Grantee Research Project Results
2003 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, 2002 through August 4, 2003
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 research 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 1 of the project.
Determine Internally Consistent Sets of Kinetic Rate Constants for Dehalogenation of Several Compound Classes by Fe0 in Aqueous Media
Measurements were made for the reaction of selected chlorosalicylic acids, chlorophenoxyacetic acids, and chloroacetanilides with unannealed, "field-grade" iron (< 50 mesh cast iron aggregates, not heat-treated) in aqueous solution. Chlorosalicylic acids included 3-, 4-, and 5-chlorosalicylic acid. The chlorophenoxy acetic acids were o- and p-chlorophenoxyacetic acid. The chloroacetanilides included the herbicides metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide) and propachlor (2-chloro-N-(1-methylethyl)-N-phenylacetamide). Analyses were preformed by high-performance liquid chromatography (HPLC), and ion chromatography (IC) was used to determine chloride release. The relative order of reactivity (resulting in dechlorination) was: chlorosalicylic acids >> chloroacetanilides >> chlorophenoxy acids. Reaction rates increased when aluminum sulfate (Al2(SO4)3) was added with the Fe0.
Initial measurements for selected chloroalkenes and chloroalkanes using the field-grade iron revealed inconsistencies, likely because of the presence of carbon and various oxides on the metal surface. Thus, electrolytic iron (99 percent purity) was used to obtain initial kinetic data. Batch experiments were conducted with the chloroethenes: tetrachloroethylene (PCE), trichloroethylene (TCE), and cis-1,2-dichloroethylene (c1,2-DCE); and the chloroalkanes: 1,1,1-trichloroethane (1,1,1-TCA), 1,1,2-trichloroethane (1,1,2-TCA), 2,2-dichloropropane (2,2-DCP), and 1,3-dichloropropane (1,3-DCP). The experiments were conducted in aqueous solutions containing N-(2-hydroxyethyl) piperazine-N-(2-ethanesulfonic acid) (HEPES) buffer to maintain pH 7.0 to 7.5 using borosilicate vials with no headspace by horizontal shaking within an anaerobic chamber under controlled light and temperature (16°C). The solutions were sampled at selected times using a gas-tight syringe and transferred to gas chromatography (GC) vials containing acetonitrile, with carbon tetrachloride as an internal standard. Chemical analyses were performed by GC with electron capture detection (GC-ECD) using ramped temperature programs. Chloride analyses were performed using IC. The chloroalkanes and chloroalkenes appeared less reactive than the chlorosalicylic acids and chloroacetanilides; however, direct comparison measurements with electrolytic iron are needed for the latter classes of compounds. Preliminary results indicate chloroalkene reaction rates of TCE > c1,2-DCE > PCE and chloroalkane reaction rates of 1,1,1-TCA >> 1,1,2-TCA > 2,2-DCP, although no reaction was observed for 1,3-DCP during a 72-hour experiment.
Measure the VAEs and DEA Cross Sections of These Compounds
We studied the DEA process in a series of nonplanar chloroalkenes in the gas phase. In these compounds, we endeavor to understand the role of moieties bearing normally empty * orbitals that are "remote" from the C-Cl bond that is broken upon electron attachment. To this end, we studied a series of ethylenic and phenyl compounds, in which the unsaturated groups are separated from the C-Cl moiety by one or more -CH2- spacer groups. The total absolute DEA cross sections were measured, and in a separate apparatus the VAEs of the temporary anion states were determined using electron transmission spectroscopy (ETS). The DEA peak cross sections are found to decrease substantially as the number of spacer groups increases, indicating the decreased coupling between the two ends of the molecules. Even with four spacer groups present, however (the largest evaluated), the DEA process still contained a contribution from attachment into the * orbital followed by transfer to the C-Cl * orbital.
When correlated with measurements of the rates of reductive remediation of chloroalkenes on zerovalent iron, we anticipate that these results will allow us to determine whether the process takes place by electron transfer first into the empty * or directly into the * orbital as we have suggested in our proposal. Initial measurements indicate associations between rate constants for reaction of chloroethenes and chlorethanes with electrolytic iron, and the VAEs and integrated half-peak cross sections for these compounds.
Additional Accomplishments
Normalized ECD response area ratios were determined for 5 chloroalkenes and 18 chloroalkanes in acetonitrile containing carbon tetrachloride as an internal standard. The area ratios were plotted against VAEs and integrated half-zero peak areas to determine relationships between ECD detector responses and ETS measurements. The initial data indicate good correlations of ECD detector responses with VAEs and integrated half-zero peak areas. Compounds with large VAEs showed low sensitivity towards ECD, although compounds with large half-zero peak areas showed high sensitivity.
Future Activities:
In the next year, we will continue making kinetic measurements for an expanded set of chloroalkanes (1,1,2-trichloro-2-methylpropane, 1,2,3-trichloropropane, 1,2-dichloro-2-methylpropane, 1,2-dichloroethane, 2,3-dichlorobutane, 1,1-dichloropropane, 1,2-dichloropropane) and additional chloroalkenes using electrolytic iron alone and in the presence of Al and Fe salts, which have been found to increase the effectiveness of Fe0 treatment (discussed in the Science and Engineering Environment Research [SEER] 2 report). We will compare reaction kinetics made with electrolytic iron to field-grade iron to determine the impact of impurities on the reaction and relationship with ETS measurements. After obtaining satisfactory data sets, we will 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 (Objective 3). Research results will be presented at one or more scientific meetings and publication will be pursued.
Journal Articles:
No journal articles submitted with this report: View all 1 publications for this projectSupplemental Keywords:
cleanup, halocarbon, restoration, chemical reduction, physics, dehalogenation, contamination, contaminate, kinetic rate constant, zerovalent iron, aqueous media, vertical attachment energy, VAE, dissociative electron attachment, DEA, molecular orbital properties, temporary anion properties, predictive model, SEER 1., 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.