POLYMER MODIFIED IRON NANOPARTICLES FOR ENVIRONMENTAL REMEDIATION

Introduction Nanoparticles are attractive for remediation of various contaminants because of their unique physiochemical properties. Various chlorinated aliphatic hydrocarbons, toxic metals, and explosives have been remediated using metal nanoparticles such as zero valent iron (Fe°) nanoparticles (nZVI). The effectiveness nZVI for groundwater remediation depends upon the effective delivery of the nanoparticles to the water/contaminate interface without flocculation and severe oxidation. To accomplish effective delivery of nZVI, a delivery system that provides for dispersability and colloidal stability of individual nanoparticles in water is required. In addition, the delivery system should protect the nZVI from severe oxidation by dissolved oxygen and water and provide an affinity for the water/contaminate interface. Colloidal stability of nZVI has been accomplished using surfactants. The hydrophobic “tails” of the surfactants physically absorb on the nZVI surface while the hydrophilic “heads” inhibit flocculation and allow for suspension in the aqueous medium. While surfactants enable colloidal stability in water, the highly reversible nature of surfactant absorption limits its application as a delivery system for ground water decontamination since desorption will be favored when the nanoparticles are transported through surfactant-free ground water. In contrast, high molecular weight, amphiphilic polymers show essentially irreversible absorption and, thus, are more suitable as a delivery system for ground water remediation. Saleh, et al. have shown that amphiphilic triblock copolymers with an A-B-C triblock microstructure are effective delivery systems for nZVI. The triblock copolymers were produced using atom transfer radical polymerization (ATRP) in conjunction with a post-polymerization ester-hydrolysis step and a post-polymerization sulfonation step. The results of the research demonstrated enhanced colloidal stability and an increased affinity for a water/organic interface provided by the amphiphilic triblock copolymer. The amphiphilic triblock copolymers synthesized and evaluated by Saleh, et al. showed promising results with respect to enhanced colloidal stability of nZVI and the creation of a thermodynamic affinity of the nanoparticles for the water/contaminant interface. However, kinetic studies showed a decrease in the rate of contaminant degradation by the polymer-modified nZVI as compared to unmodified nZVI. The reduction in contaminant degradation rate was attributed to low permeability of the contaminant through the film absorbed onto the nZVI. In addition to issues associated with contaminant degradation rate, the synthesis method required to produce the triblock copolymer structure is quite sensitive to impurities and oxygen and quite slow. Further, catalyst residues can be difficult to remove. As a result, there exists a need to prepare tailored, multifunctional polymeric materials using a costeffective, synthetic route that can serve as a highly effective delivery system for nZVI. Considering the requirements of an effective delivery system for nZVI, functionalized amphiphilic polysiloxanes are an ideal class of polymers for the application. Figure 1 provides a schematic representation of amphiphilic polysiloxane graft copolymers (APGCs) that could be easily synthesized and evaluated. The hydrophobicity of the polysiloxane polymer backbone will protect the nZVI from excessive oxidation by creating a barrier to water while also creating an affinity of the coated nanoparticles for the water/contaminant interface as illustrated schematically in Figure 2. Since polysiloxanes have a very low glass transition temperature (-120°C) and are highly soluble in most hydrocarbons, they will readily allow permeation of organic contaminants such as trichloroethene to the Fe° nanoparticle surface providing fast, efficient contaminant remediation. Additionally, the versatility of the hydrosilylation reaction enables tailoring of the polysiloxane polymer backbone chemical structure to enhance contaminant specificity.