Abstract
The aqueous chemistry of silica (SiO 2) is regulated by a number of coupled processes such as dissolution/precipitation, complexation to cations and anions in the aqueous phase and complexation to cations and anions at the particle-water interface. Further, uptake by biota is significant in parts of the hydrosphere. The dissolution of silica is strongly pH-dependent in that silicic acid (Si(OH) 4) is formed at pH values ≤ 9. In more alkaline solutions, monosilicates (SiO(OH) 3 −), SiO 2(OH) 2 2−) and polysilicates are formed. Composition and thermodynamic stability of these polysilicates are still not fully understood. The surface chemistry of silica has been extensively studied throughout the years. Based on recent surface complexation models, thermodynamic data describing acid/base properties, as well as cation (metal ion) and anion adsorption/desorption reactions have been presented. The acidic properties of a silica surface have been described by an acidity constant for surface SiOH groups with pK a(intr.) = 6.8. Further, surface complexation constants for several metal ions have been published showing a strong correlation between hydrolytic properties of the metal ion in aqueous solution and at the SiO 2-water interface.
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