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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

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Description of key information

The half-lives of the test substance in water, sediment and entire system were 4 – 4.87 d, 1.61 – 10.61 d and 4.37 to 5.94 d respectively.

Key value for chemical safety assessment

Half-life in freshwater:
4.87 d
at the temperature of:
20 °C
Half-life in freshwater sediment:
10.61 d
at the temperature of:
20 °C

Additional information

A study was conducted to evaluate the aquatic aerobic metabolism of the test substance according to OECD Guideline 308 and US EPA OCSPP Guideline 835.4300, in compliance with GLP. Water and sediment samples were collected from two different sites at Goose River, Grand Forks County, North Dakota (US) and Golden Lake, Steel County, North Dakota (US) from the surface of the water. Separate experiments were conducted using the test substance labeled in the phenyl ring (PH label) or the thiophene ring (TH label). The target dose rate for the study was 0.10 μg/mL substance based on the total water volume in the test vessel. For the high dose samples, a target concentration of 0.60 μg/mL was used to provide sufficient amounts of metabolites for identification if necessary. Using a 100 μL glass syringe, aliquots (100 μL) of the respective dose solution were delivered to each Goose River sample and 85 μL aliquots were delivered to each Golden Lake sample. After pre-incubation, individual sediment / water samples were dosed with radiolabelled test substance and were then incubated under anaerobic conditions at 20 ± 2°C for up to 129 d. Duplicate samples were collected immediately after treatment (Time 0) and after 1, 3, 7, 14, 30, 59, and 98 d of incubation at 20 ± 2°C for both groups. An additional sampling was conducted at 129 d for the PH-labeled samples. At each sampling, dissolved oxygen (DO), redox potential (ORP) and pH values were measured for each sample. Radioactivity in the water layers and sediment extracts was quantified by liquid scintillation counting (LSC). Following extraction, the residual radioactivity in the sediment was determined by combustion with subsequent radioassay. The test substance and its metabolites were quantified by high-performance liquid chromatography (HPLC) of the water phases and sediment extracts coupled with an online radioactivity detector. The identities of the test substance and its metabolites were confirmed by two-dimensional TLC, or by HPLC using a second method. The material balance was based on the sum of recovered radioactivity in the water layers, sediment extracts, bound / unextracted residues and trapped volatiles (ethylene glycol and NaOH traps).The total recoveries in the Goose River (GR) system averaged 95.2 ± 2.7% (PH label) and 92.5 ± 5.7% (TH label). In the Golden Lake (GL) system, total recoveries averaged 93.0 ± 4.0% (PH label) and 90.5 ± 7.0% (TH label). The test substance dissipates in aerobic aquatic systems through biotic degradation with eventual mineralization and formation of unextracted bound residues. Initial rapid metabolism of the test substance occurs by reductive cleavage of the N-O bond of the oxadiazole ring to form Iminoamide degradate. The iminoamide degradate is a somewhat transient intermediate that is further metabolized by amide hydrolysis to benzamidine and 2-thiophenecarboxylic acid. This hydrolysis step is almost certainly enzyme-mediated due to the exceptional stability of amide bonds to chemical hydrolysis. Mineralization of the test substance degradates to 14CO2 is a significant dissipation route. This is especially the case for the TH (thiophene) label from which large amounts of 14CO2 were produced during the study. Formation of unextracted/bound residues is considerable, especially from the PH (phenyl) label. Whereas the major route of dissipation of the thiophene portion of the test substance molecule is mineralization, formation of unextracted/bound residues with eventual slower mineralization is the major route of dissipation for the phenyl portion of the test substance molecule. Under the study conditions, half-lives of the test substance in water, sediment and entire system were observed to be 4 – 4.87 d, 1.61 – 10.61 d and 4.37 to 5.94 d, respectively (Ponte M, 2014).