Environ. Sci. Technol. 2005, 39, 1354-1358
Engineered Polymeric Nanoparticles for Bioremediation of Hydrophobic Contaminants WARAPONG TUNGITTIPLAKORN,† CLAUDE COHEN,‡ AND L E O N A R D W . L I O N * ,† School of Civil and Environmental Engineering and School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York 14850
Sorption of hydrophobic organic contaminants, such as polycyclic aromatic hydrocarbons (PAHs), to soil has been shown to limit their solubilization rate and mobility. In addition, sequestration of contaminants by sorption to soil and by partitioning in nonaqueous phase liquids (NAPLs) reduces their bioavailability. Polymeric nano-network particles have been demonstrated to increase the “effective” solubility of a representative hydrophobic organic contaminant, phenanthrene (PHEN) and to enhance the release of PHEN from contaminated aquifer material. In this study, we investigate the usefulness of nanoparticles made from a poly(ethylene) glycol modified urethane acrylate (PMUA) precursor chain, in enhancing the bioavailability of PHEN. PMUA nanoparticles are shown to increase the mineralization rate of PHEN crystal in water, PHEN sorbed on aquifer material, and PHEN dissolved in a model NAPL (hexadecane) in the presence of aquifer media. These results show that PMUA particles not only enhance the release of sorbed and NAPL-sequestered PHEN but also increase its mineralization rate. The accessibility of contaminants in PMUA particles to bacteria also suggests that particle application may be an effective means to enhance the in-situ biodegradation rate in remediation through natural attenuation of contaminants. In pump-andtreat or soil washing remediation schemes, bioreactors could be used to recycle extracted nanoparticles. The properties of PMUA nanoparticles are shown to be stable in the presence of a heterogeneous active bacterial population, enabling them to be reused after PHEN bound to the particles has been degraded by bacteria.
Introduction Hydrophobic contaminants such as polycyclic aromatic hydrocarbons (PAHs) are widespread and persistent in the environment. The hydrophobic nature of PAHs causes them to sorb strongly to soil and sorption limits the bioavailability of the contaminants. Sequestration in nonaqueous phase liquids (NAPLs) also decreases the mobility and bioavailability of hydrophobic pollutants (1, 2). Surfactant micelles have been shown in numerous studies to enhance PAH and hydrocarbon solubilization (3-8) but have had mixed results with regard to enhancing the biodegradation of these * Corresponding author phone: (607)255-7571; fax: (607)255-9004; e-mail:
[email protected]. † School of Civil and Environmental Engineering. ‡ School of Chemical and Biomolecular Engineering. 1354
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 39, NO. 5, 2005
compounds. Some investigators have reported increased rates of PAH biodegradation in the presence of surfactant micelles because of enhanced contaminant solubilization (9-11), but others have reported inhibitory effects of surfactants on biodegradation (12-14). Tiehm and Stieber (15) have suggested that the toxicity of surfactants is related to their hydrophil-lipophil balance (HLB). A high HLB value indicates low lipophilicity, and surfactants with low HLB values are thought to be capable of penetrating the lipid layer of liposomes causing cell membranes to become more permeable. Since different surfactants have different HLB values, it follows that a range of experimental results could be obtained regarding surfactant inhibition of bacteria. Kim and Weber (13) also found that preferential biodegradation of surfactant micelles causes the release and recrystalization of the associated phenanthrene which affects the efficacy of surfactant-enhanced bioremediation of contaminated soils. We have previously reported on the synthesis of nonionic amphiphilic polyurethane (APU) nanoparticles from a mixture of poly(ethylene glycol) modified polyurethane acrylate (PMUA) and polyurethane acrylate precursor chains for use in solubilizing PAHs from contaminated soil (16). The physical structure of PMUA nanoparticles is analogous to that of surfactant micelles since the particle exterior is hydrophilic and the interior is hydrophobic. PMUA nanoparticles have hydrophilic poly(ethylene glycol) (PEG) chains extending out from their surfaces and a hydrophobic core of polyurethane acrylate. PMUA nanoparticles are cross-linked; therefore, unlike surfactant micelles, they do not break up upon contact with soil and would not be expected to interact with the liposomes of microorganisms. PMUA nanoparticles have been shown to have excellent properties in terms of their ability to enhance the desorption and the mobility of phenanthrene in an aquifer sand (16). In this study, we investigate the potential of PMUA nanoparticles in the enhancement of PAH bioremediation. Nanoparticle reuse was also evaluated and is reported here.
Experimental Section Materials. A sandy aquifer media was obtained from a quarry in Newfield, NY. The organic and inorganic carbon content of the aquifer material were determined to be 0.57% and 1.43%, respectively, using the thermal conductivity method (total carbon) and the pressure calcimeter method (inorganic carbon) (17). A sieve analysis revealed that the aquifer material contained sand in grain size ranges of 0.02-0.1 mm (11.07%), 0.1-0.25 mm (38.65%), 0.25-0.5 mm (30.30%), 0.5-1 mm (12.73%), and >1 mm (1.16%). The remainder of the aquifer material consists of 4.32% silt (0.002-0.053 mm) and 1.77% clay (
