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Environmental fate & pathways

Phototransformation in soil

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The photolysis of the radiolabelled test substance was studied in a test (BASF Crop Protection, 394791, 2013) on a soil obtained from New Jersey according to OECD guideline (Phototransformation of Chemicals on Soil Surfaces, Draft Document 2002). The DT50 of the test substance was determined to be 40.7 days for the sterilized soils in the dark control samples. Thus, photolytic degradation of the test substance in soil did not result in unique metabolites (photoproducts), nor did the influence of light appear to be an important dissipation pathway in/on soil. Metabolites observed in the soil photolysis study, including the dark control, were consistent with those from the aerobic soil metabolism study.

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The photolysis of the radiolabelled test substance was studied in a test (BASF Crop Protection, 394791, 2013) on a soil obtained from New Jersey according to OECD guideline (Phototransformation of Chemicals on Soil Surfaces, Draft Document 2002 in order to determine the rate of photolytic degradation of the active substance and to identify any major photoproducts formed.

The test compound was applied to a loam soil from New Jersey at an application rate of approximately 0.5 ppm, which is equivalent to 50 g/ha based on an assumed bulk density of 1.0 g/cm3 and 1 cm depth. Separate sets of non-sterile soil samples were dosed in duplicate with nicotinic or nicotinic acid label and pyranone label. An additional set of sterilized soil samples was dosed in duplicate with the pyranone label. The soil moisture was adjusted to approximately 50 % of the maximum water holding capacity prior to application of the test material and maintained throughout the experimental period by adding deionized water to the non-sterile soils and sterilized water to the sterile soils daily, if necessary. Samples were irradiated continuously for 15 days by light from a xenon-arc lamp filtered to remove wavelengths less than 290 nm in an Atlas Suntest CPS+ unit. The average light intensity both pre- and post-study, was 558 W/m2 for the nicotinic labeled samples and was 497 W/m2 for the pyranone labeled samples. Natural sunlight intensity in the spring at 40° N latitude is 584 W/m2. Control soil samples were similarly treated and were maintained in the dark at a temperature of 20 ± 2 °C during the course of the study. Both the irradiated and the dark control samples were analyzed concurrently at 0, 1, 3, 7, 10 and 15 days after treatment for the non-sterile soils and at 0, 3, 7 and 15 for the sterile soil. Volatile residues were collected for the irradiated and the dark control samples and analyzed at the soil sampling intervals. Each 20 g sample was extracted twice with 50 mL acetonitrile and three times with 50 mL water/acetonitrile (70/30; v/v). The extracts were pooled, concentrated, and analyzed by HPLC. Reference standards were used for co-chromatography for characterization in conjunction with MS/MS analysis of select samples. The mean material balance for the nicotinic label irradiated samples ranged from 92.1-99. 7% T AR. The mean material balance for the nicotinic label dark control samples ranged from 95.5-102% TAR. The mean cumulative volatile 14C-residues from the nicotinic label irradiated and dark control samples was 0.7% TAR and 0.8% TAR, respectively.

The mean material balance for the pyranone label irradiated non-sterile samples ranged from 91.8-100% TAR, and the matched dark controls ranged from 92.6-100% TAR. Recovered volatile 14C residues were 1.3% TAR for irradiated samples and 1.1 % TAR for dark controls. The mean material balance for pyranone label sterile soil samples was 99.5-101% TAR for irradiated samples and 101-103% TAR for the dark controls. lrradiated samples produced 1.9% TAR volatile 14C residues, while dark controls generated only 0.2% TAR in volatiles. The mean amount of radioactivity extracted from the nicotinic label irradiated samples decreased with time, from 99.0% of the applied radioactivity at 0 DAT to 79.1 % on 15 DAT. The extractable residues from the nicotinic label dark controls decreased with time, from 99.0 % of the applied radioactivity at 0 DAT to 81.0% on 15 DAT. The extracted activity from the pyranone label non-sterile samples also decreased with time, from 99.8% TAR at 0 DAT to 77.9% TAR on 15 DAT for the irradiated samples and 99.8% TAR to 69.7% TAR for the dark controls over the same time range. In the irradiated pyranone labelled sterile soil samples, the extracted activity decreased from 101% TAR to 86.2% TAR over 15 days. The dark controls showed a small decrease in extractable residues, producing 101 % TAR on 0 DAT and 94.0% TAR on 15 DAT.

The mean non-extractable radioactivity (NER) from the nicotinic label irradiated samples increased with time, from 0.5% of the applied radioactivity at O DAT to 12.3% on 15 DAT. The dark controls increased to 13.7% TAR at 15 DAT.

The NER from the pyranone label irradiated non-sterile samples increased with time, from 0.5 % of the applied radioactivity at 0 DAT to 12.6% on 15 DAT, while the dark controls reached 21.8% TAR at the end of the study. For the pyranone label samples, N ER in the sterile soil, both irradiated and dark controls, was of 0.2% TAR at 0 DAT. At 15 DAT, NER in the irradiated samples increased to 11.4% TAR, while the dark controls reached 9.2% TAR. Data for the nicotinic acid and pyranone labels were pooled for the non-sterile soil for kinetic analysis. In non-sterile soil, the first-order DT so of the test substance was determined to be 32.1 days in the irradiated samples and 8.4 days in the dark control samples. Slower degradation in the irradiated, non-sterile samples may be related to the rapid drying of the soil surface in the Xenon arc lamp irradiation chamber, despite daily adjustment of soil moisture to the original level. The phenomenon of slower degradation in the irradiated samples versus the dark controls is not uncommon for compounds that are quickly degraded by microbial action in soil. For sterilized soils, the test substance first-order DT50 was 43.8 days in the irradiated samples and 40.7 days in the dark control samples. The DT50 values in sterilized soil were essentially the same under irradiated and dark conditions, demonstrating that when microbial degradation is minimized there is little, if any, impact of light on the test substance degradation kinetics.

With irradiation, the test substance degraded only to minor products in non-sterile soil, including M4401002, M440I003, and M440I024, none of which exceeded 5% TAR. At 15 DAT, the remaining of the test substance in non-sterile soil ranged from 66.8% to 67.6% TAR. In irradiated sterile soil, M4401003 was identified as a minor product reaching 1.4% TAR while 76.9% TAR remained as the test substance at 15 DAT. In dark control soil samples, the test substance produced only one major metabolite, M4401003, during 15 days of incubation, reaching 13.4% TAR in non-sterile soil. Many minor products were observed in non-sterile soil in the dark including M4401001, M4401002, M4401005, M441016 and M4401024. In sterile dark control samples, M4401003 reached 12.1 % and M4401002 was observed as a minor product.

Given that the degradation kinetics of the test substance were essentially the same in sterile soil under both irradiated and dark conditions, and no unique degradation products were observed in irradiated samples versus dark controls, it may be concluded that there is a low potential for light to impact the environmental behavior of the test substance when in or on soil surfaces.