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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

Based on read across from the xylene isomers and ethylbenzene, the streams in this category are considered to be readily biodegradable.

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

Biodegradation studies are not available for any of the streams in this category. However, it is difficult to assess the persistence of streams using standard test methods as the tests measure the properties of the whole substance but do not provide information on the individual constituents. Instead, it is more appropriate to consider whether the representative components of these streams are persistent. Therefore, data has been read across from the xylene isomers and ethylbenzene as these are the major components of the streams in this category.

Ethylbenzene and the xylene isomers are structurally similar substances, consisting of a benzene ring with one and two methyl functional groups attached respectively; the different xylene isomers having the two methyl functional groups attached at different locations. The presence and position of the second methyl functional group on the benzene ring is not expected to change the biodegradation properties of the substance significantly. The similarities in structure, water solubility and behaviour in the aquatic environment indicate that the biodegradation of the components of this stream are likely to be similar. All of the substances are relatively soluble in water, with limited potential to partition to soils or sediment. The water solubility (Yalkowsky and He 2003) and log partition coefficient (Hansch et al. 1995) of ethylbenzene, m-, o- and p-xylene are 190.7 mg/L and 3.15, 146 mg/L and 3.2, 170.5 mg/L and 3.12, and 156 mg/L and 3.15 respectively.

Data are available for all three xylene isomers from GLP-compliant guideline studies (Dr Noack 2015). The studies followed OECD guideline 301F (ready biodegradability: manometric respirometry test) using a mixture of sewage and soil micro-organisms. m-xylene reached 60% biodegradation after 5 days and 98% biodegradation after 28 days. o-xylene reached 60% biodegradation after 8 days and 94% biodegradation after 28 days. p-xylene reached 60% biodegradation after 7 days and 90% biodegradation after 28 days. The studies show that the xylene isomers are readily biodegradable, meeting the 10 day window criterion. In addition to the key studies, additional data on the biodegradability of the xylene isomers and ethylbenzene are available from a number of sources.

The EU RAR (2007) concludes that ethylbenzene is readily biodegradable. The Risk Assessment Report is peer-reviewed by the Scientific Committee on Toxicity, Ecotoxicity and the Environment (CSTEE), now renamed Scientific Committee on Health and Environmental Risks (SCHER) which gives its opinion to the European Commission on the quality of the risk assessment. This Draft Risk Assessment Report has undergone a discussion in the Competent Group of Member State experts with the aim of reaching consensus by interpreting the underlying scientific information.

Exxon Biomedical Sciences (1995) conducted an OECD 301F test with p-xylene, showing 68% biodegradation in 10 days and 87.8% biodegradation in 28 days. This GLP compliant guideline study found p-xylene to be readily biodegradable. Although greater than 20% variability was observed between replicates at the start and end of the 10 day window, all three replicates had reached >60% biodegradation by day 11, so this was not considered to have invalidated the results of the test.

Exxon Biomedical Sciences (1996) conducted a GLP-compliant study with o-xylene following OECD guideline 301F. The study was conducted with three replicates, each containing approximately 36 mg/L of test item. The biodegradation reached approximately 70% by day 28, as an average of the three replicates (47.41, 79.5 and 82.1 %), and the positive control material degraded by approximately 94% by day 28. However, only one replicate met the 10 day window criterion, with one other replicate reaching 10% degradation by day 11 but not achieving 60% degradation until day 25 and the final replicate not reaching 60% biodegradation by day 28. As the difference in the extremes of the replicates at the end of the test is greater than 20%, the validity criteria for the OECD guideline 301F were not met, however the study does show that o-xylene underwent significant degradation over the course of the test.

Bridie et al. (1979) report the results of a 5 day BOD/ThOD test. This study predates the implementation of GLP and the OECD guidelines for biodegradation screening tests. The study authors report that they followed guidelines in operation at the time, although the level of detail in the publication is limited, in particular the concentration of the test substance is not reported. The study followed an APHA 219 method (1971) for determination of the biological oxygen demand (BOD) and ASTM D1251 -67 method (1974) for determination of the chemical oxygen demand (COD). The study is used in the draft SIDS for xylenes and demonstrates that the xylene isomers can be biodegraded by non-adapted sewage sludge, with a 5 day BOD/ThOD of 80, 52 and 44% for m-, o- and p-xylene respectively.

MITI (2001) reports that 100% biodegradation (based on BOD and GCMS) was observed in an OECD 301C study with m-xylene. In a second study with p-xylene 38% biodegradation based on BOD and 92% biodegradation based on GCMS analysis was reported. An earlier study with ethylbenzene, CITI (1992), the precursor to MITI, reported 81-126% biodegradation based on BOD over two weeks in an OECD Guideline 302 C (Inherent Biodegradability: Modified MITI Test (II)). We have been unable to obtain copies of these reports so have not been able to assess their reliability or confirm that the validity criteria were met. However, as the results have been taken from regulatory review articles, they are considered to be suitable for use to support this endpoint.

BIOWIN indicates that ethylbenzene and m-, o- and p-xylene biodegrade fast (linear and non-linear model predictions), with primary biodegradation timeframes of days-weeks and ultimate biodegradation timeframes of weeks. The MITI linear and non-linear models predict that m-, o- and p-xylene will biodegrade fast and the overall ready biodegradability predictions are “yes”. However, the MITI linear and non-linear models predict that ethylbenzene will not biodegrade fast and the overall ready biodegradability prediction is “no”. The BioHCwin results estimate that the half-lives of m-, o-and p-xylene would be 4.44 days and the half-life of ethylbenzene is 5 days. A screening assessment for persistence using the combined results of some of the Biowin models (ECHA (2012) Guidance on information requirements and chemical safety assessment Chapter R11: PBT assessment) indicates that the components in this stream would not meet the screening criteria for persistence. The use of QSAR estimates to predict the biodegradability of the stream is considered valid as ethylbenzene and the xylene isomers are included in the training and validation sets of the BIOWIN and BioHCwin models.

As it is difficult to assess the persistence of streams, it is more appropriate to consider whether the representative components of these streams are persistent. Data have been read across from the xylene isomers and ethylbenzene as these are the major components of the streams in this category. All three xylene isomers are considered readily biodegradable on the basis of the results of the GLP-compliant studies following OECD guideline 301F (Dr Noack 2015). The EU RAR for ethylbenzene concludes that the substance is readily biodegradable. Overall, the available data have been used to conclude that ethylbenzene and the xylene isomers, and therefore the streams in this category, are readily biodegradable.