N-tert-butylbenzothiazole-2-sulphenamide

Endpoint summary

Additional information

Results from National Institute of Technology and Evaluation are used mostly as key studies for aquatic toxicity, because they are most sensitive. The most sensitive acute toxicity is to aquatic algae (Selenastrum capricornutum) tested according to OECD TG 201 "Alga, Growth Inhibition Test". After 72 hours of exposure, an EC50 of 0.071mg/L and a NOEC of 0.023 mg/L were obtained. For chronic fish, a read-across approach from the hydrolysis product 2-mercaptobenzothiazol (MBT) was used to support the assessment, and a NOEC of 0.041 mg/L after 89 days was received. For TBBS a long-term study is available for aquatic invertebrates (Daphnia magna) tested according to OECD TG 202 part 2 reproduction test. After 21 days of exposure, an EC50 and a NOEC of > 0.16 mg/L were obtained. For the assessment of microorganisms in biological treatment plants, an EC50 value of > 10000 mg/L after 3 hours exposure was observed.

 

TBBS can be rapidly degraded by hydrolysis and photolysis, the hydrolysis half-life was determined to be 7.76 – 9.53 h. It is very likely that in the aquatic toxicity tests the observed effects were a combination caused by the parent substance and the degradation products.

 

For hydrolysable substances used in aquatic ecotox tests, REACH Guidance Document R7b (2017), p. 86 states: :
“Where degradation is rapid (e.g. half-life < 1 hour), the available test data will frequently define the hazard of the degradation products since it will be these that have been tested. These data may be used to classify the parent substance in the normal way.
Where degradation is slower (e.g. half-life > 3 days), it may be possible to test the parent substance and thus generate hazard data in the normal manner using a suitable renewal regime. The subsequent degradation may then be considered in determining whether an acute or chronic hazard class should apply.
Where degradation rates fall between these two, testing of either parent and/or degradates should be considered on a case-by-case basis. “

 

TBBS hydrolyses with a DT50 of 1.8h (MITI 1996) or 9.53h (Monsanto 1984) thus falling in the category where either parent or degradation product could be tested. In the following considerations are discussed which kind of study is recommended:

1. In the case of the sulphenamide category, a study with the degradation product benzothiazole-2-thiol (MBT) is already available. This study (FELS test) has been performed in accordance similar to an accepted international guideline (OECD 210, FELS test) and has been evaluated as Klimisch 2.

2. There are acute and chronic ecotox studies with TBBS and the transformation products MBT and t-butylamine available. The results are presented in the table below (effect values are based on measured concentrations):

	TBBS (mg/L)	MBT (mg/L)	t-butylamine (mg/L)
Daphnia 48h-EC 50	1.3	0.71	136 (24h-EC 50, Calamari  et al. 1980)
Daphnia 21d-NOEC	>0.16	0.08	n/a
Fish 96h-LC50	1.38	0.73	28 (soft water, Calamari et al. 1980) 270 (hard water, Calamari et al. 1980)
Fish 14d-LC50	1.02	0.67 (8d-LC50)	n/a
Fish 89d NOEC	intended to use MBT	0.048	n/a
Algae 72h-EC50 72h-NOEC	0.071 0.023	0.5 0.066	16* (Calamari et al. 1980)

*96h-EC50 based on fluorimetric response. No data are given on growth rate and/or biomass. Though it is stated in the publication that the fluorimetric values were determined after 72 hours as well, the data are not presented. Therefore, the EC50 value for 72 hours has to be considered to be 16 mg/L.

 

Whereas for algae, TBBS appears to be more toxic, the effect values for TBBS and MBT in acute daphnia and fish exhibit comparable. This can also be seen in the chronic tests in daphnia. For the second transformation product t-butlyamine limited data is available, as the substance is only registered in the total tonnage band 10 - 100 tpa. However, in comparison the amine is much less toxic for the available endpoints. Moreover, the effect values all lie well above the water solubility of TBBS (1.74 mg/L, Duslo 2007).

 

As a conclusion the existing chronic fish test for MBT is sufficient to explain the toxicity of TBBS.

With respect to the overall aquatic toxicity of TBBS, it is found that the transformations products show either a comparable (MBT) or much lower toxicity (t-butylamine) than the registered substance. For the hazard assassment the toxicity to algae of the parent substance is identified as the lowest effect value relevant for PNEC derivation. Please refer to the Read Across Justification for further information.