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Environmental fate & pathways

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Fate in the environment

Endpoints of environmental fate and pathways are attached with a waiving argument for the following reasons:

Silicon is the second most abundant element in the earth’s crust mass (approx. 28 %) after oxygen. It appears as complex silicate minerals in soils and sediments, as the oxide (silica, SiO2) in crystalline form in rocks, soils and sand, and as biogenic silica in organisms such as diatoms, radiolarians or silicoflagellates plants such as grass, rushes, rice or sugar cane.

Released into the environment, synthetic amorphous silica and silicates - including calcium silicate - released into the environment are expected to be distributed mainly into soils and sediments, weakly into water and probably not at all in the air due to their physico-chemical properties, particularly low water solubility and very low vapour pressure.

Synthetic amorphous silica and silicates released into the environment are expected to combine indistinguishably with the soil or sediment due to their similarity with inorganic soil/sediment matter and will be subjected to natural processes under environmental conditions (cation exchange, dissolution, sedimentation).

Based on the chemical nature of synthetic amorphous silica and silicates (inorganic structure and chemical stability of the compound: Si-O bond is highly stable), no photo- or chemical degradation is expected. Biodegradation is not applicable to these inorganic substances.

The bioavailable form of synthetic amorphous silica and silicate is the dissolved form which exists exclusively as monosilicic [Si(OH)4] acid under environmental pH. In analogy to the general chemical reaction of weak acids and salts of weak acids with water, the water-soluble fraction of silica acts as a weak acid and, therefore, will tend to lower the pH value, while that of a silicate acts as a base tending to bind protons and, thus, raise the pH value by forming hydroxyl ions. But pH shifts which are measurable at high loadings under laboratory conditions are not expected to occur from the anthropogenic deposition in the aquatic environment of synthetic amorphous silicas and silicates due to low aquatic releases and sufficient natural buffer capacities.  Finally, these materials are supposed to combine indistinguishably with the soil layer or sediment due to their chemical similarity with inorganic soil matter.

Dissolved silica and silicates can be actively assimilated by some marine and terrestrial organisms as normal natural processes mainly related to structural function.