Determination and Comparison of the Fractal Dimensions of Coal Stack Ash Subfractions by Gas Adsorption and Small - X-Ray Scattering

Polycyclic aromatic hydrocarbons (PAH), many of which can be biochemically transformed to produce known mutagens and carcinogens, are stabilized with respect to photodegradation when adsorbed on coal fly ash. Coal fly ash is a heterogeneous substance in both chemical composition and particle morphology. In order to obtain more homogeneous samples for study, bulk coal fly ash has been separated into various fractions on the basis of chemical composition and physical properties. In previous studies, it was observed that carbonaceous and iron containing fly ash particles suppressed the photochemical degradation of sorbed PAHs. It was postulated that this inhibition to photolysis was due to an "inner filter effect", wherein PAHs are adsorbed in the surface pore structure of these particles, and are thereby screened from incident radiation. Measurement of the fractal dimension, D, of coal ash particles should result in a better understanding of the relationship between particle morphology and the photolysis of sorbed PAHs. The fractal dimension of a particle is a quantitative measurement of the irregularity of the surface. Values of the fractal dimension (D) range from 2.0 to 3.0. The larger the fractal dimension, the more irregular is the surface of the particle. The fractal dimensions of numerous coal ash subfractions have been determined by gas adsorption and small-angle X-ray scattering (SAXS). Two gas adsorption techniques have been used, the "tiling" method and the Avnir-Jaroniec (AJ) method. The "tiling" method resulted in physically meaningless values for D, whereas the AJ and SAXS methods yielded values of D in the expected range of 2.0-3.0. The D values determined by these two methods did not always agree with one another. By comparison of the particle characteristics and the assumptions made in each of the fractal models, it is suggested that the D values obtained by the AJ method are more likely to reflect the surface geometries of coal fly ash particles than the D value obtained from SAXS. In addition, no evident correlation was found between the observed photodegradation of pyrene sorbed on the ash fractions and the D values determined by the AJ method.