Cyclohexanone oxime

Toxicity to microorganisms

Description of key information

From the results of a Zahn-Wellens test on inherent biodegradability performed according to EU method C.9 (equivalent to OECD 302B; reliability category 2) inoculum toxicity of cyclohexanone oxime can safely be excluded at the initially applied nominal concentration of 656 mg/L. 

Key value for chemical safety assessment

EC10 or NOEC for microorganisms:

656 mg/L

Additional information

Inherent biodegradability of the test item cyclohecanone oxime was assessed in a Zahn-Wellens test on inherent biodegradability performed according to EU method C.9 (equivalent to OECD 302B; reliability category 2). An initial test item concentration of nominal 656 mg/L was applied, corresponding to 400 mg/L DOC (nominal). Actual test item concentration was slightly higher deduced from the measured DOC of 421 mg/L (value from inoculum blank substracted). Non-adapted sludge from a STP receiving industrial and domestic sewage was applied with a final concentration of solids of 0.5 g/L. The first DOC determination was carried out after 3 hours to account for elimination due to adsorption. This was determined to be negligible (2.7%). Adaption phase was very short, as already between day 1 and 3 degradation phase began and at day 3 already 50.2% of the test item were degraded. The final degradation level at the end of the test amounts to 96.5%. All reported degradation values are exclusive of elimination, i.e. the DOC determined after 3 hours was taken as the reference value according to the EU-guideline. Conclusions:

- Cyclohexanone oxime is inherently biodegradable, fulfilling specific criteria (≥70 % mineralisation (DOC removal) within 7 d; log phase no longer than 3d; removal before degradation occurs was below 15%, namely 2.7% and inoculum was not pre-adapted).

- Elimination due to adsorption was negligible (2.7% within 3 hours).

- Inoculum toxicity can safely be excluded at the initial nominal concentration of cyclohexanone oxime of 656 mg/L. The Zahn-Wellens test can therefore be used as key study for hazard assessment for STP microorganisms. In line with REACH guidance on Information Requirements and Chemical Safety Assessment part R.10 the initial nominal concentration of cyclohexanone oxime of 656 mg/L will be regarded as a NOEC STP-microorganisms.

This is supported by a short-term test on activated sludge respiration inhibition (reliability category 3 due to short incubation period) according to ISO 8192 (draft 1984) and similar to OECD 208. The following effect concentration for the test item cyclohexanone oxime (highest tested concentration: 3598 mg/L) were determined in this test:

Highest tested concentration with less than 20% respiration inhibition within 30 min.: 1199 mg/L

EC20 (30 min): 1500 mg/L

EC50 (30 min): 3000 mg/L