C8-C26 branched and linear hydrocarbons – Distillates

Biodegradation in soil

Description of key information

- Test material Distillates (Fischer-Tropsch), C8-26 - branched and linear (OECD 307; GLP): Indicative half-life (both biodegradation and physicochemical removal mechanisms) DT50 in biotic system of 22.4 days compared to predicted DT50 of 82.6 days in a sterile system (one soil type only).
- Test material Dodecane (OECD 307; GLP): Biodegradation half-life cannot be determined on the basis of this test because of significant losses of test substance during soil preparation; removal from test system dominated by volatilisation. DT50 considered to be < 1 day.
- Test material Hexadecane (OECD 307; GLP): Indicative half-life of 1.5 days in biotic system compred to 32.9 days in a sterile system (in one soil type only). Removal from test system dominated by biodegradation.
- Test material Eicosane (OECD 307, GLP): Indicative half-life of 11.4 days in biotic system compred to a predicted value of 122 days in a sterile system (one soil type only). Removal from test system dominated by biodegradation.

Additional information

The results of experimental studies on biodegradation in soil are summarised in the following table:

Table: Overview of tests for biodegradation in soil

Method	Results	Remarks	Reference
Test type: soil degradation   Compatible with OECD Guideline 307	Indicative half-life (both biodegradation and physicochemical removal mechanisms) DT50in biotic system of 22.4 days compared to predicted DT50of 82.6 days in a sterile system (one soil type only).	2 (reliable with restrictions) key study experimental result Test material: Distillates (Fischer-Tropsch), C8-26 - branched and linear (CAS: 848301-67-7)	Wadsley, 2011
Test type: soil degradation   Compatible with OECD Guideline 307	Indicative half-life life (both biodegradation and physicochemical removal mechanisms) 11.4 days in one soil type only.	2 (reliable with restrictions) supporting study experimental result Test material: Distillates (Fischer-Tropsch), C8-26 - branched and linear (CAS: 848301-67-7)	Wadsley, 2010
Test type: soil degradation   Compatible with OECD Guideline 307	Biodegradation half-life cannot be determined on the basis of this test because of significant losses of test substance during soil preparation; removal from test system dominated by volatilisation. DT50considered to be < 1 day.	2 (reliable with restrictions) key study experimental result Test material: Dodecane	Wadsley, 2011
Test type: soil degradation   Compatible with OECD Guideline 307	Indicative half-life of 1.5 days in biotic system compred to 32.9 days in a sterile system (in one soil type only). Removal from test system dominated by biodegradation	2 (reliable with restrictions) key study experimental result Test material: Hexadecane	Wadsley, 2011
Test type: soil degradation   Compatible with OECD Guideline 307	Indicative half-life of 11.4 days in biotic system compred to a predicted value of 122 days in a sterile system (one soil type only). Removal from test system dominated by biodegradation	2 (reliable with restrictions) key study experimental result Test material: Eicosane	Wadsley, 2011

The degradation/removal in soil has been measured in two separate screening studies (Wadsley, 2010 and 2011). The main objective of these studies was to identify any constituents of GTL Gasoil that may potentially be persistent in the terrestrial environment.

In the 2010 study the test substance was applied in acetone to one soil (type: loamy sand) at a loading rate of 1000 mg/kg dry soil (an acetone control was included in the study). There is no indication of steps being taken to minimise or prevent evaporation during test substance loading or during the test and indeed the test did not include any attempt to evaluate losses associated with abiotic processes (including volatilisation or adsorption). The soil sample was taken from a pasture-land location shortly before the study, sieved to 2 mm, and stored with watering as necessary. The soil microbial biomass in the test samples was low: OECD 307 recommends the use of a soil with 0.5 – 2.5% organic carbon (OC) and that soil microbial biomass should normally be at least 1% of the total OC. The test soil had 8.13% OC and microbial biomass 0.55 µg OC/g soil (equivalent to microbial biomass 7E-04% of the total OC). The exposure duration was 51 days, at 20°C. On the basis of this test, an overall half-life for removal of the test substance of 11.4 d was reported, which also applied to most constituents. Degradation levels were determined based on analysis of remaining test substance by GC-FID (see Figure 4.1), for which the limits of detection and quantification were equivalent to 16 and 39 mg/kg respectively, quite high in relation to the loading rates used in the study. This is a reflection of the technical difficulties in analysing UVCB substances for which the test guideline was not designed. A recovery experiment at the time of study initiation showed full recoveries but abiotic/ physicochemical removal mechanisms during the exposure time cannot be ruled out on the basis of this test. The corresponding Day 0 chromatogram can be found in the study report attached in IUCLID. The study is broadly of high quality but it should be noted that there are some important variations compared to the quoted OECD 307 guideline which affect the interpretation of the study results, particularly the use of only one soil type. However, the OECD 307 method is onlyapplicable to chemical substances (non-labelled or radiolabelled) for which an analytical method with sufficient accuracy and sensitivity is available.  The OECD 307study was not designed for complex substances and the primary objectives of the initial study were to assess:

1) the feasibility of undertaking this study with GTL Gasoil; and

2) which constituents of GTL Gasoil were potentially persistent in soil, at the specific request of ECHA. 

This is different from the objectives of the OECD 307 guideline which is primarily designed to determine a definitive rate of degradation of a specific compound (mainly crop protection products) added to a soil. The inability of the test as performed to differentiate between biodegradation and other e.g. abiotic or physical mechanisms of removal is a significant issue. Therefore in conclusion, the rate of degradation seen in this test must be considered indicative only, and is not necessarily associated with biodegradation specifically. However in view that the microbial biomass level appears to have been very low compared to the recommended test conditions, this would suggest that any biodegradation occurring in the screening test may have underestimated that which might have occurred in an OECD 307 guideline-compliant study.

The 2011 study used a similar method with some important improvements in the methodology. GTL Gasoil and three pure alkanes (dodecane; hexadecane; eicosane) were applied in a solvent carrier to one soil (type: loamy sand) at one specific loading rate per test substance (nominal loadings GTL Gasoil: 1000 mg/kg; dodecane: 10.5 mg/kg; hexadecane: 32 mg/kg; eicosane 26.6 mg/kg). No steps were taken to minimise or prevent evaporation, and indeed the recoveries even at Day 0 are significantly below 100% in several cases. This is because it is impossible to fully mix the test substances in aerobic soil which is another limitation of the OECD 307 test method for this type of material. The limitations of the test method mean that greater care is required in the interpretation of the data. In the second study, the design was altered by the inclusion of abiotic (sterile) controls as described below.  In effect for the types of substances evaluated in this study the term degradation has been used for the aerobic systems although it should be noted that this is a combination of biological degradation and losses due to physical (abiotic) factors. The sterile controls were used to ascertain a disappearance rate to account for abiotic loss processes such as losses due to volatilisation or due to significant binding to soil (non extractable residues). An ongoing ECETOC Task Force is developing guidance on the relevance of bound and non-extractable residues in environmental risk assessment[1].

As stated above, sterile controls were included for each of the four test systems. The soil sample was taken from a pasture-land location shortly before the study, sieved to 2 mm, and stored with watering as necessary. The soil microbial biomass in the test samples was adequate at 14.80 – 17.80 µg OC/g soil. The test soil had approx. 1.6% OC (equivalent to microbial biomass approx. 1% of the total OC). The exposure duration was 56 days, at 20°C. On the basis of this test, an overall half-life for removal of GTL Gasoil of 22.4 d was reported, which reflects both biodegradation and other removal factors (attributed to volatilisation and non-extractable/bound residues). Degradation levels for GTL Gasoil and the other alkanes tested in parallel were determined based on analysis of remaining test substance by GC-FID, for which the limits of detection and quantification were equivalent to 9.8 and 49 mg/kg respectively for GTL Gasoil. A recovery experiment at the time of study initiation showed good recoveries. Comparison of recoveries in biotic test system vs sterile test system gives some evidence on the contribution of physicochemical removal. The sterilisation method used in the test was validated by checks on the microbial viability in sterile systems.

The alkane studies helped provide more information on factors governing the fate of constituents of GTL Gasoil in aerobic soils.  The alkanes were added to soils at concentrations which were similar to the concentrations which they would be present in the 1000 mg/kg GTL Gasoil study. These studieswith the n-alkanes show the significance that abiotic factors have in determining the fate of such substances in OECD 307 studies. Abiotic factors (suspected to be volatilisation) were far more significant in removal of the lower carbon chain lengths. For example, even during soil dosing there were significant losses of the most volatile n-alkane (Dodecane) which continued to occur in the sterile system. Hexadecane is more volatile than Eicosane and this was also manifested in the losses from their respective sterile systems. This is reflected in the recoveries at Day 0 (after soil dosing) in which only 22-39% of dodecane was revovered compared to between 84 – 87% of hexadecane and 108-112% eicosane.  

The use of abiotic factors provided evidence that in all cases biodegradation did occur. Even in the Dodecane, in which the degradation rate (DT₅₀) could not be calculated due to the low recovery at Day 0, there was still some evidence that the remaining substance disappeared more quick in the biotic systems. For example, in the biotic treatment the Day 0 concentration of 39% had declined to 17% after 3 days and below the limit of quantification (LOQ) after 7 days. In comparison for the sterile system the Day 0 concentration of dodecane of 25% had only declined to 15% after 7 days.  For Hexadecane the DT₅₀ at 32 mg/kg was 1.5 days compared to 32.9 days for the sterile system when incubated in aerobic soil in the dark at 20 ± 2°C. Finally, Eicosane had a DT₅₀degradation rate, at 26.6 mg/kg, of 11.4 days compared to a predicted value of 122 days in the sterile system when incubated in aerobic soil in the dark at 20 ± 2°C.

The soil extract analysis over the study indicates that the GTL Gasoil (particularly the n-alkane constituents) disappears relatively quickly and the chromatography traces show the lighter n-alkanes (C14-16) at retention times of 15.1-17.7 minutes disappearing more quickly relative to the heavier n-alkanes (C18-20) at retention times of 20.1-22.3. This is to be expected owing to greater water solubility (and hence bioavailability) of the shorter chain lower molecular weight hydrocarbons. As the carbon number of the chain length increases the water solubility decreases, which hampers the uptake of alkanes by microorganisms (Rojo, 2009).

Further information about the "Application of the OECD 307 study to assess the persistence of Gas to Liquid (GTL) Fuel" may be found in the document below and in the Chemical Safety Report attached in Section 13.