as: powder versus granular, sodium versus calcium, etc. (Bouazza 2002). A compatibility test is usually conducted prior to selection.
Results from test carried out on GCLs by (Shackelford et al. 2000) of the hydraulic condiuctivity of swelling clays of nonstandard liquids (0.05 N CaSO
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) containing both monovalent cations (Na) as well as low concentrations of divalent cations (Ca) can cause significant increases in hydraulic conductivity. This holds true if the test is significantly long in duration to allow full exchange of adsorbed cations, equilibrium established.
They also show that the control of average effective stress is of more importance than controlling the hydraulic gradient while evaluating the hydraulic gradient.
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ASTM D 5084: standard test method for the measurement of the hydraulic conductivity of saturated porous materials using a fexible wall permeameter.
Proff. Kerry Rowe
Danie l s, permeability
Hydraulic conductivity is related to the mineralogy of bentonite.
The hydraulic conductivity of montmorillonite to water typically is very low (10 -8 cm/s).
Also, the large affinity of montmorillonite for water molecules and hydrated cations results in significant swelling of montmorillonite (5-10 times the dry volume) when hydrated under low effective stress. The montmorillonite content in bentonite also is reflected indirectly by the cation exchange capacity (CEC) of the bentonite. The CEC is a measure of the total adsorption capacity of a soil for cations, and increases with greater surface charge deficiency and greater specific surface of the clay mineral portion of the soil (Shackelford et al. 2000).
Figure 11.1 – Mineralogy of bentonite portion of 3 GCLs (Shackelford et al. 2000)
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Table 11.1 shows the CEC of two different geometric forms of Wyoming (Na) bentonite.
Table 11.1 – Chemical properties of two Wyoming bentonites (Shackelford et al. 2000)
Pure montmorillonite has a higher CEC than bentonite because of the impurities in the bentonite such as quartz.
Since the low hydraulic conductivity of bentonite is primarily due to adsorbed molecules associated with the montmorillonite restricting the pore spaced active in flow, bentonites are particularly sensitive to changes in the composition of the pore fluid that influence the thickness of the adsorbed layer. In particular, liquids that cause the adsorbed layer to collapse also causes the hydraulic conductivity to increase, thus bentonites with greater montmorillonite content are potentially more vulnerable to chemical attack and incompatibility based on the permeating liquid being held.
Determination of the level of cations in the permeant, level of cations in the surrounding host soil/clay/minerals/ and the cation exchange capacity of the bentonite liner.
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11.3
Diffusion
In liquids, molecular diffusion occurs by jumps of the molecules from one position to another; this arises when the energy of the molecule is high enough to rupture the bonds with the neighbouring molecules allowing the molecule to move. On average, the jump does not exceed an intermolecular spacing, and since in a liquid this is much less than in a gas, the diffusion is substantially lower. Since a liquid is virtually incompressible, the diffusion rate is independent of pressure. Elevation of temperature increases intermolecular spacing’s and the velocity of vibrations and jumps of molecules, which enhances diffusion.
In general, only about 8% -10% solids slurries of good quality swelling smectite can be produced in water. Indeed, at solids contents greater than about 8%, the viscosities of the slurries can become so high that they cannot readily be pumped by conventional equipment and gelling upon standing becomes a problem. At higher solids it becomes virtually impossible to form a uniform paste without special equipment. Thus, there is a need for slurries containing substantially greater than 8% by weight of smectite clay, which have viscosities low enough to allow pumping (Uskarci 2006). Since bentonite ore mined from bentonite deposit usually has a water content of 15 to 35%, it is primarily broken and dried in the sun or hot air to obtain bentonite ore having a water content of 5 to
10%.
Ficks Law, differential equations,
11.4
Long term permeation
The effects of long term permeation of high cation solution on the swelling ability of the bentonite layer, and maintenance of a low hydraulic conductivity need to addressed.
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Bouazza, A. (2002) Geosynthetic clay liners. Geotextiles and Geomembranes, 20 (1) pp.
3-17.
Carlson, L. (2004) Bentonite Mineralogy. Geological Survey of Finland.
Cernica, J.N. (1995) Geotechnical Engineering: Soil Mechanics. Wiley
Egloffstein, T.A. (2001) Natural bentonites—influence of the ion exchange and partial desiccation on permeability and self-healing capacity of bentonites used in GCLs.
Geotextiles and Geomembranes, 19 (7) pp. 427-444.
Ho Young, J., Benson, C.H. & Edil, T.B. (2006) Rate-limited cation exchange in thin bentonitic barrier layers. Canadian Geotechnical Journal, 43 (4) pp. 370-391.
Konta, J. (1986) Textural Variation and Composition of Bentonite Derived from Basaltic
Ash. Clays and Clay Minerals, 34 (3) pp. 257-265.
Lee, J.O., Lim, J.G., Kang, I.M. & Kwon, S. (2012) Swelling pressures of compacted Cabentonite. Engineering Geology, 129–130 (0) pp. 20-26.
Ncsu (2013) Bentonite (montmorillonite). [Online]. Available at: http://www4.ncsu.edu/~hubbe/BENT.htm
[Accessed: 03/07/2013].
Poerpressure (2013) Pore Pressure - Clay Diagenesis. [Online]. Available at: http://www.porepressure.info/Clay-Diagenesis.html
[Accessed: 03/07/2013].
Shackelford, C.D., Benson, C.H., Katsumi, T., Edil, T.B. & Lin, L. (2000) Evaluating the hydraulic conductivity of GCLs permeated with non-standard liquids. Geotextiles
and Geomembranes, 18 (2–4) pp. 133-161.
Uskarci, T. (2006) Behaviour of Bentonite Suspensions in Non-Aqueous Media. Thesis
(Masters). Middle East Technical University.
Yen, T.F. (2007) Chemical Processes for Environmental Engineering. Imperial College
Press
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