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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Biodegradation in water: screening tests: 9.7-9.9% in 28 days (CO2 evolution, OECD 301B), based on read-across from a structurally-related substance.

Key value for chemical safety assessment

Biodegradation in water:
under test conditions no biodegradation observed

Additional information

There are no reliable ready biodegradation data available for octaphenylcyclotetrasiloxane (CAS No. 546-56-5), therefore good quality data for the structurally-related substance, 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane (CAS No. 70940-00-0), have been read across.

Octaphenylcyclotetrasiloxane (CAS No. 546-56-5) and 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane (CAS No. 70940-00-0) are structurally-similar substances, both are tetrasiloxanes with phenyl functionality. The target substance octaphenylcyclotetrasiloxane (CAS 546-56-5) is a cyclic siloxane containing four Si atoms linked by oxygen. The silicon atoms are each substituted with two phenyl groups. The source substance 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane (CAS No. 70940-00-0) is a linear structure of four Si atoms, the central silicon atoms are substituted with two phenyl groups and the terminal silicon atoms are each substituted with two methyl and one hydrogen group.

 

A comparison of the key physicochemical properties is presented in the table below. The source substance has negligible biodegradability.

Table: Key physicochemical properties of octaphenylcyclotetrasiloxane and surrogate substance 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane

Property

Octaphenylcyclotetrasiloxane (CAS No. 546-56-5)

1,1,7,7-Tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane

(70940-00-0)

Molecular weight (g/mol)

793.19

530.92

log Kow

9.0

9.0

log Koc

6.0

6.0

Water solubility (mg/l)

<1.0E-05

<1.0E-05

Vapour pressure at 25°C (Pa)

<1.0E-06

<1.0E-06

The registration and read-across substances are within the Reconsile Siloxane Category of substances within which, in general, there is no evidence of any significant biodegradation. Therefore, in the context of the RAAF, Scenario 6 is considered to apply. The Category hypothesis is that read-across of “not readily biodegradable” between siloxanes is appropriate where data are not available for the substance itself. Additional information is given in a supporting report (PFA, 2017) attached in Section 13.

The table below presents ready biodegradation data for substances within the Siloxane Category.

Table: Ready biodegradation data for substances within the Siloxane Category

CAS

Name

Readily biodegradable? Yes/no

Result: Biodegradation after 28 days (%)

Guideline

Test type

Klimisch code

000107-46-0

Hexamethyldisiloxane

No

2%

OECD 301C

BOD

1

000107-51-7

Octamethyltrisiloxane

No

0%

OECD 310

CO2

1

000540-97-6

Dodecamethylcyclohexasiloxane

No

4.5%

OECD 310

CO2

1

000541-02-6

Decamethylcyclopentasiloxane

No

0.14%

OECD 310

CO2

1

000556-67-2

Octamethylcyclotetrasiloxane

No

3.70%

OECD 310

CO2

1

001873-88-7

1,1,1,3,5,5,5-heptamethyltrisiloxane

No

0%

OECD 310/ ISO Guideline No 14593

CO2

1

002627-95-4

1,1,3,3-tetramethyl-1,3-divinyldisiloxane

No

0%

OECD 301D

BOD

1

017928-28-8

1,1,1,3,5,5,5-heptamethyl- 3-[(trimethylsilyl)oxy] trisiloxane

No

0%

OECD 301F

BOD

1

017955-88-3

1,1,1,3,5,5,5-heptamethyl-3-octyltrisiloxane

No

10 – 32 % after 25 d

OECD 301B

CO2

 1

060111-47-9

3-[(dimethylvinylsilyl)oxy]-1,1,5,5-tetramethyl-3-phenyl-1,5-divinyltrisiloxane

No

2.20%

OECD 301F

BOD

1

070940-00-0

1,1,7,7,-Tetramethyl-3,3,5,5,-Tetraphenyl-Tetrasiloxane

No

9.7 and 9.9%

OECD 301B

CO2

 1

180073-46-5

Pentasiloxane, 1,1,9,9-tetramethyl-3,3,5,5,7,7-hexaphenyl-

No

5.2 and 4.4%

OECD 301B

CO2

 1

It is therefore considered valid to read-across the results for 1,1,7,7-tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane (CAS No. 70940-00-0) to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2017at) attached in Section 13.

1,1,7,7-Tetramethyl-3,3,5,5-tetraphenyl-tetrasiloxane attained 9.7 -9.9% biodegradation in 28 days (CO2 evolution).

Reference:

PFA (2017at). Peter Fisk Associates. Siloxane Category Report for Environmental Endpoints. PFA.404.114.001