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Environmental fate & pathways

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In a test according to OECD 301C thiourea could not be demonstrated to be readily biodegradable.

Thiourea may be regarded as inherently and ultimately biodegradable under the conditions of the SCAS test.

But according to the ECHA Guidance R.7b only test results from inherent biodegradability studies according to OECD 302C (MITI II study) conducted with non pre-adapted bacteria can be used to conclude that the substance is not persistent where full mineralisation occurs (> 70 % degradation). Thus, persistence of thiourea cannot be assessed based on basis of SCAS Test results as this test provides a high potential for adaption of the micro-organisms to the test substance.

The available MITI II test resulted in only minor degradation within 14 days (NITE, 2012). Therefore, a definitive conclusion on persistence cannot be drawn based on the available screening level results.

However, in addition to the screening level results data from a three year project are available. During this project the microbial degradability of thiourea at low, environmentally relevant concentrations (1 µg/L 14C-labelled thiourea) in samples taken from the estuary of the river Elbe as well as from the North Sea and the Baltic Sea was investigated (incubation at 10 °C and 20 °C, respectively). Degradation was followed by measurement of CO2 formation. The river Elbe and its estuary served as an example for a highly contaminated habitat, whereas the western Baltic Sea can be regarded as less polluted.

The biodegradability of thiourea differed strongly between habitats. Overall, the biodegradation of thiourea ranged between 28 % (after 70 d of incubation) and 87.3 % (after 14 days of incubation). These experiments demonstrate that thiourea serves as nitrogen (N) source for degrading microorganisms. Thiourea is more easily degraded in N-limited environments.

Degradation half-lives (t1/2) based on mineralisation to CO2 were derived from the degradation graphs presented in the study. Half-life in marine water and sediment samples taken in the North Sea ranged from 20 to 39 days, and 17 to 28 days, respectively. These samples were incubated at 10 °C and are therefore considered to be more representative for environmental conditions than the results obtained with Baltic Sea samples that were incubated at 20 °C. The hazard assessment is based on the mean value of the half-lives reported above (marine water t1/2 = 29.5 d, marine sediment t1/2 = 22.5 d).

Thus it could be demonstrated that thiourea can be degraded under appropriate environmental conditions and the substance shall therefore not be considered persistent (for details please refer to the chapter “PBT assessment”).

In a supporting study (Tomlinson et al., 1966) also complete degradation (mineralisation) of thiourea by activated sewage sludge could be demonstrated, as evidenced by formation of nitrate and sulphate.

Thiourea is quickly degraded in natural soil samples when applied at concentrations that are not inhibiting and/or toxic towards soil micoroorganisms (Loehr & Matthews, 1992). In this case the determined half-lives for thiourea biodegradation in soil range between 12.8 days and 18.7 days.