Octadecanoic acid, branched and linear

Biodegradation in water: screening tests

Description of key information

Octadecanoic acid, branched and linear, is determined to be readily biodegradable in accordance to Regulation (EC) No. 1907/2006 Annex XI, 1.5 1.2, based on a Weight of Evidence approach.

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Additional information

Secondary source data based on the Screening-Level Hazard Characterisation of High Production Volume Chemicals (HPV) (U.S. EPA 2007) for the Category “Tall Oil Fatty Acids and Related Substances” is available for Octadecanoic acid, branched and linear. This OECD 301B biodegradation study reported 47% degradation within 28 days using activated sludge from a municipal sewage treatment plant as inoculum. However, the original report is not available and therefore the result cannot be evaluated. As summarized in the category justification, the members of the fatty acids are divided into two sub-categories on the basis of their environmental fate and environmental toxicity. The first sub-category covers three monomeric (by-) products of the dimerization process (all readily biodegradable substances). The second sub-category covers the predominately oligomers (dimeric and trimeric products) of dimerization based on their lack of biodegradability and the environmental fate. In accordance to Regulation (EC) No. 1907/2006 Annex XI, 1.5 1.2. Weight of evidence, there is sufficient data from several independent sources of information of monomeric fatty acids C16-18 saturated and unsaturated leading to the conclusion that this substance is readily biodegradable. As summarized in the category justification, the members of the fatty acids will predominantly rapidly biodegrade. Individual degradation rates reported may vary to some degree depending on their physico chemical properties (e.g. water solubility), bioavailability and test methods employed. Also some limitations regarding the validity of single tests as discussed below do not affect the general appraisal: Consistent positive test results, i.e. readily biodegradable, from both Fatty acids, C16-18 and C18-unsaturated, branched and linear and Isooctadecanoic acid in addition to two further analogue substances octadec-9-enoic acid and stearic acid supersede the low biodegradation percentage as reported in the HPV documentation (EPA, 2009). In conclusion, aliphatic fatty acids comprising C6-C22 carbon chain length are shown to be predominantly rapidly biodegradable providing a sufficient bioavailability is given. Given that Isooctadecanoic acid is also highly branched and has a lower water solubility than the Octandecanoic acid, branched and linear,one can conclude that Octandecanoic acid , branched and linear is also readily biodegradable.

In the two screening tests for biodegradation in water for both Fatty acids, C16-18 and C18-unsatd., branched and linear, and Isooctadecanoic acid, the dimerised fatty acids of sub-category 1 were determined as readily biodegradable (Sewell, 1994, Coenen, 1990). The first biodegradation screening test with Fatty acids, C16 -18 and C18 -unsatd., branched and linear was carried out comparable to OECD guideline 301B using domestic activated sludge and a test substance concentration of 20 mg/L. The test substance attained 67 % degradation after 29 days, however the 10 d window was not met. This test is supported by a further read across substance in the sub-category 1, Isooctadecanoic acid. This acid contains the highest amount of C18 branched Fatty acids and has the highest level of saturation. The biodegradation study reported an observed degradation of 88 % of Isooctandecanoic after 28 days for the nominal test concentration of 10 mg/L and a degradation of 62 % after 28 days for the nominal test concentration of 20 mg/L, where again the 10d window was not met (Coenen, 1990). Since Octadecanoic acid, branched and linear and all other members of sub-category 1 are UVCB substances and thus consist of constituents with different chain-lengths and different degrees of branching, sequential (instead of concurrent) biodegradation can take place but all can be considered as readily biodegradable. Thus, referring to Annex I to the OECD Guideline for Testing of Chemicals ‘Revised introduction to the OECD guidelines for testing of chemicals, section 3’ (OECD, March 2006), the 10-day window should not be considered for this UVCB substance and due to a degradation of >60 % within 28 days the monomeric acids of sub-category 1 can be regarded as readily biodegradable.

Two further studies on analogue substances were included as part of a weight of evidence approach. 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 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 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 can be judged not to preclude the ready biodegradability of the fatty acids. For the second analogue substance, octadec-9-enoic acid a GLP study according to OECD 301B was conducted (Coenen, 1991). After 28 days 93 % and 75 % of octadec-9-enoic acid was biodegraded at concentrations of 10 mg/L and 20 mg/L, respectively und thus passed the 60 % degradation level. Furthermore, the degradation value at the lower test concentration of 10 mg/L met the 10-day window. According to the criteria for ready biodegradation 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.

All members of sub-category 1 are UVCB substances and thus consist of constituents with different chain-lengths and different degrees of branching, sequential (instead of concurrent) biodegradation can take place but all can be considered as readily biodegradable. Thus, referring to Annex I to the OECD Guideline for Testing of Chemicals ‘Revised introduction to the OECD guidelines for testing of chemicals, section 3’ (OECD, March 2006), the 10-day window should not be considered for this UVCB substance and due to a degradation of >60 % within 28 days the monomeric acids of sub-category 1 can be regarded as readily biodegradable.

Thus, in accordance to Regulation (EC) No. 1907/2006 Annex XI, 1.5 1.2. Weight of evidence, there is sufficient data from several independent sources of information of monomeric fatty acids C16-18 saturated and unsaturated leading to the assumption/conclusion that the members of the sub-category 1 are all readily biodegradable.