Fatty acids, C16-18, zinc salts

Biodegradation in water: screening tests

Description of key information

The assessment entity “C16-18” is a mixture of saturated C16 -C18 fatty acids. Therefore, the endpoint is addressed with publicly available data on fatty acids with the same or similar structure, including conservatively fatty acids with a shorter chain if relevant and appropriate in accordance with previously applied read-across approaches (U.S. EPA Fact Sheet, 2008).

A registration dossier shall contain information on the environmental hazard assessment (Regulation 1907/2006, Article 10). For the environmental hazard assessment of C16-18 fatty acids, the standard testing regime set out in Annexes VII to IX is adapted in accordance with Section 1.2 and 1.3 of Annex XI so that “testing does not appear to be scientifically necessary” as follows:

(I) The ecotoxic potential of C16-18 fatty acids is assumed to be negligible. Fatty acids are generally not considered to represent a risk to the environment, which is reflected in their exemption from the obligation to register (Annex V, Section 9 and Regulation (EC) No 987/2008). Non-branched aliphatic fatty acids (C5-24) “are expected to be of low toxicity by their nature”, i.e. they “are not considered PBT/vPvB, […] degrade rapidly and have a low potential for bioaccumulation” (ECHA, 2020: Integrated Regulatory Strategy Annual Report May 2020)

(II) Degradation of fatty acids:

Fatty acids are rapidly degraded in soil and water via the β-oxidation pathway and are thus not expected to accumulate in the environment (EU RAR zinc distearate, 2008). Most biodegradation studies or model estimations on fatty acids up to an alkyl chain length of C = 22 fulfil the criteria for ready biodegradability (OECD SIDS, 2014). Fatty acids, C16-18, zinc salts are readily biodegradable (93% in 28 days, TÜV Bayern, 1992; 71% in 28d, Rudolf, 1992) and are not expected to persist in the environment.

The degradation of fatty acids proceeds by β-oxidation under stepwise shortening of the alkyl chain and the formation of acetyl coenzyme A fragments, used in living cells for energy production (referenced in Madsen et al. 2001). According to HERA (2003) and references therein, the fate of fatty acid salts in aqueous systems proceeds as follows: (1) The biodegradation rate mainly depends on e.g. solubility, adsorption and physico-chemical properties. (2) The fate of fatty acid salts is strongly influenced by the poor water solubility of the respective salts. (3) The rate of metabolism decreases as chain length increases. (4) Unsaturation increases the rate of metabolism although the degree of unsaturation and positioning of double bonds is not highly significant and (5). The available data indicate that all (fatty acid salt) chain lengths up to and including C18 can be metabolised under aerobic conditions and can be considered to be readily biodegradable (referenced in HERA, 2003). “Biodegradation studies or model estimations for single and multi-component aliphatic acids generally confirm that the extent of biodegradation observed in 28 days meets the ready biodegradability criterion (> 60 %). In some cases, insufficient sampling points were included in the tests to determine whether or not the 10-day window was met and thus are insufficient to demonstrate ready biodegradability. When the 10-day window was not met or less than 60 % biodegradation was observed in 28 days, it is likely that the aliphatic acids tested were not fully in solution (OECD SIDS, 2014). Degradation results obtained for fatty acids of various chain lengths are summarized in the Table “Percentages of degradation of fatty acids of various chain lengths obtained via aerobic sludge digestion tests” below. Regarding the group of C16-18 fatty acids, the results show that this group is readily biodegradable with degradation percentages ranging from 62-100 % using different methods.

(III) Degradation of fatty acid salts:

The ready biodegradability of fatty acids as such can nevertheless be inhibited by the formation of poorly soluble salts (e.g. calcium, magnesium) that are not readily biodegradable (EU RAR zinc distearate, 2008). Former studies using the Warburg method (O2 uptake) indicate that oxidation of fatty acid salts exceeded 50%ThOD in many cases within 6-24 hours and the following conclusions were drawn for the degradation of fatty acid salts: “(1) Fatty acids sodium salts of C10-C18 can be metabolised in aerobic systems. The equivalent length calcium salts can also be metabolised, particularly if the insoluble particles are dispersed. (2) Metabolism was influenced by solubility. (3) The rate of metabolism decreased as chain length increased and (4) Unsaturation increases the rate of metabolism although the degree of unsaturation and positioning of double bonds was not highly significant” (referenced in HERA, 2003). Biodegradation ranged from 48 to 89 % degradation of all C16 and C18 fatty acid salts and the percentage degradation of e.g. Ca-stearate tested via different methods (Sturm and MITI, various carriers) ranged from 55 to 99 % (HERA, 2003).

(IV) Anaerobic degradation of fatty acids:

The aliphatic acids also undergo biodegradation under anaerobic conditions (HERA, 2003), which is supported by the fact that the above-described β-oxidation may also proceed in the absence of oxygen (Madsen et al. 2001). Results obtained in tests investigating the anaerobic biodegradability of fatty acids of various lengths are summarized in the Table “Percentages of degradation of fatty acids of various chain lengths obtained via anaerobic sludge digestion tests”.

Regarding the identification of degradation products, fatty acids are readily broken down to carbon dioxide and water in soil and water. Hence, based on the chemical structure, transformation products of environmental concern are not expected.

 

In summary, fatty acids are readily biodegradable and not persistent in the environment. Transformation products of environmental concern are also not expected. Thus, performing further biodegradation tests of C16-18 fatty acids is from a scientific point of view not expected to provide more insight into the environmental fate and is not considered scientifically necessary for the environmental hazard assessment.

 

References:

European Chemicals Agency (ECHA, 2020). Grouping speeds up regulatory action. Integrated Regulatory Strategy Annual Report May 2020.

EU Risk Assessment Report, RAR - Zinc distearate (2008), CAS No. 557-05-1 & 91051-01-3. PART 1 Environment, p. 63

Health Canada’s PMRA, Pest Management Regulatory Agency (2017). Ammonium Salt of Fatty Acid Proposed Registration Decision PRD2017-04, p. 36

HERA (2003). Human & Environmental Risk Assessment on ingredients of European household cleaning products. Fatty Acid Salts (Soap) Environmental Risk Assessment

Madsen et al. (2001). Environmental Project No. 615 Miljøprojekt, Centre for Integrated Environment and Toxicology; CETOX. Environmental and Health Assessment of Substances in Household Detergents and Cosmetic Detergent Products, p. 240.

OECD SIDS initial assessment profile- aliphatic acids (2014), CoCAM 6 September 30-October 3, Italy/ICCA, p. 41

Rudolf, A. (1992): Study report following OECD 301B: Biodegradability of Zink-12-Hydroxystearat

TÜV Bayern Sachsen E.V. (1992): Untersuchungsbericht Zinkstearat

U.S. Environmental Protection Agency, U.S. EPA (2008). Ammonium nonanoate (031802) Fact Sheet, OPP Chemical Code: 031802, p. 2

 

Table: Percentages of degradation of fatty acids of various chain lengths obtained via aerobic sludge digestion tests

Fatty acid	Test	% Degradation	Reference
C8-18	BOD test, 28 d	100	Steber and Berger 1995 referenced in Madsen et al. 2001
C10	Closed bottle test, 30 d	71 - 100	Henkel (unpublished) referenced in OECD SIDS 2014
C10	BOD test, after 1 d	23.4	[33] referenced in OECD SIDS 2014
C10	Not stated, after 5 d	60.9	[32] referenced in OECD SIDS 2014
C12	BOD test, 30 d	87	Henkel referenced in OECD SIDS 2014
C12-14	CO2 Evolution, 28 d	84	Unilever referenced in OECD SIDS 2014
C12-14	Closed bottle test, 28 d	90 – 94	Steber and Berger 1995 referenced in Madsen et al. 2001
C12-14	Modified OECD screening test, 28 d	91	Steber and Berger 1995 referenced in Madsen et al. 2001
C14	Closed bottle test, 28 d	85	[20] referenced in OECD SIDS 2014
C14	EMPA test, after 15 d	99	[21] referenced in OECD SIDS 2014
C16	BOD test, 28 d	100	Steber and Berger 1995 referenced in Madsen et al. 2001
C16-18	Closed bottle test, 28 d	62	Henkel unpublished referenced in OECD SIDS 2014
C16-18	Closed bottle test, 28 d	77	Steber and Berger 1995 referenced in Madsen et al. 2001
C16-18	Closed bottle test, 28 d	91	Painter 1992 referenced in Madsen et al. 2001
C16-18	Modified OECD screening test, 28 d	85 – 88	Steber and Berger 1995, Painter 1992 referenced in Madsen et al. 2001
C18	CO2 Evolution, 28 d	82	Unilever referenced in OECD SIDS 2014
C18	Closed bottle test, 28 d	89	Henkel unpublished referenced in OECD SIDS 2014
C18	Closed bottle test, 28 d	62	Henkel unpublished referenced in OECD SIDS 2014
C18	BOD test, 28 d	79	Steber and Berger 1995 referenced in Madsen et al. 2001
C20-22	Closed bottle test, 28 d	89	Henkel (unpublished) referenced in OECD SIDS 2014
C22	BOD test, 28 d	69	Steber and Berger 1995 referenced in Madsen et al. 2001

 

Table: Percentages of degradation of fatty acids of various chain lengths obtained via anaerobic sludge digestion tests

Fatty acid	Test	% Degradation	Reference
C12-14	CH4 evolution, 50 d	77-84	Salanitro and Diaz, 1995 referenced in Madsen et al. 2001
C12-14	14CH4 and 14CO2 evolution, 56 d	85	Madsen et al. 2001
C14	14CH4 and 14CO2 evolution, 15 d	80	Nuck and Federle, 1996 referenced in Madsen et al. 2001
C14	14CH4 and 14CO2 evolution, 69 d	77.3 ± 17.3	[20] referenced in OECD SIDS 2014
C14	14CH4 and 14CO2 evolution, 28 d	96.5	[21] referenced in OECD SIDS 2014
C16	14CH4 and 14CO2 evolution, 28 d	92-97	Steber and Wierich 1987 referenced in Madsen et al. 2001
C16	ECETOC test, 28 d	79-94	Birch et al. 1989 referenced in Madsen et al. 2001
C18	14CH4 and 14CO2 evolution, 28 d	> 90	Steber and Berger 1995 referenced in Madsen et al. 2001
C18	ECETOC test, 56 d	88	Birch et al. 1989 referenced in Madsen et al. 2001

 

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable