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Biodegradation in water: screening tests

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Description of key information

Ready result with OECD closed bottle test + additional literature info to support this result

Key value for chemical safety assessment

Biodegradation in water:
readily 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 test, inhibition by coco dipropylene triaminewas still noted at day 14. The biodegradation of coco dipropylene triaminestarted at day 21 (van Ginkel et al, 2009).Inhibitory effects of fatty amine derivatives increase with increasing chain lengths unless the substance becomes unavailable (Dean Raymond and Alexander 1977; van Ginkel, 1995). In view of the difficulty to demonstrate the true biodegradability ofcoco dipropylene triamine in the Closed Bottle test it is very unlikely thatjustifiable results can be obtained in ready biodegradability tests with longer-chainalkyl dipropylene triamines.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-chainalkyl dipropylene triamines can therefore only be assessed through read across using extrapolation.

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).dipropylene triamine is a chemical consisting ofa hydrophilic group is 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 dipropylene triamines have an alkyl groups linked directly to a primary nitrogen atom of dipropylene triamines through a covalent bond. The alkyl group may be derived from dodecyl, coco, tallow, or oleyl. 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 alkyl dipropylene triaminesisan attack on the hydrophobic part of the molecule followed by the degradation of dipropylene triamine. 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, dipropylene triamine is metabolised through ß-alanine, which also feeds into the central metabolism (Large, 1992). Dipropylene triamine(N-(3-aminopropyl)-1,3-propanediamine) is readily biodegradable (van Ginkel et al, 2010; Rothkopf and Bartha, 1984). Alkyl dipropylene triamines based on the biodegradation pathways of all moieties, are therefore completely (ultimately) biodegradable. The ultimate biodegradation of coco dipropylene triaminehas demonstrated in semi-continuously fed activated sludge (SCAS) units.

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 withcoco 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 that all N-alkyl dipropylene triaminesare readily biodegradable.