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Environmental fate & pathways

Biodegradation in water: screening tests

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Description of key information

A weight of evidence approach suggests that 2,6-xylenol biodegrades but does not meet the criterion for ready biodegradability.

Key value for chemical safety assessment

Biodegradation in water:
inherently biodegradable, not fulfilling specific criteria

Additional information

This endpoint is addressed on a weight of evidence basis. Six endpoints have been provided to form the weight of evidence; five of these are experimental values and the sixth is a Quantitative Structure Activity Relationship (QSAR) calculation (EPI Suite vs 4.0).

 

The first result was published in Applied Microbiology and Biotechnology (Puig-Grajales et al., 2000). No guideline was followed however the study was conducted using acceptable scientific methods and principles. It was therefore awarded a reliability score of 2 in accordance with the criteria for assessing data quality as set forth by Klimisch et al. (1997).

A batch anaerobic biodegradability assay was conducted with either anaerobic granular sludge treating distillery wastewater or anaerobic Rhine sediment. Studies were conducted under methanogenic, sulfate-reduction, denitrification, Fe (III)- and Mn (IV)-reducing conditions for up to 100 days.

Under the conditions of this assay, 2,6-xylenol was not biodegradable under methanogenic, sulfate-, or nitrate-reducing conditions but did convert by chemical oxidation with Mn (IV).

 

In the second study, the potential of the read-across substance 2,4,6-trimethylphenol to undergo ready biodegradation was investigated in accordance with the standardised guideline OECD 301D under GLP conditions. It was awarded a reliability score of 2.

Activated sludge was exposed to the test material under aerobic conditions for 28 days. O₂ consumption was the parameter followed to estimate biodegradation.

Under the conditions of this study, the test material showed 11.3 % biodegradation in 28 days, which did not meet the criterion for ready biodegradability (i.e., 60 % degradation in 28 days).

 

In the third study, the potential of the test material to undergo ready biodegradation was investigated in accordance with the standardised guideline OECD 301C under GLP conditions. It was awarded a reliability score of 2.

Activated sludge was exposed to the test material under aerobic conditions for 28 days. O₂ consumption measured by BOD was the parameter followed to estimate biodegradation. After the 28 day contact period, 2 % degradation was observed.

Under the conditions of this study, 2,6-xylenol was determined to be non-biodegradable.

 

The QSAR calculation was carried out using EPI (Estimation Programs Interface) Suite, a Windows-based suite of physical/chemical property and environmental fate estimation programs developed by the US EPA’s Office of Pollution Prevention Toxics and Syracuse Research Corporation (SRC). It was awarded a reliability score of 2.

The BIOWIN model was used to estimate the potential biodegradation of the test material. The model estimates aerobic and anaerobic biodegradability of organic chemicals using 7 different models. The models and results are as follows:

- Biowin1 (Linear Model Prediction): Biodegrades Fast

- Biowin2 (Non-Linear Model Prediction): Biodegrades Fast

- Biowin3 (Ultimate Biodegradation Timeframe): Weeks

- Biowin4 (Primary Biodegradation Timeframe): Days-Weeks

- Biowin5 (MITI Linear Model Prediction): Biodegrades Fast

- Biowin6 (MITI Non-Linear Model Prediction): Biodegrades Fast

- Biowin7 (Anaerobic Model Prediction): Does Not Biodegrade Fast

- Ready Biodegradability Prediction: YES

This calculation therefore estimated 2,6-xylenol to be readily biodegradable.

 

The fifth study was published in Environmental Progress (Stenstrom et al., 1989). However the documentation was insufficient for assessment and as a result the study was awarded a reliability score of 3 in accordance with the criteria for assessing data quality as set forth by Klimisch et al. (1997).

An aerobic batch reactor was used. Batch aqueous solutions were inoculated with activated sludge acclimated to 2,4-xylenol and aerated. The duration of the test was not clearly specified; however it was implied that it was less than one day.

The authors concluded that xylenol and cresol compounds with a methyl group in the 2 or 4 position degrade well. However, the reaction rate for 2,6-xylenol was relatively low.

 

The sixth study was published in Water Research (Pitter, 1976). It was comparable to a 301D guideline study with acceptable restrictions and was therefore awarded a reliability score of 2.

Activated sludge, adapted, was exposed to the test material for a contact time of 20 days under aerobic conditions. COD removal was the parameter followed to estimate biodegradation.

The rate of biodegradation was determined to be 9.0 mg COD/g/hour. 94.3 percent of the test material was removed based upon COD in an unspecified time period.

Under the conditions of this study, 2,6-xylenol was determined to be biodegradable.

 

Three studies suggest that the test material is biodegradable, the publication by Pitter (1976), the EPISUITE QSAR calculation and the poorly documented study by Stenstrom et al. (1989).

Two studies suggest that 2,6-xylenol does not meet the criterion for ready biodegradability. The study conducted in accordance with the standardised guideline OECD 301 indicated that 2,6-xylenol is not biodegradable. Further, a the reliable study conducted on 2,4,6 -trimethylphenol showed 11.3 % biodegradation in 28 days, which did not meet the criterion for ready biodegradability.

This read-across datum is potentially conservative with respect to the potential for degradation of 2,6-xylenol: As part of the study reported by Pitter the degradability of 2,4,6-trimethylphenol was also tested; 94.3 and 0 % degradation were observed for 2,6-xylenol and 2,4,6-trimethylphenol, respectively, during the test period.

 

The weight of evidence therefore suggests that 2,6-xylenol is inherently biodegradable but not readily biodegradable.