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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Only one valid ready biodegradability test result for alkyl triamines is available because these substances are very toxic to micro-organisms. All aspects important for achieving a ready biodegradability test result with these substances i.e. 1) ultimate (complete) biodegradation (demonstrated in (S)CAS test with a related substance, linear triamine, and through read across with linear triamine C12-18 and intermediates) 2) high rates of degradation (through read across with linear triamine C12-C18 and 3) occurrence of competent micro-organisms in unacclimated ecosystems (through read across with linear triamine C12-C18 are fulfilled.  The branched oleyl triamine should therefore be classified as readily biodegradable.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

Triamines (alkyl dipropylene triamines) are surfactants composed of an alkyl chain linked to either a primary (linear) or the secondary nitrogen atom (branched) of dipropylene triamine(N-(3-aminopropyl)-1,3-propanediamine). Triamines are biocidal to micro-organisms and consequently inhibitory in all ready biodegradability tests. Inhibition of growth of competent microorganisms 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 triamine C12-18 (coco dipropylene triamine) was still noted at day 14. The biodegradation of triamine C12-18 started at day 21 (van Ginkel et al, 2009). Triamine C16-C18 (tallow dipropylenetriamine) exhibited toxic effects for 28 days (Batelle, 1993). 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 of short chain triamines in Closed Bottle tests it is very unlikely that justifiable results can be obtained in ready biodegradability tests with longer alkyl-chain 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 range. The true biodegradability of long-chaintriamines can therefore only be assessed through read across using extrapolation.

Linear triamine,C12-18(“short-chain alkyl”) and branched triamine dodecyl (C12) are the only triamines available which have been used for proper ready biodegradability testing. Linear triamine C12-18, tested in the presence of silica gel was biodegraded 75% at day 28 in the Closed Bottle test. Silica gel was added to lower the test substance concentration in the aqueous phase. This test result allowed classification as readily biodegradable of linear triamine C12-18 (AkzoNobel, 2009). The initial concentration of the test substance in the Closed Bottle test is usually 2 mg/L or higher. To minimize toxic effects of branched triamine C12 an initial test concentration of 1.0 mg/L was used. At this “low” concentration a biodegradation percentage of 79% at day 28 was achieved (Akzo Nobel 2002). Hence, branched triamine C12 was classified as readily biodegradable.

Triamines are chemicals consisting of a hydrophilic group which 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 this fatty 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 alkyltriamines have an alkyl group linked directly to a either a primary (linear) or the secondary nitrogen atom (branched) of dipropylene triamine (N-(3-aminopropyl)-1,3-propanediamine) 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 tri-carboxylic 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 utilizes the hydrophilic moiety released as growth substrate. The most plausible biodegradation pathway of triamines is an attack on the hydrophobic part of the molecule followed by the degradation of dipropylene triamine. The alkyl chain is degraded through the b-oxidation cycle. In each cycle, the alkyl chain is progressively shortened by two carbons yielding one molecule of acetyl-CoA. The acetyl-CoA generated in b-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). Triamines alkyl based on the biodegradation pathways of all moieties, are therefore completely (ultimately) biodegradable.

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 linear triamine C12-18 in the presence of silica gel did not lead to a false negative result and thus to a fair interpretation of the biodegradability. A ready biodegradability test result with branched triamine C12 was obtained with a very low initial test substance concentration. The adequate ready biodegradability test result obtained and the scientific evidence that consortia of hydrophilic moiety and alkyl-utilizing micro-organisms through a joint biodegradation pathway degrade all triamines, alkyl lead to the conclusion that all triamines, alkyl are readily biodegradable.