In situ Degradation of Trichloroethylene in Soil and Groundwater with Stabilized Zero Valent Iron Nanoparticles and Catalytic Hydrodechlorination with Supported Palladium Nanoparticles
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Chlorinated solvents, such as trichloroethylene (TCE), are potent carcinogens and have been found in soil and groundwater at thousands of National Priorities List sites in the U.S. To facilitate in-situ chlorinated solvents remediation, a new class of stabilized zero valent iron nanoparticles was prepared using carboxymethyl cellulose (CMC) as a stabilizer. This study reported that CMC stabilized Fe-Pd nanoparticles could effectively degrade TCE sorbed on two model soils. Soil sorption can limit the rate and extent of the dechlorination reaction especially for organic matter-rich soils. Dissolved orgaic matter (DOM) can inhibit the degradation rate. Anionic surfactant SDS could aid in overcoming these limitations, however, its effectiveness would depend on soil type and dosage. The transport behavior of CMC-stabilized ZVI was investigated. CMC-stabilized ZVI nanoparticles can be delivered into porous media even with 200 mg/L Ca2+. The presence of 40~80 mg/L DOM as total organic carbon (TOC) had insignificant effect on breakthrough profiles of the nanoparticles, whereas metal oxides on sand grains (4.1 mg-Fe/g and 3.6 mg-Al/g) increased particle deposition by approximately 10%. A revised transport model was developed to simulate the breakthrough profiles and to evaluate the respective effects of adsorption and filtration on transport of ZVI nanoparticles. A field test of the in-situ remediation technology was conducted at the Hill Air Force Base site in Utah. The results confirmed the soil deliverability and dechlorination reactivity of CMC-stabilized Fe-Pd nanoparticles. The nanoparticles were able to be delivered at least five feet down-gradient of the injection well. Following the nanoparticles injection, rapid TCE degradation and elevated degradation products, such as ethane and ethene, were observed in the groundwater. To facilitate application of nanoscale Pd catalysts for water treatment uses, we developed and characterized a new class of supported Pd catalyst by immobilizing CMC-stabilized Pd nanoparticles on three support materials (alumina, Ambersorb 572 and Titanium-Silicalite (TS-1)). The alumina-supported Pd nanoparticles could facilitate rapid and complete hydrodechlorination of TCE. The strong adsorption of TCE and carbon contamination on Ambersorb-572 resulted in much diminished activity. TS-1 could offer superior catalytic performance, suggesting that a support of modest hydrophobicity is in favor of TCE hydrodechlorination.
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