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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Bis (2-hydroxyethyl) tallow alkylamine and 2,2’(octadec-9-enylimino)-bisethanol  as   primary fatty amines ethoxylate are poorly soluble in water and at normal test concentrations toxic to the micro-organisms in the inoculum. Silica gel may be added to reduce the exposure concentration and to facilitate the slow release of the test substance. Under these conditions, the substance is observed to be readily biodegradable in a closed bottle test.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

For primary fatty amines ethoxylates with chain lengths of C12 to C18 (saturated and unsaturated) many ready biodegradability test results in excess of 60% at day 28 are available. Biodegradation in excess of 60% was not achieved in a few tests. The low biodegradability percentages obtained in these tests should be attributed to the biocidal properties of primary fatty amine ethoxylates and should consequently be ignored.

 

A primary fatty amine ethoxylate is a chemical consisting of a 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 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 primary fatty amine ethoxylates have an alkyl chain linked directly to a nitrogen atom of diethanolamine through a covalent bond. The alkyl group may be derived from different sources like 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 biodegradation 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.

Most surfactant-degrading consortia interact commensalistically through production and release of the hydrophilic part of the molecule by alkyl chain degrading bacteria. To understand the metabolic basis of degradation by microorganisms, the pathway of octadecylbis(2-hydroxyethyl)amines has been studied (van Ginkel and Kroon, 1993). Starting from an enrichment culture a bacterium capable of degrading octadecylbis(2-hydroxyethyl)amine was isolated. Alkylbis(2-hydroxyethyl)amineswith tallow, octadecenyl and coco alkyl chains, alkanals, and fatty acids can also serve as a carbon and energy source. Simultaneous adaptation studies showed that acetate, octadecanal and octadecanoate are respired by octadecylbis(2-hydroxyethyl)amine-grown cells indicating that the long alkyl chain is utilized for microbial growth. Dehydrogenase activity present in cell-free extract of octadecylbis(2-hydroxyethyl)amine-grown cells catalysed the liberation of the alkyl chain. The biodegradation of octadecylbis(2-hydroxyethyl)amine is therefore initiated by cleavage of the Calkyl-N bond. Another dehydrogenase, producing fatty acids from alkanals was also detected in cell-free extracts of octadecylbis(2-hydroxyethyl)amine-grown cells. The fatty acids are 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. Diethanolamine is stoichiometrically produced by the pure culture of microorganisms growing with octadecylbis(2-hydroxyethyl)amine as sole source of carbon. Diethanolamine is readily biodegradable (van Ginkel and Kroon, 1993; Davis and Carpenter, 1997). Moreover, Williams and Callely (1982) isolated a bacterium capable of converting diethanolamine into biomass, carbon dioxide, water and ammonia. A consortium of microorganisms degrading the alkyl chain of alkylbis(2-hydroxyethyl)amines and diethanolamine are therefore capable of complete (ultimate) degradation of alkylbis(2-hydroxyethyl)amines.

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.

The adequate ready biodegradability test results obtained and the scientific evidence that consortia of diethanolamine and alkyl-utilizing micro-organisms through a joint biodegradation pathway degrade all primary fatty amine ethoxylates lead to the conclusion that all primary fatty amine ethoxylates (alkylbis(2-hydroxyethyl)amines)are readily biodegradable.