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Bentonite

From Wikipedia, the free encyclopedia
Not to be confused with the Ba,Ca,Mg-mixed carbonate mineralBenstonite.
For the chromium-rich ettringite mineral, seeBentorite.
For the barium titanium cyclosilicate mineral, seebenitoite.

Bentonite layers from an ancient deposit ofweatheredvolcanic ashtuffinWyoming
Gray shale and bentonites (Benton Shale;Colorado Springs, Colorado)

Bentonite(/ˈbɛntənaɪt/BEN-tə-nyte)[1][2]is anabsorbentswellingclayconsisting mostly ofmontmorillonite(a type ofsmectite) which can either be Na-montmorillonite or Ca-montmorillonite. Na-montmorillonite has a considerably greater swelling capacity than Ca-montmorillonite.

Bentonite usually forms from the weathering ofvolcanic ashinseawater,or byhydrothermal circulationthrough theporosityof volcanic ash beds,[3][4]which converts (devitrification) thevolcanic glass(obsidian,rhyolite,dacite) present in the ash into clay minerals. In the mineral alteration process, a large fraction (up to 40–50 wt.%) ofamorphoussilicaisdissolvedandleachedaway, leaving the bentonite deposit in place.[citation needed]Bentonite beds are white or pale blue or green (traces ofreducedFe2+
) in fresh exposures, turning to a cream color and then yellow, red, or brown (traces ofoxidizedFe3+
) as the exposure is weathered further.[5]

As a swelling clay, bentonite has the ability to absorb large quantities of water, which increases its volume by up to a factor of eight.[5]This makes bentonite beds unsuitable for building and road construction. However, the swelling property is used to advantage indrilling mudand groundwater sealants. The montmorillonite / smectite making up bentonite is analuminiumphyllosilicatemineral,which takes the form of microscopic platy grains. These give the clay a very large total surface area, making bentonite a valuableadsorbent.The plates also adhere to each other when wet. This gives the clay a cohesiveness that makes it useful as abinderand as an additive to improve the plasticity ofkaoliniteclay used forpottery.[6]

One of the first findings of bentonite was in theCretaceousBenton ShalenearRock River, Wyoming.TheFort Benton Group,along with others in stratigraphic succession, was named afterFort Benton, Montana,in the mid-19th century byFielding Bradford MeekandF. V. Haydenof the U.S. Geological Survey.[4]Bentonite has since been found in many other locations, including China and Greece (bentonite deposit of theMilosvolcanic island in theAegean Sea). The total worldwide production of bentonite in 2018 was 20,400,000 metric tons.[7]

Types[edit]

2:1clay mineralscrystallographic structure made of three superimposed sheets of Tetrahedra-Octahedra-Tetrahedra (TOT layer unit), respectively
Detailed molecular structure of puremontmorillonite,the best known end-member of thesmectitegroup. The interlayer space between two successive TOT layers is filled with hydratedcations(mainlyNa+
andCa2+
ions) compensating the negative electrical charges of the TOT layers and withwater moleculescausing the interlayer expansion.

In geology, the termbentoniteis applied to a type ofclaystone(a clay rock, not a clay mineral) composed mostly ofmontmorillonite(a clay mineral from thesmectitegroup). It forms by devitrification ofvolcanic ashortuff,[5]typically in a marine environment.[3][4]This results in a very soft, porous rock that may contain residual crystals of more resistant minerals, and which feels soapy or greasy to the touch. However, in commercial and industrial applications, the termbentoniteis used more generally to refer to any swelling clay composed mostly ofsmectiteclay minerals, which includes montmorillonite.[5]The undifferentiated reference to the weathered volcanic rock for the geologist or to the industrial mixture of swelling clays can be a source of confusion.

The montmorillonite making up bentonite is analuminiumphyllosilicatemineralwhose crystal structure is described as low-chargeTOT.This means that a crystal of montmorillonite consists of layers, each of which is made up of twoTsheets bonded to either side of anOsheet. TheTsheets are so called because each aluminium or silicon ion in the sheet is surrounded by four oxygen ions arranged as a tetrahedron. TheOsheets are so called because each aluminium ion is surrounded by six oxygen or hydroxyl ions arranged as an octahedron. The completeTOTlayer has a weak negative electrical charge, and this is neutralized by calcium or sodiumcationsthat bind adjacent layers together, with a distance between layers of about 1nanometer.Because the negative charge is weak, only a fraction of the possible cation sites on the surface of aTOTlayer actually contain calcium or sodium. Water molecules can easily infiltrate between sheets and fill the remaining sites. This accounts for the swelling property of montmorillonite and other smectite clay minerals.[6]

The different types of bentonite are each named after the respective dominant cation.[8]For industrial purposes, two main classes of bentonite are recognized: sodium and calcium bentonite. Sodium bentonite is the more valuable but calcium bentonite is more common.[7]Instratigraphyandtephrochronology,completelydevitrified(weathered volcanic glass) ash-fall beds are sometimes also referred to as "K-bentonites" (the illitized clay rock) when the dominant clay species isillite(a non-swelling clay).[9]However, in pure clay mineralogy, the term illite is more appropriate than "K-bentonite" (the "altered K-rock" ) because it is a distinct type of non-swelling clay while the commercial term bentonite implicitly refers to a swelling clay, asmectite(in the European and UK terminology), or amontmorillonite(in the US terminology).

Sodium bentonite[edit]

Sodium bentonite expands when wet, absorbing as much as several times its drymassin water. Because of its excellentcolloidalproperties,[10]it is often used indrilling mudforoiland gas wells and boreholes for geotechnical and environmental investigations.[11]The property of swelling also makes sodium bentonite useful as a sealant, since it provides a self-sealing, low permeability barrier. It is used to line the base oflandfills,for example. Bentonite is also part of the backfill material used at the nuclearWaste Isolation Pilot Project.[12]Various surface modifications to sodium bentonite improve somerheologicalor sealing performance in geoenvironmental applications, for example, the addition of polymers.[13]

Sodium bentonite can be combined withelemental sulfuras fertilizerprills.These permit slow oxidation of the sulfur tosulfate,a plant nutrient needed for some crops likeonionsorgarlicsynthesizing a lot of organo-sulfur compounds, and maintain sulfate levels in rainfall-leached soil longer than either pure powdered sulfur orgypsum.[14]Sulfur/bentonite pads with added organic fertilizers have been used for organic farming.[15]

Calcium bentonite[edit]

Calcium bentonite is a usefuladsorbentof ions in solution,[16]as well as fats and oils. It is the main active ingredient offuller's earth,probably one of the earliest industrial cleaning agents.[17]It has significantly less swelling capacity than sodium bentonite.[18]

Calcium bentonite may be converted to sodium bentonite (termed sodium beneficiation or sodium activation) to exhibit many of sodium bentonite's properties by anion exchangeprocess. As commonly practiced, this means adding 5–10% of a soluble sodium salt such assodium carbonateto wet bentonite, mixing well, and allowing time for the ion exchange to take place and water to remove the exchanged calcium.[19][20]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.[10]For example, residual calcium carbonates (formed if exchanged cations are insufficiently removed) may result in inferior performance of the bentonite in geosynthetic liners.[21]

Illitisation of smectite clays by potassium ions and K-bentonite rock[edit]

Illiteis the main clay constituent of potash bentonite (a rock type also known as K-bentonite or potassium bentonite). K-bentonite is a term reserved tovolcanicstratigraphyandtephrochronologyand is related to theweatheredclay rock type only. Illite, the clay mineral, is apotassium-rich phyllosilicate formed from the alteration of smectic clay in contact with groundwater rich inK+
ions.[9]Illite is a high-chargeTOTclay mineral, in which negatively charged sheets are bound relatively strongly by more numerous potassiumcations,and so it is no longer a swelling clay and has few industrial uses.[22]In contrast to the highly hydratedNa+
ions which act as "swellers" or "expanders" ions, poorly hydratedK+
ions behave as "collapsers" when exchanging withNa+
ions accessible in the interlayers space present between twoTOTlayers. DehydratedK+
ions are preferentially located in between two face-to-face hexagonal cavities formed by six joined silica tetrahedra present at the surface of the basal plane of aTOTlayer (see the corresponding figure showing an elementaryTOTlayer). Because dehydrated, theseK+
ions are sometimes said to form inner-sphere bonds with the surrounding oxygen atoms present in the hexagonal cavity hosting them. It means there is no water molecule in between theK+
ion and the oxygen atoms attached to the silica tetrahedra (T).

Applications[edit]

Making a bentonite slurry for fining after wine pressing

The main uses of bentonite are indrilling mudand as abinder,purifier,absorbent,and carrier forfertilizersorpesticides.As of around 1990, almost half of the US production of bentonite was used as drilling mud. Minor uses includefiller,sealant,andcatalystinpetroleum refining.Calcium bentonite is sometimes marketed asfuller's earth,whose uses overlap with those of other forms of bentonite.[11][23]

Drilling mud[edit]

Bentonite is used indrilling mudtolubricateandcoolthe cutting tools (drill bit), to remove cuttings, to stabilize theboreholewalls, and to help preventblowouts(by maintaining a sufficienthydraulic pressurein the well). Bentonite also curtailsdrilling fluid invasionby its propensity for aiding in the formation ofmud cake.[11]It plays an important role in theearth pressure balanceandslurry shieldvariants oftunnel boring machines.

Much of bentonite's usefulness in the drilling andgeotechnical engineeringindustry comes from its uniquerheologicalproperties. Relatively small quantities of bentonitesuspendedin water form aviscous,shear-thinningmaterial. Most often, bentonite suspensions are alsothixotropic,[24]although rare cases ofrheopecticbehavior have also been reported.[25]At high enough concentrations (about 60 grams of bentonite per litre ofsuspension,~6wt.%), bentonite suspensions begin to take on the characteristics of agel(a fluid with a minimumyield strengthrequired to make it move).[24]

Binder[edit]

Bentonite has been widely used as afoundry-sandbond inironandsteelfoundries.Sodium bentonite is most commonly used for large castings that use dry molds, while calcium bentonite is more commonly used for smaller castings that use "green" or wet molds. Bentonite is also used as a binding agent in the manufacture ofiron ore(taconite) pellets as used in thesteelmaking industry.[11]Bentonite, in small percentages, is used as an ingredient in commercial and homemade clay bodies and ceramic glazes. It greatly increases theplasticityof clay bodies and decreases settling in glazes, making both easier to work with for most applications.[26][27]

Theionic surfaceof 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 asgreen sandused for making molds insand casting.[28]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.[citation needed]

The same effluvial deposition of bentonite clay onto beaches accounts for the variety of plasticity of sand from place to place for buildingsand 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 sandcastles than others.[citation needed]

The self-stickiness of bentonite allows high-pressure ramming or pressing of the clay in molds to produce hard, refractory shapes, such asmodel rocketnozzles.[29]

Purification[edit]

Bentonites are used for decolorizing various mineral, vegetable, and animal oils. They are also used for clarifying wine, liquor, cider, beer, mead, and vinegar.[11]

Bentonite has the property ofadsorbingrelatively large amounts ofproteinmolecules from aqueous solutions. Consequently, bentonite is uniquely useful in the process ofwinemaking,where it is used to remove excessive amounts ofproteinfromwhite wines.Were it not for this use of bentonite, many or most white wines would precipitate undesirableflocculentclouds or hazes upon exposure to warm temperatures, as these proteinsdenature.It also has the incidental use of inducing more rapidclarificationof both red and white wines.[30]

Bentonite is also considered an effective low-cost adsorbent for the removal ofchromium(VI)ions from aqueous solutions (contaminated wastewater).[31]

Absorbent[edit]

Bentonite is used in a variety of pet care items such ascat litterto absorb pet waste. It is also used to absorb oils and grease.[11]

Carrier[edit]

Bentonite is used as an inert carrier forpesticides,fertilizers,andfire retardants.It helps ensure that the active agent is uniformlydispersedand that pesticides and fertilizers are retained on the plants.[11]

Filler[edit]

Bentonite is used as afillerin a wide variety of products, includingadhesives,cosmetics,paint,rubber,andsoaps.It also acts as a stabilizer and extender in these products.[11]

Sealant[edit]

The property of swelling on contact with water makes sodium bentonite useful as a sealant since it provides a self-sealing, low-permeabilitybarrier. It is used to line the base oflandfillsto prevent migration ofleachate,for confining metal pollutants ofgroundwater,and for the sealing of subsurface disposal systems forspent nuclear fuel.[32]Similar uses include makingslurry walls,waterproofing of below-grade walls, and forming other impermeable barriers, e.g., to seal off theannulusof awater well,to plug old wells.

Bentonite can also be "sandwiched" between synthetic materials to creategeosynthetic clay liners(GCLs) for the aforementioned purposes. This technique allows for more convenient transport and installation, and it greatly reduces the volume of bentonite required. It is also used to form barriers around newly planted trees to constrain the growth of its roots: in order to prevent damage to any of the nearby footpaths, parking lots, playgrounds, etc; or any of the surrounding underground infrastructures like the pipes, drainage systems, sewers, etc. Farmers use bentonite to seal retention ponds and line canals.[33]

Catalyst[edit]

High-purity calcium bentonite is treated with acid for use as a catalyst incrackingheavypetroleumfractions.[11]

Medicine[edit]

Bentonite has been prescribed as a bulklaxative,and it is also used as a base for manydermatologicformulas.[34]Granular bentonite is being studied for use in battlefield wound dressings.[35]Bentonite is also sold online and in retail outlets for a variety of indications.[36]

Bentoquatam is a bentonate-based topical medication intended to act as a shield against exposure tourushiol,the oil found in plants such aspoison ivyorpoison oak.[37]

Bentonite can also be used as adesiccantdue to its adsorption properties. Bentonite desiccants have been successfully used to protect pharmaceutical, nutraceutical, and diagnostic products from moisture degradation and extendshelf life.In most common package environments, bentonite desiccants offer a higher water adsorption capacity thansilica geldesiccants. Bentonite complies with theFDAfor contact with food and drugs.[38]

Farming in Thailand[edit]

The application of clay technology by farmers in northeast Thailand, using bentonite clay, has dramatically reversedsoil degradationand resulted in greater economic returns, with higher yields and higher output prices. Studies carried out by TheInternational Water Management Instituteand 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. Applying bentonite clays effectively improved yields of foragesorghumgrown under rain-fed conditions.[39][40]

Bentonite application also influenced the prices that farmers received for their crops. Production costs are higher, but due to higher production and the quality of the food, clay farmers could afford to invest and grow more and better food, compared to nonclay-using farmers.[41][42]

Bentonite slurry walls in modern construction[edit]

Bentoniteslurry walls(also known as diaphragm walls [43] ) are used in construction, where the slurry wall is a trench filled with a thickcolloidalmixture of bentonite and water.[44] A trench that would collapse due to the hydraulic pressure in the surrounding soil does not collapse as the slurry balances the hydraulic pressure. Forms for concrete, and rebar, can be assembled in a slurry-filled trench, and then have concrete poured into the form. The liquid concrete being denser displaces the less-dense bentonite slurry and causes the latter to overflow from the trench. This displaced bentonite slurry is then channeled to a recycling unit from which it can subsequently be reused in a new trench elsewhere on the construction site.

In addition, because the colloid is relatively impervious to water, a slurry wall can prevent the seepage of groundwater, which is useful in preventing the further spread of groundwater that has been contaminated by toxic material such as industrial waste.[44]

Ceramics[edit]

Plasticityis the property of clay that allows it to be manipulated and retain its shape without cracking after the shaping force has been removed; clays with low plasticity are known asshortornon-plastic.A small amount of bentonite added to clay can increase its plasticity, and hence ease forming of articles by some shaping techniques. However, bentonite typically contains minerals that affect the fired color of the mix,[26]and its swelling properties can make such a mix prone to significant shrinkage and potential cracking as it dries.[45]

Ceramic glazesoften contain bentonite. The bentonite is added to slow or prevent the settling of the glazes. It can also improve the consistency of application of glazes on porousbiscuit-firedware. Once a certain amount of glaze water has been absorbed by the biscuit the bentonite effectively clogs the pores and resists the absorption of further water resulting in a more evenly thick coat.[27]

Emergency response[edit]

Bentonite is used in industry and emergency response as a chemical absorbent and container sealant.

History and natural occurrence[edit]

Further information:List of countries by bentonite production

In 2018, China was the top producer of bentonite, with almost a one-quarter share of the world's production, followed by the United States and India. Total worldwide production was 24,400,000 metric tons of bentonite and 3,400,000 metric tons of fuller's earth.[46][7]

Most high-grade natural sodium bentonite is produced from the western United States in an area between theBlack HillsofSouth Dakotaand theBighorn Basinof Wyoming, and the Tokat Resadiye region of Turkey.[11]Mixed sodium/calcium bentonite is mined inGreece,Pakistan,Australia,India,Russia,andUkraine.

In the United States, calcium bentonite is mined primarily inMississippiandAlabama.[11]Other major locations producing calcium bentonite include New Zealand, Germany, Greece, Turkey, India, and China.

See also[edit]

References[edit]

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  2. ^"bentonite".Dictionary.com Unabridged(Online). n.d.
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External links[edit]

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