Reduction of the pesticides oxamyl and methomyl by Fe¹¹: Effect of pH and inorganic ligands

This work examines the effect that pH and selected inorganic ligands have on the kinetics of reactions between Fe¹¹ and two structurally related oxime carbamate pesticides, oxamyl and methomyl. In anoxic solutions containing Fe¹¹, these compounds degrade by parallel elimination and reduction pathways. Rates of Fe¹¹-independent carbamate elimination (Elcb mechanism) are proportional to [OH^-], increasing 10-fold for each unit increase in pH. In homogeneous solution, rates of carbamate reduction by 0.5 mM Fe¹¹ are relatively constant at pH <7, but increase dramatically between pH 7 and pH 8.3. At pH >8.3, Fe(OH)₂(s) precipitation occurs, and carbamates react with both solution-phase and solid-phase Fe^II. Carbamate reduction by Fe^II is not significantly affected by the presence of chloride, bromide, nitrate, perchlorate, and sulfate. In contrast, increased rates of carbamate reduction are observed in solutions containing fluoride, carbonate, and phosphate. Kinetic measurements are interpreted in terms of changing Fe^II speciation according to the expression k_red = [Fe^II]∑_ik₁α_i, where k_red is the pseudo-first-order rate constant for carbamate reduction, [Fe^II] is the total Fe^II concentration, and k_i and α_i are the second-order rate constant and fractional concentration of each Fe^II species, respectively. It follows that the overall kinetics of carbamate reduction is a function of the identity and concentration of individual Fe^II species present in solution as well as the inherent reactivity of each species with carbamates. The magnitude of k_i is related to the standard one-electron reduction potential (E_H°) of the corresponding Fe^III/Fe^II redox couple.