Degradation kinetics of tebufenozide in model aquatic systems under controlled laboratory conditions

Abstract

The hydrolysis of the insecticide tebufenozide was studied in the dark at 20 to 40°C in buffered (pH 4 to 10) distilled water, and at 20°C in unbuffered, sterilized and unsterilized stream water. Tebufenozide was very stable in acidic and neutral buffers at 20°C and the corresponding pseudo‐first‐order rate constants (k_obsd) and half‐lives (T_1/2) were 5.946 × 10^‐4 and 13.10 × 10^‐4 d^‐1, and 1166 and 529 d, respectively. The hydrolytic degradation was dependent on pH and temperature. At pH 10 and at 20,30 and 40°C, the k_obsd (10^‐4 d^‐1) and T_1/2 (d) values were 34.22, 66.72 and 130.0; and 203, 104 and 53.3, respectively. The energy of activation (E_a) values for the hydrolysis of tebufenozide at pH 4, 7 and 10, calculated from the Arrhenius plots, were 83.50, 66.71 and 50.87 kJ/mol, respectively. Tebufenozide was stable in sterilized stream water in the dark (T_1/2 = 734 d) but it degraded fairly rapidly in unsterilized stream water (T_1/2 = 181 d). Sunlight photodegradation of the chemical was slower (T_1/2 = 83.0 h) than the photolysis by ultraviolet radiations (T_1/2 values at 254 and 365 nm were 9.92 and 27.6 h, respectively); nevertheless, it was still appreciable during the summer months at 46°31’ N latitude. The differences in degradation rates between the unsterilized and sterilized stream water and the degradation of the chemical in the sterile, distilled water in sunlight, suggests that microbial processes and photolysis are the two main degradative routes for tebufenozide in natural aquatic systems.