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EC number: 231-130-8
CAS number: 7440-21-3
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.
Information on Registered Substances comes from registration dossiers which have been assigned a registration number. The assignment of a registration number does however not guarantee that the information in the dossier is correct or that the dossier is compliant with Regulation (EC) No 1907/2006 (the REACH Regulation). This information has not been reviewed or verified by the Agency or any other authority. The content is subject to change without prior notice.Reproduction or further distribution of this information may be subject to copyright protection. Use of the information without obtaining the permission from the owner(s) of the respective information might violate the rights of the owner.
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