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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Based on the available screening level tests thiourea cannot be considered biodegradable. 
Complete biodegradation could only be observed under the conditions of the SCAS tests which provide a high potential for adaption of the micro-organisms to the test substance thiourea.
Based on the available screening level tests thiourea cannot be considered readily biodegradable. The available test results on inherent biodegradability do not allow for a persistence assessment of the substance. Therefore, persistence is assessed based on higher tier biodegradation studies in surface water and sediment as well as in soil (please refer to the respective sections in IUCLID/the CSR).
Complete biodegradation could only be observed under the conditions of the SCAS tests which provide a high potential for adaption of the micro-organisms to the test substance thiourea. Thus, thiourea is considered inherently biodegradable under the conditions of the SCAS Test.

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, not fulfilling specific criteria

Additional information

One key study (Klimisch 2) for the assessment of the ready biodegradability of thiourea is available. In this study conducted by Hanstveit (1990) the biodegradability of thiourea was determined by a method derived from the OECD guideline 301C (ready biodegradability: modified MITI test), using oxygen consumption as test parameter. Thiourea was tested at concentrations of 10 and 30 mg/L. This test was specifically designed to study the biodegradability of test compounds with a relatively low carbon content and a relatively high content of oxidisable nitrogen. The test was carried out using an automatic system (Sapromat) for recording oxygen consumption and involving a test medium with higher buffer capacity than specified by the guideline. In addition to the normal controls for inoculum activity and toxicity, a control on the suitability of the test substance as a nitrogen source was included. The controls showed that the activity of the inoculum was sufficient and that the test substance exerted a toxic effect to a limited degree; the toxic effect was only slight on acetate degradation itself, but strong on other processes such as nitrification. Thiourea was not degraded in this test and is therefore considered not to be readily biodegradable. However, when thiourea served as sole nitrogen source, acetate was rapidly degraded, indicating that thiourea was also degraded under these circumstances. Determination of the dissolved organic carbon (DOC) remaining in the medium at the end of the test confirmed that thiourea was not fully degraded in the biodegradation test, but did not completely confirm its use as a nitrogen source.

On the other hand, urea is readily biodegradable. The main mode of degradation is enzymatic mineralization. The degrading microorganisms are expected to be generally capable of degrading these chemical structures even in case the oxygen atom (in urea) is replaced by sulphur (in thiourea). Therefore, biodegradation of thiourea should be generally possible.

In addition to the information on the ready biodegradability of thiourea several studies determining the inherent biodegradability of the test substance are available:

Friesel et al. (1984) investigated the biodegradation of thiourea in a SCAS Test (similar to OECD 302 A) in two test series for 84 and 29 days, respectively. The method involves exposure of the chemical to relatively high concentrations of micro-organisms over a long time period. The viability of the micro-organisms is maintained over this period by daily addition of a settled sewage feed. The conditions provided by the test are highly favourable to the selection and/or adaptation of micro-organisms capable of degrading the test compound. Thiourea was applied at 20 mg/L DOC in the test vessels. The test system consisted of 6 SCAS units (test vessels). Two vessels were used for the test substance, two were used for the blank controls, and another two vessels were used for the reference substances diethylene glycol and p-chloroaniline that were used in the first and second test series, respectively. Under the conditions of the SCAS test thiourea was biodegradable by 90-100 % after an adaption phase of six weeks (first test series). In the second test series 70-85 % biodegradation could be observed after 29 days (possibility of inoculation of activated sludge with adapted micro-organisms via ambient air).

Broecker et al. (1984) also determined the inherent biodegradability of thiourea in a test according to OECD 302A (SCAS Test). The substance (initial concentration: 13 mg DOC/L) was exposed to activated sludge of a municipal sewage treatment plant under aerobic conditions. Diethylene glycol served as positive control. Based on the data of DOC determinations 96 ± 20 % biodegradation of thiourea was observed (test duration not reported). Based on these results for thiourea that were obtained in the SCAS test, the substance is considered to be inherently biodegradable under the conditions of this test method.

It should be noted that this test method provides a high potential for adaption of the micro-organisms to the test substance. Therefore, this test could potentially overestimate biological degradation.

In contrast to the above results, NITE (2012) cites results from a test on the inherent biodegradability of thiourea according to OECD guideline 302C (MITI-II). The substance was incubated at an initial concentration of 30 ppm with activated sludge (100 ppm) for two weeks. Biodegradation of the test substance was followed by indirect measurement via determination of the biological oxygen demand (BOD) as well as by direct measurement of the test substance concentration (TOC, HPLC, and UV-VIS). After two weeks the following results were obtained:

- BOD: 2.6 % degradation

- TOC: 7.1 % degradation

- HPLC: 10.0 % degradation

- UV-VIS: 13.5 % degradation

Therefore, thiourea was assessed to be non-biodegradable based on these screening-level tests.

In addition to the tests mentioned above, Rott et al. (1982) studied the biodegradation of thiourea in the GSF-test using 14C-labelled test substance at an initial concentration of 50 µg/L. Activated sludge from laboratory culture at a concentration of 1 g solids/L served as inoculum. Under the conditions of the GSF-test thiourea degraded by 17 % in 5 days.

Conclusion:

The screening test results suggest that thiourea should not be considered as readily biodegradable.

According to ECHA Guideline R.7b, there are three tests that can be used to determine the inherent biodegradability of organic: Modified SCAS test (OECD 302 A), Zahn-Wellens/EMPA test (OECD 302 B) and modified MITI test (II) (OECD 302 C). The results for biodegradation of thiourea obtained in the two available SCAS tests (Friesel et al., 1984 and Broecker et al., 1984) were above 70 % of theoretical (DOC removal). Therefore, thiourea may be regarded as inherently and ultimately biodegradable under the conditions of the SCAS test.

In cases were DOC removal is used care must be taken to ensure that elimination did not occur through adsorption. No adsorption controls were included in both these experiments, but as the substance has a log Kow of -0.92, is non-ionizing, has a low (experimental) Koc value of 36.49 for adsorption to soil (Korte & Freitag, 1984), and a Kd value of 90 for adsorption to sludge (Freitag et al., 1985), adsorption is expected to be of limited influence on the results in the SCAS Test.

However, 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 thoiurea 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