1,2,4-Oxadiazole, 3-phenyl-5-(2-thienyl)-

Biodegradation in soil

Description of key information

Calculated aerobic soil half-lives for the test substance varied widely (about an order of magnitude) across the four soils in the study. DT50 values were 20.2-23.5 d for the Manning sandy loam (KD) soil (FOMC model), 50.8-57.1 d for the Hoyleton silt loam (IL) soil (SFO model), 141-144 d for the Webster sandy clay loam (IA) soil (HS model), and 221-277 d for the Barnes-Svea clay loam (DU) soil (HS model). The test substance dissipated in all four soils during the study by formation of bound residues and through mineralization to 14CO2.

Key value for chemical safety assessment

Half-life in soil:

277 d

at the temperature of:

20 °C

Additional information

A study was conducted to assess the aerobic soil metabolism of the test substance according to OECD Guideline 307 and US EPA OPPTS Guideline 835.4100, in compliance with GLP. The radiolabeled test substance was applied to four agricultural soils collected in the US. Separate experiments were conducted using the test substance labeled in the phenyl ring (PH label) or the thiophene ring (TH label). For each radiolabel set, individual soil samples were treated with the test substance at a target rate of 0.62 μg/g (μg test substance/g dry soil). The samples were maintained at 20 ± 2°C in the dark except for the time 0 samples, which were processed immediately after dosing. The samples were incubated at 20°C for periods up to 123 d in a constant temperature room, protected from light. The soil samples were continuously aerated throughout the incubation period and the soil moisture was adjusted to maintain a pF of between 2.0 and 2.5. Traps for volatiles included an ethylene glycol trap to collect organic volatiles and two 10% aqueous NaOH traps to collect CO2. Duplicate samples were collected at Day 0, and at least seven additional time points for each set up to four months (123 d for the PH set, 121 d for the TH set). Total volumes in each trap were measured and aliquots were radioassayed by Liquid Scintillation Counting (LSC). The test substance and its degradates were quantified by high performance liquid chromatography (HPLC) of soil extracts with co-injection of appropriate analytical reference standards. The HPLC peak assignments were confirmed by thin layer chromatography (TLC). The test substance was the major component detected in the soil extracts by HPLC throughout the study. The test substance radiolabeled with phenyl ring, declined from a maximum average of 100.6% AR at time 0 to average 17.5%, 28.1%, 53.2% and 67.7% AR in the KD, IL, IA and DU soils, respectively, at the final sampling at Day 123. The test substance labeled with thiophene ring,declined from a maximum average of 98.4% AR at time 0 to average 11.2%, 23.5%, 57.4% and 67.9% AR in the KD, IL, IA and DU soils, respectively, at the final sampling at Day 121. Calculated aerobic soil half-lives for the test substance varied widely (about an order of magnitude) across the four soils in the study. DT50 values were 20.2-23.5 d for the Manning sandy loam (KD) soil, 50.8 -57.1 d for the Hoyleton silt loam (IL) soil, 141 -144 d for the Webster sandy clay loam soil (HS model) and 221 -277 d for the Barnes-Svea clay loam soil (DU). The DT50 estimates for the DU soil are extrapolated considerably past the end of the study (the test substance represented 67.5-68.2% AR at the end of the study) and, thus, there is significant uncertainty in these estimates. Estimated DT90 values for the test substance ranged from approximately 0.5 year for the Manning sandy loam (KD) and Hoyleton silt loam (IL) soils to ≥1.5 year for the Webster sandy clay loam (IA) and Barnes-Svea clay loam (DU) soils. As the estimated DT90 values for the IA and DU soils are well past the end of the study, they are considered to be rough estimates only. The aerobic rate of dissipation of the test substance in soil did not correlate well with soil biomass (r2 = 0.17) for the soils tested in this study, but appeared to be highly correlated (r2 = 0.99) inversely with clay content of the soils (longer DT50 values in soils with higher clay content). The major degradates in both labels were soil bound residues and 14CO2. No metabolites of the test substance were observed other than occasional detections of trace amounts. The test substance dissipated in all four soils during the study by formation of bound residues and through mineralization to 14CO2 (Shepler K, 2013).