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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

The xylene isomers are considered to be readily biodegradable

Key value for chemical safety assessment

Biodegradation in water:
readily biodegradable

Additional information

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 are available from a number of sources. 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, with >60% biodegradation by day 10 showing that the 10 day window criterion had been met. 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. The study has been assigned a Klimisch score of 1 and is considered to be suitable for use for this endpoint.

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). Despite these limitations, the study has been assigned a Klimisch score of 2, with the justification "non-GLP, guideline study, published in peer-reviewed literature, acceptable with restrictions" and is considered suitable for use for this endpoint. The study is used in the draft SIDS for xylenes. The study 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 201C 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. 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 as weight of evidence for this endpoint.

QSAR predictions

The BIOWIN results pages indicate that 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”. The BioHC results estimate that the half-lives of m-, o-and p-xylene would be 4.44 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 a high probability of the substances being biodegradable. The use of QSAR estimates to predict the biodegradability of xylene isomers is considered valid as the isomers are included in the training sets of the BIOWIN and BioHC models.

Read across

m-, o- and p-xylene are all xylene isomers. They are structurally similar, consisting of a benzene ring with two methyl functional groups attached at different locations. Ortho (o-) xylene has methyl groups attached at positions 1 and 2 on the benzene ring, while meta (m-) xylene has functional groups at 1 and 3 and para (p-) xylene at 1 and 4. The position of the 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 behavior in the aquatic environment indicate that the biodegradation of the substances is likely to be similar. All three 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 m-, o- and p-xylene are 146 mg/L and 3.2, 170.5 mg/L and 3.12, and 156 mg/L and 3.15 respectively.

Conclusion

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). On the basis of the similarity of the structure and environmental fate and behaviour of the xylene isomers, the available studies have been read across between m-, o- and p-xylene. Several additional studies are available for xylene isomers showing ready biodegradability, as well as QSAR estimates and further studies showing high levels of biodegradation. The available evidence has been used to conclude that all the xylene isomers are considered to be readily biodegradable.