thiourea; thiocarbamide

PBT assessment

PBT assessment: overall result

Reference

Name:

62-56-6_Thiourea

Type of composition:

legal entity composition of the substance

State / form:

solid: particulate/powder

Related composition:

62-56-6_Thiourea

Reference substance:

62-56-6_Thiourea

PBT status:

the substance is not PBT / vPvB

Justification:

Persistence

In screening level tests (OECD test guideline 301) thiourea could not be demonstrated to be readily biodegradable. However, additional data from a three year project investigating the biological degradation of various xenobiotics in natural water and sediment samples are available. During this project the microbial degradability of thiourea at low, environmentally relevant concentrations (1 µg/L radiolabelled 14C-thiourea) in samples taken from the Elbe estuary 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 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).

 

Degradation half-lives (DT50) based on mineralization to CO2 were derived from the degradation graphs reported in the study. The DT50 in marine water and sediment samples taken in the North Sea ranged between 20-39 days and 17-28 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, which were incubated at 20 °C. The hazard assessment is based on the means of the DT50 values reported above (marine water: DT50 = 29.5 d; marine sediment: DT50 = 22.5 d).

 

Results from freshwater and freshwater sediment samples taken at Hamburg-Teufelsbrück and Cuxhaven did not reach 50 % mineralization during the total incubation time of up to 85 days. However, the experiment demonstrates that thiourea serves as nitrogen (N) source for degrading microorganisms. Thiourea is more easily biodegraded in N-limited environments:

1) Elbe estuary

Thiourea biodegradation is more extensive and faster in habitats with lower nitrogen (N) content. Therefore higher thiourea biodegradation can be found in the saltwater samples (e.g. annual average of the total N-content at the area “Großer Vogelsand/Scharhörn: 1.6 mg/L, Elbe I and Helgoland: < 0.5 mg/L) from the Elbe estuary compared to its highly eutrophic freshwater parts (e.g. annual average of the total N-content at Hamburg-Teufelsbrück, 1988: 6.2 mg/L). Thiourea is used as N source. Its mineralization proceeds slowly and continuously and takes place parallel to the degradation of other carbon (C) sources.

 

The same relationship could be observed for the sediment samples: Thiourea biodegradation increases with increasing distance from the coast.

 

2) Baltic Sea

The biodegradation of thiourea strongly differed between different sampling points: In the surface water samples from the Trave estuary (A, 1m) thiourea was completely and rapidly mineralised. Presumably the test substance was consumed by bacteria that otherwise degrade urea. These bacteria exist predominantly in wastewater and are regarded as tracers for water pollution, as urea is rated to be an anthropogenic pollutant. Thiourea competitively inhibits the absorption of urea, thus the same transport mechanism is used.

 

In the water samples that were taken close to the sediment layer from the Trave estuary (A, sw) biodegradation of thiourea was much slower compared to the water sample A, 1m. This may either be attributed to a reduced number of degrading bacteria that could immediately make use of thiourea (however, after 40 days thiourea biodegradation increased), or to the higher amount of N-sources in the water layer close to the sediment.

 

In the water sample from the Lübeck bay (B, 1m) thiourea was almost as rapidly used as in the underlying sediment sample (B, sediment). However, the extent of mineralisation was much higher in the water sample due to a lower N-content.

 

In the sediment sample B only 24 % of thiourea were mineralised, but only 41 % of thiourea could be found in the water phase above the sediment layer. This indicates high adsorption of 35 % in the silty sediment.

 

In sediment sample C, 52 % of thiourea were mineralised, with a maximum of 13 % adsorbed in the sediment, as 35 % of the test substance could be found in the water phase.

 

At station D (Oderbank) which is influenced by the river Oder 72 % mineralization of thiourea could be measured in the respective sediment sample (D, sediment). Unfortunately, no water sample could be analysed for thiourea biodegradation. Sediment sample D showed almost exactly the same biodegradation behaviour as water sample A, 1m. Analysis of the water layer above the Oderbank sediment revealed high nutrient contents. However, in this case the high NH4-N content did not inhibit thiourea mineralisation.

 

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.

 

Thus it could be demonstrated that thiourea can be degraded under appropriate environmental conditions and the substance shall therefore not be considered persistent, as:

1. Biodegradation in marine environments is usually slower compared to freshwater habitats, which is also the reason for less strict P-criteria in marine water and sediment as compared to freshwater;


2. REACH does not require registrants to investigate biodegradation in simulation tests in all environemntal compartments; this is normally restricted to the freshwater compartment and soil. For thiourea a study is available that does even surpass the requirements of REACH as investigations in seawater samples (water phase and sediment) have been conducted. Within the PBT assessment of a substance it has to be demonstrated whether a substance is generally biodegradable in the environment or not. Thiourea biodegradation can be clearly demonstrated by the results of the Rheinheimer study for the marine compartment. TFurther, in a GLP-compliant aerobic transformation test according to OECD 307 of Voelkel (2013) the degradation half-life of the substance was determined to be 0.6 days (13.6 hours, geometric mean, at 20 °C). 


3. The structural analogue urea is readily biodegradaed in the environment. The degrading microorganisms are expected to be generally capable of degrading these chemical structures even in the case that the oxygen atom (urea) is replaced by sulfur (thiourea).

 

In conclusion, thiourea cannot be considered to be a P or vP substance.

 

Bioaccumulation

Thiourea does not have a significant potential for bioaccumulation as the measured log Kow of the substance is -0.92. This is further supported by two studies:

 

- Geyer et al. (1985) exposed algae (Chlorella) to 14C-labelled thiourea for at least 24 h at room temperature (20-25 °C). The subsequently determined bioconcentration factor for the alga Chlorella was determined to be 54.

- Freitag et al. (1985) experimentally determined the bioconcentration factor of thiourea to be < 10 in the fish Leuciscus idus melanotus after 3 days of exposure.

Thus it can be concluded that thiourea has neither B nor vB properties.

 

Toxicity

Thiourea is legally classified as Carc. 2 and Repr. 2 according to EC No 1272/2008. Thus thiourea is considered to fulfil the T criteria.

 

Conclusion

Thiourea is neither persistent nor bioaccumulating, hence not a PBT or vPvB substance.