Didodecyl 3,3'-thiodipropionate

Biodegradation in water: screening tests

Description of key information

Readily biodegradable, but failing 10-day window

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Additional information

The biodegradability of the test compound (EC no. 204 -614 -1) was assessed in a Modified Sturm Test (OECD 301 B; Ciba 1989). The lower concentration resulted in 25 % degradation and the higher concentration in 57 %. Additionally, the results of an 28 d OECD 301 C (MITI 1) test are available. This test used a concentration of 100 mg/L test substance and 30 mg/L sludge and resulted in 82 % degradation based on BOD and 97 % degradation based on HPLC analysis.

In addition to these tests, an analogue substance (EC no. 211 -750 -5) was assessed. This substance only differs in the length of the aklyl side chains (C18H37 instead of C12H25). Due to the fact that the analogue substance has a longer alkyl chain length this is regarded as worst case scenario. The degradability of the analogue substance was object of several studies.

(1) An OECD 301 B (modified Sturm test) resulted in a mineralization of 2 % in 28 days using a test concentration of 23.9 mg/l and activated sludge as an inoculum (Ciba 1989). Another available record on an OECD 301 B –test reports a mineralization of 15 % in 28 days. In this test a test concentration of 11.3 mg/l and activated sludge were employed (Ciba 1989).

(2) The degradability of the analogue substance was studied by Akzo Nobel (2006) in a closed bottle test according to OECD 301 D with slight modifications using test concentrations of 2mg/l. Test substance dissolved in dichloromethane was added to 250-300 ml BOD bottles. Thereafter, the dichloromethane was evaporated during 24 hours on a roller bank before adding mineral medium and inoculum. The concentration of inoculum (secondary activated sludge derived from a municipal STP) was 2 mg dw/l. The test bottles were placed on magnetic stirrers and were incubated in the dark at 19-21 °C for 28 days. Two duplicate bottles were withdrawn for analyses of dissolved oxygen at days 7, 14, 21 and 28. The biodegradation of the compound was 71% in 28 days based on the BOD/ThOD-ratio. It reaches the pass level of 60% biodegradation in 21 days.

The modified OECD 301 D test result (71 % in 28 days) is considered to reflect more appropriately the biodegradation potential of the substance than the results of two OECD 301 B –tests (2 % in 28 days and 15 % in 28 days) as these tests were performed in exceedance of the water solubility and limited availabilty for biodegradation can be assumed.

Based on the modified OECD 301 D test result using lower concentrations of the substance, the substance is considered as readily biodegradable without meeting the 10-day window.

(3) This conclusion is supported by an 14 d OECD 301C (MITI 1) test (JETOC 1992), using 100 mg/l test substance and 30 mg/l sludge concentration,resulting in 51% degradation based ob BOD and 74% degradation based on HPLC analysis.

In conclusion, supported by the analogue substance (EC no. 211 -750 -5) which is regarded as worst case scenario due to longer alkyl side chain length, the compound (EC no. 204 -614 -1) is regarded as readily biodegradable without meeting the 10 day window.