Degradation of Polychlorinated Biphenyls and Polycyclic Aromatic Hydrocarbons in Supercritical Carbon Dioxide

• Main Authors: Yun-Chieh Chen, 陳韻潔
• Other Authors: Hwa-Kwang Yak
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/79203592271690212108

博士 === 中原大學 === 化學研究所 === 101 === Persistent organic pollutants (POPs) are defined as organic compounds that pollute and persist in the environment. Polychlorinated biphenyl (PCB) is a family of 209 congeners that well fits the classification of POPs. PCBs have high thermal stability, high chemical inertness, resistance to acid, alkali, and corrosive chemicals, and are difficult to biodegrade. Owing to their low polarity and fat solubility, they are very difficult to metabolize in animals, thus continue to accumulate in food chain in nature and ultimately affect human health. In this research, PCBs are reductively dechlorinated by zero-valent iron (Fe0) in supercritical carbon dioxide (sc CO2) at 100 atm and 300 ℃. The results show that hydrodechlorination of Aroclor 1254 after four hours of reaction is complete, with the sole product being biphenyl. These highly successful results demonstrate that zero-valent iron coupled with scCO2 may be an attractive method for the complete conversion of PCBs to the much less toxic biphenyl. Inexpensive household steel wool was also used as a reducing agent for the reductive dechlorination of PCBs in sc CO2. Untreated steel wool does not produce significant results. Steel wool treated with 6 N HCl, however, showed complete reduction of the PCBs (Aroclor 1254) used. Scanning electron microscopy (SEM) reveals formation of nanoscale pits (nanopits) on the surface of steel wool after etching with HCl. These nanopits are responsible for the high efficiency of the hydrodechlorination process. In a separate but similar theme, PCBs are completely reduced to bicyclohexyl in scCO2 by the use of hydrogen gas and a novel, lab-synthesized catalyst that contains palladium and/or platinum nanoparticles impregnated onto Santa Barbara Amorphous 15 (Pd/SBA－15). This catalyst was synthesized by mixing 1 to 4 % palladium (w/w) and/or 1 to 4 % platinum (w/w) with the mesoporous SBA－15 by the Incipient Wetness Impregnation (IWI) method. With addition of 5 atm hydrogen gas into the supercritical carbon dioxide at 100 atm and 160 ℃ with a 1% Pd/BSA-15 catalyst, all PCBs studied were completely converted to bicyclohexyl. Hydrodechlorination of PCBs with a Pt＋Pd/ SBA15 catalyst with addition of 5 atm hydrogen gas into the supercritical carbon dioxide at 100 atm and 50 ℃ was also successful, with all PCBs studied completely converted to bicyclohexyl. A 1%Pd+4%Pt/SBA-15 catalyst can also be successfully applied to hydrogenation of polycyclic aromatic hydrocarbons.