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Nanostructured Materials for Environmental Decontamination of Chlorinated Compounds

Yunfeng Lu and Vijay T. John
Tulane University, New Orleans, LA

Our research is directed towards the development of novel mesoporous materials that act as supports for zerovalent iron nanoparticles used in the breakdown of chlorinated compounds. Halogenated organic compounds, such as chlorinated aromatics, chlorinated aliphatics, and polychlorinated biphenyls, are typical of DNAPLs that are prevalent at contaminant sites. In recent years, the use of zerovalent iron has represented a promising and innovative approach to the destruction of these compounds1,2. Of particular interest is the number of publications recently that describe the use of nanoparticles of Fe in remediation through hydrodechlorination3. The enormous surface area of nanoparticles leads to enhanced efficiencies. Additionally, the colloidal nature of Fe nanoparticles indicates that these materials may be pumped to contaminated sites. Alternatively, "funnel and gate" treatment systems may be devised where porous barriers of iron particles are constructed in the path contaminated groundwater plumes4.

As a result of the high surface energy of nanoparticles, the particles tend to aggregate leading to larger units that do not maintain colloidal stability. While Fe nanoparticles may be superparamagnetic, if the particle dimensions exceed 10-15 nm, they exhibit ferromagnetism and this also leads to aggregation and dropout of solution. Finally, iron is hard to functionalize with organic compounds to attempt to maintain stability in aqueous or in organic systems.

Our technical approach combines the simplicity and affordability of the sol-gel processing techniques for ceramic synthesis with the efficiency and spontaneity of surfactant/silica cooperative assembly to manufacture nanostructured decontamination materials using a simple aerosol processing technique.

Figure 1 schematically depicts the formation of nanostructured particles by the aerosol-assisted self-assembly process5. Starting with a solution of soluble silica and surfactant in ethanol/water solvent, the aerosol apparatus atomizes the solution into droplets that undergo a drying and solidification step generating nanoparticles that are collected in a filter. During this process, solvent evaporation enriches the surfactant and silicate from the air-liquid interface of the droplet towards its interior, resulting in their co-assembly and in the formation of nanostructures growing from the interface towards the interior. Subsequent condensation reactions of silica (see the reactions 2 and 3) during the drying and heating steps freezes the mesophases and results in nanostructured particles that exhibit hexagonal, cubic, lamellar, or other nanostructures.

A wide variety of additives (e.g., metal ions, organic functional ligands, monomers, polymers, metal colloidal particles, and catalyst particles) can be readily introduced to the surfactant/silicate solutions and incorporated into the nanostructured particles. This method provides an extremely simple and efficient approach to synthesize various nanostructured composite particles. Figure 2 shows the representative transmission electron microscopy (TEM) images of nanostructured particles containing highly ordered hexagonal, cubic, or lamellar pore channels, suggesting that porous nanostructured silica particles can be readily prepared using this approach.


Figure 1. Schematic of the Formation of Nanostructured Particles Using the Aerosol-Assisted Surfactant Self-Assembly


Figure 2. Representative TEM Images of Hexagonal (Left), Cubic (Center), and Lamellar (Right) Nanostructured Silica Particles Prepared Using Aerosol-Assisted Surfactant Self-Assembly

Our Recent Results

The aerosolization technique employing surfactants as structure directing agents leads to the generation of mesoporous ceramics. The idea was to incorporate iron precursors into the solution to be aerosolized with the expectation that iron species would become entrapped in the silica particles. The hypothesis was that if zerovalent iron can be entrapped in porous silica nanoparticles, they would not aggregate. Furthermore, silica can be surface functionalized quite easily to prepare particles that are hydrophobic or hydrophilic and stay sustained in an aqueous or in an organic phase. Since silica is essentially inert, the composite would be expected to be safe.

The results are extremely interesting. We obtain shells of silica with zerovalent nanoparticles of iron. The transmission micrographs of Figure 3 illustrate the results.

  
Figure 3. TEMs of Aerosolized Silica Particles Containing Zerovalent Iron Nanoparticles

Our continuing work seeks to test the efficiency of these materials in the breakdown of trichloroethylene. We are tremendously interested in studying the transport of these particles in pump and treat technologies. Research is underway to functionalize these materials so that they partition to the organic-water interface or to the organic bulk phase for maximum efficiency.

References

  1. Gillham, R.W. In Advances in Groundwater Pollution Control and Remediation, Aral, M.M. Eds.; Kluwer Press, Dordrecht, The Netherlands, 1996.
  2. Tratnyek, P.G. Chem.Ind.1996, 13, 499.
  3. Wang, C-B.; Zhang, W.-X. Environ. Sci. Technol., 1997, 31, 2154. Ponder, S. M. ; Darah, J.G. ; Mallouk, T.E. Environ. Sci. Technol. 2000, 34, 2564. Ponder, S.M.; Darab, J.G.; Bucher, J.; Caulder, D.; Craig, I.; Davis,L.; Edelstein, N.; Lukens, W.; Nitsche, H.; Rao, L.; Shuh, D.K.; Mallouk, T.E. Chem. Mater., 2001, 13, 479.Schrick, B.; Blough, J.L.; Jones, D.; Mallouk, T.E. Chem.Mater., 2002, 14, 5140.
  4. Starr, R.C.; Cherry, J.A. Ground Water, 1994, 32, 465.
  5. Lu, Y.; Fan, H.; Stump, A.; Ward, T. L.; Reiker, T.; Brinker, C. J. Nature, 1999, 398, 223-226.
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