Reaction mass of decamethyltetrasiloxane; dodecamethylpentasiloxane; hexamethyldisiloxane; octamethyltrisiloxane

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

activated sludge respiration inhibition testing

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2001-02-15 to 2001-03-07

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Remarks:

The study was conducted according to an appropriate OECD test guideline, and in compliance with GLP.

Qualifier:

according to guideline

Guideline:

OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test

GLP compliance:

yes

Analytical monitoring:

no

Vehicle:

yes

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION (especially for difficult test substances)
- Method:  Tertiary butyl alcohol was used as a solvent in the five lower test solutions. HMDS was dosed directly in the 2 highest concentrations.

Test organisms (species):

activated sludge, domestic

Details on inoculum:

Activated sludge from a domestic sewage oxidation ditch situated at the district of Hazerswoude Dorp, The Netherlands, taken on 5th March 2001.

Test type:

static

Water media type:

freshwater

Limit test:

yes

Total exposure duration:

3 h

Post exposure observation period:

The oxygen concentration was recorded every minute during a period of about 10 minutes.

Test temperature:

20 ± 2 °C

pH:

8.1 to 8.3

Nominal and measured concentrations:

Nominal concentrations were 0, 0.10, 0.32, 1.0, 3.2, 11, 39 and 100 mg/l.

Details on test conditions:

TEST SYSTEM
- Test vessel: One litre glass beaker and BOD bottle

- Aeration: All suspensions were aerated vigorously.

TEST MEDIUM / WATER PARAMETERS
- Source/preparation of dilution water: Dilution water prepared by adding 1 ml of stock 1 to 4 to each litre of ultrapure water and store at 20 deg C in the dark with aeration. Prescription according to NEN 6634 [3].

- Vehicle: Tertiary butyl alcohol (TBA)

- Particulate matter: Activated sludge = 4.0 g (dw)/l

Reference substance (positive control):

yes

Remarks:

3,5-dichlorophenol

Duration:

3 h

Dose descriptor:

NOEC

Effect conc.:

>= 100 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

inhibition of total respiration

Remarks:

respiration rate

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

Details on results:

The two controls had a difference in respiration rate of 2.6%, less than the 15% limit for a valid study.  The validity criteria of the Guideline were fulfilled.

Results with reference substance (positive control):

A positive control test with 3,5-dichlorophenol yielded an EC50 of 13.1 mg/l.  The validity criteria of the Guideline were fulfilled.

No inhibition of the respiration rate was observed within the entire concentration range. Based on nominal test concentrations, the NOEC of HMDS was >= 100 mg/l. The EC50 was >100 mg/l.

Table 1: Results of the activated sludge respiration inhibition test

Test sample	Nominal concentration of test substance (mg/l)	Respiration rate (mg O2/g/h)	Coefficient of determination	% Inhibition respiration rate	pH
Hexamethyldisiloxane
Initial control	0	18.6	-1.0000	-1.3	8.2
Final control	0	18.1	-0.9999	1.3	8.2
	0.1	19.8	-1.0000	-0.8	8.2
	0.32	20.0	-1.0000	-9.1	8.2
	1	19.5	-1.0000	-6.4	8.1
	3.2	19.6	-1.0000	-6.9	8.1
	11	20.6	-1.0000	-12.0	8.1
	39	21.0	-1.0000	-14.1	8.1
	100	20.7	-1.0000	-12.9	8.1
3,5-dichlorophenol
Initial control	0	15.7	-0.9999	7.0	8.2
Final control	0	18.1	-0.9999	-7.0	8.2
	5	14.6	-0.9998	13.7	8.3
	12	8.9	-1.0000	47.7	8.3
	30	3.5	-0.9743	79.4	8.3

 

Validity criteria fulfilled:

yes

Conclusions:

A NOEC of >=100mg/l and EC50 >100 mg/l for toxicity to microorganisms were determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Reference 2

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

Remarks:

The study was conducted according to the appropriate OECD test guideline, and in compliance with GLP.

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

HMDS: Toxicity to microorganisms: 3-hour EC₅₀ >100 mg/l and NOEC≥100 mg/l (ASRI).

L3: Toxicity to microorganisms: 3-hour EC₅₀ >100 mg/l (ASRI), read-across from structurally-related substance L4.

L4: Toxicity to microorganisms: 3-hour EC₅₀ >100 mg/l (ASRI).

L5: Toxicity to microorganisms: 3-hour EC₅₀ >100 mg/l (ASRI), read-across from structurally-related substance L4.

Additional information

Constituent HMDS

A NOEC (3 hour) value of ≥100 mg/l and an EC₅₀ value of >100 mg/l have been determined in an activated sludge respiration inhibition study (Mayer, 2001). 

Constituent L3

There are no reliable microorganism toxicity data available for L3. A category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017). 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 from the structurally-related substance, decamethyltetrasiloxane, L4, (CAS 141-62-8), have been read across to L3.

L3 and L4 are members of the Reconsile Siloxanes Category within which, in general, the substances exhibit no evidence of significant toxicity to microorganisms.

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

In the study with L4, an activated sludge respiration inhibition 3-h EC₅₀ value of >100 mg/l was reported (Desmares-Koopmans, 2010).  

Constituent L4

A 3-hour 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 (Desmares-Koopmans, 2010). 

 

Constituent L5

There are no reliable microorganism toxicity data available for L5. A category approach is applied to this endpoint and is detailed in the Siloxane Category report (PFA, 2017). 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 L5.

L5 and L4 are members of the Reconsile Siloxanes Category within which, in general, the substances exhibit no evidence of significant toxicity to microorganisms.

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

In the study with L4, an activated sludge respiration inhibition 3-hour EC₅₀ value of >100 mg/l was reported (Desmares-Koopmans, 2010).