Bentonite

Bentonite (/ˈbɛntənaɪt/ BEN-tə-nyte)[1][2] is an absorbent swelling clay consisting mostly of montmorillonite (a type of smectite) which can either be Na-montmorillonite or Ca-montmorillonite.

In the mineral alteration process, a large fraction (up to 40–50 wt.%) of amorphous silica is dissolved and leached away, leaving the bentonite deposit in place.

[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.

The montmorillonite / smectite making up bentonite is an aluminium phyllosilicate mineral, which takes the form of microscopic platy grains.

[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.

[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.

Because the negative charge is weak, only a fraction of the possible cation sites on the surface of a TOT layer actually contain calcium or sodium.

Because of its excellent colloidal properties,[10] it is often used in drilling mud for oil and gas wells and boreholes for geotechnical and environmental investigations.

[12] Various surface modifications to sodium bentonite improve some rheological or sealing performance in geoenvironmental applications, for example, the addition of polymers.

[10] For example, residual calcium carbonates (formed if exchanged cations are insufficiently removed) may result in inferior performance of the bentonite in geosynthetic liners.

K-bentonite is a term reserved to volcanic stratigraphy and tephrochronology and is related to the weathered clay rock type only.

Because dehydrated, these K+ ions are sometimes said to form inner-sphere bonds with the surrounding oxygen atoms present in the hexagonal cavity hosting them.

[11] It plays an important role in the earth pressure balance and slurry shield variants of tunnel boring machines.

It greatly increases the plasticity of clay bodies and decreases settling in glazes, making both easier to work with for most applications.

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 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.

Were it not for this use of bentonite, many or most white wines would precipitate undesirable flocculent clouds or hazes upon exposure to warm temperatures, as these proteins denature.

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

[11] Bentonite is used as a filler in a wide variety of products, including adhesives, cosmetics, paint, rubber, some greases, and soaps.

[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 of landfills to prevent migration of leachate, for confining metal pollutants of groundwater, and for the sealing of subsurface disposal systems for spent nuclear fuel.

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.

Bentonite desiccants have been successfully used to protect pharmaceutical, nutraceutical, and diagnostic products from moisture degradation and extend shelf life.

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.

A small amount of bentonite added to clay can increase its plasticity, and hence ease forming of articles by some shaping techniques.

[46][7] 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, and the Tokat Resadiye region of Turkey.

[11] Other major locations producing calcium bentonite include New Zealand, Germany, Greece, Turkey, India, and China.