Reductive dechlorination of chlorinated pollutants by metals and organometallics.

Abstract

Reductive dechlorination is an important chemical and biological transformation in anoxic environments, particularly for highly chlorinated compounds. In nature, however, the reaction is slow and certain compounds released into the environment appear recalcitrant. This thesis examines the role of metals and organometallics in enhancing reductive dechlorination of chlorinated pollutants in different biotic and abiotic systems, and the dependence of the reaction rates and product distribution on the redox potential of the system. The ability of denatured heme protein and boiled bacterial suspensions to dechlorinate carbon tetrachloride (CT) under reducing conditions, in addition to the co-metabolic dechlorination of tetrachloroethylene by non-acclimated methanogenic consortia, support the possible non-enzymatic character of reductive dechlorination. The rates and products of reductive dechlorination in a chemical mimetic system are influenced by changes in medium and in reaction components (donor, mediator, acceptor). Donors, such as elemental Zn$\sb{\rm (s)}$ are able to reductively dechlorinate CT to less chlorinated products (even methane). The first dechlorination of a pentachlorobiphenyl is demonstrated in the presence of dithiothreitol (DTT) as donor, vitamin B$\sb{12}$ (B$\sb{12})$ as mediator and 1,4-dioxane as co-solvent, albeit at much lower rates than for the aliphatic C$\sb1$ electron acceptor CT in water. B$\sb{12}$ is a better mediator for CT dechlorination than Co(II), Ni(II) and Fe(III)-hematoporphyrin and -DTT complexes. For each ligand, the electron transfer ability of the complex depends on metal type and decreases in the order: Co, Ni, Fe. An empirical rate law relates the disappearance rate of CT to pH and both DTT and B$\sb{12}$ total concentrations, and a free radical pathway with atom transfer is proposed: one-electron reduction of B$\sb{12}$ (cyanocobalamin(III)) to B$\sb{\rm 12r}$ (cobalamin(II)) by DTT and participation of (cobalamin(II)) in the rate-determining electron transfer to CT with formation of an alkyl-cobalamin(III) intermediate. In parallel, an equation is developed linking the redox potential of the system to the concentrations of reduced, oxidized DTT, and pH. The equation coincides well with theoretical predictions based on the number of electrons involved in the oxidation of DTT and its acid-base properties.