Dibutoxy(dimethyl)silane

Biodegradation in water: screening tests

Description of key information

Biodegradation in water (screening tests): dimethylsilanediol: 0% in 28 days OECD 301A), read-across from an analogous substance; n-butanol: readily biodegradable (published literature)

Key value for chemical safety assessment

Biodegradation in water:

under test conditions no biodegradation observed

Additional information

There are no reliable ready biodegradation data available for dibutoxy(dimethyl)silane (CAS No. 1591-02-2). Dibutoxy(dimethyl)silane hydrolyses rapidly in water to dimethylsilanediol and n-butanol. The chemical safety assessment will focus on the silanol hydrolysis products, therefore good quality data for the analogous substance, dichloro(methyl)(vinyl)silane (CAS No. 124-70-9), have been read across to fill the data gap for the silanol hydrolysis product, dimethylsilanediol. Dichloro(methyl)(vinyl)silane hydrolyses very rapidly in water to methyl(vinyl)silanediol and hydrogen chloride. Dimethylsilanediol and methyl(vinyl)silanediol are structurally similar. Both contain a silicon atom which is attached to two small alkyl groups, and both have two silanol groups. Dibutoxy(dimethyl)silane has measured hydrolysis half-lives of < 8 minutes at pH 4 and 19.6°C, 1.4 h at pH 7 and 25°C and 0.59 h at pH 9 and 25°C, forming dimethylsilanediol and n-butanol. For both substances, extensive or complete hydrolysis in the course of the 28-day ready biodegradation study is to be expected.

 

The other hydrolysis products are n-butanol (CAS No. 71-36-3) and hydrogen chloride, respectively. n-Butanol is readily biodegradable (Price et al., 1974). Hydrogen chloride is inorganic and biodegradability is therefore not relevant.

Both substances are within an analogue group of substances within which, in general, there is no evidence of any significant biodegradation once hydrolysis and subsequent biodegradation of alkoxy/acetoxy groups has been taken into account.

This analogue group for the ready biodegradability endpoint consists of a number of sub-classes of substances (based on structure/functionality) with different main codes (used to represent the different sub-classes). However, read-across is carried out between substances with the same main code in most cases.

The table below presents ready biodegradation data for substances within the sub-class II-2-1 of Long chain alkoxysilanes substances, and other selected relevant substances which hydrolyse rapidly (or very rapidly) to release small-molecule silanols.

It is considered valid to read-across the results for dichloro(methyl)(vinyl)silane (CAS No. 124-70-9) to fill the data gap for the registered substance. Additional information is given in a supporting report (PFA, 2013f) attached in Section 13 of the IUCLID 5 dossier.

Table: Ready biodegradation data for substances within the relevant sub-class II-2-1 of chemicals and other selected relevant substances

CAS	Name	Readily biodegradable? Yes/no	Result: Biodegradation after 28 day		Guideline	Test type	Klimisch code	Corrected % biodegradation once alkoxy/acetoxy biodegradation is accounted for^[1]
018748-98-6	Trimethyl(octadecyloxy)silane	No		65%	OECD 301B	CO2	2	-21
1066-40-6	Hydroxytrimethylsilane	No		0%	OECD 310 and ISO Guideline N 14593	CO2	1	0
124-70-9	Dichloro(methyl)(vinyl)silane	No		-6.78% (0%)	OECD 301A	DOC	1	0

Table note:

[1]The negative values in this column are where the biodegradation rate observed in the test is less than that expected assuming only alkoxy/acetoxy parts biodegrade. This could be due to low solubility of parent substance or low rates of hydrolysis. These values should in effect be considered as zero.

In the study with dichloro(methyl)(vinyl)silane (CAS No. 124-70-9), 0% biodegradation was observed in 28 days.

Dichloro(methyl)(vinyl)silane (CAS No. 124-70-9) hydrolyses very rapidly within the timescale of the ready biodegradation study to methyl(vinyl)silanol and hydrogen chloride. No significant biodegradation is expected for the silanol hydrolysis product. Hydrogen chloride is inorganic and ready biodegradability is not relevant.

The study was conducted according to OECD 301A and is considered to be reliable. This is selected as the key study.

References: Price KS., Waggy GT., Conway RA. (1974), Brine shrimp bioassay and seawater BOD of petrochemicals. J Water Pollut Contr Fed 46: 63 -77 (cited in OECD SIDS for Butyl alcohol, CAS 71-63-3 (2005)).