Influence of CO₂ on silica in solution

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.