Octaphenylcyclotetrasiloxane

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

Study period:

09 February 2012 - 09 February 2012

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

Deviations:

no

Qualifier:

according to guideline

Guideline:

EU Method C.11 (Biodegradation: Activated Sludge Respiration Inhibition Test)

Deviations:

no

Qualifier:

according to guideline

Guideline:

ISO 8192 (Water quality - Test for inhibition of oxygen consumption by activated sludge for carbonaceous and ammonium oxidation)

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Analytical monitoring:

no

Vehicle:

no

Details on test solutions:

Silsequioxanes, phenyl (tested as received) was a clear colourless liquid that was not completely soluble in test medium at the loading rates initially prepared. Therefore, 1-Litre test bottles were filled with 200 ml of test substance mixtures in Milli-RO water (tap water purified by reverse osmosis; Millipore Corp., Bedford, Mass., USA) with initial loading rates of 2.5 times the final loading rate. These mixtures were stirred in closed dark brown bottles for 23½ hours, except in the abiotic control (25½ hours). Subsequently, 16 ml synthetic sewage feed, 250 ml sludge and Milli-RO water up to 500 ml were added resulting in the required loading rates. Optimal contact between the test substance and test organisms was ensured applying continuous aeration and stirring.

Test organisms (species):

activated sludge of a predominantly domestic sewage

Details on inoculum:

The sludge was coarsely sieved, washed and diluted with ISO-medium. A small amount of the sludge was weighed and dried overnight at ca. 105°C to determine the amount of suspended solids (3.0 g/l of sludge, as used for the test). The pH was 7.4 on the day of testing. The batch of sludge was used one day after collection; therefore 50 ml of synthetic sewage feed was added per litre of activated sludge at the end of the collection day. The sludge was kept aerated at test temperature until use.

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, the oxygen consumption was recorded for a period of approximately 10 minutes. During measurement, the sample was not aerated but continuously stirred on a magnetic stirrer. The pH and temperature were determined in the remaining part of the reaction mixture.

Test temperature:

Between 18.1 and 20.5°C.

pH:

Before addition of sludge : 7.6 - 7.7.
After the 3 hour exposure period : 7.5 - 8.3.

Dissolved oxygen:

The oxygen concentration at the start of measurements was at least 7.0 mg O2/l

Oxygen consumption:
Controls : 48 mg O2/l/h (mean value)
silsequioxanes, phenyl 10 mg/l : 42 mg O2/l/h
silsequioxanes, phenyl 100 mg/l : 49 mg O2/l/h
silsequioxanes, phenyl 1000 mg/l : 51 mg O2/l/h (mean value)

Nominal and measured concentrations:

Nominal loading rates: 10, 100 and 1000 mg/l.

Details on test conditions:

TEST SYSTEM
- Type: open
- Material, size, headspace, fill volume: All glass
- Aeration: During exposure with clean, oil-free air
- No. of vessels per concentration (replicates): 1 for 10 and 100 mg/l and 3 for 1000 mg/l
- No. of vessels per control (replicates): 2
- Biomass loading rate: 1.5 g/l suspended solids in final test mixture

TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: Tap-water purified by reverse osmosis (Milli-RO
EFFECT PARAMETERS MEASURED (with observation intervals if applicable) : The respiration rate from each vessel, in mg O2/l/hr, was calculated from the linear part of the respiration curve, which was generally between 2 and 7 mg O2/l.

TEST CONCENTRATIONS
- Test concentrations: combined limit/range-finding test: at 10, 100 and 1000 mg/l (loading rates).

Reference substance (positive control):

yes

Remarks:

3,5-dichlorophenol

Key result

Duration:

3 h

Dose descriptor:

NOEC

Effect conc.:

>= 1 000 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Remarks:

(loading rate)

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Key result

Duration:

3 h

Dose descriptor:

EC50

Effect conc.:

> 1 000 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Remarks:

(loading rate)

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

Details on results:

In the combined limit/range-finding test no inhibition of the respiration rate of the sludge was recorded at the highest loading rate tested, 1000 mg silsequioxanes, phenyl per litre.

There was no oxygen uptake from abiotic processes and the results at 1000 mg/l with a nitrification inhibitor showed that the heterotrophic inhibition of the respiration rate was comparable to the total inhibition.

Results with reference substance (positive control):

The EC50 of 3,5-dichlorophenol was in the accepted range of 2 to 25 mg/l for total respiration (4.0 mg/l).

Reported statistics and error estimates:

ECx
For the reference substance the percentage inhibition was plotted against the logarithm of the concentrations and the EC50 was determined using linear regression analysis.

For silsequioxanes, phenyl no EC50 could be calculated because the test substance proved to be non-toxic (EC50 > a loading rate of 1000 mg/l).

NOEC estimation
Since no inhibition of the respiration rate was observed at the highest loading rate tested, no statistical analysis of the data was performed.

Validity criteria fulfilled:

yes

Conclusions:

An activated sludge respiration inhibition (ASRI) 3 hour EC50 >1000 mg/l and NOEC >=1000 mg/l (OECD 209) were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

The EC50 exceeded a loading rate of 1000 mg/l.

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

EC50 for microorganisms:

1 000 mg/L

EC10 or NOEC for microorganisms:

1 000 mg/L

Additional information

There are no microorganism toxicity data available for octaphenylcyclotetrasiloxane, therefore good quality data for the structurally-related substance phenyl silsesquioxanes (CAS 70131-69-0), have been read across.

Octaphenylcyclotetrasiloxane (CAS No. 546-56-5) and phenyl silsesquioxanes (CAS No. 70131-69-0) are structurally-similar substances, both are siloxanes with phenyl functionality. Octaphenylcyclotetrasiloxane is a cyclic tetrasiloxane with two phenyl substituents at each silicon. Phenyl silsesquioxanes is a reaction mass of the monomer, dimer and oligomers of phenyltris(trimethylsiloxy)silane. Phenyltris(trimethylsiloxy)silane is a branched tetrasiloxane with a phenyl substituent on the central silicon atom and three methyl groups on the other three silicons. The other major constituents of phenyl silsesquioxanes are also branched siloxanes with phenyl and methyl substituents.The target substance and all constituents of the source substance have a very low water solubility, very high low K_ow and log Koc, high molecular weight and very low vapour pressure.

Both octaphenylcyclotetrasiloxane and silsesquioxanes, phenyl are within the Reconsile Siloxanes Category. Substances in this category tend to have low water solubility, high adsorption and partition coefficients and slow degradation rates, and category member substances exhibit no evidence of significant toxicity to sewage treatment plant microorganisms.

The table below presents microorganism toxicity data for the siloxanes category member substances. It is therefore considered valid to read-across the results for phenyl silsesquioxanes to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2017) attached in Section 13.

 

Table: Microorganism toxicity data 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

 

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 K_ow. For example, trichlorophenylsilane (CAS 98-13-5) has a very fast hydrolysis rate and the log K_ow 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 of ≥1000 mg/l (based on loading rate) was reported.