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Biodegradation in water: screening tests

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Description of key information

Biodegradation in water, screening test on read accross substance (chloromethyl)trimethoxysilane (CAS RN 5926-26-1): 44-52 % in 28 days (OECD 301F)

Key value for chemical safety assessment

Additional information

There are no reliable ready biodegradation data available for (chloromethyl)diethoxymethylsilane (CAS No. 2212-10-4), therefore good quality data for the structurally-related substance, Silane, (chloromethyl)trimethoxy- (CAS No. 5926-26-1), have been read across. Both substances hydrolyse to structurally-related silanol hydrolysis products, chloromethylsilanetriol and (chloromethyl)methylsilanediol, the other hydrolysis products being methanol and ethanol, respectively, which are readily biodegradable. Both (chloromethyl)trimethoxysilane (CAS No. 5926-26-1) and silane, (chloromethyl)diethoxymethyl- (CAS No. 2212-10-4) are within a wider analogue group which exhibit no evidence of any significant biodegradation once hydrolysis and subsequent biodegradation of alkoxy/acetoxy groups has been taken into account. It is therefore considered valid to read-across the results for silane,(chloromethyl) trimethoxy- to fill the data gap for the registered substance. In the study 44 -52 % biodegradation was observed in 28 days. The study was conducted according to OECD guideline 301F and is considered reliable. This is selected as the key study. The study is considered to be reliability 1 (reliable without restrictions).

 

  Read-across justification biodegradation

 

There is no measured data available for (Chloromethyl)diethoxymethylsilane (CAS 2212-10-4) for the biodegradation. This document describes the analogue approach for fulfilling this endpoint by read-across from a source substance (Chloromethyl)trimethoxysilane (CAS 5926-26-1), according to the Read-across Assessment Framework (RAAF)[1].

Read-across is proposed in accordance with RAAF Scenario 2: “This scenario covers the analogue approach for which the read-across hypothesis is based on different compounds which have the same type of effect(s). For the REACH information requirement under consideration, the effects obtained in a study conducted with one source substance are used to predict the effects that would be observed in a study with the target substance if it were to be conducted. The same type of effect(s) or absence of effect is predicted. The predicted strength of the effects may be similar or based on a worst case.” The read-across justification is presented (Table 5.6.4) according to RAAF scenario 2 assessment elements (AE) as outlined in Table B1 of the RAAF1:

 

Table 1: RAAF scenario 2 assessment elements (AE) as given in Appendix B (Table B1) of the RAAF1

AE A.1

Characterisation of source substance

AE A.2

Link of structural similarity and differences with the proposed Prediction

AE A.3

Reliability and adequacy of the source study

AE 2.1

Compounds the test organism is exposed to

AE 2.2

Common underlying mechanism, qualitative aspects

AE 2.3

Common underlying mechanism, quantitative aspects

AE 2.4

Exposure to other compounds than to those linked to the prediction

AE 2.5

Occurrence of other effects than covered by the hypothesis and Justification

AE A.4

Bias that influences the prediction

 

1. AE A.1 Identity and characterisation of the source substance

 

The source substance, (Chloromethyl)trimethoxysilane (CAS RN 5926-26-1) contains a chloromethyl silane group in addition with methoxy moieties. (Chloromethyl)trimethoxysilane hydrolyses rapidly to form (chloromethyl)silanetriol (1 moles) and methanol (3 moles) under conditions relevant in the environment or in vivo,with a half-life of 28 to 504 min. A half-life value of approximately 28 to 504 min at 20-25°C and pH 7 was obtained using an accepted using HYDROWIN v2.00.

The source substance has log Kow of -0.42 at 20°C (QSAR), water solubility of 1.4E+05 mg/l at 20°C (QSAR) and vapour pressure of 340 Pa at 25°C (QSAR).

 

 

2. AE A.2 Link of structural similarities and differences with the proposed prediction

The registration substance, (Chloromethyl)diethoxymethylsilane (CAS 2212-10-4), and the read-across substance, (Chloromethyl)trimethoxysilane (CAS RN 5926-26-1), are structurally similar. (Chloromethyl)trimethoxysilane (CAS RN 5926-26-1) contains a chloromethyl silane group in addition to three methoxy-groups attached to the Si atom, while in (Chloromethyl)diethoxymethylsilane (CAS 2212-10-4) two ethoxy-groups are attached to the Si atom of the chloromethyl silane group and the third ethoxy group is substituted by a methyl group.

This structural difference remains in the Si-containing hydrolysis product for the target and source substance. The Si hydrolysis product for the target substance is (chloromethyl)methylsilanediol while the Si hydrolysis product for the source substance is chloromethylsilanetriol.

 

Table 2: Physico-chemical properties

 

Property

 

Target substance

 

Source substance

 

Substance name

 

(Chloromethyl)diethoxymethylsilane  

 

(Chloromethyl)trimethoxysilane  

 

CAS number

 

2212-10-4

 

5926-26-1

 

Hydrolysis half-life at pH 4 and 20 - 25°C

 

1.1 – 24 min (QSAR)

 

1.1 – 24 min (QSAR)

 

Silanol hydrolysis product

 

(chloromethyl)methylsilanediol

 

chloromethylsilanetriol

 

Non-Si hydrolysis product

 

Ethanol

 

Methanol

 

LogKow Value

 

1.82 (QSAR)

 

-0.42 (QSAR)

 

Vapour pressure

 

25.2 hPa at 25°C (measured)

 

340 Pa at 25°C (QSAR)

 

Water solubility

 

1487 mg/L at 25°C (QSAR)

 

1.4 E+05 mg/l at 20°C (QSAR)

 

 

 

3. AE A.3 Reliability and adequacy of the source study

 

The ready biodegradability of (chloromethyl)trimethoxysilane (CAS RN 5926-26-1) was assessed in a manometric respirometry test according to OECD Guideline 301 F. The test item was investigated over a period of 28 days. The biodegradation was followed by the oxygen uptake of the microorganisms during exposure. As a reference item Aniline was tested simultaneously under the same conditions as the test item and functioned as a procedure control. After correction of the mean biochemical oxygen demand of the inoculum controls on day 2 of exposure 19% and 20% biodegradation were determined. At the end of the 10-day window, on day 12 biodegradation rates of 52 % and 53 % were measured. At the end of the 28-day exposure period degradation rates of 44 % and 52 % were found. The percentage biodegradation did not exceed 60 % within the 10- day window. The test item can therefore be considered to be not readily biodegradable.

 

4. AE A.4 Bias that influences the prediction

 

Data on the source substance (Chloromethyl)trimethoxysilane (CAS RN 5926-26-1) were read-across to the registered (target) substance (Chloromethyl)diethoxymethylsilane (CAS 2212-10-4). The source substance and the target substance have similar chemical structure and physico-chemical properties. Both substances hydrolyse at similar rate to a similar Si-containing hydrolysis product, chloromethylsilanetriol resp. (chloromethyl)methylsilanediol. The non-silanol hydrolysis product is the same, ethanol. Due to that, their ecotoxicological properties are expected to be similar. No other data for relevant substances were available.

 

5. AE A.2.1 Compounds the test organism is exposed to

 

Both substances hydrolyse very rapidly in contact with water under conditions relevant for this endpoint. Therefore, the test organism is mainly exposed to their hydrolysis products, (chloromethyl)methylsilanediol, resp. chloromethylsilanetriol. The source and target substances have been profiled using the OECD QSAR Toolbox v. 4.1. The two substances and their silanol hydrolysis products show similar profiles for all toxicological endpoints. No alert for biodegradation was detected by OECD QSAR Toolbox v.4.1. No classification for sensitization has been assigned to the substance.

 

 

6. AE A.2.2 and A.2.3 Common underlying mechanism, qualitative and quantitative aspects

 

No toxicity data are available for the target substance (Chloromethyl)diethoxymethylsilane (CAS 2212-10-4), therefore data are read-across from the structurally analogous substance (Chloromethyl)trimethoxysilane (CAS RN 5926-26-1). Both substances hydrolyse at similar rate to a similar Si-containing hydrolysis product, chloromethylsilanetriol resp. (chloromethyl)methylsilanediol. The non-silanol hydrolysis products, ethanol and methanol respectively, are not expected to be relevant for this endpoint. Moreover, they have similar physico-chemical properties. Thus, both substances are expected to have similar toxicity profiles.

 

7. AE 2.4 Exposure to other compounds than to those linked to the prediction

 

The target substance, (Chloromethyl)diethoxymethylsilane (CAS 2212-10-4), does not have any impurities of toxicological concern. The source substance, (Chloromethyl)trimethoxy-silane (CAS RN 5926-26-1), has a named impurity of approx. 1.5% which is (Chlormethyl)dimethoxymethylsilan (CAS RN 2212-11-5). It is not expected that this impurity that will release (Chloromethyl)methylsilandiol as the target substance upon hydrolysis will contribute to biodegradation of the source substance. The test substance in the study with the source substance, (Chloromethyl)trimethoxysilane (CAS RN 5926-26-1), had a purity of 97.2%.

 

8. AE 2.5 Occurrence of Other Effects than Covered by the Hypothesis and Justification

 

Not relevant.

 

[1] European Chemicals Agency (ECHA) (2015) Read-across Assessment Framework. Appendix B, Scenario 2.