Grantee Research Project Results
Final 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 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 localize at a DNAPL-water interface or contaminant source area, and to degrade DNAPL to non-toxic products. Specific project objectives are to i) determine the optimal properties of bare and surface-modified zero-valent iron nanoparticles (NZVI) for in situ treatment of the chlorinated solvent trichloroethylene (TCE), and ii) to demonstrate the ability to provide targeted delivery of reactive nanoparticles to the DNAPL-water interface or contaminant source area in saturated porous media.
Summary/Accomplishments (Outputs/Outcomes):
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 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, zeta potential. The Fe- content of RNIP ranged from 70% for freshly made particles, to ~0% 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% Fe0 are -31.7 mV (pH=7.5) and -38.8 mV (pH=8.5) in 1mM NaHCO3 and 1mM NaCl. Bare RNIP rapidly aggregates in water to form micron sized aggregates due to magnetic attractive forces between particles. 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 due to 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 20-100nm. 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 would suggest that in situ NAPL targeting is possible, however this was not realized under flow conditions in situ as described below. Stability in aqueous dispersions. Adsorbed triblock copolymers or commercially available polyelectrolytes having molecular weights ranging from 2.5 to 125 kg/mol stabilize a fraction of the RNIP dispersion in water. The degree of stabilization depends on both the adsorbed mass of polyelectrolyte and the extended layer conformation. Larger particles are not completely stabilized as the long range magnetic attractive forces are larger than the shorter range elecetrosteric repulsions afforded by the adsorbed polyelectrolyte. Implications. The resulting particle-particle interaction is agglomeration (as opposed to aggregation) in a secondary minimum which can significantly affect the mobility of nanomaterials in porous media since the agglomeration and the resulting deposition should be reversible.
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 ~4-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 a Fe-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 2 to 7 depending on the ionic species. The presence on dissolved solutes accounts for approximately ½ of the decrease in reactivity observed in groundwater compared to DI water. TCE concentration up to saturation did not affect the TCE dechlorination rate constant, but does reduce the amount of H2 evolved. The presence of adsorbed polyelectrolyte decreased RNIP reactivity by as much as a factor of 24 due to site blocking and to mass transfer resistance for TCE diffusion to the RNIP surface at higher adsorbed mass of polyelectrolyte. Commercially available NZVI products effectively degrade TCE in soil and groundwater from a marine site, however the rates of TCE dechlorination are slower than observed in laboratory reactors. The use of NZVI in regions where DNAPL is present as pools will require a significant mass of NZVI to treat the pool. 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 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. There will be a tradeoff between reactivity and mobility as adsorbed polyelectrolyte needed for mobility enhancement decreases NZVI reactivity with TCE. Delivering this mass of NZVI directly to the pool through an injection scheme will be difficult given constraints on mixing in the subsurface. NZVI is more applicable to contaminant “hot spots” such as back diffusion from a clay matrix rather than in the presence of DNAPL pools.
Mobility:
Mobility studies conducted in laboratory columns demonstrated that bare RNIP is essentially immobile at a concentration of 3 g/L due to 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:
NAPL targeting experiments in columns indicated that NZVI did not effectively target entrapped NAPL regardless of the polymer architecture. Under flow conditions, there is inadequate mixing between particles and DNAPL and not enough energy to fix the particles to the DNAPL-water interface as observed in batch studies where an ultrasonic probe provided the energy needed to form a Pickering emulsion of TCE DNAPL in water by the polyelectrolyte-coated NZVI. Implications. Alternative strategies are required to target polyelectrolyte-modified NZVI to the contaminant source zone. This can be done through velocity control during emplacement, choice of modifier type, and by leveraging ionic strength effects on mobility.
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, provides the potential to deliver nanoiron to the contaminant source zone in porous media, but modification decreases the reactivity so there will be a tradeoff between mobility and reactivity. TA better understanding of the fate and transport of nanoiron will increase their efficiency for TCE degradation, and will also identify the potential for unwanted migration and potential exposure to humans and to sensitive species in the environment.
Journal Articles on this Report : 11 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. |
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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. |
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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. |
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Liu Y, Phenrat T, Lowry GV. Effect of TCE concentration and dissolved groundwater solutes on NZVI-promoted TCE dechlorination and H2 evolution. Environmental Science & Technology 2007;41(22):7881-7887. |
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Phenrat T, Saleh N, Sirk K, Tilton RD, Lowry GV. Aggregation and sedimentation of aqueous nanoscale zerovalent iron dispersions. Environmental Science & Technology 2007;41(1):284-290. |
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Phenrat T, Saleh N, Sirk K, Kim H-J, Tilton RD, Lowry GV. Stabilization of aqueous nanoscale zerovalent iron dispersions by anionic polyelectrolytes: adsorbed anionic polyelectrolyte layer properties and their effect on aggregation and sedimentation. Journal of Nanoparticle Research 2008;10(5):795-814. |
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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. |
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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. |
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Saleh N, Sirk K, Liu Y, Phenrat T, Dufour B, Matyjaszewski K, Tilton RD, Lowry GV. Surface modifications enhance nanoiron transport and NAPL targeting in saturated porous media. Environmental Engineering Science 2007;24(1):45-57. |
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Saleh N, Kim H-J, Phenrat T, Matyjaszewski K, Tilton RD, Lowry GV. Ionic strength and composition affect the mobility of surface-modified Fe0 nanoparticles in water-saturated sand columns. Environmental Science & Technology 2008;42(9):3349-3355. |
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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.