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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:
09 February 2012 - 09 February 2012
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to
Guideline:
OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test
Deviations:
no
Qualifier:
according to
Guideline:
EU Method C.11 (Biodegradation: Activated Sludge Respiration Inhibition Test)
Deviations:
no
Qualifier:
according to
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. certificate)
Analytical monitoring:
no
Vehicle:
no
Details on test solutions:
The batch of triethoxy(octyl)silane tested was a clear yellow liquid with a purity >93%. The test substance 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) with initial loading rates of 2.5 times the final loading rate. These mixtures were stirred in closed dark brown bottles for approximately 24 hours. As a result, because hydrolysis half-life is predicted to be 22 hours at 25°C, it is likely that what was tested was a combination of parent and hydrolysis products (information obtained from the sponsor, after performance of the test). Subsequently, 16 ml synthetic medium, 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 medium 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 19.8°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 7.5 mg O2/l.

Controls: 48 mg O2/l h
Triethoxy(octyl)silane 10 mg/l: 48 mg O2/l h
Triethoxy(octyl)silane 100 mg/l: 42 mg O2/l h
Triethoxy(octyl)silane 1000 mg/l: 47 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
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
Details on results:
In the combined limit/range-finding test no statistically significant inhibition of the respiration rate of the sludge was recorded at a loading rate of 1000 mg triethoxy(octyl)silane per litre (Two Sample t-Test : α=0.05 Toxstat).

There was no significant oxygen uptake from abiotic processes and the results at 1000 mg/l with a nitrification inhibitor showed no heterotrophic inhibition of the respiration rate.
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 triethoxy(octyl)silane no EC50 could be calculated because the test substance proved to be non-toxic (EC50 > a loading rate of 1000 mg/l).

NOEC estimation
The NOEC was based on statistical analysis of the data. Data obtained for the test concentrations were compared with those obtained for the control using TOXSTAT Release 3.5, 1996, D.D. Gulley, A.M. Boelter, H.L. Bergman.
Validity criteria fulfilled:
yes
Conclusions:
Under the conditions of this present test, triethoxy(octyl)silane was not toxic to wastewater (activated sludge) bacteria up to and including a loading rate of >=1000 mg/l (NOEC).

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

Description of key information

Toxicity to microorganisms: ASRI 3 hour EC50 >1000 mg/L (loading rate) and NOEC >=1000 mg/L (loading rate), based on read-across from a structurally-related substance.

Key value for chemical safety assessment

Additional information

There are no microorganism toxicity data available for trimethoxy(octyl)silane, therefore good quality data for the structurally-related substance triethoxy(octyl)silane (CAS 2943-75-1), have been read across. Both substances undergo hydrolysis to form the same silanol hydrolysis product, octylsilanetriol. The other hydrolysis product for both substances are methanol and ethanol respectively. Both substances are expected to undergo minimal hydrolysis in the time-frame of an ASRI test.

Ethanol and methanol do not have adverse effects on microorganisms (OECD 2004a and OECD 2004b).

Trimethoxy(octyl)silane and triethoxy(octyl)silane are within a wider group of substances within which in general, there is no evidence of significant toxicity to microorganisms. This group consists of substances containing a number of different functional groups but specific read-across is between substances with similar functionality.

Table 7.4.2 presents microorganism toxicity data available for substances relevant to trimethoxy(octyl)silane. It is considered valid to read-across the results for triethoxy(octyl)silane (CAS 2943-75-1) to fill the data gap for the registered substance since the source and target substances generate the same silanol hydrolysis product.

Additional information is given in a supporting report (PFA, 2013j) attached in Section 13.


Table 7.4.2: Microorganism toxicity data for other relevant substances

CAS

Name

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

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

Guideline Number

Test method

Species

Duration (hours)

Reliability

1000-50-6

Butylchlorodimethylsilane

>340

 

88/302/EC

ASRI

 

3

1

1066-40-6

Hydroxytrimethylsilane

6670

 

OECD 209 and ISO 1892

ASRI

 

Uncertain

1

1185-55-3

Trimethoxy(methyl)silane

>100

 

OECD 209

ASRI

 

3

1

31795-24-1

Potassium methylsilanetriolate

>100

 

OECD 209

ASRI

 

3

1

87135-01-1

1,6-Bis(trimethoxysilyl)hexane

>1000

 

OECD 209

ASRI

 

3

1

17980-47-1

Triethoxyisobutylsilane

>1000

≥1000

OECD 209

ASRI

 

3

1

5894-60-0

Trichloro(hexadecyl)silane

>1000

≥1000

OECD 209

ASRI

 

3

1

16415-12-6

Hexadecyltrimethoxysilane

>1000

 

OECD 209

ASRI

 

3

1

2943-75-1

Triethoxyoctylsilane

>1000

 

OECD 209

ASRI

 

3

1

35435-21-3

Triethoxy(2,4,4-trimethylpentyl)silane

>100

 

OECD 209

ASRI

 

3

1

18395-30-7

Trimethoxy(2-methylpropyl)silane

 

EC10 1.3 ml/l

Huls AG method

oxygen consumption

P putida

5.8

2

142877-45-0

Silane, trimethoxy(1,1,2-trimethylpropyl)-

>1000

≥1000

OECD 209

ASRI

 

3

1

13154-25-1

Chlorotri(3-methyl-propyl)silane

>1000

100

OECD 209

ASRI

 

3

2

126990-35-0

Dicyclopentyldimethoxysilane

>water solubility

 

OECD 209//EU C11/Huls AG method (WOE)

ASRI/ASRI/

oxygen consumption

P. putida

3

1/1/2

139147-73-2

Silane, dichlorodicyclopentyl-

>100

 

OECD 209

ASRI

 

3

1

124-70-9

Dichloro(methyl)(vinyl)silane

>100 mg/l

 

OECD 209

ASRI

 

3

1

75-94-5

Trichloro(vinyl)silane

>100 mg/l

 

OECD 209

ASRI

 

3

1

1067-53-4

Tris(2-methoxyethoxy)vinylsilane

 

EC10 > 2 ml/L

Huls AG method

oxygen consumption

P. putida

5

2

2768-02-7

Trimethoxyvinylsilane

 

EC10 1.1 ml/l

Huls AG method

oxygen consumption

P. putida

5

2


In the study with triethoxy(octyl)silane, an activated sludge respiration inhibition, 3 hour NOEC (total respiration) value of ≥1000 mg/L and EC50 (total respiration) of 1000 mg/L were determined in a reliable study conducted according to an appropriate test method and in compliance with GLP. Under the conditions of this present test, triethoxy(octyl)silane was not toxic to wastewater (activated sludge) bacteria up to and including a loading rate of 1000 mg/L (NOEC). The EC50 exceeded a loading rate of 1000 mg/L.

Reference:

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