Silicon orthophosphate

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Additional information

Silicon orthophosphate is an inorganic salt, which easily dissociates into silicon and phosphate ions after introducing into the aquatic environment (relevant pH values range from 4 – 9). The tetravalent silicon ion and the phosphate ion will then react with the media to form different silicon and phosphate species depending on the pH and redox potential of the media.

The dissociated silicon ion hydrolyses in water and exists predominantly as orthosilicic acid H4SiO4/Si(OH)4, which is also the main species when silicon dioxide is dissolved in water. The dissociation constants of orthosilicic acid are high (pKa 9.9, 11.8, 12 & 12 at 30 °C, Lide & Frederikse 1995). The sum of soluble silicate rapidly decreases when the pH is lowered to 9. Because of these, at environmental pH values of 6.5 – 8.5 large amounts of the soluble silicate ions might be removed from the aqueous solution gradually(OECD SIDS, 2004).

In the case of free aqueous phosphate, four forms are existent. PO34- predominates in strong basic conditions, whereas H3PO4 is the main form in strong acid conditions. In the normal aquatic environment (pH 4-9), H2PO4- and HPO42- are prevalent and show equilibrium, which is relatively stable and not significantly affected by the normal environmental pH conditions.

As inorganic substance, silicon orthophosphate is not amenable to photo- or biodegradation. For silicon orthophosphate as well as its products of dissociation, bioaccumulation is not expected. As an inorganic compound, silicon orthophosphate is insoluble in n-octanol, making determination of a log Kow not feasible.

If silicon orthophosphate is released into the environment, it is expected to combine indistinguishably with the soil or sediment, due to the similarity with inorganic soil/sediment matter and will be subjected to natural processes under environmental conditions (cation exchange, dissolution, sedimentation).

Of the elemental composition of the earth’s crust, silicon is the second most abundant element after oxygen, i.e. 28% (Salminen, 2012). Silicon oxides are the most abundant compounds in the earth’s crust mass (> 60%; Salminen, 2012). Compounds of silicon, oxygen and phosphorus 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. 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, espacially diatoms as normal process mainly related to structural function. Silica is found in all natural waters with an average concentration of 8 mg SiO2/L (Salminen, 2012).

References:

Lide, D. R. and Frederikse, H. P. R., editors (1995). Handbook of Chemistry and Physics, 76th Edition. CRS Press, Boca Raton.

OECD SIDS (2004). Synthetic amorphous silica and silicates, SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 19-22 October 2004.

Salminen R. (ed.) (2012): FOREGS: Geochemical Atlas of Europe. ISBN 951-690-913-2 (electronic version).