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Environmental fate & pathways

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Of the elemental composition of the earth’s crust, SiO2makes up 59% and similar percentages are present in many sediments and soils. Thus, silicon is the second most abundant element on earth. By weathering of soil, rocks and sediments and by atmospheric deposition, silica is released into surface and ground waters from where it may be removed by precipitation and sedimentation or taken up by living organisms, especially diatoms. Dead sedimenting diatoms also contribute significantly to sediment silica (diatomaceous earth). Silica is found in all natural waters with an average concentration of 10-20 mg SiO2/L (HERA 2005).

Compounds of silicon and oxygen are ubiquitous in the environment; they are present in inorganic matter, like minerals and soils as well as in organic matter, like plants, animals and man. Silicon is an essential trace element participating in the normal metabolism of higher animals. It is required in bone, cartilage and connective tissue formation as well as participating in other important metabolic processes (HERA 2005).

Toxicokinetic data on vertebrates revealed a low potential for bioaccumulation. Ingested soluble silicates are excreted via urine and to a lesser extent via the faeces. Markedly increased and rapid urinary excretion of silica was observed when soluble sodium silicates were administered to rats (Benke & Osborn 1979), dogs (King et al. 1933), cats (King & McGeorge 1938) and guinea pigs (Sauer et al. 1959). The urinary silicon excretion half-life after administration of sodium silicate to rats via stomach tube was 24 h (Benke & Osborn 1979).

Based on these considerations no bioaccumulation is to be expected.