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Biodegradation in water: screening tests

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Description of key information

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

The ready biodegradability of fatty acids, C14-22 is assessed in a weight of evidence approach based on the results of the substance on its own and on read across from the key studies of the main constituents as well as on QSAR predictions on ready biodegradability. According to the Substance Identity Profile the representative individual fatty acids are myristic acid (C14), palmitic acid (C16), stearic acid (C18), Eicosanoic acid (C20) and docosanoic acid (C22).

Fatty acids, C14-22 was tested according to ISO 10708 (BODIS test) which is apparently similar to the “closed bottle test”, OECD 301D. Three replicates with fatty acids, C14-22 at a concentration of 100 mg/L ThOD were incubated with non-adapted activated sludge. The oxygen consumption was monitored during the 28 day test period. The results showed that fatty acids, C14-22 was degraded by 40% in average at day 28 (Richterich and Mühlberg, 2001). As described in the study report the insoluble test substance was “weighed on piece of glass […] and directly put into the test flask […]” indicating inhomogeneous distribution of the test substance. Therefore, the moderate biodegradation observed in this test is probably caused by insufficient water solubility/low bioavailability of the test substance. The impact of the low water solubility of some fatty acids on the results of the ready biodegradability tests is also 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.

In the following the available data of the individual constituents is presented.

No experimental results are available for myristic acid (C14). Reliable results obtained by the QSAR model BIOWIN v.4.10 (EPI Suite, 2010) 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 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 (sampling every second day, followed by sampling every fifth day), which might have led to the marginal failure 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, 2010) 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’) was 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, 2010) clearly predicting the substance as readily biodegradable.

Eicosanoic acid (C20) and docosanoic acid (C22) both are predicted to be readily biodegradable according to the QSAR model BIOWIN v.4.10 (EPI Suite, 2010).
These results are consistent with experimental results published in the HERA Report for a mixture of C20-C22 aliphatic acids tested according to OECD 301D. 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).

Considering the evidence of the biodegradability reported above, the substance fatty acids, C14-22 should be 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.