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Environmental fate & pathways

Phototransformation in water

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Reference
Endpoint:
phototransformation in water
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
other: Poster
Rationale for reliability incl. deficiencies:
other: Experimental conditions are not relevant from an environmental point of view.
Study type:
direct photolysis
Qualifier:
no guideline available
Principles of method if other than guideline:
The photochemical degradation of AHTN in water was studied using a Mercury high pressure lamp. After extraction with hexane, quantification followed by GC/MS and UV-detection.
GLP compliance:
not specified
Radiolabelling:
no
Analytical method:
gas chromatography
Details on sampling:
Water samples were extracted with hexane at regular time intervals.
Light source:
other: mercury lamp
Details on light source:
Photolysis is carried out using the OMNILAB TQ 150, a , at a temperature of 20 °C. Analysis by GC/MS and UV spectra (in hexane).
Duration:
10 min
Temp.:
20 °C
Reference substance:
no
Dark controls:
yes
DT50:
1.3 min
Test condition:
Mercury lamp
DT50:
20.4 min
Test condition:
without oxygen
Predicted environmental photolytic half-life:
not possible.
Transformation products:
yes
Details on results:
First-order reaction kinetics was observed with t1/2of 1.25 minutes. Tranformation products were observed, but their structures not identified. No degradation in the dark and strongly reduced degradation in the absence of oxygen.

AHTN shows fast photodegradation. In the dark there is no degradation observable. Degradation without oxygen is remarkable slower, t1/2of 20.39 minutes instead of 1.25 minutes. Even after 5 hours and at a 1000 times higher concentration no stable metabolites are observed.

Validity criteria fulfilled:
not applicable
Conclusions:
Experiment shows the potential for photochemical degradation, but the results are not relevant for a risk assessement.
Executive summary:

The photochemical degradation of AHTN in water was studied using a Mercury high pressure lamp (OMNILAB TQ 150) at 20 °C. After extraction with hexane, quantification followed by GC/MS and UV-detection. The degradation followed a first order reaction kinetics with t½ = 1.25 minutes. The reaction did not take place in the dark and the rate was strongly reduced in the absence of oxygen (t½ = 20 min.). Some unidentified reaction products were only temporarily observed. At the end of the experiment no stable metabolites were detectable even if the medium was concentrated by a factor of 1000. Although this experiment shows the potential for photochemical degradation, the experimental conditions are not relevant from an environmental point of view.

Source executive summary: EU Risk Assessement AHTN, ECB, May 2008

Description of key information

The test item rapidly degrades in photo-induced degradation experiments.

Key value for chemical safety assessment

Half-life in water:
4 h

Additional information

The EU Risk Assessment Report of the test item addressed the phototransformation in water. Photo-induced degradation of the test item was studied using a solid-phase micro-extraction fibre and UV radiation generated by low-pressure mercury lamps. The test item was dissolved in water and adsorbed on 100 µm fibres coated with polydimethylsiloxane. Photodegradation of the parent test item showed a half-life of less than 5 minutes. After 30 minutes more than 95% was degraded. Irradiation in water showed similar degradation kinetics but only slightly faster than on fibre. The tentatively identified degradation products show that the test item is not attacked on the hexamethyltetralin group but on the acetyl group, either by formation of a five-ring with aldehyde group or by removal of the oxygen moiety or by addition of an extra oxygen (Sanchez-Prado et al. 2004).

In another study by Willenborg and Butte, The photochemical degradation of the test item in water was studied using a Mercury high pressure lamp. The degradation followed a first order reaction kinetics with half life time of 1.25 minutes. The reaction did not take place in the dark and the rate was strongly reduced in the absence of oxygen (half life time of 20 min.). Some unidentified reaction products were only temporarily observed. At the end of the experiment no stable metabolites were detectable even if the medium was concentrated by a factor of 1000.

In a laboratory set-up, the test item at 1 μg/L was incubated in lake water from the Zürichsee (CH) and in distilled water at 20 ºC. The test item was subjected to UV radiation from lamps emitting between 300 and 460 nm with a maximum at 365 nm The emission was comparable to that of 24h-averaged sunlight at 50 ºN in July under clear sky conditions. Degradation of the test item followed first-order kinetics with half-lives of ≈ 4 h (photolysis rate constants 4.6 and 4.4 d-1 in lake water and in distilled water, respectively). The molar absorptivity of the test item around 300 nm in distilled water at pH 7 was circa 1000 cm-1M-1, giving a calculated quantum yield for AHTN of 0.12. The minimal differences between the photolysis rate constants determined in lake water and distilled water indicate that AHTN is degraded primarily via direct photolysis and that indirect photochemical degradation by reactive oxygen species is of minor importance. Control experiments in the dark indicated that the test item was not eliminated by other processes. The photodegradation may explain the decreased concentrations in the epilimnion of the lake in summer. The average photolysis rate for a typical winter situation, integrated over the whole depth of the Zürichsee (mean depth 50 m, attenuation 0.01 – 0.02 per cm at 315 nm) ranges from 1.0 to 2.0 * 10-3 d-1. In summer, considering only the epilimnion, the estimate is about 2 orders of magnitude higher than in winter for the whole lake: 0.10 – 0.19 d-1 (Bürge et al. 2003).

Generally, the wave length of UV ranges from 180 to 400 nm. The wave length of visible light is > 400 nm. The UV/Vis spectrum of the test item (PFW Aroma Chemicals B.V.), shows that there is absorption at wavelengths below 325 nm with peaks at 215, 257 and 297 nm. The molar absorptivity of the test item decreased from 967 cm-1.M-1at 297.5 nm to 20 cm-1M-1at 320 nm and extinguishes at higher wave lengths. The test item does not absorb at wavelengths above 325 nm (in the visible light spectrum).

 

Source: EU Risk Assessment Report AHTN, ECB (May 2008).

 

The EU Risk Assessment Report referenced to the following studies:

[1] Bürge I J, Buser HR, Müller MD and Poiger T (2003) Behaviour of the Polycyclic Musks HHCB and AHTN in Lakes, two Potential Anthropogenic Markers for Domestic Wastewater in Surface Waters. Environ. Sci. Technol. 37, 5636-5644.

[2] Sanchez-Prado L, Lourido M, Lores M, Llompart M, Garcia-Jares C, Cela R (2004). Study of the photoinduced degradation of polycyclic musk compounds by solid-phase microextraction and gas chromatography/mass spectrometry. Rapid Commun. Mass Spectrom., 18, 1186-1192.

 

The test item in water absorbs UV radiation, which is followed by a photo-induced degradation process. The available information showed that the test item rapidly photodegrades in water. Given the consistency between distilled and lake water, a half life time of 4 hours has been selected. However, in the environment, the depth of the water column plays a significant role and it should be borne into mind that the average photolysis rate is a function of the depth.