Mechanistic Investigations of Alkyl Halide Contaminant Reduction by IronEPA Grant Number: FP916405
Title: Mechanistic Investigations of Alkyl Halide Contaminant Reduction by Iron
Investigators: Cwiertny, David M.
Institution: The Johns Hopkins University
EPA Project Officer: Jones, Brandon
Project Period: January 1, 2004 through December 31, 2006
Project Amount: $111,172
RFA: STAR Graduate Fellowships (2004) RFA Text | Recipients Lists
Research Category: Academic Fellowships , Fellowship - Environmental Engineering , Engineering and Environmental Chemistry
Halogenated solvents represent a class of ubiquitous groundwater contaminants that have proven to be both difficult and costly to treat via conventional methods such as pump-and-treat. In recent years, zero-valent iron (ZVI) permeable reactive barriers (PRBs) have emerged as an effective and cost-efficient treatment alternative for these and many other species of environmental concern. Despite the widespread application of iron PRBs to solvent plumes, several questions remain regarding the chemical reactions between iron and organic contaminants. Unlike vinyl halides solvents that have garnered significant attention in both laboratory and field-scale research with ZVI, alkyl halide species, such as 1,2-dibromoethane and 1,1,1-trichloroethane, have received much less focus. Given the differences in chemical structures between these two classes of organohalides (with vinyl halides possessing a π-bond system in contrast to the fully saturated bond systems of alkyl halides), it is possible that the behavior of these compound classes in iron systems will differ substantially. There is sufficient need, therefore, for a study that fully characterizes the reactivity of alkyl halide solvents in ZVI systems. In particular, the objectives of this current study are to determine the kinetic rate constants and to elucidate the reaction pathways and product species resulting from the reduction of alkyl halide solvents by ZVI. Results of this study will allow for better design and modeling of field-scale iron PRBs, while also providing the means to develop predictive tools for alkyl halide reactivity in iron systems. Such predictive tools can be used by regulators to determine if iron PRB installation is the proper treatment approach for a given groundwater site contaminated with such species.
To achieve the stated project goals, a series of batch studies will be performed to investigate the reactivity of more than 15 alkyl halide contaminants in ZVI systems. Additional batch studies will be conducted with select alkyl halides to assess those experimental factors most influential to system reactivity. Potential factors to be investigated include the identity of the buffer employed in batch systems (Tris versus carbonate), the initial pH of the reactor system, the mixing speed of batch systems, the effect of initial oxidant concentration, and the influence of varying the amount of iron surface area. Rate constants for alkyl halide reduction will be determined by applying a pseudo-first-order kinetic model to the concentration versus time data obtained from all batch systems. Gas chromatography with either flame ionization detection (GC/FID) or electron capture detection (GC/ECD) will be used to monitor both parent species decay and product species formation in batch systems. When necessary, gas chromatography with mass spectrometry detection (GC/MS) will be used to assist in the identification of product species. Kinetic rate constants obtained from these batch studies then will be used to develop quantitative structure-activity relationships for alkyl halide solvents in batch systems. Such relationships will be constructed by comparing rate constants for alkyl halide reduction by iron to a variety of thermodynamic parameters such as carbon-halogen bond dissociation energies and the lowest unoccupied molecular orbital energy levels for each species. Additional relationships will be developed by comparing rate constants for reduction in iron systems to rate constants for these species measured with other reductants such as zero-valent zinc, Fe(II) porphyrin, and aqueous phase Cr(II) that are reported in the literature. In addition to the aforementioned batch studies, studies investigating interspecies competition in systems with multiple alkyl halide species will be conducted to gain insights into the reactivity of iron in complex mixtures of contaminants, a situation much more likely to be encountered in a contaminated groundwater plume.