Insecticide Indoxacarb metabolites JT333 and MP819 were used to assess the utility of kinetic modeling to elucidate metabolic pathways and determine the degradation kinetics of the non-extractable residue (NER) in the soil. Soil adsorption and metabolism studies were conducted in the laboratory. Data were grouped into four compartment-pools, Parent (P), extractable metabolite (MET), NER and CO2. Inverse kinetic modeling was applied to analyze the dynamic relationship of P dissipation and the formation of MET, NER and CO2 as part of the process to elucidate metabolic pathways. JT333 and MP819 share similar chemical structures, showed high affinity to soil (Kd &gt;5000 L/kg), rapid metabolism (DT50 around 4-10 days) and significant CO2 formation in soil. However, they degraded via different pathways. Formation of METs followed by their mineralization to CO2 was the major pathway for JT333. While multiple pathways were involved in the degradation of MP819, the formation of NER was predominant. The time-exposure area under the curve (ACU) of the MET was derived from the time-%concentration plot by the trapezoidal rule to facilitate the investigation of the rate limiting step in the pathway. When MET mineralization is the rate limiting step, the rate constant of P to MET metabolism is larger than the constant of MET to CO2 and greater MET AUC accumulation was observed. In both JT333 and MP819, the rate of NER degradation appeared much slower compared to the rate of P-MET metabolism and MET-CO2 mineralization, likely due to the rate-limiting step of NER release from the soil, indicating that in this situation free-state NER would be unlikely to accumulate in soil. The study reported here demonstrates the application of kinetic modeling and the AUC determination to understand the rate of metabolism/NER release relationship that would affect the accumulation of free-state NER in soil, therefore, the potential for the quantitative NER environmental risk assessment.

Application of kinetic modeling to predict the fate of bound residue degradation in soil

254th National Meeting (2017)

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