Chlorpyrifos

Endpoint summary

Description of key information

The test substance is not classified as readily biodegradable. After 28 days of incubation with activated sludge, percentage biodegradation was 22%. 

 

The evaluation of chlorpyrifos in a natural surface water demonstrated dissipation through volatilization and hydrolysis to 3,5,6-trichloro-2-pyridinol. In the definitive test, the concentration of chlorpyrifos declined as the mass balance of the test systems declined. Mineralization to CO2was negligible (<1% AR). A subsequent experiment evaluated the volatilization of chlorpyrifos by adding a foam plug to trap any volatiles in the headspace of the sample tube. Up to 95.7% of the radioactivity was found in the foam plugs and shown to be 100% unchanged chlorpyrifos. These results demonstrate that the low mass balance observed from the later time points in the main test is due to evaporation of unchanged parent compound. Chlorpyrifos degraded in the surface water with DT50values of 46 and 21 days for the low (12 μg/L) and high (126 μg/L) concentration systems, respectively.

 

The transformation of chlorpyrifos was assessed in water/sediment samples from Calwich Abbey Lake and Swiss Lake, both systems were from the UK. Chlorpyrifos was applied to the water surface and samples were incubated for up to 150 days under aerobic conditions with associated overlying waters at a sediment/water ratio of 1:3 in the dark at 20 °C. Potassium hydroxide (KOH), ethylene glycol, foam plug, and Harvey Cocktail organic volatiles traps were used in flow-through aerobic test systems to collect14CO2and any volatile organic components that evolved during this study. Up to 30% of the applied radioactivity was found in the foam plug and rinses of the tubing used in the flow-through system and shown to be 100% unchanged chlorpyrifos. Chlorpyrifos also dissipated to the sediment and reached a maximum concentration of 62.4% AR at 14 days in the sediment layer. The DT50values from the water layer were 3.3 and 5.7 days for the Calwich Abbey Lake and Swiss Lake systems, respectively. The disappearance rate from the water/sediment total systems resulted in DT50values of 15 and 20 days for the Calwich Abbey Lake and Swiss Lake systems, respectively. One major metabolite was formed in the study, 3,5,6-trichloro-2-pyridinol (max. 55.9% AR, 28 days), formed by hydrolysis of the parent compound. Several minor transformation products were also formed; in all cases these transformation products represented <5% AR. Trapped14CO2accounted for less than 10% AR, with organic volatiles trapped in the ethylene glycol and Harvey cocktail not exceeding 0.1% for either system. 

 

In studies conducted to fulfill the current OECD 307 guideline (De Vette & Schoonmade (2001) and Clark (2013a), under aerobic conditions in soil, chlorpyrifos is converted to 3,5,6-trichloro-2-pyridinol, up to a maximum of 60.2% of applied radioactivity (AR) at 14 days, by hydrolysis and microbial degradation. Further degradation to minor metabolites (<10% of applied radioactivity) including 3,5,6-trichloro-6-methoxypyridine (TMP) was observed. Mineralization to CO2generally accounted for >50% AR at study termination. Organic volatiles were not observed. Non-extractable residues (NER) accounted for up to 27% AR by study termination.

 

Under anaerobic soil conditions in a current GLP study (Kang, 2014a), chlorpyrifos is converted to 3,5,6-trichloro-2-pyridinol, up to a maximum of 82.7% AR, after 45 days of incubation by microbial degradation. 3,5,6-trichloro-2-pyridinol is further degraded to 3,6-dichloro-2-pyridinol (DCP) up to a maximum of 71.0% AR at study termination. Mineralization to CO2accounted for up to 5.5% AR. Organic volatiles were observed at <1% AR. NER accounted for up to 26.1% of the applied radioactivity by study termination. The DT50values of chlorpyrifos ranged 3-9 days in anaerobic soil.