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Ecotoxicological information

Endpoint summary

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Only very limited aquatic ecotoxicity data are available with ZMBT as test substance. ZMBT is an organic complex, in which MBT structure is present. Meanwhile MBT is often found as impurity in ZMBT with variable percentage. The dissolution of MBT determines the hazard profile of ZMBT aqueous solution; and hence a read-across approach from MBT is used to support the risk assessment of ZMBT as a worst-case.

The most sensitive acute toxicity of MBT is to aquatic algae (Selenastrum capricornutum) tested according to OECD TG 201 "Alga, Growth Inhibition Test". After 72 hours of exposure, an ECr50 of 0.5mg/L was obtained (MITI, 1999). Besides the results from acute tests, the ecotoxicity of MBT was also determined in long-term tests to three trophic levels. Anembryo-larval test to Oncorhynchus mykiss carried out in a flow-through system resulted in a NOEC of 0.041 mg/l (CMA, 1989), which is the most sensitive one in three trophic levels and hence used for further risk assessment. The described metabolites of MBT are less toxic to aquatic organisms than MBT itself.

The toxicity of ZMBT to activated sludge was studied and effect concentration was reported as 3h-EC50 of 1220 mg/l. For the assessment of microorganisms in biological treatment plants, Tomlinson (1966) studied the inhibition of MBT on the first nitrification step (oxidation of NH4 to NO2) and obtained after 2-4 h exposure an EC75 value of 3mg/l for non-adapted sludge. The effect concentration of MBT is lower than the one of ZMBT; and hence it is used as a worst case for the further risk assessment of ZMBT.