Fumes, silica

Endpoint summary

Additional information

Pure silica fume and, in general, pure amorphous silica have low aquatic toxicity. Silica is ubiquitous in aquatic environment and normally the species are well adapted to the prevailing background concentrations. The surface water background concentrations of dissolved silica are typically from a few mg/l to 20-30 mg/l. Acute lethal effects LC50 of (truly) dissolved silica can be found at the level of ca. 200-300 mg/l SiO₂. Perhaps the relatively low general toxicity of silicon is the main reason that the ecotoxicity dataset is still relatively scarce and no long-term studies can be found in the scientific literature.

Acute study results for silica fume are available for fish, daphnia and green algae. All LC50 values are > 100 mg/l. Acute effects, caused by a truly dissolved species, have not been detected in the most reported studies. Therefore, very high nominal substance test loading rate values are often reported, as a result, in short-term silica tests.

Silica fume is an industrial by-product. Because of the production process, the presence of impurities is dependent on process conditions, the by-product recovery system and the nature of the raw materials used. The impurities of commercial substances raise concerns over the Si fume substance rather than the Si-element itself. The effects seen in the aquatic short-term studies of silica fume can be seen as effects caused by impurities rather than the effects of pure silica. Nano size silica particles may have a toxicity profile deviating remarkably from the ordinary grades of silica fume particles.  

Silicon is essential to some aquatic species. As aquatic algae, diatoms (Bacillariophycea) build up their skeletons by taking up large amounts of silicic acid from water and further synthetising amorphous silica skeletal structures. Silicon is therefore an essential element for diatoms and silicon is a major factor influencing (primary) algal production in many lakes and fresh-water systems. The utilization of silica in the trophogenic zone of lakes by diatoms often reduces the surface water (epilimnetic) concentrations and includes, along with other factors, a seasonal succession of diatom species. When the concentration of silica is reduced below about 0.5 mg/l, many diatomic species cannot compete effectively with nonsiliceus algae, and their growth rates decline until the silica concentration again increases. In deep lakes, many of the dead diatom structures undergo partial dissolution before reaching the sediment (Wetzel, 1983). Silicon is not an essential element for green algae. 

The Transformation/Dissolution T/D protocol applicable for poorly water soluble substances, and in particular metals and metalloids, has been performed on Silicon (low grade) for a period of 7 and 28 days representing potential acute and chronic effects respectively. Silica fume (high grade)appears not to contain any impurities at concentrations that may be expected to cause acute or chronic toxicity to aquatic organisms and therefore they do not require any further ecotoxicity tests for classification purposes.