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Environmental fate & pathways

Biodegradation in soil

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Reference
Endpoint:
biodegradation in soil: simulation testing
Data waiving:
other justification
Justification for data waiving:
other:
Transformation products:
yes
Remarks:
Degradation is principally expected via abiotic transformation (aqueous). The transformation products expected are those identified in IUCLID section 1.10 "transformation of the registered substance", the environmental hazard assessment is based on those.
No.:
#1
No.:
#2

Description of key information

The simulation tests on 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.

Key value for chemical safety assessment

Additional information

The soil simulation test and 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. 

Many organosilicon compounds hydrolyse rapidly to a silanol and a by-product such as ethanol, hydrochloric acid 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 available evidence regarding biodegradation and persistence of organosilicon compounds is summarised in an attached document in Section 13 (PFA 2021).

The registered substance is ‘inherently biodegradable’ based on read-across from structural analogue substances. The registered substance hydrolyses rapidly in the environment (DT50 << 2 min at pH 7 and room temperature), forming hexadecylsilanetriol and hydrochloric acid. Thus, the environmental fate assessment is based on the hydrolysis products hexadecylsilanetriol and hydrochloric acid rather than the parent substance. The exposure assessment and risk characterisation for the silanol hydrolysis product of the substance is however carried out on the basis that it is not biodegradable as a worst-case scenario. Risk characterisation ratios are below 1. The second hydrolysis product, hydrochloric acid, is inorganic and therefore not susceptible to biodegradation as such, and thus is negligible for the assessment. 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 hexadecylsilanetriol and hydrochloric acid. The silanol hydrolysis product of the registered substance shows a potential for adsorption/desorption to soil and sediment based on estimated log Koc values of 5.6 (KOCWIN) and 2.4 (TGD) and an estimated log Kow value of 5 (QSAR). However, based on reliable and good quality read-across data the silanol hydrolysis product of the registered compound is assessed as ‘inherently biodegradable’ and thus does not persist in the environment and therefore, the possibility for bioaccumulation of the silanol hydrolysis product are minimized. Additionally, further testing for persistence is not expected to affect the overall outcome of the PBT/vPvB assessment and therefore not deemed necessary. The chemical safety assessment does not indicate any need to investigate further the degradation of the silanol hydrolysis product at the present time. 

References:

OECD SIDS, 2002. Hydrochloric acid - SIDS Initial Assessment Report For SIAM 19, Berlin, Germany: UNEP Publications. 

PFA (2021): Background to persistence assessment of organosilicon compounds, PFA.923.001.001, February 2021.