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Ecotoxicological information

Toxicity to microorganisms

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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 with acceptable restrictions
Remarks:
The study was conducted according to an appropriate OECD test guideline, and in compliance with GLP, and is considered to be reliability 1 (reliable without restrictions).
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.
Endpoint:
activated sludge respiration inhibition testing
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
Please see endpoint summary for justification of read-across.
Reason / purpose for cross-reference:
read-across source
Key result
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

Description of key information

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

Key value for chemical safety assessment

Additional information

There are no reliable microorganism toxicity data available for L3, therefore good quality data for the structurally-related substance, decamethyltetrasiloxane, L4, (CAS 141-62-8), 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. L3 and L4 are structurally-similar substances, both are linear siloxanes.

L3 and the source substance L4 are linear siloxanes with three silicon atoms and four silicon atoms respectively, alternated by oxygen atoms. In both L3 and L4, the Si atoms are fully methyl substituted. L3 and L4 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 L3 and surrogate substance L4

Property

L3 (107 -51-7)

L4 (141-62-8)

Molecular weight

236.54

310.69

Log Kow

6.60

8.21

Log Koc

4.34

5.16

Water solubility (mg/l)

3.4E-02 (at 23°C)

6.7E-03 mg/l at 23°C

Vapour pressure at 25°C (Pa)

5.3E+02

7.3E+02

Hydrolysis half- life at pH 7 (h)

329

728

 

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 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.

The table below 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 6 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

Duration

Reliability

540-97-6

Dodecamethylcyclohexasiloxane

>100 mg/l

 

OECD 209

3 h

1

541-05-9

Hexamethylcyclotrisiloxane

>100 mg/l

>100 mg/l

OECD 209

3 h

1

541-02-6

Decamethylcyclopentasiloxane

>2000 mg/l

 

C11

3 h

1

556-67-2

Octamethylcyclotetrasiloxane

>10 000 mg/l

 

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

3 h

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 to Escherichia coli and Staphylococcus aureus at 37°C for 24 hours

24 h

4 (WOE)

107-46-0

Hexamethyldisiloxane

>100

≥100

OECD 209

3 h

1

141-62-8

Decamethyltetrasiloxane

>100 mg/l

 

OECD 209

3 h

1

352230-22-9

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

>1000 mg/l

≥1000 mg/l

OECD 209

3 h

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 h

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 h

1

70131-69-0

Silsesquioxanes, Phenyl

>1000

≥1000

Nitrification control indicated no heterotrophic inhibition of respiration rate

OECD 209

3 h

1

99363-37-8

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

> water soluble fraction

≥ water soluble fraction

OECD 209

3 h

1

71750-79-3

dimethyl, (aminoethylaminopropyl) methyl siloxane, trimethylsiloxy-terminated

> water soluble fraction

≥ water soluble fraction

OECD 209

3 h

1

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

The study is considered to be reliability 1 (reliable without restrictions).