Date of Award

12-2007

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Environmental Engineering and Science

Advisor

Schlautman, Mark A

Committee Member

Freedman , David L

Committee Member

Carraway , Elizabeth R

Abstract

Reductive dechlorination of chlorinated organic contaminants using metalloporphyrins as catalysts may be an effective approach to treat widespread contamination. However, most previous studies have examined water-soluble rather than insoluble metalloporphyrins despite the fact that the latter category makes up the largest fraction found in nature. Recent studies have shown that addition of different cosolvents to water can activate otherwise non-reactive metalloporphyrin catalysts and facilitate the reduction of tetrachloroethylene (PCE), presumably by the mere process of making the otherwise insoluble metalloporphyrins dissolve in solution. Therefore, solubility has been advanced as a key factor in the ability of metalloporphyrins to catalyze organic contaminant degradation.
This study was designed to test the solubility hypothesis by examining whether a higher concentration of co-solvent, in this case dimethylformamide (DMF), would result in more metalloporphyrin catalyst remaining in solution and subsequent enhancement in the degradation of PCE. The primary objective was to determine the relationship between co-solvent concentration (0-10% DMF) and the rate of PCE degradation catalyzed by water-insoluble metalloporphyrins such as Fe(III) Cl-TPP and Ni-TPP. Secondary objectives were to examine whether the metalloporphyrin catalyzed degradation of PCE was dependent on different reductants and pH buffers.
Results from this study demonstrate that although addition of DMF is, in fact, required to activate insoluble metalloporphyrin catalysts, there appears to be some threshold whereby continued addition of cosolvent begins to adversely affect the catalyzed degradation of PCE. For Fe(III) Cl-TPP, the rate of PCE degradation increased from 0 to 2% DMF, remained relatively constant at 3% DMF, then declined at higher DMF concentrations. Similar trends were observed for Ni-TPP. Collectively, these findings suggest that solubility of metalloporphyrins is not the only governing factor in their ability to catalyze PCE degradation. A secondary finding was that Fe(III) Cl-TPP appears to be more effective than Ni-TPP for catalyzing PCE degradation, possibly because it may be forming dimers in water-DMF mixtures.
A secondary objective of this study was to compare the effects of different pH buffers and bulk electron donors on the rate of metalloporphyrin-catalyzed PCE degradation to better understand whether these experimental variables might play a potential role in the reaction. Titanium (III) citrate was found to be as effective an electron donor as dithionite Also, little difference was observed when using bicarbonate versus TRIS buffers, as long as the concentration of buffer was high enough to keep the pH from decreasing too much during the reaction.

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