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Toxicity to microorganisms

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Description of key information

Toxicity to microorganisms: 3 h EC50 >1000 mg/l; NOEC >=1000 mg/l (OECD 209)

Key value for chemical safety assessment

Additional information

There are no microorganism toxicity data available for 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane. A category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017a). The hypothesis for read-across is that siloxane substances exhibit no evidence of significant toxicity to sewage treatment plant microorganisms. This is consistent with a non-polar narcosis mode of action. Therefore good quality data for the structurally-related substance Silsesquioxanes, Phenyl (CAS 070131-69-0), have been read across..

Phenyl silsesquioxanes is a multiconstituent substance, and is a reaction mass of the monomer, dimer and oligomers of phenyltris(trimethylsiloxy)silane. The linear oligomers of phenyl silsesquioxanes contain a siloxane chain, where the terminal Si atoms are fully methyl substituted, and the repeating unit contains a Si atom substituted with a phenyl and a trimethylsiloxy group. The major constituents comprise 70-80% of the linear oligomers n=1-5; the minor constituents comprise 10-20% cyclic (n=3-7) and linear (n=6-7) oligomers. The combined purity of both linear and cyclic oligomers is about 85-95%. The remaining impurities are higher polymerised material and alkoxy substituted oligomers. The submission substance 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane is also a mono-constituent substance consisting of >95% of 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxaneand <5% identified impurities (phenyl-siloxane). In 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane, each terminal silicon atom is substituted with one methyl and two phenyl groups, whilst the central silicon atom is substituted with one methyl and one phenyl group.

Both substances share structurally similar ultimate silanol hydrolysis products: 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane hydrolyses to form methyl(diphenyl)silanol and methyl(phenyl)silanediol, and silsesquioxanes, phenyl hydrolyses to form phenylsilanetriol and trimethylsilanol.

Both 1,3,5-trimethyl-1,1,3,5,5-pentaphenyltrisiloxane and silsesquioxanes, phenyl are members of the Reconsile Siloxanes Category within which, in general, the substances exhibit no evidence of significant toxicity to microorganisms.

The table below presents microorganism toxicity data available for substances within the Siloxanes Category.

It is therefore considered valid to read-across the results forsilsesquioxanes, phenyl to fill the data gap for the registered substance.


 

Table: Microorganism toxicity data available for substances within the Siloxanes Category

CAS

Name

Result: E(I) C50(mg/l)

 

Result: NOEC (or EC10/ EC20) (mg/l)

 

Guideline Number

Duration (h)

 

Reliability

540-97-6

Dodecamethylcyclohexasiloxane

>100 mg/l

 

OECD 209

3

1

541-05-9

Hexamethylcyclotrisiloxane

>100 mg/l

>100 mg/l

OECD 209

3

1

541-02-6

Decamethylcyclopentasiloxane

>2000 mg/l

 

C11

3

1

556-67-2

Octamethylcyclotetrasiloxane

>10 000 mg/l

 

ISO 8192 (Test for Inhibition of Oxygen Consumption by Activated Sludge)

3

4 (WOE)

556-67-2

Octamethylcyclotetrasiloxane

little or no toxicity (24 h): 0.1 g/g solution test mat. based on: reduction in number of viable bacteria

 

Toxicity toEscherichia coliandStaphylococcus aureusat 37°C for 24 hours

24

4 (WOE)

107-46-0

Hexamethyldisiloxane

>100

≥100

OECD 209

3

1

141-62-8

Decamethyltetrasiloxane

>100 mg/l

 

OECD 209

3

1

352230-22-9

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

>1000 mg/l

≥1000 mg/l

OECD 209

3

1

2627-95-4

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

>1000 mg/l

≥1000 mg/l

Nitrification control indicated no heterotrophic inhibition of respiration rate

OECD 209

3

1

2554-06-5

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

>1000 mg/l

≥1000 mg/l

Nitrification control indicated no heterotrophic inhibition of respiration rate

OECD 209

3

1

70131-69-0

Silsesquioxanes, Phenyl

>1000

≥1000

Nitrification control indicated no heterotrophic inhibition of respiration rate

OECD 209

3

1

99363-37-8

dimethyl, methyl(3-aminopropyl) siloxane, trimethylsiloxy-terminated

> water soluble fraction

≥ water soluble fraction

OECD 209

3

1

71750-79-3

dimethyl, (aminoethylaminopropyl) methyl siloxane, trimethylsiloxy-terminated

> water soluble fraction

≥ water soluble fraction

OECD 209

3

1

 

Consideration of low functionality side chains and reactive groups

Phenyl group: The ecotoxicity of phenyls is related to non-polar narcosis. The ecotoxicity of substances with phenyl groups increases with increasing log Kow. For example, trichlorophenylsilane (CAS 98-13-5) has a very fast hydrolysis rate and the log Kow of the hydrolysis product is -0.2. Short-term toxicity data with trichlorophenylsilane determine no effects up to 100 mg/l (Springborn Smithers 2009a, b and c).

Conclusion

In the study with phenyl silsesquioxanes, an activated sludge respiration inhibition 3 hour NOEC value of1000 mg/l (based on loading rate) was reported.