Fatty acids, C14-18 and C16-18-unsatd.

Biodegradation in water: screening tests

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Additional information

The fatty acids, C14-18 (even numbered) and C16-18-unsatd.(even numbered) was tested by Bogers (1989) for its ready biodegradability according to OECD 301B and GLP. At concentrations of 10 and 20 mg/L the degradation values were 71% and 74%, respectively at test termination (28 d). The pass criterion for ready biodegradability (60% degradation reached within 10 days once exceeded 10% degradation) was barely missed. Since the sampling interval was not as narrow as recommended by the OECD guideline (sampling every second day, followed by sampling every fifth day) this might have led to the barely missing of the 10-day window. Moreover, the insoluble test substance was added to the inoculum via an inert membrane filter which probably had an impact on the bioavailability as well.

Another OECD 301B study performed according to GLP is available for that substance. The study conducted by Bogers (1989) showed that after 28 days C14-18 and C16-18-unsatd. was degraded by 81% and 70% at 10 and 20 mg/L, respectively. Again, the 10 day-window was failed barely (59.8% degradation instead of 60%, which was comparable with the reference substance that barely passed the 10-d window). Beside the low water solubility another reason for this failure might have been that the test substance was added via an membrane filter and thus probably reduced the bioavailability.
The failure of the 10-day window in biodegradation tests due to the low water solubility/bioavailability as well as to the inappropriate sampling intervals were already recognized and discussed within the framework of the SIDS Initial Assessment Report for the Category “Aliphatic Acids” (OECD, 2009) and judged not to preclude the ready biodegradability of the fatty acids.

Further, results of the QSAR model BIOWIN V.4.10 (EPI Suite, 2010) predicts ready biodegradability for C14-18 and C16-18. This method is based on the application of Bayesian analysis to ready biodegradation data for chemicals, derived collectively from all six OECD301 test methods plus OECD310.

Biodegradation in marine seawater was investigated according to the BODIS-Test (comparable to OECD 306) under GLP (Hudson, 2006). Unlike similar ready tests in freshwater this method uses natural seawater fortified with mineral nutrients and no inoculum was added in addition to the microorganisms already present in the seawater. The overall degradation monitored by the BOD concentration was 92% over 28 days and showed an inhibition of -10% to seawater bacteria. The reference substance sodium benzoate degraded by 82% at day 28.

According to the Guidance Document R.7.9.4.1 the OECD TG 306 (similar to the seawater BODIS Test) on Biodegradability in Seawater includes seawater variants of the Closed Bottle Test (OECD 301 D) and of the Modified OECD Screening Test (OECD 301 E). Degradation of chemicals in seawater has generally been found to be slower than that in freshwater tests. When a chemical attains >60% ThOD or >70% DOC removal in a Biodegradability in Seawater test, it can also be expected to fulfill the criteria for ready biodegradability. Thus it can be concluded that based on a degradation in seawater (92% over 28 days) the fatty acid C14-18 and C16-18-unsatd.can be regarded as readily biodegradable.

Overall, the substance C14-18 and C16-18-unsatd. is regarded as readily biodegradable. This judgment is consistent with the hazard assessment presented in the OECD SIDS (2009) for the category “Aliphatic Acids” where fatty acids with a carbon chain length in the range of C8 – C22 in generally and in especially the C14-18 and C16-18-unsatd. were judged to be readily biodegradable.