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Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

Additional information

Only very limited aquatic ecotoxicity data are available with ZMBT as test substance.

Short-termToxicity to Fish:

ZMBT: LC50 geometric mean = 22.4 mg/l

MBT: In a flow-through toxicity test on rainbow trout a 96-hour LC50 of 0.73 mg/l was obtained.

Comparing these two LC50 values indicates, that the short term toxicity to fish of MBT exceeds the that of ZMBT by a factor exceeding 30.

Toxicity to microorganisms:

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.

Again, the two results for ZMBT and MBT indicate a much higher toxicity to microorganisms for MBT. Although not directly comparable (EC50 vs. EC75) a higher toxicity by a factor exceeding 300 is deemed realistic.

ZMBT is an organic complex, in which the MBT structure is present. MBT is found as impurity in ZMBT with variable percentage. The MBT impurity in the substance as manufactured as well as dissociation of ZMBT determine the hazard profile of ZMBT aqueous solution. Dissociation of ZMBT is as a minimum expected as a result of ADME (adsorption, distribution, metabolism, excretion) processes in the fish, at least as soon as ZMBT reaches the stomach.

Comparing short-term Toxicity Fish data for ZMBT and MBT with Toxicity to microorganisms data for ZMBT and MBT, the hypothesis that in fact, aquatic toxicity is driven by the impurity MBT plus dissociation of ZMBT to MBT and Zn2+, appears to be supported.

Available data on ZMBT and MBT suggest that the impurity MBT is the driver for aquatic toxicity already at concentrations of about 3%. Hence a read-across approach from MBT is used to support the risk assessment of ZMBT as a worst-case, thus effectively also covering ZMBT grades containing less than 3% MBT impurity.

A respective Read-Across Strategy according to RAAF Category 1 ((Bio)transformation to common compound(s); Property of the target substance predicted to be quantitatively equal to those of the source substance or prediction based on a worst-case approach.), RAAF Category 2 (Different compounds have qualitatively similar properties; Properties of the target substance predicted to be quantitatively equal to those of the source substance or prediction based on a worst-case approach.) or a combination of both may be applicable. An update of the ZMBT dossier at ECHA with the RAAF document covering ecotoxicity and fate will be submitted in due course.

For the purpose of the risk asssessment of ZMBT, a read-across approach from MBT is used 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.