Geochemical Analysis of Leached Elements from Fly Ash Stabilized Soils

Leaching controlling mechanisms of As, Ba, Ca, Cd, Mg, Se, and Sr from soil alone, fly ash alone, and soil-fly ash mixtures were investigated at a pH range of 1.5-14 through geochemical modeling. Results of the pH-static tests from previous studies indicated that As and Se concentrations increased at alkaline conditions, while Ca, Cd, Mg, and Sr concentrations increased when the pH of the aqueous solutions were acidic. Leaching of Ba tended to increase slightly at alkaline pH values. Effluent pH, oxidation-reduction (i.e., redox) potential E-h, and element concentrations measured from pH-static tests were used as input data in the geochemical model. Geochemical modeling results indicated that release of the elements, except As and Se, are controlled by the dissolution and precipitation of oxide, hydroxide, sulfate, and carbonate solids of the elements. For a given element, these solids controlling the leaching of elements were very consistent regardless of soil and fly ash types. Model-based predictions indicated that leaching of Mg and Cd were controlled by carbonate minerals, and both carbonate and sulfate minerals were controlling solids for Ca, Ba, and Sr, depending on the pH of the leachate. According to the results of geochemical modeling analyses, leaching of As and Se were controlled by neither the dissolution nor precipitation of oxide, hydroxide solid minerals. The research reported in this paper suggests that the sorption mechanism was most probably controlling the leaching behavior of As and Se elements. (C) 2015 American Society of Civil Engineers.