DL-menthol

Biodegradation in water: screening tests

Description of key information

A closed bottle test comparable to OECD TG 301 D under aerobic conditions was performed and after 28 days 92 % of D-Menthol (CAS no. 15356-60-2) was degraded in a test with an initial concentration of 0.84 mg/l and 76 % of D-Menthol was degraded in a test with an initial concentration of 2.01 mg/l. Therefore, D-Menthol is considered being readily biodegradable (Haarmann & Reimer, 2002).

Additional information

Hypothesis for the read-across approach: REACH regulation (Corrigendum) EC 1907/2006 on page L 136/120: “Substances whose physicochemical, toxicological and ecotoxicological properties are likely to be similar or follow a regular pattern as a result of structural similarity may be considered as a group, or ‘category’ of substances. Application of the group concept requires that physicochemical properties, human health effects and environmental effects or environmental fate may be predicted from data for reference substance(s) within the group by interpolation to other substances in the group (read-across approach).” Further, ECHA guidance document “Guidance on information requirements and chemical safety assessment – Chapter R.6: QSARs and grouping of chemicals (May, 2008)” provides information on the use of read-across (page 78) as follows: “In the read-across technique, endpoint information for one chemical is used to predict the same endpoint for another chemical, which is considered to be similar in some way (usually on the basis of structural similarity). In principle, read-across can be applied to characterise physico-chemical properties, environmental fate, human health effects and ecotoxicity.” It can be stated that menthols act as a prime example of substances that are suitable for read-across since menthols are (stereo)isomeres and, thus ideally fulfill the recommended criteria of structural similarity. Further, the OECD SIDS report on menthols (2003) assessed data of the isomers L-menthol (CAS no. 2216-51-5), D-menthol (CAS no. 15356-60-2), the racemate and menthol (unspecified isomers; CAS no. 1490-04-6). The report concludes on page 4 that “the menthols can be considered as a category because of their similarity in physico-chemical, toxicological, ecotoxicological and environmental fate properties” which is another requirement to justify the application of the read-across approach. In the OECD SIDS report (2003) biodegradability is given in the range of 76 - 92 % after 28 days for D menthol (CAS no. 15356-60-2) and 79 - 92 % after 28 days for L-menthol (CAS no. 2216-51-5) determined according to OECD 301D tests. These two closed-bottle tests with D-menthol and L-menthol show clearly that both isomers are readily biodegradable. D- and L-menthol are main constituents of menthol (CAS no. 1490-04-6) that is read-across here and, thus, it can be assumed that menthol is also readily biodegradable. The study on D-menthol, showing the lower biodegradation rate of 76 - 92 %, was was used for read-across. The closed bottle test comparable to OECD TG 301 D under aerobic conditions was performed and after 28 days 92 % of D-Menthol was degraded in a test with an initial concentration of 0.84 mg/l and 76 % of D-Menthol was degraded in a test with an initial concentration of 2.01 mg/l. Therefore, D-Menthol is considered being readily biodegradable (Haarmann & Reimer, 2002). On the other hand, a MITI test (not specified, whether L- or D/L-menthol was used; in the literature source both CAS-numbers are referred) resulted in 0% oxygen consumption after 28 days of incubation (MITI, 1992). While both studies give significantly different biological degradation rates when assessing ready biodegradability it has to be considered that e.g. the OECD SIDS report (2003) record several studies on D- and L-menthol that show the ready biodegradability. At least these two constituents would be ready biodegradable even if MITI test material would have been DL-menthol (CAS no. 1490 -04 -6). Thus, it appears to be inconclusive that the MITI give 0 % biodegradation.