Registration Dossier

Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

Currently viewing:

Administrative data

Link to relevant study record(s)

Description of key information

Half-life in sediment: >192 d at 12 °C (based on read-across from HMDS (hexamethyldisiloxane))

Key value for chemical safety assessment

Half-life in freshwater sediment:
192 d
at the temperature of:
12 °C

Additional information

There is no sediment degradation data available for L5; an OECD 308 study is available with the analogous substance hexamethyldisiloxane (HMDS; CAS 107-46-0) and has been read across to the registration substance.

L5 and HMDS are members of the Reconsile Siloxane Category and are structurally similar substances. HMDS and L5 are linear siloxanes with two silicon atoms and one oxygen atom, and five silicon and four oxygen atoms, respectively. Each silicon atom is fully substituted with methyl groups. The physico-chemical properties of L5 are reasonably similar to those of HMDS, but the former should have greater stability in sediment: HMDS has a higher tendency to volatilise from sediment compared to L5, based on its higher vapour pressure (5500 Pa versus 7.8 Pa at 25°C) and lower tendency to partition to organic matter (log Koc3.0 versus 6.3) than L5. Substances that are highly absorbing are 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 L5 may be 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 L5.

Table4.1.8. 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. ( 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 (2019)