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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Reference
Endpoint:
biodegradation in water: simulation testing on ultimate degradation in surface water
Data waiving:
study technically not feasible
Justification for data waiving:
the study does not need to be conducted because the substance is highly insoluble in water
Transformation products:
no

Description of key information

Half-life in sediment: 6.91 years at 12°C (based on read-across)

Key value for chemical safety assessment

Half-life in freshwater sediment:
6.91 yr
at the temperature of:
12 °C

Additional information

There is no sediment degradation study available for ViM4Q; an OECD 308 study is available with the analogous substance octamethyltrisiloxane (L3; CAS 107-51-7) and has been read across to the registration substance.

ViM4Q and L3 are members of the Reconsile Siloxane Category and are structurally similar substances. L3 is a linear siloxane with three silicon atoms and two oxygen atoms, and each silicon atom is fully substituted with methyl groups. ViM4Q is a quaternary-branched structure of five Si atoms, with four terminal Si atoms linked to a central Si atom by Si-O-Si bonds. Each of the four terminal Si atoms is substituted by one vinyl and two methyl groups. Even though the surrogate substance and the target substance cannot be considered as close structural analogues, the property that will dominate the behaviour of the substances in the environment, and in particular in the sediment compartment, is the high adsorption potential (log Kowand Koc). The physicochemical properties of the registration substance are reasonably similar to those of L3, but the former should have greater stability in sediment: L3 has a higher tendency to volatilise from sediment compared to ViM4Q based on its higher vapour pressure (530 Pa versus <10 Pa at 20-25°C) and lower tendency to partition to organic matter (log Koc4.34 versus 6). 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 ViM4Q may be expected to be slower than L3, however since the degradation half-life for L3 leads to a conclusion of ‘vP’ for the sediment compartment based on a worst-case half-life of 6.91 years at 12°C, it is reasonable to read-across the conclusion of ‘vP’ in sediment for ViM4Q.

 

Table4.1.6Reconsile 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)

107-51-7

Octamethyltrisiloxane

Natural sediment (aerobic)

Half-life (DT50): 6.91 years at 12°C (high %OC sediment); 3.50 years at 12°C (lower % OC sediment)

1

The Dow Chemical Company (2021)

 

The chemical safety assessment according to REACH Annex I indicates that it is not necessary to conduct the simulation test on ultimate degradation in surface water, because the risk characterisation ratios (RCRs) for the aquatic compartment, even with the assumption that the parent substance is not biodegradable, are <1. Furthermore, the registration substance is highly insoluble in water, and it is considered that testing would not be feasible.