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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

Through an OECD TG 308 compliant study, it is shown that tert-dodecyl polysulfide partitions rapidly from water to sediment compartment (DT50 water 6.4-6.6 days) , where it undergoes a true degradation with DT50 = 158 - 180 days (average 169 days).

Key value for chemical safety assessment

Half-life in freshwater:
6.5 d
at the temperature of:
20 °C
Half-life in freshwater sediment:
169 d
at the temperature of:
20 °C

Additional information

Polysulfides, di-tert-dodecyl is poorly water soluble and highly adsorptive, therefore, if released in the environment adsorption to suspended matter is expected, and sediment would be the sink.

Therefore based on both expected behaviour and uses of the substance, simulation testing in water is not considered as a relevant approach for characterizing biodegradation potential and a sediment simulation test according to OECD TG 308 was offered in a testing proposal. Given the expected difficulty of the experiment, a time span of 18 months was asked to ECHA for conducting the study and updating the dossier. Only 12 months were allocated by ECHA requiring updating the dossier by March 2014. As expected, this tight and unrealistic deadline could not be met: this is the reason why a statement from the laboratory has been added to this file arguing for a planned study completion in October 2014.

The study has finally been done successfully (draft report november 2014, final report October 2015, further analytical investigations January-December 2015).

In summary, biodegradation of tert-dodecyl polysulfides in freshwater sediments has been evaluated through an OECD TG 308 compliant study. Two water/sediment systems with duly characterised different properties were used ("SL" is Organic Carbon poor, "SW" is OC rich).

The 2 water/sediment systems were incubated during 98 days in the dark at 20°C. Periodically (t = 0, 3, 7, 14, 28, 63 and 98 days), samples were removed in duplicates in order to measure concentration of TPS 32 in each phase.

It was shown that the substance migrates rapidly from water to sediment. Dissipation half-life TD50 in water was 6.4 days in "SL" water/sediment system and 6.6 days in "SW".

It was further shown that, after an increase of its concentration in the sediment phase, resulting from the partitioning (in agreement with the high Koc), the concentration decreased from 28 days onwards in both sediments. A DT50 could be calculated as being = 180 days in "SL" and 158 days in "SW" sediment. DT90 are respectively 600 and 526 days.

The anticipated degradation product, tert-dodecyl mercaptan, TDM, which could be produced from cleavage of the polysulfide chain, was quantified, but even in the TPS 32 tested sample, which is contradictory with what is known of its composition (only present in ppm range). An analytical interference has finally been identified further to investigation. TDM is therefore not identified as a stable metabolite of degradation of Polysulfides, di-tert-dodecyl.

The main conclusion is that polysulfides di-tert-dodecyl partitions rapidly from water to sediment compartment (DT50water 6.4-6.6 days), where it undergoes a true degradation with DT50global = 158 - 180 days (average 169 days).