Abiotic Reduction of Nitroaromatic Compounds by Aqueous Iron(II

Environ. Sci. Technol. 2006, 40, 3006-3012

Abiotic Reduction of Nitroaromatic Compounds by Aqueous Iron(II)-Catechol Complexes DAISUKE NAKA, DONGWOOK KIM, AND TIMOTHY J. STRATHMANN* Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, 205 North Mathews, Urbana, Illinois 61801

Complexation of iron(II) by catechol and thiol ligands leads to the formation of aqueous species that are capable of reducing substituted nitroaromatic compounds (NACs) to the corresponding anilines. No reactions of NACs are observed in FeII-only or ligand-only solutions. In solutions containing FeII and tiron, a model catechol, rates of NAC reduction are heavily dependent on pH, ligand concentration, and ionic strength. Observed pseudo-first-order rate constants (kobs) for 4-chloronitrobenzene reduction vary by more than 6 orders of magnitude, and the variability is well described by the expression kobs ) kFeL26- [FeL26- ], where [FeL26-] is the concentration of the 1:2 FeII-tiron complex and kFeL26- is the bimolecular rate constant for 4-chloronitrobenzene reaction with this species. The high reactivity of this FeII species is attributed to the low standard one-electron reduction potential of the corresponding FeIII/FeII redox couple (EH0 ) -0.509 V vs NHE). The relative reactivity of different NACs can be described by a linear free-energy relationship (LFER) with the oneelectron reduction potentials of the NACs, EH1′(ArNO2). The experimentally derived slope of the LFER indicates that electron transfer is rate determining. These findings suggest that FeII-organic complexes may play an important, previously unrecognized, role in the reductive transformation of persistent organic contaminants.

Introduction In suboxic and anoxic aquatic environments, microbial utilization of FeIII-bearing minerals as terminal electron acceptors can result in accumulation of considerable amounts of dissolved FeII; concentrations greater than 1 mM have been reported in some settings (1, 2). FeIII reduction is often enhanced in organic-rich settings (3). Thus, the co-occurrence of dissolved FeII and natural organic constituents is relevant to many natural systems. An increasing number of studies has shown the importance of metal-organic interactions in a variety of environmental phenomena, including the biogeochemical cycling of elements, nutrient acquisition, mineral weathering, and contaminant fate (4-7). Interactions between FeII and organic constituents are especially important for assessing the fate of groundwater contaminants because both of these groups can serve as electron donors for abiotic and microbial contaminant reduction processes (8-10). Previous studies report that FeII complexation by organic ligands can dramatically alter the metal’s reactivity with a * Corresponding author phone: (217)244-4679; fax: (217)333-6968; e-mail: [email protected]. 3006

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variety of oxidizing species, including O2, H2O2, CrVI, and carbamate pesticides; the presence of some ligands increases FeII reaction rates by several orders of magnitude, whereas others inhibit FeII reactions (11-14). The effects of individual organic ligands have been attributed to the influence of metal complexation on the reduction potential of the FeIII/FeII redox couple and oxidant access to the metal’s inner-coordination shell (12-14). It follows that organically complexed FeII species may contribute significantly to the metal’s overall redox reactivity in organic-rich anoxic environments (e.g., marshes, sediments) where these species account for a significant fraction of the total FeII pool (1). Yet, the full relevance of these species to the environmental fate of many important classes of aquatic and soil contaminants (e.g., nitroaromatic and halogenated organics, UVI) has not been evaluated. It is important to characterize the reactions between contaminants and FeII complexes with individual classes of organic ligands because microorganisms and plants excrete strong chelating agents that will disproportionately affect FeII speciation in relation to the overall pool of dissolved organic matter (15-17). Nitroaromatic compounds (NACs) are ubiquitous aquatic contaminants because of their widespread use as pesticides, munitions, pharmaceuticals, and industrial chemical intermediates (18). In addition, atmospheric photochemical production of significant quantities of NACs has been documented (19). Previous work, most notably by Schwarzenbach and co-workers, has demonstrated that abiotic reduction of aromatic nitro groups (I) to the corresponding aniline (IV) is an important pathway that contributes to the environmental fate of these compounds (18). The reaction proceeds via nitroso (II) and hydroxylamine (III) intermediates

Previous reports have demonstrated that NACs are reduced by FeII sorbed to mineral surfaces and FeII-containing minerals (e.g., phyllosilicate clays) (20-23). In contrast, it has generally been assumed that NACs are unreactive with most dissolved FeII species. To date, no study has systematically evaluated the reactivity of NACs with aqueous FeII species, including complexes with abundant classes of natural and synthetic organic ligands. This contribution reports for the first time on the kinetics and mechanism of reactions between NACs and aqueous FeII-catechol complexes. In comparison to other classes of oxygen-donor ligands, catechols display a unique ability to promote rapid abiotic redox reactions between FeII and NACs. Previous work showed that Fe-porphyrin complexes can act as electron-transfer catalysts for NAC reduction by cysteine (24); while metalloporphyrins are highly specialized biomolecules that evolved to facilitate intracellular redox processes (25), catechols are more representative of the metalbinding functional groups present in humic substances (10). Thus, FeII-catechol complexes may be important contributors to the documented redox activity of humic substances and their role in promoting the reductive transformation of contaminants (26-28). Furthermore, several microorganisms have been shown to excrete catechol siderophores to acquire metal nutrients (17), so FeII complexes with these exudates may represent an important class of reductants in some 10.1021/es060044t CCC: $33.50

 2006 American Chemical Society Published on Web 03/29/2006

microenvironments. The major goals of this study were to (i) identify environmentally relevant classes of organic FeIIcomplexing ligands that promote NAC reduction, (ii) quantify the effects of environmental factors such as pH, ionic strength, and FeII speciation on rates of NAC reduction, and (iii) identify molecular-scale factors and features of the controlling reaction mechanism.

Experimental Section Setup. A complete list of reagents is provided in Supporting Information. Experiments were conducted inside a atmosphere- and temperature-controlled glovebox (95% N2, 5% H2; Pd catalyst; 25 ( 2 °C; Coy Laboratory Products) because strict oxygen exclusion is required to prevent FeII oxidation. The glovebox atmosphere was continuously bubbled through a 1 M NaOH solution to remove CO2. All glassware and dry reagents were exposed to the glovebox atmosphere for several hours prior to use. Solutions were prepared in reagent-grade water (18 MΩ cm resistivity) that was deoxygenated by boiling and sparging with N2 prior to and immediately after being transfered into the glovebox. All experiments were conducted in batch reactors (40 mL acid-washed glass bottles). Experimental Procedures. Previous studies have established that dissolved Fe2+ is unreactive with NACs (20). A survey experiment was initially conducted to evaluate the general potential of different classes of FeII-organic complexes to reduce NACs. A series of solutions were prepared containing FeII (0.5 mM) and representative ligands from each class (5 mM), and the solutions were buffered at pH 7.0 (50 mM MOPS). Excess ligand was added to ensure that a significant fraction of the total FeII was complexed. Ligandonly and FeII-only controls were also prepared. 4-chloronitrobenzene (4Cl-NB) was then spiked into the reactors to achieve an initial concentration of 50 µM. Solutions were then allowed to react for 24 h, at which time aliquots were collected to analyze the concentration of 4Cl-NB and expected reduction products. Detailed kinetic studies of NAC reduction were subsequently conducted with tiron (3,5-disulfonate-substituted catechol). Tiron was selected as the model catechol because (a) the attached sulfonate groups provide for high aqueous solubility, thereby allowing us to examine a wider range of ligand concentrations and (b) no other strong metalcomplexing groups are in the structure, allowing us to isolate the reactivity of FeII-catechol complexes (hydroxybenzoic acids also contain carboxylate groups). Kinetic measurements were made over a wide range of conditions (pH, ionic strength, tiron-to-FeII ratio, NAC structure). The initial concentrations of FeII and tiron were significantly higher than the NAC to ensure pseudo-first-order reaction conditions. Individual batch reactions were prepared by mixing appropriate volumes of FeCl2, tiron, pH buffer (acetate, MES, MOPS, or TAPS), and electrolyte (NaCl or NaNO3) from stock solutions. The pH stability was verified after the mixture was equilibrated overnight in foil-wrapped reactors. Reactions were then initiated via the addition of a small volume of the parent NAC from a methanolic stock solution. Aliquots were then collected periodically and analyzed. For faster reactions (half-life < 1 h), reactions were quenched by addition of excess EDTA to the collected aliquots. The initial survey experiment showed that FeII-EDTA complexes were unreactive with 4Cl-NB, and the strong chelating agent effectively removes FeII from the tiron complexes. Tests verified that reaction rates were not affected by the pH buffers used in this study. Analytical. Concentrations of 4Cl-NB, the hydroxylamine intermediate (4Cl-HA), and the aniline product (4Cl-An) were monitored by HPLC with photodiode array detection (Shimadzu VP Series). The isocratic mobile phase was 55% methanol and 45% aqueous buffer (10 mM hydroxylamine,

pH 6.0). The stationary phase was a Spherisorb ODS-2 column (4.6 × 150 mm, 5 µm C-18 packing material) with a 10 mm guard column of the same material (Waters). 4Cl-NB and 4Cl-An concentrations were determined by comparison with external standards. The concentrations of an intermediate, presumed to be 4Cl-HA, were determined after the development of a peak-area response factor (relative to the 4Cl-NB peak) needed to close the mass balance in a single experiment conducted early in the study. This same response factor was then used in subsequent experiments. The concentration of 4Cl-HA was not determined in faster reactions because addition of the EDTA quenching agent did not stop conversion of 4Cl-HA to 4Cl-An. The concentrations of other substituted NACs were quantified for kinetic experiments in a similar manner; corresponding HA and An products were not quantified in these experiments.

Results and Discussion Survey of Organic Ligands. An initial survey was conducted to assess the potential of representative metal-chelating ligands to promote NAC reduction in the absence and presence of FeII. The ligands surveyed contain carboxylate, alcohol, amino, thiol, pyridyl, and phenolate Lewis base groups. 4Cl-NB reduction is not observed in solutions containing only FeII (ligand-free control) or ligand (FeII-free controls). Reduction of 4Cl-NB is observed when FeII is added in combination with certain ligands. Figure 1A shows that after 24 h, 4Cl-NB is reduced to the corresponding hydroxylamine intermediate (4Cl-HA) and aniline product (4Cl-An) in solutions containing FeII and the ligands shown in Figure 1B. Analysis of the ligands that form reactive complexes shows that they contain either catechol (ortho-diphenolics) or thiol groups in their structures. Whereas other classes of ligands, including polycarboxylates, polyaminocarboxylates, and mixed phenolic-carboxylates, do not promote 4Cl-NB reduction, all catechol- and thiol-containing ligands examined cause some degree of 4Cl-NB reduction when FeII is also present. The remainder of this report focuses on identifying the mechanistic factors controlling NAC reduction by FeIIcatechol complexes. A future contribution will evaluate reactions involving FeII-thiol complexes. 4Cl-NB Reduction by FeII-Catechol Complexes. Figure 2 shows the transformation of 4Cl-NB in a solution containing FeII and tiron. No reactions are observed in FeII-only or tirononly solutions (Figure 2A). In comparison, 4Cl-NB reduction is observed when both FeII and tiron are present, and the kinetics follow a pseudo-first-order rate law

ln[NAC] ) ln[NAC]0 - kobst

(2)

where [NAC] and [NAC]0 are the NAC concentration at times t and 0, respectively, and kobs (s-1) is the pseudo-first-order rate constant. The kobs values were obtained by linear regression of ln[NAC] versus time plots for each batch reaction. The line shown indicates good agreement between eq 2 and the measured data throughout the concentration range bracketed by calibration standards (1-25 µM). The Supporting Information includes a table of all measured kobs values. Figure 2B shows that 4Cl-NB reduction proceeds stoichiometrically through 4Cl-HA to 4Cl-An, as expected from eq 1. Good mass balances were observed in all reactions, and 4Cl-HA was the only intermediate detected. Nitrosobenzene intermediates (II) are too reactive to accumulate in solution (20, 24, 27), and no evidence of coupling products was observed (21). It is interesting to note that the importance of the 4Cl-HA intermediate in reaction timecourses varies with solution conditions and the identity of the FeII-complexing ligand VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Influence of different organic ligands on FeII reactivity with 4Cl-NB. (A) Chart showing the composition of 4Cl-NB solutions after reacting for 24 h in solutions containing FeII and organic ligands. Initial reaction conditions: 50 µM 4Cl-NB, 0.5 mM FeII, 5 mM ligand, pH 7.0 (50 mM MOPS buffer). (B) Structures of catechol and thiol ligands that form reactive FeII complexes. Ligand abbreviations: IDA ) iminodiacetic acid, NTA ) nitrilotriacetic acid, EDTA ) ethylenediaminetetraacetic acid, BIPY ) 2,2′-bipyridine, PYRO ) pyromellitic acid, 2,3-DMSA ) meso-2,3-dimercaptosuccinic acid, DHBA ) dihydroxybenzoic acid, THBA ) trihydroxybenzoic acid. (Figure 1). This observation corresponds with earlier reports of NAC reduction catalyzed by natural organic matter and model electron shuttles (24, 27) and indicates that the ratedetermining factors for 4Cl-NB and 4Cl-HA reduction by FeII differ. Influence of Solution Composition. A series of kinetics experiments were conducted to quantify the influence of solution conditions and identify factors that affect reaction rates. kobs was determined in solutions that were pH-buffered and ionic strength-buffered since initial experiments indicated that the reaction rates are highly dependent upon both variables. Measured kobs values increase steadily from 7.33 × 10-5 s-1 to 3.11 × 10-3 s-1 when ionic strength increases from 0.05 to 0.7 M (pH 6.0, 1 mM FeII, 10 mM tiron) via the addition of either NaCl or NaNO3 (see Table S2 and Figure S1 in Supporting Information). Subsequent experiments were conducted in solutions buffered at I ) 0.25 M. This high ionic strength permitted wide variation in the tiron concentration (0.5-74 mM) and pH (4.5-9) without affecting ionic strength. Figure 3A and B show how kobs is affected by variation of the tiron concentrations at a fixed pH and FeII concentration (Figure 3A) and variation of pH at fixed FeII and tiron concentrations (Figure 3B). At pH 6.0, increasing tiron concentration increases kobs from 1.99 × 10-6 s-1 to 3.87 × 10-3 s-1 (t1/2 decreases from 4 days to 3 min). An even greater effect is 3008

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observed when pH is varied; kobs increases from 9.69 × 10-8 s-1 to 8.91 × 10-3 s-1 (>90 000-fold increase) when pH increases from 5.0 to 6.5 (t1/2 decreases from 2.7 months to 1.3 min). At pH < 5.0 and pH > 6.5, reactions are too slow and too fast, respectively, to obtain accurate values of kobs. Instead, upper- and lower-limit estimates are provided for these conditions, representing the smallest and largest kobs values that could be accurately determined by the methods used here. FeII Speciation and Reactivity. Variations in kobs can be explained by examining how tiron solution conditions affect FeII speciation. Figure 3C and D show the predicted FeII speciation at conditions corresponding to the kinetic measurements shown in Figure 3A and B. FeII speciation was calculated using MINEQL+ with an equilibrium model provided in the Supporting Information. The Supporting Information also includes the experimental details and results of a titration procedure used to determine the conditional stability constants for formation of three FeII-tiron complexes in 0.1 M NaCl (log cKFeHL- ) 15.71 ( 0.10, log cKFeL2- ) 8.92 ( 0.04, log ckFeL26- ) 15.33 ( 0.17). Three FeII-tiron complexes (FeL2-, FeHL-, and FeL26-) are predicted to be present and to vary in concentration with changing solution conditions. Figure 3C shows that the concentration of each complex increases with increasing tiron concentration at pH 6.0 but that the relative contribution of

and molar concentration of individual reactive FeII complexes, respectively. Examination of eqs 3 and 4 shows that the contribution of individual FeII species to the overall observed reactivity is the product of its concentration and its intrinsic reactivity with the oxidant in question. In solutions containing FeII and tiron, eq 4 can be expanded to include all FeII species shown in Figure 3C and D,

kobs ) kFe2+ [Fe2+] + kFeOH+ [FeOH+] +

kFe(OH)20[Fe(OH)20] + kFe(OH)3-[Fe(OH)3-] +

kFeHL-[FeHL-] + kFeL2-[FeL2-] + kFeL26-[FeL26-] (5) Since 4Cl-NB reduction does not occur in tiron-free solutions, terms for Fe2+ and FeII-hydroxo complexes can be neglected, and eq 5 can be simplified.

kobs ) kFeHL-[FeHL-] + kFeL2-[FeL2-] + kFeL26-[FeL26-] (6)

FIGURE 2. Reduction of 4Cl-NB in solution containing 0.5 mM FeII and 10 mM tiron at pH 5.75 and I ) 0.25 M. (A) Natural log plots with pseudo-first-order model fit and comparison with FeII-only and tiron-only controls. (B) Complete time course showing 4Cl-NB disappearance and formation of reaction intermediates and products. the complexes to overall FeII speciation is minor at most conditions examined. Figure 3D shows that Fe2+ predominates at pH < 6.75, whereas FeII-tiron complexes predominate at higher pH. Concentrations of FeII-tiron complexes also increase with increasing ionic strength (data not shown). Although the concentrations of all three FeII-tiron complexes vary with changing solution conditions, individual complexes exhibit varying dependencies with respect to tiron concentration, pH, and ionic strength. A visual comparison of the trends in kobs with trends in FeII speciation suggests a parallel between reaction rates and concentrations of the 1:2 complex, FeL26-. Quantitative relationships between FeII speciation and redox reactivity with a number of substrates (e.g., O2, H2O2, CrVI, carbamates) have been reported (11-14). A relationship can be determined by assuming that the oxidizing substrate reacts in parallel with individual FeII species and that equilibrium FeII speciation is maintained throughout the course of the redox reaction (i.e., electron-transfer processes are slow in comparison to complexation/dissociation reactions). Accordingly, NAC reduction should obey a generalized rate law of the following form,

d[NAC] dt

) -kobs[NAC] ) -

∑k [FeL ][NAC] i

i

(3)

i

and the relationship between kobs and FeII speciation is described by

kobs )

∑k [FeL ] i

i

(4)

i

where ki and [FeLi] are the bimolecular rate constant (M-1s-1)

In some systems, the observed redox reactivity of FeII is controlled by a single species, and eq 6 can be further simplified to a single term involving the dominant species. As noted earlier, kobs measurements appear to parallel the predicted trends for [FeL26-]. A plot of log kobs versus log[FeL26-] (Figure 4) confirms that a strong correlation exists between kobs and the concentration of this species. Most notably, data sets for experiments where tiron concentration is varied (Figure 3A), pH is varied (Figure 3B), and ionic strength is varied (Figure S1 in Supporting Information) all fall on the same line and yield an apparent reaction order with respect to [FeL26-] that approaches unity (0.90 ( 0.06). The tight correlation shown in Figure 4 (adjusted r2 ) 0.98 for all three data sets combined) indicates that eq 6 is dominated by the last term. As a result, the other species can be neglected

kobs ) kFeL26-[FeL26-]

(7)

This result does not imply that the other FeII-tiron complexes are unreactive, only that their contribution to kobs is negligible in comparison to FeL26- (i.e., product of ki[FeLi] , kFeL26[FeL26- ]). All the data shown in Figure 4 was fit with eq 7 to obtain kFeL26- ) 3.81 ( 0.85 × 104 M-1 s-1 for 4Cl-NB reduction (uncertainty ) 95% CI). The least-squares fit, conducted using the software package Scientist, was carried out using the logarithmic form of eq 7 to prevent undue weighting of the highest kobs values, since the measured rate constants span several orders of magnitude. The inclusion of the other FeII-tiron complexes during the fitting procedure (i.e., use all terms in eq 6) does not markedly improve the fit quality, and the fit-derived values kFeL2- and kFeHL- are not significantly different from zero at the 95% confidence level. Attempts to fit the data with FeL2or FeHL- alone were unsuccessful. Model predictions using eq 7 are shown in Figures 3A and B and S1. The predictions are in excellent agreement with the kobs measurements, providing compelling evidence that the effects of changes in ionic strength, ligand concentration, or pH on kobs (more than 6 orders of magnitude variation) are all the result of the influence of these variables on the concentration of single FeII complex. The parametrized eq 7 also predicts extremely fast reduction kinetics at pH g 7 (t1/2 < 1 s). The value of kFeL26- is several orders of magnitude larger than the reported bimolecular rate constants for 4Cl-NB reactions with model hydroquinones (5.0 × 10-1 M-1 s-1 for juglone and 1.1 × 101 M-1 s-1 for lawsone), an FeII-porphyrin complex (2.7 M-1 s-1), and a hydrolyzed FeII surface complex VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Effect of tiron concentration and pH on kobs for 4Cl-NB reduction (A and B) and FeII speciation (C and D). The left panels indicate the effect of varying tiron concentration at pH 6.0, and the right panels indicate the effect of varying pH in solutions containing 10 mM tiron. Ninety-five percent confidence intervals for kobs measurements are smaller than the symbols shown. Symbols containing upward and downward arrows indicate minimum- and maximum-value estimates, respectively. Reaction conditions: 0.5 mM FeII, 25 µM 4Cl-NB, 0.5-75 mM tiron, pH 4-9, I ) 0 25 M, and 25 °C. Effect of NAC Substituents. A series of kinetic experiments were conducted to quantify the relative reactivity of a series of monosubstituted nitrobenzenes with FeII-tiron complexes. The results are summarized in Table S2 in the Supporting Information. As anticipated, the addition of electronwithdrawing ring substituents (e.g., Cl, acetyl) markedly increases NAC reduction rates, while the addition of electrondonating substituents (e.g., CH3) decreases reduction rates slightly. Figure 5 shows a relationship between the kobs values measured at pH 5.75 and the one-electron reduction potentials of the half reactions

ArNO2 + e- T ArNO2•-

EH1′(ArNO2)

(8)

The relationship is described by a linear free-energy relationship (LFER)

E1′ H(ArNO2) +b 0.059V

log kobs ) a

FIGURE 4. Apparent reaction order plot with respect to the concentration of FeL26-. Line represents linear regression of three data sets; circles represent the variation in total tiron concentration; triangles represent the variation in pH; squares represent the variation in ionic strength. in magnetite suspensions (2.2 × 101 M-1 s-1) (24, 29). This comparison suggests that FeII-catechol complexes may contribute to NAC reduction in natural settings even though the complexes are likely to be present in low concentrations. 3010

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(9)

An excellent correlation is observed and the slope a ) 1.11 indicates that the Gibbs free-energy change associated with nitroaromatic radical anion formation has a pronounced effect on the reaction rate. Schwarzenbach and co-workers reported LFER slopes of a ≈ 1.0 for the reaction of NACs with model hydroquinones and reduced natural organic matter, whereas a , 1 was observed for meta- and para-substituted NACs reacting with a model FeII-porphyrin complex and for NACs reacting with FeII ions sorbed to magnetite, Fe3O4(s) (20, 24, 27). These authors conclude that the actual transfer of the first electron is the rate-determining step when a ≈ 1.0, whereas other processes (e.g., formation of a precursor

FIGURE 5. Relationship between kobs and EH1′ of a series of substituted nitrobenzenes. Reaction conditions: 0.5 mM FeII, 10 mM tiron, 25 µM nitrobenzene, pH 5.75, I ) 0.25 M, 25 °C. complex or mass transfer) are also important when a , 1.0. The differing LFER slopes observed for FeII-tiron versus FeII-porphyrin systems is particularly noteworthy. This suggests that the relative importance of the Gibbs free-energy change for radical anion formation versus reorganization energy differs among classes of FeII complexes. Explanation for Reactivity of NACs with FeII-Catechol Complexes. The high reactivity of NACs with the 1:2 FeII-tiron complex and the lack of noted reactivity with other FeII-tiron complexes or FeII complexes with other classes of organic ligands can be explained in terms of the standard one-electron reduction potentials (EH0) of the FeIII/ FeII half reactions associated with different FeII-ligand complexes

FeIIIL + e- T FeIIL

(10)

The EH0 values can be determined from the relative stability constants (KFeL) for formation of equivalent FeII and FeIII complexes (i.e., complexes that differ only in oxidation state)

E0H ) 0.77 -

( )

RT KFeIIIL ln F KFeIIL

(11)

where 0.77 (V vs NHE) is the value of EH0 for the Fe3+/Fe2+ half reaction (30), R is the gas constant, T is absolute temperature, and F is Faraday’s constant. A full derivation of eq 11 is provided elsewhere (31). According to this expression, ligands that stabilize FeIII to a greater degree (i.e., result in larger KFeIIIL/KFeIIL values) will form FeII complexes with lower EH0 values, thereby increasing reductant strength. EH0 values can be obtained for the following half reactions associated with the FeII-tiron complexes shown in Figure 3C and D

FeIIIHL0 + e- T FeIIHL-

EH0 ) +0.352 V

(12)

FeIIIL- + e- T FeIIL2-

EH0 ) +0.045 V

(13)

FeIIIL25- + e- T FeIIL26-

EH0 ) -0.509 V

(14)

A larger listing of EH0 values for redox couples associated with the FeII-ligand complexes surveyed in this study is provided in the Supporting Information (Table S4). Examination of data for available redox couples shows that the EH0

value for the 1:2 FeII-tiron complex is the most energetically favorable reductant. A comparison of this value with other FeII/FeIII redox couples, as well as the EH1′(ArNO2) values of substituted NBs is also graphically illustrated in Figure S3. Although direct summation of EH0 (FeIII/FeII) and EH1′(ArNO2) values is not appropriate, the diagram illustrates that combining the half reaction for reduction of NACs with the half reaction for oxidation of the 1:2 FeII-tiron complex is more energetically favorable than any other FeII-ligand complex for which appropriate thermodynamic data are available. Although thermodynamic data are not currently available to estimate EH0 for FeII complexes associated with other catechol or thiol ligands, the observed reactivity of 4Cl-NB in solutions amended with these ligands suggests that similarly favorable EH0 values exist for one or more complexes with these ligands. At the same time, the lack of observed 4Cl-NB reactivity with FeII in solutions containing other classes of ligands (Figure 1), including several strong chelating agents, indicates that FeII complexes with these ligands are inherently unreactive with 4Cl-NB (i.e., ki values are close to zero). This finding suggests that there is a thermodynamic threshold required before FeII-to-NAC electron transfer will occur. Environmental Significance. Previous reports have suggested that mineral-associated FeII species (structural, adsorbed) represent the major reductants controlling NAC fate in heterogeneous aquatic environments and that dissolved FeII species are considered, for the most part, unimportant reductants in these settings (8, 20-22). Results presented here demonstrate that some classes of FeII-organic complexes are highly reactive with NACs, and previous work shows that a wide range of FeII-organic complexes rapidly react with contaminants such as CrVI and carbamoyloxime pesticides (13, 14). Preliminary work in our laboratory indicates that FeII-catechol and FeII-thiol complexes also exhibit reactivity with other classes of contaminants (nitramine explosives, polyhalogenated alkanes). Although further work is underway to critically evaluate the environmental and molecular factors controlling these redox reactions, results presented here suggest that the contribution of these species should not be ignored when assessing the natural attenuation of persistent aquatic pollutants, especially in settings that are rich in both FeII and organic matter. From an environmental relevance standpoint, it is helpful to compare NAC reactivity trends observed for FeII-tiron complexes with reactivity trends reported for FeII adsorbed to iron oxide minerals (20, 23). This comparison shows that the rates of NAC reduction by FeII-tiron complexes exhibit much greater sensitivity to changes in the redox potential of the target NAC and changes in solution pH. The implication of these heightened sensitivities is that the relative contribution of FeII-catechol complexes to overall rates of NAC reduction, in comparison with mineral-associated FeII species, can be expected to be most pronounced when the target NACs possess higher EH1′ values and when porewater pH is higher. Although tiron is a synthetic ligand, catechol and thiol groups are present in natural organic matter (NOM) (10), and microorganisms have been shown to excrete catechol siderophores to modify metal speciation in their extracellular environments (16, 17). However, the abundance of these functional groups within different sources of NOM and their effect on FeII speciation is unclear. Available reference NOM materials have been extensively treated and exposed to oxygen for long periods of time, likely destroying oxygensensitive groups such as catechols and thiols. Thus, research on the speciation and reactivity of FeII-NOM complexes should take great care to use freshly collected and preserved NOM obtained from different anoxic environments. VOL. 40, NO. 9, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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The high redox reactivity of some aqueous FeII-organic complexes also suggests a possible remediation strategy for contaminated subsurface environments. Reactive FeII complexes can be injected into contaminated groundwater to promote reductive transformation processes and enhance natural attenuation. If dissolved FeII is already present (as unreactive species), appropriate ligands can be injected to convert the metal to more reactive species in situ.

(12)

(13) (14)

Acknowledgments The ACS Petroleum Research Fund is acknowledged for support of this research. D.N. was supported by fellowships from the University of Illinois Environmental Council and the Hughes Undergraduate Research Foundation. D.K. was partially supported by a fellowship from the Korean Army. Helpful comments and suggestions were provided by three anonymous reviewers.

Supporting Information Available A complete listing of chemical reagents, a table of pKa values for ligands that form reactive FeII complexes, a table of results from all kinetics experiments, a figure showing the effect of ionic strength, experimental details and results of potentiometric titrations used to determine equilibrium constants for formation of FeII-tiron complexes, a table of equilibrium reactions used to model FeII speciation, and a table and figure comparing the EH0 values of FeII-ligand complexes. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review January 9, 2006. Revised manuscript received March 5, 2006. Accepted March 7, 2006. ES060044T