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Phototransformation in soil

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phototransformation in soil
Type of information:
experimental study
Adequacy of study:
key study
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
no guideline followed
Principles of method if other than guideline:
The study was performed to determine the rate of photolysis and thereby the photolytic DT50 of the test substance, on a soil surface at ca 20°C, under irradiation conditions which stimulate natural sunlight, to identify any photolysis products representing ≥10% of the applied radioactivity, to investigate the rate of formation and decline of the photolysis products.
GLP compliance:
Pyridinyl (specific activity: 1.16 GBq/mmol) and phenylacrylate (specific activity: 1.10 GBq/mmol) labels
Analytical monitoring:
Analytical method:
high-performance liquid chromatography
other: TLC and HPLC mass spectroscopy
Details on sampling:
At the pre-determined sampling intervals, nominally equivalent to 1, 5, 10, 20, and 30 days 50°N summer sunlight, sample was removed from the suntest instrument. All vessels were transferred back to the laboratory where they were stored in the freezer until analysed.

Most samples were extracted within 2 days of removal from the suntest. Prior to analysis they were stored in the freezer. The samples were extracted as follows:
1. Soil transferred to a 50 ml centrifuge tube
2. 7 ml of acetonitrile added to the tube
3. Shaken using an autovortex mixer for ca. 1 minute
4. Ultrasonicated for ca. 5 minutes
5. Centrifuged for 6 minutes then the liquid phase decanted off
Steps 2-5 were repeated once more with acetonitrile. The total process was repeated using 80:20 acetonitrile:water (v/v) and then 75:25 acetonitrile:0.1 M HCI (v/v). The radioactivity in all the extracts was quantified by LSC.

The extracts were stored in the freezer, until analysis by TLC. For each time point, equal proportions of extracts were combined and analyzed using three normal phase systems and two reverse phase system.

The identification of photolysis products was also confirmed by HPLC and HPLC-MS.
Details on soil:
The soil used for the study was a sandy clay loam collected from 18 Acres field, Jealott's Hill. The soil was taken at a depth of 10-15 cm. The collected soil was first sieved through a 5 mm sieve, followed by a 2 mm sieve.
Light source:
Xenon lamp
Details on light source:
The Suntest Exposure Instrument was fitted with a high powered xenon burner 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 removes 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. This unit was cooled by a double blower which directs separate air streams over the burner and photolysis tank.
Details on test conditions:
Photolysis vessel: Vessels with side arms were used for this study in order to trap volatiles. Each vessel was individually covered wtih a qurtz lid, which is suitable for general optical applications requiring good transmission in the near ultraviolet and visible range.

Photolysis tank: Treated soil samples in photolysis vessels were placed in a stainless steel tank, which was designed to enable cooling water to circulate through its base. In order to maintain the temperature of the vessels at ca. 20°C, the cooling water was circulated using a thermostatically controlled circulator.

LI-1800 Portable Spectroradiometer: Light intensity measurements were taken at the start and end of the irradiation period for each suntest apparatus and the mean of these two values was used in calculations. A baseless vessel with quartz lid attached and surrounded by a black cardboard collar was attached to the LI-1800 sensor optic probe when taking light intensity measurements.

Preparation of soil vessels: A thin layer of the 2 mm sieved soil was applied to the base of each vessel by slurrying approximately 1 g of soil in 1 mL of water then allowing the soil to equilibrate overnight at ambient temperature prior to application of the test substance. This gave layers measured <1 mm thick.

Application and irradiation of 14C-test substance: 150 µL of 14C-test substance in acetonitrile was applied to each soil layer. The soil vessels were placed on top a bed of ice during application. The solution was applied in droplets using a Hamilton syringe attached to a stepper motor. The homogeneity of the radioactivity on the soil surface was determined by applying a 150 µL aliquot of the radiochemical to a soil disc disc of the same dimensions as the soil in the vessels, followed by autoradiography using a bioimaging analyser. Aliquots (3 x 150 µL) of the application solution were dispensed into volumetric flasks and then quantified by LSC to determine the application rate.
30 d
20 °C
Initial conc. measured:
750 other: g a.i./ha
Reference substance:
Dark controls:
Key result
7 d
Test condition:
50°N summer sunlight
Remarks on result:
other: First order multi-compartment model with fixed intercept
Transformation products:
Details on results:
The radioactive recoveries from the irradiated samples ranged from 90.0-102.2%. Recoveries from dark control samples were between 100.6-101.6%.

A total of 7 photoproducts were identified: Compounds 3, 4, 8, 12, 13, 15, and CO2. In addition to these identified photoproducts, there were a number of minor unidentified photoproducts none of which exceeded 3.2% of the applied radioactivity. CO2 and compound 3 were the major photoproducts reaching levels of up to 32.2 and 28.3% of the appllied radioactivity, respectively. The other 5 photoproducts were present at levels of ≤6.6% of the applied radioactivity. The detection of both compounds 3 and 15 demonstrated the potential for cleavage between the aryl rings. Further degradation ultimately leading to complete mineralisation of the test substance to CO2.

Identity of compounds:
Compounds 3: (6-(trifluoromethyl)pyridine-1H-2-one)
Compounds 4: methyl(Z)-3-methoxy-2-{2-[6-(trifluoromethyl)pyridine-2-yloxymethyl]phenyl}acrylate
Compounds 8: 2-[6-(trifluoromethyl)pyridine-2-yloxymethyl]-benzoic acid
Compounds 12: methyl 2-hydroxy-{2-[6-(thrifloromethyl)pyridine-2-yloxymethyl]phenyl}acetate
Compounds 13: methyl 2-oxo-{2-[6-(trifluoromethyl)pyridine-2-yloxymethyl]phenyl}acetate
Compounds 15: o-phthalic acid
Validity criteria fulfilled:
The test substance which reaches the soil surface has the potential to undergo rapid photolysis.
Executive summary:

The photolysis of the test substance on a soil surface was investigated. 14C-test substance, separately radiolabeled in either the phenylacrylate or pyridinyl ring, were applied at a rate equivalent to a field application of ca. 750 g a.i./ha, to thin layers (≤1 mm) of soil in individual photolysis vessels.

The treated soils were continuously irradiated using light from a xenon arc lamp which was filtered to give a spectral distribution close to that of natural sunlight. The samples were maintained at 20 ± 1°C and were irradiated for periods up to the equivalent of ca. 30 days 50°N summer sunlight.

For each label, a single sample was taken for analysis at nominal irradiation periods equivalent to 0, 1, 5, 10, 20 and 30 days 50°N summer sunlight. A 'dark control' sample was also prepared and maintained at Ca. 20°C for the duration of the irradiation. No significant degradation was apparent in the 'dark controls' indicating that the degradation in irradiated samples was due to photodegradation only.

The overall recoveries obtained for all samples were between 90.0 and 102.2%; trapped volatiles (14CO2) accounted for up to 32.2%.

The test substance degraded extensively under the experimental conditions. Degradation followed biphasic kinetics; the estimated DT50 was 7 days of 50°N summer sunlight. 14C-test substance was the major component in all the soil extracts, accounting for between 19.1 and 24.8% of the applied radioactivity at the final sampling interval. In addition to the parent compound, two photoproducts were present at levels 10% of the applied radioactivity at any point during the course of the study. These were identified as Compound 3 (reached a maximum level of 28.3% and subsequently degraded to level of 13.1% by the end of the irradiation period) and CO2 (32.2%). Five other photoproducts were identified (Compounds 4, 8, 12, 13, and 15), and of these the most significant was Compound 15 (6.6% maximum). There were no significant unknown photoproducts (the remaining individual unidentified photoproducts were all ≤3.2% of the applied radioactivity).

This study demonstrates that the test substance which reached the soil surface has the potential to undergo rapid photolysis, resulting in the formation of a number of photoproducts which are ultimately mineralisation to CO2 (25.2 and 32.2% found in the phenylacrylate and pyridinyl labels, respectively).

Description of key information

Degradation of the test substance followed biphasic kinetics; the estimated DT50 was 7 days of 50°N summer sunlight.

Key value for chemical safety assessment

Half-life in soil:
7 d

Additional information