Silicic acid, sodium salt

Toxicity to soil microorganisms

Link to relevant study record(s)

Reference

Endpoint:

toxicity to soil microorganisms

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

other:

Description of key information

According to REACh legislation Annex XI, section 1, toxicity testing towards terrestrial organisms does not appear scientifically necessary and is being waived due to the combined evidence based on existing information.

Due to:

a)  the low amount of dissolved soluble silicates under environmental relevant conditions,

b) the fact that silicates are natural components of soil minerals and ubiquitous in soils (OECD SIDS, 2004),

c) the presumably small emission of anthropogenic soluble silicates into the terrestrial environment, and

d) the absence of adverse effects in the aquatic compartment,

there is no need to further investigate the effects of the substance and/or relevant degradation products on terrestrial organisms.

References:

OECD SIDS, 2004: SIDS initial assessment report for SIAM 18 – soluble silicates, p. 1 – 43.

Additional information

According to REACh legislation Annex XI section 1, toxicity testing towards terrestrial organisms does not appear scientifically necessary and is being waived due to the combined evidence based on existing information. Soluble silicates are inorganic compounds that consist of silicon-oxide tetrahedra as the basic structural unit in which a silicon atom is surrounded by four oxygen atoms in a tetrahedral formation. The tetrahedra are linked together via Si-O-Si bonds to form an infinite three-dimensional network and have alkali cations (sodium, potassium, or lithium) randomly spaced in the interstices (HERA, 2005; OECD SIDS, 2004; Ullmann’s, 2012). The number of alkali ions varies and is presented by the weight ratio (WR) or molar ratio (MR) of SiO2 to Na2O or K2O, respectively. The higher the molar ratio, the less alkali ions are present in the silica network, thus the less alkaline the silicates are. The molar ratio varies, for example, commercial sodium silicates have molar ratios between 0.5 to 4.0, while commercial potassium silicates have molar ratios between 1.5 to 5.0 (HERA, 2005). Soluble silicates exhibit a moderate-to strong alkalinity with a pH of 10 – 13 (CEES, 2019). The molar ratio or weight ratio controls the alkalinity of the silicates, therefore is the physical, chemical, toxicological and eco-toxicological behaviour strongly dependent on this ratio (CEES, 2019) Soluble silicates used in industry are divided into two groups: 1. Amorphous silicates, which are solidified as a glass from the melt (solid or lump glasses, often referred to as waterglass), 2. Crystalline silicates, which are products of a controlled crystallisation of silicate solutions and only exist in the sodium form (HERA, 2005; OECD SIDS, 2004). The amorphous glasses are anhydrous and soluble in water at elevated temperature and pressure (ca. 150°C and > 5 bar) resulting in silicate solutions (liquid glasses). Fine powders or granules (usually in the sodium form), which are readily dissolved in water and form silicate solutions, can be obtained by evaporation of the silicate solutions. Crystalline silicates are as well readily soluble in water (OECD SIDS, 2004). However, the water solubility depends on the pH. Above a pH of 11-12 stable solutions of monomeric and polymeric silicate ions exist. The solubility decreases rapidly when the pH is lowered to 9, leading to an increased precipitation of amorphous silica. Below a pH of 9 the majority exists as insoluble amorphous silica and only a small proportion is present as soluble monomeric silicate ions (OECD SIDS, 2004). An increasing dissolution of soluble silicates leads to an increase in pH, but not as much as can be expected, due to the buffer capacity of the silicates. The dissociation constants of silicic acid are in the range of pKa 9.9 – 12 at 30°C (Lide & Frederikse 1995) and give rise to the assumption that only a small proportion of silicate ions will be in solution under environmental pH values of 6.5 – 8.5.   The emission of soluble silicates into the terrestrial environment is possible as they are directly used in soil stabilizers and making up 3.5% of the total usage of industrial use of soluble silicates per year. Also the use of soluble silicates in the production of building materials, welding rods, ceramic binders, refractories, TiO2 has the potential for release into the terrestrial environment. However, no emission data is available (Lauriente and Sakuma, 2002). Approximately 90% of the amount of soluble silicates used per year is used in the production of detergents, soaps, cleaners, pulp, and paper, for which an emission into the hydrosphere is the main pathway in the environment. Therefore, an emission of anthropogenic soluble silicates into the terrestrial environment can be assumed to be comparably little. Nevertheless, when soluble silicates are intentionally added to or injected into soil, for instance, in applications such as sealing around landfills and soil treatment, they can react with the acidic components and polyvalent metal ions in the soil to form an impermeable gel structure. Leaching into the groundwater or transport and further spreading of silicate solutions into other areas of the soil will not take place due to the impermeable nature of the formed gel layer. Any effects on soil organism is limited to the area where the gel has formed (OECD SIDS, 2004). However, no adverse effects of soluble silicates to aquatic organisms have been reported (LC50/EC50 > 100 mg/L)  and the aquatic environment is the more relevant compartment as a recipient of anthropogenic soluble silicates, due to an estimated higher input from industry into this compartment (HERA, 2005; OECD SIDS, 2004). Furthermore, acute effects within the solubility range above 10 mg/L can be used to waive the data requirements of Annex X (Chapter R.7c: Endpoint specific guidance, 2017, page 148). In addition, one available short-term study with Apis mellifera, conducted as limit test with the  read-across substance potassium silicate, show no evidence of harmful effects or increased mortality caused by acute contact.  Since silicates are natural components of soil minerals, organism are already adapted to these components. Compounds of silicon and oxygen are ubiquitous in the environment and present in inorganic matter, such as minerals and soils, as well as in organic matter, like plants, animals and humans. In fact, about 59% of the elemental compositions of the earth’s crust is made of SiO2 and comparable proportions are obtained for many sediments and soils. Silica is the second most abundant element on earth (HERA, 2005; OECD SIDS, 2004). Silicon is also the primary constituent of the frustules of diatoms and is taken up by diatoms from the ambient water and incorporated into their skeleton (HERA, 2005). Sedimenting diatoms contribute significantly to the silica found in sediments (diatomaceous earth) (OECD SIDS, 2004). Therefore, it can be assumed that an additional input of anthropogenic soluble silicates will not lead to an increased hazard potential for terrestrial organisms, even though no data is available for terrestrial ecotoxicity tests.  In consequence, due to: a)  the low amount of dissolved soluble silicates under environmental relevant conditions,  b) the fact that silicates are natural components of soil minerals and ubiquitous in soils (OECD SIDS, 2004) and  c) the presumably small emission of anthropogenic soluble silicates into the terrestrial environment, and d) the absence of adverse effects in the aquatic compartment, there is no need to further investigate the effects of the substance and/or relevant degradation products on terrestrial organisms.  

References:

Centre Européen d’études des silicates (CEES) (2019): Soluble silicates – Chemical, toxicological, ecological and legal aspects of production, transport, handling and application. Version: CEES1010c, p. 1 – 23.

Human & Environmental Risk Assessment on ingredients of European household cleaning products (HERA), 2005: Soluble Silicates – draft- (CAS No.: 1344-09-8, 6834-92-0, 10213-79-3, 13517-24-3, 1312-76-1), p 1 – 64.

Lauriente, D.H. and Sakuma, Y. (2002): Chemical Economics Handbook, Marketing Research Report “Silicates and Silicas”. SRI International, March 2002, No. 766.4000 A.  

Lide DR and Frederikse HPR (1995). CRC Handbook of Chemistry and Physics, 75th Edition. CRC Press, Boca Raton, page 8-44. OECD SIDS, 2004: SIDS initial assessment report for SIAM 18 – soluble silicates, p. 1 – 43.

Ullmann’s (2012). Encyclopaedia of industrial chemistry – Silicates. Wiley-VHC Verlag GmbH & Co. KGaA, Weinheim. DOI: 10.1002/14356007.a23_661, Vol. 32, p. 509 – 569.

Van Dokkum, H.P., Hulskotte, J.H.J., Kramer, K.J.M. and Wilmot, J. (2004): Emission, fate and effects of soluble silicates (waterglass) in the aquatic environment. Environ. Sci. Technol., Vol. 38, p. 515 – 521.