Presentation Abstract

Delivering Polymer-Modified Fe⁰ Nanoparticles to Subsurface Chlorinated Organic Solvent DNAPL

Gregory V. Lowry, Bruno Dufour, Yueqiang Liu, Sara A. Majetich, Kris Matyjaszewski, Tanapon Phenrat, Navid Saleh, Kevin Sirk, and Robert D. Tilton
Carnegie Mellon University, Pittsburgh, PA

Introduction

Dense non-aqueous phase liquids (DNAPLs) in the subsurface remain an important and costly environmental liability. DNAPL serves as a continuous long-term source of groundwater contamination. This project integrates several basic science fields to advance a particle-based strategy for in situ DNAPL degradation by providing targeted delivery of reactive particles directly to subsurface DNAPL.

Over the past decade, laboratory and field studies have demonstrated that zerovalent iron and bimetallic colloids (Fe/Pd) can rapidly transform dissolved chlorinated organic solvents into non-toxic compounds1-3. This emerging technology also has the potential to address DNAPL contamination. The objective of this research is to develop and test reactive nanoscale particles for in situ delivery to, and degradation of, chlorinated solvents that are present as DNAPLs in the subsurface. The hypothesis under consideration is that the surfaces of reactive Fe⁰-based nanoparticles can be modified with amphiphilic block copolymers to maintain a stable suspension of the particles in water for transport in a porous matrix, as well as create an affinity for the water-DNAPL interface. Delivering reactive particles directly to the surface of the DNAPL-water interface will decompose the pollutant into benign materials, reduce the migration of pollutant during treatment, and reduce the time needed to remove residual pollution by other means, such as natural attenuation.

Research in the first two years of the project has focused on: (1) identifying suitable Fe⁰ nanoparticles and understanding the properties that control their reactivity with TCE; (2) synthesizing and characterizing amphiphilic block copolymer-modified nanoiron; and (3) evaluating the DNAPL-targeting and transport properties of the resulting polymer-modified functional nanoparticles.

Results and Discussion

Identifying suitable Fe⁰ nanoparticles and understanding the properties that control their reactivity with TCE. The TCE reaction rates, pathways, and efficiency of two types of nanoscale Fe⁰ particles were measured in batch reactors; particles synthesized from sodium borohydride reduction of ferrous iron Fe(B), and commercially available particles (RNIP) synthesized from the gas phase reduction of Fe-oxides in H₂. Particle characterization indicated many similarities between the particles, but several distinct differences between the particle types were found. TEM micrographs of the particles evaluated are given in Figure 1(a,b). Both particle types showed a core-shell morphology. RNIP particles were crystalline and had a Fe⁰ core and a magnetite (Fe₃O₄) shell. No other Fe-oxides were detected. Fe(B) (borohydride reduced particles) were amorphous α-Fe⁰, and contained 4-5 wt % boron (18 mol %). Boron precipitated on the outer shell of the particles (as borate) accounts for ~0.2-1 wt %, so the remaining boron is most likely present as a FeB_x alloy^2,3.

  
Figure 1. (a) Fe(B); (b) RNIP

Reactivity was determined under iron limited (high [TCE]) and excess iron (low [TCE]) conditions, and with and without added H2. The reactivity and efficiency of the two particle types were very different and strongly influenced by the oxide shell properties and the presence of boron. For example, the main reaction products using Fe(B) were primarily saturated (e.g., ethane, butane), while the reaction products using RNIP were primarily unsaturated (e.g. acetylene, ethane)². A concentration dependence on the TCE reaction rate and product distribution was observed. Few chlorinated intermediated were observed for either particle. The addition of H₂ to the reactor headspace increased the reactivity of Fe(B), and these particles were able to use externally supplied H₂ to reduce TCE, indicating that these particles are catalytic. RNIP particles did not display this behavior. The ability of Fe(B) to catalyze the hydrodehalogenation of TCE is attributed to their amorphous structure rather than the presence of boron in the structure3. All of the boron in Fe(B) is released during reaction with water of with TCE, which may be problematic if boron degrades water quality or has human or ecotoxicity.

Synthesis and TCE/Water partitioning of amphiphilic block copolymer-modified nanoiron. Atom Transfer Radical Polymerization (ATRP) was used to synthesize tailored block copolymers for hybrid nanoparticles^4-6. Concerning the synthesis of Fe⁰ nanomaterials, we developed a technique for building hydrophobic-hydrophilic hybrids that consist of a short anchoring poly(methacrylic acid) block, a hydrophobic poly(methylmethacrylate) protective shell, and a hydrophilic poly(styrene sulfonate) outer block (Figure 2). This PMAA_x-b-PMMA_y-b-PSS_z architecture provides the desired characteristics (i.e., water solubility, strong electrosteric repulsions, and an affinity for DNAPL). Using an emulsion procedure developed in our laboratory⁷, TCE-water emulsions were formed using nanoiron modified with these triblock copolymers (Figure 3). Figure 3 shows that a distinct shell of aggregated iron nanoparticles surrounds the TCE droplets. The flocculated particles surrounding the emulsion droplets appear to form dendritic structures, and the emulsion droplets are held apart by these aggregates of particles. The width of the aggregated nanoiron shells around the droplets is approximately 1 μm, indicating that they are approximately 5-10 particles thick. Nanoiron was not found inside the emulsion droplets, which is consistent with the facts that the emulsion was of the oil-in-water type and that the polymer-coated nanoparticles are not dispersible in pure TCE. The ability of these particles to form a highly stable emulsion phase demonstrates the ability of amphiphilic triblock copolymer modified iron nanoparticles to preferentially localize at the TCE/water interface⁶.

Figure 2. Hydrophobic-Hydrophilic Triblock Copolymer Containing a Short Anchoring Group (PMAA), Short Hydrophobic Section (PMMA), and an Extended Hydrophilic Block (PSS)

Dispersion stability and transport of modified nanoiron in water-saturated porous media. Without surface modification, RNIP particles rapidly flocculate and sediment from solution (Figure 4). Modification by each polymer increased the stability of the suspensions relative to bare RNIP (Figure 4). The suspension stability increases as the PSS block degree of polymerization increased from 462 to 650. Thus, the larger PSS block of PMAA₄₈-PMMA₁₇-PSS₆₅₀ provided stronger electrosteric repulsions and better stability improvement than PMAA₄₂-PMMA₂₆-PSS₄₆₂, as would be expected for these polymers that are otherwise quite similar to each other. Particles modified with PSS homopolymer (PSS₄₈₄) were more stable than bare RNIP, but less stable than the triblock copolymers (Figure 4). It indicates that PSS homopolymer adsorbs to RNIP to some degree, but less effectively than the triblock copolymers containing the PMAA anchor block. Polymer-modified nanoiron is significantly more transportable that unmodified nanoiron, particularly at the high (3 g/L) concentrations needed for delivery into the subsurface (Figure 5). A systematic evaluation of the geochemical (pH, I, flow velocity) will be conducted.

Figure 4. Sedimentation Curves for Bare and Polymer-Modified Iron Nanoparticle Dispersions (0.08 wt%) in Water

Figure 5. Effect of Triblock Copolymer Modification on Nanoiron Transport in a 10-cm Sand-Packed Column

References

1. Zhang, W. Nanopart. Res. 2003, 5, 323-332.
2. Liu, Y.; Majetich, S. A.; Tilton, R. D.; Sholl, D. S.; Lowry, G. V. Environ. Sci. Technol. 2005, 39, 1338-1345.
3. Liu, Y.; Choi, H.; Dionysiou, D.; Lowry, G. V. Chem. Mater. 2005, 17, 5315-5322.
4. Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H.; Pakula, T. Macromolecules 1999, 32, 8716-8724.
5. Matyjaszewski, K.; Xia, J. Chemical Reviews 2001, 101, 2921-2990.
6. Saleh, N.; Phenrat, T.; Sirk, K.; Dufour, B.; Ok, J.; Sarbu, T.; Matyjaszewski, K.; Tilton, R. D.; Lowry, G. V. Nanolett. 2005, Submitted.
7. Saleh, N.; Sarbu, T.; Sirk, K.; Lowry, G. V.; Matyjaszewski, K.; Tilton, R. D. Langmuir 2005, 21, 9873-9878.

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