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As inorganic substance, silicon orthophosphate is not amenable to photo- or biodegradation. 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.


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.