Identifying bound compounds in non-extractable residues of pesticides in soil by 4-pool kinetic analysis

This study explores the feasibility of identifying bound compounds in non-extractable residues (NERs) of pesticides in soil by 4-pool kinetic analysis. The 4-pools refer to parent compound, metabolites, NERs, and CO₂ in ¹⁴C-labeled pesticide soil degradation studies. We discovered the following two characteristic 4-pool kinetic behaviors of formation of NERs: (1) if parent compound is bound as NERs, the metabolites (m(t) in % applied radioactivity (AR)) kinetically drive the evolution of CO₂ only; and (2) if a metabolite (x) in a sequential degradation pathway is bound as NERs, m(t) is split into m₁(t) and m₂(t) at the metabolite (x) that is bound as NERs, which kinetically drive the formation of NERs and evolution of CO₂ respectively. We developed two (i.e., Parent➔NER and metabolite➔NER) 4-pool models to capture the kinetic behaviors respectively. By fitting the models to a set of 4-pool data, we not only determine whether NERs form from parent compound or metabolites but also identify the bound metabolite (x) by resolving the metabolites into M₁(t) and M₂(t) (i.e., the variables used to simulate m₁(t) and m₂(t)) and then matching m₁(t) (i.e., the sum of the bound metabolite (x) and its metabolite precursors) with M₁(t). By applying the models to ¹⁴C-labeled parent compound-dosed studies and then sequentially to metabolite-dosed studies for 7 pesticides with ≥70 % AR NERs, we identified the bound compounds, which have the moieties known to be responsible for NERs, are bound as NERs instantaneously when dosed into soil, and account for pH dependency of NER formation.