NEUTRALIZATION OF ALKALINE COAL FLY-ASH LEACHATES BY CO2(G)

The factors that control pH in alkaline fly ash leachates were investigated in a series of experiments with four fly ashes. The rate at which the fly ash leachates were neutralized by CO2 (g) absorption and subsequent calcite precipitation was directly proportional to S concentrations in the fly ash leachates because of the effects of SO42- on calcite precipitation rates. After the pH of the solutions declined from their initial high values, solution concentrations of Mg increased and further slowed calcite precipitation. The incorporation of SO42- and Mg into the calcite lattice resulted in the precipitation of a CaCO3(s) that is more soluble than pure calcite. As a result, the leachates remained oversaturated with respect to calcite after 18-135 d of reaction time. In fly ash leachates in natural systems, the rate of input of CO2(g) into the leachate is expected to control the overall rate of pH equilibrium.