Diatomaceous earth

Diatomaceous earth (also known as DE, diatomite, or kieselgur/kieselguhr) is a naturally occurring, soft, siliceous sedimentary rock that is easily crumbled into a fine white to off-white powder. It has a particle size ranging from less than 3 μm to more than 1 mm, but typically 10 to 200 μm. Depending on the granularity, this powder can have an abrasive feel, similar to pumice powder, and has a low density as a result of its high porosity. The typical chemical composition of oven-dried diatomaceous earth is 80–90% silica, with 2–4% alumina (attributed mostly to clay minerals) and 0.5–2% iron oxide.

Diatomaceous earth consists of fossilized remains of diatoms, a type of hard-shelled protist (chrysophytes). It is used as a filtration aid, mild abrasive in products including metal polishes and toothpaste, mechanical insecticide, absorbent for liquids, matting agent for coatings, reinforcing filler in plastics and rubber, anti-block in plastic films, porous support for chemical catalysts, cat litter, activator in blood clotting studies, a stabilizing component of dynamite, and a thermal insulator.

Composition

Each deposit of diatomaceous earth is different, with varying blends of pure diatomaceous earth combined with other natural clays and minerals. The diatoms in each deposit contain different amounts of silica, depending on the age of the deposit. The species of diatom may also differ among deposits. The species of diatom is dependent upon the age and paleo-environment of the deposit. In turn, the shape of a diatom is determined by its species.

Many deposits throughout British Columbia, such as the one at Graham Island, are from the Miocene age and contain a species of diatom known as Melosira granulata. These diatoms are approximately 12 to 13 million years old and have a small globular shape. A deposit containing diatoms from this age can provide many more benefits than that of an older deposit. For example, diatoms from the Eocene age (approximately 40 to 50 million years old) are not as effective in their ability to absorb fluids because older diatoms recrystallize, their small pores becoming filled with silica.

Applications

Diatomaceous earth is widely applied for insect control in grain storage.

In order to be effective as an insecticide, diatomaceous earth must be uncalcinated (i.e., it must not be heat-treated prior to application) and have a mean particle size below about 12 μm (i.e., food grade – see below).

Although considered to be relatively low-risk, pesticides containing diatomaceous earth are not exempt from regulation in the United States under the Federal Insecticide, Fungicide, and Rodenticide Act and must be registered with the Environmental Protection Agency.

Safety considerations

Inhalation of diatomaceous earth may cause irritation of the nasal passages. If an extremely large amount is inhaled, people may experience coughing, shortness of breath, and chest pain. Pool grade (also called filter grade) diatomaceous earth has been heat and chemically treated and contains high amounts of silica. It is classified as a Group 1 carcinogen to humans.

See also

• Silicosis
• Pest (organism)
• Pesticide
• Insecticide
• Fungicide
• Rodenticide

References

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