Reduction of Oxamyl and Related Pesticides by Fe^II:  Influence of Organic Ligands and Natural Organic Matter

The reduction of oxamyl and related oxime carbamate pesticides (OCPs; methomyl and aldicarb) by Fe^II 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 Fe^II, no OCP reduction by the ligands is observed. In the presence of Fe^II, the rate of OCP reduction varies by several orders of magnitude and can be described by the expression k_red = [Fe^II]∑_ik_iα_i, where k_red is the observed pseudo-first-order rate constant for OCP reduction, [Fe^II] is the total Fe^II concentration, α_i is the fraction of each Fe^II species in solution, and k_i is the second-order rate constant for OCP reduction by each Fe^II species. The reactivity of individual Fe^II species is dependent upon the standard one-electron reduction potential of the corresponding Fe^III/Fe^II redox couple (E_H°) and the availability of inner-sphere Fe^II 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 Fe^II 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.