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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Endpoint:
biodegradation in water: sediment 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
Endpoint:
biodegradation in water: simulation testing on ultimate degradation in surface water
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 ultimate degradation in surface water and sediment as well as the identification of degradation products do 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 aquatic/sediment compartment is not significant.

Key value for chemical safety assessment

Additional information

The simulation tests on ultimate degradation in surface water and sediment as well as the identification of degradation products do 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 aquatic/sediment 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 assessed as ‘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 on 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 (OECD SIDS, 2002), and thus is negligible for the assessment. Hence, 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 is considered to show potential for bioaccumulation based on its high log Pow of 5 (QSAR), which also demonstrates a potential for adsorption to sediment and soil. For the adsorption/desorption to soil and sediment log Koc values were calculated based on different models and log Koc values of 5.6 (KOCWIN) and 2.4 (TGD) were obtained. Further evidence on the bioaccumulation potential is available from a QSAR calculation method, which obtained a BCF value of 39.3 L/kg wwt. However, the given QSAR calculation contains only one substance in the training set with a C-Si-O bond. Hence, the data needs to be considered with caution and therefore no clear estimation of the bioaccumulation potential of the silanol hydrolysis product can be given. Nevertheless, based on reliable and good quality data from structural analogue read-across substances, the registered compound and its silanol hydrolysis product are assessed as being inherently biodegradable and thus are not expected to be persistent in the environment. Therefore, further testing for persistence is not necessary and 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 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.