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Diss Factsheets

Environmental fate & pathways

Biodegradation in soil

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Administrative data

Link to relevant study record(s)

Reference
Endpoint:
biodegradation in soil: simulation testing
Data waiving:
other justification
Justification for data waiving:
other:
Transformation products:
not specified

Description of key information

A soil simulation test is not considered necessary because:


 



  • The substance hydrolyses rapidly and the silanol hydrolysis product has a low log Kow value and therefore, exposure of the soil compartment is expected to be low. 

  • The risk characterisation ratios (RCRs) for the soil compartment, based on the assumption that the silanol hydrolysis product is not biodegradable, are << 1. 

  • The non-silanol hydrolysis product, methanol, is readily biodegradable.

Key value for chemical safety assessment

Additional information

The simulation test on ultimate degradation in soil as well as the identification of degradation products does not need to be conducted as:



  • in accordance with Column 2 of REACH Annex IX, the chemical safety assessment according to Annex I indicates that this is not necessary, and

  • in accordance with Section 3 of REACH Annex XI, exposure of the soil compartment is not significant.


The available evidence regarding biodegradation and persistence of organosilicon compounds is summarised in an attached document (PFA 2021). Many organosilicon compounds hydrolyse rapidly to a silanol and a by-product such as ethanol, methanol or HCl. For most organosilicon compounds, little or no degradation is observed in ready biodegradation studies once degradation of any readily biodegradable hydrolysis by-product is accounted for. This is supported by a small number of simulation studies that show limited biodegradation. Therefore, most organosilicon compounds either meet the criteria for persistence or produce transformation products that may meet the criteria for persistence based on currently available data.


 


The registered substance is ‘not readily biodegradable’ based on read-across from the structural analogue trimethoxy(propyl)silane (CAS No. 1067-25-0). The registration substance undergoes a rapid two-step hydrolysis (DT50 = 0.3 h) to methanol and methylsilanediol (first step) and then methylsilanetriol (2nd step).


The exposure assessment and risk characterisation is carried out for the second silanol hydrolysis product methylsilanetriol on the basis that it is not biodegradable. The risk characterisation ratios are below 1. The non-silanol hydrolysis product, methanol, is readily biodegradable. Therefore, further testing for biodegradation would not affect the outcome of the environmental risk characterisation.


 


Since degradation of the registration substance is expected to be principally via abiotic transformation under the aqueous conditions of a degradation simulation study, the transformation products expected in the environment are those identified as the hydrolysis products in Section 5.1.2 of the IUCLID.


 


The silanol hydrolysis product of the registered substance is considered to be not B based on its low predicted log Kow of -2.4. Therefore, further testing for persistence would not affect the overall outcome of the PBT/vPvB assessment.


 


The chemical safety assessment does not indicate any need to investigate further the degradation of the substance, or either of its hydrolysis products, at the present time.


 


Reference


PFA 2020: Background to persistence assessment of organosilicon compounds, Reference: PFA.923.001.001, February 2021.