Dodecamethylpentasiloxane

Biodegradation in water: screening tests

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 L5, 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. L5 and L3 are structurally-similar substances, both are linear siloxanes.

L5 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 L5, the Si atoms are fully methyl substituted. L3 and L5 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: Key physicochemical properties of L5 and surrogate substance L3

Property	L5 (141-63-9)	L3 (107 -51-7)
Molecular weight	348.9	236.54
Log Kow	9.3-9.5	6.60
Log Koc	5.16	4.34
Water solubility (mg/l)	7.0E-05 mg/l at 23°C	3.4E-02 (at 23°C)
Vapour pressure at 25°C (Pa)	7.8	5.3E+02
Hydrolysis half- life at pH 7 (h)	728	329

L5 and L3 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 of the IUCLID 6 dossier.

The table below presents 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 reliable. This is selected as the key study.