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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

The results of the QSAR predictions for buta-1,2-diene indicate that it is expected to degrade rapidly, with an estimated half life of 2.36 days. 

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

As buta-1,2-diene is a gas at standard temperature and pressure conducting a standard ready biodegradability test is technically difficult, and based on exposure estimates the test may not be relevant. Therefore, the use of a QSAR to predict the biodegradability of buta-1,2-diene is an appropriate technique to use a part of a weight of evidence approach. The use of BioHCwin and Biowin is appropriate for buta-1,2-diene as this compound clearly falls within the applicability domains of these models as all substructures in buta-1,2-diene are included in the models. The results from both QSAR indicate that buta-1,2 -diene is expected to biodegrade rapidly.

In addition, read-across to buta-1,3-diene adds to the weight of evidence that buta-1,2-diene is rapidly biodegradable in the environment. Various researchers have isolated microorganisms that are capable of metabolising buta-1,3-diene (e.g. Hou et al. 1983). Although this study does not follow standard guidelines for ready biodegradability tests they do demonstrate that microorganisms can biodegrade buta1,3-diene in aerobic conditions.

Annex XI of the REACH legislation states that data for an endpoint may be omitted where testing is not technically possible. Given its high volatility it would be very difficult to carry out a standard biodegradation test and for the test to provide meaningful results. It is therefore difficult to provide measured data to demonstrate the biodegradability of buta-1,2 -diene. A proprietary study was carried out in 2000 and reported that buta-1,2-diene was not readily biodegradable. However, the modified Sturm test was used which is not appropriate for volatile substances, according to OECD Test Guidelines. In addition, the investigators had to implement conditions which would not be found in the natural environment in order to get the buta-1,2-diene into the test solution. On this basis, the study has been given a Klimisch score of 3 (not reliable) and will not be used.

In place of measured data we have submitted data from two QSARs whose scientific validity has been established and which have applicability domains including buta-1,2-diene. Based on the data reported in the EU RAR for buta-1,3-diene, the likely environmental fate of buta-1,2-diene and the additional QSAR estimates of biodegradation rate provided here we believe that there is sufficient weight of evidence to conclude that buta-1,2-diene can be considered readily biodegradable for the purposes of this risk assessment.