C16-(branched), C20-(branched) and C24-(branched)-alkanes

Biodegradation in water: screening tests

Description of key information

 Tetrabutan, dest. reached a biodegradation value of 32.3 % after 28 days in a BODIS test. Therefore, the test substance is considered not readily biodegradable after 28 days. However, an enhanced ready biodegradation test according to test guideline OECD 310 demonstrated a good potential for biodegradation and reveals a biodegradation rate of 59% after 60 days. The primary degradation was further investigate by GC-MS analysis. Primary degradation of alkanes with different branching has been observed although the analytical analysis has been overlayed with abiotic elimination processes. 

Key value for chemical safety assessment

Type of water:

freshwater

Additional information

Biodegradation of Tetrabutane or fractions of Tetrabutane has been examined in two ready biodegradation test. Tetrabutane, dest. was tested for its ready biodegradation in a BODIS test according to ISO Draft (BOD Test for Insoluble Substances). With a biodegradation value of 32.3 % of the corresponding Theoretical Oxygen Demand (ThOD) after 28 days, readily biodegradability could not be shown for Tetrabutane, dest. by this test.

Additionally, biodegradation has been examined in an enhanced ready biodegradability study. According to the ECHA document chapter R.7b, the enhanced ready biodegradability test can used to confirm the potential for biodegradation. The enhanced ready biodegradability test has been performed with non-adapted activated sludge over a test period of 60 days in a Headspace test according to the test guideline OECD 310. A test item concentration of 17.7 mg/L (corresponding to 14.9 mg C/L carbon content in the test vessel) has been tested and the CO2 evolution was determined by TIC analysis during the test period.

The test item replicates reached the 10% level (beginning of biodegradation) within 14 days. The 95% confidence interval on day 28 was 39 – 44%. The biodegradation came to 59% after 60 days (confidence interval 55-63%). Overall, the test item is classified as not readily biodegradable within the study period. Nevertheless, the test item shows a very good potential for biodegradation with 3 out of 5 replicates with at least 60% biodegradation after 60 days.

The functional control reached the pass level of 60% within 7 days and the biodegradation of the reference item was not inhibited by the test item in the toxicity control. The chosen enhancements (like the extended test period or larger test vessels) were in accordance to the guidance document chapter R.7b (2017).

 

Furthermore, primary biodegradation has been examined with GC-MS analysis. To analyze the complex mixture of branched and linear alkanes (C₁₆) in the test substance, the components were grouped according to their degree of branching identified by their Kovats Retention Indices (KRI). Three groups were defined: Group 1: Grade of branching 3 + 4; Group 2: Grade of branching 3 + 2 and Group 3: Grade of branching 0 + 1. The limits of the groups were defined by their specific range of their Kovats Retentions Indices (KRI).

The test item replicates from Group 1 and Group 2 show a significant decrease in concentration to approx. the background level from the abiotic inoculum control samples until day 28. The corresponding abiotic test item replicates of these two groups show also a decrease in concentration over the whole 60 days. For Group 2, a linear decrease was observed as expected from abiotic adsorption processes as opposed to a fast decrease in the beginning and levelling out after 28 days to 60 days. After correction for the background, the primary degradation / elimination in relation to the mean value at test start shows a similar course with a plateau approximately after day 28. The level of primary degradation / elimination for Group 1 has been about 65% at test end and about 106% for Group 2 at test end.

The concentration of species of group 3 with no or one branching was in the range of the background values. Accordingly, an evaluation of the primary biodegradation was prone to deliver unrealistic values and, therefore, detailed values have not been reported. A linear increase of primary elimination in the abiotic test item replicates has been observed and might be caused by adsorption processes, e.g. to the sludge. As the mineralization data proves, this elimination or adsorption does not inhibit the biodegradation processes.

 

Overall, the enhance biodegradation test showed a good potential for biodegradation of the tested fraction of registered substance. The mineralization data demonstrates that the main part of the test substance has been degraded. This suggests that also branched alkanes must have been degraded to reach the biodegradation rate of 59% after 60 days. Based on the course of the biodegradation rate during to study period, a tiered biodegradation process of the species with different branching can be assumed. However, due to abiotic elimination processes, the analytical method has been limited to support this hypothesis.