Silicic acid, sodium salt

Sediment toxicity

Link to relevant study record(s)

Reference

Endpoint:

sediment toxicity: long-term

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, long-term toxicity testing towards sediment organisms does not appear scientifically necessary and is being waived due to the combined evidence based on existing information.

In consequence, due to:

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

b) the fact that dissolved soluble silicates are molecular species which are indistinguishable from natural dissolved silica (OECD SIDS, 2004), and

c) the lack of any adverse effects on aquatic organisms below the 100 mg/L level,

there is no need to further investigate the effects of the substance and/or relevant degradation products on sediment 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, long-term toxicity testing towards sediment 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 the physical, chemical, toxicological and eco-toxicological behaviour is 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 also well 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.   By looking at the different applications of soluble silicates and the corresponding amounts of silicates used for these applications, pathways of emissions into the environment can be assessed. About 50% of the produced silicates, including sodium and potassium silicates, in Western Europe is further processed to derivatives. About 10% of produced silicates are used for products with direct uses, such as refractories, welding rods, building industry, TiO2, and ceramic binders. An emission into the environment for the aforementioned uses is possible during production, processing, and use. However, no emission data is available. The remaining 40% of produced soluble silicates are used for the production of detergents, soaps, cleaners (21%), in pulp and paper production (15%), water and wastewater treatment (2%), as well as in soil stabilizers (3.5%) (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 and is therefore, presenting the most relevant pathway for soluble silicates into the environment, especially the hydrosphere.  Soluble silicates are removed from the aquatic environment to about 10% by sewage treatment plants and another 10% are estimated to be lost by sedimentation and adsorption in the sewer system before the sewage plant (van Dokkum et al., 2004). By entrance into the aquatic environment, soluble silicates are diluted and depolymerize rapidly to give molecular species like H4SiO4 or SiO2 [aq.], which are indistinguishable from natural dissolved silica (OECD SIDS, 2004). In fact, chemical speciation modelling data shows that under environmental pH conditions (pH 7 – 9), the majority of Si in freshwater systems is present as orthosilicic acid H2(H2SiO4), known as dissolved silicate and the biological active species. The rest is present as H(H2SiO4). Precipitation of amorphous silica H2(H2SiO4) (solid) will occur with increasing concentration of Si. The solubility of amorphous silica is independent of pH in the range 7 – 9 and about 95 mg/L of SiO2 at 25°C (van Dokkum et al., 2004). Furthermore, at a given pH dissolved silica are able to react with naturally occurring dissolved polyvalent metals, like Mg, Fe, Al, and Ca, which will lead to insoluble silicates or amorphous silicates. These products, however, appear as well in abundance in natural soils and rocks (CEES, 2019). An anthropogenic emission of soluble silicates of 88 – 121 kilo tonnes of SiO2 per year has been estimated for Western Europe. However, by comparing the anthropogenic input of soluble silicates (approx. 100 kilo tonnes SiO2/year) into the aquatic system with the natural flux (approx. 5 million tonnes SiO2/year) in Western Europe (van Dokkum et al., 2004), it shows that the anthropogenic flux is negligible (< 2%) compared to the natural flux. 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). Due to this natural adaptation of organisms to silicon, it can be assumed that the additional input of anthropogenic soluble silicates will not lead to an increased hazard potential for aquatic organisms. In fact, short-term aquatic toxicity tests show no effects at the 100 mg/L level regardless of the molar ratio or metal cation. Acute toxicity studies with fish resulted in LC50 values in the range of 210 mg/L (sodium metasilicate, MR 1) (Richterich and Mühlberg, 2001b) to 1108 mg/L (sodium silicate, MR 3.46) (Adema, 1988). The acute toxicity testing with freshwater invertebrates (Daphnia magna) gave EC50 values in the range of > 146 mg/L (potassium silicate solution, MR 3.9 – 4.1) (Richterich and Mühlberg, 2001a) to 1700 mg/L (sodium silicate, MR 3.2) (Kirch, 1997). Also the acute toxicity to algae was tested and an ErC50 of > 345 mg/L (sodium silicate, MR 3.0) (Rieche, 1995). In regard of the present data, it can be concluded that an input of anthropogenic soluble silicates into the aquatic environment and thereby into sediments is not likely to have a significant adverse effect onto organisms and the aquatic environment.  In consequence, due to: a) the low amount of dissolved soluble silicates under environmental relevant conditions,  b) the fact that dissolved soluble silicates are molecular species which are indistinguishable from natural dissolved silica (OECD SIDS, 2004), and  c) the lack of any adverse effects on aquatic organisms below the 100 mg/L level, there is no need to further investigate the effects of the substance and/or relevant degradation products on sediment organisms.  

References:

Adema DMM (1988). The acute toxicity of Natronwasserglas to Brachydanio rerio. TNO Division of Technology for Society. Report no. R88/410.

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.

Kirch A (1997). Kieselsäure, Na-Salz. Akute Daphnientoxizität, Abschlussbericht. Henkel KGaA Forschung Biologie/Produktsicherheit, Ökologie. Report no. R 9700908.

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.

Richterich K and Mühlberg B (2001a). Silicic acid, potassium salt. Daphnia magna, acute toxicity. Henkel KGaA. Final report R-0100925.

Richterich K and Mühlberg B (2001b). Silicic acid, potassium salt. Fish, acute toxicity. Final report  R-0100924. Henkel KGaA.

Rieche H-W (1995). Wasserglas 3.0 – unfiltriert, Algen-Zellvermehrungshemmtest. Abschlussbericht. Henkel KGaA Forschung Biologie,Ökologie. Report no. R 9400273.

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.