Geochemical controls of coal fly ash pH

When coal fly ash is initially mixed with water, the initial pH of resultingextract or leachate may be strongly acidic (pH 4) oralkaline (pH12). With time, however, this pH range tends to narrow becauseof geochemical buffering reactions. Because pH isthe major variable that controls the leaching of many potential groundwater contaminants, understanding the long-term pHbehavior of fly ash leachate is crucial to evaluating the environmental impacts of fly ash management. Using laboratoryextract data, kinetic-geochemical models were created to gain a better understanding of the potential buffering that influencesthe long-term pH of ash leachate. The initially low pH of acidic fly ashes may be short-lived because the acidity is neutralizedby the dissolution of calcium and magnesium oxides, is ultimately buffered by carbon dioxide yielding a pH of 7 to 8. Alkalinefly ash leachate (pH.10) tends to absorb carbon dioxide, and the resulting pH of the liquid phasewill decrease with time to apH between 8 and 9. Kinetic modeling suggests that the chemical composition of short-term laboratory extracts of coal fly ashwill not be representative of long-term leachate after equilibrating with the atmosphere.