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Environmental fate & pathways

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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. MBT is essentially non-volatile with the estimated Henry's law constant lower than 3.6*10-6 hPa m3/mol at 25°C. All the described metabolites of MBT have a HLC lower than 1 Pa m3/mol and hence are considered as non-volatile. MBT has a half-life in air of 9.5 h estimated by AOPWIN v1.92 with a rate constant of 7.95 *10-11cm3/(molecule*sec), considering an OH-concentration of 500,000 radicals/cm³. The estimated half-life in air of MBT is much shorter than 24 hours and hence no potential for long-range transport of MBT in air is expected. The distribution of MBT 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 MBT is water with 95.96 %, followed by soil with 2.00 % and sediment 2.02 %.