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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

The key study for degradation in water screening tests was a closed bottle ready biodegradability test conducted on the test substance, which showed that the substance was not readily biodegradable.  The results of another closed bottle test from a supporting study found in the published literature reported 25% biodegradation of the substance in 28 days, indicating inherent biodegradability.  Although ready biodegradability was not shown in the key study for biodegradation in water screening tests, an aerobic soil simulation study was undertaken (summarised as the key study in Section 5.2.3 of this IUCLID file).  The aerobic soil simulation study reported 70.7% in 28 days; these results showed that the substance was rapidly biodegradable, but did not show that the 10-day window was achieved. 

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable but failing 10-day window

Additional information

The substance was not shown to be readily biodegradable in a closed bottle test conducted in accordance with GLP; 0% degradation was observed at 28 days at a test substance concentration of 2 mg/L, below the threshold of 60% degradation for ready biodegradability. However, the findings of a supporting study from the published literature reported 25% degradation at 28 days at a test concentration of 1 mg/L in a closed bottle test, below the threshold of 60% degradation for ready biodegradability, but indicative of inherent biodegradability.  An aerobic soil simulation biodegradation study showed that the substance was rapidly biodegradable in soil under aerobic conditions; the soils were not pre-adapted to the substance, which was applied at a concentration of 0.376 mg/kg soil dry weight. The soil simulation study reported mineralisation of 1.2%, 14.7%, 31.1%, 49.1%, 63.5% and 70.7% in three, seven, 10, 14, 21 and 28 days; the results at these intervals showed that the substance was rapidly biodegradable, but did not show that the 10-day window was achieved.The DT50 for the substance (i.e., the parent compound) and its primary metabolite (i.e., Metabolite II) were reported as 9.7 days and 3.5 days, respectively, each based on the Single First Order Model. Therefore, the substance has been shown to be rapidly biodegradable under aerobic conditions. The experimentally-determined rapid degradation in soil is sufficient documentation for showing rapid degradation in surface water and sediment, in accordance with ECHA guidance (Guidance on the application of the CLP criteria, Version 3.0, November 2012, Section II.2.3.6, p. 498). 

The potential for respiratory inhibition by the test substance was also evaluated in the key study for biodegradation in water screening tests. Although possible inhibition of respiratory activity was indicated when oxygen concentrations of the blank controls were compared to those of the test substance vessels, the substance was not found to meet the criteria for classification as inhibitory, based on toxicity controls at a test substance concentration of 2 mg/L, and a comparison of the microbial plate counts of the control, reference substance, test substance and toxicity control solutions at day 28.