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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Ready result with alkyl dipropylene triamine in OECD closed bottle test  + literature data to justify the  read across for the "true" biodegradability of Tallow tripropylenetetramine.

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable

Additional information

Alkyl polypropylenepolyamines are biocidal to micro-organisms and consequently inhibitory in all ready biodegradability tests.Inhibition by test substances in ready biodegradability tests is best detected prior to the onset of the biodegradation of the test substance through suppression of the endogenous respiration (lower oxygen consumption in the presence of a test substance as compared to the control). In a standard Closed Bottle test, inhibition of the endogeneous respiration by oleyl tripropylenetetraminewas still noted after 56 days.Inhibitory effects of fatty amine derivatives are known especially fatty amine derivatives with long alkyl chain lengths (Dean Raymond and Alexander 1977; van Ginkel, 1995).

Due to the high toxicity of oleyl tripropylenetetramine it is very unlikely that justifiable results can be obtained in ready biodegradability tests with oleyl tripropylenetetramine. For very toxic substances, the specified high test substance concentrations are controversial because substances are present in the environment in the sub μg l-1range. The true biodegradability of long-chain oleyl tripropylenetetramine can therefore only be assessed through read across.

Cocodipropylene triamine, (“short-chain alkyl”) is the onlyalkyl dipropylene triamine available which can be used for proper ready biodegradability testing. Cocodipropylene triamine tested in the presence of silica gel was biodegraded 75% at day 28 in the Closed Bottle test. Hence this substance should be classified as readily bio­degradable (AkzoNobel GLP report, 2009). Coco dipropylene triamine is a chemical consisting ofa hydrophilic group linked to a hydrophobic moiety. Biodegradation of both moieties of surfactants requires the concerted action of at least two micro-organisms as a single organism usually lacks the full complement of enzymatic capabilities (van Ginkel, 1996). In ready biodegradability tests,the two moieties of thisfatty amine derivative are therefore degraded sequentially.The degradation curve will therefore be the sum of two growth curves.The biodegradation of the two moieties may be fully in line with the time-day window criterion when judged as separate chemicals. The time window criterion was developed on the assumption that a compound is degraded according to the “standard” growth curve in ready biodegradability tests. The time-window should therefore be ignored as a pass fail criterion for these surfactants.

Chemically alkyl polypropylenepolyamines have an alkyl group linked directly to a primary nitrogen atom of a polypropylenepolyamine through a covalent bond. Biodegradation of surfactants refers to the reduction in complexity of the chemical through metabolic activity of micro-organisms utilizing the substance as carbon and energy source.If a surfactant is to serve as a carbon and energy source for aerobic micro-organisms then it has to be converted into a form that can enter the central metabolism of micro-organisms. Normally this involves converting the surfactant into one, or more, low molecular weight intermediates of the tricarboxylic acid (TCA) cycle or compounds that feed into it. These conversions are described in pathways for cationic surfactants(van Ginkel, 2007).Although micro-organisms capable of degrading surfactants are immensely diverse, the central metabolism (b-oxidation and TCA cycle) is remarkably similar. Kluyver and Donker (1926) first described this similarity known as the unity of biochemistry. This unity is the key to justification of the use of read-across of biodegradability test results.

Detailed studies with fatty amine derivatives imply that complete mineralisation is achieved by consortia of alkyl chain utilizing and hydrophilic moiety degrading micro-organisms (van Ginkel, 1996). Most surfactant-degrading consortia interact commensalistically through production and release of the hydrophilic part of the molecule by alkyl chain degrading bacteria. Another organism utilises the hydrophilic moiety released as growth substrate. Themost plausible biodegradation pathway of oleyl tripropylenetetramineisan attack on the hydrophobic part of the molecule followed by the degradation of tripropylene tetramine. The alkyl chain is degraded through theb-oxidation cycle. In each cycle, the alkyl chain is progressively shortened by two carbons yielding one molecule of acetyl-CoA. The acetyl-CoA generated inb-oxidation enters the TCA cycle, where it is further oxidised to carbon dioxide and water. A single micro-organism can degrade both saturated and unsaturated chains with varying chain lengths. The alkyl chains are therefore completely degraded by micro-organisms with comparable potential.The hydrophilic moiety, tripropylene tetramine is metabolised through ß-alanine, which also feeds into the central metabolism (Large, 1992). Tripropylene tetramine(N,N bis(3-aminopropyl)-1,3-propanediamine) is readily biodegradable (van Ginkel et al, 2010; Rothkopf and Bartha, 1984). Oleyl tripropylenetetramine based on the biodegradation pathways of all moieties, is therefore completely (ultimately) biodegradable. The ultimate biodegradation ofoleyl tripropylenetetraminehas been shown in semi-continuously fed activated sludge (SCAS) units (van Ginkel et al, 2009).

Based on the broad substrate specificity of micro-organisms degrading fatty amine derivatives with respect to the alkyl chain length it is unlikely that the biodegradability of these surfactants differs significantly with varying alkyl chain lengths. Biocidal effects explain negative results obtained in ready biodegradability tests. A Closed Bottle test carried out with coco dipropylene triaminein the presence of silica gel did not lead to a false negative result and thus to a fair interpretation of the biodegradability. The adequate ready biodegradability test result obtained and the scientific evidence that consortia of polypropylenepolyamines- and alkyl-utilizing micro-organisms through a joint biodegradation pathway degrade all N-alkyl dipropylene triamineslead to the conclusion thatoleyl tripropylenetetramine is not persistent and may bereadily biodegradable.

It was impossible to demonstrate that oleyl tripropylenetetramine is readily biodegradable. The impossibility to obtain a ready biodegradability test result is caused by the toxicity of oleyl tripropylenetetramine in the Closed Bottle test. Demonstration of the non persistence of oleyl tripropylenetetramine through enhanced biodegradability testing (prolonged Closed Bottle test) is also considered impossible. However oleyl tripropylenetetramine is considered not persistent based on the demonstration of its ultimate biodegradation and the metabolic link to N-alkyl dipropylene triamines.