Registration Dossier

Environmental fate & pathways

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Silicon is an inorganic substance and element which may undergo transformation processes in the environment. The speciation of the substance may be different if the environmental conditions change. Abiotic (non-biological) transfomation of silicon can typically occur by physico-chemical processes such as oxidation and hydrolysis.

A thin layer of amorphous silicon dioxide (SiO2) is formed on the surface of silicon exposed to air, even at ambient temperatures (Kirk-Othmer 2001). The surface composition of high grade silicon (Si 99.1 wt%) consists of oxidized silicon. The oxidised layer on the surface passivates silicon and makes handling of silicon safer. Non passivated silicon powder is highly reactive. Silicon dust reacts vigorously with water/moisture at room temperature evolving hydrogen, generating SiO2 and heat.

Phototransformation and photolysis is not regarded as an important environmental fate process of amorphous nor crystalline silicon. Silicon absorbs photons effectively, but the reaction does not lead to remarkable decomposition/destruction of the material. In addition, the active surface of the material is normally oxidized easily by atmospheric oxygen and pure non oxidised silicon surface is not normally subject to direct light exposure.

Hydrolysis of silicon occurs at the surface of Si particles leading to more soluble products. In massive form (> 1 mm particles) the solubility is known to be slow because of small surface area of the particles. Silicon is more susceptible to hydrolysis (more soluble) in neutral and alkaline solutions than in acidic solutions. The highest hydrolysis/dissolution rates in the near pH‐neutral conditions of polycrystalline silica PCS was 58% Si dissolved after 7 days (168 hours) at 100 mg/l initial load (KTH 2010). Hydrolysis rates of silicon in distilled water, fresh surface water, brackish water and sea water may therefore be very different. Inorganic complexes of silicon (e.g. chlorides or fluorides) may therefore play important (temporary) role in dissolution/hydrolysis process of elemental silicon. However, the stable form of dissolved silicon in the environment is known to be predominantly in the form of mono silicic acid Si(OH)4. The dissociation constants of silicic acid are pKa1 9.9, pKa2 11.8, pKa3,4 12 & 12 at 30 °C and therefore it is present in the typical environmental conditions and dilute solutions as a non ionized molecule.

The speciation of 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, dissolved Si(OH)4 and following increasing concentrations, dimerized, trimerized, colloidal or in the form of aggregated colloids of different physical size or entirely as insoluble particulate matter (Gorbach 2006). Dissolved silica species form precipitates with other elements like Al and Mg and may form with these elements several types of clay minerals.

Sorption of dissolved silica in soil/sediments is known to be controlled remarkably by solid phase constituents like clay minerals and oxides and in the lesser extent by solid organic matter.

Silicon is not biodegradable. Silicon and Si(OH)4 are generally known to have no or very low potential for bioconcentration and bioaccumulation. Certain species may actively concentrate Si in their body and Si is also an essential element for some species. These issues must be documented in the CSA report but these findings must not be considered as a harmful ecotoxicological effects.