diatomaceous earth

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diatomaceous earth

diatomaceous earth: see diatom.

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

or kieselguhr

Light-coloured, porous, and friable sedimentary rock composed of the frustrules (silicate cell walls) of diatoms. It is used in industrial filtration applications; as a filler or extender in paper, paint, brick, tile, ceramics, linoleum, plastic, soap, detergent, and other products; in insulation for boilers, blast furnaces, and other high-temperature devices; as a sound insulator; and as a carrier for herbicides and fungicides. The oldest and best-known commercial use is as a very mild abrasive in metal polishes and toothpaste. Large deposits occur in California, Nevada, Washington, and Oregon; other sources are Denmark, France, Russia, and Algeria.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.

Diatomaceous earth

Diatomaceous earth also known as DE, TSS, diatomite, diahydro, kieselguhr, kieselgur and celite) is a naturally occurring, soft, chalk-like sedimentary rock that is easily crumbled into a fine white to off-white powder. This powder has an abrasive feel, similar to pumice powder, and is very light, due to its high porosity. The typical chemical composition of diatomaceous earth is 86% silica, 5% sodium, 3% magnesium and 2% iron.

Diatomaceous earth consists of fossilized remains of diatoms, a type of hard-shelled algae. It is used as a filtration aid, as a mild abrasive, as a mechanical insecticide, as an absorbent for liquids, as cat litter, as an activator in blood clotting studies, and as a component of dynamite. As it is also heat-resistant, it can be used as a thermal insulator.

Applications

Industrial

In 1866, Alfred Nobel discovered that nitroglycerin could be made much more stable if absorbed in diatomite. This allows much safer transport and handling than nitroglycerin in its raw form. He patented this mixture as dynamite in 1867, and the mixture is also referred to as guhr dynamite.

Filtration

The most common use (68%) of diatomaceous earth is as a filter medium, especially for swimming pools. It has a high porosity, because it is composed of microscopically-small, coffin-like, hollow particles. It is used in chemistry under the name Celite as a filtration aid, to filter very fine particles that would otherwise pass through or clog filter paper. It is also used to filter water, particularly in the drinking water treatment process and in fish tanks, and other liquids, such as beer and wine. It can also filter syrups and sugar. Other industries such as paper, paints, ceramics, soap and detergents use it as a fulling material.

Abrasive

The oldest use of diatomite is as a very mild abrasive and, for this purpose, it has been used both in toothpaste and in metal polishes, as well as in some facial scrubs.

Pest control

Diatomite is also used as an insecticide, due to its physico-sorptive properties. The fine powder absorbs lipids from the waxy outer layer of insects' exoskeletons, causing them to dehydrate. Arthropods die as a result of the water pressure deficiency, based on Fick's law of diffusion. This also works against gastropods and is commonly employed in gardening to defeat slugs. However, since slugs inhabit humid environments, efficacy is very low. It is sometimes mixed with an attractant or other additives to increase its effectiveness. Medical-grade diatomite is sometimes used to de-worm both animals and humans. It is most commonly used in lieu of boric acid, and can be used to help control and eventually eliminate a cockroach infestation. This material has wide application in control of insects of grain storage.

Disadvantages of using diatomaceous earth for pest control include the health risk to humans (see below), and the harm it does to many beneficial insects, including predatory beetles and bugs and many detritivores.

Absorbent

Its absorbent qualities make it useful for spill clean-up and the U.S. Center for Disease Control recommends it to clean up toxic liquid spills.

It has been employed as a primary ingredient in a type of cat litter. The type of silica used in cat litter comes from freshwater sources and does not pose a significant health risk to pets or humans.

Thermal

Its thermal properties enable it to be used as the barrier material in some fire resistant safes.

Hydroponics

Freshwater diatomite can be used as a growing medium in hydroponic gardens.

It is also used as a growing medium in potted plants, particularly as bonsai soil. Bonsai enthusiasts use it as a soil additive, or pot a bonsai tree in 100% Diatomaceous earth. Like perlite, vermiculite, and expanded clay, it retains water and nutrients while draining fast and freely allowing high oxygen circulation within the growing medium.

DNA purification

Diatomite (Celite) can be used for the removal of DNA in the presence of a highly concentrated chaotropic agent such as sodium iodide, guanidinium hydrochloride and guanidinium thiocyanate. As with other silicates, the diatomites will remove double stranded DNA but not RNA or proteins. The DNA can be extracted from the diatomites using low ionic strength buffers, including water, at neutral to slightly alkaline pH. Crude diatomites of a uniform size must first be washed in a heated acid such as 5M HCl. Calcination can further improve consistency of the material, while mild caustic treatment may improve adsorption with lower levels of chaotrophs.

Geology

Because diatomite forms from the remains of water-borne diatoms, it is found close to either current or former bodies of water. It is generally divided into two categories based upon source: freshwater and saltwater. Freshwater diatomite is mined from dry lakebeds and is characteristically low in crystalline silica content. Saltwater diatomite contains a high crystalline silica content, making it a useful material for filters, due to the sieve-like features of the crystals.

Specific varieties

TripoliteDakine is the variety found in Tripoli, Libya.
Bann clay is the variety found in the Lower Bann valley in Northern Ireland.
Moler (Mo-clay) is the variety found in northwestern Denmark, especially on the islands of Fur and Mors.

Climatologic importance

The Earth's climate is affected by dust in the atmosphere, so locating major sources of atmospheric dust is important for climatology. Recent research indicates that surface deposits of diatomaceous earth play an important role. For instance, the largest single atmospheric dust source is the Bodélé depression in Chad, where storms push diatomite gravel over dunes, generating dust by abrasion.

Safety considerations

The absorbent qualities of diatomite can result in a significant drying of the hands, if handled without gloves. The saltwater (industrial) form contains a highly crystalline form of silica, resulting in sharp edges. The sharpness of this version of the material makes it dangerous to breathe and a dust mask is recommended when working with it.

The type of hazard posed by inhalation depends on the form of the silica. Crystalline silica poses a serious inhalation hazard because it can cause silicosis. Amorphous silica can cause dusty lungs, but does not carry the same degree of risk as crystalline silica. Food-grade diatomite generally contains very low percentages of crystalline silica. Diatomite produced for pool filters is treated with heat, causing the formerly amorphous silicon dioxide to assume its crystalline form.

In the United States, the crystalline silica content in the dusts is regulated by the Occupational Safety and Health Administration (OSHA), and there are guidelines for the maximum amounts allowable in the product and in the air near the breathing zone of workers.