Trichlorosilane

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:

2009-09-04 to 2009-09-04

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, and was considered to be reliability 1 (reliable without restrictions).

Qualifier:

according to guideline

Guideline:

OECD Guideline 209 (Activated Sludge, Respiration Inhibition Test

Deviations:

no

GLP compliance:

yes

Analytical monitoring:

no

Vehicle:

no

Details on test solutions:

PREPARATION AND APPLICATION OF TEST SOLUTION
- Method: 54 µl of the test substance, corresponding to 50 mg, was pipetted below the surface of the
Milli-RO-water and stirred briefly to mix (5 minutes). Subsequently, synthetic sewage feed and sludge were added (final volume 500 ml) and the test was initiated. A loading rate of 100 mg/l was tested in duplicate. Optimal contact between the test substance and test medium was ensured by applying continuous aeration and stirring during the exposure period.

Test organisms (species):

activated sludge of a predominantly domestic sewage

Details on inoculum:

- Source: Activated sludge from municipal sewage treatment plant: ‘Waterschap de Maaskant', 's-Hertogenbosch, The Netherlands, receiving predominantly domestic sewage.

- Preparation of inoculum for exposure: The sludge was coarsely sieved to remove large particles. The sludge was then washed by centrifuging at 1000 g for 10 minutes. The supernatant was decanted and replaced with ISO-medium. The washing step was performed three times. A small amount of the sludge was weighed and dried overnight at ca. 105°C to determine the amount of suspended solids (3.8 g/l of sludge, as used for the test). The pH was 7.8 on the day of testing. The batch of sludge was used within two days after collection; therefore 50 ml of synthetic sewage feed was added per litre of activated sludge at the end of each day. The sludge was kept aerated at test temperature until use.

- Initial biomass concentration: 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

Test temperature:

18.5 to 19.0 °C

pH:

8.4

Details on test conditions:

TEST SYSTEM
- Test vessel: 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): Duplicate

- No. of vessels per control (replicates): Two controls without test substance were tested in each test series, one at the start and one at the end.

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.)

EFFECT PARAMETERS MEASURED (with observation intervals if applicable) :
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. Oxygen meter: WTW inolab Oxi 730 supplied with a WTW CellOx 325 oxygen electrode, electrolyte type ELY/G.

OTHER TEST CONDITIONS
- Adjustment of pH: The pH of synthetic sewage feed was adjusted with 1 M NaOH (Merck) to 7.4.


TEST CONCENTRATIONS
- Spacing factor for test concentrations: Performance of a limit concentration at 100 mg/l is acceptable, because 100 mg/l is far above the water solubility of Tetraethyl orthosilicate and thus exceeds the expected concentration in a Sewage Treatment Plant.

Reference substance (positive control):

yes

Remarks:

3, 5-dichlorophenol

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

Results with reference substance (positive control):

The 3-hour EC50 of 3,5-dichlorophenol was in the accepted range of 5-30 mg/l

Validity criteria fulfilled:

yes

Conclusions:

An activated sludge respiration inhibition 3 hour EC50 value of >100 mg/L was determined in a reliable study conducted according to an appropriate test protocol, and in compliance with GLP.

Description of key information

Toxicity to microorganisms: ASRI 3 hour EC₅₀ >100 mg/L (OECD 209) based on read-across from a analogous substance.

Additional information

There are no microorganism toxicity data available for trichlorosilane (CAS 10025-78-2), therefore good quality data for the analogous substance, tetraethyl orthosilicate (CAS 78-10-4), have been read across. Both substances undergo hydrolysis to form the same silanol hydrolysis product, silicic acid.

Tetraethyl orthosilicate hydrolyses rapidly with a half-life of 4.4 hours at pH 7 and 25°C to give monosilicic acid and ethanol.

Trichlorosilane (CAS 10025-78-2) hydrolysesvery rapidly with a half-life of <0.3 minutes at 25°C and pH 7 (analogue read-across). This half-life relates to degradation of the parent substance to give silanetriol and HCl. The Si-H bonds of silanetriol are expected to react rapidly (half-life <12 hours at 25°C and pH 7) to produce hydrogen and monosilicic acid.

In view of the rapid hydrolysis, it is the silanol hydrolysis product that is relevant for environmental risk assessment.

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.

Ethanol does not have adverse effects on microorganisms (OECD 2004).

Trichlorosilane (CAS 10025-78-2) and tetraethyl orthosilicate (CAS 78-10-4) are within an analogue 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 trichlorosilane. It is considered valid to read-across the results for tetraethyl orthosilicate (CAS 78-10-4) 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: Microorganism toxicity data relevant for the substance

CAS	Name	Main code	Result: E(I)C50 (mg/L)	Result: NOEC (Or EC10/EC20) (ml/L)	Guideline	Test method	Species	Duration (hours)	Klimisch code
78-10-4	Tetraethyl orthosilicate	I-4-Q	>100		OECD 209	ASRI		3	1
18765-38-3	Tetrakis(2-butoxyethyl) orthosilicate	I-4-Q		EC10 >1.7	Huls AG method	oxygen consumption	P. putida	5	2
68412-37-3	Silicic acid (H4SiO4), tetraethyl ester, hydrolyzed	I-4-Q		EC10 >2	Huls AG method	oxygen consumption	P. putida	5	2

In the study with tetraethyl orthosilicate (CAS 78-10-4), an activated sludge respiration inhibition (ASRI) 3-hour EC₅₀ value of >100 mg/L was obtained for the substance using a relevant test method and in compliance with GLP.

The hydrolysis rate for tetraethyl orthosilicate is 4.4 hours at pH 7 and 25°C, and so it is likely to hydrolyse to monosilicic acid to a lesser extent than trichlorosilane would hydrolyse over the timescale of the ASRI test. Therefore, microorganisms toxicity data for Silicic acid (H₄SiO₄), tetraethyl ester, hydrolysed (CAS 68412-37-3), which is a member of the same analogue group, has been read-across as supporting data. Microorganism toxicity data from a non-standard oxygen consumption study with Pseudomonas putida are available for Silicic acid (H₄SiO₄), tetraethyl ester, hydrolysed.

A 5.42 hour EC₁₀ of >2 ml/L for toxicity to Pseudomonas putida was determined for silicic acid (H₄SiO₄), tetraethyl ester, hydrolysed in a reliable study conducted according to generally accepted scientific standards and described in sufficient detail, and was carried out in compliance with GLP, with acceptable restrictions. The restrictions were that a reference substance was not used to check the sensitivity of the microorganisms, and an emulsifier was used which could have affected the results.

References:

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

OECD (2004): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 19-22 October 2004, Ethanol, CAS 64-17-5.