Mechanistic Investigations of Fe(0) Reactions with OrganohalidesEPA Grant Number: R828164
Title: Mechanistic Investigations of Fe(0) Reactions with Organohalides
Investigators: Roberts, A. Lynn , Fairbrother, D. Howard
Institution: The Johns Hopkins University
EPA Project Officer: Lasat, Mitch
Project Period: September 1, 2000 through August 31, 2002
Project Amount: $225,000
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Engineering and Environmental Chemistry
Description:This research will investigate several poorly-understood aspects of the reaction of alkyl and vinyl halides with Fe(0): (a) effect of organohalide structure on reaction rates and products; (b) influence of intra- and interspecies competition on contaminant transformation rates; (c) effect of oxide coatings and surface sulfur species on reaction rates and products. We hypothesize that mechanisms through which alkyl and vinyl halides react are fundamentally different. For alkyl polyhalides, reactions are initiated by dissociative electron transfer. Reactivity should therefore increase sharply with increasing halogenation (and therefore one-electron reduction potential or E1 value). For vinyl halides, data obtained by our research group demonstrates clearly that reactivity decreases markedly with increasing halogenation (E1). We have tentatively interpreted this to reflect a mechanism in which the rate-limiting step involves conversion of a p-bonded surface-bound intermediate to a di-s bonded surface-bound intermediate.
Approach:To test these hypotheses, we propose to investigate the kinetics and products pertaining to reaction of Fe(0) (and associated Fe(II)/Fe(III) (hydr)oxide phases) with a wide array of alkyl and vinyl halides in aqueous solution in batch and column reactors, as well as in an electrochemical cell coupled to a surface analysis chamber to facilitate studies of surface composition changes via scanning Auger Electron Spectroscopy. Our investigations will emphasize the effects of: (a) degree of halogenation and identities of halogens present; (b) presence of bulky alkyl substituents near the reaction center; (c) intra- and interspecies competition for surface sites; and (d) effect of iron surface composition on reaction kinetics and products. Reaction products will be carefully identified, and particular attention will be paid to mass balance.
Expected Results:This research will substantially augment the existing database of kinetic constants for organohalide reaction with Fe(0), and will also provide practitioners with information concerning potentially persistent products that might be of regulatory concern in the effluent from a permeable barrier. The fundamental insight into reaction mechanism and how reactivity and selectivity are influenced by the evolving composition of the reactive surface will facilitate application of this increasingly popular treatment approach to new organohalide contaminants.
Improvements in Risk Management:
The ability to predict reaction products (and their likely longevity in a permeable Fe(0) barrier), as well as the kinetic models encompassing intra- and interspecies competitive effects, will facilitate cost-effective design of permeable barriers.