KWRRI Research Reports


In Kentucky, approximately 3 million tons of coal fly ash are produced annually at a disposal cost around $20 per ton. Moreover, disposal is becoming a major issue because of the ash's potential to contaminate surface and groundwater with arsenic, boron, heavy metals, etc. Knowledge on the chemistry of fly ash is essential in developing a methodology that can predict release rate(s) and concentration(s) of chemical constituents of environmental concern (pollutants). Currently, there is major concern in the state how to dispose of safely the fly ash generated from the combustion of coal by electrical generating plants. Safe disposal of fly ash with respect to surface and groundwater protection depends on having the know-how and technology to evaluate the potential of a given fly ash to release toxic pollutants and 2) having the know-how to do something about it, assuming that a given fly ash is shown to have the potential to pollute. Kentucky is in major need of the above technologies because a major portion of its electrical needs comes from coal-fired electricity generating plants. The results of this study showed that Kentucky fly ashes were made of three types of solids: 1) chemically water stable solids (SiO, FeO, AlO), 2) chemically water reactive solids (SO4, BO3). and 3) metal-oxides (CaO, K2O) unstable at the pH range of natural water. The selected fly ashes varied from acidic to alkaline because of the chemical make-up of the source coal. Physical appearance of the samples tested varied depending on coal type and furnace. All fly ash samples were mainly composed of glass-like porous beads that varied in chemical composition with respect to Al/Si/Fe ratio and varied in pH from extremely low (pH near 3) to near pH 11. Alkaline fly ash samples were associated with high boron levels and exhibited extremely low potential pH buffering capacity. Potentiometric titrations revealed a fly ash PZCpH somewhere around 4.6 which was approximately midway between the PZCpH of iron-oxides and SiO2. Also, these data revealed that fly ash surfaces exhibited an apparent pH-dependent positive charge. A positive charge of approximately 40 cmolc kg-1, and a negative charge of approximately 40 cmolc kg-1 with intrinsic protonation and dissociation constants of 106.2 and 10-7.8 , respectively). Little if any charge was exhibited between pH 4 to 8.5. Low pH buffering capacity, low pH dependent charge and relatively low PZCpH appeared to make the fly ash samples tested extremely sensitive to pCO2 with respect to pH and boron release. Increasing pCO2 increased boron release but pCO2 had no influence on nickel release. This suggested that nickel was most-likely strongly chemisorbed. Nickel and cadmium adsorption isotherms showed that adsorption maximum took place above pH 6. The acidic fly ash showed a greater metal adsorption potential than the alkaline fly ash. Because boron (the major pollutant detected in the fly-ash samples tested) is weakly held, one should avoid burying such "fresh" fly-ash in water permeable waste disposal sites.

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Funding Information

The work upon which this report is based was supported in part by funds provided by the United States Department of the Interior, Washington, D.C. as authorized by the Water Resources Research Act of P.L 101-397.