Contents file · Web viewThe clay mineral montmorillonite is part of a basic 3 group of minerals also containing kaolinites and illites with the lattice structure of the minerals

Bentonite Mineralogy Literature ReviewPreferential Modification of Bentonite Structure

MSc.

By Luke Molloy

Supervised by Dr Michael McLaughlin

Luke Molloy [email protected]

Contents

Contents.............................................................................................................................. i

1 Background and uses...................................................................................................1

2 Mineralogy and molecular structure...........................................................................2

3 Particle size and stability in suspension.......................................................................4

3.1 Nano bentonite....................................................................................................4

4 Swelling capacity.........................................................................................................4

5 The law of mass action................................................................................................5

6 Isomorphous substation..............................................................................................5

7 Cation exchange capacity............................................................................................6

8 Van der Waals Attractive Forces..................................................................................7

9 Particle associations in clay suspensions.....................................................................7

10 Particle size and size distribution.............................................................................7

11 Permeability, mass transport mechanisms..............................................................7

11.1 Diffusion......................................................................................................... 10

11.2 Long term permeation....................................................................................11

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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 10-9m) 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, Al3+, is replaced

by a magnesium ion, Mg2+. 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.

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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 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/m3. 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 Ca2+ 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 7.4 shows the 2-1 crystal lattice structure of a layered

clay particle.

Figure 7.4 – Alumina silicate clay particle structure

Figure 7.5 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 7.5 – 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 10-9 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 CaSO4) 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.

ASTM D 5084: standard test method for the measurement of the hydraulic conductivity

of saturated porous materials using a fexible wall permeameter.

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Proff. Kerry Rowe

Daniels, 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 9.6 – Mineralogy of bentonite portion of 3 GCLs (Shackelford et al. 2000)

Table 9.1 shows the CEC of two different geometric forms of Wyoming (Na) bentonite.

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Table 9.1 – 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.

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

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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. WileyEgloffstein, 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 Ca-bentonite. 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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http://www.porepressure.info/Clay-Diagenesis.html

http://www4.ncsu.edu/~hubbe/BENT.htm

Documents

Contents file · Web viewThe clay mineral montmorillonite is part of a basic 3 group of minerals also containing kaolinites and illites with the lattice structure of the minerals