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Toxicity to microorganisms

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Reference
Endpoint:
activated sludge respiration inhibition testing
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2008-09-12 to 2008-12-04
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to
Guideline:
OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test
Qualifier:
according to
Guideline:
EU Method C.11 (Biodegradation: Activated Sludge Respiration Inhibition Test)
Qualifier:
according to
Guideline:
other: EPA OPPTS 850.6800 (Modified Activated Sludge, Respiration Inhibition Test for Sparingly Soluble Chemicals)
GLP compliance:
yes
Vehicle:
no
Details on test solutions:
PREPARATION AND APPLICATION OF TEST SOLUTION (especially for difficult test substances)
- Method: Amounts of parent material (50, 90, 160, 280, 500mg) were each separately dispersed, using a plastic disposable syringe, in approximately 250mL of water and subjected to ultrasonication for approximately 15 minutes. Synthetic sewage (16 ml), activated sewage sludge (200 ml) and water were added to a final volume of 500 ml.

- Evidence of undissolved material (e.g. precipitate, surface film, etc): At all test concentrations solid lumps of test material were observed dispersed throughout and adhered to the sides and bottom of the test flask. These were considered to be condensation products as a results of the reaction of the parent substance with water.
Test organisms (species):
activated sludge of a predominantly domestic sewage
Details on inoculum:
The activated sewage sludge was maintained on continuous aeration in the laboratory at a temperature of approximately 21°C.

- Initial biomass concentration: 3.8 g/L
Test type:
static
Water media type:
freshwater
Limit test:
yes
Total exposure duration:
3 h
Post exposure observation period:
The rate of respiration was determined after 3 hours contact time.
Test temperature:
21±1°C
pH:
7.5-7.8
Nominal and measured concentrations:
100, 180, 320, 560 & 1000 mg trichloro(phenyl)silane / L, equivalent to 74, 133, 236, 414, 740 mg phenylsilanetriol / L. It was considered justifiable to conduct the test using some test concentrations in excess of the water solubility level of the test substance in order to assess the effect of the test substance as a whole, i.e. the effect of an excess amount of undissolved test substance can be assessed.
Details on test conditions:
TEST SYSTEM

- Test vessel:

- Material, size, headspace, fill volume: 500mL conical flask

- Aeration: 0.5-1 L/min with clean, oil-free compressed air

- No. of vessels per concentration (replicates): one

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

TEST MEDIUM / WATER PARAMETERS

- Source/preparation of dilution water: laboratory tap water dechlorinated by passage through an activated carbon filter (Purite Serise 500) and partly softened (Elga Nimbus 1248D Duplex water softener).



OTHER TEST CONDITIONS

- Adjustment of pH: yes

- Light intensity: normal laboratory lighting


TEST CONCENTRATIONS

- Spacing factor for test concentrations: 100, 180, 320, 560 & 1000 mg trichloro(phenyl)silane / L ( equivalent to 74, 133, 236, 414, 740 mg phenylsilanetriol / L)

- Range finding study: Yes

- Test concentrations: 100 & 1000 mg trichloro(phenyl)silane / L, equivalent to 74 & 740 mg phenylsilanetriol / L, were used in range-finding study

- Results used to determine the conditions for the definitive study: significant inhibition of respiration was observed at 1000mg trichloro(phenyl)silane/L, however, there was no significant effects on respiration at 100mg trichloro(phenyl)silane/L. Based on this, concentrations of 100, 180, 320, 560 & 1000 mg trichloro(phenyl)silane / L were selected for the initial study.
Reference substance (positive control):
yes
Remarks:
3,5-dichlorophenol
Duration:
3 h
Dose descriptor:
EC50
Effect conc.:
> 1 000 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Remarks:
Equivalent to 740 mg phenylsilanetriol/L.
Basis for effect:
inhibition of total respiration
Remarks:
respiration rate
Remarks on result:
other: pH adjusted
Duration:
3 h
Dose descriptor:
NOEC
Effect conc.:
>= 1 000 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Remarks:
Equivalent to 740 mg phenylsilanetriol/L.
Basis for effect:
inhibition of total respiration
Remarks:
respiration rate
Remarks on result:
other: pH adjusted
Details on results:
The two controls had a difference in respiration rate of 8%, which is less than the 15% limit required by the guideline for a valid study.
Results with reference substance (positive control):
A positive control test with 3, 5-dichlorophenol yielded EC50s of 8.3 mg/l in the initial test and 7.3 mg/L in the definitive test, which are within the accepted range of 5 to 30 mg/l required by the guideline for a valid study.

Table 1: Oxygen consumption rates, percentage inhibition values and pH values in the definitive test after 3 hours contact time

Test sample

Nominal conc. (mg/l)

Initial oxygen reading (mg O2/l)

Final oxygen reading (mg O2/l/)

Oxygen consumption (mg O2/l/min)

% Inhibition

respiration rate

pH

0 hours

3 hours

 

 

Initial control

0

6.4

2.6

0.48

-

7.6

8.0

Final control

0

6.6

3.1

0.44

-

7.7

8.0

 

 

Parent material*

100

6.2

2.7

0.44

4

7.7**

7.8

180

6.3

2.7

0.45

2

7.7**

7.8

320

5.9

2.3

0.45

2

7.7**

7.8

560

6.2

3.0

0.46

0

7.5**

7.8

1000

6.3

2.7

0.45

2

7.6**

7.7

 

 

3,5-dichlorophenol

3.2

7.0

3.5

0.35

24

7.5

8.1

10

7.9

6.3

0.16

65

7.5

8.2

32

8.3

7.7

0.06

87

7.6

8.2

      *concentrations based on mg of trichloro(phenyl)silane/L                                                           

      **pH after adjustment to that of the dechlorinated tap water.

Initial test results: EC50 360 mg/l; NOEC 180 mg/l (based on mg of trichloro(phenyl)silane/L). The pH of test solutions dropped to 6.5 at 320 mg/l, 2.3 at 560 mg/l and 1.5 at 1000 mg/l.

      

Validity criteria fulfilled:
yes
Conclusions:
Activated sludge respiration inhibition 3hr EC50 value of >1000 mg/l (Nominal. Equivalent to >740 mg/l phenylsilanetriol) and 3hr NOEC value of ≥1000 mg/l (Nominal. Equivalent to ≥740 mg/l phenylsilanetriol). This indicates that the substance was not toxic to waste water (activated sludge) bacteria at a nominal concentration of 1000 mg/l (Nominal. Equivalent to 740 mg/l phenylsilanetriol).

Description of key information

Toxicity to microorganisms: Activated sludge respiration inhibition 3 hour EC50 value of >1000 mg/L (nominal) and NOEC value of ≥1000 mg/L (nominal) read-across from the structurally-related substance trichloro(phenyl)silane.

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 dichloro(methyl)(phenyl)silane, therefore good quality data for the structurally-related substance trichloro(phenyl)silane (CAS 98-13-5), have been read across. Both substances hydrolyse to structurally-related hydrolysis products, methylphenylsilanediol and phenylsilanetriol respectively; the other hydrolysis product being hydrochloric acid.

Hydrochloric acid is not expected to have adverse effects on microorganisms in a sewage treatment plant, where the pH is maintained within a favourable range.

Dichloro(methyl)(phenyl)silane and trichloro(phenyl)silane are within a wider analogue group than the aryl analogue group. Substances within this wider analogue group, in general, exhibit no evidence of significant toxicity to microorganisms. This wider analogue group for the toxicity to microorganisms endpoint consists of a number of different functional groups but specific read-across is between substances with similar functionality.

Table 7.4.2 presents microorganism toxicity data for substances relevant to the registered substance.

It is considered valid to read-across the results for trichloro(phenyl)silane (CAS 98-13-5) to fill the data gap for the registered substance since the source and target substances generate structurally analogous silanol hydrolysis products. Additional information is given in a supporting report (PFA 2013j) attached in Section 13.


Table 7.5.2. Microorganism toxicity data for relevant substances

CAS

Name

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

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

Guideline Number

Test method

Duration

Reliability

98-13-5

Trichloro(phenyl)silane

>1000 mg/l (Phenylsilanetriol >740mg/l) with pH adjustment

≥1000 mg/l (Phenylsilanetriol 740mg/l) with pH adjustment

OECD 209

ASRI

3 hr

1


A 3 hr EC50 of >1000 mg/L and NOEC of ≥1000 mg/L were determined for trichloro(phenyl)silane. Trichloro(phenyl)silane hydrolyses very rapidly to phenylsilanetriol and was used in the test as parent substance on the understanding that it would hydrolyse rapidly to produce the phenylsilanetriol. Initial and definitive tests were carried out, under the same conditions but pH regulation was employed in the definitive test as significant reduction in pH of test vessels occurred in the initial test due to hydrolysis of trichloro(phenyl)silane producing hydrogen chloride, and thus toxic effects may be related to hydrogen chloride rather than phenylsilanetriol. Effects of hydrogen chloride on microorganisms are limited to those that result from changes to pH in unbuffered media, therefore it is appropriate to use the results from the definitive test with pH adjustment.

Reference:

PFA (2013j). Peter Fisk Associates, STP Microorganism toxicity Main Analogue Group report, PFA.300.003.006