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 FeII. Carbamate reduction by FeII 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 FeII speciation according to the expression kred = [FeII]∑ik1αi, where kred is the pseudo-first-order rate constant for carbamate reduction, [FeII] is the total FeII concentration, and ki and αi are the second-order rate constant and fractional concentration of each FeII species, respectively. It follows that the overall kinetics of carbamate reduction is a function of the identity and concentration of individual FeII species present in solution as well as the inherent reactivity of each species with carbamates. The magnitude of ki is related to the standard one-electron reduction potential (EH°) of the corresponding FeIII/FeII redox couple.
ASJC Scopus subject areas
- Environmental Chemistry