Environ. Sci. Technol. 2007, 41, 1200-1205
Using Gas-Phase Molecular Descriptors to Predict Dechlorination Rates of Chloroalkanes by Zerovalent Iron SATHAPORN ONANONG,† S T E V E D . C O M F O R T , * ,† PAUL D. BURROW,‡ AND PATRICK J. SHEA† School of Natural Resources, University of Nebraska-Lincoln, Lincoln, Nebraska 68583, and Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska 68588
Reductive dehalogenation of chlorinated compounds is the most important process occurring within the zerovalent iron (Fe0) barrier. The relative reaction rates of individual halocarbons with Fe0 can vary considerably. This variability has been the stimulus for using various chemical descriptors for a priori predictions of transformation rates via linear free-energy relationships (LFERs). Our objective was to determine the efficacy of four molecular descriptors to describe the transformation rates of three chloromethanes, three chloroethanes, and six chloropropanes by Fe0. This was accomplished by generating an internally consistent set of rate constants under controlled environmental conditions (16 °C, anaerobic) and regressing the surfacearea normalized rate constants (kSA) against (i) energy of the lowest unoccupied molecular orbital (ELUMO); (ii) vertical attachment energies (VAE); (iii) thermal electron attachment rate constants; and (iv) the molar response from a commercial electron capture detector (ECD). Results showed good correlations between kSA’s and all four descriptors (r2: 0.72-1.0), but a separate trend line was required for the chloromethanes and the chloro- ethanes/ propanes. Given the availability and ease with which ECD response can be obtained, this physical measurement may provide a practical means of determining relative rates of reactivity of various halocarbons in permeable reactive iron barriers.
Introduction Chlorinated alkanes comprise some of the most commonly observed environmental contaminants at hazardous waste sites. Widely used in industry as chemical solvents and manufacturing intermediates, annual worldwide production for some chloroalkanes exceeds 105 metric tons (1). High demand and use of chloroalkanes has resulted in inadvertent releases to the environment. Methylene chloride and 1,1,1trichloroethane rank among the top ten organic groundwater pollutants observed worldwide (1). Given their recalcitrant, toxic, and mobile character in soil-water environments (2, * Corresponding author phone: (402) 472-1502; fax: (402) 4727904; email:
[email protected]. † School of Natural Resources. ‡ Department of Physics and Astronomy. 1200
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 41, NO. 4, 2007
3), developing and implementing removal technologies for chloroalkanes in groundwater is a major focus of many environmental engineering firms. Using zerovalent iron (Fe0) in permeable reactive barriers to dechlorinate chloroalkanes in groundwater has become an established technology (4, 5). Over the past decade, several research groups have determined destruction kinetics for a variety of chlorinated alkanes and alkenes with Fe0. Previous literature reviews have shown that the reactivity of individual halocarbons with Fe0, as quantified by surface-area rate constants (kSA), can vary by an order of magnitude or more (6, 7). Yet, despite this variability, the range in observed rate constants for individual compounds is modest when compared to the 5 orders of magnitude observed among the various halocarbons (6, 7). Given the variability in chemical reactivities among halocarbons, attempts have been made to correlate the dehalogenation process with a number of chemical descriptors in hopes of generating a predictive tool for applied applications. A number of possible measurements are useful in such correlations such as bond dissociation energies (8, 9), electronegativities of the leaving groups (9), and one-and two-electron potentials (10) but in general these parameters are only available for a select number of compounds. This has been the stimulus for the development of quantum-chemical descriptors that are readily computable for a wide range of molecules. Differences in chemical reactivity toward Fe0 are linked, at least in principle, to differences in the susceptibility of the molecule to accept electrons. Of the quantum-chemical descriptors, the energy of the lowest unoccupied molecular orbital (ELUMO) is perhaps the most justified because it represents the frontier orbital into which electron-transfer takes place. Scherer et al. (6) first reported that Fe0-generated kSA’s for twelve chloroalkanes and alkenes successfully correlated with molecular LUMO energies, with LUMO dependence accounting for 83% of kSA variability. This good correlation across chemical classes (alkanes, alkenes) implied that the orbital energy was the key player independent of symmetry or other molecular properties (i.e., saturation, unsaturation). Burrow et al. (11) subsequently regressed the kSA’s of Scherer et al. (6) against vertical attachment energies, a gas-phase measure of the ease of reduction. Specifically, vertical attachment energies (VAEs) represent the energy required to add an electron to the neutral molecule in its ground state geometry. Because VAEs can readily be determined by gas-phase electron scattering techniques, such as electron transmission spectroscopy (ETS) (12), they respresent the physical quantities that LUMO energies approximate, and a number of comparisons between VAEs and LUMO energies have been successfully demonstrated (13-16). By using measured VAEs, Burrow et al. (11) scaled LUMO energies computed for different molecular families such as the σ* and π* orbitals of the chloro-alkanes and alkenes and showed that both LUMO energies and VAEs should only be employed to predict dechlorination rates within each family of chlorinated hydrocarbons. In addition to vertical attachment energies, two other gasphase molecular descriptors that may correlate with the dehalogenation process are the thermal electron attachment rate constant and electron capture detector (ECD) response. Thermal attachment rate constants are highly correlated with VAEs (17), increase with degree of chlorination, and can be obtained from various electron-swarm methods (18) as well as from electron beam measurements in which cross sections for production of Cl- are determined by the dissociative electron attachment (DEA) process: RCl + e- f [RCl•,-] f 10.1021/es061746l CCC: $37.00
2007 American Chemical Society Published on Web 01/18/2007
R• + Cl- (19), where [RCl•,-] represents a temporary radical anion. The molar response of an electron capture detector could be employed as a variation of an electron-swarm method. Its use in correlating with dehalogenation rates is less intuitive but justified by the fact that the ECD is one of the most sensitive gas chromatograph detectors available and unlike flame ionization detectors, which respond with nearly uniform sensitivity to most hydrocarbons, ECD response varies tremendously among halocarbons (e.g., degree of chlorination) and between chemical classes due to its VAE dependence. The ECD detector consists of a cavity with two electrodes and a radiation source that emits β-radiation (e.g., 63 Ni). The collision between electrons and the carrier gas produces plasma containing electrons and positive ions. When electronegative compounds enter the ECD, they immediately undergo attachment reactions with some of the free electrons and reduce the number of electrons remaining in the electron cloud. Because the primary mechanism occurring in the ECD arises from attachment and formation of temporary anions that subsequently dissociate or are collisionally stabilized, good correlations were obtained between ECD response and the attachment rate constants determined in an electron beam apparatus as well as molecular descriptors such as VAEs and thermal DEA cross sections (20). Given that Burrow et al. (11) previously showed good correlations between VAEs and transformation rates of halocarbons by Fe0, the correlation between ECD response and kSA’s of various chloroalkanes was investigated. Reductive dechlorination of chloroalkanes by Fe0 takes place by direct electron transfer between Fe0 and substrate at the metal oxide film coating the iron surface. This is a concerted, dissociative process (i.e., RX + e- f R• + X-) that results in the formation of a carbon-centered radical, R• (21, 22). In dilute aqueous systems, the radical accepts a second electron from Fe0 and undergoes protonation (hydrogenolysis) or elimination of the adjacent chlorine atom (βelimination), resulting in the formation of a carbon-carbon double bond (23). For compounds containing more than one chlorine atom only at the R-carbon position (e.g., 1,1,1,trichloroethane), germinal dechlorination (R-elimination) can also occur (24) via the loss of two chlorine atoms from the same carbon (no CdC produced). Factors reported to influence the dechlorination rates of chlorinated alkanes by Fe0 include the Fe0 source (7, 25); metal loading (26); mixing rates (21); initial concentration of reactants (7, 27); pH (21, 27); and electrolyte concentration (28, 29). These variables need to be controlled in order to obtain an internally consistent set of rate constants. Our objective was to determine an internally consistent set of kSA’s of several chloroalkanes with Fe0 under similar environmental conditions and relate them to the four molecular descriptors, namely ELUMO, VAEs, thermal attachment rate constants, and ECD response. Defining these relationships has the potential to provide a means of predicting the relative transformation rates of commonly, and perhaps less frequently, encountered chloroalkanes in groundwater.
Experimental Section Chemicals and Materials. Chlorinated compounds were purchased as standards from various manufacturers and used as received. These chemicals included carbon tetrachloride (PCM, HPLC grade, MCB Manufacturing Chemists, Inc.), chloroform (TCM), preserved with 0.75% (v/v) ethanol (Fisher), dichloromethane (DCM, 99%, Aldrich), 1,1,1,2tetrachloroethane (1112TeCA, 99%, Aldrich), 1,1,1-trichloroethane (111TCA, +99%, Aldrich), 1,1,2-trichloroethane (112TCA, 98%, Aldrich), 1,1,2-trichloro-2-methylpropane (112TCMP, 98%, TCI), 1,2,3-trichloropropane (123TCP, 99%, Aldrich), 1,2,3-trichloro-2-methylpropane (123TCMP, 98%
TCI), 1,2-dichloro-2-methylpropane (12DCMP, 98%, Aldrich), 2,2-dichloropropane (22DCP, 98%, Aldrich), 1,2-dichloropropane (12DCP, 99%, Aldrich), commercial-grade Al2(SO4)3 (Dragon Chemical Corp.), and electrolytic Fe0 (99%,
