In this study we developed a 4-pool (parent compound, combined metabolites, non-extractable residues, and CO₂) kinetic model to facilitate speciation of non-extractable residues (NERs) of pesticides in soil. The model is developed to capture the characteristic kinetic behaviors of formation of NERs from parent compound and metabolites. Namely, if NERs form from parent compound, the combined metabolites (M(t)) mineralize to CO₂ only. However, if NERs form from a metabolite, the combined metabolites (M(t)) can be resolved into two components M₁(t) and M₂(t) at the metabolite that binds as NERs, with M₁(t) and M₂(t) kinetically driving NER formation and mineralization respectively. By leveraging the characteristic behaviors, the model not only provides a definitive answer on the source (parent compound vs. metabolites) of NERs but also identifies the kinetic driving metabolite for NER formation. The 4-pool kinetic analysis narrows the possible bound species down to kinetic driving metabolite itself or one of its degradation products. We tested the approach on an herbicide with >90% NERs. The model was applied first to the 4-pool data collected with the herbicide of three ¹⁴C-labels and then to the 4-pool data from ¹⁴C-labeled metabolite-dosed studies. It is found that two Stage 1 metabolites 5-OH and 7-OH kinetically drive NER formation, but both convert to 5,7-di-OH before bound as NERs. The same amount of NERs as in the herbicide studies was generated when the soils were dosed with ¹⁴C-labeled 5,7-di-OH. NERs could also be included as bound 5,7-di-OH in the kinetic pathway fit with major soil metabolite. The study demonstrates that the bound chemical species of NERs can be identified by applying the 4-pool kinetic analysis to ¹⁴C-labeled pesticide-dosed and then metabolite-dosed studies sequentially along degradation pathway.

Kari Lynn

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Speciation of non-extractable residues of pesticides in soil via 4-pool kinetic analysis

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