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Environmental fate & pathways

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After oxygen, silicon is next abundant in the earth’s crust in the form of silicate minerals. Silicon is not known to be present naturally in the environment in its reduced elemental form. If released in the environment as its elemental metallic form Si (0), it is oxidized by atmospheric oxygen or hydrolysed to Si (IV) silica species (SiO2, in aq. solution Si(OH)4). The rate of these transformations to oxidized forms is highly dependent on particle size/surface area of released silicon particles and environmental conditions.

Oxidized silicon as Si(IV) in fresh water or seawater can occur in both suspended and dissolved forms and is partitioned over a number of chemical species. In dilute solutions (< 100 mg/L) silicon is as dissolved Si(OH)4, and in more concentrated solutions as dimerized, trimerized, colloidal or in the form of aggregated colloids of different physical size or entirely as insoluble solid particulate matter.

Silicon is practically non volatile in the environment. Amorphous silica in its solid particulate form is completely non volatile and immobile substance. Dissolved silica is also non volatile and partitions mainly in aquatic and soil compartments.

Adsorption of dissolved silica to soil and sediments is not strong and it is relatively mobile in soil. Adsorption takes place primarily to inorganic materials and in lesser extent to organic material. A dynamic adsorption/desorption equilibrium between soil/sediment particles and water keeps the surface water aquatic concentrations of dissolved silica relatively constant (ca. 2-15 mg/l). Higher concentrations can be found in ground waters. Very strong binding of silica to soil materials may takes place if dissolved silica species form precipitates with other elements like Al and Mg and forms several types of clay minerals with these elements.