N,N-dicyclohexylbenzothiazole-2-sulphenamide

Biodegradation in water and sediment: simulation tests

Description of key information

A sediment simulation study is waived based on the fact that DCBS has been found to

be vP in a soil simulation test. Therefore no reason exists to further investigate the biodegradation behaviour.

Additional information

Currenta 2013b:

An inherent biodegradation study with model substrates for sediment (10 mg/L DCBS, adsorbed on particles) demonstrates limited mineralisation with a half-life of DCBS of about 60 days. One test vessel was eventually contaminated with NaOH solution. Therefore, this vessel was omitted from the final evaluation which was finally based on the three remaining vessels. A mean degradation rate of 3.3 % at day 28 and 31% at day 60 was achieved.

In their response to this study, the evaluating member state Germany outlined: The inherent biodegradability tests

(Currenta 2013b, Currenta 2014b) show experimental shortcomings. One test vessel with test substance in both

studies has undesignedly and extremely higher pH value (both > pH 9.4) than all the other

test vessels with test substance and those with blank tests (all < pH 7.1). These two test

vessels show extremly high biochemical oxygen demand (BOD) values, which wrongly

indicate a biodegradation of the test substance beyond 100%. This influences the calculated

mean value, erroneously. Following the guideline, these two test vessels have to be

removed from any further assessment and conclusion. The mean value of the degradation

(%) of the remaining three test vessels for both studies was calculated following the

procedure of calculation by the author of the studies. The mean degradation reached after

28 days is 3.3 °h for Currenta 2013c and 28.7 % for Currenta 2013a. It can be therefore

concluded that DCBS is NOT inherently biodegradable.

Currenta 2014b:

In a second test inherent test with a low concentration of DCBS (starting concentration 50 µg/L) adsorbed on silical gel particles, the parent compound and 6 metabolites were analysed and quantified. Within the first 14 days, DCBS is degraded from 13.3 µg/L to 2.2 µg/L reflecting a degradation of about 80%. thus meeting the requirement to be inherently degradable.

Even short after start at day 0, MBT was found as the major metabolite.The other metabolites MeSBT, BT, BTon, BTSO3H and DCHA were found as intermediates. Finally after 61 days DCBS and all metabolites are almost completly degraded with concentration the region of 0.1 to 2 µg/L. This test is regarded to be invalid as the analytical method appears not stable enough and the recoveries and the mass balance are not satisfactory.

In their response to this study the evaluating member state Germany outlined: Two other screening tests on inherent biodegradability (Currenta, 2013b, Currenta 2015b) are not reliable

because purity was unclear, the mass balance at the start of the experiment is not

confirmed, and pH values are partly implausible or not reported. Nevertheless, MBT (CAS

149-30-4) and Dicyclohexylamine (CAS 101-83-7) were identified as primary degradation

products suggesting that hydrolysis, already before the start of the experiment, is the

dominant degradation process in these tests and not biological degradation.

UBA 2014a/b:

The study has been performed by well-known laboratories and organisations (ECT laboratories, Flörsheim (Germany); Fraunhofer Society Schmallenberg (Germany) supervised by Umweltbundesamt Dessau (Germany) and evaluated by an external expert J. Ranke, Grenzach-Wyhlen (Germany) The study contains a water-sedement test according to OECD 308 performed with two sediment types, however some deficiencies were detected. Deficiencies: 1.The study was not performed under GLP. Further, the study director passed away during the test, and the report was finalised by successors. Therefore some relevant information is missing. For example, details of spiking are not described. 2. The purity and identity of the radiolabelled material was not proven in the solution used for spiking. It was thought by the external expert J. Ranke that the material could perhaps be disintergrated. Evaluation of the Registrant however does not support this thesis. Just the Currenta 2014 study also showed that an intact DCBS substance rapidly disintegrated after spiking the water phase. Thus unexpected low concentrations of the parent compound were found even at day 0 but mayer concentrations of metabolites or hydrolysis products, e.g. MBT. 3. Just in the first days of the test, the recovery of the radioactivity was questionable. It seems that radioactivity was lost during processing of the samples. Although certain question stay even after evaluation of an external expert (J. Ranke), the degradation behaviour of DCBS can be characterised. One test yiels a half-life time with an acceptable standard deviation below chi2 = 15%. The second test has a somewhat higher standard deviation (chi2 = 22%) but half-lives can be calculated using different kinetic models. As a result of these supporting information it can be held for true: 1. DCBS rapidly disintegrates after spiking the water phase 2. Intact DCBS tends to adsorb to sediment with time 3. DCBS in sediment degraded either by adsorption loss or by mineralisation 4. After day 14 NERs and mineralisation are observed in the range of 20 to 30% after 300 days. 5. Several attempts to describe a kinetic model have been applied. 6. For the entire system, half- lives obtained with different models are in the range of 65 to 111 days at 12°C 7. As a conclusion, methodological shortcomings do not allow the study to be used for the assessment of the persistency. As the identity and concentration of the test substance remains unclear, the study is finally considered to be invalid.