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2002 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. H.
Institution: Johns Hopkins University
EPA Project Officer: Krishnan, Bala S.
Project Period: September 1, 2000 through August 31, 2002
Project Period Covered by this Report: September 1, 2001 through August 31, 2002
Project Amount: $225,000
RFA: Exploratory Research - Environmental Chemistry (1999)
Research Category: Engineering and Environmental Chemistry
Description:
Objective:The objective of this research project is to gain a better mechanistic understanding of how granular iron surface area and surface composition, organohalide structure, and co-contaminants affect the rate of removal and product distributions in reactions of organohalides with granular iron. These studies will be used to determine the predominant reaction pathways in reactions between granular iron and alkyl and vinyl halide contaminants, such as 1,1,1-trichloroethane (1,1,1-TCA) and cis-and trans-dichloroethylene (DCE), all of which are ubiquitous groundwater contaminants that pose significant health risks when encountered in drinking water. By applying the mechanistic insights obtained from experiments conducted with a select number of probe compounds, we intend to develop a high- quality database of kinetic, mechanistic, and product information for a wide variety of alkyl halide contaminants. Once completed, this database can be used to develop models for predicting the feasibility of treating alkyl halide contaminants whose reactions with Fe(0) have not been investigated previously.
Progress Summary:To accomplish these objectives, a variety of analytical approaches are being employed, including electrochemistry and surface spectroscopy, as well as traditional batch studies that yield kinetic and product information. An additional aspect of this research project is the development and implementation of a custom electrochemical cell coupled to a surface analysis chamber. There is compelling evidence to suggest that surface composition plays a crucial role in controlling environmental liquid/solid reactions at mineral surfaces and during organohalide remediation processes involving so-called "zero valent" iron. Consequently, understanding the role of interfacial chemistry in environmental processes is crucial for developing the mechanistic insight needed to anticipate reaction rates and product distributions. To correlate the effects of surface composition with reaction rates and selectivity in environmental processes, we have designed an electrochemical cell coupled to a surface analysis chamber. In addition to the surface analysis capabilities, the availability of ultra-high vacuum conditions can be exploited as a means to prepare and characterize model surfaces. The strength of this approach derives from the ability to control and modify the initial surface composition while retaining the ability to make measurements on the impact of surface chemical composition on reaction rates. Results obtained from this research project will, therefore, allow for the development of a more complete understanding of the factors controlling the reactivity of granular iron systems, allowing for the optimization of contaminant removal by granular iron permeable reactive barriers (PRBs).
During Year 1 of the project, a nonlinear, non-first-order relationship between the overall rate constant (kobs) for 1,1,1-TCA reduction and granular iron particle surface area was observed in batch systems. Surface area-normalized rate "constants" (kSA) for granular iron batch systems were not in fact constant, but rather varied by more than one order of magnitude as iron loading was increased. Rather than being first-order in iron loading, reaction rates exhibited close to a square-root dependence on iron loading. These results indicate a fundamental weakness of the surface area-normalized kinetic model traditionally applied to granular iron batch systems. Such behavior complicates attempts to use kSA values as a metric in comparing reactivity of granular iron with different organohalides. Extrapolation of kSA values obtained in batch systems to the modeling and design of PRBs that operate at significantly higher iron mass loadings also could compromise PRB performance. The results of subsequent batch studies demonstrated that the nonlinear relationship could not be attributed to factors such as mixing method, mixing speed, buffer composition, initial pH, intraspecies competitive effects, or kinetic limitation by external mass transfer.
The goal of recent batch studies has been to develop a mechanistic explanation for this nonlinear relationship between granular iron surface area and kobs for 1,1,1-TCA degradation. Mass balances performed on available hydrogen in batch systems revealed a significant sink of hydrogen; we infer this sink to involve the granular iron surface. Protons associated with the surface can either protonate surface iron hydroxide sites or be reduced to atomic hydrogen. Correlations between kobs for 1,1,1-TCA degradation and the estimated concentration of surface-associated hydrogen determined by mass balance are linear and first-order, implying that adsorbed nascent hydrogen may play a significant role in governing reactivity in granular iron batch systems. This result not only suggests the identity of a possible reactive surface species in granular iron batch systems, but the concentration of surface-associated hydrogen determined by mass balance also provides a more accurate descriptor for granular iron reactivity than total available surface area that is used traditionally. Having an accurate descriptor of granular iron reactivity will greatly enhance the design and implementation of field-scale granular iron PRBs. In addition, knowledge of the reactive surface species responsible for contaminant degradation will allow for a more complete understanding of organohalide degradation pathways, thereby helping to minimize the formation of hazardous byproducts from the reaction with granular iron.
Currently, additional batch studies are being conducted to elucidate the role of adsorbed nascent hydrogen species in the reduction of 1,1,1-TCA by granular iron particles. One approach has been to modify the granular iron surface with additives such as metallic nickel, palladium, and copper that are expected to increase the formation of surface-bound atomic hydrogen species in batch systems. Batch studies conducted thus far suggest that the addition of these metals to the iron surface increases the rate of 1,1,1-TCA reduction. Moreover, increasing the amount of the metal additive on the granular iron surface appears to favor the formation of completely dehalogenated products such as ethane and ethylene, both of which are environmentally benign. Further experiments are being conducted to determine if this observed increased rate of removal and shift in product distribution asachieved with these bimetallic reductants is directly attributable to increased concentrations of surface-associated hydrogen species resulting from the presence of the metal surface additives.
Our recent construction of an electrochemical cell, coupled to an ultrahigh vacuum chamber (UHV) equipped with an x-ray photoelectron spectroscopy system (XPS), has allowed us to begin exploring the influence of surface composition on reaction rates and pathways. The experimental system allows us to characterize model iron surfaces and transfer them to the liquid cell without exposing the sample to the ambient atmosphere. Initial studies are focusing on understanding the relationship between surface composition, open circuit potential, and rates of reaction.
Because the electrochemical cell is an open system that must be purged continuously with nitrogen to maintain anaerobic conditions, it was necessary to identify a model organohalide with a very low Henry's Law constant so that the compound would remain in the aqueous phase during the course of a reaction. We have synthesized a probe compound, 4-(3,4-dibromobutoxy)benzoic acid, that meets this requirement. This vicinal dibromide is reduced on the iron surface to form only one product, the corresponding alkene, 4-(3-butenyloxy)benzoic acid. Both the parent compound and the reduction product are easily quantified using high-performance liquid chromatography with ultraviolet-visible absorbance detection.
We currently are using this probe compound to monitor changes in reaction kinetics that may accompany the changes in open circuit potential we observe as the iron electrode’s surface composition evolves. Specifically, we are exploring the effects produced by the addition of a second metal to the iron surface. When a more noble metal, such as nickel, is plated onto the iron surface, the open circuit potential of the system moves towards more positive values. Over time, however, the potential at the surface returns to the value observed for the iron surface before plating. XPS analysis of the surface indicates that the decrease in open circuit potential occurs over the same timescale associated with the disappearance of the XPS Ni signal, suggesting that the growth of an iron oxide layer may be responsible for the observed changes. Observation of the reaction kinetics with the probe compound on the timescale of this evolution of the surface will allow us to better understand the rate enhancement effects observed with bimetallic reductants.
Future Activities:The activities remaining on this project will focus on two areas. The first area involves the application of the mechanistic information acquired thus far to various other alkyl halide species. Batch systems will be used to investigate reactions of a wide variety of chlorinated and brominated alkyl polyhalide contaminants that pose a significant threat to groundwater supplies to develop a high-quality database containing kinetic, mechanistic, and product information. This database then will be used to develop linear free-energy relationships by relating the experimentally determined rate data to a variety of molecular descriptors, including one-electron reduction potentials (E1), two-electron reduction potentials (E2), and bond dissociation energies. Such linear free-energy relationships may be valuable as a means of predicting reactivity and potential selectivity of alkyl halides not investigated previously in a laboratory study, as well as providing mechanistic insights into reaction pathways for families of compounds.
The second area of research remaining involves further application of the UHV-coupled electrochemical cell. Future research will include exploring options for regenerating the surface once the bimetallic reductant loses reactivity. In particular, we are interested in understanding the relationship between oxide thickness, open circuit potential, and reactivity. If acid washing thins the oxide layer and re-exposes the nickel, this may be accompanied by a return to higher open circuit potentials and an increase in reactivity towards the probe compound. Additionally, we will conduct comparative studies aimed at elucidating any trend in reactivity based on the galvanic series of corrosion potentials for various bimetallic systems.
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) |
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groundwater remediation, chlorinated organic solvents, sustainable industry, sustainable business, sustainable environment, technology for a sustainable environment, toxics, waste, chemistry, engineering, environmental engineering, hazardous, hazardous waste, national recommended water quality, cleaner production, pollution prevention, volatile organic compounds, VOCs, chlorinated solvents, halogens, reaction engineering. , Toxics, Sustainable Industry/Business, Scientific Discipline, Waste, RFA, Technology for Sustainable Environment, Sustainable Environment, Chemistry, Hazardous Waste, Environmental Engineering, cleaner production/pollution prevention, Groundwater remediation, Hazardous, National Recommended Water Quality, Engineering, reactivity, Volatile Organic Compounds (VOCs), chlorinated solvents, halogens, environmental chemistry
Relevant Websites:
http://www.jhu.edu/~dogee/roberts/
Progress and Final Reports:
2001 Progress Report
Original Abstract
Final Report