Influence of Surface Composition on the Kinetics of Alachlor Reduction by Iron Pyrite

Even though naturally occurring iron sulfide minerals have previously been shown capable of promoting reductive dehalogenations, the role of surface composition has not been fully investigated. Some researchers have proposed that sulfur species represent redox-active moieties on iron pyrite surfaces. Results from this study indicate that neitherthe stoichiometric (100) pyrite surface, nor monosulfide defects, play direct roles in the observed reduction of the herbicide alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl)acetanilide]. Pyrite surfaces were initially characterized by X-ray photoelectron spectroscopy (XPS); the samples were then transferred to a liquid cell coupled to the ultrahigh vacuum chamber. Aliquots were periodically removed from the liquid cell to monitor the appearance of the reductive dechlorination product, 2',6'-diethyl-N-(methoxymethyl)acetanilide. In experiments with unaltered pyrite (100) surfaces, rates of reaction decreased over time, even though no change in surface composition could be discerned via XPS. In contrast, ion-bombarded surfaces, which are dominated by monosulfide species, exhibit an initial induction period of low reactivity during which the highly defective surface is oxidized by water. Only after the monosulfide defects undergo oxidation does the rate of alachlor reduction increase, precluding a direct role for these defects as alachlor reductants.