Direct and Sensitized Dechlorination of Polychlorinated Biphenyls (PCBs).

摘要

Production of polychlorinated biphenyls (PCBs) was banned more than 30 years ago because of their adverse effects on the environment and to human health. However, PCBs entered the environment during their use, legal and illegal disposal, and remain there because of their persistence. Large quantities also remain in storage waiting for treatment.;For the alkaline 2-propanol system, the first step was to study details of the plausible pathways and formation of intermediates specific to the chain reaction. The results reported in Chapter II show that two steps of different mechanistic characteristics, direct photolysis and electron transfer, contribute to determination of the relative reactivity of the PCB congeners. Additional results also show that the direct step can have a non-quenchable component, while the electron transfer step is efficiently quenched by oxygen. To take full advantage of the chain reaction, de-aeration of the solution is required (Appendix.I). Failure of complete oxygen quenching of the excited state of PCBs led to analysis of the nature of the excited states involved (Chapter III). Sufficient evidence for a contribution from a singlet excited state is provided.;It is also possible to take advantage of the chain reaction specific to basic 2-propanol at longer wavelength than 254 nm by using an appropriate sensitizer. Potential reagents are those capable of producing the chain carrier of the reaction, acetone radical anion. Literature review and the results of a few tests are collected in Appendix II.;In the second part of the thesis, sensitized reaction in an organic solvent/water system with methylene blue (MB) is described. MB was chosen based on successful dechlorination of chloroaromatic compounds in its presence under visible light. However such reactions were performed in the presence of a sacrificial electron donor. Mechanistic studies (Chapter III) revealed that the active species in this system was actually the reduced form of MB, known as Leuco Methylene Blue (LMB). LMB was produced from MB by a know reaction stimulated by red light. LMB, which was excited under blue to near UV light, transferred an electron to the ground state of PCBs. LMB can also be produced thermally, which moves the research one step further toward using a monochromatic LED light source without the red light region. For thermal production of LMB, sodium borohydride (NaBH4) was chosen (Chapter VI). Interestingly, NaBH4 has a double role in the process of dechlorination of PCB's, it not only maintains a steady-state concentration of LMB, but also promotes dechlorination. Thermally reduced MB was successfully applied under conventionally used visible and black light irradiation, as well as in a novel LED reactor.;Among effective technologies for PCB remediation, applicable at low temperatures, reductive photodechlorination technologies are of special interest because of the lower risk of producing additional toxic products. Photodechlorination of PCBs in alkaline 2-propanol under irradiation at 254 nm seemed promising because of efficient initiation of a chain reaction. The first part of this thesis reports a detailed study on direct and sensitized dechlorination of PCBs in alkaline 2-propanol to support development of field processes for removal of PCB contamination. The second part reports on an efficient sensitizer in a mixed organic/water solvent system irradiated at 350 nm, which has an efficiency comparable to that of direct dechlorination in alkaline 2-propanol irradiated at 254 nm. The ultimate goal is to use commercially available LED light sources for dechlorination of PCBs, to replace conventional mercury light sources.