Mineral surface catalysis of reactions between Fe^II and oxime carbamate pesticides

This study examines the reduction of oxime carbamate pesticides (oxamyl, methomyl, and aldicarb) by Fe^II in aqueous suspensions containing twelve different (hydr)oxide and aluminosilicate minerals. In the absence of Fe^II, mineral surfaces have no apparent effect on the pathways or rates of oxime carbamate degradation. In anoxic suspensions containing Fe^II and mineral surfaces, rates of oxime carbamate reduction are significantly faster than in equivalent mineral-free homogeneous solutions. Rates increase with increasing surface area loading (mineral surface area per volume of suspension) and pH. Kinetic trends are interpreted in terms of changes in Fe^II speciation. Quantitative modeling indicates a first-order dependence on total adsorbed Fe^II concentration and no significant dependence on adsorbed oxime carbamate concentration. Bimolecular rate constants describing the reactivity of adsorbed Fe^II with dissolved oxamyl decrease in the following order: silicon dioxide #2 > silicon dioxide #1 ≫ hematite #2 > titanium dioxide #1 > hematite #1 > titanium dioxide #2 > silicon dioxide #3 > aluminum oxide > kaolinite #1 > kaolinite #2 > goethite ≫ titanium dioxide #3. Possible factors responsible for the increased reactivity of adsorbed Fe^II, as well as for the relative reactivity of Fe^II adsorbed on different surfaces, are discussed. Results from this study demonstrate that mineral surfaces present in subsurface environments can substantially catalyze the reduction of oxime carbamate pesticides by Fe^II. Overall rates of pesticide degradation may be under predicted by > 1 order of magnitude if the effects of mineral surfaces are not accounted for.