Influence of metal speciation in landfill leachate on Kaolinite sorption

The sorption onto kaolinite of Pb, Cd, Ni and Cu from a landfill leachate was studied in relation to the metal speciation in the liquid phase. Metal speciation was determined by two different experimental procedures based on the exchangeability on a cation chelating resin (Chelex100) and on the separation by dialysis with membranes at different molecular weight (MW) cut off. The speciation procedures were applied on the leachate before and after equilibration with clay, in order to determine the contribution of the different fractions to the total sorption. As determined by the MW-based procedure, large fractions of dissolved metals were associated to substances with high MW (> 1000 and > 12000 daltons), even if these substances represent only 18% of the total organic content (as determined by chemical oxygen demand, COD). These high-MW fractions contribute to metal sorption onto kaolinite, as also confirmed by the concurrent removal of COD and phenolic substances. As determined by the exchange-based procedure, the main contribution to metal sorption derived from free/labile (rapidly exchangeable) or slowly exchangeable Fractions. However, Pb and Cd were also removed from the stable/inert (not exchangeable) fraction. Because metal sorption is partially due to species that are not exchangeable on Chelex100 and partially to species that have MW more than 1000 daltons, these experimental evidences strongly support that free metals are not the only species participating in the sorption onto kaolinite and that some metal species are adsorbed without previous dissociation.