Kaolinite sorption of Cd, Ni and Cu from landfill leachates: influence of leachate composition

Heavy metal speciation in landfill leachates plays a significant role in de termining the mobility during the percolation through soils. The complexati on characteristics of landfill leachate directly affects heavy metal solubi lity and the extent of the interaction with soils, lowering or raising the sorbed amount depending on the relative affinity of the complexed metal and uncomplexed form to soil adsorption sites. In this paper, the adsorption o f Cd, Ni and Cu onto kaolinite from three leachates (collected from landfil l at different fermentation stage) is studied, also in comparison with meta l speciation by two different operative procedures. The heavy metals, at th eir natural concentration, were divided into operational classes according to an exchange-based procedure and by fractionation on the basis of molecul ar weight (exchange onto Chelex100 resin and ultrafiltration, respectively) . All the experiments were performed also on synthetic solutions designed a ccording to leachate composition and theoretical speciation. The experiment al results have shown leachate complexing capacity is strongly dependent on landfill age, and that broad parameters such as COD, DOC, pH, ionic streng th and VFA concentration are not able to predict it. It is notheworthy that the strong complexing capacity of leachate can cause extraction of metals from the solid phase instead of adsorption from the liquid one.