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Environmental fate & pathways

Phototransformation in soil

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Description of key information

DT50 (days) = 17.6 (irradiated samples), 10.7 (dark controls)

Key value for chemical safety assessment

Additional information

Photo-transformation of [Isothiazol-3-14C, carboxamide-14 C]Isotianil in soil was studied by Hellpointner & Weiss (2013). The study was conducted according to GLP and followed US EPA Test Guideline OPPTS 835.2410, OECD Guideline draft (Phototransformation of Chemicals on Soil Surfaces), and DRAFT SANCO 11802/2010/rev 1 in accordance with Regulation (EC) No 1107/2009, 2011. The route and rate of photo-transformation was investigated on a silt loam soil (pH 6.2 (CaCl2), OC 1.9%) from Burscheid, Germany, at a nominal application rate of 200 g Isotianil / ha for approx. 10 days at 20 ± 1 °C and at a soil moisture of about 55 ± 5% of MWHC.

Radiolabeled [Isothiazol-3-14C, carboxamide-14C]Isotianil was applied directly to the surface of the soil samples at a nominal concentration of about 20.4 µg / test system with a surface of 10.2 cm² (3 g soil, dry weight). The treated samples were continuously

exposed to artificial irradiation (xenon lamp with < 290 nm cut-off filter, 640 W/m²). In addition, dark controls were set up. All test vessels were connected to traps for the collection of CO2 and organic volatiles. Samples were taken in duplicate after 0, 1, 2, 4,

6.24, 8.24 and 9.53 days of incubation for the determination of the amounts of the parent compound and its transformation products. The soil samples were extracted three times with 10 mL acetonitrile/water (80/20, v/v) at ambient temperature, once with 10 mL acetonitrile/water (50/50, v/v) at aggravated conditions (microwave extraction, about 70 °C) and once with 10 mL methanol/water (50/50, v/v) at aggravated conditions (microwave extraction, about 50 °C). The residues (Isotianil and transformation products) of the combined extracts were determined by liquid scintillation counting (LSC) and by reversed phase HPLC with radioactivity detection. The amounts of volatiles and non-extractable residues were determined by LSC and combustion/LSC, respectively. Identification of the parent compound in the stock solution was done by NMR, HPLC-MS and HPLC-MS/MS. The major transformation product in the extracts was identified by HPLC co-chromatography with reference substance.

The mean material mass balances were 94.8% (RSD: 1.4%) and 95.0% (RSD: 2.1%) of the applied radioactivity (AR) in the irradiated and dark samples, respectively. For irradiated test systems, the extractable radioactivity remained on a high level and varied between 88.2% (DAT-9.53) and 95.9% of AR (DAT-0) throughout the incubation period of 10 days. The amount of non-extractable residues (NER) was low and increased to a maximum of 4.3% of AR at DAT-8.24. For dark test systems, the extractable radioactivity remained on a high level and varied between 90.9% (DAT-2) and 96.4% of AR (DAT-4). NER was low and increased to a maximum of 1.6% at the end of study. In the irradiated test systems Isotianil decreased to 65.4 ± 2.2% of AR at the end of the study. Just one transformation product accounting for > 10% of AR was detected and identified as DCIT-acid. It reached a maximum content of 19.2 ± 1.7% of AR at the end of the study. One minor transformation product (ROI 2) increased up to 3.0% of AR. In addition, two minor metabolite peak zones (each individual accounted for ≤ 1.0% of AR) were characterized according to their retention time. The diffuse radioactivity not assigned to individual peaks was 1.7% of AR at maximum. Mean 14CO2 formation in the irradiated samples increased up to 1.5% of AR at the end of study. Formation of volatile organic compounds (VOC) was insignificant (values of ≤ 0.01% AR, always).

In the extracts of dark samples Isotianil decreased to 49.9 ± 3.1% of AR at the end of the study. One major transformation product was detected and identified as DCIT-acid. It reached a maximum proportion of 44.5 ± 0.0% of AR. No minor transformation products were detected in the extracts of dark soil samples. Diffuse radioactivity not assigned to individual peaks accounted for up to 1.3% of AR. Mean 14CO2 formation in the irradiated samples increased up to 1.1% of AR; formation of VOC was insignificant, again.

Based on the experimental DT50 value of 17.6 days for irradiated samples, the DT50 of Isotianil under kind of extreme environmental conditions is calculated to be 50.5 solar summer-days at Phoenix, Arizona, USA. There was no difference in the degradation pathway of dark and irradiated soil samples, despite the fact that Isotianil in dark samples degraded much faster and formed higher amounts of the degradation products, i.e. of DCIT-acid.

From this study, it is evident that photo-transformation of Isotianil on soil surface does not contribute to the elimination of this compound from the environment. No metabolites of Isotianil formed exclusively by sunlight irradiation on soil surface have to be considered for risk assessments.