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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): 7% and 9% in 28 days (EU Method C.4-C, MITI), read-across from an analogous substance.

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 [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS No: 85857-16-5), therefore good quality data for the analogous/structurally related substance, [2-(perfluorohexyl)ethyl]triethoxysilane (CAS No: 51851-37-7), have been read across. In contact with water, [2-(perfluorohexyl)ethyl]trimethoxysilane (CAS No: 85857-16-5) hydrolyses rapidly to form [2-(perfluorohexyl)ethyl]silanetriol and methanol. The hydrolysis products of [2-(perfluorohexyl)ethyl]triethoxysilane are [2-(perfluorohexyl)ethyl]silanetriol and ethanol.

Ethanol and methanol are readily biodegradable (OECD 2004a and OECD 2004b).


[2-(Perfluorohexyl)ethyl]trimethoxysilane (CAS No: 85857-16-5) and [2-(perfluorohexyl)ethyl]triethoxysilane (CAS No: 51851-37-7) 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 III-17 of highly fluorinated chains. [2-(Perfluorohexyl)ethyl]trimethoxysilane (CAS No: 85857-16-5) and [2-(perfluorohexyl)ethyl]triethoxysilane (CAS No: 51851-37-7) are members of this sub-class.

It is therefore considered valid to read-across the results for [2-(perfluorohexyl)ethyl]triethoxysilane (CAS No: 51851-37-7) to fill the data gap for the submission substance. Additional information is given in a supporting report (PFA, 2013f) attached in Section 13.


Table: Ready biodegradation data available for substances within the relevant sub-class III-17 of chemicals



Main code

Readily biodegradable? Yes/no

Result: Biodegradation after 28 day



Corrected % biodegradation once alkoxy/acetoxy biodegradation is accounted for[1]






Annex V C4-C












2,4,6 -Trimethyl-2,4,6-tris(3,3,3-trifluoropropyl)cyclotrisiloxane







[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.

[1a] study conducted in accordance with relevant test method and in compliance with GLP

[1b] study conducted using a test method that is comparable to a relevant OECD/EU test method and in compliance with GLP


Two reliable ready biodegradation studies are available for [2-(perfluorohexyl)ethyl]triethoxysilane. In the first study (Diefenbach, 1997), a ready biodegradation study was conducted in accordance with EU Method C.4 and in compliance with GLP. A biodegradation value of 9% in 28 days was obtained for [2-(perfluorohexyl)ethyl]triethoxysilane. Similarly, in second study (Omura, 2005), a biodegradation value of 7% DOC removal in 28 days was determined for [2-(perfluorohexyl)ethyl]triethoxysilane using a relevant test method and in compliance with GLP. The results are considered to be reliable and are used as weight of evidence.

The studies are considered to be reliability 1 (reliable without restrictions); the read across of the result is considered to be reliability 2 (reliable with restrictions).


OECD (2004a): SIDS Initial Assessment Report for SIAM 19, Berlin, Germany, 18-20 October 2004, Methanol, CAS 67-56-1.

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

PFA, 2013f, Peter Fisk Associates, Biodegradation Main Analogue Group report, PFA.300.005.007