Silicic acid, aluminum sodium salt

Endpoint summary

Description of key information

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

Since no studies investigating the hydrolysis of Silicic acid, aluminum sodium salt (CAS 1344-00-9, NAS) are available, a read-across to a structurally related silicic acid species, synthetic amorphous Silicic acid, aluminum magnesium sodium salt (CAS 12040-43-6, SMAS) was conducted.

The test item was considered to be stable (following OECD 111, t(0.5) at 25 °C is greater than 1 year). Therefore, the target substance synthetic amorphous Silicic acid, aluminum sodium salt (CAS 1344-00-9, NAS) is expected to be hydrolytically stable as well.

OECD 105 measurements (see section 4.8 water solubility) show a low water solubility of this substance resp its forms, this is supported also by measurement according to OECD 29. Here, the maximum concentration of total dissolved Si and Al was determined in order to identify compounds of the substance which undergo either dissolution or rapid transformation. The following concentrations of Si and Al were stated on day 28 with 1 mg/L nominal load: 64.7 µg/L Si and 3.48 µg/L Al.

 

Furthermore, 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 and in plants such as grass, rushes, rice or sugar cane. Released into the environment, synthetic amorphous silica and silicates - including synthetic amorphous Silicic acid, aluminum sodium salt (CAS 1344-00-9, NAS) - are expected to be distributed mainly into soils and sediments, weakly into the water and probably not at all in the air due to its 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). The same applies to aluminum which is the third most abundant element (approx. 8%) in the Earth's crust. 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 incl. NAS is the dissolved form that exists exclusively as monosilicic [Si(OH)4] acid and low concentrations of Al³⁺ ions under environmental pH conditions.

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