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Environmental fate & pathways

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Although N-cyclohexylbenzothiazole-2-sulphenamide (CBS) is not readily biodegradable with a biodegradability of 0% observed in 28 days and also not inherently biodegradable with no biodegradation observed during 35 days exposure to adatped bacterial inoculum, it is abiotically unstable in aquatic environment with a hydrolysis half-life of 12.5 hours at pH 7.0 and 25°C and a half-life of direct phototransformation in water of 26 minutes.


 


The main abiotic degradation products of CBS and its metabolites (e.g. MBT and MBTS) are BT and its further product as BTon. In the EU-Risk Assessment (2008), BT and BTon are reported as persistent under environmental conditions. Another described metabolite from CBS is MeSBT, which is also reported as resistant both to biodegradation and photolysis.


 


CBS is bioaccumulative with an estimated BCF of 924.7 L/kg based on the log Pow of 5.


 


In order to fulfill the information requirements especially for the PBT properties of CBS, the registrant proposes to perform a bioaccumulation test according to OECD guideline 305 dependent on the outcome of the assessment of the P criterion. For this reason, the Registrant has already proposed to first perform a simulation study in soil.
1. If the outcome of the preceding simulation test leads to the conclusion that CBS is P/vP, a bioaccumulation test shall be performed.
2. If the conclusion is not P/vP, a bioconcentration test shall not be performed.


 


Should establishing stable conditions for CBS not prove feasible owing to the hydrolytic behaviour of CBS, the test will be performed with a degradation product which is potentially found to be P/vP in the soil simulation test and if it fulfills the screening criterion for the B assessment.


 


According to EU-risk assessment from 2008, the described degradation products of CBS, MBT, BT, BTon, MBTS, MeSBT and MeBT are not bioaccumulative (BCF< 100). Simultaneously, CBS has a high adsorption potential in soil/sludge with the adsorption coefficient (log Koc) of 3.3332 - 4.1254. The adsorption coefficients of the described metabolites of CBS distribute also in the same range as the one of CBS.


 


CBS is essentially non-volatile with the estimated Henry's law constant lower than 3.74*10-4 hPa m3/mol at ambient temperature (e. g. 21 -25°C). Most of the described metabolites of CBS, such as MBT, MBTS, BTon, BT, and MeSBT are also non-volative, but only MeBT has an estimated HLC of 2.75 Pa m3/mol, which is higher than 1 Pa m3/mol and hence considered as slightly volatile. CBS has a half-life in air of 0.202 days (4.8 hours) estimated by AOPWIN v1.93 with a rate constant of 79.4902*10-12 cm3/(molecule-sec), considering an OH-concentration of 500,000 radicals/cm³. The estimated half-life in air of CBS is much shorter than 48 hours and hence no potential for long-range transport of CBS in air is expected.


 


The distribution of CBS in a "unit world" was calculated according to the Mackay fugacity model level I based on the physico-chemical properties. The main target compartment for CBS is soil of 46.6 % and sediment of 47.1%, followed by water of 5.89 %.