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The ready biodegradability of ETU (Imidazolidine-2 -thione) was evaluated in a study performed in accordance with OECD testing guideline 301 F and GLP requirements. The maximum level of biodegradation was 0 % in 28 days. The lag phase was not determined because no degradation was observed. Therefore, according to these results, ETU is considered as not readily biodegradable.

As ETU is not B (log Kow < 3), no further testing on biodegradation of ETU is needed in the framework of the PBT/vPvB assessment. ETU may degrade in soil and water, but its metabolites are not PBT/vPvB and are not dangerous for the environment according to CLP criteria (EC 1272/2008). Further biodegradation studies in the framework of the PBT/vPvB assessment are therefore not required. Under the assumption of ETU being not biodegradable (worst-case scenario), the chemical safety assessment indicate no risk for any compartment (water, sediment or soil). No further biodegradation testing is therefore required in the framework of the CSA neither, in compliance with annex IX, section 9.2, column 2 of the REACH regulation EC 1907/2006. The rationale is described in the position paper reproduced below and also attached as a copy in the "biodegradation in water and sediment" section.

POSITION PAPER ON THE DEGRADATION AND BIODEGRADATION OF ETHYLENETHIOUREA (CAS 96-45-7) IN SOIL AND SURFACE WATER

I - Introduction

Ethylenethiourea (ETU) is a non-readily biodegradable substance according to the OECD 301F study data available in the dossier. It is also not expected to bioaccumulate in animals (log Kow < 3). Therefore, being not B, ETU is not considered as PBT or vPvB under both REACh EC 1907/2006 and CLP EC 1272/2008 regulations. In addition, adequate toxicity and environmental data are available for ETU according to CLP regulation. ETU is not classified for any acute or chronic environmental hazard. The degradation and biodegradation of ETU in soils and water has been investigated in several studies. This position paper synthesizes the results and highlights the fact than when ETU degrades, it is partially converted into CO2 and three main metabolites in both soil and water, of which none is classified as dangerous for the environment according to CLP regulation and none should be considered as PBT or vPvB.

II - Degradation and biodegradation in soils

Several studies on biodegradation of ETU in soil using 14C radiolabelled molecules are available. (Rhodes, 1977) investigated [14C]ETU biodegradation on the field, using stainless steel cylindrical tubes driven into the ground to isolate undisturbed columns. The radiolabelling was done on both carbons of the ethyl group. The application was around 2.24 mg/dm². Soil [14C] radioactivity and [14C]ETU content were analysed after 1, 4, 12 and 52 weeks exposure. The analytical procedure indicated good recovery of [14C]ETU in spiked samples (>90%). ETU metabolites were identified by mass spectrometry and comparison of mass and infrared spectra with reference compounds.

The half-life of ETU was less than one week, indicating that ETU is rapidly degradable in soil. In addition, only 58% of the14C initially introduced into the soil was recovered after one week, suggesting ultimate biodegradation of the other 42%, as observed in another study on ETU biodegradation (Kaufman and Fletcher, 1973). 79% of the 14C remaining in the soil after one week was identified as ethyleneurea, also known as 2-imidazolidone (CAS 120-93-4). The rest of the extracted radioactivity (i.e 12% of the initial 14C radioactivity) was qualified as “polar material” but not further identified.

Johannesen et al. (1996) tested the ability of a crop field soil extracted at various depth (surface, 60 cm and 100 cm depth) on [14C]ETU ultimate biodegradation (70 µg/kg soil). Biodegradation seemed depth-dependent.14CO2represented 20% and 41% of the initial 14C initially introduced in the surface soil after 24 hours and 28 days respectively at 21°C. This is in accordance with the results of Kaufmann and Fletcher (1973) and Fomsgaard and Kristensen (1999) who found similar values of ultimate biodegradation in soil (43% and 40 -60% respectively) within short and long incubation periods (4 and 60 days respectively). 14CO2 represented 23% and 17% of the initial 14C initially introduced after 109 days at 10°C in soil sampled at 60 and 100 cm depth respectively. Johannesen et al. (1996) did not look for ETU metabolites.

 

III - Degradation and biodegradation in water

ETU is very stable in pure water and is not hydrolysable (Cruickshank and Jarrow, 1973). However, some studies indicate that it is sensitive to UV exposure, especially in the presence of photosensitizer.

Rhodes (1977) exposed aqueous solutions of 100 ppm [14C]ETU to mercury vapour UV lamps for 8 hours in the absence or the presence of a photosensitizer (acetone 0.1M). The degradation products were identified as hydantoin (CAS 461-72-3), Jaffe’s base (CAS 484-92-4), glycin (56-40-6) and ethyleneurea. The degradation pattern was similar in both treatments (Table 1). Jaffe’s base formation resulted from the dimerization of ETU with ejection of one sulphur atom and the formation of a carbon-nitrogen bond. Dimerization may result from the high concentration tested in the experiment (100 mg/L) and may not happen in the wild where concentrations are very low. This concentration is indeed not environmentally relevant as five hundred times lower concentrations would be expected in water in worst cases (see CSA). Such low concentrations should not lead to much dimerization because two ETU molecules have much smaller chance to encounter and react with each other. Jaffe’s base is therefore not expected to be formed by ETU in significant amounts in the natural environment.

In the absence of photosensitizer, Ross and Crosby (1973) failed at observing ETU degradation under natural sunlight (California) or UV light after 24 hours. When adding photosensitizers (acetone, rhodamin or riboflavin) in significant amount (several milligrams per liter), ETU disappearance occurred in less than 4 hours. In accordance with Rhodes (1977), ethyleneurea and glycin were identified as the main metabolites. The presence of hydantoin was also suspected, but thought to rapidly turn into ethyleneurea. The experiment was very short and nothing is known about long-term photodegradation of ETU under natural sunlight without any sensitizer. Both experiments also identified sulphate ions as the major transformation product of the S atom of ETU. Sulphates are naturally present in the environment and are not considered as PBT/vPvB or dangerous for the environment according to CLP criteria. Apart from a unique ready biodegradability test, there is little information on ETU biodegradation in water. However, if ETU degrades, there is strong evidence that the main metabolites would be among hydantoin, ethyleneurea and glycine. These metabolites are readily biodegradable and do not present any hazard for the environment according to CLP criteria.

 

 

% of total 14C

Compound

No photosensitizer

Acetone (0.1 M)

Hydantoin

9.3

24.4

Ethyleneurea

13.9

7.3

Jaffe’s base

10.9

16.5

Glycine

63

49.4

Table 1: Percentage of metabolites formed after exposure of aqueous [14C]ETU to UV for 8 hours in presence or absence of photosensitizer, after(1977).

IV – Metabolite hazard identification

The main expected metabolites of ETU degradation in soil and/or water are glycine, ethyleneurea, hydantoin and sulphate (Table 2).

 

Chemical name

Ethylenethiourea

Ethyleneurea

Glycin

Hydantoin

5,5-dimethylhydantoin

Sulphate

CAS number

96-45-7

120-93-4

56-40-6

461-72-3

77-71-4

7757-82-6

Skeletal formula

 

Ready biodegradability

No

Yes

Yes

Yes

Yes

n.a.

Log Kow

-0.67

-1.16

-3.21

-1.69

-0.48

n.a.

Bioaccumulation

Not expected

Not expected

Not expected

Not expected

Not expected

No

CLP classification

None

None

None

None*

None

None

 

Table 2: Information about ready biodegradability, bioaccumulation potential and CLP classification of ethylenethiourea and its main metabolites. * hydantoin ecotoxicity characteristics are read-acrossed from 5,5-dimethylhydantoin. Sulphate characteristics are taken from the sodium sulphate disseminated dossier.

IV.1 - Glycine

Glycine was registered to ECHA in the framework of the REACh regulation EC 1907/2006. Its disseminated dossier indicates that it is readily biodegradable, not expected to bioaccumulate (log Kow < 3) and not classified for the environment according to CLP regulation EC 1272/2008. Therefore, the degradation product glycine is not considered as PBT/vPvB and is not considered as dangerous for the environment (it is in fact an amino acid naturally found in proteins of the living kingdom).

IV.2 - Ethyleneurea

Ethyleneurea was registered to ECHA in the framework of the REACh regulation EC 1907/2006. Its disseminated dossier indicates that it is readily biodegradable, not expected to bioaccumulate (log Kow < 3) and not classified for the environment according to CLP regulation EC 1272/2008. Therefore, the degradation product ethyleneurea is not considered as PBT/vPvB and is not considered as dangerous for the environment.

IV.3 - Hydantoin

Hydantoin is readily biodegradable according to an OECD 301C MITI test (data available on the JCHECK website, see the reference section for further information). Hydantoin is not expected to bioaccumulate (log Kow < 3 ; Hansch et al., 1995). Information on hydantoin toxicity for the environment is scarce. Nevertheless, information is available on the dimethylated form of hydantoin (5,5-dimethylhydantoin), registered under the REACh regulation (CAS 77-71-4). Its disseminated dossier indicates that it is readily biodegradable, not expected to bioaccumulate (log Kow < 3). The partition coefficients of both substances are close (Table 2). 5,5-dimethylhydantoin is not classified for the environment according to CLP regulation EC 1272/2008. Therefore, the degradation product hydantoin should not be considered as PBT/vPvB and should not be considered as dangerous for the environment.

IV.4 – Sulphate

Sodium sulphate was registered to ECHA in the framework of the REACh regulation EC 1907/2006. Sodium sulphate is inorganic. It naturally occurs in the wild. Its disseminated dossier indicates that it is not bioaccumulative, not considered as PBT/vPvB and not classified in any hazard category for the environment according to CLP regulation EC 1272/2008.

V - Conclusions

Rapid and complete degradation of ETU can be observed in soils (, 1977). In addition, partial mineralization is also observed (Rhodes, 1977 ; Johannesen et al., 1996 ; Fomsgaard and Kristensen, 1999). In water, hydrolysis does not occur, but photolysis gives the same main metabolites as in soil. The main metabolites of ETU in soil and water are glycine, ethyleneurea and hydantoin. These substances (or analogue) all have sufficient and adequate data for hazard identification. These substances are not considered as PBT or vPvB or classified in any hazard category according to CLP regulation. In addition, they are all readily biodegradable. It is therefore unlikely that they may cause any harm to the environment. Regarding PBT/vPvB classification, ETU is not expected to bioaccumulate (log Kow < 3). ETU is therefore not considered as PBT or vPvB. In the absence of any study on degradation of ETU in water, it is yet difficult to estimate the persistency of ETU in this compartment. However, it can be concluded from this paper that if ETU were degradable in water, the main metabolites poses negligible risks to the environment. Moreover, even if considering that ETU fulfils persistency criterion in water, ETU does not fulfil PBT or vPvB criteria, being not B. In addition, the environmental risk assessment does not indicate unacceptable risk for water or soil compartments when considering ETU as persistent in a worst case (RCR = 0.6 for freshwater ; RCR = 0.09 for marine water ; RCR < 0.01 for soil).Therefore, following annex IX, section 9.2.1.3, column 2 of the REACH regulation EC 1907/2006, no further testing is required on biodegradation of ETU in water as the chemical safety assessment does not indicate the need to. Conducting an OECD 309 study on biodegradation of ETU in surface water is not necessary.

References

Cruickshank P.A., Jarrow H.C., 1973. Ethylenethiourea degradation. J. Agr. Food Chem. 21, 333-335.

Fomsgaard I.S., Kristensen K., 1999. ETU mineralization in soil under influence of organic carbon content, temperature, concentration, and depth. Toxicol. Environ. Chem. 70, 195-220.

Jacobsen O.S., Bossi R., 1997. Degradation of ethylenethiourea (ETU) in oxic and anoxic sandy aquifers. FEMS Microbiology Reviews 20, 539-544.

Hansch. C., Leo A., Hoekman D., 1995.  Exploring QSAR. Hydrophobic, Electronic, and Steric Constants.  ACS Professional Reference Book. Washington, DC: American Chemical Society.

Jcheck, Japan Chemicals Collaborative Knowledge database. Biodegradation in water: screening tests.http://www.safe.nite.go.jp/jcheck/template.action?ano=4945&mno=9-1026&cno=461-72-3&request_locale=en

Johannesen H., Nielsen A.B., Helweg A., Fomsgaard I.S., 1996. Degradation of [14C]ethylenethiourea in surface and subsurface soil. Sci. Total. Environ. 191, 271-276.

Kaufman D.D., Fletcher C.L., 1973. Degradation of ethylenethiourea in soil. Abstracts of the 165thMeeting, 9-13 April 1973,, Am. Chem. Soc. Pest., 1 p.

RhodesC.R., 1977. Studies with manganese [14C]ethylenebis(dithiocarbamate)([14C]Maneb) fungicide and [14C]ethylenethiourea([14C]ETU) in plants, soil, and water. J. Agr. Food Chem. 25, 528-533.

Ross R.D.,D.G., 1973. Photolysis of ethylenethiourea. J. Agr. Food Chem. 21, 335-337.