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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Biodegradation in water (screening tests): 65% in 28 days (OECD 301B) read-across from a structurally related substance.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable but failing 10-day window

Additional information

There are no reliable ready biodegradation data available for dimethylbis(octadecyloxy)silane (CAS 29043-70-7), therefore good quality data for the structurally-related substance, trimethyl(octadecyloxy)silane (CAS 18748-98-6), have been read across.

 

The target and source substance have similar physico-chemical properties and hydrolysis rates; both are very poorly soluble, low volatility and highly absorbing. Hydrolysis rates of >15 h are predicted for both substances. The target substance, dimethylbis(octadecyloxy)silane (CAS 29043-70-7), has two octadecyloxy groups and two methyl groups bonded to a central silicon atom. The source substance, trimethyl(octadecyloxy)silane has a central silicon atom substituted by one octadecyloxy group and three methyl groups. Both substances are predicted to hydrolyse at a similar rate (>15 hours), though hydrolysis rates in the environment are expected to be slower and limited by the high absorption potential and low solubility of the substances. Dimethylbis(octadecyloxy)silane and trimethyl(octadecyloxy)silane have structurally similar silanol hydrolysis products, dimethylsilanediol and trimethylsilanol respectively, whilst the non-silanol hydrolysis product is octadecanol for both substances. Octadecanol is considered to be inherently biodegradable (OECD 1993). Any biodegradation observed for the parent can be attributed principally to the C18 hydrocarbon chain. Since the source substance contains one octadecyloxy group, whilst the target substance contains two octadecyloxy groups, the read-across can be considered conservative.

 

It is therefore considered valid to read-across the results for trimethyl(octadecyloxy)silane (CAS 18748-98-6) to fill the data gap for the registered substance.

In the study with trimethyl(octadecyloxy)silane (CAS 18748-98-6) 65% biodegradation was observed in 28 days. The study was conducted according to OECD Guideline 301B and is considered reliable. This is selected as the key study.

OECD (1993): Screening Information Data Set (SIDS) of 1-octadecanol (CAS No 112-92-5). OECD High Production Volume Chemicals Programme (1993).