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Environmental fate & pathways

Biodegradation in water: screening tests

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In a MITI test according to EU method C. 4 the ready biodegradation of CBS was investigated (Bayer, 1989). After 28 days 0 % degradation of the test substance was determined and hence CBS under test conditions is not biodegradable. Even with adapted bacterial inoculum CBS was observed to be 0% biodegradable during 35 days incubation (Monsanto, 1979). In the EU-risk assessment, information regarding the biodegardabilities of the main metabolites of CBS, such as MBT, MBTS, BT, BTon, MeSBT and MeBT, is also summarized and discussed. The described metabolites are also not readily biodegradable. Only when the inoculi were adapted under special conditions, certain level of degradation of MBT, BT, BTon, and MeSBT was observed. Such processes are expected in biological treatment plant, but not in the environment.

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The concentrations (100 mg/l) used in the study of Bayer (1989) are much higher than the water solubility of CBS (i.e. 0.32 mg/l). Although CBS can be rather rapidly hydrolyzed with a half-life of 12.5 hours, the amount of CBS used was sufficient to maintain CBS at saturation level in aqueous phase. As the main hydrolysis product, benzothiazole (BT) has a toxicity to microorganisms (3h-EC50) of 650 mg/l (Yo-shioka et al., 1986). The original concentrations from CBS are not high enough to reach the critical level of BT's toxicity to microorganisms, and hence no inhibition due to hydrolysis products is expected.