Diatomaceous earth

Diatomaceous earth ( /ˌdaɪ.ətəˌmeɪʃəs ˈɜːrθ/, 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.[1]

A sample of food grade diatomaceous earth

Diatomaceous earth consists of fossilized remains of diatoms, a type of hard-shelled protist. 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, a thermal insulator, and a soil for potted plants and trees like bonsai.[2][3]

Diatomaceous earth as viewed under bright field illumination on a light microscope. This image of diatomaceous earth particles in water is at a scale of 6.236 pixels/μm, the entire image covers a region of approximately 1.13 by 0.69 mm.
SEM photo of diatomaceous earth

Specific varieties
  • Tripolite 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.
  • Freshwater-derived food grade diatomaceous earth is the type used in United States agriculture for grain storage, as feed supplement, and as an insecticide. It is produced uncalcinated, has a very fine particle size, and is very low in crystal silica (<2%).
  • Salt-water-derived pool / beer / wine filter grade is not suitable for human consumption or effective as an insecticide. Usually calcinated before being sold to remove impurities and undesirable volatile contents, it is composed of larger particles than the freshwater version and has a high crystalline silica content (>60%).

Microbial degradation

Certain species of bacteria in oceans and lakes can accelerate the rate of dissolution of silica in dead and living diatoms; by using hydrolytic enzymes to break down the organic algal material.[30][31]

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. Research shows that significant dust comes from the Bodélé depression in Chad, where storms push diatomite gravel over dunes, generating dust by abrasion.[32]

Safety considerations

Inhalation of crystalline silica is harmful to the lungs, causing silicosis. Amorphous silica is considered to have low toxicity, but prolonged inhalation causes changes to the lungs.[33] Diatomaceous earth is mostly amorphous silica, but contains some crystalline silica, especially in the saltwater forms.[34] In a study of workers, those exposed to natural D.E. for over 5 years had no significant lung changes, while 40% of those exposed to the calcined form had developed pneumoconiosis.[35]Today's common D.E. formulations are safer to use as they are predominantly made up of amorphous silica and contain little or no crystalline silica.[36]

The crystalline silica content of D.E. is regulated in the United States by the Occupational Safety and Health Administration (OSHA), and there are guidelines from the National Institute for Occupational Safety and Health setting maximum amounts allowable in the product (1%) and in the air near the breathing zone of workers, with a recommended exposure limit at 6 mg/m3 over an 8-hour workday.[36] OSHA has set a permissible exposure limit for diatomaceous earth as 20 mppcf (80 mg/m3/%SiO2). At levels of 3000 mg/m3, diatomaceous earth is immediately dangerous to life and health.[37]

In the 1930s, long-term occupational exposure among workers in the cristobalite D.E. industry who were exposed to high levels of airborne crystalline silica over decades were found to have an increased risk of silicosis.[38]

Today, workers are required to use respiratory-protection measures when concentrations of silica exceed allowable levels.

Diatomite produced for pool filters is treated with high heat (calcination) and a fluxing agent (soda ash), causing the formerly harmless amorphous silicon dioxide to assume its crystalline form.[36]

See also
  • Biomineralization – Process by which living organisms produce minerals
  • Diatom – A class of microalgae, found in the oceans, waterways and soils of the world
  • Frustule
  • Fuller's earth – Any clay material that can decolorise oil or other liquids
  • Perlite – Amorphous volcanic glass
  • Rock flour
  • Silica aerogel
  • Siliceous ooze
  • Zeolite – Microporous, aluminosilicate minerals commonly used as commercial adsorbents and catalysts

References
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