L-menthan-3-one

Biodegradation in water: screening tests

Description of key information

Substance is readily biodegradable. 

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Additional information

Four studies on ready biodegradability are available. Caspers & Müller (2000) and Eisner (2012) used a test item which is very closely related to the submission item, i.e. ca. 85% L-menthan-3 -one and ca. 15% D-isomenthone. In the study of King (1996) a test item was used containing 41% L-menthan-3-one and 8.5% D-isomenthone (i.e. the natural menthones), plus 41% D-menthone and 8.5% L-isomenthone (i.e. the synthetic menthones). Müller (1999) used a test item containing 17.6% L-menthan-3-one and 32.25% D-isomenthone (i.e. the natural menthones), plus 17.6% D-menthone and 32.25% L-isomenthone (i.e. the synthetic menthones). All in all it is to be considered that the variety of test items used in these studies are describing the different varieties of the registration item as laid out in section 1.2.  

According to ECHA "Guidance on information requirements and chemical safety assessment Chapter R.7b: Endpoint specific guidance" (2012), page 209, for substances where a range of degradation data is available, a Weight of Evidence approach should be employed. Two studies applying the Closed Bottle Test are available. While in the study of Caspers and Müller (2000) the 60% pass level has been failed (46%), in the Closed Bottle Test of Müller (1999) the pass level has been reached (63%). In the manometric respirometry test following OECD 301F of Eisner (2012) the pass level has been missed. It should however be stressed that it is conceivable that during the test runs a volatile substance such as L-menthane-3-one might be expelled to a certain extent from the aqueous phase into the gas head space of the respirometer cell via the violent stirring action of the test water. As the portion of the test item in the gas phase cannot be accessible anymore for biodegradation by the dispersed inoculum, this may offer a possible explanation why the 60% pass level has been missed closely (52%). Taking further into account the fast degradation rate before eventually reaching the plateau level of 52%, these observations at least provide strong supporting indication for ready biodegradability of the test item, even though strictly speaking only inherent biodegradability criteria have been fulfilled in this study. In a CO2 Evolution Test of King (1996) the 60% pass level has also been reached (99.5%). This study however is of lower reliability as the CO2 Evolution Test design is not considered as suitable for volatile substances according to OECD TG 301.

When interpreting these data, again it shall be referred to ECHA Guidance R.7b (2012). On page 179 the guidance is stating that ready biodegradability tests may sometimes fail because of the stringent test conditions, in general, and the differences among the individual tests in terms of their stringency. Consistent positive test results from tests therefore should generally supersede negative test results. Furthermore, according to page194 of this guidance, when conflicting results in ready biodegradability tests are obtained the positive results could be considered valid irrespective of negative results, when the scientific quality of the former is good and the positive test results are well documented, i.e. guideline criteria are fulfilled, including use of non-adapted inoculum (United Nations GHS (Rev.1) 2005).

Summarising there is one study of Müller (1999) directly indicating ready biodegradability as the 60% pass levels has been reached. There is additional supporting evidence for ready biodegradability from the manometric respirometry study of Eisner (2012), where the narrowly missing of the pass level can possibly be explained by the relatively high volatility of the test item. Even though being of lower reliability, the study of King also indicates ready biodegradability of the test item. Taking together the results as retrieved from the studies of Müller, Eisner, and King, it is to be concluded that this collective evidence outweighs the negative study outcome of the study of Caspers and Müller. Based on these reasons the substance is considered as readily biodegradable.