Environ. Sci. Technol. 2008, 42, 4494–4499
Reactivity Characteristics of Nanoscale Zerovalent Iron-Silica Composites for Trichloroethylene Remediation TONGHUA ZHENG, JINGJING ZHAN, JIBAO HE, CHRISTOPHER DAY, YUNFENG LU, GARY L. MCPHERSON, GERHARD PIRINGER, AND VIJAY T. JOHN* Department of Chemical and Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118
Received September 18, 2007. Revised manuscript received December 17, 2007. Accepted January 22, 2008.
Spherical silica particles containing nanoscale zerovalent iron were synthesized through an aerosol-assisted process. These particles are effective for groundwater remediation, with the environmentally benign silica particles serving as effective carriers for nanoiron transport. Incorporation of iron into porous sub-micrometer silica particles protects ferromagnetic iron nanoparticles from aggregation and may increase their subsurface mobility. Additionally, the presence of surface silanol groups on silica particles allows control of surface properties via silanol modification using organic functional groups. Aerosolized silica particles with functional alkyl moieties, such as ethyl groups on the surface, clearly adsorb solubilized trichloroethylene (TCE) in water. These materials may therefore act as adsorbents which have coupled reactivity characteristics. The nanoscale iron/silica composite particles with controlled surface properties have the potential to be efficiently applied for in situ source depletion and in the design of permeable reactive barriers.
Introduction Chlorinated hydrocarbons such as trichloroethylene (TCE) form a class of dense non-aqueous-phase liquid (DNAPL) contaminants in groundwater and soil that are difficult to remediate. They have a density higher than water and settle deep into the sediment from which they gradually leach out into aquifers, causing long-term environmental pollution. Remediation of these contaminants is of utmost importance for the cleanup of contaminated sites (1–3). In recent years, the reductive dehalogenation of such compounds using zerovalent iron (ZVI) represents a promising approach for remediation (4–24). The overall redox reaction using TCE as an example is C2HCl3 + 4Fe0 + 5H+ f C2H6 + 4Fe2+ + 3Clwhere gaseous products such as ethane result from complete reduction. The environmentally benign nature of ZVI and its low cost are attractive to the development of such remediation technologies. * Corresponding author phone: 504-865-5883; e-mail: vj@ tulane.edu. 4494
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008
Nanoscale iron particles often have higher remediation rates resulting from their increased surface area (25–33). More importantly, the colloidal nature of nanoiron indicates that these particles can be directly injected into contaminated sites for source depletion or, alternatively, be devised to construct permeable reactive barriers for efficient TCE remediation (34–40). For successful in situ source depletion of pure-phase TCE, it is necessary for injected nanoscale ZVI to migrate through the saturated zone to reach the contaminant. The transport of colloidal particles such as nanoiron through porous media is determined by competitive mechanisms of diffusive transport, interception by soil or sediment grains, and sedimentation effects as shown through the now-classical theories of colloid transport (36, 41–44). The Tufenkji-Elimelech model (45) which considers the effect of hydrodynamic forces and van der Waals interactions between the colloidal particles and soil/sediment grains is a significant improvement over these earlier models and predicts optimal particle sizes between 200 and 1000 nm for zerovalent iron particles at typical groundwater flow conditions (46, 47). At particle sizes exceeding 15 nm, however, ZVI exhibits ferromagnetism, leading to particle aggregation and a loss in mobility (48). The particles by themselves are therefore inherently ineffective for in situ source depletion. One of the common methods to increase nanoiron mobility is to stabilize the particles by adsorption of organic molecules on particle surface (49–53). The adsorbed molecules enhance steric or electrostatic repulsions between particles to prevent their aggregation. Functionalization of ZVI nanoparticles with organic ligands is another alternative, but such functionalization is not easy and it is unclear if the reactivity of ZVI is retained. We describe here a method to entrap ZVI nanoparticles in spherical porous silica sub-micrometer particles using an aerosol-assisted process. Our previous work has shown that an aerosol-assisted reactor is a facile approach to prepare mesoporous silica particles in the sub-micrometer (typically 100–800 nm) size range, containing a variety of materials incorporated into the silica matrix (54–56). We apply the aerosol-assisted process here to prepare functional nanocomposites of zerovalent iron in porous silica. The postulated advantages behind the work are the following: (1) entrapment of ZVI into porous silica may make the ZVI less prone to aggregation, while maintaining reactivity; (2) silica is environmentally benign, and entrapment of ZVI into porous silica reduces the safety concerns of nanoiron hazards of fire and explosion when exposed to air (57); (3) the aerosol-assisted process would be able to efficiently synthesize the porous colloidal silica in the optimal size range for transport, and scale-up to produce large quantities of the material would also be feasible; (4) functionalization of silica is extremely simple, and there are several methods of silica functionalization that could be exploited to allow maximum contact of ZVI with the contaminant (TCE). The fourth point is especially relevant from two perspectives. First, it would be a significant advantage to target the delivery of ZVI so that the particles transport efficiently through the saturated zone and then effectively partition to the water-TCE interface upon encountering regions of bulk TCE. Second, if silica can be functionalized appropriately, the sparingly soluble purephase TCE in water would partition to the silica, increasing local concentrations and accessibility to the ZVI nanoparticles. This paper focuses on the synthesis of silica particles containing zerovalent iron nanoparticles and an understanding of its reactivity with dissolved TCE. In particular, we describe the properties of silica functionalized with alkyl 10.1021/es702214x CCC: $40.75
2008 American Chemical Society
Published on Web 05/16/2008
FIGURE 1. (a) Structure of silica precursors used in the aerosolassisted process, (b) schematic of the aerosol reactor for particle synthesis, and (c) schematic of reaction in an aerosol droplet. (ethyl) groups. The hypothesis is that the hydrocarbon moieties serve as adsorption sites for TCE, allowing access to ZVI entrapped in the particles. Reaction characteristics of this system are compared with characteristics of nonfunctionalized silica containing ZVI nanoparticles.
Experimental Procedures Materials. All chemicals used for synthesis were purchased from Sigma-Aldrich and were used as received: ferric chloride hexahydrate (FeCl3 · 6H2O), tetraethyl orthosilicate (TEOS), ethyl triethoxysilane (ETES), sodium borohydride (NaBH4), and palladium acetate (Pd(OAc)2). Structures of the silica precursors are shown in Figure 1a. Silicas made with ETES and TEOS have the ethyl functionalities throughout the material, while silicas made with TEOS are surface-terminated solely with hydroxyl groups. Sample Preparation. In a typical synthesis, 4.0 g of FeCl3 · 6H2O was first dissolved in 15 mL of water. To this solution, 4.2 g of TEOS and 1.8 mL of 0.1 M HCl were added for the synthesis of nonfunctionalized silicas. The resulting solution was aged for 0.5 h under stirring. The aerosol process was carried out in an apparatus depicted in Figure 1b. The precursor was first atomized to form aerosol droplets, which were then sent through a drying zone and heating zone where preliminary solvent evaporation and silica condensation occur. The temperature of the heating zone was held at 400 °C. The resulting particles were collected by a filter maintained at 100 °C. Particles with ethyl groups were synthesized by a similar procedure but with 2.6 g of TEOS and 1.6 g of ETES instead of 4.2 g of TEOS in the precursor solution. Figure 1c is a representation of the solidification reaction in an aerosol droplet containing the entrapped iron species with solvent evaporation and silica hydrolysis/condensation. Ferric chloride in the as-synthesized particles was reduced to ZVI through either liquid-phase NaBH4 reduction or gasphase H2 reduction. For NaBH4 reduction, 0.4 g of particles was dispersed into 10 mL of water followed by dropwise addition of a 2 mL NaBH4 water solution (0.031 g/mL). The particles were centrifuged and washed by water several times before use. For H2 reduction, 0.4 g of particles was thermally reduced at 400 °C for 48 h under the flow of H2/N2 (9% H2).
FIGURE 2. TEM images of (a) Fe(B)/silica reduced by NaBH4, (b) Fe(B)/ethyl-silica reduced by NaBH4, (c) Fe(H)/silica reduced by H2, and (d) electron diffraction pattern of Fe(H)/silica illustrating polycrystallinity in the sample. To deposit palladium, the reduced particles were dispersed in a dilute 10 mL Pd(OAc)2 ethanol solution (0.0025 g/mL), centrifuged, and washed by water several times before use. The palladium deposition procedure followed is similar to that used by Wang and Zhang (25). Characterization. Particle size, morphology, and crystal structure were characterized using transmission electron microscopy (TEM, JEOL 2011, operated at 120 kV voltage), and X-ray diffraction (XRD, Siemens, D 500, using Cu KR radiation at 1.54 Å.). The porosity of the particles was measured by the nitrogen sorption technique at 77 K (Micromeritics, ASAP 2010). The samples were degassed at 200 °C prior to the measurement. Specific surface areas were determined using the Brunauer– Emmett–Teller (BET) equation. Analytical. After reduction, 0.4 g of the particles was dispersed in 10 mL of water and placed in a 40 mL reaction vial capped with a Mininert valve. To this vial, 10 mL of 40 ppm TCE stock solution was added to reach an overall TCE concentration of 20 ppm. The reaction of particles with TCE was monitored through headspace analysis with gas chromatrography (GC) using a HP 6890 gas chromatograph equipped with a J&W Scientific capillary column (30m × 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 to ensure adequate peak separation between TCE, chlorinated, and nonchlorinated reaction products.
Results and Discussion Particle Characterization. We adopt the nomenclature Fe(B)/silica to depict ZVI nanoparticles obtained by reduction with NaBH4 entrapped in porous silica particles with TEOS as the precursor and Fe(B)/ethyl-silica to depict ZVI nanoparticles obtained by reduction with NaBH4 entrapped in porous silica based particles using ETES and TEOS as the precursor. Fe(H)/silica represents ZVI in silica particles with TEOS as the precursor reduced at high temperatures with H2. Figure 2 shows representative TEM images of silica particles with ZVI. As seen in Figure 2a, the Fe(B)/silica particles are spherical with sizes in the sub-micrometer range, VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4495
FIGURE 3. XRD for (a) Fe(B)/silica reduced by NaBH4, (b) Fe(B)/ ethyl-silica reduced by NaBH4, and (c) Fe(H)/silica reduced by H2 indicating a polycrystalline sample.
TABLE 1. BET Surface Area, Pore Volume, and Calculated Density for (a) Fe(B)/Silica Reduced by NaBH4, (b) Fe(B)/Ethyl-Silica Reduced by NaBH4, and (c) Fe(H)/Silica Reduced by H2
(m2/g)
BET surface area pore volume (cm3/g) porosity (%) particle effective density (g/cm3)
a
b
c
55.1 0.0011 0.56 5.05
229.6 0.05 20.15 4.03
36.2 0.001 0.51 5.05
characteristic of particles synthesized by the aerosol-assisted method. The presence of nanoiron with higher electron contrast inside the silica matrix indicates an even distribution of nanoiron throughout the silica without aggregation. On the basis of the Fe/Si ratio in the precursor solution, the iron comprises 40% by weight of the particle. Fe(B)/ethyl-silica particles with hydrophobic ethyl groups reduced by NaBH4 are shown in Figure 2b, and particles are more porous compared with Figure 2a. The Fe(H)/silica particles are shown in Figure 2c, and thermal treatment in a reducing atmosphere of hydrogen leads to particle sintering as illustrated by the clearly defined larger iron nanoparticles. The electron diffraction pattern (Figure 2d) shows that the nanoiron is polycrystalline, which is further confirmed by the presence of the Fe (100) peak at 45° in the X-ray diffraction data of Figure 3c. In comparison, samples reduced by sodium borohydride (Fe(B)/silica and Fe(B)/ethyl-silica) indicate poor crystallinity, representative of extremely small nanoparticles (Figure 3a,b). We have carried out X-ray photoelectron spectroscopy (XPS) studies following the procedures of Sun and co-workers (58) and have verified the presence of zerovalent iron in the sample through the shoulder at a binding energy of 707 eV. The binding energy peak patterns are identical to those reported by Sun and co-workers (58) and are therefore not presented here. Surface area and pore volumes of the three materials are listed in Table 1. The Fe(B)/ethyl-silica particles show a higher BET surface area and pore volume as a result of ETES addition in the precursor, where the ethyl groups serve as a porogen during particle formation (59). With the exception of the Fe(B)/ethyl-silica, the other particles have low porosities. The high surface area and porosity of Fe(B)/ ethyl-silica will likely ensure entry of NaBH4 solution to reduce the iron within the porous silica particles. The particle densities in Table 1 were calculated by weighting the relative densities of Fe and silica with the observed porosities. Reactivity Studies. Reaction characteristics of systems containing ZVI particles reduced by NaBH4 are shown in Figures 4-7. All ZVI-containing systems are eventually able to remove TCE from solution. H2 reduced ZVI (Fe(H)/silica) exhibits a lower destruction rate than the NaBH4 systems, but we do not consider this system further, since the hightemperature reduction process is destructive to silicas that are functionalized with organic moieties. 4496
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008
FIGURE 4. Reaction kinetics for Fe(B)/ethyl-silica (solid circles), Fe(B)/silica (open circles), and Pd/Fe(B)/ethyl-silica (solid triangles) over 8 h. M/M0 is the fraction of TCE remaining in solution. The remarkable aspect of reaction in the composite particles is the characteristics of the Fe(B)/ethyl-silica system which shows an immediate sharp reduction of the TCE peak to ∼45% of its original value followed by a much slower reaction rate (Figure 4, solid circle). We have repeated the experiment several times with independently prepared samples of the material. In all cases the same qualitative results are observed; allowing for the inherent variability of the aerosolization process, we observe an almost instantaneous reduction of 40–80% of the TCE peak area followed by a slow decrease. We explain the apparent enhancement of TCE remediation by Fe(B)/ethyl-silica particles as a consequence of TCE partitioning to the hydrophobic ethyl groups of the functionalized silica. To prove this concept, we have prepared ethyl silicas but without the iron component. When these particles are exposed to TCE-containing solutions, there is an immediate and sharp reduction of solution concentration (again between 30 and 80% due to inherent variability in the aerosol process), due to the adsorption of TCE onto the alkyl groups. The Fe(B)/ethyl-silica sample exhibits an average adsorption of 0.56 mg of TCE/(g of silica) (or 56% of the total TCE) at the TCE concentration used in this study. This is an important aspect of the design of these materials for TCE decontamination. The inclusion of alkyl groups in the silicas clearly acts as an adsorbent. It is also noteworthy that the TCE dechlorination rate for the Fe(B)/ ethyl-silica particles is only slightly higher than that for Fe(B)/ silica particles after the initial sharp drop in concentration due to adsorption. We do not have a clear explanation for this observation. The role of diffusional restrictions, the reestablishment of adsorption equilibrium after initial reaction from high concentration adsorbed regions, are all factors that determine the overall observed rate. The overriding observation, however, is that there is significant initial adsorption but that reaction proceeds to eventual destruction of the TCE. Figure 4 also illustrates a significant increase of the reaction rate upon deposition of palladium to the ZVI system for Pd-Fe(B)/ethyl-silica particles. The catalytic effect of Pd in dramatically enhancing reaction rates has been discussed in detail in the literature (25, 60, 61). The role of Pd is to dissociatively chemisorb hydrogen produced by redox reactions on Fe0 (60). Additionally chlorinated hydrocarbons adsorb strongly on Pd, leading to the reduction of the chlorinated species through surface reaction with the adsorbed hydrogen (60, 61). We have carried out the reaction simply to demonstrate that the composite particles prepared through the aerosol-assisted process also exhibit the enhanced reaction rates described in the literature. With such fast kinetics, it is difficult to decouple the adsorption step from the reaction step as is evident in the reaction with Fe(B)/ ethyl-silica.
FIGURE 6. TCE removal from solution and gas product evolution rates for Fe(B)/silica. M/M0 is the fraction of the original TCE remaining and P/Pf is the ratio of the gas product peak to the gas product peak at the end of 96 h.
FIGURE 7. TCE removal from solution and gas product evolution rates for Fe(B)/ethyl-silica. M/M0 is the fraction of the original TCE remaining, and P/Pf is the ratio of the gas product peak to the gas product peak at the end of 96 h.
FIGURE 5. Representative GC trace of headspace analyses showing TCE degradation and reaction product evolution: (a) at 0 h, (b) after 8 h reaction with Fe(B)/ethyl-silica particles, and (c) after 1 h reaction with Pd/Fe(B)/ethyl-silica particles. The chlorinated products include vinyl chloride (VC), cis-dichloroethylene (cis-DCE) and trans-dichloroethylene (trans-DCE). Figure 5 illustrates representative GC traces of the TCE and reaction products. Small traces of chlorinated products are observed even after 8 h of reaction in the nonpalladized iron-silica system. It is noteworthy that palladized particles produce more saturated ethane instead of ethene and only a trace amount of toxic chlorinated products were observed in the final reaction products. Figures 6 and 7 illustrate the kinetics of reaction for extended periods, showing the eventual significant removal of TCE in both systems of Fe(B)/ silica (Figure 6) and Fe(B)/ethyl-silica (Figure 7). It is interesting to note an almost linear removal rate for the Fe(B)/ silica, indicating that the effective reaction rate is almost zero order with respect to TCE up to very low concentrations of the contaminant. Further evidence that the initial drop in TCE concentration is due to adsorption is seen by examining the product generation data of Figure 7. The fact that the
very rapid drop in TCE solution concentration is not reflected in a sharp rise in product generation indicates that it is adsorption that is responsible for the initial sharp drop in concentration. The adsorption of TCE on the alkyl groups of ethylfunctionalized silica is an important concept that leads to a particular advantage in using these composite particles for TCE remediation. In the aqueous phase, the surface alkyl groups would stick close to the surface of the silicas and are effective in adsorbing solubilized TCE, thereby increasing local concentrations of TCE in the vicinity of the iron nanoparticles. When in contact with a bulk TCE phase, the alkyl groups would extend, increasing the effective volume of the particles and decreasing the effective density, thereby allowing a degree of stability in the organic phase. The covalent attachment of alkyl groups on a carrier (silica) is certain to maintain the viability of these particles during subsurface transport. In summary, nanoscale zerovalent iron particles have been incorporated into silica particles using an aerosol-assisted process and subsequent reduction. While the sub-micrometer size particles make possible their effective transport through soil, a uniform and a high iron percentage in the particles ensures efficient TCE remediation without nanoiron aggregation. More importantly, the silanol groups on silica support allow us to control surface properties to increase the affinity between particles and TCE. The modification of silica particles by ethyl groups has been demonstrated to be a viable approach to increase TCE adsorption. Synthesis of particles with longer hydrocarbon groups and with varying compositions of silica precursors would allow a finer level of tuning surface characteristics to optimize reactivity, adsorption, partitioning, and transport through sediments. VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4497
These systems with controlled surface properties are therefore of interest in developing in situ remediation technologies. In continuing work, we are exploring the use of surfactants to further template silica mesostructures for enhanced surface areas and accessibility to entrapped iron.
Acknowledgments We gratefully acknowledge funding from the Environmental Protection Agency (Grant EPA-GR832374) and NASA (Grant NAG-1-02070). Dr. Ulrike Diebold is acknowledged for assistance with XPS characterization.
Literature Cited (1) Dowideit, P.; von Sonntag, C. Reaction of ozone with ethene and its methyl- and chlorine-substituted derivatives in aqueous solution. Environ. Sci. Technol. 1998, 32, 1112–1119. (2) Cowell, M.; Kibbey, T.; Zimmerman, J.; Hayes, K. Partitioning of ethoxylated nonionic surfactants in water/NAPL systems: Effects of surfactant and NAPL properties. Environ. Sci. Technol. 2000, 34, 1583–1588. (3) Nutt, M.; Hughes, J.; Wong, M. Designing Pd-on-Au bimetallic nanoparticle catalysts for trichloroethene hydrodechlorination. Environ. Sci. Technol. 2005, 39, 1346–1353. (4) Matheson, L.; Tratnyek, P. Reductive dehalogenation of chlorinated methanes by iron metal. Environ. Sci. Technol. 1994, 28, 2045–2053. (5) Orth, W.; Gillham, R. Dechlorination of trichloroethene in aqueous solution using Fe0. Environ. Sci. Technol. 1996, 30, 66–71. (6) Fennelly, J.; Roberts, A. Reaction of 1,1,1-trichloroethane with zero-valent metals and bimetallic reductants. Environ. Sci. Technol. 1998, 32, 1980–1988. (7) Huang, C.; Wang, H.; Chiu, P. Nitrate reduction by metallic iron. Water Res. 1998, 32, 2257–2264. (8) Devlin, J.; Klausen, J.; Schwarzenbach, R. Kinetics of nitroaromatic reduction on granular iron in recirculating batch experiments. Environ. Sci. Technol. 1998, 32, 1941–1947. (9) Gotpagar, J.; Lyuksyutov, S.; Cohn, R.; Grulke, E.; Bhattacharyya, D. Reductive dehalogenation of trichloroethylene with zerovalent iron: surface profiling microscopy and rate enhancement studies. Langmuir 1999, 15, 8412–8420. (10) Li, T.; Farrell, J. Reductive dechlorination of trichloroethene and carbon tetrachloride using iron and palladized-iron cathodes. Environ. Sci. Technol. 2000, 34, 173–179. (11) Uludag-Demirer, S.; Bowers, A. Adsorption/reduction reactions of trichloroethylene by elemental iron in the gas phase: the role of water. Environ. Sci. Technol. 2000, 34, 4407–4412. (12) Butler, E.; Hayes, K. Factors influencing rates and products in the transformation of trichloroethylene by iron sulfide and iron metal. Environ. Sci. Technol. 2001, 35, 3884–3891. (13) Hu, H.; Goto, N.; Fujie, K. Effect of pH on the reduction of nitrite in water by metallic iron. Water Res. 2001, 35, 2789–2793. (14) Doong, R.; Chen, K.; Tsai, H. Reductive dechlorination of carbon tetrachloride and tetrachloroethylene by zerovalent silicon-iron reductants. Environ. Sci. Technol. 2003, 37, 2575–2581. (15) Su, C.; Puls, R. In situ remediation of arsenic in simulated groundwater using zerovalent iron: Laboratory column tests on combined effects of phosphate and silicate. Environ. Sci. Technol. 2003, 37, 2582–2587. (16) Su, C.; Puls, R. Nitrate reduction by zerovalent iron: Effects of formate, oxalate, citrate, chloride, sulfate, borate, and phosphate. Environ. Sci. Technol. 2004, 38, 2715–2720. (17) Támara, M.; Butler, E. Effects of iron purity and groundwater characteristics on rates and products in the degradation of carbon tetrachloride by iron metal. Environ. Sci. Technol. 2004, 38, 1866–1876. (18) Miehr, R.; Tratnyek, P.; Bandstra, J.; Scherer, M.; Alowitz, M.; Bylaska, E. Diversity of contaminant reduction reactions by zerovalent iron: role of the reductate. Environ. Sci. Technol. 2004, 38, 139–147. (19) Dries, J.; Bastiaens, L.; Springael, D.; Agathos, S.; Diels, L. Competition for sorption and degradation of chlorinated ethenes in batch zero-valent iron systems. Environ. Sci. Technol. 2004, 38, 2879–2884. (20) Li, T.; Farrell, J. Electrochemical investigation of the rate-limiting mechanisms for trichloroethylene and carbon tetrachloride reduction at iron surfaces. Environ. Sci. Technol. 2001, 35, 3560– 3565. 4498
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 42, NO. 12, 2008
(21) Kenneke, J.; McCutcheon, S. Use of pretreatment zones and zero-valent iron for the remediation of chloroalkenes in an oxic aquifer. Environ. Sci. Technol. 2003, 37, 2829–2835. (22) Satapanajaru, T.; Shea, P.; Comfort, S.; Roh, Y. Green rust and iron oxide formation influences metolachlor dechlorination during zerovalent iron treatment. Environ. Sci. Technol. 2003, 37, 5219–5227. (23) Arnold, W.; Roberts, A. Pathways and kinetics of chlorinated ethylene and chlorinated acetylene reaction with Fe(0) particles. Environ. Sci. Technol. 2000, 34, 1794–1805. (24) Jeen, S.; Gillham, R.; Blowes, D. Effects of carbonate precipitates on long-term performance of granular iron for reductive dechlorination of TCE. Environ. Sci. Technol. 2006, 40, 6432– 6437. (25) Wang, C.; Zhang, W. Synthesizing nanoscale iron particles for rapid and complete dechlorination of TCE and PCBs. Environ. Sci. Technol. 1997, 31, 2154–2156. (26) Xu, Y.; Zhang, W. Subcolloidal Fe/Ag particles for reductive dehalogenation of chlorinated benzenes. Ind. Eng. Chem. Res. 2000, 39, 2238–2244. (27) Lowry, G.; Reinhard, M. Pd-Catalyzed TCE dechlorination in groundwater: Solute effects, biological control, and oxidative catalyst regeneration. Environ. Sci. Technol. 2000, 34, 3217– 3223. (28) Schrick, B.; Blough, J.; Jones, A.; Mallouk, T. Hydrodechlorination of trichloroethylene to hydrocarbons using bimetallic nickeliron nanoparticles. Chem. Mater. 2002, 14, 5140–5147. (29) Joo, S.; Feitz, A.; Waite, T. Oxidative degradation of the carbothioate herbicide, molinate, using nanoscale zero-valent iron. Environ. Sci. Technol. 2004, 38, 2242–2247. (30) Tee, Y.; Grulke, E.; Bhattacharyya, D. Role of Ni/Fe nanoparticle composition on the degradation of trichloroethylene from water. Ind. Eng. Chem. Res. 2005, 44, 7062–7070. (31) Dror, I.; Baram, D.; Berkowitz, B. Use of nanosized catalysts for transformation of chloro-organic pollutants. Environ. Sci. Technol. 2005, 39, 1283–1290. (32) Liu, Y.; Majetich, S.; Tilton, R.; Sholl, D.; Lowry, G. TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties. Environ. Sci. Technol. 2005, 39, 1338–1345. (33) Liu, Y.; Choi, H.; Dionysiou, D.; Lowry, G. Trichloroethene hydrodechlorination in water by highly disordered monometallic nanoiron. Chem. Mater. 2005, 17, 5315–5322. (34) Wüst, W.; Köber, R.; Schlicker, O.; Dahmke, A. Combined zeroand first-order kinetic model of the degradation of TCE and cis-DCE with commercial iron. Environ. Sci. Technol. 1999, 33, 4304–4309. (35) Astrup, T.; Stipp, S.; Christensen, T. Immobilization of chromate from coal fly ash leachate using an attenuating barrier containing zero-valent iron. Environ. Sci. Technol. 2000, 34, 4163–4168. (36) Elliott, D.; Zhang, W. Field assessment of nanoscale bimetallic particles for groundwater treatment. Environ. Sci. Technol. 2001, 35, 4922–4926. (37) Yabusaki, S.; Cantrell, K.; Sass, B.; Steefel, C. Multicomponent reactive transport in an in situ zero-valent iron cell. Environ. Sci. Technol. 2001, 35, 1493–1503. (38) Morrison, S. Performance evaluation of a permeable reactive barrier using reaction products as tracers. Environ. Sci. Technol. 2003, 37, 2302–2309. (39) Casey, F.; Ong, S.; Horton, R. Degradation and transformation of trichloroethylene in miscible-displacement experiments through zerovalent metals. Environ. Sci. Technol. 2000, 34, 5023– 5029. (40) Shimotori, T.; Nuxoll, E.; Cussler, E.; Arnold, W. A polymer membrane containing Fe0 as a contaminant barrier. Environ. Sci. Technol. 2004, 38, 2264–2270. (41) Yao, K.; Habibian, M.; O’Melia, C. Water and waste water filtration: Concepts and applications. Environ. Sci. Technol. 1971, 5, 1105–1112. (42) Rajagopalan, R.; Tien, C. Trajectory analysis of deep-bed filtration with the sphere-in-cell porous media model. AIChE J. 1976, 22, 523–533. (43) Spielman, L. Particle capture from low-speed laminar flows. Annu. Rev. Fluid Mech. 1977, 9, 297–319. (44) Ponder, S.; Darab, J.; Mallouk, T. Remediation of Cr(VI) and Pb(II) aqueous solutions using supported, nanoscale zero-valent iron. Environ. Sci. Technol. 2000, 34, 2564–2569. (45) Tufenkji, N.; Elimelech, M. Correlation equation for predicting single-collector efficiency in physicochemical filtration in saturated porous media. Environ. Sci. Technol. 2004, 38, 529– 536.
(46) Hydutsky, B.; Mack, E.; Beckerman, B.; Skluzacek, J.; Mallouk, T. Optimization of nano- and microiron transport through sand columns using polyelectrolyte mixtures. Environ. Sci. Technol. 2007, 41, 6418–6424. (47) He, F.; Zhao, D. Manipulating the size and dispersibility of zerovalent iron nanoparticles by use of carboxymethyl cellulose stabilizers. Environ. Sci. Technol. 2007, 41, 6216–6221. (48) Phenrat, T.; Saleh, N.; Sirk, K.; Tilton, R.; Lowry, G. Aggregation and sedimentation of aqueous nanoscale zerovalent iron dispersions. Environ. Sci. Technol. 2007, 41, 284–290. (49) Sayles, G.; You, G.; Wang, M.; Kupferle, M. DDT, DDD, and DDE dechlorination by zero-valent iron. Environ. Sci. Technol. 1997, 31, 3448–3454. (50) Alessi, D.; Li, Z. Synergistic effect of cationic surfactants on perchloroethylene degradation by zero-valent iron. Environ. Sci. Technol. 2001, 35, 3713–3717. (51) Quinn, J.; Geiger, C.; Clausen, C.; Brooks, K.; Coon, C.; O’Hara, S.; Krug, T.; Major, D.; Yoon, W.; Gavaskar, A.; Holdsworth, T. Field demonstration of DNAPL dehalogenation using emulsified zero-valent iron. Environ. Sci. Technol. 2005, 39, 1309–1318. (52) Saleh, N.; Phenrat, T.; Sirk, K.; Dufour, B.; Ok, J.; Sarbu, T.; Matyjaszewski, K.; Tilton, R.; Lowry, G. Adsorbed triblock copolymers deliver reactive iron nanoparticles to the oil/water interface. Nano Lett. 2005, 5, 2489–2494. (53) He, F.; Zhao, D. Preparation and characterization of a new class of starch-stabilized bimetallic nanoparticles for degradation of chlorinated hydrocarbons in water. Environ. Sci. Technol. 2005, 39, 3314–3320.
(54) Lu, Y.; Fan, H.; Stump, A.; Ward, T.; Rieker, T.; Brinker, C. Aerosolassisted self-assembly of mesostructured spherical nanoparticles. Nature 1999, 398, 223–226. (55) Zheng, T.; Zhan, J.; Pang, J.; Tan, G.; He, J.; McPherson, G.; Lu, Y.; John, V. T. Mesoporous carbon nanocapsules from enzymatically polymerized poly(4-ethylphenol) confined in silica aerosol particles. Adv. Mater. 2006, 18, 2735–2738. (56) Zheng, T.; Pang, J.; Tan, G.; He, J.; McPherson, G.; Lu, Y.; John, V. T.; Zhan, J. Surfactant templating effects on the encapsulation of iron oxide nanoparticles within silica microspheres. Langmuir 2007, 23, 5143–5147. (57) Li, A.; Tai, C.; Zhao, Z.; Wang, Y.; Zhang, Q.; Jiang, G.; Hu, J. Debromination of decabrominated diphenyl ether by resinbound iron nanoparticles. Environ. Sci. Technol. 2007, 41, 6841– 6846. (58) Sun, Y.-P.; Li, X.-Q.; Cao, J.; Zhang, W.-X.; Wang, H. P. Characterization of zero-valent iron nanoparticles. Adv. Colloid Interface Sci. 2006, 120, 47–56. (59) Zhao, W.; Gu, J.; Zhang, L.; Chen, H.; Shi, J. Fabrication of uniform magnetic nanocomposite spheres with a magnetic core/ mesoporous silica shell structure. J. Am. Chem. Soc. 2005, 127, 8916–8917. (60) Muftikian, R.; Fernando, Q.; Korte, N. A method for the rapid dechlorination of low molecular weight chlorinated hydrocarbons in water. Water Res. 1995, 29, 2434–2439. (61) Schreier, C.; Reinhard, M. Catalytic hydrodehalogenation of chlorinated ethylenes using palladium and hydrogen for the treatment of contaminated water. Chemosphere 1995, 31, 3475–3487.
ES702214X
VOL. 42, NO. 12, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
4499
