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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: 0% in 28 days (OECD 310), 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 L4, therefore good quality data for the structurally-related substance, octamethyltrisiloxane, L3 (CAS 107-51-7), have been read across. The choice of read-across substance is based on structural similarities between the surrogate and target substance and similarities in their physico-chemical properties. L4 and L3 are structurally-similar substances, both are linear siloxanes.

L4 and the source substance L3 are linear siloxanes with four silicon atoms and three silicon atoms respectively, alternated by oxygen atoms. In both L3 and L4, the Si atoms are fully methyl substituted. L3 and L4 possess similar physicochemical properties. A comparison of the key physicochemical properties is presented in the table below. Both substances have negligible biodegradability and similar moderate hydrolysis rates.

Table 1: Key physicochemical properties of L4 and surrogate substance L3

Property

L4 (141-62-8)

L3 (107 -51-7)

Molecular weight

310.69

236.54

Log Kow

8.21

6.60

Log Koc

5.16

4.34

Water solubility (mg/l)

6.7E-03 mg/l at 23°C

3.4E-02 (at 23°C)

Vapour pressure at 25°C (Pa)

7.3E+02

5.3E+02

Hydrolysis half- life at pH 7 (h)

728

329

 

L4 and L3 are within the 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 of the IUCLID 6 dossier.

 

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

Table 2: Ready biodegradation data for substances within the Siloxane Category

CAS

Name

Readily biodegradable? Yes/no

Result: Biodegradation after 28 days (%)

Guideline

Test type

Reliability

000107-46-0

Hexamethyldisiloxane

No

2%

OECD 301C

BOD

1c

000107-51-7

Octamethyltrisiloxane

No

-3.70%

OECD 310

CO2

1a

000540-97-6

Dodecamethylcyclohexasiloxane

No

4.5%

OECD 310

CO2

1a

000541-02-6

Decamethylcyclopentasiloxane

No

0.14%

OECD 310

CO2

1a

000541-05-9

Hexamethylcyclotrisiloxane

No

0.06%

OECD 310

CO2

1a

000556-67-2

Octamethylcyclotetrasiloxane

No

3.70%

OECD 310

CO2

1a

001873-88-7

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

No

0%

OECD 310/ ISO Guideline No 14593

CO2

1a

002627-95-4

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

No

0%

OECD 301D

BOD

1a

017928-28-8

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

No

0%

OECD 301F

BOD

1a

017955-88-3

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

No

10 – 32 % after 25 d

OECD 301B

CO2

 1a

060111-47-9

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

No

2.20%

OECD 301F

BOD

1a

069304-37-6

1,3-dichloro-1,1,3,3-tetraisopropyldisiloxane

No

0% (BOD); 100% (GC)

OECD 301C

BOD

1a

070940-00-0

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

No

9.7 and 9.9%

OECD 301B

CO2

 1a

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

 1a

352230-22-9

Siloxanes and Silicones, di-Ph, Ph (trimethylsilyl)oxy

No

0%

OECD 301F

BOD

1a

 

In the study with L3, 0% biodegradation was observed in 28 days.

The study was conducted according to OECD 310 and is considered to be reliable. This is selected as the key study.