Comparison of Leaching of Pb(Ⅱ), Cd(Ⅱ), As(Ⅴ), and Cr(Ⅵ) from Cementitious Wastes Using Acetic Acid and Landfill Leachates

摘要

The Toxicity Characteristic Leaching Procedure (TCLP) has been widely used to characterise the suitability of solid wastes for disposal in landfills. However, the applicability of this test for wastes disposed in different landfill types is often questionable. This paper investigates the leaching profiles of cement-stabilised heavy metal ions, namely Pb (Ⅱ), Cd (Ⅱ), As(Ⅴ), and Cr(Ⅵ), using acetic acid (a TCLP leaching fluid) and leachates from two Australian landfill sites (a municipal landfill and a non-putrescible landfill) as the leaching fluids. It was found that the leaching profiles of Pb, Cd, As, and Cr using the municipal landfill leachate were different to those using the acetic acid and the non-putrescible landfill leachate. The leaching profiles of the four metal ions using the non-putrescible landfill leachate were similar to those using the acetic acid. The presence of high amounts of organic and inorganic compounds in the municipal landfill leachate produced different leaching mechanisms and profiles for these metal ions compared to the acetic acid and the non-putrescible systems. It is postulated here that the organic compounds present in the municipal landfill leachate formed complexes with the cations (Pb and Cd), increasing the mobility of these ions. Moreover, the organic compounds in the municipal landfill leachate induced a reducing environment in the leachate, causing the reduction of Cr(Ⅵ) and As(Ⅴ) to Cr(Ⅲ) and As(Ⅲ) respectively. It is also postulated that the presence of carbonates in the municipal landfill leachate affected the stability of calcium arsenate, since carbonate would compete with arsenate for calcium at high pH, forcing arsenate into the solution.