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Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

STABILITY

Phototransformation in air

Supporting information has been provided in the form of data taken from a peer reviewed database (Syracuse Research Corporation). No details on methodology are provided and the study was awarded a reliability score of 2 in accordance with the criteria for assessing data quality set forth by Klimisch et al. (1997).

The Atmospheric OH Rate Constant was reported as: 6.59E-11 cm³/molecule-sec at 25 °C.

Hydrolysis

In accordance with section 1 of Annex XI it I considered to be justified to omit this study on the basis that 2,6-xylenol does not contain any functional groups which under normal environmental (pH) conditions will be hydrolysable.

Phototransformation in water

The database of the Syracuse Research Corporation (2010) reports an experimentally determined OH rate constant of 6.59E-11 cm³/molecule/s at 25 °C (Kwok and Atkinson, 1994). This rate constant agrees with a half life of 5.84 h under typical environmental conditions. One reliable study was identified on phototransformation of 2,6 -Xylenol in water using direct sunlight (Faust and Hoigne, 1987). This study measured a half life of 7.4 h at the surface and 19 h at a 1 m depth. In natural water (Greifensee), a half-life of 1.9 d was measured in the upper 1 m of the water column.

BIODEGRADATION

Biodegradation in Water

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.

Biodegradation in Water and Sediment

In accordance with column 2 of Annex IX, this study need not be conducted if direct and indirect exposure of sediment is unlikely. While some limited data were identified and are referenced in this dossier, no further study needs to be performed as exposure is unlikely. 2,6-Xylenol is used as a chemical intermediate in the manufacture of polymers and certain pesticides. The Chemical Safety Assessment indicated no need to investigate further the degradation of the substance and it is therefore considered to be justified to omit this study.

Biodegradation in Soil

In accordance with column 2 of Annex IX, this study need not be conducted if direct and indirect exposure of soil is unlikely. 2,6-Xylenol is used as a chemical intermediate in the manufacture of polymers and certain pesticides. The Chemical Safety Assessment indicated no need to investigate further the degradation of the substance and it is therefore considered to be justified to omit this study.

BIOACCUMULATION

Bioacummulation Aquatic / Sediment

In accordance with Column 2 of Annex IX, this study does not need to be conducted if the substance has a low potential for bioaccumulation (for instance a log Kow less than or equal to 3). 2,6-Xylenol has a log Kow value of 2.3, therefore aquatic bioaccumulation data are not needed.

TRANSPORT AND DISTRIBUTION

Adsorption/Desorption

In accordance with section 9.3.1 of Column 2 of Annex VIII, the study does not need to be conducted if based on the physicochemical properties the substance can be expected to have a low potential for adsorption (e.g., the substance has a low octanol water partition coefficient). 2.6-Xylenol has a log Kow of 2.3, indicating a low potential for adsorption. It is therefore considered to be justified to omit this study.

For the environmental assessment the EUSES calculated value (Koc = 164 L/kg; non-hydrophobics (default QSAR)) was used.

Supporting information is available in the form of a published study. Although not conducted to a standardised guideline, the study was well-documented study and performed according to scientific methods. It was therefore awarded reliability score of 2 in accordance with the criteria for assessing data quality set forth by Klimisch et al. (1997).

The adsorption to soil was investigated using the batch equilibrium method. Solutions of 2,6-xylenol were added to samples of five types of soil and shaken at room temperature for up to five days. Data fitted to Freundlich equation.

Phenol adsorption by soils is very much time-dependent, with up to 5 days for the adsorption reactions to reach equilibrium in water-saturated soils. Freundlich isotherm constants were derived.

Henry's Law Constant

Three key pieces of data to address this endpoint are provided; two are experimental values and the third is an estimation by calculation. All were awarded a reliability score of 2 in accordance with the criteria for assessing data quality set forth by Klimisch et al. (1997).

 

A Quantitative Structure Activity Relationship (QSAR) calculation (EPI Suite vs 4.0) was carried out to determine the Henry’s law constant of 2,6-xylenol. By definition, the Henry's law constant is the ratio of vapour pressure to solubility and is determined by calculation. EPISUITE determined the constant to be 0.000004669 atm m³/mol.

 

The first experimental value was published in Phys. Chem. Chem. Phys. in a well-documented publication. The Henry’s Law constant was directly measured using a dynamic equilibrium system based on the water/air equilibrium at the interface within the length of a microporous tube. The measurements were conducted over the range 278 to 293 K in both deionized water and 35 g/ L solution of NaCl.

The Henry's Law Constant (HLC) for 2,6-xylenol was determined to be 250 M/atm at 20 °C (back calculated to 0.41 Pa m³/mol via: H (Pa m³/mol) = 101.325 / HLC (M/atm)).

 

The second experimental value was published in Environ. Sci. Technol. In a well-documented study performed according to scientific principles.

The determination of Henry's Law Constants for organic species found to be emitted from shale oil waste waters were determined by the use of a gas-stripping vessel redesigned with a 12 mm i.d. tube but had approximately the same height, so that a much smaller volume of sample water could be used for Henry Law constant determinations. Air was introduced through a glass frit and allowed to bubble through the column of liquid. Henry's Law constants for individual species were calculated from the slope of a plot of the logarithm of the integrated area counts of a chromatographic peak vs. the volume of air which had been bubbled through the water column.

The Henry's Law Constant for 2,6-xylenol measured for a single component dissolved in water was 0.67 atm m³/mol.10E5 (0.68 Pa m³/mol) at 25 °C and for a single mixture in water was 0.76 atm m³/mol.10E5 (0.77 Pa m³/mol) at 25 °C.

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