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Silicon orthophosphate is an inorganic salt, which easily dissociates into silicon and phosphate ions after introducing into the aquatic environment (relevant pH values range from 4 – 9). The tetravalent silicon ion and the phosphate ion will then react with the media to form different silicon and phosphate species depending on the pH and redox potential of the media.

The dissociated silicon ion hydrolyses in water and exists predominantly as orthosilicic acid H4SiO4/Si(OH)4, which is also the main species when silicon dioxide is dissolved in water. The dissociation constants of orthosilicic acid are high (pKa 9.9, 11.8, 12 & 12 at 30 °C, Lide & Frederikse 1995). The sum of soluble silicate rapidly decreases when the pH is lowered to 9. Because of these, at environmental pH values of 6.5 – 8.5 large amounts of the soluble silicon species might be removed from the aqueous solution gradually (OECD SIDS, 2004).

In the case of free aqueous phosphate, four forms are existend. PO34- predominates in strong basic conditions, whereas H3PO4 is the main form in strong acid conditions. In the normal aquatic environment (pH 4-9), H2PO4- and HPO42- are prevalent and show equilibrium, which is relatively stable and not significantly affected by the normal environmental pH conditions.

Due to the ready dissociation in aqueous solution, the assessment of ecotoxicity of silicon orthophosphate will be based on the products of dissociation. Data from phosphoric acid, potassium salt (2:3), dihydrate (CAS No. 6922-99-4) and dipotassium hydrogenorthophosphate (CAS: 7758 -11 -4) are used to cover the aquatic toxicity of the phosphate moiety of the substance. Data from silicon dioxide/synthetic amorphous silica (CAS No. 7631-86-9) and sodium aluminum silicate (CAS No. 1344-00-9) are used to cover the aquatic toxicity of the silicon moiety of the substance. This read across approach is in accordance with Regulation (EC) No 1907/2006, Annex XI, 1.5. “Grouping of substances and read across approach” and is explained within the read across justification in section 13.

In water, silicon orthophosphate dissociates into ionic silicon and ionic orthophosphate. The source substances which were chosen for the analogue read across approach yield the same ions as the target substance (either ionic silicon or ionic orthophosphate) when dissolved in water. The presence of the respective counterions, i.e. ionic potassium in the case of phosphoric acid, potassium salt (2:3), dehydrate, and ionic sodium are known to cause no toxic or other adverse effects to aquatic organisms in the concentration ranges tested and are therefore not prejudicial to the chosen read across approach. This statement is also applicable to the chosen read across substance, silicic acid, aluminum sodium salt, which dissociates into sodium, aluminum and silicate ions. Aluminum ions are known to hydrolyze in water, forming different aluminum species, depending on the pH and redox potential of the media. The main species under environmental pH conditions are aluminum hydroxide Al(OH)3 and/or aluminum oxide hydrate Al2O3 H2O. Based on the experimental result of the toxicity to aquatic algae using silicic acid, aluminum sodium salt (CAS No. 1344-00-9; Lebertz, 1998), it can be confirmed that also in the case of the different aluminum ion species, no adverse effects to algae of such a counter ion is present and the read across thus fully justified.

For the phosphate specific toxicity, data from phosphoric acid, potassium salt (2:3), dihydrate were used to read across to silicon orthophosphate. For this substance short-term toxicity data are available for all three trophic levels (fish, invertebrates and algae).In all reports, no toxic effects occurred up to 100 mg/L which was the maximum concentration tested. Therefore, EC/LC50 values > 100 mg/L and for algae a NOErC ≥ 100 mg/L were stated. The absence of aquatic toxicity for the phosphorus moiety of silicon orthophosphate is in line with a recent summary assessment for phosphorus acid (OECD SIDS, 2009).

For the silicon specific toxicity to fish and invertebrates, data from silicon dioxide are used to read across to silicon orthophosphate.For both trophic levels, no toxic effects were observed. Therefore, an LL0 (96h) of 10,000 mg/L for fish and an EL50 (48h) > 10,000 mg/L for invertebrates were assumed. For the silicon specific toxicity towards algae, data from the source substance sodium aluminum silicate are provided. As no inhibition of algal growth and no abnormalities occurred, an ErL50 (72h) > 10,000 mg/L was stated. The absence of aquatic toxicity for the silicon moiety of silicon orthophosphate is in line with summary assessments for synthetic amorphous silica and silicates (OECD SIDS, 2004).

For the toxicity to microorganisms data from dipotassium hydrogenorthophosphate (CAS: 7758 -11 -4) are used to assess the impact of the phosphorus moiety. No inhibition of microbial respiration occurred up to 1000 mg/L which was the highest test concentration. No data are available for the silicon moiety. However, based on the absence of aquatic toxicity of silicates in general, and since silicon dioxide is known to be utilized by aquatic organisms such as diatoms, radiolarians and silicoflagellates and thus being a part of the environment, the absence of toxicity for activated sludge in sewage treatment plants can be safely assumed.This assumption is supported by public assessments for synthetic amorphous silica and silicates (OECD SIDS, 2004).

References:

OECD SIDS (2004). Synthetic amorphous silica and silicates, SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 19-22 October 2004.

OECD (2009). SIAP SIDS Inititial Assessment Profile for Phosphoric acid; CAS No. 7664-38-2; UNEP Publications.