Silicon dioxide

Endpoint summary

Description of key information

Amorphous silica is a naturally occurring substance. The synthetic form (synthetic amorphous silica, SAS) is of higher amorphous purity than the naturally occurring amorphous silica and does not contain contaminants. Both natural amorphous silica and SAS have the tendency to aggregate and agglomerate but are not expected to undergo any transformation in the atmospheric or terrestrial compartment, apart from dissolution by water and precipitation in sediments. Based on the physico-chemical nature and structure of SAS, no photo- or chemical degradation is expected. The hydrolysis process is considered a rate-limiting step in the dissolution of SAS in water. Once released and dissolved into the environment no distinction can be made between the initial forms of silica. At normal environmental pH, dissolved silica [Si(OH)4] exists as orthosilicic acid which is the bioavailable form for aquatic organisms and plays an important role in the biogeochemical silicon cycle in the natural environment. Dissolved silica is a major nutrient for many aquatic systems and certain terrestrial plants. Freshwater and marine organisms such as diatoms, radiolarians and many plants use silicic acid to build up their skeletons or other structures. Grasses may contain up to 10% dry weight of silica. Due to its inherent physico-chemical properties, such as the absence of lipophilicity as well as the capability of organisms to eliminate absorbed SAS components, bioaccumulation is not expected.