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Geochemical Journal
Geochemical Journal An open access journal for geochemistry
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Influence of CO2 on silica in solution

Ghansityam D. Sharma
Geochemical Journal, Vol. 3, No. 4, P. 213-223, 1970

ABSTRACT

Laboratory experiments show that H4SiO4 (monomeric silica) up to 80ppm in seawater is stable under all CO2 pressures from 0 to 17.2 × 103 kg/m2. CaCO3, AlCl3·6H2O, MgCl2, and standard clays, when added individually, did not remove silica from solutions containing silica from 50 to 80ppm under these CO2 pressures. Addition of CaCO3 and AlCl3·6H2O together, however, removed silica from solution in inverse proportion to the CO2 pressure. CO2 serves to inhibit the removal of silica from solution containing CaCO3 and aluminum chloride. The CaCO3 functions as a buffer to maintain the acid to neutral pH. The final reaction appears to follow: 2H4SiO4 + Al3++ 2CaCO3 + H2O = AISi2O5(OH)·4H2O + 2Ca2+ + HCO3- + CO2. In an open system, where CO2 can escape to the atmosphere, the removal of silica from solution is controlled by the available AlCl3·6H2O and CaCO3. Electron microscopy and infrared analyses confirm that the principal precipitates were clayey sediments and trace amounts of sepiolite. This suggests an intimate interrelationship between the geochemical cycles of carbon dioxide, calcium carbonate, aluminum, and silica in nature.

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