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Toxicity to soil microorganisms

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Endpoint:
toxicity to soil microorganisms
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study scientifically not necessary / other information available
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Description of key information

Key value for chemical safety assessment

Additional information

Due to the hydrolysis half-life of the substance (4.5 hours at pH 7 and 20°C), the exposure of the terrestrial compartment is expected to be predominantly to the hydrolysis products. Exposure of soil is via the partitioning of the substance to the WWTP sludge. The sludge may then be spread onto soil, depending on the practices of the WWTP. The substance will have undergone significant hydrolysis by the time it reaches the soil, therefore exposure and chemical safety assessment of the terrestrial compartment is based on the degradation products (3-chlorpropyl)silanetriol and methanol.

(3-Chloropropyl)silanetriol:

Toxicity was observed in the aquatic microorganism test, read across from a structural analogue, dichloro(3-chloropropyl)methylsilane (CAS 7787-93-1) (3-hour EC50 value of 400 mg/l and EC10 value of 197 mg/l (ASRI) (Werner, 1997)).

 

ECHA guidance Chapter R.7c states “Where inhibition of sewage sludge microbial activity has been observed in Annex VIII testing, a test on soil microbial activity will additionally be necessary for a valid PNEC to be derived.” A toxicity to soil microorganisms test is therefore required.

 

However, prior to terrestrial toxicity tests being carried out, the technical feasibility of testing the silanol hydrolysis product, (3-chloropropyl)silanetriol, would need to be assessed. To achieve the test concentrations required by terrestrial testing guidance (up to 1000 mg/kg), aqueous stock solutions would need to be prepared at very high concentrations, well above 1000 mg/l. Silanetriols are susceptible to condensation reactions. Condensation of (3-chloropropyl)silanetriol is expected to become important at loadings above about 1000 mg/l causing the formation of insoluble polymeric particles (sols) and gels over time. It therefore may not be possible to test terrestrial organisms at high enough test concentrations to meaningfully assess the terrestrial toxicity of (3-chloropropyl)silanetriol. Prior to any testing, the feasibility of dosing at concentrations up to 1000 mg/kg dw would have to be assessed. Further details on technical feasibility of terrestrial testing with silanetriol substances are given in Annex 5 of the CSR and attached in Section 13 of IUCLID. 

Details on how the PNEC and the risk characterisation ratio have been derived can be found in IUCLID Section 6.0, and Chapters 7, 9 and 10 of the Chemical Safety Report.