Tetradecamethylhexasiloxane

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Endpoint:

activated sludge respiration inhibition testing

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Qualifier:

according to guideline

Guideline:

OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test

GLP compliance:

yes

Analytical monitoring:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION (especially for difficult test substances)
- Method: 59 µl of the test substance, corresponding to 50 mg, was pipetted below the surface of the Milli-RO-water (tap water purified by reverse osmosis; Millipore Corp., Bedford, Mass., USA) and stirred briefly to mix (5 minutes). Subsequently, synthetic sewage feed and sludge were added.

Test organisms (species):

activated sludge of a predominantly domestic sewage

Details on inoculum:

- Preparation of inoculum for exposure: Coarsely sieved then washed and diluted with ISO medium.

- Amount suspended solids: 3.8 g/l of sludge, as used for the test

Test type:

static

Water media type:

freshwater

Limit test:

yes

Total exposure duration:

3 h

Post exposure observation period:

After the 3-hour contact time, a well mixed sample of the contents was poured into a 300 ml oxygen bottle, and the flask was sealed with an oxygen electrode connected to a recorder, forcing the air out of the vessel. Oxygen consumption was measured and recorded for approximately 10 minutes. During measurement, the sample was not aerated but continuously stirred on a magnetic stirrer.

Test temperature:

20.1 - 20.4°C

pH:

8.2 - 8.3

Nominal and measured concentrations:

A loading rate of 100 mg/l was tested in duplicate. Performance of a limit concentration at 100 mg/l is acceptable, because 100 mg/l is far above the water solubility of Decamethyltetrasiloxane and thus exceeds the expected concentration in a Sewage Treatment Plant.

Details on test conditions:

TEST SYSTEM
- Test vessel:
- Material, size, headspace, fill volume: All glass, approximately 300 ml oxygen bottles and 1 litre test bottles.

- Aeration: Strong aeration with clean, oil-free air.

- No. of vessels per concentration (replicates): Loading rates of 100 mg/l tested in duplicate.

- No. of vessels per control (replicates): Two replicates.

- No. of vessels per reference (replicates): Three concentrations, each one replicate.


TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: Tap-water purified by reverse osmosis (Milli-RO) and subsequently passed over activated carbon and ion-exchange cartridges (Milli-Q) (Millipore Corp., Bedford, Mass., USA).

Reference substance (positive control):

yes

Remarks:

3,5-dichlorophenol

Duration:

3 h

Dose descriptor:

EC50

Effect conc.:

> 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Results with reference substance (positive control):

- Results with reference substance valid? Yes. The EC50 of 3,5-dichlorophenol was in the accepted range of 2 to 25 mg/l

Controls (C) and Decamethyltetrasiloxane (T): pH, oxygen concentration, oxygen consumption and percentage inhibition of the respiration rate

Flask	Loading rate (mg/l)	Oxygen conc. at the start (mg O2/l)	Oxygen consumption (mg O2/l/h)	% Inhibition respiration rate	pH
Cstart T	0	7.0	38	-	8.3
Cend T	0	7.9	41	-	8.3
			∆ 8%
T 1	100	5.8	41	-4¹	8.2
T 2	100	6.1	40	-1¹	8.2

¹: Negative values indicate stimulation of the respiration rate of the sludge. The values are considered as not significant.

Validity criteria fulfilled:

yes

Conclusions:

An EC50 value of >100 mg/l for toxicity to microorganisms was determined in a reliable ASRI study conducted according to an appropriate test protocol, and in compliance with GLP.

Description of key information

Toxicity to microorganisms: EC₅₀ >100 mg/l (ASRI), read-across from a structurally-related substance.

Additional information

There are no reliable microorganism toxicity data available for L6.

A category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017at). 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, decamethyltetrasiloxane, L4, (CAS 141-62-8), have been read across to L6.

L6 and L4 are members of the Reconsile Siloxanes Category within which, in general, the substances exhibit no evidence of significant toxicity to microorganisms.

L4 is a linear siloxane chain with four silicon atoms, connected by three oxygen atoms. All silicon atoms present are fully substituted with methyl groups. L6 is a structurally related linear siloxane, with six silicon atoms and five oxygen atoms.

Table 7.4.2 presents microorganism toxicity data available for substances within the Siloxanes Category.

Additional information is given in a supporting report (PFA, 2017at) attached in Section 13 of the IUCLID dossier.

Table: Microorganism toxicity data for substances within the Siloxane Category

CAS	Name	Result: E(I)C50(mg/l)	Result: NOEC (or EC10/ EC20) (mg/l)	Guideline Number	Test method	Duration (h)	Reliability
540-97-6	Dodecamethylcyclohexasiloxane	>100		OECD 209	ASRI	3	1
541-05-9	Hexamethylcyclotrisiloxane	>100	>100	OECD 209	ASRI	3	1
541-02-6	Decamethylcyclopentasiloxane	>2000		C11	ASRI	3	1
107-46-0	Hexamethyldisiloxane	>>100	≥100	OECD 209	ASRI	3	1
141-62-8	Decamethyltetrasiloxane	>100		OECD 209	ASRI	3	1
352230-22-9	Siloxanes and Silicones, di-Ph, Ph (trimethylsilyl)oxy	>1000	≥1000	OECD 209	ASRI	3	1
2627-95-4	1,1,3,3-tetramethyl-1,3-divinyldisiloxane	>1000	≥1000	OECD 209	ASRI	3	1
2554-06-5	2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane	>1000	≥1000	OECD 209	ASRI	3	1
N/A (EC 939-487-8)	Phenyl silsesquioxanes	>1000	≥1000	OECD 209	ASRI	3	1
99363-37-8	dimethyl, methyl(3-aminopropyl) siloxane, trimethylsiloxy-terminated	> water soluble fraction	≥ water soluble fraction	OECD 209	ASRI	3	1
71750-79-3	dimethyl, (aminoethylaminopropyl)methyl siloxane, trimethylsiloxy-terminated	> water soluble fraction	≥ water soluble fraction	OECD 209	ASRI	3	1

In the study with L4,an activated sludge respiration inhibition 3-hour EC₅₀value of >100 mg/l was reported.

Reference:

Peter Fisk Associates (2017at) Siloxane Category Report for Environmental Endpoints. Owner company: Reconsile. Report date: 2017-06-02