Disodium disilicate

Ecotoxicological Summary

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:

PNEC aqua (freshwater)

PNEC value:

7.5 mg/L

Marine water

Hazard assessment conclusion:

PNEC aqua (marine water)

PNEC value:

7.5 mg/L

STP

Hazard assessment conclusion:

PNEC STP

PNEC value:

28 mg/L

Assessment factor:

10

Extrapolation method:

assessment factor

Sediment (freshwater)

Hazard assessment conclusion:

PNEC sediment (freshwater)

PNEC value:

29.4 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Sediment (marine water)

Hazard assessment conclusion:

PNEC sediment (marine water)

PNEC value:

29.4 mg/kg sediment dw

Extrapolation method:

equilibrium partitioning method

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:

PNEC soil

PNEC value:

1.47 mg/kg soil dw

Extrapolation method:

equilibrium partitioning method

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:

PNEC oral

PNEC value:

106 mg/kg food

Assessment factor:

30

Additional information

The crystalline structure of layered delta-sodium disilicate is completely destroyed by dissolution in water yielding an aqueous solution which is identical to a solution of amorphous sodium disilicate (CAS No. 1344-09-8) and so called water glas is formed. In consequence, the argumentation form the soluble silicates HERA risk assessment Report (2005) is adopted. The primary hazard of commercial soluble silicates is their moderate-to-strong alkalinity, which can be harmful to aquatic life. However, most of natural aquatic ecosystems are slightly acid or alkaline and usually their pH values fall within the range of 6 – 9, and due to the high buffer capacity of these ecosystems pH effects of released soluble silicates to aquatic organisms are very unlikely. Consequently, the PNEC derived from artificial laboratory test systems overestimate the effects of soluble silicates to aquatic organisms in ecosystems. Therefore, the PNEC was derived from the ubiquitous SiO₂ background concentration in the environment. The median values in the US were reported to be 17 mg SiO₂/L for ground waters and 14 mg SiO₂/L for streams (Davis 1964). The world-wide mean concentration in rivers is 13 mg SiO₂/L (Edwards and Liss 1973) and in Europe is 7.5 mg SiO₂/L (Jorgensen et al. 1991). Therefore, the conservative PNEC for aquatic organisms is 7.5 mg SiO₂/L and was used for the final risk characterisation.

References:

Davis SN (1964). Silica in streams and ground water. Am. J. Sci. 262, 870-891.

Edwards AMC and Liss PS (1973). Evidence of buffering of dissolved silicon in fresh waters. Nature 243, 341-342.

Hera Risk Assessment Report Soluble Silicates, February 2005

Jorgensen SE, Nielsen SN, Jorgensen LA (1991): A Handbook of ecological parameters and ecotoxigology, Elsevier : Amsterdam, The Netherlands.

Conclusion on classification

Acute toxicity:

Algae (Scenedesmus subspicatus) is the most sensitive species with a EC50 value of 44.1 mg/L (pH adjustment to pH 7.6 at the start of the test, but no pH adjustments during the test). All other EC50 values for fish and Daphnia were far above the trigger value of 100 mg/L. According to 67/548/EEC and CLP (GHS) disodium disilicate (delta-crystalline) has not to be classified regarding acute toxicity to the environment as the lowest LD50 value is above 1 mg/L.

Chronic toxicity:

No test on biodegradability of the test substance is available as the substance is of inorganic nature. In a hydrolysis study, it was concluded that the crystalline structure of disodium disilicate (delta-crystalline) is completely destroyed by dissolution in water yielding an aqueous solution which is identical to a solution of amorphous sodium disilicate (CAS No. 1344-09-8). In addition log Koc and BCF values are expected to be very low as the substance is not soluble in octanol.

With regard to classification disodium disilicate (delta-crystalline) is thus regarded as degradable in nature and has in conclusion not to be classified regarding chronic toxicity to the environment according to 67/5487EEC and CLP (GHS).