Dissolution of Heavy Metals from Contaminated Soils and Its Prevention by Zeolite

Abstract

Contaminated soils were prepared by impregnating four soils of different origins with heavy metals, Cd, Cu and Pb, to concentrations of 300 mg/kg-dry soil. The effects of pH on the dissolution of heavy metals from the contaminated soils in aqueous solution were investigated. For all soils, the heavy metal dissolution increased markedly when the initial pH of the solution was 2 or below. In the range of initial pH of 4–10, the dissolution of heavy metals was low and almost constant. The pH buffering actions of the soils varied with their place of origin. When soils with high contents of humic substances or other organic matter were immersed in solutions with an initial pH of 12–14, the amount of dissolved Cu increased due to complexation by the dissolved organic compounds. Stabilization experiments were carried out by adding zeolite synthesized from coal fly ash to soil in a mixing tank, and by making pellets of soil and zeolite. The amount of heavy metals in the solution was decreased by the cation exchange action of zeolite and by increasing the solution pH. In the mixing tank, the addition of zeolite showed a stabilizing effect on Cd, Cu and Pb, compared with the absence of zeolite. With pellets of soil and zeolite, the amounts of Cd, Cu and Pb in the solution decreased to less than 2 mg/dm³. Thus the stabilization of heavy metals in contaminated soil can be achieved by addition of zeolite.