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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

Key value for chemical safety assessment

Half-life in freshwater sediment:
1 200 d
at the temperature of:
24 °C

Additional information

There are no sediment degradation data available for D6; an OECD 308 study is available with the analogous substance decamethylcyclopentasiloxane (D5, CAS 541-02-6) and has been read across to the registration substance.

D6 and D5 are members of the Reconsile Siloxane Category and are structurally similar substances. D6 is a cyclic siloxane with six dimethylated silicon atoms linked by six oxygen atoms. D5 is a directly analogous structure with five dimethylated silicon atoms and five oxygen atoms. The substances have similar physicochemical properties: high molecular weight (445 and 370 respectively), low water solubility (both insoluble, at 0.0051 and 0.017 mg/l respectively), high log Kow(8.87 and 8.07 respectively) and high log Koc(5.9 and 5.2 respectively). 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 D6 may be expected to be slower than D5, however since the degradation half-life for D5 leads to a conclusion of ‘vP’ for the sediment compartment, it is reasonable to read-across the conclusion of ‘vP’ in sediment for D6.

Sediment degradation half-lives of 1200 days under aerobic conditions at 24°C and 3100 days under anaerobic conditions at 24°C were determined for the analogous substance, D5, in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

The major degradation products, under aerobic and anaerobic conditions, were dimethylsilanediol and non-extractable silanols, while carbon dioxide and methane generation was minimal, indicating complete mineralisation of D5 or its degradation products is very slow.

Under aerobic conditions, D5 degradation in non-sterilised samples was significantly faster than that in the chemically sterilised samples, suggesting that the degradation of D5 in the sediment might not be purely abiotic.

The studies were carried out using a modified version of the OECD Guideline 308 (Aerobic and Anaerobic Transformation in Aquatic Sediment Systems), to account for the combination of high air/water partitioning coefficient and low water solubility of the substance.

Table: 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 (2009a)

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 (2009b)

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 (2010)

 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) 

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 substance is highly insoluble in water. In addition, in accordance with Column 2 of REACH Annex IX, the simulation test on ultimate degradation in surface water does not need to be conducted as the chemical safety assessment according to Annex I indicates that this is not necessary,because the risk characterisation ratios (RCRs) for the aquatic compartment, even with the assumption that the parent substance is not biodegradable, are <1.

 

References:

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

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

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

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