Fatty acids, C16-22

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, C16-22. Therefore, read across from the key studies of the most relevant aliphatic fatty acids was performed. According to the Substance Identity Profile stearic acid (C18), eicosanoic acid (C20) and docosanoic acid (C22) are the most representative structures for fatty acids, C16-22.
Therefore the data for stearic acid (C18, CAS 143-07-7), oleic acid as surrogate (C18’, CAS 112-80-1) as well as QSAR predictions on ready biodegradability of eicosanoic acid (C20, CAS 506-30-9) and docosanoic acid (C22, CAS 112-85-6) were considered to deduce the biodegradability of fatty acids, C16-22.

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 reached within 10 days once exceeded 10% degradation) was barely missed. However, the sampling interval was not as small 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 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.

The analogue substance oleic acid, unsaturated (9-Octadecenoic acid, (Z)-) was also assessed for ready biodegradability.

Coenen (1991) conducted a GLP study according to OECD 301B. After 28 days 93% and 75% of oleic acid were biodegraded at concentrations of 10 mg/L and 20 mg/L, respectively und thus pass the 60% degradation level. Furthermore, at the lower test 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.

Reliable results of the QSAR model BIOWIN v.4.10 (EPI Suite, 2010) predicts ready biodegradability for both substances eicosanoic acid and docosanoic acid. 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.

These results are consistent with experimental results published in the HERA Report for a mixture of C20-C22 aliphatic acids tested according to OECD 301 D. An overall degradation rate of 89% as well as ready biodegradation was stated for the mixture of C20-C22 aliphatic acids in the HERA report (a reliability score of 1 is given for the results).

Overall, the substance fatty acids, C16-22 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.