Benzenepropanal, .beta.-methyl-3-(1-methylethenyl)-

Biodegradation in water: screening tests

Description of key information

The substance is considered readily biodegradable based on read-across from an appropriate structural analogue.

Key value for chemical safety assessment

Biodegradation in water:

readily biodegradable

Additional information

Four biodegradation studies are available for the analogue substance, 3-(3-isopropylphenyl)butanal, which have all been performed in line with GLP and standardised guidelines with a sufficient level of detail to assess the quality of the studies. 3-(3-isopropylphenyl)butanal (source substance) is structurally similar to 3-(3-isopropenylphenyl)butanal (target substance). The only difference between the two substances is the nature of the alkyl substituent on the benzene ring (i.e. isopropyl versus isopropenyl). This minor structural difference is not expected to affect the biodegradation potential of the two substances. Therefore, this endpoint is being addressed on the basis of read-across.

 

Different ready protocols have given conflicting results. The read-across substance was shown to be readily biodegradable in the Manometric Respirometry Test (OECD 301F, Givaudan 2011) achieving 74% biodegradation after 28 days and fulfilling the 10-day window criterion (16% on day 14 and 68% on day 24). In contrast, no positive biodegradation of the test item was observed in the two MITI tests (OECD 301C, Givaudan 1989 and 1993b). However, the concentration of test item used in the MITI tests was higher than that used in the 301F test (100-120mg/l compared with 30mg/L). This would result in mass transfer issues (water solubility of test item is 40mg/L) and potential inhibition to the inoculum. Indeed, the negative degradation levels of -9% observed in study 1993b suggest a toxic effect (i.e. respiration is reduced compared to the control), as does the curve for the test substance and reference substance together. In addition, the activated sludge respiration inhibition test (see endpoint 6.1.7) resulted in 53% inhibition at a concentration of 100mg/L after 3 hours exposure. Therefore the two MITI tests are considered to give false negative results due to the high test concentrations used and have been disregarded.

Ready biodegradability tests must be designed so that positive results are unequivocal. Therefore positive test results generally supersede negative test results. To this end, the 301F study (Givaudan, 2011) was considered to be the 'key' study to address ready biodegradability.

High levels of biodegradation (100% in 28 days) were observed in the inherent 302C study (Givaudan 1993a). Although the test item was tested at the higher test concentration of 100mg/L, inhibition is less of an issue due to the higher concentration of inoculum used in the inherent test.

 

In summary, the read-across substance is considered to be easily biodegraded under aerobic conditions.The biodegradation of the read-across substance and 3-(3-isopropenylphenyl)butanal (target substance) are anticipated to be similar due to their structural similarity (i.e.common carbon skeleton) and physico-chemical properties (for further details see read-across justification document, attached in section 13). Therefore, 3-(3-isopropenylphenyl)butanal is also considered to be readily biodegradable based on read-across from an appropriate structural analogue.