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Environmental fate & pathways

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In view of their chemical structure and inorganic nature, soluble silicates are not expected to be photodegraded. The basic structural unit of soluble silicates is a tetrahedral arrangement of four oxygen atoms surrounding a central silicon atom. Tetrahedra are linked with each other via Si-O-Si bonds resulting in an infinite three-dimensional network where the oxygen atoms at the corners of a given tetrahedron are shared with neighbouring tetrahedra. Not all corners in the tetrahedra are shared; the negative charge of unshared oxygen atoms is balanced by the presence of sodium cations which are randomly spaced in the interstices of the silicate structure.

With respect to hydrolysis stability is to large extent depending on the pH regime. The basic consideration is that silica dissolves according to: SiO2+ H2O = Si(OH)4. At low concentrations most species are present as monomers, at higher concentrations polymerisation will occur. Most soluble silicates are in the form: M2O x mSiO2x nH2O where M = alkali metal, predominantly Na, but also K. The index m (molar ratio) ranges between 0.5 - 4, most commonly m = 3.3. Above pH 10.6 the solutions are chemically stable. The increase of ionic strength accelerates nucleation and deposition and decreases the SiO2solubility. Coating of surfaces by organic matter may hamper dissolution, but at the same time Si(OH)4may form complexes with organic matter, a process which favours dissolution (Falcone 1997).