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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

 The test substance is considered to be inherently biodegradable (37% after 60 days in a closed bottle test-OECD testing guideline 301D).

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable

Additional information

A closed bottle test was conducted to assess the ready biodegradability of the test substance according to slightly modified OECD, EU and ISO Test Guidelines in compliance with GLP. A solution of the test substance at 1 mg/L was inoculated with undiluted, non-adapted, river water activated sludge and placed in closed bottles in the dark for 28 days. Due to the low solubility of the test substance, the test solution was prepared using an emulsion of silicone oil and water (1:1) with 0.5 g/L of Tween 85. The degradation of the test substance was assessed by the determination of the dissolved oxygen concentration (DOC) on Days 7, 14, 21 and 28. Control solutions containing the reference substance, sodium acetate (6.7 mg/L), together with abiotic control and inhibition control were used for validation purposes.

The test substance was biodegraded by 22% at Day 28 and 37% at Day 60 (during the prolonged closed bottle test). Although the test substance was not readily biodegradable, the level of degradation reached on the last day demonstrates that it should be classified as inherently biodegradable (i.e. >20% biodegradation).

The test was valid as shown by an endogenous respiration of 1.2 mg/L and by the total mineralization of the reference compound, sodium acetate. Sodium acetate was degraded 83% of its theoretical oxygen demand after 14 days. Finally, the most important criterion was met by oxygen concentrations >0.5 mg/L in all bottles during the test period.

Under the study conditions, the test substance was considered to be inherently biodegradable.

Furthermore, a study conducted to increase the bioavailability indicated that the poor water solubility leading to slow desorption and dissolution rates of the test substance at high concentrations was responsible for affecting the biodegradation kinetics. The biodegradation curves of substances with limited bioavailability were linear instead of the anticipated S-shape. The partial degradation (i.e. 37%) was therefore caused by limited bioavailability.

However, since the test substance is expected to be present in the environment at concentrations in the range of µg/L or lower, complete degradation can be expected under these conditions within a shorter time period (van Ginkel, 2013).

Further testing is planned to better characterise the biodegradation potential of this highly insoluble substance in water.