Influence of Organic Ligands on the Reduction of Polyhalogenated

Aug 28, 2007 - Experimental work demonstrates that polyhalogenated alkanes (PHAs) are rapidly reduced in aqueous solutions containing FeII complexes w...
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Environ. Sci. Technol. 2007, 41, 6740-6747

Influence of Organic Ligands on the Reduction of Polyhalogenated Alkanes by Iron(II) ADAM L. BUSSAN AND TIMOTHY J. STRATHMANN* Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews, Urbana, Illinois 61801

Experimental work demonstrates that polyhalogenated alkanes (PHAs) are rapidly reduced in aqueous solutions containing FeII complexes with organic ligands that possess either catechol or organothiol Lewis base groups in their structure and are representative of extracellular ligands and metal-complexing moieties within humic substances (tiron, 2,3-dihydroxybenzoic acid, 3,4-dihydroxybenzoic acid, 2,3,4-trihydroxybenzoic acid, 3,4,5-trihydroxybenzoic acid, thioglycolic acid, and 2,3-dimercaptosuccinic acid). In solutions containing FeII-tiron complexes, 1,1,1-trichloroethane (1,1,1-TCA) is reduced quantitatively to acetaldehyde, a product previously reported for reactions with CrII, but not with Fe-based reductants. Observed pseudo-first-order rate constants for 1,1,1-TCA reduction by FeII-organic complexes (k ′obs) generally increase with increasing pH and ligand concentration when FeII concentration is fixed. For the FeII-tiron system, k ′obs is linearly correlated with the concentration of the 1:2 FeII-tiron complex (FeL26-), and kinetic trends can be described by k ′obs ) kFeL26- [FeL26-], where kFeL26-is the bimolecular rate constant for PHA reaction with the 1:2 FeII-tiron complex. Comparing reaction rates for 14 polyhalogenated ethanes and methanes reveals linear free energy relationships (LFERs) with molecular descriptors for PHA reduction (DR-X ′, ∆G0′, and ELUMO), with the strongest correlation being obtained using carbon-halogen bond dissociation energies, DR-X ′. The collective experimental results are consistent with a dissociative one-electron transfer process occurring during the rate-limiting step.

Introduction Characterizing the influence of biogeochemical factors and natural constituents on FeII speciation and redox reactivity is of considerable importance because the metal is one of the most abundant reductants found in suboxic and anoxic aquatic and soil environments (1), and biogenic FeII species can contribute to the environmental fate of anthropogenic contaminants through coupled biotic-abiotic processes (eq 1) (2, 3).

* Corresponding author phone: (217)244-4679; fax: (217)333-6968; e-mail: [email protected]. 6740

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To date, most studies on contaminant reactions with FeII have focused on quantifying the reactivity of mineralassociated FeII species (adsorbed and structural FeII (2-7)) because past reports concluded that they are much more reactive than dissolved FeII species (4). However, recent work demonstrates that some organically complexed aqueous FeII species exhibit considerable reactivity with contaminants (8-11), suggesting that FeII complexes with organic constituents (with dissolved, mineral-bound, and cell-bound) may be important contributors to the natural attenuation of persistent contaminants. Naturally occurring organic compounds, whether exuded from microorganisms or resulting from breakdown of plant and animal material, contain a wide range of oxygen-, nitrogen-, and sulfur-based Lewis base functional groups, including carboxylic, phenolic, amino, heterocyclic N, thiol, and hydroxamate groups (12-15). Some of these moieties have strong attractive interactions with metal ions (16, 17), and natural organic constituents dissolved in subsurface pore waters and coated on mineral surfaces can markedly influence the speciation of iron and other metal ions (16, 18). Thus, a thorough understanding of the factors controlling the redox reactivity of organically complexed iron species is needed to better predict the biogeochemical cycle for iron and its influence on contaminant fate. Complexation of FeII by low molecular weight organic ligands and natural organic matter (NOM) alters the metal’s redox reactivity by modifying the energetics of the FeIII/FeII redox couple as well as access to the metal’s inner coordination shell (8, 9). The effect of complex formation on the energetics of the iron redox couple can be quantified according to eq 2,

EH0 ) 0.77 -

( )

KFeIIIL RT ln F KFeIIL

(2)

where EH0 is the standard one-electron reduction potential of the FeIII/FeII redox couple (V vs NHE), 0.77 is the EH0 value for the aqueous Fe3+/Fe2+ redox couple, R is the universal gas constant, T is absolute temperature, F is Faraday’s constant, and KFeIIIL and KFeIIL are stability constants for formation of the equivalent FeIII- and FeII-ligand complexes, respectively. According to eq 2, ligands which form more stable complexes with FeIII than FeII (i.e., KFeIIIL/KFeIIL > 1.0) lower EH0, thereby making FeII a stronger reducing agent. Most common natural organic ligand donor groups are FeIIIstabilizing, so NOM complexation of FeII is expected to enhance the metal’s redox reactivity. However, coordinative saturation of FeII by strong chelating ligands (e.g., EDTA) diminishes FeII reactivity with some oxidants by limiting access to the metal’s inner coordination shell, thereby inhibiting inner-sphere electron transfer processes (9, 19). Polyhalogenated alkanes (PHAs), some of which are known to be highly toxic, are among the most common subsurface contaminants at National Priority List sites (20). Reductive dehalogenation processes are a major route for PHA degradation in suboxic and anoxic environments, resulting in formation of lesser halogenated and nonhalogenated products (21). Recent reports suggest that abiotic reactions with FeII may contribute significantly to the natural attenuation of PHAs (22, 23). Laboratory investigations demonstrate that halogenated methanes and ethanes are reduced by mineral-associated FeII species (adsorbed and structural) (5-7, 24) and FeII-containing coenzymes (25, 26). Observed rates and products of these reactions are highly variable and are dependent upon both the identity of the 10.1021/es071108i CCC: $37.00

 2007 American Chemical Society Published on Web 08/28/2007

reductant and the prevailing environmental conditions (5, 27). Although PHA reactions with specialized FeII-porphyrin coenzyme complexes are well established (25, 26), very little is known about PHA reactivity with FeII-organic complexes that are more representative of natural organic matter (NOM) that is ubiquitous in terrestrial and aquatic environments. Curtis and Reinhard (28) report that PHAs are reduced more rapidly in aqueous solutions containing both FeII and NOM than in solutions containing the individual components, but they do not identify the mechanism responsible for the observed synergism. This study reports for the first time on the abiotic reduction of PHAs by aqueous FeII complexes with low molecular weight organic ligands that possess Lewis base donor groups that are representative of metal-complexing moieties within NOM and microbial siderophores. Studying the reactivity of PHAs with well-defined FeII-organic complexes enables us to identify individual mechanisms by which more poorly defined NOM contributes to PHA natural attenuation and remediation. Specific objectives of this study were to (i) test the hypothesis that PHAs can be reduced by aqueous FeIIorganic complexes that have previously been shown to reduce other classes of persistent organic contaminants (e.g., nitroaromatics, N-heterocyclic nitramines) (10, 11); (ii) identify specific classes of organic ligands that promote FeII reactivity with PHAs; (iii) quantify the effects of changing solution composition, FeII speciation, and PHA structure on reaction rates; and (iv) use results from experimental studies to gain insights into the controlling reaction mechanisms.

Experimental Section Experimental Setup. A complete listing of chemicals used in this study is provided in the Supporting Information (SI). Kinetic experiments were setup and initiated inside a temperature-controlled anoxic glove box (25 ( 2 °C, 95% N2, 5% H2), and deionized water, reagents, and glassware were all deoxygenated using procedures described previously (11). Two different batch reactor setups were used to monitor PHA reactions with FeII-organic complexes. Most experiments with 1,1,1-trichloroethane (1,1,1-TCA), the model PHA used to examine the effects of ligand identity and solution composition, were conducted in foil-wrapped 160-mL glass serum vials. Vials were initially filled to 100 mL with aqueous solution containing the FeII salt (0.5 mM), desired organic ligand (0.5-50 mM), pH buffer (50 mM), and electrolyte (NaCl to set I ) 0.25 M). After adjusting the pH and equilibrating, vials were crimp-sealed with Teflon-lined butyl rubber septa. Reactions were then initiated by spiking the vials with 5 µmol of 1,1,1-TCA (50 µL of a methanolic stock solution containing 0.1 M 1,1,1-TCA and 0.002 M n-heptane as an internal standard) and vigorously mixing. At selected intervals, 200 µL headspace samples were then collected using a gastight syringe and immediately analyzed by gas chromatography (GC; method details provided in the SI). Vials were laid sideways between sampling to prevent headspace contact with the septa. Reactions were monitored for 3 half-lives or 1 month, whichever occurred first. For reactions with expected completion times >10 h, vials were maintained inside the glovebox and removed briefly when collecting headspace samples. For faster reactions, vials were submerged in a constant temperature water bath (25 °C) near the GC to allow for more frequent sample collection. Measurement at the end of each experiment confirmed that pH never varied by >0.1 units during reactions. The reactor setup described above could be used for experiments with 1,1,1-TCA because the reactions were slow compared to aqueous-gas exchange rates for PHAs, and equilibrium could be assumed for the latter process (29). A second headspace-free reactor setup was used to quantify the reactions of 14 related polyhalogenated methanes and

ethanes with FeII-tiron complexes, many of which react at much faster rates than 1,1,1-TCA where continuous aqueous-gas equilibrium could not be assumed. Reactor solutions were prepared and preequilibrated in the same manner described above, then drawn into a 50 mL glass syringe, and any gas headspace was expelled prior to initiating the reaction. Reactions were then initiated by adding the target PHA and an internal standard (0.1 µmol n-heptane or 50 µmol 1-chloro-3-fluorobenzene) from a methanolic stock solution by syringe and vigorously mixing. At selected intervals, 4 mL of solution from the syringe was then dispensed into separate crimp-sealed 15 mL serum vials containing a 5 mL solution of 100 mM EDTA to quench any further reaction of the PHAs. Headspace samples from the EDTA-quenched vials were then collected and analyzed after allowing time for the quenched solutions to reach aqueousgas equilibrium. EDTA is a strong metal chelating agent that has previously been used to quench reactions of FeII-ligand complexes with other classes of organic contaminants (911). It is an effective quenching agent for reactions with PHAs because (i) FeII-EDTA complexes are unreactive with PHAs, (ii) the high stability of FeII-EDTA chelate complexes combined with the elevated EDTA concentration in the quenching solution suppresses the concentration of all other FeII complexes (17), and (iii) FeII ligand exchange rates are orders-of-magnitude faster than the rates of PHA reduction observed in this study (30). Analytical. Headspace gas samples were analyzed by GC with either flame ionization detection (GC-FID) or electron capture detection (GC-ECD). Acetaldehyde was analyzed as a product in select batch reactions by gas chromatographymass spectrometry (GC-MS). Detailed descriptions of the methods are provided in the SI. Kinetic Modeling. Apparent pseudo-first-order rate constants (kobs; s-1) for individual batch reactions were determined by fitting the measured data for total moles of PHA per vial (nPHAt) with a linearized form of the integrated firstorder rate law:

ln(nPHAt) ) ln(nPHA0) - kobst

(3)

where nPHA0 is the initial moles of PHA present and t is time. To enable comparison of rate constants measured with the two experimental setups, kobs values were converted to the equivalent rate constants that would be obtained in headspace-free reactor systems (k ′obs; s-1) using a procedure described by Burris et al. (29). For reactions performed in headspace-free reactors, the two rate constants are equivalent (i.e., k ′obs ) kobs). However, for reactions monitored by continuous headspace analysis, rate constants need to be corrected to account for fw, the fraction of the total PHA mass present in the aqueous phase (k ′obs ) kobs/fw).

Results and Discussion Reduction of 1,1,1-Trichloroethane. Previous contributions from the authors demonstrate that nitroaromatic and Nheterocyclic nitramine contaminants (e.g., RDX, HMX) are rapidly reduced by FeII complexes with organic ligands containing either catechol (ortho-diphenol chelates) or thiol Lewis base groups (10, 11). Results shown in Figure 1 and Figure S1 in SI confirm the hypothesis that PHAs are also reduced by the same FeII-organic complexes, strengthening the conclusion that such FeII species may be important contributors to the natural attenuation of a wide range of contaminants. Figure 1A shows that 1,1,1-TCA is converted quantitatively to acetaldehyde (CH3-CHO) in aqueous solution containing FeII and tiron, a model catechol ligand. The reactive species is most likely a FeII-tiron complex since no reaction occurs in FeII-only and tiron-only controls. Results from a ligand screening experiment (Figure S1 in the SI) VOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. (A) Time course showing the reduction of 1,1,1-TCA in solution containing 0.5 mM FeII and 10 mM tiron at pH 9 and I ) 0.25 M (headspace-free reactor setup). Lines show pseudo firstorder model fit of 1,1,1-TCA disappearance and the predicted formation of acetaldehyde assuming no lag. (B) Structures of catechol and organothiol ligands that form FeII complexes capable of reducing 1,1,1-TCA (experimental data data provided in Figure S1 in the SI). Ligand abbreviations: 2,3-DMSA ) meso-2,3-dimercaptosuccinic acid; DHBA ) dihydroxybenzoic acid; THBA ) trihydroxybenzoic acid. shows that 1,1,1-TCA reduction also occurs in solutions containing FeII and a number of other ligands that possess either catechol or thiol donor groups in their structure (Figure 1B). In contrast, no reduction of 1,1,1-TCA is observed when FeII is added to solutions containing ligands that lack these moieties (e.g., citric acid, EDTA, salicylic acid). Catechol and thiol donor groups are key components of humic and nonhumic fractions of natural organic matter, and a variety of microorganisms excrete catecholate siderophores under nutrient stress conditions (12, 13, 15). The disappearance of 1,1,1-TCA follows a pseudo-firstorder rate law and formation of acetaldehyde occurs without a lag time. This is evident by comparison of the measured acetaldehyde formation data with concentrations predicted by assuming quantitative conversion of degraded 1,1,1-TCA to acetaldehyde. Acetaldehyde formation was confirmed in replicate experiments and analysis of the reactor headspace and hexane extracts of the aqueous phase showed no significant concentrations of products typically reported for reduction of 1,1,1-TCA by FeII and Fe0 reductants (lesser halogenated C2 products, C2 hydrocarbons, and C4 coupling products (7, 24, 31, 32)). Acetaldehyde formation has been reported for 1,1,1-TCA reduction by CrII (33), but this is the first report, to our knowledge, of its formation in reactions with FeII. Castro and Kray (33) outline a 2-electron reduction scheme for conversion of geminal trihalides to the corresponding aldehydes involving hydrolytic trapping of carbenoid intermediates (eq 4):

Additional product studies are needed to determine if acetaldehyde formation predominates for all reactive FeII6742

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organic complexes and all solution conditions or if it is unique to reactions with FeII-tiron complexes under the conditions examined. GC-FID headspace analysis from 1,1,1-TCA reactions with several FeII-organic complexes (Figure S1) indirectly supports the acetaldehyde pathway because none of the other commonly reported reduction products were detected (e.g., dichloroethane, chloroethane, ethane, ethene), but further studies are still needed to positively confirm acetaldehyde formation in these reactions. Furthermore, the proposed reaction scheme can be further confirmed using free radical and carbene trapping methods to isolate the suspected intermediates (2). The first step in eq 4 is believed to be a rate-determining dissociative single-electron transfer. Results from kinetic studies, to be discussed later, are consistent with this mechanism. The contrast between products observed here and those reported in earlier studies with Fe-based reductants suggest that the molecular environment surrounding the initial one-electron reduction product in this system may be unique from other Fe-based reductants. Cr(H2O)62+ is a oneelectron donor like FeII, and its reactions with organohalides have been shown to occur by an inner-sphere electron transfer mechanism (34). In comparison, there remains considerable debate about the nature of electron transfer reactions between reduced iron species and PHAs. Effects of Solution Composition. Kinetic measurements of 1,1,1-TCA reduction by 0.5 mM FeII were collected in solutions containing selected ligands over a wide range of pH and ligand concentrations to assess the influence of solution composition (Figure 2). Observed reaction rates vary widely over the range of conditions examined, with reaction half-lives ranging from 1 month (reactions slower than these could not be monitored in our system because 1,1,1-TCA volatility results in slow nonreactive losses). For all FeII-ligand systems examined, k ′obs increases with increasing pH when FeII and ligand concentrations are fixed (Figure 2A), a trend consistent with the degree of metal complex formation with very weak acid donors like phenolic and thiol groups. The degree of pH dependence varies considerably among the ligands. Whereas k ′obs varies 26fold from pH 6.5-9.0 in solutions containing FeII and the dithiol ligand 2,3-DMSA, a higher than 500-fold variation is observed over a smaller pH range (7.5-9.0) in solutions containing FeII and either 3,4-DHBA or 3,4,5-THBA. With the exception of 2,3-DMSA, the rate of 1,1,1-TCA reduction increases with increasing ligand concentration when FeII concentration and pH are fixed (Figure 2B). Measured values of k ′obs increase by approximately 2 ordersof-magnitude when concentrations of the catechol ligands are increased from 0.5 to 50 mM, indicating that the concentration of reactive FeII-catechol complexes increases with increasing [ligand]/[FeII] ratio. For solutions containing FeII and 2,3-DMSA, rates of 1,1,1-TCA reduction reach a peak value when the ligand concentration is 1 mM and then decrease as the ligand concentration is further increased. Although this trend may result from formation of less reactive FeII-DMSA complexes at elevated [ligand]/[FeII] ratios, a definitive conclusion cannot be made because earlier tests found that FeII-DMSA are unstable (11), so the lower apparent reaction rates at high [ligand]/[FeII] could be due to accelerated decomposition of FeII-DMSA complexes rather than changes in reactivity with 1,1,1-TCA. Further studies are needed to characterize the kinetics and mechanism for decomposition of these complexes under different conditions. Correlation of Reactivity with FeII Speciation. In the FeIItiron system, kinetic trends can be interpreted in terms of changing FeII speciation because an equilibrium model for FeII speciation was previously developed (10); similar information is not available for the other ligands examined in

FIGURE 2. Effects of solution conditions on k ′obs for 1,1,1-TCA reduction by FeII-organic complexes. Panel A shows the effects of varying pH in solutions containing 10 mM of each ligand, and Panel B shows the effects of varying ligand concentration at a fixed pH (pH 7.5 for Tiron, pH 8.0 for all other ligands). Reaction conditions: 5 µmol 1,1,1-TCA, 0.5 mM FeII, I ) 0.25 M. Solid line represents predicted k obs ′ in FeII-tiron solutions using eq 5.

FIGURE 3. Apparent reaction order plot showing a linear relationship between the pseudo-first-order rate constant for reduction of 1,1,1-TCA and the concentration of the 1:2 FeII-tiron complex (FeL26-). Error bars represent 95% confidence levels (smaller than symbol if not shown), and slope uncertainties represent 95% confidence levels. Data for the reduction of 4-chloronitrobenzene (ref 10) and RDX (ref 11) shown for comparison. Figure 2. Stability constants for FeII-tiron complexes with three distinct stoichiometries (FeHL-, FeL2-, and FeL26-; L-4 ) deprotonated ligand) were determined by fitting potentiometric titration data measured under strict anoxic conditions (see the Supporting Information for ref 10). FeII speciation varies with changing solution composition, and the calculated speciation at conditions corresponding to kinetic measurements shown in Figure 2 is provided in the SI (Figure S2). The predominant FeII species progresses from Fe2+ f FeL2- f FeL26- when either pH or tiron concentration are increased; FeHL- does not predominate under any of the

conditions examined. Visual comparison of 1,1,1-TCA kinetic trends with corresponding FeII speciation calculations reveals similar trends for k ′obs and the concentration of the 1:2 FeIItiron complex (FeL26-). A quantitative correlation between the two parameters is confirmed by data presented in Figure 3. All 14 kinetic measurements of 1,1,1-TCA reduction in the FeII-tiron system (pH 7-9, 0.5-50 mM tiron) collapse onto a single line when plotted versus the concentration of FeL26-; an apparent first-order dependence is indicated by the slope value approaching unity. This finding suggests that FeL26is primarily responsible for 1,1,1-TCA reduction in the FeIIVOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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tiron system, and the observed variations in k ′obs result from the effects of changing concentration of this species at different solution conditions. Similar relationships are obtained for the reduction of 4-chloronitrobenzene and RDX (data provided in Figure 3 for comparison (10, 11)), and reaction kinetics are well described by a second-order rate law that assumes FeL26- is the only reactive FeII species:

-

d[C] ) kobs[C] ) kFeL26-[FeL26-][C] dt

(5)

where [C] is the molar concentration of the target contaminant, and [FeL26-] and kFeL26- are the concentration of 1:2 FeII-tiron complex and bimolecular rate constant (M-1 s-1) for its reaction with the target contaminant, respectively. Results of a least-squares fit of the corresponding logarithmic relationship between k ′obs and [FeL26-] to the data shown in Figure 3 show that kFeL26- ) 9.48((1.63) × 10-2 M-1 s-1 (uncertainty ) 95% CI; fitting performed using Scientist for Windows; Micromath). Fits are not markedly improved when additional terms for FeL2- and/or FeHL- are added to the kinetic model, and these fits as well as those performed using FeL2- or FeHL- alone yield values of kFeL2- and kFeHL- that are not significantly different from zero at the 95% confidence level. Model predictions of k ′obs using kFeL26- and calculated speciation data in the FeII-tiron system are provided in Figure 2. The predictions agree closely with measured kinetic trends, accounting for >400-fold variation in k ′obs. The model slightly underpredicts the measured rate constants at the highest pH conditions, suggesting that another FeII complex not considered in the current FeII speciation model (e.g., one expected to form at high pH, such as Fe(OH)L27-) may also contribute to 1,1,1-TCA reduction at elevated pH conditions. The reactivity of the 1:2 FeII-tiron complex with 1,1,1TCA and other contaminants is attributed to the low standard one-electron reduction potential (EH0) of the associated FeIII/ FeII redox couple,

FeIIIL25- + e– h FeIIL26-;

EH0 ) -0.509 V

(6)

which is much more reducing than the EH0 values of FeII complexes that exhibit no reactivity with 1,1,1-TCA (e.g., +0.353 V for FeII-citrate, +0.08 V for FeII-EDTA, +0.045 V for 1:1 FeII-tiron complex; extensive list provided in the Supporting Information of ref 10). As reported for 4-chloronitrobenzene and RDX (10, 11), there appears to be a threshold value of EH0 before FeII complexes will exhibit measurable reactivity with 1,1,1-TCA; the lack of reactivity observed in FeII-salicylate solutions suggests that reactive complexes must have corresponding EH0 values lower than -0.276 V (EH0 value of the 1:2 FeII-salicylate complex). Unfortunately, kinetic trends observed with 2,3-DMSA, 3,4-DHBA, and 3,4,5-THBA in Figure 2 cannot be analyzed in terms of FeII speciation and EH0 because the necessary FeII- and FeIII-ligand stability constants are not available. Furthermore, because the structures of these ligands contain multiple types of Lewis base groups (e.g., phenolic and carboxylic groups for 2,3-DHBA), such an analysis would be further complicated by the need for microscopic stability constants to distinguish between FeII species with equivalent stoichiometry but different structure (e.g., for 2,3-DHBA, FeLcan form when FeII is chelated by either 2 phenolate groups or 1 phenolate and 1 carboxylate group), since it is expected that these species will exhibit markedly different redox reactivities with PHAs. Influence of Contaminant Structure. The value of kFeL26- determined for 1,1,1-TCA in the FeII-tiron system is much smaller than equivalent rate constants determined 6744

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for reduction of 4-chloronitrobenzene (kFeL26- ) 3.81 × 104 M-1 s-1 (10)) and RDX (kFeL26- ) 7.31 × 102 M-1 s-1 (11)). The general reactivity pattern (4-chloronitrobenzene . RDX . 1,1,1-TCA) agrees with trends reported for other reductants (6, 26, 35, 36), although the FeII-tiron system is the first, to our knowledge, to directly compare all three contaminant classes in the same experimental system. Comparison of the relative magnitudes of kFeL26- obtained for 4-chloronitrobenzene versus 1,1,1-TCA (400 000-fold difference between the 2 compounds) with the relative magnitudes of comparable kinetic parameters obtained for reactions with a model FeIIporphyrin complex, kFeP (only 85-fold difference) shows that the reactivity of the FeII-tiron complex is much more sensitive to the target functional group undergoing reduction (nitro group versus alkyl halide) than is the FeII-porphyrin complex. The influence of PHA structure was further evaluated by measuring rate constants for reduction of 14 PHAs (seven ethanes and seven methanes containing chlorine, bromine, and fluorine substituents) in the FeII-tiron system. Estimated values of kFeL26-, listed in Table S3 in the SI, span nearly two and half orders-of-magnitude, further demonstrating that the reactivity of FeII-organic complexes is highly sensitive to the structure of the electron acceptor. Qualitatively, structure-reactivity trends are similar to those reported for other reductants (5, 26, 31). That is, halogenated methanes exhibit higher reactivity than comparably substituted ethanes, reduction rates increase with increasing degree of halogenation, brominated species tend to be more reactive than comparably chlorinated analogues, and reduction rates decrease when chloro substituents are replaced with fluoro substituents. Linear Free Energy Relationships. The relationship between PHA structure and reactivity can sometimes be quantified through linear free energy relationships (LFERs) developed using appropriate molecular or reaction descriptor variables (φ),

logk ) b + aφ

(7)

where b and a are constants estimated by least-squares regression, with the latter representing the sensitivity of the reaction to the descriptor variable. Figure 4 shows scatter plots of log kFeL26- versus three descriptors commonly used for PHA reductions (37). When the entire group of test compounds is considered together, the strongest correlation is obtained using carbon-halogen bond dissociation energies (DR-X ′) (Figure 4A).

log kFeL26- ) 33.4((5.7) - 0.121((0.022)DR-X ′; r2(adj) ) 0.92, SE ) 0.54 (8) Successful LFERs using DR-X ′ have also been reported for the reduction of PHAs in other systems, and they have been interpreted as evidence that carbon-halogen bond cleavage occurs during the rate-limiting step (31, 37). The regressionderived value of a in eq 8 is similar to that reported for reduction of PHAs by mercaptojuglone (0.144 ( 0.036) (37), but significantly larger than those reported for reactions with a FeII-porphyrin complex (0.0781 ( 0.0112) (37) and FeS(s) (0.034 ( 0.016) (31). The greater sensitivity to changing PHA structure for reactions with FeII-tiron complexes than reactions with the FeII-porphyrin complex is also consistent with the greater difference in reactivity among contaminant classes already noted. No correlation between reactivity and the free energy (∆G 0′) for one-electron transfer (i.e., R - X + e- f R• + X-) is obtained when all the tested compounds are considered together (Figure 4B; r2(adj) ) -0.07). However, strong

FIGURE 4. Correlations of bimolecular rate constants for reduction of polyhalogenated alkanes by FeII-tiron complexes (kFeL26-) with three descriptor variables: (A) DR-X ′, (B) ∆G 0 ′, and (C) ELUMO. Circles represent PHAs containing bromine leaving groups and triangles indicate those with chlorine leaving groups. Uncertainties of individual rate constants at the 95% confidence level are smaller than the symbols shown. Solid lines indicate linear regression lines corresponding to eqs 8-11, and dotted lines represent 95% confidence intervals. Molecular descriptor variables from ref 37.

correlations are obtained when the compounds are separated into those containing chlorine as the most favorable leaving group,

log kFeL26- ) 2.72((0.53) - 0.0784((0.0164)∆G0 ′; r2(adj) ) 0.95, SE ) 0.26 (9) and those containing bromine as a leaving group.

log kFeL26- ) 13.0((3.8) - 0.168((0.062)∆G0 ′; r2(adj) ) 0.92, SE ) 0.51 (10) Similar clustering of these two PHA subgroups was reported for reactions with an FeII-porphyrin and mercaptojuglone (37). The authors attributed the correlations observed within each subgroup to systematic covariation in the ∆G 0 ′ values with DR-X ′ among the test compounds. The observation that the slope derived for eq 9 is smaller than the slope obtained for eq 10 suggests that the free energy change associated with alkyl radical formation may be less important in the rate-limiting reaction step for dechlorination reactions than debromination reactions. Figure 4C shows that the reactivity of PHAs in the FeIItiron system is also slightly correlated with the energy of the lowest unoccupied molecular orbital (ELUMO).

log kFeL26- ) -5.35((2.14) - 0.0171((0.0052)ELUMO; r2(adj) ) 0.80, SE ) 0.86 (11) The correlation is not improved by considering different PHA subgroups separately. This finding indicates that the energy associated with donation of an electron to the PHA is at least partially rate limiting. Collectively, the LFERs obtained for reduction of PHAs by the FeII-tiron complex are consistent with a rate-limiting step involving dissociative one-electron transfer. For 1,1,1-TCA, this mechanism is also supported by the observed acetaldehyde product. Bivariate LFERs of the form developed by Perlinger et al. (37) were not considered here because the correlation with DR-X ′ is already satisfactory, especially when the collective uncertainties associated with experimental rate constant measurements and computational estimation methods of the descriptor variables is considered. LFERs correlating the redox reactivity of different FeIIligand complexes with EH0 values of the corresponding FeIII/ FeII redox couples have been previously reported for reactions

with other contaminants (8, 9). Although such a LFER might also be expected for reactions with PHAs, one cannot be developed at this time because, with the exception of tiron, the necessary FeII speciation information and EH0 values are unavailable for the tested ligands that form reactive FeII complexes (Figure 1B).

Environmental Significance Results of this study demonstrate rapid reduction of PHAs by FeII complexes with ligands containing donor groups representative of natural organic constituents. Even though model FeII complexes were used in experiments, findings suggest that reactions involving FeII complexes with natural organic ligands may be of considerable environmental significance even if they are less abundant than mineralassociated FeII species. Reduction of 1,1,1-TCA in FeII-tiron solutions yields nonhalogenated acetaldehyde as the sole organic product, which is naturally occurring, bioavailable, and environmentally benign at low concentrations. Acetaldehyde is more desirable than the halogenated products reported for reactions with other FeII-based reductants (e.g., 1,1-dichloroethane (7, 24, 31)), so remediation of 1,1,1-TCA contamination using FeII-organic complexes may be a promising strategy for preventing formation of more persistent halogenated products. Before such remedial strategies can be adopted, though, more in-depth product studies need to be conducted for reactions involving a broader range of FeII-organic complexes and PHAs. The unique nature of the acetaldehyde product also suggests that its formation at contaminated sites may be a diagnostic for the contribution of reactions involving FeII-organic complexes to natural attenuation. Because specific donor groups were found to affect FeII reactivity the greatest, remediation efforts could be tailored to introduce organic amendments that are rich in these groups (with or without FeII, depending on in situ concentrations) or to promote in situ formation of such groups in soil organic matter (e.g., increase phenolic content using oxidants or enzymatic amendments). LFERs like those developed here may also be useful to cleanup efforts, enabling prediction of the reactivity of a wider range of halogenated contaminants. Ongoing work aimed at developing LFERs for the reactivity of different FeII-organic complexes will further expand predictive capabilities. VOL. 41, NO. 19, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Finally, the wide variation in reactivity of different FeII species observed here and in other recent work raises the need for better tools for characterizing the speciation of iron in natural environments. Currently, we are limited to operational determination of speciation, such as dissolved versus sorbed FeII and strongly versus weakly complexed FeII, but results of this and other studies show that such operationally defined FeII species are poor predictors of redox reactivity with contaminants. Instead, it appears that functional group-specific Fe-ligand bonding information is needed to provide an accurate assessment of FeII redox reactivity.

Acknowledgments The donors to the ACS Petroleum Research Fund and the University of Illinois Research Board are acknowledged for support of this research. A.B. was partially supported by a Deuchler fellowship provided to the University of Illinois. David Cwiertny (UC Riverside) provided helpful advice and discussion throughout the study.

Supporting Information Available A complete list of reagents, detailed gas chromatography methods, a table listing results from batch kinetics experiments, a plot showing results from a survey of 1,1,1-TCA reactivity with representative FeII-organic ligand complexes, a speciation model and plots in the FeII-tiron system at conditions corresponding to kinetic measurements shown in Figure 2, and a table of bimolecular rate constants and PHA molecular descriptors. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review May 11, 2007. Revised manuscript received July 19, 2007. Accepted July 20, 2007. ES071108I

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