ARTICLE pubs.acs.org/IECR
Carbothermal Synthesis of Aerosol-Based Adsorptive-Reactive IronCarbon Particles for the Remediation of Chlorinated Hydrocarbons Jingjing Zhan,† Bhanukiran Sunkara,† Jingjian Tang,† Yingqing Wang,† Jibao He,‡ Gary L. McPherson,§ and Vijay T. John*,† †
Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118, United States Coordinated Instrumentation Facility, Tulane University, New Orleans, Louisiana 70118, United States § Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, United States ‡
ABSTRACT: Spherical ironcarbon nanocomposites were synthesized through a facile aerosol-based process and a subsequent carbothermal reduction. The distribution and immobilization of iron particles throughout the carbon microspheres prevents nanoiron aggregation, allowing the maintenance of particle reactivity. The carbon microspheres allow adsorption of TCE, thus removing dissolved TCE rapidly and facilitating reaction by increasing the local concentration of TCE in the vicinity of iron particles. The strongly adsorptive property of the composites may prevent release of toxic chlorinated intermediate products. The nanoscale composite particle size is in the optimal range for effective transport through groundwater saturated sediments. It is also shown that carbothermal treatment of the composite material leads to highly porous carbon materials containing zerovalent iron species, a necessary reactive component in the reaction pathway. The mesoporous structure generated allows access to internal reactive sites.
’ INTRODUCTION The objective in this research is to develop a potential high throughput method to produce carbon-based materials containing zerovalent iron nanoparticles for the environmental remediation of chlorinated hydrocarbons. Chlorinated compounds such as trichloroethylene (TCE) are pollutants commonly found in the environment. They have a specific gravity greater than water and thus migrate deep into the subsurface from which they gradually dissolve into aquifers, causing problems of persistent environmental pollution and difficulties in cleanup.1,2 The use of nanoscale zerovalent iron (NZVI) particles represents a promising approach to the remediation of such compounds through reductive dechlorination following the simplified redox mechanism shown below.35 C2 HCl3 þ 4Fe0 þ 5Hþ f C2 H6 þ 4Fe2þ þ 3Cl Compared to conventional granular iron powders, the potential advantages of the use of nanoparticles include the high reactivity as a result of their increased specific surface area6 and the fact that the extremely small particle size intrinsically enhances colloidal stability when aggregation is prevented, allowing direct injection into sites of contamination as a particle slurry.7 However, the intrinsic ferromagnetism of NZVI particles larger than 1520 nm leads to rapid aggregation, decreasing the reactivity and limiting the mobility of NZVI in the subsurface.812 Prior studies have shown that the aggregation of NZVI particles can be inhibited dramatically by the addition of hydrophilic or amphiphilic polymers such as guar gum,13 poly (acrylic acid) (PAA),14 starch,15 carboxymethyl cellulose (CMC),9,16 and triblock copolymers (PMAA-PMMA-PSS)17 on the NZVI particle surface. These adsorbed polymer molecules inhibit NZVI aggregation and enhance r 2011 American Chemical Society
solution stability through steric hindrance and/or electrostatic repulsion. However, they may decrease the reactivity by blocking reactive surface sites that are intrinsically necessary for the reaction.18 Another option is to distribute and immobilize NZVI particles onto solid supports typically silicas10,11 and carbons,1922 generating NZVI-support composites. In addition to preventing the aggregation of NZVI particles, the main feature of these hydrophobically designed supports, such as alkyl-functionalized silica and hydrophobic carbons,14,23 is their effectiveness in adsorbing TCE, thereby increasing local concentrations of TCE in the vicinity of the NZVI particles and facilitating reaction. A second requisite of an efficient material to decontaminate chlorinated compounds is the property of efficient transport through groundwater saturated sediments. Filtration theory predicts that the migration of colloidal particles through porous media such as soil is typically dictated by Brownian diffusion, particle interception, and gravitational sedimentation.24 The Tufenkji-Elimelech (T-E) model is perhaps the most comprehensive model to describe these effects in the presence of interparticle interactions,25 with the governing equation 0:052 Nvdw þ 0:55AS NR1:675 NA0:125 η0 ¼ 2:4AS NR0:081 NP0:715 e 1=3
0:053 þ 0:22NR0:24 NG1:11 Nvdw
Special Issue: Ananth Issue Received: April 12, 2011 Accepted: August 29, 2011 Revised: August 14, 2011 Published: August 29, 2011 13021
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throughout the carbon matrix and avoid the use of sodium borohydride using a carbothermal method of reduction. This is a simple step where the iron oxide particles generated by the aerosol process are essentially reduced by the carbon support matrix through pyrolysis in an inert atmosphere, leading to the evolution of carbon dioxide. The potential distinct advantages of this process include the following (1) NZVI particles that are evenly distributed throughout the carbon support allowing a high loading of NZVI, (2) reduction of the precursor iron species to zerovalent iron occurring through a carbothermal step taking advantage of the carbon that is present on the support, instead of adding NaBH4, and (3) the fact that the carbothermal treatment introduces a high porosity in ironcarbon composites since part of the carbon is reacted away to CO and CO2 during the reduction of iron oxide, thus allowing the entry of contaminants to reactive sites. Additional factors include the fact that the starting materials are environmentally benign and economical and the whole process is operated through a semicontinuous method, facilitating large scale synthesis for eventual in situ application. The incorporation of iron into carbons through the aerosol route has been recently explored in the literature including work from this laboratory.26,27 Applications to the environmental remediation of chlorinated hydrocarbons through the use of the carbothermal process to modulate porosity for this application are the new aspects of the current work. Figure 1. (a) Schematic of aerosol-based process and (b) schematic of synthesis route to synthesize ironcarbon composites.
’ EXPERIMENTAL SECTION
where η0 is the collector efficiency, simply defined as the ability of the sediment to collect migrating particles, thus limiting transport through the subsurface. The first term on the right characterizes the role of particulate diffusion on the collector efficiency, while the second and third terms describe the effects of interception and sedimentation, respectively. The model predicts a minimum in collector efficiency (implying optimal transport) over the particle size range of 0.1 to 1 μm. Supporting zerovalent iron (10100 nm) on a carbon matrix with particle sizes in this range therefore indicates a potential in designing materials that have optimal transport properties in addition to reactive and adsorptive properties.10 Indeed, this is the objective of our work in trying to generate high throughput materials in the optimal size range. In recent work from our laboratory, we have demonstrated that an aerosolbased process followed by liquid phase reduction using NaBH4 can be employed to prepare Fe/C composites, where nanoscale zerovalent iron particles are supported on the surface of carbon particles.26 During the aerosol process, the “chemistry in a droplet” concept leads to submicrometer-sized Fe/C composites (100 nm1 μm), which are in the optimal size range for effective transport through sediments.10,14 Importantly, aerosol-based carbon microspheres serve as strong adsorbents for TCE increasing local concentrations at the ZVI reaction sites, thereby enhancing the driving force of reaction. While our recent work has shown the feasibility of the aerosol process, the reactivity of zerovalent iron is obtained through reduction with NaBH4 which adds to the costs of the process. In addition, the use of sodium borohydride generates large amounts of hydrogen during reduction operation, impeding the scale-up of the process.23 In the current paper, we retain the merits of our earlier work to use the aerosol-based technology to produce ironcarbon composites but attempt to distribute NZVI particles
Materials. All chemicals were used as received without further treatment. Sucrose (ACS reagent) and trichloroethylene (TCE, 99%) were purchased from Sigma-Aldrich. Ferrous sulfate heptahydrate (FeSO4 3 7H2O) (certified ACS) was supplied by Fisher Scientific. Deionized (DI) water, generated with a Barnstead E-pure purifier (IA) to a resistance of approximately 18 MΩ, was used in all experiments. Sample Preparation. The aerosol-based technique was employed to prepare ironcarbon particles. In a typical synthesis, 6 g of sucrose and 5 g of FeSO4 3 7H2O were dissolved in 50 mL of water to form a starting precursor. During the aerosol process, the precursor solution was first aerosolized to droplets which were then carried by nitrogen gas through a heating zone where solvent evaporation, sucrose carbonization, and iron phase transition occur. The flow rate of nitrogen gas was 2.5 L/min, and the heating was done in a 100 cm tube with a furnace length of 38 cm leading to a superficial velocity of 2.7 cm/s. The temperature of the heating zone was held at 1000 °C. The resulting black powder, containing magnetite (Fe3O4) and carbon (hereafter referred to as Fe3O4C), was collected over a filter maintained at 100 °C to facilitate drying. The effluent gas through the filter was bubbled through water to bring it to ambient conditions before discharge. To obtain zerovalent ironcarbon composites, a carbothermal reduction process was used, where the collected Fe3O4C composites were thermally reduced by the carbon of the particles to produce CO2 and zerovalent iron. In the carbothermal process, the Fe3O4C powder is placed in a crucible boat in a quartz tube within a tube furnace and subjected to a temperature of 720 °C for 10 h under flowing nitrogen. The tube is purged with nitrogen gas for 30 min before heating, and the sample is allowed to cool down to room temperature in a nitrogen atmosphere after the carbothermal step. Furthermore, a mild passivation step using deoxidized water or ethanol (95%) is carried out before sample removal from the tube furnace to prevent spontaneous ignition of 13022
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Figure 2. (a) SEM and (c) TEM illustrate the morphology of aerosol-based Fe3O4C composites. (b) and (d) are SEM and TEM images of an individual Fe3O4C particle, respectively.
the zerovalent iron nanoparticles when contacted with the oxygen in air. For brevity, we adopt the nomenclature FeC to depict zerovalent iron nanoparticles incorporated in the carbon matrix. Characterization. Transmission electron microscopy (TEM, JEOL 2010, operated at 200 kV voltage) and field emission scanning electron microscopy (SEM, Hitachi S-4800, operated at 20 kV) were used to characterize the size and the morphology of the particles. The porosity of the particles was measured by the nitrogen sorption technique at 77 K using a surface area and porosimetry analyzer (Micromeritics, ASAP 2010). The samples were degassed at 200 °C prior to the measurement. Specific surface areas were determined using the BrunauerEmmett Teller (BET) equation. Powder X-ray diffraction (XRD) patterns were recorded using a Rigaku diffractometer, with a step size of 0.1°, a scanning speed of 5°/min, and Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was conducted with a Scienta ESCA-300 high-solution X-ray photoelectron spectrometer (HR-XPS). A Kα X-ray beam at 3.8 kW was generated from an Al rotating anode. Analytical. TCE adsorption and dechlorination effectiveness was tested in batch experiments. In detail, 0.5 g of the as-prepared FeC composite particles was added to a 40 mL vial, followed by the addition of known quantities of diluted Pd(OAc)2 in acetone solution (6 103 M) to load the Pd catalyst onto FeC composite particles. The catalyst loading step is similar to the
procedure described in the literature.9,15,21,28,29 Here, the palladium salt was completely reduced by zerovalent iron and is rapidly deposited onto the iron surface following a Pd(II)Fe(0) replacement reaction:30,31 Fe þ Pd2þ f Fe2þ þ PdV After evaporation of acetone, 20 mL of a 20 mg/L TCE stock solution was added and, then, the vial was capped with a Mininert valve. The reaction was monitored through headspace analysis using a HP 6890 gas chromatography (GC) equipped with a J&W Scientific GS-GASPRO capillary column (30 m 0.32 mm) and flame ionization detector (FID). Samples were injected splitless at 220 °C. The oven temperature was held at 75 °C for 2 min, ramped to 150 °C at a rate of 25 °C/min, and finally held at 150 °C for 10 min.
’ RESULTS AND DISCUSSION Particle Synthesis. Figure 1a illustrates the schematic of the aerosol apparatus, consisting of an atomizer, a heating zone, and a filter. Starting with a homogeneous aqueous solution containing sucrose and iron(II) sulfate, a commercial aersolization unit (Model 3076, TSI, Inc., St Paul, MN) atomizes the solution into droplets that undergo a heating step and drying step, generating particles that are collected on a filter. Figure 1b is a representation 13023
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Figure 3. (a) SEM and (c) TEM show the morphology of FeC composites obtained after carbothermal reduction. (b) and (d) are SEM and TEM images of an individual FeC particle, respectively.
of the formation route of the FeC composites. When these aerosol droplets pass through the heating zone, the precipitation and phase transition of iron species is concomitant with the dehydration and carbonization of sucrose under high temperature conditions leading to the formation of Fe3O4C composites with Fe3O4 nanoparticles incorporated in the carbon matrix. In the subsequent carbothermal reduction process, a “self-redox” reaction occurs within these Fe3O4C composites, where the entrapped Fe3O4 particles are thermally reduced by carbon frameworks (in excess), resulting in the formation of final product FeC composites. The final weight percentage of zerovalent iron in the FeC composites was approximately 40%. This content was determined by weighing the residual solid (Fe2O3) of a known mass of FeC composites after calcination under 500 °C in the air for 4 h. It is noteworthy that FeC composites can also be obtained by reduction of Fe3O4C using hydrogen at 500 °C for 5 h. In both cases, we have found FeC composites with the same requisite characteristics. Our earlier work has shown that the aerosol process is a facile approach to prepare silica-based materials in the submicrometer (typically 100800 nm) size range and applied to environmental remediation.10,11 Here, we expand this technology to produce carbon-based particles using inexpensive materials as the carbon source. Interestingly, for aerosol-based processes with a short residence time (∼14 s), carbon products from the sucrose
precursor are only obtained in the presence of the iron salt and carbonization is not observed if sucrose is used as the sole precursor. Our explanation is that iron cation (Fe2+) may act as a Lewis acid catalyst to promote the dehydration and carbonization process of sucrose.27 We also note that acidifying the precursor through the addition of dilute sulfuric acid also catalyzes carbon microsphere formation. Moreover, the iron oxide nanoparticles (Fe3O4) may serve as seeds for the growth of carbon spheres similar to the formation of carbon nanotubes and carbon nanocages.32,33 Recent pioneering work by Atkinson and coworkers have also showed the formation of ironcarbon composites through the aerosol process using highly basic precursor solutions and hydrogen reduction to prepare zerovalent iron.27 Particle Characterization. The morphology and microstructure of the composite particles were analyzed through scanning and transmission electron microscopy. Figure 2 shows representative SEM and TEM images of Fe3O4C composites obtained from the aerosol process prior to carbonization. The Fe3O4C composites are spherical, with sizes ranging from 100 nm to 1 μm, characteristic of particles synthesized by the aerosol-based technique. The relatively rough surface particle surfaces indicate a porous morphology. The TEM images of Figure 2c,d are more illustrative in clarifying the presence of Fe3O4 nanoparticles which are visualized clearly as the dark spots due to high electron density. Figure 3 shows typical images of the final product FeC 13024
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Figure 4. (a) Nitrogen adsorptiondesorption isotherms for FeC and Fe3O4C composite microspheres; (b) the BJH pore size distribution derived from the desorption branch of the isotherm of composite microspheres.
composites obtained after the carbothermal reduction process. The SEM images appear to show zerovalent iron clusters on the surface of the particles which maintain a porous morphology. The TEMs in Figure 3c,d further confirm the presence of zerovalent iron nanoparticles in the porous carbon spheres, where the zerovalent iron particles show enhanced contrast due to the higher electron density of iron. The larger iron particle sizes in the carbonized sample is perhaps due to the sintering that occurs during the extended carbonization treatment. It is noted that FeC composites appear to be more porous than the Fe3O4C composites. We consider the increase of porosity to be due to the carbon consumption during the carbothermal reduction. Figure 4a shows the N2 adsorption isotherms obtained for these composites. Surface areas at 77 K were calculated on the basis of the BrunauerEmmetTeller (BET) method. For Fe3O4C and FeC composites, the BET surface areas were found to be 130 m2/g and 224 m2/g, respectively, and the corresponding BarretJoynerHalenda (BJH) desorption pore volumes were determined to be 0.282 cm3/g and 0.4 cm3/g. Both samples exhibit a IV-type isotherm with H3-type hysteresis loop on the basis of the IUPAC (International Union of Pure and Applied Chemistry) classification.34 The type IV isotherm is associated with the capillary condensation taking place in mesopores, and H3-type hysteresis loop is attributed to asymmetric slit-shape mesopores. Such isotherms indicate an interconnected porous system and percolating high pore connectivity.35,36 In agreement with the TEM images of Figures 2 and 3, the evolution of pore structure from Fe3O4C composites to FeC composites is confirmed from the BJH pore size distribution, as illustrated in Figure 4b. Clearly, a narrow pore size distribution at a
Figure 5. (a) XRD patterns and (b) XPS full survey scan of FeC and Fe3O4C particles.
mean value of 3.4 nm is validated for the Fe3O4C composites. In contrast, there are broad peaks in the range of 4 to 8 nm for FeC composites, which may be a consequence of the increased porosity of the material created through the reduction of iron oxides using carbon. The presence of zerovalent iron is verified by the XRD pattern shown in Figure 5a. For particles collected directly from the aerosol process prior to carbothermal treat, magnetite peaks dominate the pattern indicating that the starting material FeSO4 transformed primarily to Fe3O4 during aerosolization. In the XRD pattern of FeC composites, strong peaks at the 2θ of 45°, 65°, and 82° reveal the presence of zerovalent iron (α-iron). Additionally, there are minor signatures based on the γ-iron crystalline phase and FeO which is formed during the passivation step. The results of X-ray photoelectron spectroscopy (XPS) studies are shown in Figure 5b. XPS peaks at 710.0 and 723.3 eV are related to the binding energies of Fe (2p3/2) and Fe (2p1/2) of iron oxide, suggesting that the surface of iron particles consisted of a layer of iron oxides, as reported in the literature.37 In addition, the peak centered at 284.5 eV represents the binding energy of C (1s),38 showing the presence of carbon in FeC and Fe3O4C composites. Adsorption and Reactivity Studies. Figure 6 illustrates reactivity characteristics of FeC composites when contacted with dissolved TCE, where TCE removal from solution and gas phase product evolution rates are shown. In all experiments, we also add extremely small amounts of Pd (typically 0.050.1 wt %) 13025
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Figure 7. Comparison of adsorption capacity of humic acid, FeC, and commercial activated carbon. In all experiments, 20 mL of a 20 ppm TCE solution and 0.2 g of sorbents (humic acid or carbon materials) were used.
Figure 6. TCE removal from solution and gas product evolution rates for palladized FeC. Pd/FeC ratio: (a) 0.05% and (b) 0.1%. M/M0 is the fraction of the original TCE remaining, and P/Pf is the ratio of the gas product peak to the final gas product. The red circles in both figures show an immediate sharp decrease of the dissolved TCE concentration to its original value.
to catalyze the dissociative chemisorption of H2 and facilitate reaction with the following mechanism:7,3942 Fe f Fe2þ þ 2e 2Hþ þ 2e f H2 pd
H2 sf 2Hðactivated atomic hydrogenÞ 5H þ C2 HCl3 þ 3e f C2 H6 þ 3Cl Here, the iron component of the PdFe material behaves as an electron donor, whereas palladium serves as a catalyst for hydrogen activation and the formation of dissociated hydrogen species which acts as the reducing agent for TCE dechlorination. The overriding observation in Figure 6 is the sharp decrease of solution TCE level as soon as the reactive particles are added, a consequence of TCE adsorption. In the case of 0.05% Pd deposited FeC composites as shown in Figure 6a, we note an immediate sharp decrease of the dissolved TCE concentration to 3% of its original value followed by a slower decrease. The initial sharp decrease is due to TCE partitioning from solution to the carbon through strong adsorption. This is an important aspect to the design of these materials as the phenomenon
Figure 8. Conceptual schematic showing potential applicability of FeC composites for the adsorption and simultaneous dechlorination of chlorinated hydrocarbon contaminants.
leads to enhanced reactant concentrations in the vicinity of the reactive zerovalent iron sites. The ratio of accessible zerovalent iron in the FeC composites was determined by weighing the residual solid (C and unreachable Fe) of a known mass of FeC composites after reaction with 37% HCl to etch out the component of zerovalent iron. Through this method, the percentage of accessible zerovalent iron is approximately 93 wt %. The rate at which gas products evolve is indicative of the observed reaction kinetics of TCE. Since adsorption is rapid, reaction is the rate controlling step and it is possible to calculate a pseudofirst order rate constant by 13026
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Industrial & Engineering Chemistry Research following the evolution of the lumped gas phase products. For FeC composites with 0.05% Pd, the apparent reaction rate constant, kobs, is approximately 1.2 h1 and the mass-normalized reaction constant, km, is 0.12 L hr1 g1 on the basis of the mass of zerovalent iron. Clearly, the results of reactivity characteristics indicate no inhibitory aspect in the reaction rate upon TCE adsorption, and we can thus consider the adsorption/desorption step of TCE as being in equilibrium with the overall rate controlled by the surface reaction associated with TCE dechlorination over iron and Pd.31 It is noteworthy that the bare FeC composite (without Pd) is also eventually able to remove TCE from solution to nontoxic products, but it exhibits a much lower destruction rate (0.003 L hr1g1) than the palladized system, in accordance with the literature.43 Figure 6b also illustrates a significant increase of the reaction rate upon the deposition of more palladium to the FeC system. For FeC composites with 0.1% Pd, the apparent reaction rate constant, kobs, is approximately 9.2 h1 and the mass-normalized reaction constant, km, is 92 L hr1 g1 on the basis of the mass of zerovalent iron. Despite the fast kinetics, the reaction is still reaction controlled in comparison to the adsorption step. In fact, the first data point taken after 15 min is simply a consequence of allowing the system to stabilize after adding the particles and initiating reaction. In independent experiments with just carbon microspheres, the adsorption step is completed within 2 min of
Cads TCE Cwater TCE
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initiating contact between carbon and TCE. We also note that the environmentally toxic intermediates such as dichloroethene (C2H2Cl2) and vinyl chloride (C2H3Cl) are below the limit of detection using our GC procedure, and the entire product range is based on the light gases as described in our recent paper.26 The fact that toxic intermediates are not observed is perhaps again due to the strong adsorptive characteristics of the carbon and reemphasizes the value of incorporating ZVI onto carbon particles. Any chlorinated intermediates remain adsorbed on the carbons until they are reacted away to the light gases, primarily ethane and ethylene but include a tiny amount of butane and butene. Hence, the adsorptive-reactive system proposed here is well-suited for TCE remediation as it also provides a strong sequestration mechanism in addition to reactive decontamination. Figure 7 compares adsorptive capacities of the aerosol-based FeC composites with humic acid (the major natural organic constituent of soil) and commercial activated carbon. In all experiments, 20 mL of a 20 ppm TCE solution and 0.2 g of particles were used. Clearly, the adsorption of TCE on FeC (∼94.5%) is much higher than that of humic acid (∼30%) and comparable to that of commercially available granular and irregularly defined activated carbons (∼95%). We have also calculated the partition coefficient for TCE adsorption on the aerosol-based FeC composites using the comprehensive definition of Phenrat and co-workers44
8 2 ! ! 39 > > Air Air > C C > > TCE TCE Air Air 4 5> > > ½ðC Þ V ðC Þ V þ V V hs hs water water > > þ þ TCE TCE ref ads > > = < KHTCE KHTCE ref ads ! ! ¼ Kp ¼ > > Mads CAir > > TCE > > > > þ > > TCE > > F K ; : ads H ads
where Cads TCE the concentration of TCE on the adsorbent (mol/L), Cwater TCE is the concentration of TCE in the water phase (mol/L), CAir TCE is the concentration of TCE in the headspace (mol/L), Vhs and Vwater are the volumes of the headspace and water, respectively (L), Mads is the mass of the adsorbent (g), and Fads is the density of the adsorbent (g/L). The subscripts “ref” and “ads” refer to the + system without and with the adsorbent. KTCE is the Henry’s law H constant for TCE partitioning in water, with a value of 0.343 at 25 °C.45 The measured partition coefficient for TCE adsorption on humic acid is 89.5. In comparison, Kp for the adsorption of TCE on aerosol-based FeC composites is 7275, constituting an almost 81fold increase in adsorption capacity. The implication of strong adsorption on the aerosol-based carbon is the ability to establish a driving force for chlorinated compounds to desorb from natural organic matter and partition to the carbon containing zerovalent iron which leads to destruction of the TCE. The inclusion of the carbon microsphere in the system to act as an adsorbent is therefore an extremely important concept in the design of a multifunctional system for TCE decontamination. Considering that numerous organic hydrophobic contaminants are impeded by natural material in the subsurface or soil layers,46 we envision that FeC composites will also play a collection role for hydrophobic pollutants in the in situ remediation technology, leading to highly effective remediation of contaminated sediments.
’ SUMMARY Figure 8 is a schematic summarizing the hypothesis behind the design of these composite particles and their adsorption and dechlorination characteristics. Our earlier experimental work has shown that FeC composites in the size range produced by the aerosol process transport effectively in groundwater saturated sediment10,21,22,26 as predicted by the T-E filtration theory, and for brevity, the aspect is not further detailed here. Additional aspects of colloidal stabilization of these particles with inexpensive polyelectrolytes are also addressed in our earlier papers.10,21 The conceptual basis of the technology involves injecting these materials into contaminated groundwater saturated sediments containing chlorinated hydrocarbons traveling as a plume with the groundwater. As soon as the FeC composite particles enter into the contaminated site, the carbon component acts as an adsorbent for dissolved TCE and concentrates TCE onto the particles. Consequently, reaction with the TCE proceeds through the zerovalent iron and Pd catalysis leading to destruction of the chlorinated compound. We also note that the hydrophobic nature of the carbon allows partitioning of the colloidally stabilized particles into interfacial regions when TCE as a bulk phase (in pools within bedrock) is in contact with water.21,43 The anchoring of the particles at such interfaces allows continuation of reaction. Further studies are in progress to understand the long-term fate and transport of these materials. 13027
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Industrial & Engineering Chemistry Research In summary, the aerosol process is an effective and scalable process to produce carbon microspheres with supported iron species for reductive dechlorination of compounds such as trichloroethylene and other chlorinated hydrocarbons. The technology proposed has significant other implications. Carbon is a ubiquitous support for catalysts, for its inertness, for its ability to act as a strong adsorbent, and for the fact that noble metals can be easily recovered from carbon supports by burning off the carbon.47 The carbothermal process may also be an efficient way to modify internal particle porosities in carbon supported catalysts. The aerosol method to generate metal supported carbons in a facile process may therefore have significant applicability to the design of new catalysts.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: (504) 865-5883. Fax: (504) 865-6744.
’ ACKNOWLEDGMENT We wish to thank Dr. Shaochun Li in Department of Physics and Engineering Physics at Tulane University for his assistance with the XPS analysis. Funding from the Environmental Protection Agency (EPA-GR832374) and the National Science Foundation (Grant 0933734) is gratefully acknowledged. V.J. is deeply grateful to Professor M.S. Ananth who was an inspiration to him during his studies at IIT Madras. Professor Ananth has influenced the life and career of almost four decades of IIT graduates through his exemplary scholarship and leadership. ’ REFERENCES (1) Gatmiri, B.; Hosseini, A. H. Conceptual model and mathematical formulation of NAPL transport in unsaturated porous media. In Geoenvironmental engineering: Integrated management of groundwater and contaminated land; Yong, R. N., Thomas, H. R., Eds.; Thomas Telford: London, 2004; pp 6775. (2) Al-abed, S. R.; Chen, J., Transport of trichloroethylene (TCE) in natural soil by electroosmosis. In Physicochemical Groundwater Remediation; Smith, J. A., Burns, S. E., Eds.; Kluwer academic/Plenum: New York, 2001; pp 91114. (3) Wang, C. B.; Zhang, W. X. Synthesizing Nanoscale Iron Particles for Rapid and Complete Dechlorination of TCE and PCBs. Environ. Sci. Technol. 1997, 31 (7), 2154–2156. (4) Liu, Y.; Majetich, S. A.; Tilton, R. D.; Sholl, D. S.; Lowry, G. V. TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties. Environ. Sci. Technol. 2005, 39 (5), 1338–1345. (5) Zhang, W. X. Nanoscale iron particles for environmental remediation: an overview. J. Nanopart. Res. 2003, 5, 323–332. (6) Liu, Y.; Choi, H.; Dionysiou, D.; Lowry, G. V. Trichloroethene hydrodechlorination in water by highly disordered monometallic nanoiron. Chem. Mater. 2005, 17 (21), 5315–5322. (7) Nyer, E. K.; Vance, D. B. Nano-scale iron for dehalogenation. Ground Water Monit. Remediat. 2001, 2, 41–46. (8) Phenrat, T.; Saleh, N.; Sirk, K.; Tilton, R. D.; Lowry, G. V. Aggregation and sedimentation of aqueous nanoscale zerovalent iron dispersions. Environ. Sci. Technol. 2007, 41 (1), 284–290. (9) He, F.; Zhao, D.; Liu, J.; Roberts, C. B. Stabilization of Fe-Pd nanoparticles with sodium carboxymethyl cellulose for enhanced transport and dechlorination of trichloroethylene in soil and groundwater. Ind. Eng. Chem. Res. 2007, 46 (1), 29–34.
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