Leaching Behavior and Mechanisms Controlling the Release of Elements from Soil Stabilized with Fly Ash

In situ soil stabilized with fly ash has become a practical and economical solution for soil stabilization. Understanding the pH-dependent leaching behavior and mechanisms controlling release of major and trace elements from soil-fly ash mixture is important for assessing the environmental impacts associated with using fly ash in soil stabilization. A pH-dependent leaching test was used to investigate the leaching behavior of soil-fly ash mixtures used in roadway construction. Three broad leaching behaviors as a function of pH were observed from the soils, the fly ashes and the soil-fly ash mixtures: (i) leaching of Ca, Cd, Mg, and Sr follows a cationic pattern where the concentration decreases monotonically as pH increases; (ii) leaching of Al, Fe, Cr, Cu, and Zn follow an amphoteric pattern where the concentration increases at acidic and alkaline pH; and (iii) leaching of As and Se shows both the behaviors following oxyanionic pattern where concentration increases at acidic and alkaline pH and those show anomalous leaching behavior. Consistency in leaching behavior for many elements was observed, which is believed to be due to solubility-controlled. The difference and inconsistency for As and Se are likely due to different controlling mechanism, such as sorption, or solid-solution formation.