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Biodegradation in water: screening tests

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Description of key information

The biodegration potential of Diquat C16 -18 ( N,N,N',N',N''-Pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride; CAS no 1211950-04-7) has been assessed in an inherent test (SCAS test) in combination with an optimized CBT-test.
The results reveal that Diquat C16 -18 is quickly degraded into an intermediate for which only limited or no biodegradation is expected.
Based on mass balance calculations a guestimated structure of the intermediate is given. Attempts for identifying the possible intermediate(s) using LC-MS/MS were not successful. The most likely structure of the intermediate is a Diquat, acetate or butyrate (when 50% biodegradation is achieved in the CBT) but the alkyl chain could also be two or four C2H4's longer (when e.g. 30% biodegradation is achieved in the CBT)

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, not fulfilling specific criteria

Additional information

N,N,N’,N’,N’’-Pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride is 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 N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride was still noted after 56 days (van Ginkel et al 2010). Reduction of the toxicity of fatty amine derivatives in ready biodegradability tests has been achieved through the addition humic acid and silica gel (van Ginkel et al, 2008). The true biodegradability of N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride can therefore only be assessed in ready biodegradability tests with the help of either silica gel or humic acid. 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. N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride tested in the presence of humic acid was biodegraded 50% (non GLP) and 28% (GLP) (AkzoNobel report, 2010a) at day 28 in the Closed Bottle test. The biodegradation increased only slightly in the prolonged Closed Bottle tests (Akzo Nobel report, 2010a; van Ginkel et al, 2010). These results strongly indicate that only part of the test substance is readily degraded.

Biocidal effects of N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride explain negative results obtained in ready biodegradability tests. Closed Bottle tests carried out with silica gel did give biodegradation results showing partial degradation of N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride within 28 days. Partial degradation i.e. oxidation of the alkyl chain leads to the formation of water soluble, non-toxic, nitrogen containing substances.

Partial degradation of by N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride was also found in a semi continuously fed activated sludge (SCAS) test. In this test removal percentages of organic carbon of approximately 70% were achieved. Organic nitrogen of the test substance was recovered completely during this period. Total recovery of the organic nitrogen and the 30 to 40% of the organic carbon demonstrates that the hydrophilic moiety of the surfactant is persistent.

Detailed studies with fatty amine derivatives imply that partial degradation has to be the result of the activity of alkyl chain degrading micro-organisms releasing the hydrophilic part of the molecule (van Ginkel 1996).The alkyl chain is degraded through the ß-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 ß-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. Complete degradation of N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride was not anticipated because of the two quaternary nitrogen atoms in the hydrophilic moiety. Biodegradation percentages of <60 were found after 14 days in the MITI test and in the Closed Bottle Test (OECD 301 C and D) for other structurally related cationics containing two nitrogen atoms (Tatsumi et al 2000, 2001; Tehrani-Bagha et al, 2007). N,N’–bis(2-carboxymethyl)-N,N,N’,N’-tetramethyl-1,2-ethaneammonium dibromide, and N,N’-bis(2-hydroxyethyl)-N,N,N’,N’-tetramethyl-1,3-propanediammonium dibromide, degraded less than 20 % also showing that hydrophilic moieties with two quaternized nitrogen are persistent in ready biodegradability tests. Based on the biodegradation pathways and the inability of microorganism to utilize the hydrophilic moiety of N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride either an attack on the far-end of the alkyl chain or a C-Nalkyl cleavage is expected. These initial attacks will result in N-acetate-N,N,N’,N’,N’-pentamethylmethyl-1,3-propaneammonium, N-butyrate-,N,N,N’,N’,N’-pentamethyl-1,3 -propaneammonium, N,N,N’,N’,N’-pentamethyl-1,3-propaneammonium and/or N,N,N’,N’,N’-pentamethyl-1,3-propaneammonium (Figure)

Identification of the biodegradation products of N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride with LC-MS/MS  did unfortunately fail (Hermansson, 2010). Nonetheless, based on the column used for the liquid chromatography, it has been shown that the biodegradation products formed in the SCAS units are more hydrophilic than the parent compound.

Finally, the effluent of the SCAS unit containing the water-soluble biodegradation products of N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride are non-toxic to both algae and Daphnids whereas effluent from the control unit spiked with the parent surfactant was toxic( Van Ginkel and Geerts, 2010; Akzo Nobel 2010a, 2010b). This non-toxicity is in line with the possible biodegradation products (Figure). These biodegradation products are not surface-active and consequently not (less) toxic.

In conclusion N,N,N’,N’,N’’-pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride is readily converted into non toxic substances which are persistent in (ready) biodegradability tests. 

The most likely structure of the intermediate is a Diquat, acetate or butyrate based on general biodegradation pathways of surfactants.

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