Effects of pH on the leaching mechanisms of elements from fly ash mixed soils

A laboratory pH-dependent batch water leach tests were conducted to investigate the leaching behavior of Al, Zn, Cr, Cu, Fe and Mn from soil–fly ash mixtures used in highway constructions under the influence of pH. Soil fly ash mixtures were prepared from six different soils (clayey and sand) and seven different fly ashes (C-type and high carbon fly ash) collected from Wisconsin, Minnesota and Maryland. Mn showed cationic leaching patterns where Mn concentrations decreased monotonically as pH increased, while Al, Cr, Cu, Fe, and Zn showed amphoteric leaching patterns where concentrations of these metals increased at acidic and alkaline pH. An increase in fly ash content increased the leached metal and mineral concentrations from soil–fly ash mixtures. Consistency in leaching behavior for many elements was observed between fly ash and soil–fly ash mixtures. High concentration of dissolved organic carbon in leachate was likely responsible for the increased leaching of Cu from the mixtures with soil having high organic matter. The Geochemical Model MINTEQA2 was used to determine the dominant oxidation states of leached metals and define their leaching controlling mechanisms. The results of geochemical modeling analyses indicated that releases of the elements were solubility-controlled. For a given element, the solubility-controlling solids were found to be very consistent. Oxide and hydroxide minerals control leaching of Al, Fe, Zn, and Cr, and leaching of Cu was controlled by oxide and/or carbonate minerals.