Distillates (Fischer-Tropsch), heavy, C18-50-branched, cyclic and linear

Biodegradation in soil

Description of key information

Soil degradation (OECD 307): Half-life 35.6 days in one soil type only

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   OECD Guideline 307	Half-life 35.6 days in one soil type only.	2 (reliable with restrictions) key study experimental study Test material: Distillates (Fischer-Tropsch), Heavy, C18-50 – branched, cyclic and linear	Bayliss, 2012a
Test type: soil degradation   OECD Guideline 307	Half-life 35.8 days in one soil type only.	2 (reliable with restrictions) supporting study experimental study Test material: GTL Base Oil 3	Bayliss, 2012b
Test type: soil degradation   OECD Guideline 307	-	3 (invalid) disregarded study experimental study Test material: Distillates (Fischer-Tropsch), Heavy, C18-50 – branched, cyclic and linear	Wadsley, 2011

 

A screening study of rate of degradation in one soil of GTL Base Oil Distillates has been conducted (Bayliss, 2012a). In this study the test substance was applied in hexane to one soil (type: loamy sand) at a loading rate of 1000 mg/kg dry soil (a hexane control was included in the study). A sterile control was included to evaluate losses association with abiotic processes (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 microbial biomass of the biotic soil samples was determined to be 8.12 mg C/100 g dry soil at the start of the study and 10.98 mg C/100g dry soil at the end of the 120 day incubation period. The organic carbon content of the soil was 1.1%, thus meeting the recommendation of the test guideline by the end of the study, although the microbial biomass was slightly lower than recommended on Day 0 (0.5 – 2.5% OC; microbial biomass at least 1% of the total OC).

The study duration was 120 days and samples were incubated in the dark under aerobic conditions at 20 ± 2 °C. Degradation levels were determined on the basis of infra-red spectroscopy analysis of test substance extracted from the soil samples. The limits of detection and quantification for the analytical method were 5.53 and 11.05 mg/l, respectively.

For the biotic samples, recovery of test substance on Day 120 was 15% whereas for abiotic samples the recovery was 54%. The corresponding half-lives for ‘disappearance’ of the test material were reported as 35.6 and 101.9 days, respectively. For the sterile control, test substance disappearance is attributable to abiotic processes, most likely due to adsorption, given the low volatility of the constistuents present in the material. A significant degree of test substance disappearance in the biotic samples can be attributed to biodegradation.

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. In particular, OECD 307 is clear that if the objective of the test is to draw conclusions on rate, then a minimum of four varied soil types must be tested. Here only one soil has been tested.

A second study is available (Bayliss, 2012b), following a similar protocol to that described above but where the lowest viscosity grade product (GTL Base Oil 3) was tested and the contact time was 56 days. In this study, the microbial biomass of the biotic soil samples was determined to be 14.8 mg C/100 g dry soil at the start of the study and 17.8 mg C/100g dry soil at the end of the 56 day incubation period. The organic carbon content of the soil was 1.6%, thus meeting the recommendation of the test guideline by the end of the study, although again the microbial biomass was slightly lower than recommended on Day 0.

The limits of detection and quantification for the analytical method were 4.60 and 23 mg/l, respectively.

For the biotic samples, recovery of test substance on Day 56 was 33% whereas for abiotic samples the recovery was 99%. The corresponding half-lives for ‘disappearance’ of the test material were reported as 35.8 days and >1 year, respectively. The results indicate that for the low viscosity product, removal is mainly due to degradation rather than abiotic processes.

A third study (Wadsley, 2011) was disregarded as sterility of the abiotic controls was not maintained during the test.