Soluble Silica in Nature

In addition to the silica carried into the sea by fresh water, additional soluble silica comes from the suspended colloidal clays and related minerals. Tests show that common colloidal silicates like clay will dissolve in seawater sufficiently to give a silica concentration of 10 ppm (35). 

Concentrations of silica of around 2 ppm were reached in dilute salt solution with mica and kaolin and up to 15 ppm with montmorillonite (36). When seawater was enriched with soluble silica to 25 ppm SiOa, it remained at that level for a year in the absence of these minerals, but when the latter were then added, the silica was removed from solution down to the 2-15 ppm level that was reached when the minerals alone were added. Since many ocean waters contain 2-10 ppm SiOj, it is possible that this value is reached as the equilibrium solubility of colloidal aluminosilicate in suspension. The above experiment is consistent with the fact that in pure water, pure a morphous silica dissolves to give a concentration of monosilicic acid of 100-110 ppm, but in the presence of polyvalent metal cations such as iron, aluminum, and other metals, colloidal silicates are formed with a much lower solubility with respect to monosilicic acid. Her (37) has shown that soluble aluminum reduces the solubility of amorphous silica from about 110 to less than 10 ppm. 

Willey (38, 39) has studied the natural interaction of soluble alumina and soluble silica in 0.6 N sodium chloride solution. Addition of aluminum ion to 200 ppm Si(OH)4 retards polymerization. Probably there is formed a colloidal complex which reacts as monomer when put Into the strongly acidic molybdate reagent. A very low concentration of soluble silica also.causes the precipitation of alumina. 

Soluble silica as determined by the molybdate method is not necessarily present as Si(OH)4. Bogdanova (40) reported that in natural waters that contained only about 5 ppm total silica, 4-9% of the silica was polymeric but was converted to monomer by acid. It is most likely that the polymeric silica was actually very small colloidal particles of aluminum silicate that liberated monomer when acidified..  

Silica is continuously removed from seawater by biochemical processes. Diatoms and sponges as well as plants remove silica which is stored within the organism. Although Calvert (41) believes that the concentration of silica in the sea is mainly controlled by biological activity. Harder (42) reports that amorphous hydroxides of Al, Fe, Mn, or Mg can react with and precipitate soluble silica, thus reducing the concentration to as low as 3 ppm. Both processes arc no doubt operative. 

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In view of the universal presence of at least some soluble silica in natural waters, it is not surprising that it is considered harmless in food and drink. Monomeric Si(OH)4 penetrates all body liquids and tissues at concentrations less than its solubility (0.01%) and is readily excreted (187a, 205). In human blood, in both corpuscles and plasma, the silica concentration is 1 ppm and in bovine blood 2.2 ppm. 

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