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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: 15.3% in 28 days (O2 consumption; taking into account potential for nitrification). Not readily biodegradable, read-across from a structurally-related substance. No significant biodegradation of the silanol hydrolysis product is expected.

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 2,2,4,4,6,6-hexamethylcyclotrisilazane (CAS 1009-93-4), therefore good quality data for the structurally-related substance, 1,1,1,3,3,3-hexamethyldisilazane (999-97-3), have been read across. Both substances have similar silanol hydrolysis products, dimethylsilanediol and trimethylsilanol respectively. The other hydrolysis product is ammonia/ammonium ions in both cases.

Ammonia/ammonium ions are inorganic and biodegradation is not relevant.

2,2,4,4,6,6-hexamethylcyclotrisilazane (CAS 1009-93-4) and 1,1,1,3,3,3-hexamethyldisilazane (999-97-3) 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-3-1 of substances containing Si-N bonds. 2,2,4,4,6,6-hexamethylcyclotrisilazane (CAS 1009-93-4) and 1,1,1,3,3,3-hexamethyldisilazane (999-97-3) are members of this main code.

It is therefore considered valid to read-across the results for 1,1,1,3,3,3-hexamethyldisilazane (999-97-3) to fill the data gap for the registered 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 main code (II-3-1) of chemicals



Main code

Readily biodegradable? Yes/no

Result: Biodegradation after 28 days (%)


PFA Klimisch code

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






"Biodegradation study of chemical substances by microorganisms and the like"[2]












1-Propanamine, 3-[(2,2-dimethyl-1-aza-2-silacyclopent-1-yl)dimethylsilyl]-







[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.


[2]Stipulated in "Testing Method for New Chemical Substances etc." (Kanhogyo No.5, Yakuhatsu 615, 49Kikyoku No.392, July 13, 1974 )

In the study with 1,1,1,3,3,3-hexamethyldisilazane (CAS 999-97-3), 15.3% biodegradation was observed in 28 days. 1,1,1,3,3,3-hexamethyldisilazane (CAS 999-97-3) hydrolyses within the timescale of the ready biodegradation study to trimethylsilanol and ammonia/ammonium ions. The biodegradation observed in the study is attributable to the biodegradation of the silanol hydrolysis product. This is consistent with the expectation that significant biodegradation is not expected for the silanol hydrolysis product. The study was conducted according to EC method C4-E and is considered reliable. This is selected as the key study. The study is considered to be reliability 1 (reliable without restrictions); the read across of the result is considered to be reliability 2 (reliable with restrictions).


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