AVS 49th International Symposium
    Surface Science Monday Sessions
       Session SS2-MoM

Paper SS2-MoM3
Influence of Surface Composition on the Pyrite Mediated Dechlorination Kinetics of Alachlor

Monday, November 4, 2002, 9:00 am, Room C-112C

Session: Oxide Structure and Surface Chemistry
Presenter: H. Fairbrother, Johns Hopkins University
Authors: H. Fairbrother, Johns Hopkins University
D.L. Carlson, Johns Hopkins University
M.M. McGuire, Johns Hopkins University
A.L. Roberts, Johns Hopkins University
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The effects of different pyrite surface compositions on the dechlorination kinetics of the herbicide alachlor were investigated. Chloroacetamides, including alachlor, are an important class of herbicides, which are encountered as ubiquitous contaminants in surface and groundwater. Currently, the understanding of the processes controlling the environmental fate and transformations of alachlor is somewhat fragmented. Alachlor can be dechlorinated by Fe(0). Because several iron sulfide minerals have been shown to dechlorinate low molecular weight alkyl and vinyl halides, we investigated the reactivity of alachlor with pyrite (FeS@sub 2@), a common minor constituent of sediments and aquifers. We found that the primary product is deschloroalachlor, the result of reductive dechlorination. This process may be important in natural systems, as previous work has identified deschloroalachlor in groundwater. In this study, we have employed a liquid cell directly coupled to a surface analysis chamber to examine the effect of surface defect sites on the pyrite mediated dechlorination kinetics of alachlor. The influence of surface chemical composition on the dechlorination of the organohalide alachlor (a herbicide) by pyrite was studied by XPS in conjunction with GC-MS. On natural pyrite surfaces, the dechlorination process exhibited first-order kinetics, indicative of a limited number of surface sites that were consumed during reaction. In contrast, pyrite surfaces modified by ion bombardment, which are dominated by sulfur vacancies, exhibited markedly different reaction kinetics characterized by an initial induction period of low reactivity during which time the surface is oxidized by water. These results indicate that monosulfide species are not responsible for alachlor reduction on pyrite. In a broader sense this study illustrates the intimate relationship that exists between surface chemical composition and reactivity at the liquid-solid interface.