Bentonite - WikiHQ

upright=1.3|thumb|Bentonite layers from an ancient deposit of [[weathering|weathered volcanic ash tuff in Wyoming]]

thumb|Gray shale and bentonites ([[Benton Shale; Colorado Springs, Colorado)]]

Bentonite ( ) is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) 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 of volcanic ash in seawater, or by hydrothermal circulation through the porosity of volcanic ash beds,

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. Bentonite has since been found in many other locations, including China and Greece (bentonite deposit of the Milos volcanic island in the Aegean Sea). The total worldwide production of bentonite in 2018 was 20,400,000 metric tons.

Types

thumb|2:1 [[clay minerals crystallographic structure made of three superimposed sheets of Tetrahedra-Octahedra-Tetrahedra (TOT layer unit), respectively]]

thumb|Detailed molecular structure of pure [[montmorillonite, the best known end-member of the smectite group. The interlayer space between two successive TOT layers is filled with hydrated cations (mainly and ions) compensating the negative electrical charges of the TOT layers and with water molecules causing the interlayer expansion.]]

In geology, the term bentonite is applied to a type of claystone (a clay rock, not a clay mineral) composed mostly of montmorillonite (a clay mineral from the smectite group). It forms by devitrification of volcanic ash or tuff, 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. 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, a smectite (in the European and UK terminology), or a montmorillonite (in the US terminology).

Sodium bentonite

Sodium bentonite expands when wet, absorbing as much as several times its dry mass in water. Because of its excellent colloidal properties, 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 of landfills, for example. Bentonite is also part of the backfill material used at the nuclear Waste Isolation Pilot Project. Various surface modifications to sodium bentonite improve some rheological or sealing performance in geoenvironmental applications, for example, the addition of polymers.

Sodium bentonite can be combined with elemental sulfur as fertilizer prills. These permit slow oxidation of the sulfur to sulfate, a plant nutrient needed for some crops like onions or garlic synthesizing a lot of organo-sulfur compounds, and maintain sulfate levels in rainfall-leached soil longer than either pure powdered sulfur or gypsum. Sulfur/bentonite pads with added organic fertilizers have been used for organic farming.

Calcium bentonite

Calcium bentonite is a useful adsorbent of ions in solution, as well as fats and oils. It is the main active ingredient of fuller's earth, probably one of the earliest industrial cleaning agents. It has significantly less swelling capacity than sodium bentonite.

Calcium bentonite may be converted to sodium bentonite (termed sodium beneficiation or sodium activation) to exhibit many of sodium bentonite's properties by an ion exchange process. As commonly practiced, 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.

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

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 to volcanic stratigraphy and tephrochronology and is related to the weathered clay rock type only. Illite, the clay mineral, is a potassium-rich phyllosilicate formed from the alteration of smectic clay in contact with groundwater rich in ions.

Drilling mud

Bentonite is used in drilling mud to lubricate and cool the cutting tools (drill bit), to remove cuttings, to stabilize the borehole walls, and to help prevent blowouts (by maintaining a sufficient hydrostatic pressure in the well). Bentonite also curtails drilling fluid invasion by its propensity for aiding in the formation of mud cake. although rare cases of rheopectic behavior have also been reported. At high enough concentrations (about 60 grams of bentonite per litre of suspension, ~6wt.%), bentonite suspensions begin to take on the characteristics of a gel (a fluid with a minimum yield strength required to make it move).

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.

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.

Purification

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

Bentonite is also considered an effective low-cost adsorbent for the removal of chromium(VI) ions from aqueous solutions (contaminated wastewater).

Absorbent

Bentonite is used in a variety of pet care items such as cat litter to absorb pet waste. It is also used to absorb oils and grease. 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.

Bentonite can also be "sandwiched" between synthetic materials to create geosynthetic 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.

Catalyst

High-purity calcium bentonite is treated with acid for use as a catalyst in cracking heavy petroleum fractions. Granular bentonite is being studied for use in battlefield wound dressings. Bentonite is also sold online and in retail outlets for a variety of indications.

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.

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 most common package environments, bentonite desiccants offer a higher water adsorption capacity than silica gel desiccants. Bentonite complies with the FDA for contact with food and drugs.

Farming in Thailand

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. Applying bentonite clays effectively improved yields of forage sorghum grown under rain-fed conditions.

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.

Bentonite slurry walls in modern construction

Bentonite slurry walls (also known as diaphragm walls

) are used in construction, where the slurry wall is a trench filled with a thick colloidal mixture of bentonite and water.

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.

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 porous biscuit-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.