Environ. Sci. Technol. 2002, 36, 3536-3541
Reductive Dechlorination of 1,1,2,2-Tetrachloroethane W I L L I A M A . A R N O L D , * ,† P A U L W I N G E T , ‡,§ A N D CHRISTOPHER J. CRAMER‡ Department of Civil Engineering, University of Minnesota, 500 Pillsbury Drive SE, Minneapolis, Minnesota 55455, and Department of Chemistry and Supercomputing Institute, University of Minnesota, 207 Pleasant Street SE, Minneapolis, Minnesota 55455
Products of the transformation of organic pollutants in the environment are often predicted based on the structure of the parent compounds. In some cases, however, multiple products may result from the same reaction pathway. In this study, the reduction of 1,1,2,2-tetrachloroethane (1,1,2,2TeCA) is investigated both experimentally and computationally. Experimental results and data available in the literature reveal that the ratio of Z-1,2-dichloroethylene (Z-DCE) to E-1,2dichloroethylene (E-DCE) produced from the reductive β-elimination of 1,1,2,2-TeCA is approximately 2:1, and this ratio is independent of the reductant used. The exception is iron metal, which results in a ratio of 4.5:1. Computational results reveal that the 1,2,2-trichloroethyl radicals (1,1,2TCA•) formed upon the transfer of the first electron are nearly isoenergetic and are in rapid equilibrium. Thus, the conformer population of the 1,1,2,2-TeCA does not dictate the product distribution. Using Marcus theory, it is demonstrated that the Z:E ratio of 2:1 reflects the relative rates of the two possible electron transfer steps to the two radical conformers. Further analysis of the thermochemistry of the reaction reveals that this ratio of rate constants should be essentially independent of the thermodynamic driving force, which is consistent with the experimental results. The different observed product distribution when iron metal is the reductant is hypothesized to result from an organometallic intermediate. The reduction of the 1,1,2,2TeCA is an overall two-electron process, but the fact that the radicals equilibrate at a rate more rapid than the transfer of the second electron suggests that reductants employed act as decoupled single electron-transfer agents.
Introduction The impact of a pollutant is determined not only by its persistence in the environment but also by the identities and lifetimes of any products formed via its transformation. In several cases, degradation products are known to be more toxic that the parent compound. For example, vinyl chloride, which is a known human carcinogen, is commonly observed as a product from reductive dehalogenation of the common groundwater pollutant trichloroethylene (TCE). Similarly, the moderately toxic pesticide Naled is reduced by organic thiolates to form the highly toxic dichlorovos (1). Thus, it is * Corresponding author phone: (612)625-8582; fax: (612)626-7750; e-mail:
[email protected]. † Department of Civil Engineering. ‡ Department of Chemistry and Supercomputing Institute. § Present address: Computer Chemie Centrum, Na ¨ gelsbachstrasse 25 D-91025 Erlangen, Germany. 3536
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FIGURE 1. Reaction pathways of 1,1,2,2-TeCA in aquatic systems under reducing conditions. of vital importance for environmental engineers and chemists to be able to predict not only reaction rates of environmental contaminants but also final product distributions. Structural features often permit potential degradation products to be identified a priori. For small, halogenated alkanes, the presence of a good leaving group on a primary carbon allows nucleophilic substitution (SN2) to take place at low activation energies. Similarly, an R-β hydrogenhalogen pair is a structural cue for elimination (E2, dehydrohalogenation). Small halogenated molecules are also subject to reductive processes such as hydrogenolysis (the replacement of a halogen with a hydrogen), reductive R-elimination (if geminal halogens are present), and reductive β-elimination (dihaloelimination; if vicinal halogens are present). In many situations, however, multiple products may result from a single reaction pathway. As shown in Figure 1, the reduction of 1,1,2,2-tetrachloroethane (1,1,2,2-TeCA) may give rise to cis-1,2-dichloroethylene (cis- or Z-DCE) or transdichloroethylene (trans- or E-DCE) via reductive β-elimination. In addition, 1,1,2-trichloroethane (1,1,2-TCA) may result from hydrogenolysis, and TCE is the product of the dehydrochlorination reaction. The rates of reduction of the DCE isomers to vinyl chloride vary, as does the selectivity between hydrogenolysis and reductive β-elimination for these species. Thus, a method that allows the prediction of not only which products will be produced but also the relative amounts of these products would prove a useful tool for accurate assessment of the environmental impacts of organic contaminants. Computational chemistry tools have thus far proved successful in this respect (2-6). This study focuses on the reduction of 1,1,2,2-TeCA. 1,1,2,2-Tetrachloroethane was a common solvent prior to World War II, and substantial releases have also occurred in the recent past (7, 8), leading to its detection as a groundwater contaminant. Among the chlorinated ethanes, 1,1,2,2-TeCA is unique in that the reductive β-elimination process may give rise to two products. Although the reduction of 1,1,2,2TeCA has been studied in variety of systems, only a portion of these report product distributions (8-13). From these data, it appears that the product distribution resulting from the reductive β-elimination of 1,1,2,2-TeCA (i.e. the Z:E ratio of the resulting DCE isomers) is independent of the reducing agent employed. 10.1021/es025655+ CCC: $22.00
2002 American Chemical Society Published on Web 07/11/2002
The objective of this study is to establish why the product ratio resulting from the dichloroelimination of 1,1,2,2-TeCA appears to be insensitive to the reducing agent. Additional data for the reduction of 1,1,2,2-TeCA in aqueous systems are obtained, and the product distributions are compared with previously published data. Computational chemistry methods are then used to identify the step which determines the product distribution. Theory rationalizes the observed product distributions based on the rate of transfer of the second electron in the reduction process.
Experimental Section Chemicals. CrCl2 (99.99%, Aldrich), CuCl (95%, Acros), 1,1,2,2TeCA (98%, Aldrich), Z-DCE (97%, Aldrich), E-DCE (98%, Aldrich), TCE (ACS grade, Fisher), 1,1,2-TCA (97%, Aldrich), Trizma hydrochloride (Sigma), potassium iodide (Mallinckrodt), sulfuric acid (96%, Mallinckrodt), and ethylenediamine (100% anhydrous, Fisher) were used without further purification. Stock solutions of the chlorinated ethanes and ethylenes were prepared in methanol. Electrolytic Fe(0) (Fisher, 100 mesh) was acid washed as described previously (14, 15). Milli-Q water and buffer solutions were deoxygenated by sparging with purified argon. Experimental Systems. All experiments were carried out in serum bottles (160 mL, nominal volume) sealed with Teflon faced butyl rubber septa (Chrom Tech). Reactors were prepared in an anaerobic chamber (5% H2:95% N2, Coy). Each reactor contained 100 mL of water or buffer solution with the remaining volume left as headspace. For the Cr(II) system, a CrCl2 stock solution was diluted to the desired concentration (13 mM) with deoxygenated 5 mM sulfuric acid (pH 2) (16). At this pH, the Cr(II) exists as the hexaaquocomplex, and the dehydrohalogenation of the 1,1,2,2-TeCA is negligible. When Cu(I) was used as the reductant, the reactors contained a solution of 6.5 mM CuCl and 50 mM ethylenediamine prepared using deoxygenated Milli-Q water and adjusted to pH 8 using deoxygenated 1 M HCl. Reactors in which iron metal was used as the reductant contained a 50 mM pH 7 Trizma buffer (15) and a metal loading of either 0.5 or 5 g per 100 mL. When employing iodide as a reductant, an unbuffered 1 M solution of KI was used. To initiate an experiment, the reactor was spiked with the methanolic solution of 1,1,2,2-TeCA to give an initial concentration of 125-150 µM. Analytical Methods. Headspace samples (200 µL) were withdrawn from the reactors at selected intervals after injecting an equivalent volume of purified argon. Samples were analyzed by a gas chromatograph (Trace GC, ThermoQuest) equipped with a GS-GasPro column (J&W Scientific), an FID detector, a split/splitless injector, and a PC driven data acquisition system (ChromQuest v. 2.53). Standards were prepared in serum bottles with the same air:water ratio and analyzed in an identical manner. Computational Methods. The absolute gas-phase free energy for molecule A, Gg,A, is computed as the sum of the Born-Oppenheimer electronic ground-state energy (including nuclear repulsion energy), zero-point vibrational energy, and translational, rotational, vibrational, and electronic thermal contributions (computed assuming ideal gas behavior for a rigid rotor with all vibrations treated as harmonic oscillators and the electronic partition function equal to the ground-state degeneracy) (17). The absolute free energy in aqueous solution is computed by adding to the gas-phase free energy the standard-state free energy of solvation. To convert free energies of one-electron reduction half reactions to standard reduction potentials relative to the normal hydrogen electrode (NHE), 4.44 eV is subtracted from the negative of the free energy of reaction, defined here for irreversible electron transfer as the sum of the gas-phase vertical electron attachment energy and the differential
solvation free energy of the anion product compared to the neutral radical reactant. To compute absolute free energies, different levels of electronic structure theory were employed. All molecular geometries were optimized at the density functional level of theory. The particular functional employed was Becke’s (18, 19) three-parameter exchange-correlation operator which combines exact Hartree-Fock exchange (17) with Slater’s (20) local exchange functional and Becke’s (21) 1988 gradient correction to Slater’s functional and for correlation employs the gradient corrected correlation functional of Lee, Yang, and Parr (22). This combination is denoted B3LYP (23). Analytic frequency calculations were carried out to verify the nature of all stationary points (minima or transition states) and to calculate zero-point vibrational energies and thermal contributions to the enthalpy and free energy at 298 K. All these calculations employed the augmented correlationconsistent valence-triple-ζ basis set of Dunning (aug-ccpVTZ) (24-26). Standard-state solvation free energies in water (dielectric constant ) 78.3) were calculated using the SM5.42R/BPW91/ DZVP aqueous solvation model (27) based on empirical atomic surface tensions and self-consistent reaction field calculations with CM2 class IV charges (28) obtained with the DZVP basis set (29); these calculations employed the gasphase B3LYP/aug-cc-pVTZ geometries (the “R” in SM5.42R implies that the model was designed to use gas-phase geometries kept Rigid in the liquid solution phase). All calculations were carried out with a locally modified version of the Gaussian 98 electronic structure program suite (30, 31).
Results and Discussion Experimental Studies. The observed products from the reduction of 1,1,2,2-TeCA by Cr(II), Cu(I), and Fe(0) are displayed in Figure 2, along with a similar plot for Zn(0) (11). All products detected and the Z:E dichloroethylene ratios are given in Table 1. Ratios of Z:E dichloroethylene were determined by averaging the concentrations determined at each time point and by fitting the parent compound decay and product formation to a pseudo-first-order kinetic model and taking the ratio of the rate constants for the formation of the Z and E isomers. Both methods yielded statistically identical results. For Cr(II), the only observed products are Z- and E-DCE with the Z:E ratio of 2.4:1. No TCE or 1,1,2TCA was observed. Because the Cu(I) experiment was performed at pH 8, the dehydrohalogenation process is faster than reduction, and TCE is the dominant product observed. Approximately 20% of the 1,1,2,2-TeCA is degraded via reductive β-elimination, however, with an observed Z:E ratio of the dichloroethylenes of 1.5:1. The Z:E ratio for Fe(0) is 4.5:1 at both iron loadings employed. In addition, the hydrolysis rate for the 1,1,2,2-TeCA (pH 7 half-life of ∼50 days (32, 33)) is not rapid enough to account for the TCE produced over the time scale of the experiments in reactors containing either 5 g (monitored for 2.4 h) or 0.5 g (monitored over 31 h) of Fe(0). At short times, the TCE concentration exceeded that predicted from the hydrolysis rate by at least an order of magnitude. A further discussion of the Fe(0) system is provided below. Dehydrohalogenation was the only observed reaction in the 1 M KI solution. Table 2 lists the observed products and the Z:E ratios of the dichloroethylene isomers determined by previous investigators (8-13). The Z- and E-DCE isomers resulting from dihaloelimination are generally the dominant products, and the resultant Z:E ratio ranges from 1.8:1 to 2.4:1. Also of note is that the results obtained in this work for Cr(II) and Cu(I) in water are consistent with those of Mochida et al. (9) in polar organic solvent or polar organic solvent/water mixtures. The similar values of the Z:E ratio observed for vitamin B12 and hematin as reductants as well as for biotic reduction VOL. 36, NO. 16, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Z-DCE (b) and E-DCE (O) produced upon the reduction of 1,1,2,2-TeCA by (a) 13 mM Cr(II), (b) 6.5 mM Cu(I) with 50 mM ethylenediamine, (c) 3.1 g Fe(0)/L, and (d) 0.63 g Zn(0)/L. The data for Zn(0) is reprinted from Arnold, W. A.; Ball, W. P.; Roberts, A. L. Polychlorinated ethane reaction with zerovalent zinc: pathways and rate control. J. Contam. Hydrol. 1999, 40, 183-200. Copyright (1999), with permission from Elsevier Science.
TABLE 1. Results from the Reduction of 1,1,2,2-TeCA products reductant
major
minor
Cr(II)a Cu(I)-ethylenediamine Fe(0) iodide
Z-DCE, E-DCE TCEc Z-DCE, E-DCE TCE
Z-DCE, E-DCE TCE
C mass balance, %
ratio of Z:E 1,2-dichloroethylene
102 102 98 100
2.4 ( 0.3b 1.5 ( 0.3 4.5 ( 0.1 NA
a This experiment was conducted at pH 2. The dehydrohalogenation product, TCE, was not observed. b Errors represent 95% confidence limits based on the average of the Z:E ratio determined at each time point. c The TCE production observed is consistent with the predicted dehydrohalogenation rate of 1,1,2,2-TeCA at pH 8.
carried out by methanogenic bacteria suggest that metal cofactors produced by the methanogens are responsible for the dechlorination of 1,1,2,2-TeCA. One possibility to explain the nearly constant 2:1 Z:E ratio of the DCE products (excluding Fe(0)) is that the reaction is under thermodynamic control, for this ratio corresponds closely to the ratio that would be expected if the system were in thermodynamic equilibrium based on thermodynamic data (15) and our computational calculations. Experiments with E-DCE and the reducants used in this work, however, demonstrate that no equilibrium with Z-DCE is established under the various sets of experimental conditions. Reanalysis of the data from Arnold and Roberts (34) does reveal that over the time scale of days at a loading of 156 g Zn(0)/L slight conversion of E- to Z-DCE may occur, but this process would be insignificant on the time scale (
