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Phototransformation in water

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phototransformation in water
Type of information:
experimental study
Adequacy of study:
key study
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Study type:
indirect photolysis
according to
EPA Guideline Subdivision N 161-2 (Photodegradation Studies in Water)
according to
other: SETAC-EUROPE Guideline: Assessing environmental fate and ecotoxicity of pesticides, 10.0 aqueous photolysis
GLP compliance:
Pyridinyl (specific activity: 1.90 GBq/mmol) and phenylacrylate labelled (specific activity: 2.01 GBq/mmol)
Analytical method:
high-performance liquid chromatography
other: TLC and 2-D TLC
Details on sampling:
Vessels were taken at the pre-determined sampling intervals, nominally equivalent to 6, 12, 18, 24, and 30 days equivalent of 50°N summer sunlight. The corresponding outlet traps were also removed for analysis.

Each sample was transferred back to the laboratory, and the volume determined by weight. The contents of the photolysis vessel were pipetted into a glass volumetric flask (15 mL) and the vessel rinsed with acetonitrile, these rinsings being used to make the sample up to volume. The samples were then quantified by LSC, decanted into foil covered scintillation vials and stored at -10°C for later analysis.

The dark control samples were removed at the same time as the final irradiated samples (approximately 30 days).

The 2 M sodium hydroxide and ethanolamine traps were quantified by measuring the total volume of sample and taking aliquots (3x200 µL) for LSC analysis. The ORBO 100 sieves were extracted by washing twice with acetonitrile into volumetric (25 mL), making up to volume and taking aliquots (3x250 µL) for LSC analysis.
An aqueous buffer (pH 7) was prepared by mixing 0.01 M sodium acetate trihydrate (399 mL) with 0.01 M acetic acid (1mL). The buffer was sterilized by autoclaving (the pH being confirmed after autoclaving).
Light source:
Xenon lamp
Details on light source:
A xenon burner was used to irradiate the samples. A correlated colour temperature of approximately 6500 K was defined as the best representation of sunlight (designated D65).
The sunset exposure instrument was fitted with a high powered xenon lamp made from high grade quartz. This allows the passage of radiation ranging from short-wave ultraviolet to middle-wave length infrared. A system of special mirrors and filters removed infrared radiation, but allows the passage of ultraviolet and visible light with a spectral distribution that closely approximates to D65 radiation. Hence, the emission spectrum produced was closely equivalent to the global radiation of natural sunlight. The burner was mounted in a cylindrical parabolic reflector to produce approximately parallel radiation. The unit was cooled by a double blower which directs seperate air streams over the burner and photolysis tank.
Type of sensitiser:
other: Water
Details on test conditions:
- Photolysis vessel: Each vessel is individually covered with a quartz lid, which was suitable for general optical applications requiring good transmission in the near ultraviolet and visible range.
- Photolysis tank: Treated aqueous samples in photolysis vessels were placed in a stainless steel tank, which was designed to enable cooling water to circulate through its base.
- LI-1800 portable spectroradiometer: Light intensity measurements were taken at a height equivalent to that of the samples, at the centre of the photolysis tank.
- Radiochemicals: The amount of radiochemical required to achieve a nominal concentration of 1.4 µg/mL test substance in 8 mL of buffer was calculated using the specific acitivity.
- Sterilisation: Photolysis vessels, glass vials, silicon tubing, air filters and pH 7 buffer were autoclaved at 121°C, 15 psi for 20 minutes. A glass pipette and hamilton syringe used for application were sterilized by soaking in 70% ethanol (aqueous).
- Dark control: Treated photolysis vessels, wrapped in aluminium foil, to act as dark control samples, were placed in a waterbath at 25 ± 1°C, along with an identical untreated vessel for temperature monitoring. The dark control were analysed at the final sample interval.
30 d
25 °C
Initial conc. measured:
1.36 other: µg/mL
30 d
25 °C
Initial conc. measured:
1.43 other: µg/mL
Reference substance:
Dark controls:
Test performance:
No microbial growth was observed on any of the nutrient agar plates confirming that sterility had been maintained throughout the entire irradiation period. Thus, it can be concluded that the observed degradation of the test substance was not microbially mediated.

The total radioactive recoveries from irradiated samples ranged from 90.3-102%. Recoveries from dark control samples were 98.6 and 96.3% for the pyridinyl and phenylacrylate samples, respectively. This variation in recovery was within acceptable limits, which demonstrated that the test solution was homogenous during the application period.
Key result
20.3 d
Test condition:
pH 7; 50°N summer sunlight
Transformation products:
methyl (Z)-3-methoxy-2-{2-[6-(trifluoromethyl)pyridin-2-yloxymethyl]phenyl}acrylate (isomer of parent) and methyl 2-hydroxy-{2-[6-trifluoromethyl)pyridin-2-yloxymethyl]phenyl}acetate
Details on results:
The calculated application rates were 1.36 and 1.43 µg/mL for the pyridinyl and phenylacrylate labels, respectively.

Parent remained as the major single radioactive component in all irradiated samples. Only two photodegrates occured at >10% of applied activity at any point in the study. These were identified as compound 4 (methyl(Z)-3-methoxy-2-{2-[6-(trifluoromethyl)pyridine-2-yloxymethyl]phenyl}acrylate) and compound 12 (methyl 2-hydroxy-{2-[6-(thrifloromethyl)pyridine-2-yloxymethyl]phenyl}acetate), reaching maximum levels of 11.7-14.2% and 14.5-15.3% respectively, compound 4 declined to 8.3-9.1% at the final sampling interval. Compound 3 [(6-(trifluoromethyl)pyridine-1H-2-one)] was present at low levels throughout the study and did not exceed 1.9% at any time.

Analysis of the dark controls showed 94.4 and 94.9% parent test substance remaining at the final time point for the pyridinyl and phenylacrylate labels, respectively. Thus, demonstrating that the observed degradation was photolytic.
Validity criteria fulfilled:
The test substance degradation was observed in the irradiated samples.
Executive summary:

Sterile aqueous solutions of the test substance, separately labelled in the pyridinyl and phenylacrylate rings were prepared at 1.36 µg/mL and 1.43 µg/mL respectively, in buffer at pH 7.

Treated samples were continuously irradiated using light from a xenon arc lamp, The light was filtered to give a spectral distribution close to that of natural sunlight. Samples were irradiated for defined periods, up to the equivalent of approximately 30 days of 50°N summer sunlight, at 25 ± 1°C. For each radiolabel, samples were taken for analysis at nominal irradiation periods equivalent to 0, 6, 12, 18, 24, and 30 days 50°N summer sunlight. Dark controls were prepared and maintained at 25 ± 1°C for the duration of the irradiation, and analyzed at the final sampling time.

The estimated DT50 (the time required for the test substance to reduce to 50% of its initial concentration) was 20.3 days 50°N summer sunlight (19.3 days Florida summer sunlight [30°N]).

Radioactive recoveries ranged from 90.3-102% of applied. Parent was the major component in all irradiated samples, accounting for 36.7-40.7% of applied radioactivity at 30 days. Only two photodegradates occurred at levels >10% of applied radioactivity throughout the study, these were identified as compounds 4 (methyl(Z)-3-methoxy-2-{2-[6-(trifluoromethyl)pyridine-2-yloxymethyl]phenyl}acrylate) and 12 ( methyl 2-hydroxy-{2-[6-(thrifloromethyl)pyridine-2-yloxymethyl]phenyl}acetate), accounting for 8.3-14.2 and 2.0-15.3% respectively. Compound 3 ((6-(trifluoromethyl)pyridine-1H-2-one)) was identified at low levels and did not exceed 1.9% at any sampling interval. No discrete unidentified photodegradate accounted for >5.9% of the applied radioactivity at any sampling time.

Diffuse areas of radioactivity (remainders), accounted for up to 10.6% of applied radioactivity. This activity did not contain any discrete components and was due to large numbers of photodegradates present at very low levels. Small amounts of volatile radioactivity (6.9% and 2.5% for the pyridinyl and phenylacrylate samples respectively), were evolved up to day 30, the majority of which was likely to be 14CO2.

Sample sterility was maintained throughout the study and no significant degradation occurred in the dark control samples, indicating that the degradation observed in the irradiated samples was solely due to photodegradation.

Description of key information

Half-life = 20.3 days at 50ºN summer sunlight; pH 7

Key value for chemical safety assessment

Half-life in water:
20.3 d

Additional information