Chlorpyrifos

Endpoint summary

Description of key information

The hydrolysis of chlorpyrifos has been found to be independent of pH below pH 7 with a half-life of approximately 72 days in this region. At alkaline pH, hydrolysis is dependent on pH with a measured half-life for chlorpyrifos of 16 days at pH 9, 25 deg C. Under conditions encountered in the environment, where other dissipative processes act on the chemical, hydrolysis will tend to be a minor route for dissipation of the chemical. The average aqueous photolysis half-life of chlorpyrifos was approximately 30 days and could provide a significant route of degradation for this compound in the aquatic environment. Photolysis is not a major route of degradation in soil systems. 

 

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

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. Under anaerobic conditions in soil, the DT50values of chlorpyrifos ranged 3-9 days.

 

BMF values in Catfish are 0.045, 0.0085, and 0.16 for whole fish, muscle, and viscera tissue, respectively. Indicating that chlorpyrifos residues accumulated via the food chain will likely be equivalent to 20% or less of concentrations of the compound in foodstuffs. BCF values in Rainbow trout are 1374 ± 321 (whole fish); 725 ± 136 (fish fillet).  

 

Koc values ranged from 2785 -7965 mL/g at 20 °C and test indicated that the test substance degraded during equilibration with some phases showing radiopurities as low as 67% 14C-chlorpyrifos. The only degradation product observed in the adsorption/desorption study was shown to be 3, 5, 6-trichloro-2-pyridinol (TCP).