Grantee Research Project Results
Final 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 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 were 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.
Summary/Accomplishments (Outputs/Outcomes):
Surface area-normalized rate constants (KSA) of 11 chloroalkanes were determined for reactions with electrolytic Fe0 (99% purity) in HEPES-buffered aqueous solution and correlated with four molecular descriptors: (1) estimated energies from lowest unoccupied molecular orbitals (LUMOs or VOEs); (2) VAEs; (3) thermal electron attachment rate constants; and (4) electron capture detector (ECD) response, to establish useful linear free energy relationships (LFERs) for exploring applications in predicting reductive dechlorination kinetics by Fe0. We found that attachment rate coefficients are well correlated with ECD response, and the latter decreases exponentially with increasing energies of the lowest anion states. ECD measurements also were carried out in monochloroalkanes substituted with unsaturated ethenyl and phenyl moieties, and the response was shown to depend strongly on mixing between the unsaturated π* and the C-Cl σ* temporary anions as exhibited by the VAEs of these states. The results showed good correlations between the chloroalkene and phenylchloride ECD responses and the VAEs for the mixed states.
We observed a decrease in rates of reaction with Fe0 as the number of chlorines decrease on the most chlorinated carbon(s) and their correlations with all descriptors exhibited a moderate degree of covariance (r2 = 0.645, 0.632, 0.682, and 0.539 for VAEs, attachment rate constants, ECD response and VOEs, respectively). Improved correlations with all descriptors were obtained when the reaction rates of each chloroalkane family were plotted separately. Kinetic data from the chloromethane family gave very good correlations with all descriptors (r2 > 0.965) compared to other families. The remaining chloroethane and chloropropane rates fell on the same regression lines with a relatively high degree of covariance (0.793 < r2 < 0.930). Nevertheless, 1,2,3-trichloropropane and 1,2-dichloropropane reacted with Fe0 at much slower rates and were considered as outliers in all correlations (except the correlation with VAEs). Surprisingly, reaction rates of 1,2,3-trichloro-2-methylpropane and 1,2-dichloro-2-methylpropane were significantly faster than the former two compounds. This indicates that the methyl group on the second carbon of both compounds has an effect on degradation rates with Fe0, likely because of differences in the structural orientation or stability of intermediates resulting from initial electron transfer. Chloroalkanes containing two or more chlorine atoms at α- and β-positions in the molecules (e.g., 1,1,2-trichloroethane) can be dechlorinated via both hydrogenolysis and reductive β-elimination reactions (α-elimination for 1,1,1-trichloroethane). This may explain the greater scatter in the data when plotted against VAEs and attachment rate constants, which are most associated with single electron transfer. Chloroalkane rate constants were highly correlated with all four descriptors, particularly VOEs and ECD response. Reaction rates of chloromethanes showed better correlations than those of chloroethanes and chloropropanes. We conclude that VOEs and ECD response, as well as VAEs and attachment rate constants, may be potentially useful in predicting dechlorination rates of chloroalkanes treated with Fe0.
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, ecology assessment models, contaminated aquifers, permeable reactive barriers, reductive treatment., Scientific Discipline, Geographic Area, Waste, Water, Contaminated Sediments, Remediation, State, Ecology and Ecosystems, Groundwater remediation, Environmental Engineering, 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.