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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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There is no sediment degradation study available for Vi2-L2; 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.

Vi2-L2 and HMDS are members of the Reconsile Siloxane Category and are structurally similar substances. HMDS and Vi2-L2 are both linear siloxanes with two silicon atoms and one oxygen atom. In the source substance HMDS, each silicon atom is fully substituted with methyl groups, whereas in the registration substance Vi2-L2 each Si atom is bonded to one vinyl group and two methyl groups. The vinyl groups of Vi2-L2 are not expected to have a significant effect on the biodegradation potential. The physicochemical properties of Vi2-L2 are similar to those of HMDS, and they are expected to have similar stability in sediment: HMDS has a higher tendency to volatilise from sediment compared to Vi2-L2, based on its higher vapour pressure (5500 Pa for HMDS versus 1700 Pa for Vi2 -L2 at 25°C) and a similar tendency to partition to organic matter (log Koc3.0 for HMDS versus log Koc3.2 for Vi2-L2). 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 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 Vi2-L2.

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) 

 

References:

Dow Corning Corporation (2010b). Aerobic and anaerobic transformation of decamethylcyclopentasiloxane (14C-D5) in aquatic sediment systems. Report no.: 10886-108.

Dow Corning Corporation (2009c). Anaerobic transformation of octmethylcyclotetrasiloxane (14C-D4) in aquatic sediment systems. Study no.: 11101-108.

Dow Corning Corporation (2009b). Aerobic transformation of octamethylcycloctetrasiloxane (14C-D4) in aquatic sediment systems. Study no.: 10885-108.

The Dow Chemical Company (2019): Hexamethyldisiloxane (HMDS): Aerobic Transformation in Aquatic Sediment Systems (study report), Testing laboratory: Toxicology and Environmental Research and Consulting The Dow Chemical Company Midland, Michigan 48674, Owner company; Reconsile, Study number: 181004, Report date: 13 February 2019