1,3-dihydro-4(or 5)-methyl-2H-benzimidazole-2-thione, zinc salt

PBT assessment

PBT assessment: overall result

PBT status:

the substance is not PBT / vPvB

Justification:

Persistence:

An OECD 309 simulation study in freshwater with suspended sediment was conducted with the substance ZMB2 (Section 5.2.2). It was concluded in the study that ZMB2 dissociated upon contact with the surface water to form MB2 as no detectable residues of ZMB2 were observed at any of the sampling intervals, including 0 h, throughout the study. It can be concluded that ZMB2 would not be persistent in surface water and does not fulfill the criteria of a persistent (P) substance according to ECHA Guidance Chapter R11 - PBT Assessment (i.e. the DT50 in freshwater was concluded to be ≤40 days). For the primary degradation product, MB2, which was observed at concentrations exceeding 10 % of applied radioactivity, the increase and subsequent decrease of residue concentrations in the test system were plotted using a single first order (SFO) kinetic model. The DT50 of MB2 was estimated to be 17.8 days and therefore does not fulfill the criteria of a persistent (P) substance according to ECHA Guidance Chapter R11 - PBT Assessment (i.e. DT50 in freshwater was concluded to be ≤40 days). Four other major transformation products (i.e. concentrations of residues were observed at ≥10 %AR through the incubation period); desthio-MB2, MB2 -sulfonic acid, MB2 -acetic acid and hydroxy-MB2 were identified by LC/MS analysis. Reliable degradation kinetics could not be estimated for these three substances as residue concentrations did not clearly decline by the end of the test incubation.

Bioaccumulation:

According to ECHA Guidance Chapter R11 – PBT Assessment, a substance fulfils the screening criteria for bioaccumulation (B) when the Log Pow is >4.5. ZMB2 was determined to have a Log Pow in the range of 2.5 - 3.07. ZMB2, therefore, is not considered as bioaccumlating (B).

A bioaccumulation study on ZMB2 is not available at the time of submission. According to 1.1.2 of Annex XIII of Regulation EC No. 1907/2006, a substance fulfils the bioaccumulation criterion (B) when the BCF in aquatic species is higher than 2,000.

Using EPIWIN v4.10, a Quantitative structure–activity relationship (QSAR),modellingtool, the BCF of ZMB2 using default values is calculated as 38.28.

In addition, data from the close structural analog, MB2 (CAS number 53988-10-6) are used to address the endpoint. Using EPIWIN v4.10,modelling tool, the BCF of MB2 using default values is calculated as 1.017.

Based upon the ZMB2 data and MB2 read across data, ZMB2 is considered to have a BCF of <2,000 and does not fulfil the criteria of B.

in Silico profiling of the three major transformation products; desthio-MB2, MB2-sulfonic acid, MB2-acetic isomers and hydroxy-MB2 identified in the OECD 309 simulation study (Section 5.2.2) and their structural analogues was conducted. A number of physical chemical and environmental fate properties based on (Q)SAR predictions and experimental data were compared and contrasted between the target substances and their respective source substances. Endpoint data relating to the bioaccumulation potential of the substances was investigated thoroughly. For each substance, the log Kow, a key indicator endpoint for bioaccumulation potential, was predicted to be < 2.1. Further, BCF values for each of the major transformation products was predicted to be <2000 L/kg wet wt. It was concluded that desthio-MB2, MB2-sulfonic acid, MB2-acetic acid and hydroxy-MB2 did not meet the criteria for bioaccumulation (B) according to ECHA Guidance R11 - PBT Assessment.

Toxicity:

No chronic aquatic invertebrate data are available on the substance ZMB2. Data from the close structural analog, MB2 (CAS number 53988-10-6) are used to address the endpoint. Based on analysis of the structural similarities between MB2 and ZMB2, ZMB2 is likely to exhibit similar chronic toxicity to aquatic invertebrates. The MB2 21 day NOEC (reproduction) (Daphnia magna) is 0.0346 mg/L (measured) (Currenta, 2012). This same value is considered representative of ZMB2 chronic aquatic toxicity.

Based upon the above, ZMB2 is considered to not be toxic to the environment. Based on results from an oral (dietary) Combined Repeated Dose Toxicity Study with the Reproduction / Developmental Toxicity Screening Test (OECD TG 422) in Sprague-Dawley rats (Safepharm, 2002), ZMB2 is classified as toxic for reproduction Category 2 with a specific target organ toxicity after repeat exposure (STOT-RE) of Category 2 according to Regulation EC No 1272/2008.

Based on the above, ZMB2 is considered to be Toxic (T) in accordance with 1.1.3 of Annex XIII of Regulation EC No 1907/2006.

Likely routes of exposure:

The primary route of ZMB2 release will be to wastewater.

ZMB2 may be released to air through use processes, however, the vapour pressure is 1.87E-011 Pa at 25 ºC and therefore air exposure is expected to be negligible. 

Release to soil may occur from spreading of STP sludge’s, however this is advised against.