Grantee Research Project Results
2004 Progress Report: Using Vertical Attachment Energies to Predict Dehalogenation Rates of Environmental Contaminants (SEER I)
EPA Grant Number: R829422E02Title: Using Vertical Attachment Energies to Predict Dehalogenation Rates of Environmental Contaminants (SEER I)
Investigators: Burrow, Paul D. , Comfort, S. D.
Institution: University of Nebraska at Lincoln
EPA Project Officer: Chung, Serena
Project Period: August 5, 2002 through August 4, 2004 (Extended to August 4, 2005)
Project Period Covered by this Report: August 5, 2003 through August 4, 2004
Project Amount: $177,831
RFA: EPSCoR (Experimental Program to Stimulate Competitive Research) (2001) RFA Text | Recipients Lists
Research Category: EPSCoR (The Experimental Program to Stimulate Competitive Research)
Objective:
The objectives of this Science and Engineering Environmental Research (SEER) I project are to: (1) determine internally consistent sets of kinetic rate constants for dehalogenation of several classes of compounds by zerovalent iron (Fe0) in aqueous media; (2) measure the vertical attachment energies (VAEs) and dissociative electron attachment (DEA) cross sections of these compounds; and (3) integrate molecular orbital and temporary anion properties (unoccupied molecular orbital energies, VAEs, and DEA cross sections) into a predictive model for describing rates of dehalogenation by Fe0.
Progress Summary:
Progress was made on Objectives 1 and 2 during Year 2 of the project.
Determining Internally Consistent Sets of Kinetic Rate Constants for Dehalogenation of Several Compound Classes by Fe0 in Aqueous Media
An expanded set of chloroalkanes was chosen for further measurements of kinetic rate constants for dehalogenation reactions with electrolytic Fe0 (99% purity) in HEPES-buffered aqueous solution. Additional batch experiments were conducted under anoxic conditions with the following selected chloroalkanes: 1,1-trichloro-2-methylpropane (1,1,2-TCMP), 1,2,3-trichloropropane (1,2,3-TCP), 1,2-dichloro-2-methylpropane (1,2-DMP), 1,1-dichloropropane (1,1-DCP), and 1,2-dichloropropane (1,2-DCP). The results indicate chloroalkane reaction rates of 1,1,1-TCA > 1,1,2-TCMP > 1,1,2-TCA > 2,2-DCP > 1,2-DMP, whereas no measurable degradation was observed for 1,2,3-TCP, 1,2-DCP, 1,2-DCA, and 1,3-DCP during 24-hour experiments. Although a set of rate constants was obtained from the 24-hour experiment, reactions with the electrolytic iron were slow under the conditions of the experiment and degradation tended to stop after 24 hours. Based on results from previous research, aluminum sulfate was added to the solution to increase treatment effectiveness. Initial results indicated an increase in 1,2,3-TCP and 1,1,2-TCMP reaction rates. Reaction kinetics are being similarly determined for the remaining chloroalkanes.
Measuring VAEs and DEA Cross Sections of the Compounds
Rate constants derived from objective 1 were correlated with VAEs and integrated half zero peak areas. Good correlations were obtained. It is expected that better correlations will be achieved with the addition of aluminum sulfate.
In addition to the specific research project objectives, project-related research produced valuable results concerning the correlation with gas chromatography (GC) electron capture detector response. GC with electron capture detection (GC/ECD) has been used widely for quantitative analysis of halocarbons, including chlorinated solvents and pesticides in water and soil because of its sensitivity in the part per trillion range. The nature of the ECD response depends on the amount and electron capture rate constant of the analyte. The most fundamental way to determine the relative ability of compounds to accept electrons is by measuring their VAEs using gas-phase electron scattering techniques. The VAE corresponds to the kinetic energy required to form the anion by attachment of a free electron to a neutral molecule and can be approximated by calculating the lowest unoccupied molecular orbital energy. Zero peaks appear near the zero energy of impacting electrons in the DEA cross section. Conceptually, the VAEs are inversely related to the magnitudes of the zero peaks as well as the ECD response. In an attempt to correlate ECD response with VAEs, zero peaks, and kinetics of dehalogenation by Fe0, a series of chloroalkanes was selected and rates determined under controlled conditions.
Normalized-concentration GC/ECD area ratios of fifteen chlorocarbons to the CCl4 internal standard showed very good correlations when plotted with VAEs and integrated half zero peak areas. Compounds with high VAEs tend to show low sensitivity towards ECD, whereas compounds with high integrated half zero peak areas show high sensitivity. The higher the degree of chlorination, the lower the VAE and the higher the integrated half zero peak area.
Because dechlorination of the chloroalkanes by reaction with Fe0 is assumed to occur by electron transfer to the molecules, the surface area-normalized rate constants (KSA) of chloroalkanes, measured under controlled experimental conditions, were compared with the normalized-concentration area ratios. The results showed that a good correlation was obtained. Compounds with VAEs greater than that of 2,2-DCP (1.41 eV) showed no measurable degradation when treated with Fe0. As an exception, 1,2,3-TCP with relatively low VAE (1.2 eV) showed little reaction with Fe0. This may be because of the symmetry and stability of the molecule. The experimental results show good correlations between the KSA with ECD response, VAEs, and thermal DEA cross sections. The lower the VAEs and the higher the near zero energy DEA cross sections of chloroalkanes, the greater the ECD response and KSA of the compounds. We conclude that ECD response, as well as descriptors determined from electron scattering techniques, may be potentially useful in predicting the dehalogenation rates of chlorinated contaminants treated with Fe0.
Future Activities:
We will make the remaining measurements for the chloroalkenes and chloroalkanes using electrolytic iron alone and in the presence of aluminum and iron salts, which have been found to increase the effectiveness of Fe0 treatment (discussed in the 2003 and 2004 SEER II [R829422E03] reports). Reaction kinetics obtained with electrolytic iron will be compared to kinetics using field-grade iron to determine the impact of impurities on the reaction and relationship with electron transmission spectroscopy measurements. After obtaining satisfactory data sets, molecular orbital and temporary anion properties (unoccupied molecular orbital energies, VAEs, and DEA cross sections) will be integrated into a predictive model describing rates of dehalogenation by Fe0 (Objective 3).
A Ph.D. student working on this research project will complete his program by December 2005. A manuscript is in preparation for submission in the near future. We anticipate one additional manuscript will be submitted for publication and at least one additional presentation will be given during the next year.
Journal Articles:
No journal articles submitted with this report: View all 1 publications for this projectSupplemental Keywords:
cleanup, halocarbon, restoration, chemical reduction, environmental chemistry, physics, Science and Engineering Environmental Research, SEER, vertical attachment energies, VAEs, dissociative electron attachment, DEA,, Scientific Discipline, Geographic Area, Waste, Water, Remediation, Contaminated Sediments, State, Ecology and Ecosystems, Environmental Engineering, Groundwater remediation, sediment treatment, predictive understanding, reductive treatment, remediation technologies, hazardous waste, zero valent iron, contaminated soil, chlorinated organic compounds, dehalogenation, permeable reactive barriers, contaminated groundwater, verticle attachment, halogenated hydrocarbons, water quality, contaminated aquifers, ecology assessment modelsProgress 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.