The degradation of two oxime carbamate pesticides, oxamyl and methomyl, was investigated in anoxic solutions containing various metal ions and reducing agents. In reagent-free solutions, these carbamates degrade slowly via base-catalyzed elimination. Rates of carbamate degradation are accelerated by FeII, CuI, and CuII, but not by several other metal ions and reducing agents. In the presence of FeII, carbamates undergo a net two-electron reduction that is coupled to the sequential one-electron oxidation of two FeII ions. The observed products are a substituted nitrile, methanethiol, and methylamine. A radical intermediate is inferred by polymerization of the radical scavenger acrylonitrile. Redox kinetics (i) vary with carbamate identity, (ii) exhibit first-order dependence on both FeII and carbamate concentration, (iii) are relatively independent of pH, (iv) follow Arrhenius temperature dependence, and (v) are only indirectly influenced by the presence of O2. Coordinatively saturated FeII complexes (FeIIEDTA2- and FeII(CN)64-) react with oxamyl at rates equal to and greater than hexaquo FeII, respectively, indicating that an inner-sphere FeII-carbamate coordination complex is not required for electron transfer. Experimental results indicate that CuI reduces the carbamates by the same mechanism as FeII but at much higher rates. In contrast, CuII acts as a catalyst for both elimination and reduction reactions.
ASJC Scopus subject areas
- Environmental Chemistry