Silicon

Endpoint summary

Additional information

Silicon is practically taken always present in soil minerals and dissolved in soil pore water. If elemental silicon “metal” is released to the soil the surfaces of silicon particles are normally already oxidized or will be oxidized rapidly. The actual rate of oxidization is highly dependent on particle size and environmental conditions. Transformation/dissolution studies have shown that at least the tested silicon powder samples did not react rapidly in contact with water. The highest transformation/dissolution rate was 58 % in one week test period (100 mg/l load in saline water at pH 7.4). Anthropogenic and potentially bioavailable silicon in soil is therefore present in oxidized form as silica and not at the elemental stage. Soil living species are primarily exposed to dissolved silica which is present in soil pore water. The maximum solubility of silica in water is limited by intrinsic physical chemical properties of silicon (polymerization/colloids forming). Short term acute aquatic tests have shown that at a concentration > 160 mg/L (at 20-25°C), SiO₂solid colloids are being formed that are responsible for the toxic effect.Colloid forming takes place at lower concentration at reduced and therefore environmentally more relevant temperatures.

It has been observed that the concentration of silica in soil pore water normally remains relatively constant and clearly below the saturation/polymerization concentrations. Adsorption of soluble silica to soil minerals is a more favorable fate process than remaining in the aqueous phase in high concentrations. On the other hand the dynamic equilibrium helps in keeping concentrations relatively stable since desorption may rapidly increase the soil pore water concentration to some extent. Ultimately it is the mineral composition of soil in a geological area that in the long term determines the concentration levels of dissolved silica found regionally.

Terrestrial toxicity of silica is expected to be low since soil living species are well adapted to the presence of silica in soil and soil pore water. Silica is also essential for some soil-living species. No studies or information has been located showing toxicity of silica to soil-living species. Based on the results of short term pelagic tests (algae, fish) acute lethal effects can be avoided if the soil pore water concentration remains below the colloid forming/saturation concentration. Adaptation to high natural silica concentration may be species sensitive and the most tolerable species favors the extreme conditions. In extreme cases, species may live in soil type called “diatomaceous earth” also known as “diatomite’ or “kieselgur’, which is a naturally occurring, siliceous sedimentary soil type and mineral. It has a particle size ranging from less than 1 micron to more than 1 millimeter, but typically 10 to 200 microns. The typical chemical composition of oven dried diatomaceous earth is 80 to 90% amorphous silica, with 2 to 4% alumina (attributed mostly to clay minerals) and 0.5 to 2% iron oxide.