Degradation of toxic chloroethylenes and chloroaromatics by vitamin B12-based reductive dechlorination and by hydroxyl radical-based oxidative dechlorination reactions.

摘要

This dissertation work involved the study of dechlorination reactions of selected chloroethylenes and chloroaromatics at room temperature by reductive and oxidative techniques. The dechlorination results were established by the disappearance rates of the parent chlorinated compound, and simultaneous chloride formation. The kinetics of these reactions were studied in detail, and the significant parameters affecting reaction rate were identified.;Specifically, the reductive dechlorination of trichloroethylene and cis-dichloroethylene was achieved by the cobalt-centered biomolecule vitamin B12. The natural oxidation state of cobalt in vitamin B12 is +3. When the cobalt center is reduced to an oxidation state of +1, it becomes a strong nucleophile, and can reduce chlorinated organics. Thus, the transformation of cobalt is critical. Literature studies have reported conventional approach of utilizing a bulk reducing agent such as titanium(III) citrate to achieve this transformation. This dissertation work has explored a novel electrochemical approach to effectively accomplish this transformation. This comprised of immobilizing vitamin B12 in an electronically conducting polypyrrole film. The immobilized biomolecule was utilized for the continuous electrochemical dechlorination of saturated solutions of both trichloroethylene and cis-dichloroethylene. The rate of dechlorination by the novel electrochemical method was found to be significantly higher than that obtained by the conventional chemical technique.;The hydroxyl radical-based oxidative dechlorination of 2,4,6-trichlorophenol was accomplished using gluconic acid as the chelate in Fenton reaction. Although the chelate can be obtained from commercial sources, this study investigated a novel approach of the simultaneous generation of chelate gluconic acid and H2O2 needed for free radical generation in Fenton reaction, by an enzymatic method. Specifically, the enzyme glucose oxidase was studied with respect to potential on-site remediation applications. The rate of oxidation of glucose to generate gluconic acid and H2O2 was established using the enzyme in free and immobilized forms. An immobilized enzyme packed bed reactor was modeled to predict the bed lengths required to achieve complete conversion of glucose. The dechlorination rate of 2,4,6-trichlorophenol was found to be a function of the chelate concentration. A mechanistic kinetic model incorporating the effects of chelation was developed, and the value of rate constant for the reaction of trichlorophenol with hydroxyl radicals, was estimated.;Keywords. Reaction kinetics, immobilization, enzyme glucose oxidase, Fenton reaction, polypyrrole.