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Bentonite (/ˈbɛntənaɪt/BEN-tə-nyte)[1][2] is anabsorbent swellingclay consisting 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 ash inseawater, or byhydrothermal circulation through theporosity of volcanic ash beds,[3][4] which converts (devitrification) thevolcanic glass (obsidian, a volcanic glass with a chemical composition equivalent torhyolite) present in the ash into clay minerals. In the mineral alteration process, a large fraction (up to 40–50 wt.%) ofamorphoussilica isdissolved andleached away, 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 mud and 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 abinder and as an additive to improve the plasticity ofkaolinite clay used forpottery.[6]
One of the first findings of bentonite was in theCretaceousBenton Shale nearRock River, Wyoming. TheFort Benton Group, along with others in stratigraphic succession, was named afterFort Benton, Montana, in the mid-19th century byFielding Bradford Meek andF. V. Hayden of the U.S. Geological Survey.[4] Bentonite has since been found in many other locations, including China and Greece (bentonite deposit of theMilos volcanic island in theAegean Sea). The total worldwide production of bentonite in 2018 was 20,400,000 metric tons.[7]
In geology, the termbentonite is applied to a type ofclaystone (a clay rock, not a clay mineral) composed mostly ofmontmorillonite (a clay mineral from thesmectite group). It forms by devitrification ofvolcanic ash ortuff,[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 termbentonite is used more generally to refer to any swelling clay composed mostly ofsmectite clay 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 analuminiumphyllosilicatemineral whose crystal structure is described as low-chargeTOT. This means that a crystal of montmorillonite consists of layers, each of which is made up of twoT sheets bonded to either side of anO sheet. TheT sheets are so called because each aluminium or silicon ion in the sheet is surrounded by four oxygen ions arranged as a tetrahedron. TheO sheets are so called because each aluminium ion is surrounded by six oxygen or hydroxyl ions arranged as an octahedron. The completeTOT layer has a weak negative electrical charge, and this is neutralized by calcium or sodiumcations that 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 aTOT layer 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] Instratigraphy andtephrochronology, 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 expands when wet, absorbing as much as several times its drymass in water. Because of its excellentcolloidal properties,[10] it is often used indrilling mud foroil and 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 somerheological or sealing performance in geoenvironmental applications, for example, the addition of polymers.[13]
Sodium bentonite can be combined withelemental sulfur as fertilizerprills. These permit slow oxidation of the sulfur tosulfate, a plant nutrient needed for some crops likeonions orgarlic synthesizing 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 is a usefuladsorbent of 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 exchange process. As commonly practiced, this means adding 5–10% of a soluble sodium salt such assodium carbonate to 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]
Illite is the main clay constituent of potash bentonite (a rock type also known as K-bentonite or potassium bentonite). K-bentonite is a term reserved tovolcanicstratigraphy andtephrochronology and is related to theweathered clay 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-chargeTOT clay 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 twoTOT layers. 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 aTOT layer (see the corresponding figure showing an elementaryTOT layer). 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).
The main uses of bentonite are indrilling mud and as abinder, purifier,absorbent, and carrier forfertilizers orpesticides. As of around 1990, almost half of the US production of bentonite was used as drilling mud. Minor uses includefiller,sealant, andcatalyst inpetroleum refining. Calcium bentonite is sometimes marketed asfuller's earth, whose uses overlap with those of other forms of bentonite.[11][23]
Bentonite is used indrilling mud tolubricate andcool the cutting tools (drill bit), to remove cuttings, to stabilize theborehole walls, and to help preventblowouts (by maintaining a sufficienthydrostatic pressure in the well). Bentonite also curtailsdrilling fluid invasion by its propensity for aiding in the formation ofmud cake.[11] It plays an important role in theearth pressure balance andslurry shield variants oftunnel boring machines.
Much of bentonite's usefulness in the drilling andgeotechnical engineering industry comes from its uniquerheological properties. Relatively small quantities of bentonitesuspended in water form aviscous,shear-thinning material. Most often, bentonite suspensions are alsothixotropic,[24] although rare cases ofrheopectic behavior 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 strength required to make it move).[24]
Bentonite has been widely used as afoundry-sand bond iniron andsteelfoundries. 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 theplasticity of clay bodies and decreases settling in glazes, making both easier to work with for most applications.[26][27]
Theionic 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 asgreen sand used 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 rocket nozzles.[29]
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 ofadsorbing relatively large amounts ofprotein molecules from aqueous solutions. Consequently, bentonite is uniquely useful in the process ofwinemaking, where it is used to remove excessive amounts ofprotein fromwhite wines. Were it not for this use of bentonite, many or most white wines would precipitate undesirableflocculent clouds or hazes upon exposure to warm temperatures, as these proteinsdenature. It also has the incidental use of inducing more rapidclarification of 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]
Bentonite is used in a variety of pet care items such ascat litter to absorb pet waste. It is also used to absorb oils and grease.[11]
Bentonite is used as an inert carrier forpesticides,fertilizers, andfire retardants. It helps ensure that the active agent is uniformlydispersed and that pesticides and fertilizers are retained on the plants.[11]
Bentonite is used as afiller in a wide variety of products, includingadhesives,cosmetics,paint,rubber, somegreases, andsoaps. It also acts as a stabilizer and extender in these products.[11]
The property of swelling on contact with water makes sodium bentonite useful as a sealant since it provides a self-sealing, low-permeability barrier. It is used to line the base oflandfills to 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 theannulus of 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]
High-purity calcium bentonite is treated with acid for use as a catalyst incracking heavypetroleum fractions.[11]
Bentonite has been prescribed as a bulklaxative, and it is also used as a base for manydermatologic formulas.[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 ivy orpoison oak.[37]
Bentonite can also be used as adesiccant due 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 gel desiccants. Bentonite complies with theFDA for contact with food and drugs.[38]
The application of clay technology by farmers in northeast Thailand, using bentonite clay, has dramatically reversedsoil degradation and resulted in greater economic returns, with higher yields and higher output prices. Studies carried out by TheInternational 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. Applying bentonite clays effectively improved yields of foragesorghum grown 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]
Bentoniteslurry walls (also known as diaphragm walls[43]) are used in construction, where the slurry wall is a trench filled with a thickcolloidal mixture 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]
Plasticity is 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 asshort ornon-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 glazes often 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-fired ware. 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]
Bentonite is used in industry and emergency response as a chemical absorbent and container sealant.
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 Hills ofSouth Dakota and theBighorn Basin of 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 inMississippi andAlabama.[11] Other major locations producing calcium bentonite include New Zealand, Germany, Greece, Turkey, India, and China.
