Hexadecyltrimethoxysilane

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:

15 December 2011 - 15 December 2011

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: This study has been performed according to OECD and/or EC guidelines and according to GLP principles

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:

The batch of Hexadecyltrimethoxysilane tested was a clear colourless liquid with a purity of 98.3% and not completely soluble in test medium at the loading rates initially prepared. The substance has a very low water solubility (prediction / substance: 6.5 E-4 mg/l, hydrolysis products: 1.5 mg/l), and degrades in contact with water (the predicted hydrolysis half-life is 4.6 h, information obtained by the sponsor).

The test substance was pipetted below the surface of the Milli-RO-water and stirred briefly to mix (5 minutes). Subsequently, synthetic sewage feed and sludge will be added (see section 6.4) and the test was initiated applying a 3 hour contact time. Optimal contact between the test substance and test organisms was ensured applying continuous aeration and stirring. The solution of the test substance in water was pH neutral.

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 7.7 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:

Oxygen consumption was measured and recorded for a period of 11 to 27 minutes after the 3 hours exposure period.

Test temperature:

Between 19.6 and 21.2°C

pH:

pH before additionof sludge: 7.2 - 7.4
pH after the 3-hour exposure periode: 8.0 - 8.2, except in the abiotic control (6.9)

Dissolved oxygen:

The oxygen concentration at the start of measurement was at least 6.9 mg O2/l

(Mean) respiration rates:
Control: 50 mg O2/l/h
10 mg/l: 48 mg O2/l/h
100 mg/l: 43 mg O2/l/h
1000 mg/l: 44 mg O2/l/h

Nominal and measured concentrations:

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

Details on test conditions:

TEST SYSTEM
- Type (delete if not applicable): open
- Material, size, headspace, fill volume: All glass
- Aeration:During exposure with clean, oil-free air
- No. of vessels per concentration (replicates): 10 mg/l (1), 100 mg/l (1), 1000 mg/l (3)
- 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 water).

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.

Reference substance (positive control):

yes

Remarks:

3,5-dichlorophenol

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

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:

No statistically significant inhibition of the respiration rate of the sludge was recorded at a loading rate of 1000 mg Hexadecyltrimethoxysilane per litre (Two Sample t-Test: α=0.05 Toxstat).

There was no oxygen uptake from abiotic processes and the results at 1000 mg/l with a nitrification inhibitor showed a slightly higher heterotrophic inhibition of the respiration rate than the total 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 (9.8 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 Hexadecyltrimethoxysilane 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 Hexadecyltrimethoxysilane was not toxic to waste water (activated sludge) bacteria at or below a loading rate of 1000 mg/l (NOEC).

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

Description of key information

NOEC (3h) ≥ 1000 mg/L and EC50 > 1000 mg/L (OECD 209)

Additional information

The effect of hexadecyl(trimethoxy)silane (CAS No. 16415-12-6) on the respiration rate of activated sludge was investigated according to OECD 209 and GLP. Activated sludge of a predominantly domestic sewage was exposed to nominal test concentrations of 10, 100 and 1000 mg/L. No inhibition in respiration rate of the sludge was recorded in any of the concentrations tested. Hence, the NOEC (3 h) is ≥ 1000 mg/L. The EC50 of the reference substance, 3,5-dichlorophenol, was in the accepted range of 2 to 25 mg/L for total respiration (9.8 mg/L). Therefore, the test was considered to be valid.