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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Readily biodegradable, but failing 10-day window; Brunswik-Titze, Gartiser & Jäger (2003)

Key value for chemical safety assessment

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

Additional information

Reliable data are available for the biodegradation of isononyl phosphate in water screening tests and is included in the dossier. Additionally, reliable data are available for the analogue, Phosphoric acid, 2-ethylhexyl ester (CAS 1645 -31 -7) and are included in the dossier.

One key study with isononyl phosphate and three supporting studies with the analogue,

Phosphoric acid, 2-ethylhexyl ester were identified, where the test item was assessed for it's ready biodegradability in accordance with internationally recognised OECD methods (OECD 301B).

Brunswik-Titze, Gartiser & Jäger (2003) conducted a reliable biodegradation study for isononyl phosphate following OECD 301B guidelines and was GLP compliant. Activated sludge was exposed to the test substance at 10,000 mg/L for 28 days. Results indicated 32.0 - 66.3 % of the test item was degraded within 28 days after acidification with HCl. The test item didn't reach the pass level for ready biodegradability (60% ThCO2 and 10 d-window). The variability of the degradation extents of the parallel vessels indicates a low reproducibility of the results probably due to the poor water solubility of the test item, although the validity criterion of 20 % difference of replicate values at the end of the 10-d-window is met. Based on results of this test, the test substance can be considered readily biodegradable, failing the 10 -day window.

Although considered a valid study, Brunswiik-Titze (2005) extrapolated the beginning of the 10 -day window based on day 0 (0 % degradation) and day 4 (19 % degradation) to conclude that the 10 -day window had been reached. This assumption is subject to considerable error, especially considering the day 14 biodegradation was only 65 %, and as such, this study was not used in the application of the biodegradability criteria.

The study by Clarke (2010) assessed the ready biodegradability of the test item using OECD methods 301B. This study was comparable, in that results supported the conclusion that the test item was readily biodegradable, but failing the 10 -day window.

The study by Clarke (2010) was used as the conclusive evidence for the bioderadation in water: screening tests endpoint.

For the purpose of assessing the ready biodegradability of multi-constituent substances using standard screening tests, the 10 -day window should not be considered, as it can only be interpreted correctly if a normal S-shaped degradation pattern is observed (UN/SCEGHS/15/INF.35, 2008). For multi-constituent substances, sequential biodegradation of the individual structures can take place, and in such cases, the 10 -day window is a poor indicator of the biodegradation kinetics and should not be applied to interpret the results of the test. The studies by Clarke (2010) clearly indicate two phases of degradation.

The following criterion is therefore used to assess the biodegradability of isononyl phosphate (UN/SCEGHS/15/INF.35, 2008):

"Multi-constituent substances do not need to fulfil the specified levels of biodegradation within a 10 days of the start of biodegradation. Such samples surpassing the 60 % limit value within the standard test duration of 28 days are considered as readily biodegradable substances".

isononyl phosphate is therefore considered a readily biodegradable multi-constituent substance