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Phototransformation in soil

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Degradation of [pyrimidinyl-5-14C]substance in Maryland sandy loam soil under irradiated and dark control samples was rapid and biphasic. Mean DT50 and DT90 values are 0.605 and 2.05 days respectively. DT50 and DT90 values for dark control samples are 0.83 and 2.83 days, respectively. Degradation pathways in both sets of samples were similar with the degradates forming and subsequently declining earlier in the dark control samples. Soil photolysis is not a major degradation pathway, microbial degradation is the primary degradation mechanism. EPA N 161 -3, Atkins 1999a
Degradation of [phenyl-U-14C]substance was monophasic in sterile Maryland sandy loam soil under irradiated and dark control conditions. Half-life values for the irradiated and dark control samples are 32.3 and 22.0 days, respectively. The rate constants are 0.02147 and 0.03157 for the irradiated and dark control samples, respectively. The DT50 and DT90 values for the irradiated samples are 28.94 and 103.90 days, respectively. DT50 and DT90 values for the dark control samples are 19.86 and 70.84 days, respectively. Soil photolysis is not a degradation route, substance is degraded by microbial action and most likely due to hydrolysis, EPA N 161 -3, Atkins 1999b

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EPA N 161 -3, Atkins 1999a: The test substance photodegradation in soil was investigated using EPA Guideline Subdivision N 161-3 (Photodegradation Studies on Soil). In a soil photolysis study, the degradation of [pyrimidinyl-5-14C]test substance in irradiated and dark control samples was rapid and biphasic. The DT50 and DT90 values for the Definitive I dark control samples are 0.60 days and 2.07 days, respectively. DT50 and DT90 values for the Definitive II irradiated samples are 0.61 and 2.03 days, respectively. DT50 and DT90 values for the Definitive II dark control samples are 0.83 and 2.83 days, respectively. Degradation pathways in both sets of samples were similar with the degradates forming and subsequently declining earlier in the dark control samples. Soil photolysis is not a major degradation pathway. Microbial degradation is the primary degradation mechanism.

Test substance was shown to be reactive under the conditions of this study. By Day 9 in both Definitive I and II, test substance had degraded below the radioactivity detection limit. Degradation products observed in both the irradiated and dark control samples (Definitive I and II) were CGA-293731, CGA-293730 and CGA-380963, sequentially. The initial degradate, CGA-293731, was present by the Hour 5 sampling and reached a maximum concentration of 50.8 (Hour 28), 61.0 (Hour 23) and 48.4% (Day 2) of the applied radiocarbon (mean of replicates) for Definitive I dark control and Definitive II irradiated and dark control samples, respectively. The secondary degradate, CGA-293730, was first observed in the Hour 23 samples and reached a maximum concentration of 81.0 (Day 9), 78.3 (Day 30) and 85.0% (Day 9) of the applied radiocarbon (mean of replicates) for the Definitive I dark control and Definitive II irradiated and dark control samples, respectively. Succeeding degradate CGA-380963 appeared at Day 6, accounting for 14.5, 5.6 and 13.2% of the applied radiocarbon (mean of replicates) by Day 30 for the Definitive I dark control and Definitive II irradiated and dark control samples, respectively. No unknowns were detected throughout either 30-day incubation.

EPA N 161 -3, Atkins 1999b: The test substance photodegradation in soil was investigated using EPA Guideline Subdivision N 161-3 (Photodegradation Studies on Soil). In a photolysis study with sterile soil, following 31(30 IRR) days of incubation, only 50.4 and 40.3% (mean) of the applied radiocarbon remained as [phenyl-U-14C]test The degradation of [phenyl-U-14C]test substance in the sterile Maryland sandy loam soil irradiated and dark control soil samples was retarded and monophasic (one compartment). Half-life values for the irradiated and dark control samples are 32.3 and 22.0 days, respectively. The rate constants are 0.02147 and 0.03157 for the irradiated and dark control samples, respectively. The DT50 and DT90 values for the irradiated samples are 28.94 and 103.90 days, respectively. DT50 and DT90 values for the dark control samples are 19.86 and 70.84 days, respectively. Photolysis is not a degradation route for test substance and that it is degraded by microbial action. Degradation was most likely, due to hydrolysis.

The major degradation products formed were CGA-293731, accounting for a maximum of39.9 and 52.7% of the applied radiocarbon (mean of replicates) and CGA-293730, accounting for 3.3 and 0.6% (mean) of the applied radiocarbon in the irradiated and dark control samples, respectively, at Day 31(30 IRR). CGA-98166 was present only in the irradiated samples, accounting for only 0.3% of the applied radiocarbon at Day 31 (30 IRR). Additional minor unknown degradates ≤2.3% (mean) of the applied radiocarbon, were observed during the study. Material balance throughout the study was 101.2 ± 1.2 and 101.0 ± 1.1% (mean± standard deviation) of the applied radiocarbon for the irradiated and dark control samples, respectively.

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