Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium (K), sodium (Na), calcium (Ca), and aluminum (Al). Experts debate a number of 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, has been used for clay beds of uncertain origin. For industrial purposes, two main classes of bentonite exist: sodium and calcium bentonite. In stratigraphy and 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 montmorillonite and kaolinite. Kaolinite-dominated clays are commonly referred to as tonsteins and are typically associated with coal.

Types

Sodium bentonite

Sodium bentonite expands when wet, absorbing as much as several times its dry mass in water. Because of its excellent colloidal properties, it is often used in drilling mud for 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 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 to line the base of landfills to prevent migration of leachate. It is also used to form a barrier around newly planted trees to constrain root growth so as to prevent damage to nearby pipes, footpaths and other infrastructure.

Sodium bentonite can also be "sandwiched" between synthetic materials to create geo-synthetic clay liners (GCL) 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 addition of polymers.

Calcium bentonite

Calcium bentonite is a useful adsorbent of ions in solution, as well as fats and oils, being a main active ingredient of fuller's earth, probably one of the earliest industrial cleaning agents. 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). In common usage, 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. 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 those of natural sodium bentonite. For example, residual calcium carbonates (formed if exchanged cations are insufficiently removed) may result in inferior performance of the bentonite in geosynthetic liners.

Potassium bentonite

Also known as potash bentonite or K-bentonite, potassium bentonite is a potassium-rich illitic clay formed from alteration of volcanic ash.

Uses

Much of bentonite's usefulness in the drilling and geotechnical engineering industry 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 litre of 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 mud used to curtail drilling fluid invasion by its propensity for aiding in the formation of mud cake.

Bentonite can be used in cement, adhesives, ceramic bodies, and cat litter. Bentonite is also used as a binding agent in the manufacture of taconite pellets 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 pyrotechnics to make end plugs and rocket engine nozzles.

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 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 rocket nozzles. 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 protein molecules from aqueous solutions. Therefore, it is 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 can also be used as a desiccant due to its adsorption properties. Bentonite desiccants have been successfully used to protect pharmaceutical, nutraceutical and diagnostic products from moisture degradation and extend shelf life. In fact, in the most common package environments, Bentonite Desiccants offer a higher adsorption capacity than silica gel desiccants. Bentonite complies with the FDA for contanct with food and drugs.

Medical

Bentonite has been prescribed as a bulk laxative, and it is also used as a base for many dermatologic formulas.

Bentoquatam is a bentonate-based topical medication intended to act as a shield against exposure to urushiol, the oil found in plants such as poison ivy or poison oak.

In Thai farming

Application of bentonite clay technology made a big contribution to rejuvenating degraded soils in Northeast Thailand, which both improved yields and farmer incomes.

Over the past 40 years, Northeast Thailand has undergone significant changes in land use. Farming systems moved from being subsistence agriculture to being commercial agriculture, typically characterized by paddy rice production in the lowlands and sugarcane/cassava production in the uplands. However, the intensification of these production systems degraded soil chemical properties in ways that are best described as nutrient/resource mining operations. As a consequence of these changes, productivity and production systems declined, as soils became depleted of their nutrients and water-retaining properties.

The application of clay technology by farmers in Northeast Thailand, using bentonite clay, has dramatically reversed soil degradation and resulted in greater economic returns, with higher yields and higher output prices. Studies carried out by The International Water Management Institute and partners in 2002–2003 focused on the application of locally sourced bentonite clays to degraded soils in the region. These applications were carried out in structured field trials. Results from these studies showed that applying bentonite clays effectively improved yields of forage sorghum grown under rain-fed conditions.

Cumulative dry matter production over a two-year period ranged from 0.22 tons per hectare under control treatment applying normal fertilizer only, to 23 tons per hectare using an application of 50 tons per hectare of bentonite. Yields rose to 36 tons per hectare when a combination of 50 tons per hectare of bentonite and 10 tons per hectare of leaf litter was applied. These and several other studies conclusively demonstrated that introducing clay-based materials such as bentonite and termite mound materials significantly and persistently improve the productivity of degraded, light–textured soils.

Three years after the conclusion of this project, a survey was carried out on 250 farmers, equally split between those farmers that had adopted clay-based approaches versus those that had not. The purpose was to assess the economic effects of the project. Using different methods, an economic assessment was carried out. Although the responses were, in essence, agronomic effects, they also tended to cause major changes in farm economies, especially concerning the type and composition of different agricultural supplies and enhancing marketability. Variations in the quantity and composition of these yield-increasing supplies explain differing productivity levels and the return on investment of farms that used clay applications versus the farms that did not.

Apart from its role of changing the nature and composition of farm supplies, bentonite application also influenced the prices that farmers received for their crops. The average output price for farmers using clay technologies was 18% higher than that for non-clay users; this suggests that either clay-using farmers go for high value crops (as in vegetable farms) or they receive a higher price for their produce, due to better quality (e.g., from organic rice and integrated farms). Production costs are higher, but, due to more production and the quality of the food, clay farmers could afford to invest and grow more and better food, compared to non clay-using farmers. For example, the average per-hectare cost of clay-using farms was 57% higher than that for non-users, but the per-hectare gross revenue of farms using bentonite clay technologies was twice that of non-clay-using farms. Since the net values of the treated and control groups were compared, clay application led to a net benefit of about 120%.

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 Wilbur C. Knight in 1898, after the Cretaceous Benton Shale near Rock River, Wyoming. Other modern discoveries include montmorillonite discovered in 1847 in Montmorillon in the Vienne prefecture 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 Bighorn Basin of Wyoming. Mixed sodium/calcium bentonite is mined in Greece, Australia, India, Russia, and the Ukraine. In the United States, calcium bentonite is mined primarily in Mississippi and Alabama. Other major locations producing calcium bentonite include Germany, Greece, Turkey, India, 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 also used to refer to attapulgite, in terms of mineralogy, a distinct clay mineral but one exhibiting similar properties.

See also

Illite

Medicinal clay

References

External links

Further reading

Brady, G.S., Clauser, H.R., & Vaccari, J.A. (2002). ''Materials handbook.'' (15th ed.) New York: McGraw-Hill.

Hosterman, J.W. and S.H. Patterson. (1992). ''Bentonite and Fuller's earth resources of the United States'' [U.S. Geological Survey Professional Paper 1522]. Washington, D.C.: United States Government Printing Office.

Milne, G.W.A. (Ed.). (2005). ''Gardner's commercially important chemicals: Synonyms, trade names, and properties.'' Hoboken, N.J.: Wiley-Interscience.

Category:Phyllosilicates Category:Aluminium minerals Category:Calcium minerals Category:Sodium minerals Category:Medicinal clay

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