C8-C26 branched and linear hydrocarbons – Distillates

Endpoint summary

Additional information

Summary and discussion on biodegradation

A recent review is available which summarises the major metabolic pathways for biodegradation of linear and branched alkanes by microorganisms (Rojo, 2009). It is beyond the scope of this work to review this in depth, but it is relevant and useful for consideration of the primary degradation step for GTL distillates constituents. Under aerobic conditions, which are of most significance for conventional understanding of persistence and exposure, oxidation using O₂is understood to be the major breakdown mechanism and various microbial alkane hydroxylase enzymes are of particular significance. In general the initial activation to form a primary alcohol is the first step, followed by oxidation to the aldehyde and subsequently the carboxylic acid. The carboxylic acids undergo β-oxidation. Oxidation may occur at both ends of longer chains, creating a ω-hydroxy fatty acid. Degradation of very long chains and branched structures is a more specialised process but several bacterial strains are known to degrade such structures. Degradation of ring structures is not discussed in this review.

A persistence assessment for C9-12iso-alkanes has been made and published (as a draft) by the EU PBT working group (ECB, 2008a). The focus is two similar multi-branched UVCB substances (ca. 1.7 – 3 branches per molecule; predominantly C10-11 or C11-12 structures present in two separate commercial products). This summary reports several results and conclusions. In biodegradation tests of the C10-11 form, results of 41% in 41 days (OECD 301F, non-acclimated inoculum); 48% in 41 days (OECD 301F, acclimated inoculum); 21.9% in 28 days (OECD 301F, predominantly domestic sewage sludge inoculum); 99.96% removal (physical removal not ruled out) in 45 days (fresh pond water), and 99.19% removal (physical removal not ruled out) in 49 days (sea water) are reported. In biodegradation tests of the C11-12 form, results of 6% in 28 days (OECD 301F, non-acclimated inoculum); 48% in 90 days (OECD 301F, acclimated inoculum); 98.1% removal (physical removal not ruled out) in 45 days (fresh pond water), and 97.7% removal (physical removal not ruled out) in 49 days (sea water) are reported (all from EMBSI studies, full details were not given and full studies were not available to the working group).

A second persistence assessment for penta-branched-alkanes has been made and published by the EU PBT working group (ECB, 2008b). This reports an OECD 301D test result using triisobutylene of 3% degradation in 28 days (BOD; from MITI 1992 study).

Summary and discussion on degradation

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 micro-organisms 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 using the registration substance and its analogues, two 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. In the present context, such conclusions for the whole substance are in any case not meaningful for the chemical safety assessment, including the hazard assessment.

BIOHCWIN (via the refined method described in the CSR) 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 results show that individually all constituents less than C20 are expected to be rapidly biodegradable.

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.