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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

 The half life (DT50) of the CAS 297730-93-9 was estimated to be between 91 and 224 years in aerobic sediments and between 189 and 415 years in anaerobic sediments at 12 deg C (OECD 308).

Key value for chemical safety assessment

Half-life in freshwater sediment:
91 yr
at the temperature of:
12 °C

Additional information

The aerobic and anaerobic biodegradation of CAS 297730-93-9 (HFE-7500) was examined in two OECD 308 studies performed with two freshwater sediments. Due to the volatile nature of the test chemical, both tests were conducted in individual VOA vials spiked with HFE 7500 and containing sediment, water and headspace. HFE-7500 was administered to the cultures by applying it onto a dried freshwater sediment and dosed to the cultures by blending the dosed sediment with each test freshwater sediment and associated lake water. The experimental setup included sterile controls, sterile and bioactive blanks, positive and negative controls, toxicity controls, and sterile water controls (no sediment, with and without HFE-7500). For the aerobic study, the incubation was done with intermittent supplementation of oxygen (air) when oxygen levels were measured below 10% in the headspace based on weekly measurements. For the anaerobic test, sediments and cultures were handled in a glove box under nitrogen, and once closed were removed from the glove box and left sealed until sampling. For both studies, the vials were incubated in the dark at 12 deg C.

Possible pathway of degradation of HFE-7500 involves metabolism of the ethoxy group to acetyl or formyl, with subsequent hydrolysis to secondary perfluoroalcohol.  Such chemistries are known to undergo dehydrofluorination, in this case forming a well-characterized branched perfluoroketone which is unstable in water.  The products of the putative metabolic intermediate are expected to be perfluorobutyric acid (PFBA), along with 1H-heptafluoropropane (HFC-227) and/or hexafluoropropene. The biodegradation of HFE-7500 was evaluated by quantitative GC/MS analysis for HFE-7500 in all three phases of the test and control cultures, and full-scan GC/MS to measure volatile products HFC-227 and hexafluoropropene in headspace gas, and unknown volatile and semi-volatile products in all three phases of cultures. The anticipated biotransformation product PFBA, and control substances, were measured in the aqueous and sediment phases by sensitive LC/MS/MS analysis. Following the conclusion of 160 days incubation and if justified by low mass balance, select culture extracts were also qualitatively evaluated by high resolution LC-qTOF analysis to evaluate for potentially missed unknown water-soluble biotransformation products of HFE-7500. This analysis was only performed for the aerobic study.  

In the anaerobic study, the mass balance for summed HFE-7500 and biotransformation products measured in bioactive cultures, versus dosed HFE 7500 was within 70-110% of the initial amount spiked. In the aerobic study, the mass balance was lower than 70% but the observed loss of HFE-7500 did not appear to be due to biological activity since similar HFE-7500 losses were also observed in the sterile controls. This is also supported by the fact that no other degradation products could be identified using LC-qTOF and GC/MS that would account for the observed mass loss. It is hypothesized in the report that the observed loss of HFE 7500 in the aerobic test system was due to potential leakage from the test systems, losses during oxygen replenishments, or nonrecoverable HFE-7500 irreversibly bound to test system components. As stated above, in the anaerobic study, which used a very similar test system but did not require periodic opening of the caps for the oxygen replenishment, the mass balance was consistently within 70 to 110%. This may suggest that even if some irreversibly bound HFE-7500 formed during the study, it wasn't enough to affect the mass balance of the parent, and that the oxygen replenishment method may have introduced opportunities for unquantified test material loss. 

In both studies, trace level formation of PFBA was observed in the bioactive cultures (<1 mol% conversion over 160 days) which showed an increasing trend over time. The PFBA formation was fit using first order kinetics and the half life (DT50) of the HFE-7500 based on conversion to PFBA was estimated to be between 91 and 224 years in the aerobic sediments and between 189 and 415 years in the anaerobic sediments. In both studies, PFBA was also observed in sterile controls and sterile and bioactive blanks but generally at lower levels than the bioactive cultures. DT50s reported based on fits of PFBA in the sterile controls were: 485 years for one of the aerobic sediments and 197 years for one of the anaerobic sediments.  HFC-227 was also detected in the anaerobic study at trace levels (up to 0.19 mol%) in the bioactive cultures.  In the aerobic study, HFC-227 was occasionally detected but without a clear trend with time, and later timepoints were also affected by an increase in LOQ due to the oxygen replenishment procedure. Similarly to PFBA, HFC-227 was also detected in sterile controls and sterile and bioactive blanks for both studies but generally at lower levels than the bioactive cultures. HFC-227 data were not fit in either study. The other possible volatile product, hexafluoropropene, was not detected under either aerobic or anaerobic conditions.