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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Biodegradation: not ready biodegradable and not inherently biodegradable.

Key value for chemical safety assessment

Additional information

TNS is not Ready Biodegradable based on the results of a ready biodegradation test performed on TNS. Approximately 12% biodegradation was observed in the test for ready biodegradation. Furthermore, TNS is not Inherently Biodegradable based on the results of an inherent biodegradation test performed on TNS. Less than 20% biodegradation was observed in the test as measured by change in Biochemical Oxygen Demand (BOD) and in TNS concentration.

Additional biodegradation studies were conducted that indicated primary biodegradation of one or more constituents of TNS resulting in the formation of metabolites. Specifically, biodegradation of polyfluoroalkyl phosphates (PAPs) via enzyme-catalyzed hydrolysis and oxidation resulted in formation of 6:2 fluorotelomer alcohol (6:2 FTOH) and terminal acids such as perfluorohexanoic acid (PFHxA). Supporting biodegradation studies with 6:2 FTOH indicated rapid primary biodegradation to metabolites including: 1) 5:2 secondary fluorotelomer alcohol (5:2 sFTOH), 5:3 Acid [F(CF2)5CH2CH2COOH], and perfluorohexanoic acid (PFHxA) in an activated sludge test system; 2) perfluoropentanoic acid (PFPeA), 5:3 acid [F(CF2)5CH2CH2COOH], perfluorohexanoic acid (PFHxA), perfluorobutanoic acid (PFBA), and 4:3 acid [F(CF2)4CH2CH2COOH] in a natural water / sediment system; and 3) perfluorohexanoic acid (PFHxA), perfluoropentanoic acid (PFPeA), perfluorobutanoic acid (PFBA), 5:3 acid [F(CF2)5CH2CH2COOH], 4:3 acid [F(CF2)4CH2CH2COOH], and 5:2 secondary alcohol (5:2 sFTOH) in a sandy loam soil.

Similar results were obtained in a supporting 56 day inherent biodegradability study on TNS in which the formation of a number of breakdown products was monitored. Contrary to the normal practice of measuring DOC changes over time to assess inherent biodegradability, this study involved the analytical quantification of eight substances that are potentially formed from biodegradation of the test substance. Progressive increases in the terminal substances PFPeA, PFHxA, perfluoroheptanoic acid (PFHpA) and 5:3 Acid were observed in all replicates over time. In a supporting anaerobic biodegradability study in digested sludge, the formation of terminal substances was observed as well with yields of 5:3 acid and PFHxA. In these studies, the degradation of the constituents was not monitored separately but rather monitored as a whole through the determination of fluorotelomer alcohol equivalents. It was not the intent or purpose of these biodegradation studies to directly assess mineralization of the test substance or its constituents. Therefore, it’s not clear from these studies whether all constituents biodegraded or only one or a part of them. Whereas the biodegradation of a disubstituted PAP (diPAP) has been observed in one of the studies mentioned above, the breakdown was much slower than that of monosubstituted PAPs (monoPAPs). However, it cannot be concluded whether the tris- or pyro-phosphates degraded in these studies.

In summary, one or more constituents of TNS undergo primary degradation but not ultimate biodegradation in the environment. So, conservatively, the rate constant for elimination in sewage treatment plant is 0 k (h-1).