Grantee Research Project Results
2001 Progress Report: Mechanistic Investigations of Fe(0) Reactions with Organohalides
EPA Grant Number: R828164Title: Mechanistic Investigations of Fe(0) Reactions with Organohalides
Investigators: Roberts, A. Lynn , Fairbrother, D. Howard
Institution: The Johns Hopkins University
EPA Project Officer: Aja, Hayley
Project Period: September 1, 2000 through August 31, 2002
Project Period Covered by this Report: September 1, 2000 through August 31, 2001
Project Amount: $225,000
RFA: Exploratory Research - Engineering, Chemistry, and Physics) (1999) RFA Text | Recipients Lists
Research Category: Water , Land and Waste Management , Air , Safer Chemicals
Objective:
The objective of this project is to investigate poorly understood aspects of the reactions between alkyl and vinyl polyhalides and Fe(0). At present, such barriers are principally being used as a means of treating chlorinated ethenes in groundwater; their applicability for treatment of halogenated alkanes, commonly encountered as co-contaminants, is less well understood. Particular interest is being paid to those factors such as chemical structure, inter- and intraspecies competition, and Fe(0) surface area that govern the rate of reaction of alkyl and vinyl halides with Fe(0), thereby controlling their removal in Fe(0)-based permeable reactive barriers. The present investigations are largely being conducted using Fe(0) batch systems, although the usefulness of other techniques for determining reaction rates (such as involving electrochemical methods) also is under investigation. Ultimately, we plan to develop a high-quality database of kinetic, mechanistic, and product information for common organohalides that can be used to develop models for predicting the reactivity of alkyl and vinyl halides not previously investigated. We also intend to investigate the effect of the Fe(0) surface composition on reaction rate through the use of an electrochemical cell coupled to an ultrahigh vacuum surface analysis chamber. With this device, we can control the composition of surface iron oxide species, allowing us to determine the influence of Fe(II)/Fe(III) (hydr)oxide film composition (and oxide film thickness) on organohalide reactivity with Fe(0). This project will provide a more complete understanding of the factors controlling the reactivity and hence utility of Fe(0) systems.
Progress Summary:
The focus of the project thus far has been to gain a better understanding of the role of Fe(0) surface area in governing the rate of reaction in batch experiments with the alkyl halide 1,1,1-trichloroethane (and, to a lesser extent, 1,1,2,2-tetrachloroethane). For reactions involving Fe(0), current practice in the literature is to invoke a surface area-normalized kinetic model in which a linear, first-order relationship is assumed between Fe(0) surface area and the pseudo first-order rate constant for reaction. Careful studies to date in our group have demonstrated that the relationship between the pseudo first-order rate constant and Fe(0) surface area deviates from first order and, therefore, is nonlinear. Experiments have ruled out a wide number of possible explanations for this behavior (such as mass transfer-limited kinetics, pH and buffer effects, inter- or intra-species competition, and variable experimental parameters such as mixing techniques). Additional batch experiments are under way in search of a cause for this behavior. Similar results have been observed for other compounds and by other investigators, though not widely recognized.
This finding is significant as it could dramatically alter the manner in which rate constants for reactions with Fe(0) are reported in literature. These results suggest that the current trend of reporting surface area-normalized rate constants is inadequate, and that extrapolation of rate constants determined in such a manner to the design of Fe(0) permeable reactive barriers could lead to significant errors.
These findings illustrate the complex nature of batch systems for investigating reactions between Fe(0) and alkyl halides. We consequently are investigating alternative analytical methods to validate the results obtained from our batch studies. Electrochemical methods have the potential to yield kinetic rate information, mechanistic insights, and product distributions for reactions of an Fe(0) electrode with organohalides that can confirm the reactivity trends observed in batch systems.
In addition to these studies, construction of the coupled electrochemical cell and surface analytical chamber has now been completed. The apparatus consists of a custom-made liquid cell (with ports for electrodes and gas purging) attached to an ultra-high vacuum surface analysis chamber (with capabilities for x-ray photoelectron spectroscopic (XPS) analysis). It provides for the transfer of oxygen-sensitive samples from the liquid cell to the surface analysis chamber without exposure to the ambient atmosphere. Preliminary experiments using the liquid cell have been successful, and extension of its use to Fe(0) systems will begin in the immediate future.
Future Activities:
In the immediate future, work will begin on the development of a high-quality database containing kinetic, mechanistic, and product information for environmentally relevant alkyl halide species. Batch systems will be used to investigate a wide variety of chlorinated and brominated alkyl polyhalide contaminants that pose a significant threat to groundwater supplies. This database then will be used to develop predictive tools such as linear free energy relationships by relating the experimentally determined rate data to a variety of molecular descriptors such as one-electron reduction potentials (E1), two-electron reduction potentials (E2), and bond dissociation energies (BDEs). Such linear free energy relationships are valuable as they can be used as a predictive means for estimating the reactivity of an alkyl halide not previously investigated in a laboratory study.
In addition, we will continue work in electrochemical techniques as an alternative method for investigating Fe(0) reactivity. We plan to investigate a similar range of alkyl halide species electrochemically as were investigated in batch studies.
Finally, studies with an electrochemical cell coupled to a surface analysis chamber will be initiated. The use of this cell will allow us to answer many significant questions regarding the role of the iron oxide surface coating and its effect on Fe(0) reactivity. In particular, reactivity and selectivity can be studied as a function of the nature and thickness of the (hydr)oxide coating. Other potential topics for investigation include the effect of surface impurities (such as sulfur) on Fe(0) reactivity.
Journal Articles on this Report : 1 Displayed | Download in RIS Format
Other project views: | All 3 publications | 2 publications in selected types | All 2 journal articles |
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Cwiertny DM, Roberts AL. On the nonlinear relationship between kobs and reductant mass loading in iron batch systems. Environmental Science & Technology 2005;39(22):8948-8957. |
R828164 (2001) R828164 (2002) |
Exit Exit |
Supplemental Keywords:
groundwater remediation, chlorinated organic solvents, VOCs., RFA, Scientific Discipline, Toxics, Waste, Sustainable Industry/Business, National Recommended Water Quality, cleaner production/pollution prevention, Sustainable Environment, Chemistry, Technology for Sustainable Environment, Hazardous Waste, Engineering, Environmental Engineering, Groundwater remediation, Hazardous, reaction engineering, halogens, environmental chemistry, reactivity, Volatile Organic Compounds (VOCs)Relevant Websites:
http://www.jhu.edu/~dogee/roberts/ Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.