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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable

Additional information

Five studies on the biodegradation in water are available. Four studies investigated the ready biodegradability and one investigated the inherent biodegradation of the test item.

In the key study on ready biodegradability,O,O,O- triphenyl thiophosphate was assessed over a 28 day period by the modified Sturm test (OECD 301B) (Springborn Smithers 2009). The nominal test concentration was 0.26 µg a.s./L (two replicates), which was in the range of the water solubility of the test substance. CO2measurements showed up to 17.8 and 19.3% mineralisation after 29 days. Further investigations during experimental phase using radiolabelled test item showed that after 29 days only 51.5% and 60.8% of the radioactivity present consisted of the test item, whereas the remaining radioactivity consisted of its degradation products. Thus, the test item is partially completely biodegraded over a 28 day period under the test conditions, up to 19% is mineralized and additionally up to 48.5% of the test item is transformed into its transformation and degradation products. The positive control substance of the ready biodegradation test (sodium benzoate) was 90.6 % degraded after 29 days. Phenol can be identified from the chromatogram of the degradation products: From the analytical measurements in this study it can be concluded that Phenol will be formed. RP HPLC was used and separation is based on hydrophobicity. The lower the content of phenolic groups of the analyte, the more polar the analyte is and eluation from the column starts earlier. Using 60% methanol at the beginning Phenol would correspond to the peak at 2.6 minutes. The peaks at 12.3 minutes correspond to the substance containing only two phenolic goups. (for further information see the attachments of the key study in IUCLID chapter 5.2.1, modified Sturm test, Springborn Smithers 2009).

The inherent biodegradability of the radiolabelled test item was assessed over a 28 day period by the MITI-test (II) according to OECD 302C (Springborn Smithers 2008). The nominal test concentration was 0.26 µg a.s./L, which was in the range of the water solubility of the test substance. The test material attained up to 59.5 and 66.8 % biodegradation after 28 days whereas the positive control substance (ethylene glycol) was 98.2 % degraded 28 days. These results indicate that the test item can be classified as inherently biodegradable under conditions of the test.

In a supporting study, the ready biodegradability of O,O,O- triphenyl thiophosphate

was assessed over a 28 day period by the modified Sturm test (OECD 301B) in concentrations of 10 and 20 mg/L. No biodegradation (0 -2%) was observed. In line with the key study on ready biodegradation described above, the results indicate that the test item cannot be classified readily biodegradable under conditions of the test. However, the very low solubility of the substance may have reduced the availability and hence the degradability of the substance in the test. For characterisation of the persistency of the substance the test-set-up of the supporting study was not sufficient due to the low solubility of the test item.

Two supporting studies on ready biodegradability with O,O,O-triphenylphosphorotioate in a lower purity are available. In a Closed Bottle Test according to OECD 301D no degradation was observed after 28 days using 3 mg/L test substance (Huntington 1998). In a study according to OECD 301C (Institute of Ecotoxicology, Gakushuin University 1999) no degradation was observed after 28 days using 100mg/L test item. Considering that the substance consists of several adsorptive constituents with poor water solubility, the limited bioavailability in the test design could be the reason for the lack of observable biodegradation.

In these studies the biodegradation of the single constituents was not analysed separately. With respect to the limited solubility of O,O,O- triphenyl thiophosphate, the bioavailability of the test item was limited and the applied methods not sensitive enough to measure degradation.

Overall, the results of supporting studies are not considered to be contradictory to the results of the key study. The methodology of these studies was not sensitive enough to measure the degradability.

O,O,O- triphenyl thiophosphate is primarily degradable under the conditions of the test. The determined degradation product phenol is readily biodegradable.