Fatty acids, C8-18

Biodegradation in water: screening tests

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Additional information

Since no study on ready biodegradability is available for fatty acids, C8-18, the biodegradability is assessed in a weight of evidence approach based on the results of the most relevant aliphatic fatty acid structures contained in the substance.
According to the Substance Identity Profile lauric acid (C12, CAS no. 143-07-7), myristic acid (C14, CAS no. 544-63-8), palmitic acid (C16, CAS no. 57-10-3) and stearic acid (C18, CAS no. 57-11-4) are the most representative structures for fatty acids, C8-18.
Experimental data as well as QSAR predictions were considered to deduce the biodegradability of fatty acids, C12-18 and C18-unsatd.

Two biodegradability tests are available for lauric acid indicating ready biodegradability. Richterich and Mühlenberg (2001) conducted a closed bottle test according to OECD 301D. Under the test conditions, 86% oxygen consumption at a substance concentration of 2 mg/L and 62% at 5 mg/L were observed. The 10-day window was identified. 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). Thus, lauric acid is readily biodegradable.

Results of 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 OECD301 test methods plus OECD310.

Palmitic acid 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.

Stearic acid, saturated was tested by Bogers (1989) for its 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 has to be 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.
Reliable results of the QSAR model BIOWIN V.4.10 (EPI Suite, 2012) predicts ready biodegradability for stearic acid.

Overall, fatty acids, C8-18 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.