Bentonite is an absorbent
aluminiumphyllosilicate generally impure clayconsisting mostly of montmorillonite. There are a few types of bentonites and their names depend on the dominant elements, such as K, Na, Ca, and Al. As noted in several places in the geologic literature, there are some nomenclatorial problems with the classification of bentonite clays. Bentonite usually forms from weathering of volcanic ash, most often in the presence of water. However, the term bentonite, as well as a similar clay called tonstein, have been used for clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium and calcium bentonite. In stratigraphyand tephrochronology, completely devitrified (weathered volcanic glass) ash-fall beds are commonly referred to as K-bentonites when the dominant clay species is illite. Other common clay species, and sometimes dominant, are montmorillinite and kaolinite. Kaolinite dominated clays are commonly referred to as tonsteins and are typically associated with coal.
Sodium bentonite expands when wet, possibly absorbing several times its dry mass in water. Because of its excellent colloidal properties (see Odom ref below) it is often used in
drilling mudfor oil and gas wells and for geotechnical and environmental investigations.
The property of swelling also makes sodium bentonite useful as a sealant, especially for the sealing of subsurface disposal systems for spent nuclear fuel [http://www.skb.se/templates/SKBPage____8806.aspx] [http://www.tdx.cesca.es/TESIS_UPC/AVAILABLE/TDX-0412105-095450/] and for quarantining metal pollutants of groundwater. Similar uses include making
slurry walls, waterproofing of below-grade walls and forming other impermeable barriers: e.g., to seal off the annulus of a water well, to plug old wells, or as a liner in the base of landfillsto prevent migration of leachate.
Sodium bentonite can also be "sandwiched" between synthetic materials to create geo-synthetic liners (GSL) for the aforementioned purposes. This technique allows for more convenient transport and installation and it greatly reduces the volume of sodium bentonite required.
Various surface modifications to sodium bentonite improve some rheological or sealing performance in geoenviromental applications, for example the addidtion of polymers [Theng, B.K.G. 1979. Formation and Properties of Clay Polymer Complexes. Developments in Soil Science 9. Elsevier, Amsterdam, 362pp] .
Calcium bentonite is a useful adsorbent of ions in solution. [Lagaly G., 1995. Surface and interlayer reactions: bentonites as adsorbents. P 137-144] [In Churchman, G.J., Fitzpatrick, R.W., Eggleton R.A. Clays Controlling the Environment. Proceedings of the 10th International Clay Conference, Adelaide, Australia. CSIRO Publishing, Melbourne] as well as fats and oils, being a main active ingredient of
Fuller's Earth, probably one of mankind's first industrial cleaning agents [R.H.S, Robertson, 1986. Fuler's Earrth. A History of calcium montmorillonite. Volturna, Press, U.K.] . Calcium bentonite may be converted to sodium bentonite (termed sodium beneficiation or sodium activation) to exhibit many of sodium bentonite's properties by a process known as "ion exchange" (patented in 1935 by Germans U Hofmann and K Endell). Commonly this means adding 5-10% of a soluble sodium salt such as sodium carbonate to wet bentonite, mixing well, and allowing time for the ion exchange to take place and water to remove the exchanged calcium. Fact|date=November 2007 Some properties, such as viscosity and fluid loss of suspensions, of sodium beneficiated calcium bentonite (or sodium activated bentonite) may not be fully equivalent to natural sodium bentonite. [Odom, I.E., 1984. Smectite clay minerals: properties and uses. Philosophical Transactions Royal Society, London, A., 311, 391-409.] For example, residual calcium carbonates (formed if exchanged cations are insufficiently removed) may result in inferior performance of the bentonite in geosynthetic liners [Guyonnet, D., Gaucher, E., Gaboriau, H., Pons C.-H., Clinard, C., Norotte, V. Didier, G. 2005. Geosynthetic clay liner interactions with leachate: correlation between permeability, microstructure and surface chemistry. Journal of Geotechnical Engineering, Vol. 131, page740-749.]
"Pascalite" is a commercial name for the calcium bentonite clay.
Uses for both types
Much of bentonite's usefulness in the drilling and
geotechnical engineeringindustry comes from its unique rheological properties. Relatively small quantities of bentonite suspended in water form a viscous, shear thinning material. Most often, bentonite suspensions are also thixotropic, although rare cases of rheopectic behavior have also been reported. At high enough concentrations (~60 grams of bentonite per litreof suspension), bentonite suspensions begin to take on the characteristics of a gel(a fluid with a minimum yield strength required to make it move). For these reasons it is a common component of drilling mudused to curtail drilling fluid invasionby its propensity for aiding in the formation of mud cake.
Bentonite can be used in
cement, adhesives, ceramicbodies, and cat litter. Bentonite is also used as a binding agent in the manufacture of taconitepellets as used in the steelmaking industry. Fuller's earth, an ancient dry cleaning substance, is finely ground bentonite, typically used for purifying transformer oil. Bentonite, in small percentages, is used as an ingredient in commercially designed clay bodies and ceramic glazes. Bentonite clay is also used in pyrotechnicsto make end plugs and rocket nozzles, and can also be used as a therapeutic face pack for the treatment of acne/oily skin. Clearasil, an acne cream, uses bentonite as an agent to absorb excess sebum, clearing pores.
The ionic surface of bentonite has a useful property in making a sticky coating on sand grains. When a small proportion of finely ground bentonite clay is added to hard sand and wetted, the clay binds the sand particles into a moldable aggregate known as green sand used for making molds in
sand casting. Some river deltas naturally deposit just such a blend of such clay silt and sand, creating a natural source of excellent molding sand that was critical to ancient metalworking technology. Modern chemical processes to modify the ionic surface of bentonite greatly intensify this stickiness, resulting in remarkably dough-like yet strong casting sand mixes that stand up to molten metal temperatures.
The same effluvial deposition of bentonite clay onto beaches accounts for the variety of plasticity of sand from place to place for building
sand castles. Beach sand consisting of only silica and shell grains does not mold well compared to grains coated with bentonite clay. This is why some beaches are much better for building sand castles than others.
The self-stickiness of bentonite allows high-pressure ramming or pressing of the clay in molds to produce hard, refractory shapes, such as
model rocketnozzles. Indeed, to test whether a particular brand of cat litter is bentonite, simply ram a sample with a hammer into a sturdy tube with a close-fitting rod; bentonite will form a very hard, consolidated plug that is not easily crumbled.
Bentonite also has the interesting property of adsorbing relatively large amounts of
proteinmolecules from aqueous solutions. It is therefore uniquely useful in the process of winemaking, where it is used to remove excessive amounts of protein from white wines. Were it not for this use of bentonite, many or most white wines would precipitate undesirable flocculent clouds or hazes upon exposure to warmer temperatures, as these proteins denature. It also has the incidental use of inducing more rapid clarification of both red and white wines.
Bentonite is used in medicine as a bulk laxative [http://cancerweb.ncl.ac.uk/cgi-bin/omd?bentonite] and for
Also known as potash bentonite or K-bentonite, potassium bentonite is a potassium rich illitic clay formed from alteration of volcanic ash. [
[http://www.answers.com/topic/potassium-bentonite Potassium bentonite] . "McGraw-Hill Dictionary of Scientific and Technical Terms". Retrieved June 12, 2008. Answers.com
History and natural occurrence
In 2005, U.S. was the top producer of bentonite with almost one-third world share followed by China and Greece, reports the
British Geological Survey.
The absorbent clay was given the name bentonite by an American geologist sometime after its discovery in about 1890 — after the Benton Formation (a geological
stratum, at one time Fort Benton) in Montana's Rock Creek area. Other modern discoveries include montmorillonitediscovered in 1847 in Montmorillonin the Vienneprefecture of France, in Poitou-Charentes, South of the Loire Valley.
Most high-grade natural sodium bentonite is produced from the western United States in an area between the Black Hills of South Dakota and the Big Horn Basin of Wyoming. Mixed sodium/calcium bentonite is mined in
Greece, Australia, India, Russia and the Ukraine. In the United States, calcium bentonite is primarily mined in Mississippi and Alabama. Other major locations producing calcium bentonite include Germany, Greece, Turkey, and China.
It should be noted that in some countries like the UK and US, calcium bentonite is known as
fuller's earth, a term which is also used to refer to attapulgite, a mineralogically distinct clay mineral but exhibiting similar properties.
In the news
In the immediate aftermath of the
2001 anthrax attacks, bentonite was alleged by some anonymous government sources to have been part of the weaponization recipe. This was politically significant as it had been used in the Iraqi biological warfare program. However, these claims were consistently denied by US officials when speaking in public.Fact|date=August 2008
Wikimedia Foundation. 2010.