pH and fly ash type effect on trace metal leaching from embankment soils

A laboratory research program was initiated to assess the environmental suitability of highway embankment structures built with high carbon fly ash and C-type fly ashes. Two different laboratory leaching tests, water leach tests (WLTs) and column leach tests (CLTs), were conducted to study boron (B), manganese (Mn), molybdenum (Mo), and selenium (Se) leaching from the soil/fly ash mixtures. The results indicated that an increase in fly ash content increased the pH of the effluent solutions and generally increased the leached metal concentrations significantly. The mass of leached metals in WLTs was higher than those in CLTs with few exceptions. It was concluded that pH had great effects on the leaching of B, Mn, Mo, and Se metals and an increase in pH yielded an increase in the leached concentrations of B, Mo and Se while it caused a decrease in the Mn concentrations in the aqueous solutions. Fe2O3 and Al2O3 contents of the fly ashes decreased the leached Mn concentrations while no significant effect on leaching of Mo, Se and B metals was observed. Unburned carbon content of fly ashes did not have a direct effect on metal leaching.