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EC number: 200-543-5
CAS number: 62-56-6
The study of Voelkel (2013) had the highest quality of all studies, following GLP and the OECD 307 with full mass balance and metabolite assessment. The study showed the lowest DT50 values covering four different soils with various soil properties. The SFO degradation rates were calculated according to the current FOCUS guideline. Therefore, the results of the study of Voelkel (2013) are the most reliable. In the OECD Test Guideline 307 it is explicitly stated that “solvents which inhibit microbial activity, such as chloroform, dichloromethane and other halogenated solvents, should be avoided”. However, in the study of Loehr and Matthews (1992) methylene chloride (= dichloromethane) was used for the application solution of the test substances (1 % of the soil). In the study of Voelkel (2013) methanol was used as a solvent (0.01 % of the soil) in an amount two orders of magnitude lower than in the Loehr and Matthews (1992) study. Therefore, it cannot be excluded that the longer DT50 values of the study of Loehr and Matthews (1992) compared to Voelkel (2013) are attributed to the use of thesolvent methylene chloride that may have inhibited the microbial activity and thus the degradation of thiourea. In a pre-experiment, the used concentrations were determined to have no toxic effects on the microorganisms present in the soil samples. However, when applied with methylene chloride, the solvent may have reduced the degradation rate of thiourea. The longer DT50 in the study of Loehr and Matthews (1992) with higher application rates than in the study of Voelkel (2013) do not necessarily prove a concentration dependency of the degradation, but the effect of the use of methylene chloride compared to very little solvent used in the study of Voelkel (2013). Overall, the results obtained by Voelkel (2013) are most reliable, and a degradation half-life of 0.6 days (13.6 hours, geometric mean, at 20 °C) is considered relevant and conservative for the risk assessment.
In an Expert Statement the applied methods and the outcome of experimental studies on the biodegradation of thiourea published by Frederick et al. (1957), Lashen and Starkey (1970) and Loehr and Matthews (1992), as well as of an unpublished guideline study carried out by Voelkel (2013), was evaluated and discussed. The study by Voelkel (2013) showed the lowest DT50 values covering four different soils with various soil properties. Thiourea degraded very fast in all four soils with DT50 values between 7.0 and 18.2 hours. The main degradation pathway of thiourea in soil proceeded through formation of minor transient metabolites, mineralization, and formation of low amounts of bound residues. The mineralization of thiourea was extremely high in all four soils tested. After 72 hours of incubation, CO2 reached mean levels of 77.5 %, 75.0 %, 72.6 % and 71.2 % AR for soils I to IV, respectively. No other volatile products were formed. The amount of bound residues increased during incubation to mean values of 8.8 %, 13.2 %, 16.0 % and 14.1 % AR for soils I to IV, respectively, at the last sampling date. In the Expert Statement it was demonstrated that the study of Voelkel (2013) fulfils all current requirements for studies on degradation in soil. It is the only study performed under GLP and providing a full mass balance. Based on the experimental results of this study a half-life of 0.6 days (13.6 hours, geometric mean, at 20 °C) was derived as key value. Significant methodological defincies were however idientified for the three publications. Therefore, the results obtained by Voelkel (2013) are considered the most reliable. In conclusion, the degradation half-life of 0.6 days (13.6 hours, geometric mean, at 20 °C) of this study is regarded relevant and conservative for the risk assessment.
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