Reduction of oxamyl and related pesticides by FeII: influence of organic ligands and natural organic matter.

The reduction of oxamyl and related oxime carbamate pesticides (OCPs; methomyl and aldicarb) by FeII is an important pathway for the degradation of these compounds in soil and groundwater. A series of batch kinetic experiments was carried out to assess the effects that selected carboxylate and aminocarboxylate ligands have on these reactions. In the absence of FeII, no OCP reduction by the ligands is observed. In the presence of FeII, the rate of OCP reduction varies by several orders of magnitude and can be described by the expression k(red) = [FeII]sigma(i)k(i)alpha(i) where k(red) is the observed pseudo-first-order rate constant for OCP reduction, [FeII] is the total FeII concentration, alpha(i) is the fraction of each FeII species in solution, and k(i) is the second-order rate constant for OCP reduction by each FeII species. The reactivity of individual FeII species is dependent upon the standard one-electron reduction potential of the corresponding FeIII/FeII redox couple (E(H)o) and the availability of inner-sphere FeII coordination sites for bonding with Lewis base donor groups within the OCP structure. A linear free energy relationship is proposed. Kinetic measurements demonstrate that natural organic matter from the Great Dismal Swamp facilitates OCP reduction by FeII in the same manner as the individual organic ligands. Results from this study improve our understanding of the pathways and rates of pesticide degradation in reducing subsurface environments, especially those rich in organic matter.