2-mercaptoethanol

Endpoint summary

Description of key information

Tests on two trophic levels were performed to examine the terrestrial toxicity of 2-mercaptoethanol. Earthworms were the most sensitive species for acute exposure, with an LC50 (14-d) of 908 mg/kg soil dw. The EC50 and EC10 (28-d) for nitrate formation of soil microflora were determined to be > 1000 and 330 mg/kg soil dw, respectively.

Additional information

The uses of 2-mercaptoethanol do not rule out the potential for soil exposure and as such, an assessment of terrestrial risk is required. The adsorption and biodegradation data available for the substance do not indicate a potential for 2-mercaptoethanol to have high adsorption or high persistence in soil, but available ecotoxicity data indicate that the substance is very toxic to aquatic organisms.

The biodegradability of 2-mercaptoethanol under anaerobic and aerobic conditions is well investigated. Based on the results of the OECD 309 study, the substance degrades in the aquatic environment with a half-life of 0.079 days, so reaches a level of >70% within a period of 28 days and therefore the substance is considered rapidly biodegradable.

The organic carbon adsorption coefficient (Koc) has been predicted using a QSAR model (PCKOCWIN) and is estimated to be 1.325 (Log Koc 0.122). 2-mercaptoethanol is therefore not expected to partition to soil from water and, after exposure to soil, significant adsorption to the solid soil phase (e.g. clay) is not expected. Since the log Koc is below 3 and the substance has no cationic properties, exposure of soil after binding to sewage sludge is unlikely.

Acute tests on three trophic levels were performed to examine the aquatic toxicity of 2 -mercaptoethanol. Daphnia magna were shown to be the most sensitive species revealing an EC50 (48 h) of 0.4 mg/L. Fish and alga turned out to be less sensitive providing a LC50 (96 h) and ErC50 (72 h) of 37 mg/L and 19 mg/L respectively. Concerning chronic data on aquatic toxicity, a NOEC of 1.7 mg/L was determined for algae (growth rate) while the NOEC for daphnids was measured to be 0.0632 mg/L, based on mortality (the most sensitive endpoint). Thus, 2-mercaptoethanol is considered acutely and chronically very toxic for aquatic organisms.

On the basis of the available data, 2-mercaptoethanol meets the criteria for soil hazard category 2 and, as such, following the approach for the screening assessment, the risk assessment was initially conducted on the basis of the Predicted Environmental Concentration (PEC) divided by the Predicted No Effect Concentration (PNEC) (derived using equilibrium partitioning method based on the aquatic toxicity data and the modelled partitioning behaviour). Confirmatory short-term soil toxicity tests were conducted on terrestrial invertebrates (earthworms) and terrestrial micro-organisms.

Tests on two trophic levels were performed to examine the terrestrial toxicity of 2-mercaptoethanol. Earthworms were the most sensitive species for acute exposure, with an LC50 (14-d) of 908 mg/kg soil dw. The EC50 and EC10 (28-d) for nitrate formation of soil microflora were determined to be > 1000 and 330 mg/kg soil dw, respectively.

As the initial risk assessment using equilibrium partitioning for the soil compartment and the updated risk assessment using the terrestrial toxicity data did not indicate risk no further toxicity testing on soil organisms is considered necessary and the Annex IX and X higher tier tests on soil organisms have been waived.