Dioctadecyl disulphide

Endpoint summary

Additional information

The ready biodegradability of the test item was determined with non adapted activated sludge in the Manometric Respirometry Test for a period of 28 days. The study was conducted according to OECD guideline 301 F. The test item concentrations selected as appropriate were 20 mg/L.

In order to check the activity of the test system sodium benzoate was used as functional control. The pass level > 60 % was reached after 2 days. The biodegradation rate came to a maximum of 98 % on day 26.

In the toxicity control containing both test and reference item 59 % degradation occurred within 14 days. After 28 days the biodegradation came to 67 %. The degradation of the reference item was not inhibited by the test item.

The 1st test item replicate reached the 10 % level (beginning of biodegradation) after 11 days, and the 2ndtest item replicate after 9 days. The 60 % pass level was not reached within 28 days. After 28 days the mean biodegradation was 32 %.

To confirm the potential for biodegradability of the test item a second enhanced Manometric Respirometry Test with non-adapted activated sludge was conducted for a period of 60 days. The study was conducted according to OECD guideline 301 F. The test item concentration selected as appropriate was 15 mg/L.

In order to check the activity of the test system aniline was used as functional control. The pass level > 60 % was reached after 14 days. The biodegradation reached a maximum of 96 % degradation on day 43.

The mean of all three test item replicates reached the 10 % level (beginning of biodegradation) within 30 days. The 60 % pass level was not reached. After 60 days the mean biodegradation was 20 %.

In the second study conducted from November 2012 to January 2013 it was confirmed that the test item has a potential for degradation, but the biodegradation progresses only slowly.

It is assumed that the degradation is decelerated by the low water solubility and the resulting low bioavailability of the test item in the test solution. Furthermore, the biodegradation might be influenced by the number of competent degraders present in the activated sludge inoculum. 

In comparison with the study on ready biodegradability conducted in September 2011 (acc. to OECD 301F standard test), the biodegradation was slower as expected. In the study on ready biodegradability a mean biodegradation of 32 % was reached after 28 days. It is assumed that the lower biodegradation in the current study was due to a lower concentration of competent degraders in the activated sludge used for inoculation.

The composition of the bacterial population of activated sludge is subjected to natural fluctuations, mainly caused by variations in the sewage flow and seasonal changes. From experience it is known that during drier periods with little rain, as it often takes place in summer and early autumn, the sewage flow is lower and the biodegradation potential and microbial diversity of the activated sludge increases. This effect influences mainly the biodegradation results of substances which show a certain biodegradation but are not readily biodegradable. It is assumed that under these conditions the microbial adaptation to degradation of not readily biodegradable substances is enlarged.

A third study was conducted in accordance with ECHA Compliance Check, which required identification of degradation products. The OECD TG 309 was recommended as the preferred test, if technically feasible.

Based on experiments demonstrating very low water solubility and similarly low solubility in other solvents, it was concluded that the test item is highly insoluble and therefore OECD TG 309 is not feasible to conduct.
Therefore, an enhanced OECD TG 310 test in combination with LC-HRMS (high resolution mass spectrometry) analysis for detection and identification of metabolites was performed.

Metabolites were investigated and mineralisation of the test item was determined in an enhanced screening test with a non-adapted activated sludge over a test period of 62 days in the Headspace Test. The study was conducted from November 2021 to the end of January 2022, following OECD 310 at the test facility. Investigations about metabolites were done by LC-HRMS analysis. The mineralisation of the test item was followed by TIC analyses of the quantity of CO2 produced by the respiration of bacteria.
To check the activity of the test system sodium benzoate was used as functional control. The percentage degradation of the functional control reached the pass level of 60 % within 10 days and came to a maximum of 92 % on day 16. The 95 % confidence interval on day 28 was 84 – 96 %.
The test item replicates reached the 10% level (beginning of biodegradation) within 10 days. The biodegradation on day 28 was 20 % and reached 48 % until test end (day 62). The 95 % confidence interval on day 62 was 30 – 66 %.
A plateau phase did not appear to have been reached after 62 days, and one replicate even reached 70% degradation after 62 days. This reinforces the assumption that the substance would be degraded even further.
The validity criteria according to the guideline were fulfilled.
No metabolites were detected. Putatively, the alkyl chains of the test item can be completely mineralised. But, higher mineralisation of the test item is slow due to the low water solubility of the test item linked to a limited bioavailability.

In comparison to the study on ready biodegradability according to OECD 301F enhanced, the biodegradation according to OECD 310 was faster and reached 48% after 62 days compared to 20% after 60 days, likely due to more competent degraders in the activated sludge.

Also the results from BIOWIN (v4.10) indicate biodegradation and confirm the results seen in the biodegradation tests:

   Biowin1 (Linear Model Prediction)    :                Biodegrades Fast

   Biowin2 (Non-Linear Model Prediction):             Does Not Biodegrade Fast

   Biowin3 (Ultimate Biodegradation Timeframe):  Weeks-Months

   Biowin4 (Primary  Biodegradation Timeframe):  Days-Weeks

   Biowin5 (MITI Linear Model Prediction)    :        Readily Degradable

   Biowin6 (MITI Non-Linear Model Prediction):     Readily Degradable

   Biowin7 (Anaerobic Model Prediction):              Biodegrades Fast

   Ready Biodegradability Prediction:                    NO

In conclusion, the substance is not readily biodegradable but the potential for biodegradation and mineralisation is confirmed. No metabolites/degradation products were detected.