The solubility limit for silica in various waters vary, it is estimated, however, at approximately 120 mg/L at 25☌. Dissolved silica species polymerisation path or aqueous silica solubility was originally described by Kiselev and then adopted by Iler and then later developed by Bergna others ( Figure 1). Because aqueous silica polymerisation depends on number of oxygen atoms attached to the particular group of aqueous silica the term ‘Practical’ silica solubility was introduced in that research to be able more accurately define aqueous silica chemistry and polymerisation processes. Let us start with summary of the set of conclusions which the author found in the past research. Contrarily, not all highly super-saturated silica streams lead to scaling of membrane surfaces as it was shown in the past research. Chemical reactions between these silica species and cations and anions often present in the Concentration Polarisation (CP) layer in super-saturation conditions during the reverse osmosis chemical separation of water molecules are commonly lead to irreversible silica scale formation on the membrane surface. These dissolved silica species (Q 0 < Q 1 < Q 2 < Q 3 aggregation) ( Figure 1) can be presented in various ionisation states which depend on the pH of solutions and silica concentrations and presence of other anions and cations. Soluble or dissolved (reactive) silica contains different forms of silica or silica species ( Figure 1) monomer, dimers, trimmers and other polymers of silicic acid in different solutions. Practical silica solubility or the solubility defined empirically is a key for prevention of silica polymerisation in RO desalination systems. Behaviours of aqueous silica species were studies using reverse osmosis (RO) desalination systems and 29Si NMR techniques and coagulation to gain better understanding of aqueous silica species polymerisation and practical implications of these techniques.
In this chapter, the past and recent studies on various aqueous silica species and its impact are discussed. For seawater the composition is relatively balanced though, this might not explain low silica precipitation in seawater desalination. It appears that some silica species behave as organics. This phenomenon of silica chemistry can be explained by presence of various silica species, which frequently define silica solubility and physicochemical reactions. Temperature, pH and ionic strength have a substantial influence on the solubility of amorphous silica and forms of silica present in a solution. It is difficult to define precisely the term ‘aqueous silica’ as there is an array of silica species possible. For instance average concentration of silica in some groundwater like coal seam gas water ranges between 0.1 and 80.0 ml/L. The dissolution process of silica and silicates from rocks into water is mainly due to hydrolysis of silica-oxygen-silica bonds, resulting in the liberation of silicic acid (Si(OH)4) and silicates into aqueous phase. The concentration of dissolved silica in natural waters is determined by a buffering mechanism which is thought to require the sorption and desorption of dissolved silica by soil particles. The processes involve ion substitution and chelate forming reactions which remove mineral lattice cations. Dissolved silica (SiO2) is supplied to the environment by chemical and biochemical weathering processes despite the fact that dissolved silica has many stable and unstable dissolved forms (silica species).
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