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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Inherently biodegradable

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable

Additional information

Since squalene is omnipresent in the environment, biodegradation would not seem to be of particular importance, and in fact mechanisms of degradation have been recorded, for instance; “Analyses of the degradation products of natural and synthetic rubbers isolated from various bacterial cultures indicated without exception that there was oxidative cleavage of the double bond in the polymer backbone. A similar degradation mechanism was postulated for the cleavage of squalene, which is a triterpene intermediate and precursor of steroids and triterpenoids.”, as reported in Rose & Steinbüchel, 2005.

 

The biodegradation of squalene has anyway been tested according to the OECD Guideline 301F for the testing of ready biodegradability. The methodology for testing of ready biodegradability according to 301F (Manometric respirometry) is considered to be suitable for poorly-soluble and, possibly, adsorbing substances such as squalene.

Two tests were performed, with initial testing at a concentration of 100 mg squalene/L which resulted in squalene not being readily biodegradable. The maximum biodegradation of squalene was 42.6% degradation at the end of the 28-day testing, which is below the 60% threshold that needs to be obtained within a 10-day window in the test, in order for squalene to be considered readily biodegradable. Maximum degradation at the end of the 10-day window was actually 24.7%.

 

However, due to the fact that squalene was tested at a relatively high concentration (i.e. 100 mg/L) and that a Teflon stir bar was used, some concerns arose regarding the bioavailability of squalene within the test. Therefore it was decided to perform the same test again, however, this time at a lower concentration (taking care to fulfil the validity criteria for ThOD) and replacing the Teflon stir bar with a non-sorbing glass stir bar to avoid possible sorption.

 

The second ready biodegradability test resulted in a maximum biodegradation at the end of the test of 80.1%, while biodegradation at the end of the 10-day window was actually 54.4%, close to the pass level of 60%. Although squalene strictly can thus not be qualified as being readily biodegradable, since the OECD Guidelines for this type of testing are very strict, what the test confirms, is the inherent possibility for biodegradation of squalene.

 

Possible environmental exposure of squalene according to the here supported use will be after showering through releases to the sewage system and subsequent follow-up by the sewage treatment plant (STP). Pass, or residence times within a sewage system are normally 1-2 days according to traditional exposure scenarios for, for instance, biocidal uses. As mentioned before, in the STP, aeration and STP-microorganisms can further degrade squalene and abiotic degradation of squalene is increased under aerobic conditions (Mouzdahir, et al., 2001).

 

Because of this, squalene should be considered as inherently biodegradable and therefore accumulation/releases exceeding the natural releases by humans, animals and plants, are not foreseen.

 

Rose, K., & Steinbüchel, A. (2005). Biodegradation of natural rubber and related compounds: Recent insights into a hardly understood catabolic capability of microorganisms.Applied and environmental microbiology, 71(6), 2803–2812