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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:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Study type:
direct photolysis
Qualifier:
according to
Guideline:
OECD Guideline 316 (Phototransformation of Chemicals in Water - Direct Photolysis)
Deviations:
no
Qualifier:
according to
Guideline:
other: U.S. EPA Fate, Transport and Transformation Test Guidelines, OPPTS 835.2240, Photodegradation in Water, October 2008.
Deviations:
no
Qualifier:
according to
Guideline:
other: MAFF Japan, Guideline 12 Nousan-8147/2-6-2 Studies of Photolytic Fate in Water, November 2000.
Deviations:
no
GLP compliance:
yes
Radiolabelling:
yes
Analytical method:
high-performance liquid chromatography
mass spectrometry
other: Liquid scintillation counting
Light source:
Xenon lamp
Light spectrum: wavelength in nm:
>= 290 - <= 800
Duration:
30 d
Temp.:
25 °C
Initial conc. measured:
5 other: µg/mL
Reference substance:
yes
Dark controls:
yes
Preliminary study:
In the preliminary study the material balance of radioactivity in individual samples ranged from 99.61-101.22% of the applied radioactivity
Test performance:
In the main study the material balance of radioactivity in individual samples ranged from 93.63-102.34% of the applied radioactivity.
Key result
DT50:
50.6 h
Test condition:
Irradiated pH 7 Buffer
Key result
DT50:
> 720 h
Test condition:
Dark Control pH 7 Buffer
Key result
DT50:
67 h
Test condition:
Irradiated Natural Water
Key result
DT50:
> 720 h
Test condition:
Dark Control Natural Water
Predicted environmental photolytic half-life:
The photolysis half-life of the test substance in sterile pH 7 buffer was 2.1 days under an irradiation:dark cycle of 12 h:12 h. In sterile natural water the photolytic half-life was 2.8 days under an irradiation:dark cycle of 12 h:12 h.
Transformation products:
yes
No.:
#1
Conclusions:
Photolytic degradation of the test substance was observed with one degradation product identified as IN-RUB93. Minimal degradation was observed in dark control samples. The photolysis half-life of the test substance in sterile pH 7 buffer was 2.1 days under an irradiation:dark cycle of 12 h:12 h. In sterile natural water the photolytic half-life was 2.8 days under an irradiation:dark cycle of 12 h:12 h. The results of this experiment indicate that photolysis is a mechanism of degradation of the test substance.
Executive summary:

The rate and route of photochemical degradation of [14C]-test substance (radiolabelled in the methylene, pyridine or fused pyrimidine positions) were determined in sterile 0.01 M pH 7 phosphate buffer or sterile natural water. Each test was conducted at a nominal concentration of 5.0 μg a.i./mL under a cycle of 12 hours irradiation and 12 hours darkness, using a xenon arc lamp for approximately 15 days irradiation (30 days in total) at 25 ± 1°C.

This study was conducted according to the guidelines OPPTS 835.2240 and OECD 316.

Methylene, pyridine or fused pyrimidine labeled [14C]-test substance was added, at a nominal rate of 5.0 μg a.i/mL, to 0.01 M pH 7 phosphate buffer in individual quartz glass photolysis vessels. The three forms of labeled [14C]-test substance were also added, at a nominal rate of 5.0 μg a.i/mL, to natural water in individual quartz glass photolysis vessels. All irradiated samples were placed under a xenon arc lamp within a Suntest accelerated exposure table unit and maintained at 25 ± 1°C. The lamp was equipped with filters to eliminate emitted wavelengths of <290 nm and reduce wavelengths greater than 800 nm to give a spectral distribution similar to natural sunlight. Non-irradiated (dark control) samples were also prepared for each radiolabel and test system and maintained in the dark at 25 ± 1°C.

Individual samples were analysed immediately following test item application (zero time) and after the following time periods of irradiation; 4, 8, 16, 20, 44, 116, 235 and 355 hours. Dark control samples were sampled and analysed at the equivalent 20, 44, 235 and 355 hours irradiation (ca 32, 80, 463 and 703 total hours in darkness); zero time served the irradiated and dark control systems. Samples were analyzed by HPLC with on-line radiochemical detection. In the pH 7 test solutions, the initial test item concentrations in the test incubates for all radio-labeled forms ranged from 4.52 to 5.22 µg a.i./mL. In the natural water test, the initial test item concentration range was 4.98 to 5.33 µg a.i./mL.

Material balance, calculated as the percent of applied radioactivity (% AR), was maintained in the irradiated and dark control samples throughout the study. Negligible applied radioactivity was detected in the volatile trapping agents. In the preliminary study the material balance of radioactivity in individual samples ranged from 99.61-101.22% of the applied radioactivity. In the main study the material balance of radioactivity in individual samples ranged from 93.63-102.34% of the applied radioactivity.

High performance liquid chromatography (HPLC) analysis showed the test substance remained relatively unchanged in the pH 7 buffer and natural water dark control samples. In these incubates, the calculated concentration of the test substance ranged between 92.18% AR and 101.20% AR in all samples.

In each of the irradiated test systems, it was demonstrated that [14C]-test substance degraded under irradiation. One major photodegradation product, identified as IN-RUB93, was detected in all irradiated test systems. This metabolite increased over the course of the study, reaching the maximum levels. Unidentified components consisted of multiple degradation products with no individual component accounting for >5% AR.

The % AR values from both pH 7 buffer and natural water samples were fitted to a simple first order kinetic model to derive half-life (DT50), rate constant and r2 values. Reliable degradation kinetics could not be determined for the dark control samples, as the extrapolated DT50 and DT90 values were considerably longer (ca 700 and ca 2300 days, respectively) than the study duration (30 days).

The photolysis half-life of the test substance in sterile pH 7 buffer was 2.1 days under an irradiation:dark cycle of 12 h:12 h. In sterile natural water the photolytic half-life was 2.8 days under an irradiation:dark cycle of 12 h:12 h.

Based on the data generated in this study, the test substance is susceptible to photolysis in pH 7 sterile buffer and natural water under irradiation using a xenon arc lamp to simulate natural sunlight. The test substance was stable in the dark control samples under experimental conditions.

Description of key information

The test substance is susceptible to photolysis in pH 7 sterile buffer and natural water under irradiation using a xenon arc lamp to simulate natural sunlight. The photolysis half-life was 50.6 hours in pH sterile buffer and 67 hours in sterile natural water. In each of the irradiated test systems, it was demonstrated that [14C]-test substance degraded under irradiation. One major photodegradation product, identified as IN-RUB93, was detected in all irradiated test systems. This metabolite increased over the course of the study. Unidentified components consisted of multiple degradation products with no individual component accounting for >5% AR. The test substance was stable in the dark control samples under experimental conditions.

Key value for chemical safety assessment

Additional information