Dr. Kevin M. Sirk, Professor Gregory V. Lowry, Professor Robert D. Tilton, Dept. Chem Eng (Center for Complex Fluids Engineering), Civil & Environmental Engineering
Groundwater contamination sites exist all over the world including the United States where the most problematic sites are Superfund sites. Groundwater contamination in the US is often found to contain chlorinated compounds, (i.e. trichloroethylene (TCE), tetrachloroethene (PCE), chloroform, carbon tetrachloride, 1,1,2,2-tetrachloroethane, etc.). These compounds are sparingly soluble in water, and have been used historically in a variety of US manufacturing processes such as metal degreasing agents, solvents, dry cleaning agents, and additives to enhance product properties.
Exposure to this class of compounds has been linked to toxic side effects in humans and wild life. Specifically, TCE exposure in humans has been linked to several types of cancers. (1) A broader range of toxic organic compounds that include these chlorinated species and are studied by the Environmental Protection Agency (EPA) and the Department of Health and Human Services are generally called Dense Non-Aqueous Phase Liquids (DNAPLs) which refers to the fact that these compounds settle or partition into underground subsurface regions and that they are not highly soluble in water. Figure 1 illustrates DNAPL contamination in and around groundwater and subsurface regions.
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Also, as shown in Figure 1, DNAPL contaminants can reside in subsurface regions in various states. The residual phase of DNAPL is characterized as DNAPL that becomes entrapped within the pore spaces of the soil. The dissolved DNAPL phase is that fraction of DNAPL that dissolves into the groundwater and then gets transported away from the source zone, contaminating a much larger volume of the groundwater and subsurface. Finally, a DNAPL pool is a highly concentrated reservoir of settled DNAPL. This free phase DNAPL existing in pools is also referred to as a “source zone.”
One attractive remediation technology to treat TCE in situ is the use of zero valent iron (ZVI) or nanoscale zero valent iron (NZVI). ZVI and NZVI have been shown to react with TCE in the presence of water to form less toxic products such as ethylene, acetylene, and ethane (3-7).
The reduction of TCE to ethane follows the stoichiometry (5):
C2HCl3+4Fe0+5H+ → C2H6+4Fe2-+3Cl-
Therefore, an approach to remediating TCE is to mobilize NZVI particles in the subsurface and target sources of contamination material. With this approach, one could reduce the time and cost it would take to clean up contaminated sites. Because of their small size relative to subsurface pores, nanoscale zero valent iron (NZVI) offers the potential to be transported in the groundwater, perhaps even to be targeted and delivered preferentially to subsurface source zones. NZVI particles offer high reactivity by virtue of their large specific surface area (10). The only issue is that particle size be maintained and aggregation be reduced.
Several groups including research groups at Carnegie Mellon University, have modified NZVI with a variety of surface modifiers to reduce aggregation rates and enhance the transport of NZVI in model packed sand columns and a number of field sites(8-14).
Stabilizing the NZVI has been pursued via adsorbance of novel amphiphilic block copolymers as surface modifiers. Systematic variations in polymer composition and solution conditions were made to gain mechanistic insight into the relevant colloidal and interfacial processes needed for optimal targeting of the NAPL.
Electrophoretic mobility measurements were made on the Malvern Zetasizer Nano ZS. Samples of the bare and polymer modified RNIP were prepared at a concentration of 25 mg/L in 1mM NaHCO3. HCl or NaOH were used to adjust the pH of the solutions between pH range of 3.5 to 10.
Bare NZVI also known as RNIP (reactive nano iron particles) have poor colloidal stability and therefore transport poorly through model porous media (packed sand columns), so their use as a remediation agent is severely limited due to these problems (12). One potential way to overcome these limitations of bare RNIP is to modify the surface of the particles. Ideally, any surface modifier used in this application will have to meet three basic criteria:
A multidisciplinary approach to this problem has been taken and therefore members from the Departments of Chemistry, Civil & Environmental Engineering and Chemical Engineering at Carnegie Mellon University have worked together to design a triblock copolymer system as a modifier for the NZVI system. Specifically, the copolymers used in these studies include different molecular weights and composition of Polymethylacrylic acid, (PMAA), Polymethylmethacrylate (PMMA), Polybutylmethacrylate (PBMA) and Polystyrene-4-sulfonate(PSS).
Carboxylic acids are known to bind strongly to iron oxide surfaces and therefore the PMAA block is primarily used to attach to the NZVI particles. Additional polymeric components in this copolymer formulation have been adjusted to provide varying degrees of water solubility, dispersability and surface activity.
The goal is to produce a negatively charged surface on the NZVI particles via the adsorption of the negatively charged copolymers. This is expected to then provide an electrostatic repulsion between the iron oxide particles and aquifer surfaces, i.e. thereby increasing the adhesion resistance, improving particle stability and improving transport through porous media.
Figure 2, shows the electrophoretic mobility results of bare ZVNI particles and coated ZVNI particles with several block copolymer surface modifications. The bare ZVNI particles are very close to their isoelectric point in the pH region known to be the typical pH range for groundwater, pH 6 to 8. Figure 2 indicates that the isoelectric point for the bare particles is just below pH 6. Therefore, focusing on the important pH region for groundwater, Figure 2 indicates that the bare ZVNI particles are only slightly negatively charged. Even at pH 8 the electrophoretic mobility remains below -2 µm cm/V s for the bare particles.
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In comparison, adsorption of any of the block copolymer modifiers increased the negative electrophoretic mobility and shifted the isoelectric point to below pH 3. The surface modified ZVNI particle results, Figure 2, indicate a significant increase in the magnitude of electrophoretic mobility for the NZVI particles coated with the various block copolymers, i.e. the electrophoretic mobility ranges from -3 to -4 µm cm/V s for these samples across the pH range of pH 6 to 8.
The higher electrophoretic mobility values are associated with higher effective surface charge and greater electrostatic repulsion, leading to greater dispersion stability.
In conclusion, the results of this research indicates that by enhancing the NZVI dispersion stability, i.e. increasing the electrophoretic mobility of the NZVI samples, the particles do exhibit increased mobility in porous media and additional findings indicate that the modified NZVI particles also promotes pollutant targetability.
These materials are currently being used in field studies to promote groundwater remediation. Electrophoretic mobility (or zeta potential) measurements provide an understanding of the particle surfaces and interfacial behavior. Only by understanding the surface chemistry will it be possible to design the ideal polymer interfaces that will minimize adhesion and promote subsurface transport in order to optimize water remediation efforts in the future.
(1)http://www.epa.gov/ttn/atw/hlthef/tri-ethy.html
(2) Heiderscheidt, JL, DNAPL source zone depletion during in situ chemical oxidation (ISCO): Experimental and modeling studies. Ph.D. Dissertation. Colorado School of Mines, Golden, CO. (2005)
(3) Stroo, H; Unger, M.; Ward, C. H.; Kavanaugh, M.; Vogel, C.; Leeson, A.;Marquesee, J.; Smith, B. Remediating chlorinated solvent source zones. Environ. Sci. Technol., (2003), 37, 225a-232a.
(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, 1338-1345.
(5) Schrick, B.; Hydutsky, B.; Blough, J.; Mallouk, T. Delivery vehicles for zerovalent metal nanoparticles in soil and groundwater. Chem. Mat. (2004), 16, 2187-2193.
(6) Liu, Y.; Choi, H.; Dionysiou, D.; Lowry, G. V. Trichloroethenehydrodechlorination in water by highly disordered monometallic nanoiron. Chem. Mat. (2005), 17, 5315-5322.
(7). Liu, Yueqiang; Lowry, Gregory V. 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.
(8). Phenrat, T; Saleh, N; Sirk, K; Tilton, R.D.; Lowry, G.V. Aggregation and Sedimentation of Aqueous Nanoscale Zerovalent Iron Dispersions. Environmental Science & Technology (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. Industrial & Engineering Chemistry Research (2007), 46(1), 29-34.
(10) Saleh, N; Sirk, K; Liu, Y; Phenrat, T; Dufour, B; Matyjaszewski, K; Tilton, R.D.; Lowry, G.V. Surface Modifications Enhance Nanoiron Transport and NAPL Targeting in Saturated Porous Media. Environmental Engineering Science (2007), 24(1), 45-57.
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(12) Cantrell, K.J.; Kaplan, D.I.; Gilmore, Tyler J. Injection of colloidal Fe0 particles in sand with shear-thinning fluids. Journal of Environmental Engineering (1997), 123(8), 786-791.
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