Grantee Research Project Results
2005 Progress Report: Developing Functional Fe(0)-based Nanoparticles for In Situ Degradation of DNAPL Chlorinated Organic Solvents
EPA Grant Number: R830898Title: Developing Functional Fe(0)-based Nanoparticles for In Situ Degradation of DNAPL Chlorinated Organic Solvents
Investigators: Lowry, Gregory V. , Matyjaszewski, Krzysztof , Majetich, Sara A. , Tilton, Robert D.
Institution: Carnegie Mellon University
EPA Project Officer: Aja, Hayley
Project Period: May 1, 2003 through October 31, 2007
Project Period Covered by this Report: May 1, 2005 through October 31, 2006
Project Amount: $358,000
RFA: Environmental Futures Research in Nanoscale Science Engineering and Technology (2002) RFA Text | Recipients Lists
Research Category: Nanotechnology , Safer Chemicals
Objective:
The project premise is that the surfaces of reactive Fe0 nanoparticles can be modified by amphiphilic block copolymers to be transportable in water through a porous matrix, to preferentially partition at a dense nonaqueous phase liquid (DNAPL)-water interface and to degrade DNAPL to nontoxic products. The specific objectives of this research project are to: (1) demonstrate the ability to provide targeted delivery of reactive nanoparticles to the DNAPL-water interface in saturated porous media; (2) determine the optimal properties of zero-valent iron nanoparticles and increase the DNAPL degradation efficiency relative to unmodified particles; and (3) retain reactive particles at the DNAPL-water interface long enough to be fully utilized.
Progress Summary:
Nanoparticle Synthesis
Two iron nanoparticles were investigated, reactive nanoscale iron particles (RNIP, Toda American, Inc.) and Fe0 nanoparticles (Fe(B)) synthesized in our laboratory using aqueous sodium borohydride reduction of dissolved iron. RNIP particles are synthesized commercially by reduction of Fe-oxides in H2 gas. The particles size and the specific surface area determined by N2 gas adsorption (N2-BET) of each particle are similar. The primary difference between the particle types is their degree of crystallinity. The amorphous nature of Fe(B) makes it more reactive with trichloroethylene (TCE) and with water (see below).
Implications. Commercially available RNIP has the desired reactivity with chlorinated solvents. RNIP is preferable to Fe(B) because the particles are commercially available and inexpensive, do not contain boron, which may be a groundwater contaminant, and remain reactive for longer at a given pH.
Characterization of Bare Nanoparticle Dispersions in Water
Bare RNIP was characterized for its Fe0 content, magnetic properties, and zeta potential. The iron-content of RNIP ranged from 70 percent for freshly made particles to approximately 0 percent for particles that had been in water for 2 years. The Fe0 content will therefore depend on when the particles are removed from the stored slurry and used.
The zeta potentials of RNIP with 14.3 and 26 percent Fe0 are -31.7 mV (pH = 7.5) and -38.8 mV (pH = 8.5) in 1 mM NaHCO3 and 1 mM NaCl. Bare RNIP rapidly aggregates in water to form micron sized aggregates. Subsequently, these aggregates assemble themselves into fractal, chain-like clusters and are rapidly sedimented from solution. The rate of aggregation was a function of the particle concentration and the saturation magnetization of the particles.
Implications. The rapid aggregation necessitates the use of surface coatings to make them mobile in the subsurface.
Polymer Synthesis and Surface Modification
Polymer and Nanoparticle-Polymer Synthesis. Poly(methyl methacrylate) (PMMA) is a good candidate for the hydrophobic blocks. Sulfonated polystyrene (PSS) is a good hydrophilic block because of its high water solubility and degree of charge. A ratio 1/5–1/10 between hydrophobic and hydrophilic blocks is sufficient to provide water solubility of the hybrid nanoparticles. Nanoiron with an amphiphilic polymer shell (~2 mg/m2) was synthesized. It is water soluble and forms more stable particle suspensions than bare nanoiron. Polymer architecture has a measurable effect on the amount adsorbed to the iron surface. The presence of a hydrophobic block adds an additional driving force for adsorption that increases the total surface excess concentration. Removing the PMAA anchor block decreases sorption but not substantially, indicating that the anchor block may not be needed. Desorption of the polymers from the particle surface is limited, indicating irreversible adsorption of these polymers.
Implications. Polymers adsorb fairly strongly (steep isotherm) and essentially irreversibly to the iron surfaces in sufficient quantity to stabilize nanoparticle suspensions in water. Therefore it is possible to modify the iron particle surfaces to provide them with NAPL-water targeting ability.
Characterization of Polymer-modified Nanoparticles
Particle Size Distributions. The polymer modified RNIP showed a bimodal distribution of sizes, with the majority of particles having average particles diameters between 20-100 nm DNAPL-water partitioning. In emulsification studies, PSS-coated silica particles and nanoiron coated with PMAA42-b-PMMA26-b-PSS466 both demonstrated their ability to preferentially locate at the NAPL/water interface.
Implications. The PMAA42-PMMA26-PSS466 triblock polymer-modified nanoiron and PSS-grafted particles can target the TCE/water interface ex situ. Their ability to target the interface and their small size suggests that in situ targeting is possible and that an inexpensive PSS polymer may provide targeting of the NAPL-water interface.
Nanoparticle Reactivity
Fe(B) is highly reactive and transformed TCE into ethane (80%) and C3-C6 coupling products with a surface-area normalized rate constant that is approximately four-fold higher than RNIP. All Fe0 in Fe(B) was accessible for TCE dechlorination. The amorphous nature of Fe(B) enabled it to activate H2 and use it for TCE hydrodechlorination and is responsible for the very high reactivity of these particles. RNIP particles yielded acetylene under iron-limited conditions and ethylene using excess iron. Some Fe0 in the RNIP particles was unavailable for TCE dechlorination and remained in the particles. Adsorbed polymers decreased the particle reactivity by a factor of 4 to 10, depending on the polymer used. pH had a large effect of the lifetime of RNIP. Particle lifetime ranged from 9 months at pH = 8.9 to 2 weeks at pH = 7. RNIP reactivity with TCE did not change over the lifetime of the particles, indicating that the formation of an iron-oxide layer on the particles as they oxidize does not inhibit the reaction with TCE; however, the rate of H2 evolution does slow down as the particles age. Common groundwater anions and cations had no effect on H2 evolution but slowed the TCE dechlorination rate by a factor of two to seven depending on the ionic species. The presence on dissolved solutes accounts for approximately one-half of the decrease in reactivity observed in groundwater compared to deionized water. TCE concentration up to saturation did not affect the TCE dechlorination rate constant but does reduce the amount of H2 evolved.
Implications. Although Fe(B) dechlorinated TCE more rapidly that RNIP, Fe(B) particles generate H2 quickly in water and have a limited lifetime at pH = 8 to 9. Oxidant loading (TCE) and pH are important geochemical parameters affecting the efficiency and lifetime of nanoscale zero-valent iron (NZVI). NZVI application is better suited for source zone remediation than for plume treatment because there is a higher Fe0 utilization efficiency at high TCE levels. The lifetime of NZVI in DNAPL source zones is likely to be only a few weeks, so repeat injections may be needed on short time scales. The presence of commonly encountered groundwater anions should not limit the applicability of NZVI or greatly affect its performance.
Mobility
Mobility studies conducted in laboratory columns demonstrated that bare RNIP is essentially immobile at a concentration of 3 g/L because of aggregation and to attachment to sand grains, but modified RNIP is mobile as the coatings minimize attachment to sand grains and aggregation. The mobility enhancement depended on the type of modifier used and correlates with the ability of a modifier to minimize attachment to sand grains rather than the ability to minimize aggregation. Ionic strength and the presence of divalent cations are important geochemical parameters affecting the mobility of modified RNIP. Increasing ionic strength and the concentration of Ca2+ greatly reduces mobility. Modifiers such as the large molecular weight triblock copolymers that provide electrosteric repulsions provide the best mobility enhancement (but also reduce the reactivity the most).
Implications. Surface modification can enable nanoparticle transport in groundwater aquifers, making it possible to deliver reactive nanoparticles to subsurface contaminants, but also highlights to the need to control the migration of these particles once released into the subsurface as the health effects of these particles have not yet been established. Tri-block copolymers that are high molecular weight polyelectrolytes provide with electrosteric stabilization that can resist high cation concentration and are necessary for enhanced transport, particularly in divalent enriched groundwater, but there will be a tradeoff between mobility and reactivity.
In Situ NAPL Targeting
A preliminary experiment in two-dimensional flow cell showed polymer-modified nanoiron aggregating near entrapped NAPL in the cell. Nanoiron was not retained in regions of the cell without NAPL. Column experiments suing dodecane-coated sand indicate that polymer architecture, particularly the hydrophobe/hydrophile ratio in the triblock copolymers, affect their ability to localize at the NAPL water interface.
Implications. This preliminary experiment suggests that polymermodified nanoiron may have the ability to target entrapped NAPL in situ under flow conditions expected in natural aquifers, but more experiments are necessary to verify the hydrogeochemical conditions conducive to targeting. The architecture of the polymer can be modified to optimize NAPL targeting.
Summary
Two common types of nanoiron have been fully characterized in terms of their particle size, specific surface area, crystallinity/morphology, Fe0 content, and reactivity with TCE and with water. Commercially available Fe0/Fe3O4 core-shell nanoparticles have the desirable properties for in situ remediation: high reactivity with TCE over the lifetime of the particles and lifetimes ranging from 2 weeks at pH = 7.5 to 9 months at pH = 8 to 9. The presence of groundwater solutes does not inhibit reactivity by more than a factor of 7. Surface modification with triblock copolymers enhances the mobility of nanoiron in water saturated porous media and provides the potential to deliver nanoiron to the DNAPL/water interface, but modification decreases the reactivity so there will be a tradeoff between mobility and reactivity. The ability to target entrapped NAPL under in situ conditions will depend on the hydrodynamics as well as the NAPL architecture and on the polymer architecture. These aspects of the problem are under investigation in the last year of the project. A better understanding of the fate and transport of nanoiron will increase their efficiency for TCE degradation and also will identify the potential for unwanted migration and potential exposure to people and biota.
Future Activities:
The investigators did not report any future activities.
Journal Articles on this Report : 6 Displayed | Download in RIS Format
Other project views: | All 75 publications | 13 publications in selected types | All 11 journal articles |
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Liu Y, Majetich SA, Tilton RD, Sholl DS, Lowry GV. TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties. Environmental Science & Technology 2005;39(5):1338-1345. |
R830898 (2003) R830898 (2004) R830898 (2005) R830898 (Final) |
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Liu Y, Choi H, Dionysiou D, Lowry GV. Trichloroethene hydrodechlorination in water by highly disordered monometallic nanoiron. Chemistry of Materials 2005;17(21):5315-5322. |
R830898 (2004) R830898 (2005) R830898 (Final) |
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Liu Y, Lowry GV. Effect of particle age (Fe0 content) and solution pH on NZVI reactivity: H2 evolution and TCE dechlorination. Environmental Science & Technology 2006;40(19):6085-6090. |
R830898 (2005) R830898 (Final) |
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Saleh N, Phenrat T, Sirk K, Dufour B, Ok J, Sarbu T, Matyjaszewski K, Tilton RD, Lowry GV. Adsorbed triblock copolymers deliver reactive iron nanoparticles to the oil/water interface. Nano Letters 2005;5(12):2489-2494. |
R830898 (2005) R830898 (Final) |
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Saleh N, Sarbu T, Sirk K, Lowry GV, Matyjaszewski K, Tilton RD. Oil-in-water emulsions stabilized by highly charged polyelectrolyte-grafted silica nanoparticles. Langmuir 2005;21(22):9873-9878. |
R830898 (2004) R830898 (2005) R830898 (Final) |
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Sarbu T, Lin K-Y, Ell J, Siegwart DJ, Spanswick J, Matyjaszewski K. Polystyrene with designed molecular weight distribution by atom transfer radical coupling. Macromolecules 2004;37(9):3120-3127. |
R830898 (2004) R830898 (2005) R830898 (Final) |
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Supplemental Keywords:
groundwater, VOC, trichloroethylene, TCE, NAPL, subsurface remediation, catalysis, zero valent iron, palladium, block copolymers, environmental chemistry, environmental engineering, source zone remediation, nanotechnology, reductive dechlorination, bimetallic particles, interdisciplinary research,, RFA, Scientific Discipline, Waste, Water, TREATMENT/CONTROL, Ecosystem Protection/Environmental Exposure & Risk, Sustainable Industry/Business, Remediation, Environmental Chemistry, Sustainable Environment, Restoration, Technology, Technology for Sustainable Environment, New/Innovative technologies, Chemistry and Materials Science, Aquatic Ecosystem Restoration, Engineering, Chemistry, & Physics, Environmental Engineering, waste reduction, in situ remediation, DNAPL, remediation technologies, nanotechnology, environmental sustainability, reductive degradation of hazardous organics, environmentally applicable nanoparticles, aquifer remediation design, groundwater remediation, acuatic ecosystems, degradation rates, sustainability, reductive dechlorination, hazardous organics, groundwater contamination, innovative technologies, pollution prevention, contaminated aquifers, reductive detoxification, recycleRelevant Websites:
http://www.ce.cmu.edu/~glowry/ Exit
Progress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.