Transport Characteristics of Nanoscale Functional Zerovalent Iron

Oct 30, 2008 - In this paper we report the transport characteristics of nanoscale zerovalent iron entrapped in porous silica particles and prepared th...
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Environ. Sci. Technol. 2008, 42, 8871–8876

Transport Characteristics of Nanoscale Functional Zerovalent Iron/Silica Composites for in Situ Remediation of Trichloroethylene JINGJING ZHAN,† TONGHUA ZHENG,† GERHARD PIRINGER,‡ CHRISTOPHER DAY,† GARY L. MCPHERSON,§ YUNFENG LU,† KYRIAKOS PAPADOPOULOS,† AND V I J A Y T . J O H N * ,† Departments of Chemical and Biomolecular Engineering, Earth and Environmental Sciences, and Chemistry, Tulane University, New Orleans, Louisiana 70118

Received February 7, 2008. Revised manuscript received October 5, 2008. Accepted October 6, 2008.

Effective in situ remediation of groundwater requires the successful delivery of reactive iron particles through soil. In this paper we report the transport characteristics of nanoscale zerovalent iron entrapped in porous silica particles and prepared through an aerosol-assisted process. The entrapment of iron nanoparticles into the silica matrix prevents their aggregationwhilemaintainingtheparticles’reactivity.Furthermore, the silica particles are functionalized with alkyl groups and are extremely efficient in adsorbing dissolved trichloroethylene (TCE). Because of synthesis through the aerosol route, the particles are of the optimal size range (0.1-1 µm) for mobility through sediments. Column and capillary transport experiments confirm that the particles move far more effectively through model soils than commercially available uncoated nanoscale reactive iron particles. Microcapillary experiments indicate that the particles partition to the interface of TCE droplets, further enhancing their potential for dense non-aqueous-phase liquid source-zone remediation.

Introduction The widespread occurrence of dense non-aqueous-phase liquids (DNAPLs) in groundwater and in soil is of serious environmental concern (1-3). These contaminants transport rapidly in the subsurface to create a contaminant plume as a result of vertical migration due to their density being higher than that of water. The contaminants spread laterally, due to capillary forces, adsorption, medium spatial variability, and dissolution (4). Consequently, they pose a serious risk to drinking water supplies. Trichloroethylene (TCE), a chlorinated DNAPL and suspected carcinogen, has been widely used as a solvent and degreasing agent in many industries (5). Because of intentional disposal and inadvertent leakage, TCE is a commonly detected groundwater contaminant. Various strategies have been explored for TCE remediation, such as soil vapor extraction, pump-and-treat, * Corresponding author e-mail: [email protected]; phone: (504) 8655883; fax: (504) 865-6744. † Department of Chemical and Biomolecular Engineering. ‡ Department of Earth and Environmental Sciences. § Department of Chemistry. 10.1021/es800387p CCC: $40.75

Published on Web 10/31/2008

 2008 American Chemical Society

thermal treatment, bioremediation, and permeable reactive barriers (6-9). Compared to these conventional approaches, the in situ direct injection of reactive zerovalent iron into the contaminated subsurface would be the preferable method because it may more directly access and target the contaminants (10, 11). Effective in situ remediation of groundwater requires the successful delivery of reactive decontamination agents to the subsurface. Prior studies have shown that nanoscale iron particles are a preferred option for reductive dehalogenation of TCE due to their environmentally benign nature, high efficiency, and low cost (12-16). However, bare nanoiron particles have a strong tendency to agglomerate due to their high surface energies and intrinsic magnetic interactions, forming aggregates that plug and inhibit their flow through porous media (11, 17-22). Extensive efforts have been made to reduce aggregation and increase nanoiron mobility. Techniques include the use of polymers, surfactants, starch, modified cellulose, and vegetable oils as stabilizing layers to form more stable dispersions (17-19, 22-29). These methods enhance steric or electrostatic repulsions of particles to prevent their aggregation and may be effective if the physically adsorbed stabilizers are retained during particle migration through sediments. Our focus in this paper is the development of carriers of nanoscale zerovalent iron particles that are able to move readily through the subsurface without aggregation of the iron. Filtration theory predicts that the migration of colloidal particles through porous media such as soil is typically dictated by Brownian diffusion, interception, and gravitational sedimentation (30). The Tufenkji-Elimelech model is perhaps the most comprehensive model to describe these effects in the presence of interparticle interactions (31), with the governing equation η0 ) 2.4AS1⁄3NR-0.081NPe-0.715NvdW0.052 + 0.55ASNR1.675NA0.125 + 0.22NR-0.24NG1.11NvdW0.053 (1) where η0 is the collector efficiency, simply defined as the probability of collision between migrating particles and sediment grains. The first term on the right characterizes the effects of particle diffusion on the collector efficiency, while the second and third terms describe the effects of interception and sedimentation. However, the Tufenkji-Elimelech equation does not provide the complete representation of particle transport, which also involves concepts such as bridging and attachment between the particles and the surfaces of soil grains. For brevity, we limit the discussion of eq 1 to demonstrating the dependence of the collector efficiency on the zerovalent iron particle size as shown in Figure 1a. As seen in the figure, the collector efficiency is minimized at a particle size range of 0.1-1 µm, which implies that this is the optimal size range for colloid particles to migrate through the soil. Figure 1b is an optical micrograph of commercially available ZVI nanoparticles, the reactive nanoiron particles (RNIP-10DS, which is uncoated or bare RNIP) from Toda Kogyo Corp. While the intrinsic particle size of these particles is on the order of 30-70 nm, aggregation to effective sizes over 10 µm makes them ineffective for transport through soil, as earlier reported (21). In related work from this laboratory (32), it has been established that significant amounts of iron can be incorporated into a silica matrix by an aerosol-assisted process. In this process, silica precursors such as tetraethyl orthosilicate (TEOS) and ethyltriethoxysilane (ETES) together with iron precursors are aerosolized, with the aerosol droplets VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. (a) Collector efficiency dependency on the particle size as predicted by the Tufenkji-Elimelech filtration model. The two curves represent the theory for particles with different densities (7.8 × 103 kg/m3 for zerovalent iron and 4 × 103 kg/m3 for composite particles of iron and silica). (b) Optical micrograph of a commercial nanoscale ZVI (RNIP-10DS from Toda Kogyo Corp.) suspension (60 mg/L in deionized water) showing significant aggregation.

FIGURE 2. Schematic of the aerosol-assisted process for the preparation of Fe/ethylsilica composite particles. The reactions occur in a solvent aerosol droplet. passing through a high-temperature zone. During this process, silicates hydrolyze and condense in the droplet, entrapping the iron species. The “chemistry in a droplet” process leads to submicrometer-sized particles of silica containing iron nanoparticles which are then collected on a filter. Since the particles are essentially made with silica and iron, they are environmentally benign. Of particular relevance also is the use of alkyl groups attached to the silica through the use of alkylsilane precursors such as ETES. These groups introduce porosity into the silica. Additionally, these organic groups play an important role in that they serve as adsorbents for the TCE, thus bringing the organic contaminant to the vicinity of the iron species and facilitating reaction. In our earlier paper we describe reactivity characteristics of these composite particles and indicate that the particles are effective in TCE remediation (32). In this follow-up work, we report the characteristics of composite Fe/ethylsilica particles regarding transport through model sediments.

Materials and Methods Chemicals. All chemicals including iron(III) chloride hexahydrate (FeCl3 · 6H2O) (97%), TEOS, ETES, sodium borohydride (NaBH4), and TCE (99%) were purchased from Sigma-Aldrich and used as received. Deionized (DI) water generated with 8872

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FIGURE 3. (a) Transmission electron micrograph of Fe/ ethylsilica composite particles. (b) Particle size distribution of Fe/ethylsilica based on TEM analysis. (c) Optical micrograph of an Fe/ethylsilica suspension (60 mg/L in DI water) showing insignificant aggregation of particles. a Barnstead E-pure purifier (IA) to a resistance of approximately 17 MΩ was used in all experiments. Sample Preparation. The aerosol-assisted technique was employed to prepare Fe/ethylsilica particles, and the experimental setup is described in an earlier paper (32). In brief, 4.0 g of FeCl3 · 6H2O was first dissolved in 15 mL of water followed by the addition of 2.5 g of TEOS and 1.5 g of ETES. The resulting solution was aged for 0.5 h under stirring. The precursor solution was atomized to form aerosol droplets, which were then passed through a drying zone and heating zone where preliminary solvent evaporation and silica condensation occurred. The temperature of the heating zone was held at 350 °C. The resulting particles were collected by a filter paper maintained at 100 °C. Reduction of the synthesized particles was achieved by dropwise addition of an excess amount of sodium borohydride water solution.

FIGURE 4. A schematic of the Fe/ethylsilica particles showing potential applicability to adsorption of dissolved TCE and characteristics when contacted with a bulk TCE phase. After cessation of visible hydrogen evolution, the particles were centrifuged and washed three times with water. Commercially available nanoscale ZVI, termed RNIP (RNIP-10DS, lot no. 070201) was supplied by Toda Kogyo Corp. (Onoda, Japan). The material was stored as a ∼300 g/L aqueous slurry at pH ≈ 12. Prior to use, the RNIP-10DS suspension was ultrasonicated for 5 min to break any aggregates that might exist. Experimental and Analytical Methods. Macroscopic methods (column breakthrough tests) were combined with microscopic methods (capillary and microcapillary experiments) to study particle mobility and partitioning, and are described together with the results. Transmission electron microscopy (TEM; JEOL 2010, operated at 120 kV voltage) and optical microscopy (Olympus IX71, Japan) were used to characterize the size and morphology of the particles.

Results and Discussion Particle Size Characteristics. The aerosol-assisted technology is a simple and efficient method to obtain colloidal spherical particles generated in a continuous, ∼6 s process (33, 34). The process is shown schematically in Figure 2. As the precursor-containing aerosol droplet passes through the heated zone of the furnace, hydrolysis and condensation of silicates leads to the formation of spherical silica particles containing FeCl3. Treatment with NaBH4 then leads to ZVI nanoparticles within the silica matrix. The use of ETES in the procedure leads to alkyl-functionalized silicas. Figure 3a is a transmission electron micrograph of the Fe/ethylsilica particles, and the fairly homogeneous distribution of Fe with a higher electron density is represented by the darker spots throughout the particles. BET surface areas of these materials are on the order of 250 m2/g, representing a porous material (32) where ingress of TCE to the silica particle interior and access to immobilized ZVI nanoparticles in the interior should be feasible. Figure 3b represents a histogram of particle size obtained by analysis of multiple TEM micrographs of the composite particles from at least three independent preparations of the Fe/ethylsilica composites. While there is a large distribution of particle sizes inherent in the aerosol process where there is little control of the droplet size, we clearly see that the particle size range is on the order of 0.1-1 µm, which is the optimal particle size for transport through soil as predicted by the Tufenkji-Elimelech correlation (31). The calculated mean and standard deviation are 358 and 249 nm, respectively. Finally, Figure 3c is an optical micrograph of the Fe/ethylsilica composite particles, where it is clearly observed that the particles do not aggregate into larger entities. Transport Characteristics. Figure 4 is an idealized schematic illustrating the hypothesis behind the design of these composite particles and their transport and partitioning characteristics. In transport through an aqueous phase, the alkyl groups of the silica do not extend out into the aqueous phase but stay confined to the silica surface. At the same time, they serve as adsorbents for dissolved TCE and concentrate TCE onto the particles serving as sponges for dissolved TCE (32). Additionally, there is reaction when the

TCE comes into contact with the entrapped ZVI nanoparticles in the silica matrix (32). When in contact with a bulk phase of TCE, it is envisioned that the alkyl groups can extend out into the solvent, thereby increasing the hydrodynamic diameter and decreasing the effective density of the colloidal particle. Our hypothesis is therefore that the extension of alkyl groups into the solvent might help stabilize the particles in the organic phase. The mobility of these composite particles through sediments was tested using column and capillary transport experiments. In each case, we compared the characteristics of the Fe/ethylsilica composite particles with those of commercial ZVI nanoparticles (the RNIP-10DS particles supplied by Toda Kogyo Corp.). The column tests were based on a 50 mL glass buret packed with standard Ottawa sand (EMD, CAS 14808-60-7, Fisher) after particles smaller than 300 µm were sieved out. The sand was packed to a volume of 10 mL, and the measured porosity of the packing was 0.32 as measured by comparing the weight of the dry column with that of the water-saturated column (11). A small glass wool plug at the each column bottom prevented the loss of the sand. The column was saturated with water prior to addition of the iron-containing suspension. After a 10 mL aliquot of the bulk suspension (either RNIP-10DS or Fe/ ethylsilica) was fed, each column was flushed with 60 mL of DI water at a flow rate of 18 mL/min (superficial velocity of 20 cm/min). The concentration of iron-containing particles in the bulk suspension was 3 g/L, considered a threshold for economical application in the field (20). Continuous water flow was provided by a large water-filled separatory funnel connected to the top of the buret. Figure 5 illustrates the setup and the elution results, with panels 1-3 (Figure 5a) indicating results for the commercial RNIP-10DS and panels 1′-3′ (Figure 5b) indicating results for the Fe/ethylsilica composite. Panels 1 and 1′ depict the overall buret and collection flask, panels 2 and 2′ depict conditions at the top of the sand column, and panels 3 and 3′ depict conditions at the bottom of the sand column. The results indicate that most of the RNIP-10DS was trapped within the first few centimeters of the column, and visible penetration does not exceed the middle of the column. In marked contrast to this poor transportability, Fe/ethylsilica particles reached the column bottom and eluted efficiently with collection of the particles in the conical flask as shown in panel 1′. Retention of RNIP-10DS at the top of the column after 60 mL of water flushing is observed in panel 2, while Fe/ethylsilica particles travel through the column and are trapped on the glass wool at the bottom of the sand column (panel 3′) in addition to being collected in the collection flask. Breakthrough curves (Figure 5c) were obtained by monitoring the turbidity of elutes with a nephelometric turbidimeter (DRT100B, HF Scientific, Inc., Fort Myers, FL). The curves indicate that almost 70% of the Fe/ethylsilica particles were eluted from the Ottawa sand column under the specified conditions. In contrast, no significant elution of RNIP-10DS particles (bare RNIP) was observed. Following the procedure of Saleh and co-workers (20), we can calculate a “sticking coefficient” R ranging from VOL. 42, NO. 23, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. (a) Elution characteristics of RNIP-10DS particles in vertical columns with flow rate 18 mL/min (linear velocity 20 cm/min). Panels 1-3 illustrate accumulation of RNIP-10DS at the top of the column and negligible collection at the bottom of the column. (b) Elution characteristics of Fe/ethylsilica particles with flow rate 18 mL/min. Clear transport and collection in the collection flask are seen in panel 1′. Panel 2′ shows depletion of the particles at the top of the packing, and panel 3′ shows some collection of the particles on the glass wool at the bottom of the column. (c) Elution profiles of RNIP-10DS and Fe/ ethylsilica. M/M0 represents the fraction of particles that are eluted. 0.17 (for 100 nm particles) to 0.38 (for 300 nm particles) on the basis of a 10 cm column length. 8874

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FIGURE 6. (a) Experimental setup to study transport in horizontal capillaries (flow rate 0.1 mL/min (liner velocity 5 cm/ min), sand length 3 cm, particle suspension concentration 3 g/L, injected suspension volume 0.03 mL). (b) (i) RNIP-10DS particles retained in the capillary after flushing with water for 1 h. (ii) The initial suspension plug is largely intact in the capillary containing Fe/ethylsilica after 5 min of flushing. (iii) The initial particle suspension plug has dispersed into the sand packing the capillary containing Fe/ethylsilica after 1 h of flushing. The particles have moved through the capillary. (c) Optical micrographs of the capillaries after 1 h of flushing showing sediment grains and the particles. (i) RNIP-10DS is retained primarily at the capillary entrance. (ii) Fe/ethylsilica remnant clusters are dispersed throughout the capillary. To gain further insights into the mobility of these particulate systems, we conducted experiments on transport through capillaries, following the particles through optical microscopy. In the experiment, glass melting point tubes with both ends open (1.5-1.8 mm i.d. × 100 mm length, Corning, NY) were used as capillaries. In all cases, the capillary tubes were placed horizontally, after a 3 cm length was packed with wet Ottawa sand. A continuous water flow at a flow rate of 0.1 mL/min (superficial velocity of 5 cm/min) was provided by a syringe pump, and the exit point of the capillary was capped with a small glass wool plug (Figure 6a). After 30 µL of the particle suspension (3 g/L) was injected into the inlet of the capillary, water flushing was initiated and an inverted optical microscope was used to observe the pore-scale transport of the iron-containing particles. Figure 6b illustrates photographs of the capillaries with panel i depicting the capillary containing RNIP-10DS particles after 1 h of water flushing, panel ii depicting the capillary containing Fe/ ethylsilica after 5 min of water flushing, and panel iii depicting the same capillary after 1 h of water flushing. The macroscopic images clearly indicate RNIP-10DS accumulation at the inlet,

FIGURE 7. (a) Micrograph of the microcapillary containing Fe/ ethylsilica particles. The micropipet injector can be seen on the left. (b) Micrograph of the microcapillary after injection of a TCE droplet. Accumulation of Fe/ethylsilica particles at the interface is observed. while the Fe/ethylsilica particles are distributed more uniformly throughout the packed capillaries even after 5 min, and in 1 h many of the Fe/ethylsilica particles have cleared the capillary. Figure 6c illustrates the optical microscope images of the capillary with panel i again depicting the RNIP10DS transport and panel ii depicting transport with Fe/ ethylsilica. Agglomerates and large clusters of RNIP-10DS are observed at the capillary inlet (panel i), while Fe/ethylsilica particles are more uniformly located in small clusters throughout the capillary and appear to have adsorbed to the sand surfaces. No effort was made to deaerate the samples, and the brown-yellow color seen in some of the images is attributed to surface oxidation of ZVI to iron oxides. We do note, however, that both the column and capillary experiments, while indicating potential of the Fe/ethylsilica particles, do not necessarily indicate applicability to field conditions. A variety of flow rates and soil conditions have to be considered before field application becomes a realistic possibility. Partitioning Characteristics. The partitioning characteristics of the particles were investigated by a microcapillary microscopy technique (35). The microcapillary is fabricated by drawing a heated capillary tube to an internal diameter of approximately 200 µm. The microcapillary instrumentation allows injection into these microcapillaries using a microinjection technique (35). A microcapillary was filled with the Fe/ethylsilica suspension and inserted into a capillary holder mounted horizontally on an inverted microscope stage. Figure 7a illustrates the microcapillary containing the Fe/ ethylsilica suspension with a micropipet tip inserted from the left. A TCE droplet was injected into the microcapillary using the micropipet controlled by a microinjector (model IM-200, Narishige, Japan). Figure 7b illustrates the TCE droplet which quickly becomes covered with the particles. Over a long period of time (>48 h), the droplet slowly

decreases in size and eventually disappears. We have not attempted to characterize the deterioration of the drop as it is difficult to decouple ZVI-induced intrinsic TCE reaction from TCE dissolution into the bulk aqueous solution and slow evaporation from the ends of the microcapillary, which is difficult to control. It is the initial partitioning of the particles to the TCE drop that is the focus of the experiment. In visual observations with a two-phase water-TCE system in a vial, we observe that the particles partition primarily to the water-TCE interface. The interfacial partitioning of the particles may be due to the fact that they exhibit some amphiphilicity due to surface-terminated silanol hydroxyl groups imparting hydrophilicity. Interfacial partitioning may be an advantage as it indicates that the particles can be transported along the water-TCE interface during groundwater flow. In principle, the particles have a solid density (4 g/mL) (32) greater than that of TCE (1.46 g/mL), but the submicrometer colloidal dimensions together with the surface functionalities allow positioning at the water-TCE interface and in the bulk of the TCE. The combined access to water and TCE at the interface may also facilitate the reductive dehalogenation of TCE. In summary, this work demonstrates that reactive Fe/ ethylsilica particles move much more easily through model soils (Ottawa sands) than commercially available RNIP-10DS. The following are beneficial characteristics of these composites: (1) The particles are reactive (32). (2) There is no aggregation of zerovalent iron since the nanoparticles are entrapped in porous silicas. (3) The particle size is in the optimal range for mobility through soils and sediments. (4) The alkyl functionalities on the silica allow adsorption of TCE, thus removing dissolved TCE and perhaps facilitating reaction by increasing TCE concentrations in the vicinity of the iron (32). (5) The particles partition significantly to the interface of bulk water-TCE, further facilitating mobility and access to bulk TCE. (6) The composite particles are environmentally benign, and the alkyl groups are amenable to microbial degradation. Finally, the aerosol process is conducive to scaleup as it is a virtually continuous process limited only by the batch requirements of particle collection on a filter. There are inherent limitations of the process, most particularly diffusional restrictions to reaction in the interior of the composite particle. Our continuing work seeks to further enhance reactivity characteristics of the composites through surfactant templating of the pore structure. Larger scale demonstrations as further aspects of continuing work will be aimed at bringing the technology to practice.

Acknowledgments Funding from the Environmental Protection Agency (Grant EPA-GR832374) is gratefully acknowledged. We are also grateful to Toda Kogyo Corp. for providing a sample of RNIP10DS.

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