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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Administrative data

Link to relevant study record(s)

Description of key information

Sediment degradation half-life [HMDS] : 34 d at 24°C (preliminary study)

Key value for chemical safety assessment

Additional information

There are no data available for the registration substance itself. The registration substance is a multi-constituent substance containing four constituents, decamethyltetrasiloxane (L4, CAS 141-62-8), dodecamethylpentasiloxane (L5, CAS 141-63-9), hexamethyldisiloxane (HMDS, CAS 107-46-0) and octamethyltrisiloxane (L3, CAS 107-51-7). For the environmental hazard assessment, it is therefore relevant to discuss the biodegradation of the constituents.

An OECD 308 study is available with the constituent hexamethyldisiloxane (HMDS; CAS 107-46-0). The degradation half-life for HMDS leads to a conclusion of ‘vP’ for the sediment compartment based on a worst-case half-life of 192 d at 12 °C. This conclusion is read-across to all constituents of the registration substance. However, the Registrants note that the Reconsile Consortium further intend to conduct a Sediment Simulation Test (OECD TG 308, including the identification of the transformation products) with L3 in accordance with an ECHA final decision No. SEV-D-2114330871-53-01/F received for octamethyltrisiloxane (CAS 107-51-7; EC 203-497-4) to verify this conclusion.

L4, L5 and HMDS are members of the Reconsile Siloxane Category and are structurally similar substances. Each constituent is a linear siloxane where each silicon atom is fully substituted with methyl groups. Siloxanes are highly absorbing and are therefore expected to have slow degradation rates in sediment. Available data indicate that degradation of siloxanes is predominantly abiotic, with the formation of hydrolytic products. Mineralisation rate is expected to be very slow. The degradation of L4 and L5 is expected to be slower than HMDS, however since the degradation half-life for HMDS leads to a conclusion of ‘vP’ for the sediment compartment based on a worst-case half-life of 192 d at 12 °C, it is reasonable to read-across the conclusion of ‘vP’ in sediment for the longer-chain constituents.

Reconsile Siloxane Category Simulation test data for degradation in water and sediment

CAS

Name

Sediment type

Results

Klimisch code

Reference

556-67-2

Octamethylcyclotetrasiloxane

Natural sediment (aerobic)

Half-life (DT50):

242 d in sediment at 24 °C (pH 7.9 after acclimation)

1

Dow Corning Corporation (2009b)

556-67-2

Octamethylcyclotetrasiloxane

Natural sediment (anaerobic)

Half-life (DT50):

365 d in sediment at 24 °C (pH 7.9 after acclimation)

1

Dow Corning Corporation (2009c)

541-02-6

Decamethylcyclopentasiloxane

Natural sediment (aerobic and anaerobic)

Half-life (DT50):

1200 d in sediment at 24°C (Non-sterilised. Aerobic.)

2700 d in sediment at 24°C (Sterilised. Aerobic.)

Approximately 3100 d in sediment at 24°C (Non-sterilised. Anaerobic. (a good trend of degradation was not able to be established, so half-life is approximate))

800 d in sediment at 24°C (Sterilised.. Anaerobic.)

1

Dow Corning Corporation (2010b)

 107 -46 -0

 Hexamethyldisiloxane

 Natural sediment (aerobic)

  

Half-life (DT50): 192 d at 12°C (high %OC sediment); 53 d at 12°C (lower % OC sediment)

 1

The Dow Chemical Company (2018)