INFLUENCE OF METAL SPECIATION IN LANDFILL LEACHATES ON KAOLINITE SORPTION

The sorption onto kaolinite of Pb, Cd, Ni and Cu from a landfill leach ate was studied in relation to the metal speciation in the liquid phas e. Metal speciation was determined by two different experimental proce dures based on the exchangeability on a cation chelating resin (Chelex 100) and on the separation by dialysis with membranes at different mol ecular weight (MW) cut off. The speciation procedures were applied on the leachate before and after equilibration with clay, in order to det ermine the contribution of the different fractions to the total sorpti on. As determined by the MW-based procedure, large fractions of dissol ved metals were associated to substances with high MW (> 1000 and > 12 000 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 als o confirmed by the concurrent removal of COD and phenolic substances. As determined by the exchange-based procedure, the main contribution t o metal sorption derived from free/labile (rapidly exchangeable) or sl owly 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 exper imental evidences strongly support that free metals are not the only s pecies participating in the sorption onto kaolinite and that some meta l species are adsorbed without previous dissociation. (C) 1998 Elsevie r Science Ltd. All rights reserved.