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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

64 % degradation after 28 days and 66 % degradation after 35 days (OECD 301F)

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

In a ready biodegradation study 64% degradation was achieved after 28 days. The biodegradation rate reached after the 10 -day window (day 6 to 16) is 58%. However, Patchouli oil is a complex mixture composed of structurally related constituents (approximately 65% sesquiterpenes and 35% sesquiterpene alcohols) that are expected to have similar degradation potential. The degradation curve will therefore be the sum of many growth curves. The 10-day time window criterion was developed on the assumption that a test substance is degraded according to a “standard” growth curve in ready biodegradability tests. The OECD Guidelines for Testing of Chemicals, Section 3. Part 1: Principles and strategies related to the testing of degradation of organic chemicals (adopted 23 March 2006), states that if a substance consists of “constituents with different chain-lengths, degree and/or site of branching or stereo-isomers” and “it is anticipated that a sequential biodegradation of the individual structures is taking place, then the 10-day window should not be applied to interpret the results of the test.” The 10-day time-window should therefore be ignored as a pass / fail criterion for Patchouil Oil. Thus, Patchouli oil should be regarded as readily biodegradable according to this test based only on the biodegradation percentage of 64% at day 28.

ECHA guidance on PBT assessment (Chapter R.11) notes that it is difficult to assess persistence of complex substances containing many constituents using biodegradation methods that measure summary parameters (such as evolution of carbon dioxide or theoretical oxygen demand). In the case of multi-consituent and UVCB substances, the following general strategy is suggested for P assessment (R.11.1.4.2). If the substance consists of homologous structures and is shown to meet the stringent ultimate ready biodegradation test criterion (>60% in 28 days), it can be concluded that the underlying constituents comprising the complex substances are not expected to be persistent. Patchouli oil is a complex substance composed of structurally related constituents (approximately 65% sesquiterpenes and 35% sesquiterpene alcohols) that are expected to have similar degradation potential due to common or very similar carbon skeletons. It achieved > 60% biodegradation in a ready biodegradation test and as such it is reasonable to conclude that the underlying constituents of Patchouli oil will not persist.