Doctoral Dissertations

Date of Award

12-1991

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Chemistry

Major Professor

E. L. Wehry, Bleb Mamantov

Abstract

In the production of electricity from coal, particulate matter (fly ash) is exhausted into the atmosphere. Association of polycyclic aromatic hydrocarbon (PAH) molecules, frequently carcinogenic and mutagenic by-products of inefficient combustion, with fly ash invariably leads to stabilization of the PAH towards photodecomposition by sunlight, thus increasing the residence time of the PAH in the environment. Previous research has indicated that ashes high in carbon content are the most effective at stabilizing PAH to photodegradation. To assess the importance of the surface characteristics of ash particles in this stabilization process, three areas were explored. First, the distribution of available surface area in a collected stack ash sample was analyzed to determine the contributions of different particle types to the bulk ash surface area. Second, gas-solid chromatographic methods were used to assess the energetics of the interaction between pyrene, a four-ring PAH, and the surfaces of ash subfractions in order to explore the possibility that strong surface interactions lead to the photostabilization of the PAH molecule. Third, since PAH are known to efficiently fluoresce in the adsorbed state, and since the fluorescence of some PAH changes in a distinctive manner under the influence of the surface environment, the possibility of using a PAH molecule to "report back" on the surface characteristics of coal fly ash surfaces was explored. Measurement of the specific surface areas of several subfractions of a number of ash samples by inert gas adsorption revealed that significant differences in specific surface area existed among various types of ash particles. It was found that those particles high in carbon content were at times two orders of magnitude greater in surface area (on a per gram basis) than the more gravimetrically plentiful mineral components of the ashes. Thus, these particles would be expected to contribute significantly to the available surface area for adsorption, more so than their mass distribution in the bulk ash indicates. Secondly, from studying the manner in which the surface area changed as a function of particle diameter in the subfractions, it could be determined that the surfaces of the carbonaceous particles of a bituminous coal fly ash were "rougher", on a molecular scale, than the ferromagnetic (and possibly mineral) subfraction of that same ash. A lignite ash sample also displayed surface scaling characteristics indicative of a very rough surface morphology. In most cases studied in these ashes, indications that the surface morphology changes with particle size precludes the use of the surface scaling properties of the larger particles from being used to estimate the surface character of the difficult-to-obtain fly ash of small particle diameter (≥1 μm Obtaining reliable heat of adsorption measurements of pyrene on coal fly ash fractions by gas-solid chromatography was found to be very difficult to achieve, requiring extensive instrumental and methodological development. Adsorption isotherms calculated using the elution by characteristic point method (ECP) were found to be highly irreproducible due principally to the failure of the low volatility PAH to achieve equilibrium between the gas phase and the surface of the coal fly ash. A pre-column method to remove solvent before it could interact with the adsorbent surface and thus compete with the pyrene for adsorption sites was devised. Chromatograms were digitized and analyzed using a LOTUS program to calculate the distribution isotherm. Isotherms obtained over a narrow temperature range allowed calculation of the heat of adsorption of the PAH on the surface of the ash. Results of these measurements indicate that PAH display, in general, higher affinities for carbonaceous and ferromagnetic coal fly ash particles than for mineral ash particles. Lastly, studies using laser induced fluorescence of adsorbed PAH to indicate surface environments proved inconclusive. Principally due to instrumental limitations, fluorescence of adsorbed PAH has not been obtained from coal fly ash samples, although spectra were obtainable from adsorbed pyrene on silica gel and from pyrene in solution. In conclusion, carbonaceous particles were shown to be highly effective surfaces upon which PAH may adsorb. This fact implies that it is the unique surface structure, both morphological and elemental, of the carbonaceous particles which contributes to the stabilization of PAH to photodecomposition. One possible consequence of the affinity of PAH for carbonaceous particles is a decreased bioavailability of PAH due to their irreversible adsorption on the surfaces of carbonaceous coal fly ash particles in the environment or at the point of origin of the particle.

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