Environ. Sci. Technol. 2007, 41, 5277-5283
Particle Size and Aggregation Effects on Magnetite Reactivity toward Carbon Tetrachloride PETER J. VIKESLAND,* APRIL M. HEATHCOCK, ROBERT L. REBODOS, AND K. ERIK MAKUS The Charles E. Via Jr. Department of Civil and Environmental Engineering, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24060
Nanoparticulate magnetite is found in many natural and engineered environments. This study characterized the reactivity of this material toward carbon tetrachloride (CCl4). Particle diameter plays an important role, with nominal 〈9 nm〉 magnetite suspensions exhibiting greater reactivity on both mass (km) and surface area normalized (kSA) bases than 〈80 nm〉 magnetite suspensions. For the 〈9 nm〉 suspension, the aggregation state of the particles affects the measured km values. At 0.001 M ionic strength and pH 7, km ()0.052-0.139 L g-1 h-1) was as much as seven times larger than at 1 M (km ) 0.025-0.030 L g-1 h-1). This decrease in reactivity with an increase in ionic strength is related to the measured diameter of the aggregates present in solution, thus implicating aggregate size as an important variable. This work is the first to indicate that both particle size and aggregation state must be considered when evaluating the reactivity of nanoparticle suspensions with groundwater contaminants.
Introduction Under anoxic conditions magnetite (Fe3O4) is topotactically oxidized by protons in water to produce maghemite (γ-Fe2O3), a structural polymorph of hematite (1, 2):
Fe3O4 + 2 H+ f γ-Fe2O3 + Fe2+ + H2O
(1)
This reaction is strongly affected by solution pH and, although rapid at low pH, is expected to be negligible for pH > 7 (2). In the presence of aqueous oxidants (1, 3-6), parallel oxidation reactions occur, as illustrated for carbon tetrachloride (CCl4):
3 Fe3O4 + CCl4 + H+ f 4 γ-Fe2O3 + Fe2+ + CHCl3 +
Cl- (2)
In this reaction, structural FeII donates an electron to surface associated CCl4, which then undergoes reductive dehalogenation via dissociative electron transfer wherein C-Cl bond cleavage and electron transfer occur simultaneously (5, 7). This one electron transfer produces trichloromethyl radical (‚CCl3), which can react to form either the hydrogenolysis product, chloroform, or intermediates such as dichlorocarbene (:CCl2) and trichlorocarbanion (-:CCl3) that decay to * Corresponding author e-mail :
[email protected]; phone: 540231-3568; fax: 540-231-7916. 10.1021/es062082i CCC: $37.00 Published on Web 06/28/2007
2007 American Chemical Society
CO, methane, and formate. The final product distribution is dictated by solution pH and by the reactions of ‚CCl3 with system components such as hydrogen donors (5, 6, 8-10). Magnetite is a ubiquitous environmental constituent and can be found in weathered clays and soils, in atmospheric aerosols, and in recently deposited marine and freshwater sediments (2, 11, 12). Magnetite also forms as a result of either abiotic corrosion processes (13) or as a bacterial metabolic byproduct (14). Past studies examining magnetite reactivity toward groundwater contaminants have typically been conducted either with FeII added to the suspension (6, 15-17) or with relatively large magnetite particles (>40 nm diameter) produced using recipes such as those in the compilation by Schwertmann and Cornell (18). The presence of surface adsorbed FeII is expected to increase the suspension reactivity (6, 15) and thus the results may not reflect those that would occur under FeII limited conditions. Furthermore, experiments employing larger magnetite particles may underestimate the reactivity of this mineral due to diffusive limitations on the availability of redox active FeII. A maghemite coating develops at the particle-water interface when magnetite is oxidized. For magnetite oxidation to proceed once this coating forms, structural FeII must diffuse to the particle surface to maintain charge electroneutrality (1, 3, 19). After the regenerated surface FeII is oxidized, the cycle repeats until the particle is completely oxidized. Because larger diameter particles have longer diffusive pathlengths, a significant percentage of structural FeII within a particle may be diffusively unavailable. It is the presence of an oxidized maghemite coating that enables particles that are predominantly magnetite to persist in oxic environments (3, 20). In addition to the potential for diffusive limitations to decrease the overall reactivity of bulk magnetite relative to smaller particles, there is also a growing body of evidence that indicates particle reactivity increases with a decrease in crystallite size. Within the nanodomain (dimension 18.1 MΩ). Water was deaerated by boiling for 30 min and then sparging with either argon or nitrogen gas for an additional 30 min while boiling continued. Deaerated water was removed from the heating source, capped, and transferred to an anaerobic chamber. Reagent grade chemicals were used in all experiments. Particle Synthesis. Nanoparticulate magnetite (referred to as 〈9 nm〉 magnetite, see Supporting Information) was synthesized in the 95%/5% N2/H2 atmosphere of an anaerobic glovebox (Coy Laboratory Products Inc.) by dropwise (∼1 drop/s) addition of a mixture of 0.2 M FeCl3 and 0.1 M FeCl2 to a well-mixed solution of 1 M NaOH in 1 M NaCl (29). The final ratio of iron solution to base solution was kept at 3:2 to keep the pH above 12. Mixing was maintained by use of an overhead mixer, glass stirring shaft, and PTFE blade. To remove excess salts, precipitated nanoparticles were magnetically separated, decanted, and rinsed with deaerated water at least three times. Following the final rinse, the particles were diluted with water and stored in a polypropylene container in the anaerobic glovebox. The final suspension pH was between 10.5 and 11.5. Larger magnetite particles (〈80 nm〉 magnetite, see Supporting Information) were synthesized using established methods (30). Preliminary experiments suggested that particle aging did not impact magnetite reactivity. Nonetheless, to minimize potential aging effects on particle reactivity, 〈9 nm〉 magnetite particles were prepared within 96 h of experiment initiation. Supporting Information Table S1 lists the batches of 〈9 nm〉 magnetite used. To determine the reproducibility of the suspension preparation procedure, three separate batches were synthesized and their reactivity toward CCl4 was found to be the same (Figure S4). This similarity indicates that differences in particle reactivity caused by batch-to-batch variability are minor. Particle Characterization. As detailed in the Supporting Information, the particle size and morphology of the synthesized particles were characterized by X-ray diffraction (XRD), Brunauer-Emmett Teller (BET) surface area analysis, and transmission electron microscopy (TEM). The Supporting Information also describes the procedures used to obtain particle sizes and surface areas from the XRD (SAXRD), BET (SABET), and electron microscopy (SAEM) results. The isoelectric point (IEP) and in-situ aggregate size of the magnetite suspensions were evaluated using a Malvern Zetasizer 3000 HS. For the IEP measurements, a series of magnetite suspensions with initial pH values ranging from pH 4.5 to 12 were analyzed. The pHIEP was estimated to be 6.5 ( 0.3 by the graphical relationship between the measured electrophoretic mobility and the pH of the solution (Figure S5). This value is similar to the pHIEP previously reported for particles produced using this technique (29). The in-situ aggregate size of dilute magnetite suspensions was determined using cumulant analysis of dynamic light scattering (DLS) data collected at a given ionic strength. For these experiments, NaCl was added to a sonicated magnetite suspension and the average aggregate size (Zavg) after 25 min was measured. Kinetic Experiments. Kinetic experiments were conducted by exposing magnetite solutions to chlorinated 5278
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organic compounds. CCl4 was chosen as the model contaminant due to its prevalence in the environment and the extensive amount of research that has described the reductive dehalogenation of CCl4 by iron materials (5, 6, 8, 9, 17). Chloroform (CHCl3) is one of the primary products formed by CCl4 reduction, so kinetic experiments were also conducted with CHCl3 to determine if the particles could reduce CHCl3. One experiment was conducted using trichloroethylene (TCE) to determine if magnetite could reduce this compound. The experiments with both CHCl3 and TCE indicated that neither compound was readily reduced by 〈9 nm〉 magnetite after 120 h. No further experiments were conducted with either species. All reactivity experiments were carried out in ∼61 mL glass vials that were stored in the anaerobic chamber and allowed to equilibrate for a minimum of 24 h. Reactor vials were prepared by mixing an aliquot of magnetite stock with 50 mM HEPES (4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, purity >99%) buffered water at a given ionic strength under headspace free conditions. NaCl was the primary salt used to set ionic strength; however, CaCl2 and NaClO4 were also employed. For simplicity, the reported ionic strength values do not consider the speciation of the zwitterionic HEPES buffer. Particle concentrations ranged from 1 to 10 g/L, with most experiments at 5 g/L. The pH was set by adding NaOH or HCl into each reactor until the desired pH was achieved. The vials were capped and sealed with aluminum seals and PTFE lined septa. Prepared vials were equilibrated for a period of 8-96 h prior to spiking them with an aliquot of CCl4, CHCl3, or TCE (each in 100% methanol). Following compound addition, the reactors were removed from the glovebox and placed on a bottle roller. Prior to sampling, the reactors were taken off the roller and placed on a magnet to enhance particle separation. At that point, 250 µL of supernatant was withdrawn using the dualsyringe technique (31) and mixed with 2.75 mL of deionized water in a 20 mL glass autosampler vial. The vials were capped and sealed with aluminum caps and septa. Control reactors without magnetite were set up for each experiment under identical solution conditions. CCl4, CHCl3, and TCE were quantified by headspace analysis using a headspace gas analyzer and carbon monoxide was measured using a reduced gas analyzer as described in the Supporting Information. Temperature and Mixing Effects. To examine potential temperature effects, the dechlorination reactions were studied over a range of temperatures (3.6-36 °C). At 22 °C, two mixing regimes were studied. The reactor vials equilibrated at a designated temperature for ∼8 h prior to CCl4 addition. Mixing of the vials was achieved by a bottle roller at 3.6 and 22 °C, and the rotating mechanism of a water bath at 22, 30, and 36 °C. As discussed in the Supporting Information, mass transfer effects are expected to be minimal for discrete nanoparticles, but are expected to become increasingly important as effective particle size increases due to aggregation.
Results and Discussion The kinetics of CCl4 degradation in the presence of magnetite can be described by eq 3 wherein CCl4 disappearance is proportional to the dissolved CCl4 concentration ([CCl4]) and the number of magnetite surface sites ([>Fe3O4]):
d[CCl4] ) - kCT[>Fe3O4]R[CCl4]β dt
(3)
where kCT is the overall rate constant for CCl4 degradation and both R and β are reaction orders. Under the conditions employed in this study, CCl4 degradation followed pseudofirst-order kinetics and the rate was independent of the initial CCl4 concentration (data not shown) indicating that [>Fe3O4]
FIGURE 1. Determination of the apparent magnetite surface site reaction order (r). Contrary to expectations, (r) was not constant and appeared to vary with the solution conditions. The error in the slope reflects the 95% confidence interval. Conditions: [CCl4]0 ≈ 100 µM, 50 mM HEPES buffer. was always in excess and that β ) 1. The pseudo-first-order rate constant kobs,CT is defined as:
kobs,CT ) kCT[>Fe3O4]R
(4)
kobs,CT values were obtained via nonlinear regression (SigmaPlot version 9) of collected CCl4 vs time data. A summary table (Table S1) of the collected results may be found in the Supporting Information. Interestingly, for the 〈9 nm〉 suspensions the magnetite surface site reaction order, R, was found not to be constant and varied between 0.71 and 1.44 in three separate mass loading studies (Figure 1). This non-integer, non-constant reaction order suggests that the number of reactive surface sites does not increase in a straightforward manner with an increase in surface loading. As discussed in the sections that follow, for a fixed magnetite concentration of 5 g/L, changes in the aggregation state of the particles have a significant effect on the CCl4 degradation rate. Similarly, changes in the magnetite mass load are also expected to affect aggregate structure (32). Because the experiments to determine R were done at different pH values and at different ionic strengths (both of which affect the aggregation state), the number of available surface sites under one set of conditions apparently differs from those under another, thus leading to variable R values. We note that a prior study with ferrihydrite nanoparticles (33) also reported surface mediated reaction orders that differ from one and thus this is not a magnetite specific phenomenon, but may be a general characteristic of nanoparticle suspensions. In many studies involving surface mediated reactions, observed rate constants such as kobs,CT are normalized either by the particle mass load or by the particle surface area to obtain either a mass normalized rate constant (km; L g-1 h-1) or a surface area normalized rate constant (kSA; L m-2 h-1). For nanoparticle suspensions, however, the available reactive surface area may differ from the dry particle surface areas determined via conventional techniques (e.g., SAEM, SAXRD, SABET) due to particle aggregation (8). In addition, Cwiertny and Roberts (34) recently showed that surface area normalization can lead to potential misinterpretations of results for
FIGURE 2. Comparison of carbon tetrachloride reduction rates in suspensions of 〈80 nm〉 and 〈9 nm〉 magnetite. Although both suspensions contained 5 g/L of magnetite, even on a surface area normalized basis the rate of carbon tetrachloride loss was considerably faster for the 〈9 nm〉 suspension. Conditions: [CCl4]0 ≈ 114 µM, [Fe3O4]0 ) 5 g/L, pH 7.8, 50 mM HEPES buffer, 0.1 M NaCl. Curves correspond to first-order fits and error bars reflect the standard error of triplicate reactors. systems where R differs from one. For these reasons, we primarily report km values. Particle Size Effects on CCl4 Degradation Rates. Under identical solution conditions, the km values for CCl4 degradation are higher for 〈9 nm〉 magnetite ()0.0096 L g-1 h-1 at pH 7; )0.057 L g-1 h-1 at pH 7.8) than those for 〈80 nm〉 magnetite ()0.00046 L g-1 h-1 at pH 7, )0.0006 L g-1 h-1 at pH 7.8; Figure 2). Some of the difference in reaction rates can be accounted for by the larger available surface area of the 〈9 nm〉 suspension. However, even after normalization by SABET, the calculated kSA values for the 〈9 nm〉 particles remain an order of magnitude larger than those for 〈80 nm〉 magnetite. The higher rates observed with 〈9 nm〉 magnetite could be the result of differences in the available reactive surface area for suspensions of different particle sizes (8), quantum confinement effects, or differences in the diffusive availability of FeII in each suspension. To evaluate this later possibility, particle conversion rates estimated using a diffusion in a sphere model (3, 19) were used to calculate theoretical CCl4 reduction rates as a function of particle radius. Under the boundary condition that FeII is immediately oxidized when it reaches the surface, this model can be written as:
( )
Mt Dt ) 6π-1/2 2 M∞ r
1/2
-3
Dt r2
(5)
where Mt/M∞ is the fractional conversion of magnetite into maghemite at time t, D is the diffusion coefficient for FeII within the crystalline matrix (≈1.25 × 10-20 cm2 s-1; ref 3), and r is the particle radius. From eq 5, it is apparent that the time required for complete oxidation of a discrete magnetite particle is a function of the particle radius (Figure S6a). This particle oxidation model was used to predict the CCl4 reduction rate by using a fixed mass to volume ratio of 1 g/L and assuming the following: (1) spherical particles, (2) a Fe3O4 unit cell (length ) 0.839 nm) of 8 formula units (30), (3) the reaction is limited by the rate of diffusion of FeII within the particle (e.g., FeII at particle surface is zero), (4) each VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Mass normalized pseudo-first-order carbon tetrachloride reduction rates increase with pH and are a function of the particle size and the ionic strength of the medium. The values for the mass loading experiments were obtained from the slope of a plot of kobs,CT vs mass concentration. Raw data may be found in Table S1. [CCl4]0 ≈ 100 µM, 50 mM HEPES buffer. particle is present in a discrete nonaggregated form, and (5) two electrons are transferred when CCl4 is converted to carbon monoxide. Using this approach it was predicted that the CCl4 reduction rate should be a function of particle radius (Figure S6b). From these simulations it was found that a doubling of the particle radius from 5 to 10 nm leads to a 4× increase in the predicted half-life of CCl4. We note, however, that the effects of particle size on the half-life of CCl4 are expected to be complicated by the aggregation state of the particles. As discussed below, the magnetite suspensions tested in this study are highly aggregated and it is not known how potential differences in aggregation state (which may be a function of the primary particle size) would affect the available reactive surface area and thus the predicted CCl4 reduction rate. A second complication that limits a straightforward interpretation of Figure S6b is that the derivation of eq 5 assumes that any FeII which diffuses to the surface is immediately oxidized. Although such an assumption may be reasonable for nanoparticles with high surface reactivity (3), it neglects the potential build-up of FeII that may occur under some conditions. Under those circumstances a boundary condition that accounts for surface FeII is required in order to integrate the diffusion in a sphere model. Even with the aforementioned caveats, we anticipate that the trends depicted in Figure S6 illustrate the fundamental effect that changes in particle size have on FeII availability and should thus reflect how particle radius affects CCl4 reduction. Experiments are ongoing to measure particle transformation rates and relate them to CCl4 degradation. These studies will provide further insight into the effects of particle radius on contaminant transformation and will help 5280
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delineate the factors responsible for the observed higher reactivity of the 〈9 nm〉 suspension. As depicted in Figure 3, the rates observed in our 〈80 nm〉 suspension correlate with those obtained previously (5) under similar ionic strength conditions (e.g., 0.1 M NaCl), but in the absence of HEPES buffer. This similarity indicates that the presence of the buffer did not alter the reactivity of bulk magnetite toward CCl4. A prior study (35) had suggested that tertiary amine buffers such as HEPES may have a modest rate enhancing effect on the initial one-electron transfer to CCl4, however, such an effect did not appear to occur here. pH Effects on CCl4 Degradation Rates. As the solution pH was increased from 6 to 8.5, the reactivity of the 〈9 nm〉 suspensions increases (Figure 3). The magnitude of this increase was a function of the ionic strength (the calculation of which does not consider buffer speciation), with lower ionic strength solutions exhibiting faster rates. This pH dependency has previously been attributed to the protonation and deprotonation of surface FeII (5):
>FeIIIOFeIIOH / >FeIIIOFeIIO- + H+
(6)
With an increase in pH, this equilibrium shifts such that a greater number of sites are present in the more reactive deprotonated form. Our results support this conclusion; however, given that a decrease in ionic strength from 0.1 to 0.001 M leads to as much as a 5× increase in km at a given pH value, we suggest that pH mediated particle aggregation also plays an important role. Solution pH and ionic strength have previously been shown to affect magnetite aggregation (36, 37). Colloidal interactions in nanoparticle solutions can be largely described
by the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory (38). Counterions surround the charged particle surface to maintain electroneutrality and form an electric double layer (EDL). The stability of the suspension is directly related to the magnitude of the EDL. When the EDL is large, the suspension remains stable because particle-particle repulsion is also large. When the EDL is small, particles may approach one another more closely and van der Waals and magneto-dipole interactions promote particle aggregation. In general, for pH values one unit above or below the IEP of the suspension, an increase in ionic strength leads to compression of the EDL and enhanced particle aggregation. Near the IEP, however, there is no charge screening and the effects of ionic strength changes are expected to be minimal. The IEP of the 〈9 nm〉 suspension was pH 6.5 ((0.3) and thus particle aggregation is expected to decrease as the solution pH increases above and below this value. This pH effect is magnified at low ionic strength and hence the measured rates are higher under those conditions. Near the IEP, charge screening does not occur and differences in ionic strength are not expected to affect particle aggregation. This effect, when coupled with the overall decrease in the magnetite surface reactivity at low pH causes the observed km values to become ionic strength independent near pH 6 (Figure 3). Ionic Strength Effects on CCl4 Degradation Rates. To support our aggregation hypothesis, a series of experiments were undertaken using three different electrolytes: sodium chloride (NaCl), calcium chloride (CaCl2), and sodium perchlorate (NaClO4) at ionic strengths ranging from 0.001 to 1 M (this corresponds to a concentration range of 0.001-1 M for the monovalent salts and 0.0004-0.4 M for CaCl2). At pH 7, the effects of an increase in NaCl over this ionic strength range were relatively modest, leading to a 40% decline in measured km values (data not shown). At pH 7.8, however, which is greater than one pH unit away from the IEP, the effects of changes in ionic strength were more significant with declines in km for a given electrolyte as large as 82% (Figure 4a). For the NaCl solutions, the observed reactivity was highest for the solution with the smallest aggregate size and considerably lower for the solutions with larger aggregates (Figure 4b). Consistent with the aggregation hypothesis, the rate constant for CCl4 degradation in the presence of CaCl2 at an ionic strength of 0.001 M was 2.0× lower than that in the presence of NaCl and 2.7× lower than that in the presence of NaClO4 at the same ionic strength. Via the Schultz-Hardy rule (38), divalent Ca2+ should enhance particle aggregation to a greater extent at low ionic strength than monovalent Na+ (36). As ionic strength increases, the electrolyte identity becomes less important and at 1 M ionic strength all solutions exhibit the same reactivity toward CCl4. This result is consistent with coagulation theory, which indicates that at ionic strength values above the critical coagulation concentration that aggregation is independent of electrolyte identity (36). The difference in CCl4 reactivity in the presence of the two monovalent salts, NaCl and NaClO4, at low ionic strength was unexpected, but may reflect either the reduced driving force for reductive dechlorination in the presence of NaCl or the formation of ClO4- surface complexes that enhance surface reactivity. Because Cl- is one of the products of the dissociative electron-transfer process, it would be expected that the presence of mM quantities of this anion will decrease the reaction rate relative to systems that do not contain Cl(39). In addition, recent reports suggest that perchlorate surface complexes may enhance cation retention/sorption at a fixed pH, while the analogous chloride surface complexes cause a decrease in cation retention/sorption (40). For magnetite particles undergoing topotactic conversion into
FIGURE 4. Effect of ionic composition on (A) the mass normalized pseudo-first-order carbon tetrachloride dechlorination rate constant and (B) the average aggregate size in NaCl solution at pH 8.0. For part (A) [CCl4]0 ≈ 100 µM, 50 mM HEPES buffer, 5 g/L 〈9 nm〉 magnetite, pH 7.8. Aggregate size (Zavg) in part B was determined using cumulant analysis of dynamic light scattering data. Error bars for the km values reflect the standard error for triplicate reactors and the error bars for Zavg reflect the variation in Zavg between 20 and 30 min. maghemite, the formation of ClO4- surface complexes could lead to enhanced FeII concentrations in the interfacial region, which would cause the observed reaction rates to be higher. Danielsen (41) evaluated CCl4 degradation by bulk magnetite and observed that her kSA values decreased as ionic strength increased between 0.008 and 0.11 M at pH 7, 8, and 10. Under her reaction conditions, the effects of ionic strength were more mild (kSA declines
