Registration Dossier

Ecotoxicological information

Endpoint summary

Administrative data

Description of key information

Additional information

SMBT is an organic salt of strong base NaOH and weak acid of MBT and it is solely marketed as aqueous solution. The presence of MBT determines the hazard profile of SMBT aqueous solution; and hence the read-across approach is used to support the risk assessment of SMBT. Only one valid ecotoxicity data is available with aqueous SMBT as test substance; and this effect concentration (48h-EC50) of pure SMBT is 9.5 mg/l to daphnia. It is much higher than the one of MBT (0.71 mg/l); and hence the effect concentration of MBT is taken as a worst case consideration for the risk assessment of SMBT.

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 SMBT aqueous solution (44.6%) to activated sludge was studied and effect concentration was reported as 3h-EC50 of 857 mg/l. To exclude the dilution effect of water, the effect concentration of 382 mg/l should be used. 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 SMBT; and hence it is used as a worst case consideration for the further risk assessment of SMBT.