Fatty acids, C8-24

Biodegradation in water: screening tests

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Additional information

No study on ready biodegradability is available for fatty acids, C8-C24. Therefore, read across from the key studies (experimental results) of the individual fatty acids representing the main constituents was done as well as QSAR predictions on ready biodegradability. According to the Substance Identity Profile lauric acid (C12), myristic acid (C14), palmitic acid (C16), stearic acid (C18) and oleic acid (C’18) are the most representative structures for fatty acids, C8-24

Two biodegradability tests are available for lauric acid (C12) that demonstrate ready biodegradability. Richterich and Mühlenberg (2001) conducted a closed bottle test according to OECD 301D. Under the test conditions, 86% oxygen consumption at a test concentration of 2 mg/L and 62% at 5 mg/L were observed. The 10-day window was met. In the short summary of Geisel and Berger (1998), the biodegradability was tested according to EU Method C.4 –E. The test also showed lauric acid to be readily biodegradable (85% within 30d at 2 mg/L of test substance and > 55% at 5 mg/L of test substance, passing 10-day window). The substance, lauric acid, is thus readily biodegradability.

No experimental results are available for myristic acid (C14). Reliable results obtained by the QSAR model BIOWIN V.4.10 (EPI Suite, 2012) predicts ready biodegradability for myristic acid. This method is based on the application of Bayesian analysis to ready biodegradation data for chemicals, derived collectively from all six OECD 301 test methods plus OECD 310.

Palmitic acid (C16) was tested according to the ISO 10708 (BODIS test) which is apparently similar to the “closed bottle test”, OECD 301 D. Three replicates with palmitic acid at concentrations of 100 mg/L COD were incubated with non-adapted activated sludge. The oxygen consumption was monitored during the 28 day test period. The results clearly showed that palmitic acid was degraded by 65% in average at day 28 (Börner,1994).
According to the opinion of the SCIENTIFIC COMMITTEE ON HEALTH AND ENVIRONMENTAL RISKS on “Compatibility of the ISO standard 10708 (biodegradability test method) with the ultimate biodegradability requirements imposed through Annex III of Regulation 648/2004 of Parliament and of the Council”, the BODIS-Test/ISO 10708 shows the same variability and biodegradation results obtained with other OECD screening tests (SCHER, 2005). Since the pass criterion of 60% degradation within 28 days of the ISO 10708 is consistent with those obtained with the OECD ready biodegradability methods, palmitic acid can be regarded as readily biodegradable.

The assessment of the ready biodegradability of stearic acid (C18, saturated) is assessed on experimental results as well as on QSAR calculations.
Stearic acid, saturated was tested by Bogers (1989) for ready biodegradability according to OECD 301B and GLP. At concentrations of 10 and 20 mg/L the determined degradation values were 72% and 71%, respectively at test termination (28 d). The pass criterion for ready biodegradability (60% degradation within 10 days once exceeded 10% degradation) was barely missed. However, the sampling interval was not as narrow as recommended by the OECD guideline which might have led to the barely missing of the 10-day window.
The failure of the 10-day window in biodegradation tests due to 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.
Reliable results of the QSAR model BIOWIN v.4.10 (EPI Suite, 2012) predicts ready biodegradability for stearic acid. Thus it can be concluded that stearic acid is readily biodegradable.

Coenen (1991) conducted a GLP study according to OECD 301B. After 28 days 93% and 75% of oleic acid (C18’) were biodegraded at concentrations of 10 mg/L and 20 mg/L, respectively und thus passed the 60% degradation level. Furthermore, at the lower concentration of 10 mg/L the 10-day window was met. According to the criteria for ready biodegradation oleic acid (9-Octadecenoic acid, (Z)-) is readily biodegradable. Since the reference substance itself failed the pass criterion for validity (60% degradation was not reached within 14 d), the study should had been repeated.
The ready biodegradability of oleic acid is supported by the QSAR calculation BIOWIN v.4.10 (EPI Suite, 2012) clearly predicting the substance as readily biodegradable.

Overall, the substance fatty acids, C8-24 predominantly containing fatty acids of the aliphatic structure C12-C18 is regarded as readily biodegradable. This judgment is consistent with the hazard assessment presented in the OECD SIDS (2009) for the category “Aliphatic Acids Category” where aliphatic fatty acids with a carbon chain length in the range of C8 – C22 were judged to be readily biodegradable.