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Environmental fate & pathways

Phototransformation in soil

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Reference
Endpoint:
phototransformation in soil
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to
Guideline:
OECD Guideline draft (Phototransformation of Chemicals on Soil Surfaces)
Deviations:
no
Qualifier:
according to
Guideline:
other: U.S. EPA Fate, Transport and Transformation Test Guidelines, OPPTS 835.2410, Photodegradation on Soil, October 2008
Deviations:
no
GLP compliance:
yes
Radiolabelling:
yes
Analytical monitoring:
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
% Moisture:
25.2
Temp.:
20 °C
Initial conc. measured:
5 mg/kg soil d.w.
Reference substance:
yes
Dark controls:
yes
Key result
DT50:
12.1 d
Test condition:
Irradiated Moist Soil; Single First Order
Key result
DT50:
227.8 d
Test condition:
Dark Control Moist Soil; Single First Order
Transformation products:
yes
Remarks:
IN-RPA19 and 3-trifluoromethylbenzoic acid
Conclusions:
The DT50 and DT90 values for the test substance in irradiated system were 12.1 days and 40.1 days, respectively. In non-irradiated (dark control) samples the DT50 and DT90 values were calculated as 227.8 days and 756.7 days, respectively, however these values are extrapolated well beyond the study duration.
The results of this experiment indicate that the test substance degrades photolytically on soil when exposed to artificial sunlight under laboratory conditions.
Executive summary:

The photodegradation of [14C]-test substance on the surface of a silty clay loam soil (pH 5.9, 1.9% organic carbon) from Illinois, USA was investigated at 20 ± 1°C for up to 30 days while being exposed to simulated sunlight and

intervening dark cycles. The study was conducted according to the methods outlined in the guidelines U.S. EPA OPPTS 835.2410 and OECD Guidelines for the Testing of Chemicals, Proposal for a New Guideline, Phototransformation of Chemicals on Soil Surfaces, January 2002.

Test item ([pyridine-2,6-14C]-test substance, [fused pyrimidine-3-14C]-test substance and [methylene-14C]-test substance) was applied to thinly-layered soil (moist soil samples at ca 75% field capacity and dry soil samples) to achieve a nominal concentration of 5.0 mg a.i./kg soil. The samples were irradiated under artificial irradiation using a SunTest®accelerated exposure table unit equipped with filters to eliminate wavelengths of <290 nm. An identical treatment to the same soil for each radiolabel was also made and kept in the dark as a control. The three sites of radiolabelled [14C]-test substance served as replicates.

The moist irradiated soil samples were analyzed at 0, 4, 8 and 16 hours and 1, 2, 5 and 15 days total irradiance and the dry soil samples at 0, 1 and 5 days total irradiance. The moist dark control soil samples were analyzed at 0, 4, 8 and 16 hours and 1, 2, 5 and 15 days post application. The soil samples were extracted three times with acetonitrile. Selected samples were further extracted where extraction efficiency remained <90% AR after three acetonitrile extracts. Extracts were analyzed by reverse-phase HPLC to determine the composition of the soil extracts. Identification of the degradation products was performed by LC-MS analysis.

The material balance for the moist irradiated test systems were quantitative (except for one sample at 87.45% AR) with individual values ranging between ca 92% to 106% AR over the incubation period. The material balance for the dry irradiated test systems were quantitative with individual values ranging between ca 90% to 105% AR over the incubation period. The material balance for the moist non-irradiated (dark control) test systems were also quantitative with individual values ranging between ca 98% to 106% AR over the incubation period. Extractable radioactivity recovered from moist irradiated soil samples was quantitative at zero time (ca 96-97% for all 3 radiolabels) and decreased to ca 56% to 71% AR by Day 15 total irradiation. Non-extractable residues reached a maximum of ca 33% AR in the moist irradiated samples by the end of the study. Radioactivity associated with 14CO2 reached a maximum of 1.6% AR in the moist irradiated samples following 15 days total irradiation. Radioactivity associated with volatile organics remained at levels close to or below the limit of quantification throughout the incubation period. Extractable radioactivity recovered from dry irradiated soil samples was quantitative at zero time (ca 98% to 99% AR for all 3 radiolabels) and decreased to ca 83% to 88% AR by Day 5 total irradiation. Non-extractable residues reached a maximum of ca 10% AR in dry irradiated samples. Radioactivity associated with 14CO2 reached a maximum of ca 1.5% AR in the dry irradiated samples following 5 days total irradiation. Radioactivity associated with volatile organics remained at levels close to or below the limit of quantification throughout the incubation period. Extractable radioactivity recovered from moist dark control soil samples was quantitative at zero time (ca 96-97% AR for all 3 radiolabels) and decreased slightly to ca 91% to 92% AR by Day 15. Non-extractable residues reached a maximum of ca 9.5% AR in moist dark control samples. Radioactivity associated with 14CO2 and volatile organics remained at levels close to or below the limit of quantification throughout the incubation period.

HPLC analysis of moist irradiated soil extracts indicated that [14C]-test substance declined from quantitative levels at Day 0 to ca 43% to 47% AR following 15 days total irradiation. A polar photodegradation product was detected in the pyridine and methylene label samples only and reached a maximum value of 9.7 % AR following 5 days total irradiation in the pyridine samples and 10.6% AR following 15 days total irradiation in the methylene samples. This polar component was structurally identified by LC-MS and then confirmed by co-chromatography as the metabolite IN-RPA19. Another photodegradation product was detected in the pyrimidine samples only and reached a maximum of 20.6 % AR following 5 days total irradiation before declining to 12.0% AR at Day 15 total irradiation. LC-MS analysis identified this component as 3-trifluoromethylbenzoic acid. Multiple minor degradation products were detected but not identified as none of these individually accounted for greater than the identification trigger level (approaching or exceeding 10% AR). HPLC analysis of dry irradiated soil extracts indicated that [14C]-test substance declined from quantitative levels at Day 0 to ca 67% to 74% AR following 5 days total irradiation. Degradation products detected in the moist irradiated soil samples were also detected in the dry irradiated soil samples. IN-RPA19 was detected in the pyridine and methylene label samples and reached a maximum of 7.4% AR following 5 days total irradiation. 3-Trifluoromethylbenzoic acid was detected in the pyrimidine samples following 5 days total irradiation reaching 14.7 % AR. Multiple minor degradation products were detected but not identified as none of these were above the identification trigger level (10% AR). HPLC analysis of moist dark control soil extracts indicated that [14C]-test substance declined from quantitative levels at Day 0 to ca 90-91% AR following 15 days in dark conditions. No degradation products >10% AR were detected at any sampling interval.

Degradation of [14C]-test substance in irradiated soil samples occurred primarily via photolytic degradation and generated IN-RPA19 and 3-trifluoromethylbenzoic acid as major metabolites. The degradation was similar in both moist and dry soil layers indicating that the process is reasonably attributed to physico-chemical processes on the soil surface.

The DT50 and DT90 values of [14C]-test substance were calculated using a simple first-order (SFO) model. The DT50 and DT90 values for the test substance in irradiated system were 12.1 days and 40.1 days, respectively. In non-irradiated (dark control) samples the DT50 and DT90 values were calculated as 227.8 days and 756.7 days, respectively, however these values are extrapolated well beyond the study duration.

The results of this experiment indicate that the test substance degrades photolytically on soil when exposed to artificial sunlight under laboratory conditions.

Description of key information

When exposed to artificial sunlight under laboratory conditions, the test substance degraded photolytically on soil. The DT50 and DT90 values for the test substance in irradiated system were 12.1 days and 40.1 days, respectively.

Key value for chemical safety assessment

Additional information