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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

Biodegradation in water and sediment: simulation tests: half-lives 242 days (aerobic conditions) and 365 days (anaerobic conditions) at 24°C (based on read-across). In the exposure assessment, a degradation half-life in bulk sediment of 365 days at 25°C is used as a worse case.

Key value for chemical safety assessment

Half-life in freshwater sediment:
365 d
at the temperature of:
25 °C

Additional information

There is no sediment degradation study available for 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (Vi4 -D4), therefore good quality data for the structurally-related substance, octamethylcyclotetrasiloxane (CAS 556-67-2), have been read across.

The registration substance has an average purity of >80% Vi4-D4, with <15% 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane Vi5-D5 (CAS 17704-22-2; Impurity 1) and <10% 2,4,6-trimethyl-2,4,6-trivinylcyclotrisiloxane Vi3-D3 (CAS 3901-77-7; Impurity 2) present as impurities. Read-across studies are in place as supporting studies, to consider the properties of the impurities. Sediment degradation data are not relevant for Vi3-D3 as it hydrolyses rapidly; a data waiver is applicable. Data for Vi5-D5 are read-across from decamethylcyclopentasiloxane D5 (CAS 541-02-6).

Vi4-D4, D4, Vi5 -D5 and D5are members of the Reconsile Siloxanes Category. This Category consists of linear/branched and cyclic siloxanes which have a low functionality and a hydrolysis half-life at pH 7 and 25°C >1 hour and log Kow>4. There is a limited amount of sediment degradation data available with siloxanes. 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.

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 (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)

Additional information on the structure of the category and the supporting evidence for the application of the Scenario is given in a supporting report (PFA, 2017) attached in Section 13 of the IUCLID dossier.

A comparison of the key physicochemical properties of the registration substance and relevant impurity, and the target substances, is presented in the table below. All substances have negligible biodegradability and similar moderate hydrolysis rates. The degradation half-life for D4 and D5 leads to a conclusion of ‘vP’ for the sediment compartment based on a worst-case half-life of 365 d at 24°C, it is reasonable to read-across the conclusion of ‘vP’ in sediment for Vi4D4.

Table: Key physicochemical properties of Vi4-D4 and Vi5-D5 and surrogate substance D4 and D5

CAS Number

2554-06-5

556-67-2

17704-22-2

541-02-6

Chemical Name

2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane

Octamethylcyclotetrasiloxane

2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (Impurity 1 (Vi5-D5))

Decamethylcyclopentasiloxane (D5)

Ultimate Si hydrolysis product

Methylvinylsilanediol

Dimethylsilanediol

Methylvinylsilanediol

Dimethylsilanediol

Molecular weight (parent)

344.66

296.62

431

370.8

Molecular weight (hydrolysis product)

104.18

92.17

104.18

92.17

log Kow(parent)

6.47

6.49

9.0

8.07

Water sol (parent)

0.0073 – 0.0088 mg/l at 23°C

0.056 mg/l

9.1E-06 mg/l

0.017 mg/l

Vapour pressure (parent)

93.5 Pa

132 Pa

0.6 Pa

33 Pa

Hydrolysis t1/2at pH 7 and 25°C

approximately 63 hours

69-144 hours

1600 hours

1590 hours

 

It is therefore considered valid to read-across the results for D4 and D5 to fill the data gap for the registered substance.

Degradation in sediment has been shown to be slow, and may be predominantly abiotic. D4 degrades in sediment to form hydrolytic products and mineralisation rate is likely to be very slow.

A sediment (Lake Pepin) degradation half-life of 242 days at 24°C and under aerobic conditions was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

The major degradation products were hydrolytic products, such as dimethylsilandiol and non-extractable silanols, while 14C-CO2 generation was minimal, indicating complete mineralisation of D4 or its degradation products is very slow.

D4 degradation in non-sterilised samples was significantly faster than that in the chemically sterilised samples, suggesting that the degradation of D4 in the sediment might not be purely abiotic.

A sediment (Lake Pepin) degradation half-life of 365 days at 24°C and under anaerobic conditions was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

The major degradation products were hydrolytic products, such as dimethylsilandiol and non-extractable silanols, while 14C-CO2 generation and 14C-methane generation is minimal, indicating complete mineralisation of D4 or its degradation products is very slow.

D4 degradation in non-sterilised samples was almost the same as that in the chemically sterilised samples, suggesting that the degradation of D4 in the sediment under anaerobic conditions may be predominantly abiotic.

These two studies form a weight-of-evidence approach to biodegradation in sediment.

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. In the exposure assessment, a degradation half-life in bulk sediment of 365 days at 24°C is used as a worse case.

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.