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Environmental fate & pathways

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There are very limited data available on the environmental fate and behavior of ZMBT. ZMBT is an organic complex, in which MBT structure is present. The dissolution of MBT determines the hazard profile of ZMBT aqueous solution; and hence the environmental fate and behavior of MBT is used as read-across approach to support the risk assessment of ZMBT.

MBT is not readily biodegradable with a biodegradability of 2.5% observed in 14 days and also not inherently biodegradable with only 2% biodegradability observed during 35 days exposure to adapted bacterial inoculum. On the other hand MBT is also hydrolytically stable at pH 7 and ambient temperature, but it is abiotically unstable in aquatic environment in presence of atmospheric oxygen with 60% conversion to MBTS within 2 hours at pH 6.5 and in presence of light with a direct photolysis half-life in water of ca. 30 mins at 32-40°C. The main abiotic degradation products of MBT and its described metabolites (e. g. MBTS) are BT (95-16-9) and its further product as BTon (934-34-9). In the appendix A of EU-risk assessment for CBS (95-33-0), BT and BTon are reported as persistent under environmental conditions. Another described metabolite from MBT is MeSBT (615-22-5), which is also reported in EU-risk assessment as resistant both to biodegradation and photolysis. MBT is not bioaccumulative with a measured BCF of 8 at 0.01 mg/l determined on Cyprinus carpio in a flow-through system. The described metabolites of MBT are also not bioaccumulative. Simultaneously, MBT has moderate adsorption potential in soil/sediment with the adsorption coefficient (log Koc) of 2.51 – 3.55. The adsorption coefficients of the described metabolites of MBT distribute also in the same range as the one of MBT. Considering the lower water solubility of ZMBT in comparison to MBT but higher Log Pow, the adsorption potential of ZMBT is expected to be higher than MBT. However the lower log Koc of 2.51 is used to estimate PNECs for sediment and soil compartment as a worst case.

ZMBT is essentially not volatile with the Henry's law constant H lower than 1.53*10-12 Pa * m³/mole at 25°C from aquatic media according to the criteria of Thomas (1990). ZMBT has a half-life in air of 4.3 h estimated by AOPWIN v1.92 with a rate constant of 9.03 *10-11 cm3/(molecule*sec), considering an OH-concentration of 500,000 radicals/cm³. The estimated half-life in air of ZMBT is much shorter than 24 hours; and hence no potential for long-range transport of ZMBT in air is expected.