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Environmental fate & pathways

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As inorganic substances and in view of their chemical structure, soluble silicates are not amenable to photo- and biodegradation.

 

With respect to hydrolysis stability depends to a large extent on pH, where solutions are chemically stable above pH 10.6. The basic consideration is that silica dissolves according to: SiO2+ H2O = Si(OH)4. At low concentrations most species are present as monomers, at higher concentrations polymerisation will occur.

 

Due to the low vapour pressure, volatilisation is also not expected.

 

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 European rivers in mean concentrations of 7.5 mg SiO2/L (Jorgensen et al. 1991) and 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 (Benke & Osborn 1979, King et al. 1933, King & McGeorge 1938, Sauer et al. 1959). Based on these considerations no bioaccumulation is to be expected. A secondary poisoning can also be excluded, since the bioaccumulation potential is low.