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Biodegradation in water: screening tests

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Description of key information


 

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

In general terms, degradation can be seen to relate to the carbon range present in test material. This is consistent with water solubility limiting the rate of uptake by microorganisms.

In studies of any multi-constituent test substance, there will be uptake of the more bioavailable constituents first. If homologous series are present, it is possible that microorganisms will adapt to the general structural types present, but it is inevitable that rates will overall appear to be slower than for pure substances.

Therefore, where studies show high rates of degradation this can be considered to be indicative of the potential for high degradation in the environment, and such studies should be given higher weight in any overall assessment.

Although degradation was achieved at varying levels in the available biodegradation in water screening tests, three biodegradability studies conducted on samples of GTL Gasoil indicate that the substance is considered to be readily biodegradable (ignoring the inapplicable 10-day window criterion). Degradation behaviour of structurally-similar substances in other screening studies in seawater are consistent with the conclusion that GTL Gasoil is readily biodegradable.

The results of ready biodegradation studies with GTL Base Oil 3 vary, typically attaining between 40 and 70% degradation in 28 days. Based on weight of evidence the the GTL Base Oil 3 is considered to be readily biodegradable, a view recently endorsed by the UK Centre for Environment, Fisheries and Aquaculture Science (Cefas) when they reviewed the data supplied for GTL Base Oil 3 under the OSPAR HOCNF (Oslo and Paris Commission Harmonised Offshore Chemical Notification Format) for their Offshore Chemical Notification Scheme (OCNS).

BIOHCWIN (via the refined method described in Section 4.1.2.1 of the Chemical Safety Report) presents an easily applied method to estimate primary aerobic biodegradation half-lives in surface water. Complete removal (mineralisation) could take more time. Waste water treatment plants are likely to have higher concentrations of microorganisms and also high levels of other primary carbon sources. Therefore higher levels and rates of biodegradation in WWTP could be anticipated compared to half-lives predicted by BIOHCWIN. The measured results show that individually all constituents less than C20 are expected to be rapidly biodegradable. Constituents of Hydrocarbons, C16-C22, n-alkanes, isoalkanes, <2% aromatics above C20 may potentially be persistent or very persistent as defined in the REACH Technical Guidance, Chapter R.11, (ECHA, May 2008), but they will all degrade completely given sufficient time.

The expected biodegradability of Hydrocarbons, C16-C22, n-alkanes, isoalkanes, <2% aromatics is confirmed by read-across from ready biodegradation studies (OECD 301F) with related substances, Hydrocarbons, C18-C24, isoalkanes, <25 aromatics and Hydrocarbons, C15-C19, n-alkanes, isoalkanes, <2% aromatics which attained 74% and 73% biodegradation in 28 days respectively. There is a lack of data on degradation in sediments, which may be anaerobic. It is therefore difficult to draw conclusions regarding persistence in freshwater and marine sediments.

The available measured data for degradation in soil indicate that after 51 days contact time, constituents of GTL Gasoil were not detectable. It was not firmly established whether this is due to biodegradation, loss by evaporation or that the constituents were irreversibly bound to the soil matrix. Scientific judgment would suggest that it is probably a combination of all three. Similarly, studies with full-range GTL Base Oil Distillates and GTL Base Oil 3 indicated a significant degree of test substance removal, most of which can be attributed to biodegradation processes.

Since the constituents of Hydrocarbons, C16-C22, n-alkanes, isoalkanes, <2% aromatics are within the carbon number range covered by the read across substances, it can be concluded, based on the available weight of evidence, that a significant degree of removal, including biodegradation, will occur in soil.

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