Environ. Sci. Technol. 2006, 40, 4976-4982
QSAR Study of the Reduction of Nitroaromatics by Fe(II) Species DALIZZA COLO Ä N , * ,† ERIC J. WEBER,† AND JAMES L. ANDERSON‡ U.S. Environmental Protection Agency, National Exposure Research Laboratory, 960 College Station Road, Athens, Georgia 30605-2720 and Department of Chemistry, University of Georgia, Athens, Georgia 30602-2556
The development of predictive models for the reductive transformation of nitroaromatics requires further clarification of the effect of environmentally relevant variables on reaction kinetics and the identification of readily available molecular descriptors for calculating reactivity. Toward these goals, studies were performed on the reduction of a series of monosubstituted nitrobenzenes in Fe(II)-treated goethite suspensions. The energy of the lowest unoccupied molecular orbital, ELUMO (B3LYP/6-31G*,water), of the nitrobenzenes was capable of explaining 99% of the variability in the rates. Results of experiments in which the surface area loading of ferric oxides was systematically varied indicate that (i) the reactivity of mineral-surface-associated Fe(II), Fe(II)surf, toward the reduction of p-cyanonitrobenzene (CNNB) decreased in the order hematite > goethite > lepidocrocite > ferrihydrite and (ii) the surface density of Fe(II)surf did not play a crucial role in determining the observed reactivity trend. CNNB was reduced in Fe(II)only control experiments in a pH range of 7.28-7.97 with a pH dependency consistent with the transformation of Fe(II) to Fe(OH)3 or related oxides. The pH dependency of the reduction of CNNB in Fe(II)-treated ferric oxide suspensions (pH 6.1-7.97) could be accounted for by the oxidation of Fe(II)surf, forming an Fe(III) oxide.
Introduction Nitroaromatics (NACs) are common pollutants in soils and subsurface environments because of their use in the munitions industry, dye industry, pesticides, and industrial syntheses (1). Reduction is the predominant degradation pathway for nitroaromatics in natural anoxic environments. There is considerable evidence in the literature which suggests that Fe(II) associated with iron minerals surfaces, Fe(II)surf, is the dominant chemical reductant in anoxic systems (24). The increased reducing capability of Fe(II)surf over Fe(II)aq has been attributed to the augmented electron density in Fe(II)surf through complexation with mineral surface hydroxyl groups that act as sigma-donor ligands (5, 6). More recently, Fe(II) in aqueous ferric oxide suspensions has been found capable of transferring electrons to the bulk oxide (i.e., hematite, goethite, and ferrihydrite), the newly formed Fe(III) layer being similar to the underlying ferric oxide (7). In the context of the work described herein, Fe(II)surf refers to Fe(II) * Corresponding author phone: (706)355-8223;
[email protected]. † U.S. Environmental Protection Agency. ‡ University of Georgia. 4976
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lost from solution, which could involve surface complexation and Fe(II) oxidation due to electron transfer to bulk oxide. Other environmentally relevant species that have been observed to be reduced in Fe(II)/Fe(III) systems include chromium(VI) (8), nitrite (9), nitrate (10), radionuclides (11), uranium(VI) (12), oxime carbamate pesticides (13), herbicides (14), monochloramine (15), nitroaromatic explosives (16), and halogenated aliphatic compounds (17, 18). It has been proposed that the underlying ferric oxide matrix does not influence the reactivity of Fe(II)surf toward the reduction of nitroaromatics (4). On the other hand, the rate of reduction of p-chloronitrobenzene was (i) 1 and 2 orders of magnitude faster in magnetite than in goethite (pH 7 and 8) and lepidocrocite (pH 7), respectively (19), and (ii) 4, 3.5, and 3 orders of magnitude faster in goethite, lepidocrocite, and magnetite than in hematite suspensions (20). Other reported reactivity trends in similar systems include (i) the reduction of oxamyl, which was 1-1.5 orders of magnitude faster in hematite than in goethite suspensions (13), (ii) the reduction of monochloramine whose rate decreased in the order of magnetite > goethite > lepidocrocite ≈ hematite > ferrihydrite, and (iii) the reduction of chromate, which was about equally accelerated by goethite and lepidocrocite (8). Overall, these data indicate that the oxide matrix is a relevant environmental parameter that justifies further research to be performed both in the area of evaluating the kinetics of reducible chemicals as well as elucidating the driving force for the oxidation of the Fe(II) species involved. Although there has been interest in the study of the reductive transformation of organic pollutants in the environment for more than two decades, there is still a need to quantitatively address and systematically study the effect environmentally relevant variables could have on the reduction of nitroaromatics by mineral-surface-associated Fe(II). This study aims to (i) explore chemical parameters that would serve the purpose of predicting the reactivity of nitrobenzenes in Fe(II)-treated ferric oxides systems, (ii) determine the effect of surface area loading of ferric oxides, which are abundant and commonly found in the environment (i.e., hematite, goethite, lepidocrocite, and ferrihydrite), in Fe(II)-treated suspensions on the measured rate of reduction of pcyanonitrobenzene (kCNNB) and on the initial concentration of Fe(II) lost from solution prior to treating with p-cyanonitrobenzene ([Fe(II)surf,o]), and (iii) determine the relative reactivity of several Fe(II)-treated ferric oxide systems as a function of pH. These results will be useful in building transformation simulators for the fate of nitroaromatics in the environment. p-Cyanonitrobenzene (CNNB) was selected to study the effect of several environmentally relevant variables on nitroaromatic reduction because (i) it sorbs negligibly to goethite, log Kow ) 1.24 at 21 °C (21), simplifying the kinetic analysis, and (ii) p-cyanoaniline (CNAN), its reduction product, pKa ) 1.95 at 21 °C (22, 23), should only negligibly bind to goethite, improving the organic chemical mass balance.
Experimental Section Synthesis of Mineral Phases. Goethite (R-FeOOH) was prepared as in Atkinson et al. (24) and Torrents and Stone (25). Hematite (R-Fe2O3), lepidocrocite (γ-FeOOH), and 2-line ferrihydrite (∼Fe(OH)3) were prepared as in Schwertmann and Cornell (26). Additional details are offered in the Supporting Information. Chemicals. See Supporting Information. 10.1021/es052425x CCC: $33.50
2006 American Chemical Society Published on Web 07/06/2006
Molecular Modeling Calculations. See Supporting Information. Batch Studies. Individual degradation experiments, including the appropriate control studies, were performed in batch systems inside an anoxic glovebox (Coy Lab) filled with 95% N2:5% H2 at 21 ( 1 °C (for details on sample handling and operation of the glovebox, see Supporting Information). The following components of the reaction mixture were added to a 59-mL borosilicate glass serum bottle containing the appropriate amount of mineral whenever necessary: water, sulfonic acid buffer (Sigma; MES for pH 6.1 and 6.6; MOPSO for pH 6.8; MOPS for pH 7.0, 7.28, and 7.5; and EPPS for pH 7.67 and 7.97), sodium chloride (Aldrich), and Fe(II)aq; the serum bottles were closed with a Teflon-lined gray butyl septa and crimped. Sulfonic acid buffer and Fe(II)aq were added to the batch systems to give 25 mM and a total of 375 µM, respectively. The time allowed for the equilibration of ferrous ion in the system was 24 h, unless otherwise specified. The suspension was mixed at 49 rpm on an end-over-end rotor. The time zero for the kinetic experiments was defined as the moment of spiking the stock solution of NB, which was prepared in degassed methanol (g99.9, Fisher), for an initial concentration of 15 µM into the reaction mixture. At predefined times 1.3-mL samples were withdrawn while continually being hand-swirled and filtered through a 0.2µm PTFE syringe filter unit into a microcentrifuge tube. To stop the reaction, 1 mL of the filtrate was transferred to a 2-mL LC vial containing 35 µL of deoxygenated 60% perchloric acid (J. T. Baker). As proposed by Klausen (19), the difference in time between the addition of the NB stock solution to the reaction mixture and the end of the filtration period is taken as the reaction time. Analysis of Organic Compounds and Fe(II). See Supporting Information. Reduction Experiments. The reductive transformation of a series of monosubstituted nitrobenzenes in individual Fe(II)-treated goethite (0.142 g/L) suspensions was studied at pH 6.6. The set of chemicals in the training set consisted of p-cyanonitrobenzene (CNNB), p-acetylnitrobenzene (AcNB), p-chloronitrobenzene (ClNB), p-bromonitrobenzene (BrNB), nitrobenzene (HNB), and p-methylnitrobenzene (MeNB). Fe(II)aq was monitored throughout the reaction of CNNB. Further reduction studies of CNNB were performed in individual Fe(II)-treated ferric oxide suspensions to evaluate the effect of the surface area loading of oxides (SAL, m2/L) and pH. See the Supporting Information for detailed experimental conditions.
Results and Discussion Reduction of p-Substituted Nitrobenzenes and QSAR Analysis. CNNB was reduced in Fe(II)-treated goethite suspensions with a pseudo-first-order rate constant of 1.58 h-1 with the concurrent formation of CNHA, which was further reduced at a rate of 0.0667 h-1 leading to the formation of CNAN with a mass balance that fluctuated between 97% and 99% of the initial CNNB concentration (Figure 1). Because it takes six electrons to fully reduce one molecule of CNNB, Fe(II)aq consumption was divided by 6 in Figure 1 to provide the same scale for both Y axes. Fe(II)aq consumption from the start to the end of its reaction with CNNB in the system was 101 µM, resulting in an electron mass balance of 112%. In addition, we studied the reduction of a set of parasubstituted NBs with varying chemical properties (Table 1 and Figure 2) in goethite suspensions. The NBs were not reduced in Fe(II) solutions at pH 6.6. A pseudo-first-order rate constant was calculated for the reduction of the NBs in Fe(II)-treated goethite suspensions. In all cases a mass balance of g90% for product formation was achieved. The pseudo-first-order approximation was used to assess the
FIGURE 1. Reduction of 15 µM CNNB in Fe(II)-treated goethite suspension with the concurrent consumption of Fe(II)aq and formation of CNHA and CNAN (a). First 3 h of the reduction experiment (b). Conditions: 375 µM Fe(II) (initially added), 7.2 m2/L goethite, 17 µM Fe(II)surf, and pH 6.6. kinetic behavior of the NBs in the various experimental conditions throughout this work. One of the objectives of this work was to determine the molecular descriptors that serve as predictors of the chemical reactivity of NBs in Fe(II)/goethite systems using the observed rate of reduction as the measurable parameter. The oneelectron reduction potential (Eh1′), electron affinity (EA), and energy of the lowest unoccupied molecular orbital (ELUMO) were evaluated for their predictive capability regarding the reduction of NBs. We chose to evaluate the use of the Eh1′ because it is widely accepted that the rate-determining step for the reduction of nitroaromatics is the gain of the first electron, the reason good correlations between log kNBs and this parameter are usually obtained (19, 29). The next step was to explore other molecular descriptors that we expected to correlate at least as well as Eh1′ did with log kNBs but with the advantage that they can be calculated theoretically without requiring additional laboratory work (4, 27, 30) or extensive and time-consuming evaluation of existing data in the literature (31), as is often the case for the determination of Eh1′ values. The determination of Eh1′ values will always include, directly or indirectly, errors associated with laboratory measurements. Molecular descriptors calculated by molecular modeling software packages and computational tools such as SPARC (SPARC Performs Automated Reasoning in Chemistry) are internally consistent; hence, the regression analysis should fit only the variability in the measured response of the system under study to the extent that the calculations are valid. Toward this goal, we evaluated the predictive capability of the electron affinity (EA) provided by the SPARC online calculator (28). EA represents the energy difference associated with the gain of an electron (28), which should correlate with the ease or difficulty of the reduction of a compound. In addition, motivated by the fact that ELUMO has been successfully employed in toxicological studies to build QSARs VOL. 40, NO. 16, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Kinetic Data and Molecular Descriptors for p-Substituted Nitrobenzenes compd
k (1/h)a
SDb
Eh1 (V)c
EA (eV)d
ELUMO (eV)e
ELUMO (eV)f
ELUMO (eV)g
ELUMO (eV)h
CNNB AcNB ClNB BrNB HNB MeNB
1.58 0.854 0.284 0.267 0.161 0.105
0.070 0.018 0.0069 0.017 0.00096 0.0045
-0.340 -0.353 -0.459 -0.445 -0.486 -0.524
1.32 1.41 1.20 1.17 1.00 0.954
0.0238 0.0341 0.0488 0.0477 0.0607 0.0645
0.0314 0.0372 0.0500 0.0489 0.0579 0.0597
-0.1165 -0.1079 -0.0979 -0.0877 -0.0892 -0.0852
-0.1119 -0.1073 -0.0974 -0.0978 -0.0937 -0.0926
a Average pseudo-first-order rate constant calculated from the linear regression of concentration data (ln[XNB]/[XNB] ) vs time (h) for duplicate o experiments. b Standard deviation. c One-electron reduction potential (27). d SPARC calculated electron affinity (28). e Calculated lowest unoccupied f molecular orbital energies using HF/6-31G*. Calculated lowest unoccupied molecular orbital energies in the aqueous solution using HF/631G*,water. g Calculated lowest unoccupied molecular orbital energies using B3LYP/6-31G*. h Calculated lowest unoccupied molecular orbital energies in aqueous solution using B3LYP/6-31G*,water. i R2 for the linear regression of log kNB vs the molecular descriptors. Initial conditions: 15 µM NB, 375 µM Fe(II), 7.2 m2/L goethite, and pH ) 6.6.
FIGURE 2. Best fit for the reduction rate constant of p-substituted nitrobenzenes, derived from their reaction in Fe(II)-treated goethite suspensions, versus the (a) one-electron reduction potential, (b) electron affinity, (c) ELUMO(HF/6-31G*), (d) ELUMO(HF/6-31G*,water), (e) ELUMO(B3LYP/6-31G*), and (f) ELUMO(B3LYP/6-31G*,water). Initial conditions: 15 µM NB, 375 µM Fe(II), 7.2 m2/L goethite, and pH 6.6. for nitroaromatics (32-34), we decided to explore its potential as a molecular descriptor for nitroaromatic reduction. We expected NB reactivity to correlate with ELUMO because the latter is the energy of the molecular orbital that will contain the first gained electron in the course of reduction. ELUMO 4978
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was calculated in both the gas and the aqueous phase using Hartree-Fock (ELUMO(HF/6-31G*) and ELUMO(HF/6-31G*,water)) and density functional theory (ELUMO(B3LYP/6-31G*) and ELUMO(B3LYP/62 31G*,water)). Linear relationships (R ) 0.834-0.990) were obtained for the correlation between log kNBs and the chemical
method previously reported by Neta (30). Although the applicability of ELUMO demonstrated in this work is limited to p-substituted nitrobenzenes, we think it has the potential to be a good predictor of the reduction of other nitroaromatics (i.e., o- and m-nitrobenzenes, polynitroaromatics, and nitroPAHs) in Fe(II)-ferric oxide systems given the success of its use for various types of nitroaromatic compounds in toxicological studies (32-34). A possible explanation for the relatively poor correlation obtained with the use of EA is that it includes the relaxation of the anion radical (28), an aspect that is not reflected in the rate-determining step for electron transfer. Effect of the Type and Surface Area Loading of Ferric Oxide. The type of ferric oxide and availability of sites for the formation of surface-associated Fe(II) species could play a significant role in determining the rate of reduction of NACs. Surface area loading of oxides (SAL; surface area, m2, per liter of suspension) has been reported to affect the reduction rate of ClNB in Fe(II)-treated magnetite suspensions; however, an assessment of the concentration of Fe(II)surf,o as a function of SAL and its relationship to the reduction rate of p-chloronitrobenzene was not reported (19). In this study the SAL had a remarkable effect on kCNNB as well as on the concentration of Fe(II)surf,o in the four oxide systems under study (Figure 3 and Supporting Information Table S1). We found that kCNNB and the concentration of Fe(II)surf,o increased in an approximately linear fashion with respect to the SAL in each of the oxide systems (Figure 3a and 3b, respectively). However, kCNNB increased by 5, 4, 3.5, and 3 orders of magnitude with only a 2 orders of magnitude increase in SAL for hematite, goethite, lepidocrocite, and ferrihydrite, respectively. The rate of increase was much greater at the lowest loadings, approaching linearity at the higher loadings. Sorption of CNNB to the system components was not observed.
FIGURE 3. Effect of oxide surface area loading on the (a) reduction rate constant of CNNB and (b) initial concentration of oxide surfaceassociated Fe(II) in Fe(II)-treated hematite, goethite, lepidocrocite, and ferrihydrite suspensions. (c) Dependence of the reduction rate constant of CNNB on the initial concentration of surface-associated Fe(II). Initial conditions: 15 µM CNNB, 375 µM Fe(II), pH 6.6, and 0.1 M NaCl. descriptors that were evaluated (Figure 2a-f for best fits and Table 1). The results for ELUMO, for both Hartree-Fock (HF/6-31G*) and density functional theory (B3LYP/6-31G*), were generally most successful (Figure 2). For both HF/6-31G* and B3LYP/ 6-31G*, the inclusion of solvation in the models improved the quality of the fit; in the case of B3LYP, solvation had a more significant effect. The results indicate tha t ELUMO(B3LYP/6-31G*,water) was the descriptor that best explained the variability in the reactivity data for the reduction of the NBs, slightly better than the predictive capability of ELUMO(HF/6-31G*,water). The correlation obtained with ELUMO(B3LYP/6-31G*,water) was better than the correlation generated with the use of the Eh1′, which included some recently experimentally determined values (27) using the same
kCNNB increased as a function of the concentration of Fe(II)surf,o in an approximately linear fashion (Figure 3c and Supporting Information Table S1). The best linear fits for the dependence of kCNNB on the concentration of Fe(II)surf,o in each oxide system are shown in Figure 3c. The reactivity trend of the mineral-surface-associated Fe(II), Fe(II)surf, toward the reduction of p-cyanonitrobenzene under varying SAL conditions decreased in the following order: hematite > goethite > lepidocrocite > ferrihydrite. The changes in surface density of Fe(II)surf,o (i.e., [Fe(II)surf,o]/surface area) did not give an indication of being a key parameter influencing the reactivity of one oxide over another (i.e., hematite and ferrihydrite had lower Fe(II)surf,o densities, while they were more and less reactive, respectively, than either goethite or lepidocrocite). pH Effect. This work studied the effect of pH on the mineral-mediated reduction of CNNB by measuring the rate of reduction of CNNB in Fe(II)-treated hematite, goethite, lepidocrocite, and ferrihydrite suspensions in which the pH was systematically varied from 6.1 to 7.97. We found that kCNNB increased by 5, 4, 4, and 3 orders of magnitude when exposed to Fe(II)-treated suspensions of lepidocrocite, hematite, goethite, and ferrihydrite, respectively (Figure 4a and Supporting Information Table S2). Plots of log kCNNB vs pH showed a linear relationship for the systems studied (Figure 4a). The coefficients of correlation for these relationships were 0.990, 0.994, 0.994, and 0.954 for hematite, lepidocrocite, goethite, and ferrihydrite, respectively. kCNNB showed a dependency on Fe(II)surf,o (Supporting Information Figure S1), which was not nearly as linear as was the case in the SAL-variation experiment. Deviation from linearity in the kCNNB vs Fe(II)surf,o relationship as well as the expected tendency of a higher concentration of Fe(II)surf,o as pH increases could be related to errors in the Fe(II) analysis. VOL. 40, NO. 16, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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reactive Fe(II)-only systems after a certain period of time. The fact that the reduction of CNNB in Fe(II)-only systems was only observed for pH g 7.28 suggests that at these higher pHs the oxidation of Fe(II) is driven by the stabilization of an Fe(III) oxide or hydroxide species because the formal potential for these processes will shift more negative as pH increases. This will have the effect of increasing the effective rate constant for the reaction as the thermodynamic and kinetic driving force becomes more favorable. The pH dependency in Fe(II) solutions could be accounted for by the formation of amorphous iron oxide (eq 1), or related iron (III) oxides, upon oxidation of Fe(II) as it reacts with CNNB since pH equilibria are generally facile. The observation of a stoichiometric dependence on three protons further supports the assertion that the rate-limiting step for reduction of CNNB involves a single electron transfer, since three protons are released per electron transferred according to eq 1. Marcus theory of electron transfer shows that the activation barrier is linearly related to the free energy change during the reaction, even if the equilibrium proton transfer step is not directly involved in the activation step (35).
FIGURE 4. Dependence of the reduction rate constant of CNNB on (a) pH in Fe(II)-treated hematite, goethite, lepidocrocite, and ferrihydrite suspensions and Fe(II) solutions; (b) Eh of Fe(II)-treated ferric oxide suspensions calculated from the solubility product equilibrium constant of the oxide proposed as being formed from the Fe(II)surf oxidation. Initial conditions: 15 µM CNNB, 375 µM Fe(II), 1.3 m2/L ferric oxide, 25 mM sulfonic acid buffer, and 0.1 M NaCl. Reaction order plots were built for pH- and SAL-variation experiments (Supporting Information Figure S2). We did not find clear evidence of the presence of more than one redoxactive species through this type of examination, as was the result of this type of evaluation for the reduction of RDX by Fe(II)-treated magnetite suspensions (7) in which deviation of the higher pH’s data points from the Fe(II) loading variation experiment suggested the presence of different surface species with greater reactivity as (i) higher pH’s data points were clustered together with SAL-variation experiment for all oxides suspensions, (ii) SAL-variation experiments provided steeper responses than pH-variation experiments for Fe(II)-treated hematite, goethite, and ferrihydrite suspensions, and (iii) a roughly equivalent distribution of pHvariation data points around the SAL-variation results was observed for lepidocrocite. The usefulness of log kCNNB vs log [Fe(II)surf] plots for the determination of the apparent reaction order with respect to the concentration of Fe(II)surf,o, provided by the slope of the plots, for the systems under study is questionable since results suggest unrealistic reaction orders as high as five (Supporting Information Figure S2). CNNB was reduced in Fe(II)-only systems in a pH range of 7.28-7.97 (Figure 4a and Supporting Information Table S2). The slope for the pH dependence of the logarithm of the pseudo-first-order rate constants for CNNB reduction in Fe(II)-only experiments was close to 3 (m ) 2.79 and R2 ) 0.968). Figure S3 (see Supporting Information) shows the linear portion of a first-order plot for the reduction of CNNB in an Fe(II) solution at pH 7.97; deviation from first-order kinetics, likely due to the formation of Fe(III) particles as a consequence of the reduction of CNNB, was observed for all 4980
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The oxidation of other Fe(II) species (i.e., Fe(OH)+ and Fe(OH)2) likely present in the system at minute concentrations could account for the divergence of the slope for the relationship of log kCNNB/Fe(II) vs pH from 3. Ferrous carbonate complexes were not expected to play a significant role because all aqueous solutions and suspensions used in this work were prepared with carbonate-free Nanopure water (see Supporting Information, Chemicals). Although we cannot completely rule out the formation of oxide nanoparticles before addition of the reducible organic chemical, as reported by Klupinski, et al. (36) in similar reaction systems, this scenario was very unlikely to occur in our systems based on our observations: no detectable losses of Fe(II)aq before addition of CNNB and the absence of (i) cloudiness, (ii) development of a yellow color (which can indicate the presence of ferric oxides), and (iii) oxygen at a statistically significant level. The reactivity of the oxides toward the reduction of CNNB could be influenced by factors other than SAL, concentration of Fe(II)surf,o, and pH, for example, (i) Fe(II)surf speciation, (ii) accessibility of Fe(II)surf, and (iii) thermodynamics of the Fe(II)/Fe(III) redox couple (13). Recently, two research groups reported that Fe(II)surf,o, upon oxidation, can use the initially present bulk ferric oxide in a suspension as a template for crystalline growth. Chun, et al. reported that Fe(II)surf would oxidize and form goethite upon reduction of p-chloronitrobenzene in Fe(II)-treated goethite suspensions (37). Williams and Scherer (7), using 57Fe Mo¨ssbauer spectroscopy, found that in the absence of a reducible organic chemical (i) electron density from Fe(II)surf,o was transferred to the bulk of hematite and that Fe(II)surf,o was oxidized using this oxide as a template for crystalline growth and (ii) Fe(II)surf,o would oxidizesin the absence of a reducible organic chemicalsand form the bulk oxide in individual experiments involving Fe(II)-treated goethite and ferrihydrite suspensions. Also, they reported that the reduction of nitrobenzene would not proceed in the absence of Fe(II)aq, even though some Fe(II) had been sorbed to the bulk oxide present in the system. Aligned to Williams and Scherer’s findings, Park and Dempsey reported that Fe(II)surf would not be oxidized by oxygen even after consumption of all detectable Fe(II)aq in Fe(II)-treated ferric oxide systems (38). In an effort to gain more insight into the driving force of the Fe(II) oxidation in ferric oxide suspensions, we explored
upon oxidation used the ferric oxide as a template in a process that should result in crystalline growth. For this interpretation to be valid, the data for the SAL-variation experiment should collapse onto the same line. The relationship of the logarithm of the SAL-normalized kCNNB vs Eh values of Fe(II)-treated ferric oxides lied in approximately one line (R2 ) 0.842) with the exception of the hematite system, which showed a steeper response (Figure 5b).
Acknowledgments We thank Phillip Sawunyama for his advice on molecular modeling calculations and John Washington for useful discussions. This paper has been reviewed in accordance with the U.S. Environmental Protection Agency’s peer and administrative review policies and approved for publication. Mention of trade names or commercial products does not constitute endorsement or recommendation for use by the U.S. EPA.
Supporting Information Available
FIGURE 5. (a) Dependence of the reduction rate constant of CNNB (kCNNB) on Eh of Fe(II)-treated hematite (4), goethite (b), lepidocrocite (O), and ferrihydrite (2) suspensions calculated from the solubility product constant (Kso) of the oxide proposed as being formed from the Fe(II)surf oxidation. (b) Dependence of the surface area loadingnormalized kCNNB on Eh for Fe(II)-treated ferric oxide suspensions, including best fit for goethite, lepidocrocite, and ferrihydrite combined. Initial conditions: 15 µM CNNB, 375 µM Fe(II), pH ) 6.6, and 0.1 M NaCl. the correlation of the kinetic data of CNNB against the calculated reduction potential (Eh) for the Fe(II)/Fe(III) redox couples for each of the conditions for the pH- (Figure 4b) and SAL-variation experiments (Figures 4b and 5). The chemical equations formulated for the oxidation of Fe(II)surf, which was assumed to be the dominant responsible species for the reduction of CNNB, in each Fe(II)-treated ferric oxide system were of the form
FeXOYHZ(s) + (2Y - Z)H+ + Xe- ) XFe(II) + YH2O
(2)
The pH dependency described in these equations (i.e., 3) closely follows our findings reflected in the slopes of the log kCNNB vs pH (Figure 4a). The Eh values were calculated from the solubility product constants (Kso) and activities of the species involved (Supporting Information Table S3), assuming Fe(II)surf would oxidize forming an oxide similar to the bulk oxide with which it was associated (eq 2). One caveat of these Eh calculations (an example of them is provided in the Supporting Information) is that due to the lack of data for Fe(II)surf in the literature, the activity coefficient for Fe(II) was used to calculate the Kso-derived Eh. Even though the Eh calculations may not represent accurate values, we found an interesting tendency worth mentioning. The log kCNNB vs Eh relationships for the various systems in which pH and SAL were varied (Figures 4b and 5a, respectively) showed a general trend: for a given pH, the CNNB reduction proceeded faster at more negative Eh values as predicted by Marcus theory. One possible interpretation of these results is that Fe(II)surf
Description of the anoxic glovebox, commercial sources or procedures for the synthesis of chemicals, detailed experimental conditions of reduction experiments, description of molecular modeling calculations, analytical methods for the determination of organic chemicals and Fe(II)aq. Experimental and calculated data for the effect of the type and surface area loading of ferric oxides and effect of pH on the reduction of p-cyanonitrobenzene in Fe(II)-treated hematite, goethite, lepidocrocite, and ferrihydrite suspensions. Plots of (i) the dependence of the rate of reduction of CNNB on surfaceassociated Fe(II) in a pH-variation experiment, (ii) apparent reaction order for the reduction of CNNB by surfaceassociated Fe(II), and (iii) ln [CNNB]/[CNNB]o versus time in a Fe(II) solution. This information is available free of charge via the Internet at http://pubs.acs.org.
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Received for review December 2, 2005. Revised manuscript received May 31, 2006. Accepted June 1, 2006. ES052425X
