Kinetic and Microscopic Studies of Reductive Transformations of

Depassivation of Aged Fe by Ferrous Ions: Implications to Contaminant Degradation. Tongxu Liu , Xiaomin Li .... Chemosphere 2012 89 (7), 789-795 ... C...
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Environ. Sci. Technol. 2006, 40, 3299-3304

Kinetic and Microscopic Studies of Reductive Transformations of Organic Contaminants on Goethite CHAN LAN CHUN,† R. LEE PENN,‡ AND W I L L I A M A . A R N O L D * ,† Department of Civil Engineering, University of Minnesota, 500 Pillsbury Dr. SE, Minneapolis, Minnesota 55455-0116, and Department of Chemistry, University of Minnesota, 207 Pleasant St. SE, Minneapolis, Minnesota 55455-0431

Reactions mediated by iron mineral surfaces play an important role in the fate of organic contaminants in both natural and engineered systems. As such reactions proceed, the size, morphology, and even the phase of iron oxide minerals can change, leading to altered reactivity. The reductive degradation of 4-chloronitrobenzene and trichloronitromethane by Fe(II) associated with goethite (R-FeOOH) was examined by performing sequential-spike batch experiments. The particle size and size distribution of the pre- and postreaction particles were quantified using transmission electron microscopy (TEM). Results demonstrate that the degradation reactions result in goethite growth in the c-direction. Furthermore, pseudo-firstorder reaction rate constants for the degradation of 4-chloronitrobenzene and trichloronitromethane and for the loss of aqueous Fe(II) decrease dramatically with each subsequent injection of organic compound and Fe(II). This result indicates that the newly formed material, which TEM and X-ray diffraction results confirm is goethite, is progressively less reactive than the original goethite. These results represent an important step toward elucidating the link between mineral surface changes and the evolving kinetics of contaminant degradation at the mineralwater interface.

Introduction In both natural environments (e.g., groundwater and wetlands) and engineered systems (e.g., Fe0 permeable reactive barriers and water distribution systems composed of ductile iron pipe), iron oxide minerals mediate the removal of many prevalent water contaminants. Specifically, surface-bound Fe(II) has emerged as a potential reductant for various organic contaminants (1-8), heavy metals (9-11), and disinfectants (12). Previous work has demonstrated that, in the presence of iron oxide surfaces, Fe(II) is a potent reductant and that reaction rates involving oxidized organic contaminants are dramatically accelerated in comparison to solution-phase rates. Prior studies have examined the impact of dissolved Fe(II) concentration (3, 4), the type of mineral in suspension (1, 5, 6, 12), suspension pH (1, 4, 13, 14), and Fe(II) surface speciation (2, 7, 11) on the reduction rates of contaminants * Corresponding author phone: (612)625-8582; fax: (612)626-7750; e-mail: [email protected]. † Department of Civil Engineering, University of Minnesota. E-mail for C.L.C.: [email protected]. ‡ Department of Chemistry, University of Minnesota. E-mail for R.L.P.: [email protected]. 10.1021/es0600983 CCC: $33.50 Published on Web 04/12/2006

 2006 American Chemical Society

to elucidate the reactivity of the surface-bound Fe(II). A common observation is that reduction kinetics strongly depend on the extent of Fe(II) adsorption, which is affected by both solution pH and dissolved Fe(II) concentration. Besides using the total amount of adsorbed Fe(II), reactivity of surface-bound Fe(II) has been also successfully modeled using single or multiple reactive Fe(II) surface complexes that are pH dependent (1, 7, 11, 12) and using mineral identity (4-6, 12). For example, the reduction rate of U(VI) was successfully explained using two surface complexes, ≡FeOFe+ and ≡FeOFeOH° at different pH values (11). Klupinski et al. (7) found, however, improved correlation between reduction kinetics of pentachloronitrobenzene and the surface density of adsorbed Fe(II) as compared to a relationship that accounted for Fe(II) surface speciation. Similarly, the surface density of adsorbed Fe(II) was noted as a practical approximation for the concentration of reactive sites for degradation of hexachloroethane (6) and a variety of disinfection byproducts (8) by the Fe(II)-goethite system. Furthermore, the nature of the reactive Fe(II) surface species on goethite has been explored using wet chemical extraction (15, 16) and direct spectroscopic measurements (17). Incomplete recovery of adsorbed Fe(II) from iron oxide adsorbents implied formation of strongly bound Fe(II) surface complexes or incorporation of Fe(II) by production of stable phases such as magnetite (Fe3O4) (15, 16). Direct spectroscopic results provided evidence that electron transfer between adsorbed Fe(II) and iron minerals was similar to that between Fe(II) and Fe(III) in mixed-valent iron minerals such as magnetite (17). Williams and Scherer (17) have also shown that both the presence of aqueous Fe(II) and surfacebound Fe(II) are necessary ingredients for reductions mediated by performing experiments in which the dissolved Fe(II) was removed while the surface-bound Fe(II) was retained. In those experiments, no significant reactivity of surfacebound Fe(II) was observed (17). Our current understanding of the nature of the reactive sites responsible for contaminant reduction remains limited. Knowledge gained from batch kinetic experiments relies solely on observed changes in solution to describe reactions occurring at mineral surfaces. Even though some studies have attempted to identify and quantify adsorbed Fe(II) on iron minerals (15-17), the observations were not directly related with reaction kinetics. Moreover, calculations from kinetic studies often assume that the surface does not change as the reaction progresses. As reactions proceed, however, the size, morphology, and even the phase of iron oxide minerals can change, leading to an evolving chemical reactivity that may not resemble the initial state of the solid reactants. Studies have shown that rates of reactions mediated by iron oxide mineral surfaces can increase (via formation of a new iron oxide phase; 1) or decrease (18, 19) with time, demonstrating that the mineral surface is dynamic throughout the course of the reaction. Recent studies have combined surface characterization and solution phase measurements to improve understanding of the processes involved in surface reactions. For example, the heterogeneous oxidation of manganese(II) at mineral surfaces was successfully monitored using a combined microscopic and spectrometric method (20, 21). Moreover, a model for the dissolution of a manganese oxide was developed using data from atomic force microscopy (AFM) in a fluid cell and X-ray photoelectron spectroscopy and Mn(II) measurements from solution samples collected during the AFM experiments (22). In terms of iron (hydr)oxides, it was shown that Fe(II) oxidation occurred at the goethite VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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mineral surface by measuring goethite growth on specific faces using AFM within a fluid cell and knowing the oxidation rate of Fe(II) in solution (23). Using a similar approach, the formation of iron (hydr)oxide was correlated with the reduction rates of As(V), Hg(II), and trichloroethylene (TCE) in an Fe(II)-phlogopite system (24). The objective of our study is to elucidate the link between mineral surface changes and evolving reactivity for the Fe(II)-goethite system using a combination of analytical chemistry methods and materials characterization (i.e., X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM)). Specifically, the reductive degradation of organic contaminants by Fe(II)-goethite was investigated by performing sequential-spike experiments in batch reactors. Ultimately, reduction kinetics are compared with physical, quantifiable changes in goethite particles.

Experimental Section Chemical sources and purities are provided in the Supporting Information. Iron Minerals. Goethite (R-FeOOH) nanorods were prepared by aging an aqueous suspension of six-line ferrihydrite at 90 °C for 3 days. The six-line ferrihydrite was synthesized by addition of 0.4799 M NaHCO3 to 0.4000 M FeNO3‚9H2O using a peristaltic pump followed by microwave annealing and dialysis (25). Particle size, size distribution, and X-ray diffraction results are provided in the Supporting Information. Sequential-Spike Batch Experiments. Sequential-spike batch experiments were carried out under anaerobic conditions at room temperature (22 ( 3 °C). Batch reactors consisted of 123 mL serum bottles containing 50 mM MOPS buffer (pH 7), 0.65 g/L goethite, and no headspace. The reactor was spiked with ferrous chloride to achieve a total Fe(II) concentration of 1 mM and was equilibrated overnight. The reactors were wrapped with aluminum foil to prevent potential photolysis reactions. To initiate each reaction, the reactor was spiked with a methanolic stock of organic contaminant to achieve a starting concentration of 100 µM. 4-Chloronitrobenzene (4-Cl-NB) and trichloronitromethane (TCNM) were selected as the target organic contaminants. 4-Cl-NB is a model compound for nitroaromatic herbicides and explosives, and TCNM has been used as a fumigant and detected as a disinfection byproduct in drinking water. Reactors were magnetically stirred throughout the experiment. At desired time intervals samples were taken via simultaneous injection of deoxygenated MOPS buffer and withdrawal of 0.5 mL of the suspension to avoid the introduction of headspace. Samples were filtered though 0.2 µm PTFE Gelman Acrodisk syringe tip filters and analyzed for 4-Cl-NB and aqueous ferrous iron concentrations by highperformance liquid chromatography and a modified Ferrozine method (26), respectively. For gas chromatography analysis of TCNM, each 0.5 mL unfiltered aqueous sample was extracted with 1.0 mL of n-pentane. When the parent compound was completely degraded, the remaining aqueous Fe(II) concentration in the reactor was measured. On the basis of this concentration, Fe(II) was added to return the aqueous Fe(II) concentration to 1 mM and the reactor allowed to reequilibrate overnight. The next reaction was initiated 24 h later by respiking the organic contaminant to a level of 100 µM. Each reactor was spiked a total of five consecutive times. Sampling and analysis were conducted in an identical manner for each spiking. TEM Observation and XRD Analysis. Prior to each respike, a sample of the goethite suspension (1 mL) was collected and characterized using an HRTEM (FEI Techni T12) equipped with a charge-coupled device camera for image acquisition. Each goethite suspension sample was filtered though a 0.2 µm RTTP Isopore membrane using a reusable syringe filter holder, and then Milli-Q water was passed 3300

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FIGURE 1. Reduction rate constants of organic contaminant (b, (a) 4-Cl-NB, (b) TCNM) and rate constants of aqueous Fe(II) loss (4) as a function of number of injections in the presence of 0.65 g/L goethite (initial), 1 mM total Fe(II), and an initial concentration of 100 µM 4-Cl-NB or TCNM at pH 7. Each reaction has the same solution conditions. The error bars are 95% confidence intervals. through the membrane to wash the collected goethite particles. Particles on the membrane were resuspended in Milli-Q water by shaking and sonication. One drop of the diluted suspension was placed on a holey carbon-coated TEM grid and allowed to dry in air. HRTEM images were taken at a minimum of 5 different locations on the grid, and a total of 25-30 images were analyzed per sample. Lengths and widths of goethite particles were measured from calibrated images using Digital Micrograph software (Gatan Inc. v.3.8.2). For XRD analysis, particles remaining in the reactor after completion of the sequential-spike experiments were washed with Milli-Q water using a centrifugedecantation method and then freeze-dried. A PANalytical X’Pert PRO X-ray diffractometer equipped with an X’Celerator detector and cobalt source was used. Additional analytical methods are described in the Supporting Information.

Results Reactivity of Fe(II)-Goethite. Fe(II) associated with goethite nanorods (Fe(II)-goethite) reduced 4-Cl-NB to 4-chloroaniline and rapidly degraded TCNM to nitromethane via reductive dechlorination. Plots of ln(C/C0) versus time were linear, and thus degradation of organic compounds was welldescribed by pseudo-first-order kinetics (see Supporting Information). Aqueous Fe(II) loss was fit by both pseudozero-order and pseudo-first-order rate models, but pseudofirst-order fitting appears most reasonable (see Supporting Information). Figure 1a shows the change in pseudo-firstorder rate constant for the reduction of 4-Cl-NB and the pseudo-first-order rate constant for loss of aqueous Fe(II) from solution with each injection. These data clearly show that despite replenishing the 4-Cl-NB and total Fe(II) concentrations to their initial levels with each injection, rate constants for both 4-Cl-NB reduction and aqueous Fe(II)

FIGURE 2. TEM images of (a) goethite particles before reaction and (b) goethite particles after the fifth reaction with 4-Cl-NB. (c) A magnified image of prereaction goethite particle tips and (d) a high-resolution TEM image obtained from the image of (b) are also shown. The postreaction particles are longer, and the tips are no longer faceted. The d spacing in image (d) shows that the newly formed material is goethite. loss consistently decreased with each injection. In fact, reactivity of 4-Cl-NB decreased by 77% when comparing the pseudo-first-order rate constants of the first and fifth injections. In the case of TCNM (Figure 1b), the degradation was so rapid that the pseudo-first-order rate constant of the first injection could not be precisely measured (≈2.44 min-1). Nevertheless, the decrease in pseudo-first-order rate constants of TCNM degradation for subsequent injections demonstrates a similar drop (∼76%) in reactivity. Although the reduction mechanisms for TCNM (reductive dechlorination) and 4-Cl-NB (reduction of nitro-group) differ, the decrease in reactivity observed was similar. TEM Observation of Solid-State Materials before and after Reaction. Figure 2a shows a representative TEM image of the prereaction goethite particles, and Figure 2b shows a representative TEM image of goethite particles after the fifth injection of 4-Cl-NB. Figure 3 shows histograms of length measurements for goethite nanorods reacted with 4-Cl-NB as a function of injection. The histograms demonstrate that the length of the particles increases with each injection. On the basis of the t-test (two-sample assuming unequal variances, R ) 0.05), the average length measured after each reaction was significantly different from the previously measured length (see Supporting Information). After the fifth injection, the goethite particles are 55% longer than the prereaction particles (Figure 4a). The particle size distribution, reflected both in the histograms and by the standard deviations, broadened with each injection. The increasing size distribution breadth indicates a distribution of growth rates. In other words, if all particles were equally reactive, the breadth of the size distribution would be constant. In contrast, particle widths did not change on the basis of the t-test (Figure 4b; see Supporting Information). This indicates that the goethite nanoparticles grew in only one directions parallel to the c-axis. Applying the same statistical tests to the particle size measurements of goethite particles used in TCNM experiments demonstrates a similar trend (Figure 4c,d). While the overall, acicular morphology of the goethite nanoparticles was retained in the postreaction goethite

FIGURE 3. Particle size distributions for goethite particles before and after reactions of 4-Cl-NB as a function of injection number. Length was measured along the crystallographic c axis of goethite. The average length (l) and the standard deviation (σ) are indicated. The average sample size is approximately 400 particles. particles, the texture of the goethite particle tips changed dramatically after reaction with the organic contaminants (Figure 2c,d). Before reaction, the goethite particle tips are strongly faceted and well described as (021) facets. After VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 4. Particle length (l) and width (w) distributions for goethite particles before reaction and after the fifth reaction with (a, b) 4-Cl-NB and (c, d) TCNM. The average values of length and width and the standard deviation (σ) are indicated. The average sample size is approximately 400 particles. reaction, goethite tips can no longer be described by (021) facets and, in fact, are no longer well-faceted and appear roughened. Furthermore, a high-resolution image of postreaction goethite particles (Figure 2d) shows that the newly formed material is clearly goethite because the lattice fringes are continuous over the entire length of the particle. That is to say, there is no evidence of the precipitation of a new phase as a result of Fe(II) oxidation during these reactions. XRD analyses confirmed this result (see Supporting Information).

Discussion Goethite particles grew only in the c-direction with each exposure to ferrous iron and 4-Cl-NB or TCNM. In our system, the growth of goethite can be attributed to either preferred oxidation of Fe(II) and reduction of the organic contaminant at a specific crystallographic surface or growth by precipitation of dissolved Fe(III) species. The latter possibility is unlikely because our experiments were conducted under anoxic conditions and, on the basis of calculations using Visual MINTEQ (v.2.30; 27), the concentration of aqueous Fe(III) species is predicted to be low. Also, the results of a control experiment (Supporting Information) without TCNM or 4-Cl-NB demonstrate that the aqueous Fe(II) concentration was stable for the duration of the sequential spike experiments (5 days) after the initial 20% drop in concentration due to adsorption. This implies anoxic conditions were maintained and precipitation of aqueous Fe(II) onto goethite was negligible. Thus, the redox reaction most likely occurs on the goethite surface. To ascertain if goethite growth occurs as the direct result of the redox reaction at the goethite surface, we performed mass balance calculations on the basis of the stoichiometry 3302

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of the reaction and the particle size (i.e., length) and size distribution measurements. The predicted length of newly formed goethite was estimated from the number of moles of organic contaminants reduced by Fe(II)-goethite (see Supporting Information for details). Consumption of organic contaminants was used in the calculation rather than loss of aqueous Fe(II) from solution because aqueous Fe(II) was consumed via multiple pathways including adsorption, oxidation, and ultimately incorporation into goethite. The actual length of goethite was taken as the mean value measured via the particle size analysis. Figure 5 shows a close match between the experimentally measured and predicted lengths of the goethite particles. The mass balance calculation supports the hypothesis that oxidation of Fe(II) to Fe(III) by reduction of organic contaminants followed by the incorporation of the product Fe(III) at the goethite surface drives the crystallization of new goethite on the original particles. The experimental measurements reveal that particles increase in length and that production of new particles is negligible. On the basis of the above, the changes in goethite particle dimensions can be related to the observed reduction kinetics. Even though each injection cycle restored contaminant and Fe(II) to their initial amounts, reactivity decreased substantially with each subsequent injection. Because the particles only grow in the c-direction, we can conclude that growth occurs only at the particle tips. The observed changes in morphology could be caused by the oxidation of Fe(II) induced by organic contaminants, by aging of the suspension, or by both. Previous results have shown that goethite nanorods do not significantly grow in aqueous suspensions (pH 4-12), even at elevated temperatures (60120 °C) and over hundreds of hours (28). Thus, we conclude

FIGURE 5. Comparison between the actual, experimentally measured lengths using particle size analysis and the predicted lengths calculated from the stoichiometry of the reaction with 4-Cl-NB (b) and TCNM (O). The line is a 1:1 line. that goethite growth results from Fe(II) oxidation by the organic contaminants. TEM images further show that the tips roughen with continued reaction. Considering only the goethite particle tips, roughening is likely to lead to a small increase in surface area, but this fundamental change is accompanied by an overall drop in (021) surface area (see Supporting Information). This observation suggests that the roughened tips are substantially less reactive than the initially well-faceted tips. The decreased reactivity could arise from several possibilities. The roughened surface may affect uptake of Fe(II) onto the goethite particles, the complexes of the Fe(II) formed on the surface, and the incorporation of Fe(III) for crystal growth. The tips of the goethite nanorod particles used in this study are bound by (021) faces. The (021) faces have been reported as sites favorable to iron adsorption and attachment compared to other faces due to the atomic structure and surface coordination of oxygen on the (021) face (23, 29-31). The roughened goethite tips may not uptake Fe(II) as effectively as the original tips due to the loss of (021) faces. Quantitatively, this hypothesis is supported by decreased adsorption of Fe(II) onto goethite after each reaction with 4-Cl-NB. The initial amount of Fe(II) adsorbed onto the goethite decreased with consecutive injections from 20.0% of the total Fe(II) concentration prior to the first reaction to 3.8% before the fifth reaction (see Supporting Information). This may explain the bulk of the reactivity loss. Additional factors may also play a role in the reduced reactivity. Different crystal faces have different surface charges, and changes in morphology result in changes in the average surface charge of a crystal (32). Such changes are expected to influence the amount and reactivity of adsorbed Fe(II). In addition, the speciation of Fe(II) surface complexes bound on the roughened (or other, less favorable) surfaces may be different from complexes on the (021) facet due to changes in surface coordination of oxygen atoms. On the basis of the idea that specific surface complexes of Fe(II) may be responsible for reduction of organic contaminants (11, 24), different distributions of Fe(II) surface complexes would alter the reactivity. Lastly, roughened tips may affect the subsequent incorporation of Fe(III) for growth. The steric environment of the (021) face favors the incorporation of Fe atoms for growth because the surface contains structural

units that can readily accept iron atoms into an arrangement that results in goethite crystal growth (23, 31). Because the tips roughen upon reaction, the Fe(III) produced may not be readily incorporating into the growing goethite crystal and may even block subsequent Fe(II) adsorption. Overall, the goethite particles in our system have lost the (021) faces (which are dominant in the reactivity and growth of goethite) with consecutive reactions on the same surfaces. It can be suggested that the sites responsible for contaminant reduction by Fe(II)-goethite reside on the (021) surface and that decreased adsorption of Fe(II) onto the active tips caused by the loss of (021) faces as the tips grow is largely responsible for the observed decrease in reactivity with each exposure to the organic contaminant. The fact that (021) faces seem to react faster than other faces is comparable to results using oxygen as oxidant reported by Wiedler et al (23). Even though their study focused on the growth rates of (100) and (110) faces, they remarked that the (021) faces grew more rapidly than (100) and (110) faces (23). Interestingly, the progressive decrease in reactivity toward 4-Cl-NB, which degrades by nitro-group reduction, is similar to the progressive decrease in reactivity toward TCNM, which degrades by reductive dechlorination. This suggests that the reaction is “crystal-chemical controlled” (i.e., the abundance of reactive sites/the ability of the surface to take up Fe(II) is more significant in determining reactivity than the reduction mechanism). This study highlights the need for microscopic studies to relate bulk solution phase kinetics to changes in the mineral surface. Materials characterization techniques (i.e., XRD and HRTEM) provide valuable information regarding specific crystallographic relationships between the underlying goethite, newly formed materials, and locations where reactions occur. Our results confirm that normalization by surface area is not sufficient to describe/compare the reduction rate of contaminants by Fe(II)-goethite (6-8). In this work, the reactive surface has been conclusively identified as the (021) face and has been shown to progressively decrease in its overall contribution to the total surface area (see Supporting Information for (021) as a percentage of total surface area). Normalizing to total surface area would artificially augment the progressive decrease in observed reactivity. Additionally, our results emphasize that the assumption of a static surface when describing reduction in Fe(II)-goethite systems is inadequate. On the basis of the results of this work, the reactivity of Fe(II)-goethite appears to be controlled by crystallography and is independent of the nature of the electron-transfer reaction itself or the identity of the electron acceptor. Reduction mediated by goethite nanoparticles in the presence of Fe(II) may be pertinent to natural environments. Ferric colloids have been detected in sediment porewaters (33), and goethite nanoparticles have been found in coastal aquifer sands (34). Also, Zee et al. found that nanogoethite is the predominant reactive phase in lake and marine sediments (35). Natural systems and the goethite or iron oxide particles they contain are certain to be more complex than the model goethite nanorods used herein. Even so, it may be reasonable to hypothesize that the reactivity of natural sediment containing iron (hydr)oxides can be attributed to goethite nanoparticles that will change dramatically with continued reaction.

Acknowledgments This work was supported by three grants from the National Science Foundation (BES-0332085 to W.A.A., CAREER0346385 to R.L.P., and MRI EAR-0320641). We thank the Department of Civil Engineering, University of Minnesota, for providing a Sommerfeld fellowship to C.L.C. Parts of this work were carried out in the Minnesota Characterization VOL. 40, NO. 10, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Facility, which receives partial support from NSF through the NINN program. Thanks are also extended to the three anonymous reviewers for their thoughtful comments.

Supporting Information Available Chemical sources and purities, characterization methods, additional analytical methods, properties of goethite nanorods, a schematic diagram of goethite nanorods, kinetic modeling, t-test results for goethite particles, plots of degradation of 4-Cl-NB and TCNM in sequential spike experiments, plots of loss of aqueous Fe(II) in sequential spike experiments, calculation of the predicted lengths of new goethite via mass balance, Visual MINTEQ input and output, XRD patterns of the pre- and postreaction goethite, aqueous Fe(II) concentration in control experiments, surface area of goethite nanoparticles after sequential spike experiments, and Fe(II) sorbed onto goethite in sequential-spike experiments with 4-Cl-NB. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review January 17, 2006. Revised manuscript received March 15, 2006. Accepted March 16, 2006. ES0600983